B R'mili

A Boreave

A Meme

P Vernoux

M Leblanc

L Noel

S Raux

B D'anna

A Boréave

L Noël

Physico-Chemical Characterization of Fine and Ultrafine Particles Emitted during Diesel Particulate Filter Active Regeneration of Euro5 Diesel Vehicles

Keywords: DPF active regeneration, Euro5, Diesel exhaust, black carbon, ultrafine and fine particles, chemical composition, TEM-EDX, AMS

published or not. The documents may come

Introduction

Motor vehicles, especially those powered by Diesel engines, have often been cited as a leading source of ambient ultrafine particles (UFPs) emission. [START_REF] Argyropoulos | Concentration levels and source apportionment of ultrafine particles in road microenvironments[END_REF][START_REF] Hama | Sub-micron particle number size distribution characteristics at two urban locations in Leicester[END_REF][START_REF] Hofman | Ultrafine particles in four European urban environments: Results from a new continuous long-term monitoring network[END_REF][START_REF] Morawska | Ambient nano and ultrafine particles from motor vehicle emissions: Characteristics, ambient processing and implications on human exposure[END_REF] UFPs are defined as particles having an aerodynamic diameter of 100 nm or less, although they are not a major factor in mass-based particulate matter (PM) measurements in the environment, they are the dominant contributor to the overall number of particles in cities and near busy roads and highways. [START_REF] Morawska | Ambient nano and ultrafine particles from motor vehicle emissions: Characteristics, ambient processing and implications on human exposure[END_REF] A growing concern arises in the public health community about the contribution of the UFPs to the overall health impacts of particulate matter (PM) and their ability to bypass the human body's natural respiratory filtration systems. [START_REF]Understanding the health effects of ambient ultrafine particles[END_REF] Population based epidemiological studies as well as toxicological and clinical studies indicate a strong association between PM exposure and adverse health outcomes. [START_REF] Pope | Health effects of fine particulate air pollution[END_REF][START_REF] Laden | Association of fine particulate matter from different sources with daily mortality in six US cities[END_REF][START_REF] Li | Ultrafine particulate pollutants induce oxidative stress and mitochondrial damage[END_REF] Particles deposit in different parts of the lung according to their aerodynamic diameter, resulting in varying degree of toxic potency. For example, the ultrafine particle fraction may be more toxic than the accumulation or coarse mode due to their smaller sizes and higher exposed surface area that becomes available for adsorption of potentially toxic organic species. [START_REF] Li | Ultrafine particulate pollutants induce oxidative stress and mitochondrial damage[END_REF][START_REF] Oberdorster | Toxicology of Ultrafine particles: in vivo studies[END_REF] In urban areas, vehicles are a major emission source of ambient UFPs both in terms of number and mass. [START_REF] Morawska | Ambient nano and ultrafine particles from motor vehicle emissions: Characteristics, ambient processing and implications on human exposure[END_REF] In 2012, Diesel exhaust was classified as being carcinogenic to humans (IARC group 1), as well as labeled a cause for other health concerns such as lung and heart diseases. [START_REF]IARC Diesel and gasoline engine exhausts and some nitroarenes[END_REF][START_REF][END_REF] Since 1993 European transportation regulation has focused on reducing PM mass, CO 2 and NOx emissions. From 2011, regulation on particle number (PN) emission has been introduced (Euro5b) limiting PN to 6.0 × 10 11 #/km (particle diameter (Dp) > 23 nm). 12 In Diesel cars, PM reduction is achieved by combining Diesel Oxidation Catalyst (DOC) and Diesel Particulate Filter (DPF). The main role of the DOC is to oxidize CO and unburnt organic compounds. The DOC, on the other hand, oxidizes NO to form NO 2 which assists PM oxidation in the DPF. [START_REF] Haralampous | Reaction and diffusion phenomena in catalyzed diesel particulate filters[END_REF][START_REF] Kandylas | NO2-assisted regeneration of diesel particulate filters: a modeling study[END_REF][START_REF] Konstandopoulos | Fundamental studies of diesel particulate filters: transient loading, regeneration and aging[END_REF][START_REF] Shrivastava | Kinetics of soot oxidation by NO2[END_REF] The operation principle of DPF systems is based on the separation of the airborne particles from the gas stream through filtration or physical deposition (i.e. inertial impaction, interception, Brownian diffusion and thermophoresis mechanisms). [START_REF] Konstandopoulos | Fundamental studies of diesel particulate filters: transient loading, regeneration and aging[END_REF][START_REF] Adler | Ceramic Diesel Particulate Filters[END_REF][START_REF] Choi | The effects of filter porosity and flow conditions on soot deposition/oxidation and pressure drop in particulate filters[END_REF][START_REF] Herner | Effect of advanced aftertreatment for PM and NO x reduction on heavy-duty diesel engine ultrafine particle emissions[END_REF][START_REF] Mayer | Particle filter retrofit for all diesel engines[END_REF][START_REF] Wade | Diesel particulate trap regeneration techniques[END_REF] Filtration efficiency is very high and a 95 % reduction PM can be easily achieved. [START_REF] Burtscher | Physical characterization of particulate emissions from diesel engines: a review[END_REF][START_REF] Majewski | Diesel emissions and their control[END_REF] Continuous soot deposition on the DPF's walls forms a deep-bed "soot-cake", which significantly improves filtration efficiency. [START_REF] Choi | The effects of filter porosity and flow conditions on soot deposition/oxidation and pressure drop in particulate filters[END_REF][START_REF] Konstandopoulos | Update on the science and technology of diesel particulate filters[END_REF] To avoid excessive back pressure on the engine, regeneration of the accumulated soot is periodically required, and it is commonly accomplished by an exothermic reaction. The so-called "active regeneration" is initiated by the post-injection in the engine's cylinders during the late expansion stroke or directly in the exhaust line in order to generate exothermic energy across the DOC. [START_REF] Fino | Open issues in oxidative catalysis for diesel particulate abatement[END_REF][START_REF] Salvat | Passenger car serial application of a particulate filter system on a common rail direct injection diesel engine[END_REF][START_REF] Van Setten | Science and technology of catalytic diesel particulate filters[END_REF] Combustion of soot by oxygen requires temperatures above 600°C which are rarely achieved in the exhaust of Diesel vehicles. Soot combustion temperature can be reduced by the a catalytic washcoat deposited in inlet channels of the DPF to boost NO oxidation reaction into NO 2 which can oxidize soot, called catalyzed DPF (CDPF). [START_REF] Setiabudi | The role of NO2 and O-2 in the accelerated combustion of soot in diesel exhaust gases[END_REF][START_REF] Twigg | Will Emission Control[END_REF] Another common system, called Fuel Borne Catalyst (FBC), is based on the addition of metallic salts or organometallic compounds into the engine combustion chamber. [START_REF] Majewski | Diesel emissions and their control[END_REF][START_REF] Ntziachristos | Effects of a catalysed and an additized particle filter on the emissions of a diesel passenger car operating on low sulphur fuels[END_REF][START_REF] Song | The role of fuel-borne catalyst in diesel particulate oxidation behavior[END_REF] Upon combustion the additive produces nanoparticles of metal oxides which are intimately mixed with soot particles and stored on the DPF walls. As the regeneration process starts and the temperature rises, the oxygen of the metal oxide catalyses the oxidation of the soot layer from around 550°C instead of 650-700°C without any catalysts. [START_REF] Zhang | Determination of active site densities and mechanisms for soot combustion with O2 on Fe-doped CeO2 mixed oxides[END_REF] During active regeneration process, the filter temperature sharply increases while the pressure gradually drops, due to the soot-cake combustion. [START_REF] Beatrice | Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies[END_REF][START_REF] Ko | Effect of active regeneration on time-resolved characteristics of gaseous emissions and size-resolved particle emissions from light-duty diesel engine[END_REF][START_REF] Cauda | PM0.1 Emissions during Diesel Trap Regeneration[END_REF] This process is characterized by high emission of PM. Bimodal particle distributions are typically observed during DPF regeneration. [START_REF] Beatrice | Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies[END_REF][START_REF] Bergmann | On-road and laboratory investigation of low-level PM emissions of a modern diesel particulate filter equipped diesel passenger car[END_REF][START_REF] Bikas | Regulated and Non-Regulated Pollutants Emitted during the Regeneration of a Diesel Particulate Filter[END_REF][START_REF] Campbell | Transient Gaseous and Particulate Emissions Measurements on a Diesel Passenger Car including a DPF Regeneration Event[END_REF][START_REF] Dwyer | Emissions from a diesel car during regeneration of an active diesel particulate filter[END_REF][START_REF] Giechaskiel | Particle Measurement Programme (PMP): Particle Size and Number Emissions Before, During and After Regeneration Events of a Euro 4 DPF Equipped Light-Duty Diesel Vehicle[END_REF][START_REF] Guo | Performance of a catalyzed diesel particulate filter system during soot accumulation and regeneration[END_REF][START_REF] Mathis | Comprehensive particle characterization of modern gasoline and diesel passenger cars at low ambient temperatures[END_REF][START_REF] Mohr | Particle emissions from diesel passenger cars equipped with a particle trap in comparison to other technologies[END_REF] The nucleation mode has mainly been attributed to semi-volatile compounds released at high temperatures and originating from stored volatile material that combines engine lubricating oil and sulfuric acid/ sulfates [START_REF] Bergmann | On-road and laboratory investigation of low-level PM emissions of a modern diesel particulate filter equipped diesel passenger car[END_REF][START_REF] Bikas | Regulated and Non-Regulated Pollutants Emitted during the Regeneration of a Diesel Particulate Filter[END_REF][START_REF] Campbell | Transient Gaseous and Particulate Emissions Measurements on a Diesel Passenger Car including a DPF Regeneration Event[END_REF][START_REF] Dwyer | Emissions from a diesel car during regeneration of an active diesel particulate filter[END_REF][START_REF] Biswas | Oxidative Potential of Semi-Volatile and Non Volatile Particulate Matter (PM) from Heavy-Duty Vehicles Retrofitted with Emission Control Technologies[END_REF][START_REF] Cauda | Secondary nanoparticle emissions during diesel particulate trap regeneration[END_REF][START_REF] Yamada | Mechanisms of Increased Particle and VOC Emissions during DPF Active Regeneration and Practical Emissions Considering Regeneration[END_REF] or to the fragmentation of soot particle aggregates. [START_REF] Beatrice | Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies[END_REF] A recent study reported direct evidence of gaseous sulfuric acid (GSA) emissions in modern Diesel engines exhaust. [START_REF] Arnold | First Online Measurements of Sulfuric Acid Gas in Modern Heavy-Duty Diesel Engine Exhaust: Implications for Nanoparticle Formation[END_REF] GSA emissions increased with sulfur containing fuel but also during high engine load, when catalyst temperature is high. [START_REF] Arnold | First Online Measurements of Sulfuric Acid Gas in Modern Heavy-Duty Diesel Engine Exhaust: Implications for Nanoparticle Formation[END_REF] The accumulation mode during DPF regeneration was mainly associated to carbonaceous soot particles coated with small fraction of ash and organic compounds. [START_REF] Beatrice | Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies[END_REF][START_REF] Cauda | PM0.1 Emissions during Diesel Trap Regeneration[END_REF] The quantification of the volatile fraction of particles in car exhaust is not straightforward and can be strongly affected by condensation and coagulation phenomena, sometimes not originating in the engine but in the sampling installation. [START_REF] Giechaskiel | Measurement of automotive nonvolatile particle number emissions within the European legislative framework: A review[END_REF] Despite these difficulties, the impact of exhaust PM on air pollution and human health cannot be properly assessed without a better estimation of this volatile fraction. Here, we report PM measurements during the DPF active regeneration of two Euro5 Diesel cars equipped with most commonly used after-treatment devices at a chassis dynamometer test bench. A new analytical approach was adopted and provided access to novel physico-chemical and morphological features of the fine and ultrafine particles emitted (including the PM volatile fraction). A compact Time-of-Flight Aerosol Mass spectrometer (c-ToF-AMS, Aerodyne Inc. USA) was used to investigate size-resolved mass spectra of the volatile PM fraction. Particle morphology and elemental composition were investigated sampling at specific times through porous TEM grids using the mini particle sampler (MPS).

Experimental set-up and methodology

Vehicles and fuels specification

Two vehicles with different Diesel engines and after treatment technologies were evaluated during the testing phases; both were equipped with a common-rail direct injection and meet Euro 5 emission standard.

1. Vehicle 1: fitted with a close coupled DOC and non-catalyzed DPF combined with a Fuel Borne Catalyst (FBC-DPF).

2. Vehicle 2: fitted with two DOCs and a catalyzed DPF (CDPF).

The main vehicles characteristics are summarized in Table 1. The after-treatment configurations evaluated in this study corresponded to the most widespread Euro5 technologies and Diesel exhaust control configurations for passenger cars. The fuel used during the experiments met the EN590 standard representative of current commercial fuels with low sulfur content less than 2mg/kg.

2.2.

Measurement protocol

The tests were conducted at IFP Energies nouvelles facility. Dedicated instrumentation was deployed at the dynamometer test bench to follow exhaust emissions (exhaust composition particle number, particle composition and morphology) and vehicle parameters (vehicle speed, exhaust gases temperature and pressure, pressure drop across the DPF) in order to characterize the active regeneration process of the two DPFs. The schematic of the experimental set-up is shown in Figure 1.

The exhaust was sampled using a two-stage dilution and sampling system with perforated tube and ejector diluter (FPS-4000 Dekati ® ). The FPS-4000 was used with a dilution ratio of about 7:1, calculated from the CO 2 concentrations continuously measured in the exhaust line and downstream of the FPS-4000 corresponding to sampling rate of about 21 L/min. The FPS dilution system allows to sample at the tailpipe using constant temperature and dilution ratios. It can therefore provide a better assessment of the volatile fraction of PM with respect to the CVS where PM emission are measured using variable dilution ratios and temperatures. [START_REF] Louis | Dilution effects on ultrafine particle emissions from Euro 5 and 1 Euro 6 diesel and 2 gasoline vehicles[END_REF] Particulate emissions were monitored by several instruments. A Scanning Mobility Particle Sizer with Faraday Cup Electrometer (SMPS+E GRIMM) equipped with a small Differential Mobility Analyzer (Vienna Type DMA) was chosen for particle number concentration and size distribution measurements. Particles were charged using an annular dielectric barrier discharge. The SMPS+E was operated with a sheath air flow of 3 L/min and an aerosol flow of 0.3 L/min. Aerosols size distribution measurements between 2 and 35 nm were achieved every 37 seconds. Data inversion is done according to the standard ISO 15900 (standard 2009).

On-line time resolved measurements of sub-micrometer particle composition were performed using a compact time-of-flight (c-TOF Tofwerk) aerosol mass spectrometer (AMS, Aerodyne Inc. USA). The AMS operating principles, calibration procedures and analysis protocols are described in details elsewhere. [START_REF] Drewnick | Aerosol quantification with the Aerodyne Aerosol Mass Spectrometer: detection limits and ionizer background effects[END_REF][START_REF] Drewnick | A new time-of-flight aerosol mass spectrometer (TOF-AMS)-Instrument description and first field deployment[END_REF] The instrument provides quantitative sizeresolved mass spectra of the non-refractory components empirically defined as vaporizable species over a 1s time interval at 700°C and 10 -7 torr. [START_REF] Drewnick | A new time-of-flight aerosol mass spectrometer (TOF-AMS)-Instrument description and first field deployment[END_REF] These typically include organic matter, ammonium nitrate and sulfate, chloride and water. This instrument combines the mass spectrometer system with aerodynamic particle sizing and thus provides information on the aerodynamic size distribution of particles. Particles were sampled through a critical orifice of 100 µm at approximately 85 mL/min. C-ToF-AMS data analysis was done with AMS Analysis Toolkit Squirrel v.1.57. using Squirrel software and a cumulative peak fitting and iterative residual analysis. [START_REF] Muller | Enhanced spectral analysis of C-TOF Aerosol Mass Spectrometer data: Iterative residual analysis and cumulative peak fitting[END_REF] The inlet system has 100% transmission efficiency of particles in the vacuum aerodynamic diameter range 70-500 nm and partial transmission of particles in the 25-70 nm and 500 nm-1.5 μm ranges. The instrument was calibrated with ammonium nitrate particles. Gas-phase corrections were made by HEPA-filtered air sampled at the beginning or the end of each test.

A Thermo Scientific Model 5012 MAAP (Multi-Angle Absorption Photometer) was used for Black Carbon (BC) mass concentration measurements of vehicle 2 (FBC-DPF) at a single nominal wavelength at 670 nm. The instrument measures both light attenuation and scattering at specific angles 0°, 130° and 165° reducing significantly the influence of scattering, even if material other than BC is also sampled in the filter. [START_REF] Petzold | Multi-angle absorption photometry -a new method for the measurement of aerosol light absorption and atmospheric black carbon[END_REF] The technique has been mainly applied to ambient measurements but it has also been successfully deployed to combustion particles. [START_REF] Petzold | Multi-angle absorption photometry -a new method for the measurement of aerosol light absorption and atmospheric black carbon[END_REF] Correction for measurement artifacts at high BC mass concentration levels were taken into account as suggested by Hyvarinen et al. [START_REF] Hyvärinen | Correction for a measurement artifact of the Multi-Angle Absorption Photometer (MAAP) at high black carbon mass concentration levels[END_REF] A mini-particle sampler (MPS) was used to collect exhaust particles. This technique, based on filtration through TEM-porous grids, enables sampling of particles directly on a specific support minimizing additional sample preparation procedure and sampling artifact. [START_REF] Le Bihan | Particle Sampling by TEM Grid Filtration[END_REF] A solenoid valve system was used to perform sampling at specific times of the regeneration events with a flow rate of 0.3 l/min. The deposited particles were then investigated by TEM, coupled with energy-dispersive X-ray (EDX) with respect to their size, morphology and elemental composition (JEOL 2010F microscope operated at 200 kV).

2.3.

Test plan

To verify DPF regeneration repeatability three regenerations were performed for each vehicle. One after the vehicle arrival, thereafter, the DPF was loaded driving for approximately 600 km on random successive cycles (10 New European Driving Cycle (NEDC), 6 World harmonized Light-duty Test Cycle (WLTC), 4 Common Artemis Driving Cycle (CADC) and 12 urban part of CADC, steady speed haulages arose from the NEDC and with different conditions of engine starts (from cold to hot)). A random order of driving cycles was implemented to avoid systematic bias. A decoy was then applied to the DPF back pressure sensor to simulate an increase in the DPF loading and the need of a DPF cleaning. This decoy forced the engine control unit (ECU) to initiate an active regeneration process and to switch to the dedicated strategy once the pressure drop before and after the DPF increased. The DPF regenerations were always initiated at constant vehicle speed of about 90 km/h. The engine load remained constant during the whole regeneration process and was set to achieve an upstream DPF temperature of about 390 °C, in order to characterize the regeneration process with similar and stable initial conditions for both DPFs. Figure 1 shows the schematic of experimental set-up at the chassis dynamometer test bench.

Results and Discussion

As mentioned above, three regenerations were realized for each vehicle, for brevity, only the results from the first regeneration of each vehicle will be presented and discussed in details here. The second and the third regenerations showed very strong similarities, therefore, only the second ones are reported in the Supplementary Information (SI) document. Temporal evolution of the volatile fraction of PM monitored by a cTOF-AMS together with methane (tracer for the post injection) and temperature profile upstream DPF are reported in Figure 2a and 2b for the FBC-DPF and CDPF vehicles, respectively.

Temperature profiles for the two vehicles (brown line) are slightly different even if the average upstream DPF temperature is approximately 610 °C. Temperature variability can have various causes, such as different conductivity of the DPF material and/or different length of exhaust lines, position in the pipe and different post-injection strategy. For the FBC-DPF vehicle (Figure 2a), the temperature first quickly increased up to 600 °C, followed by slower increase to 630 °C and a oscillatory behavior between 600 °C and 630 °C until postinjection end. Such temperature oscillations are most probably due to the regulation of the fuel post-injection in order to guarantee the required regeneration temperature and to avoid filter breakup. For the CDPF vehicle (Figure b), as the catalytic after-treatment is located in close-coupled, a sharp increase of temperature up to 620°C was observed involving a temperature overshoot at the beginning of the regeneration process before stabilization at 610 °C.

Regeneration of the FBC-DPF vehicle (Figure 2a) generated a progressive emission of PM composed of BC (black line) up to a maximum value of 450 µg/m 3 , condensable material up to 85 µg/m 3 at the end of regeneration. The condensable fraction of particles was composed by inorganic material (ammonium bisulfate, sulfuric acid, traces of nitrate) and organic material. The pie chart in Figure 2a shows the relative weight percentage of each volatile species : bisulfate 51%, organics 28%, ammonium 10%, sulfuric acid 8% and nitrate 3%, averaged during 600 seconds from the regeneration start. Good filtration efficiency, close to 90%, calculated using BC data, was rapidly restored after post-injection end. BC emissions dropped to 50-60 µg/m 3 and remained stable for 1800 seconds then the engine was turned off. AMS measurement were interrupted just after the DPF regeneration end. Figure S2 shows the good repeatability of BC emission for the three regenerations of the FBC-DPF vehicle. BC time profiles were explained by the reduced filtration efficiency of the DPF due to the oxidation of the soot layer throughout the regeneration. The required time to rebuild the soot cake is related to both soot concentration in the exhaust and DPF walls porosity. [START_REF] Beatrice | Detailed characterization of particulate emissions of an automotive catalyzed DPF using actual regeneration strategies[END_REF][START_REF] Cauda | Secondary nanoparticle emissions during diesel particulate trap regeneration[END_REF] Figure S1a shows the time profile and chemical composition of the condensable fraction observed for the second regeneration. A very similar time profile was observed even if considerable less sulfur containing particles were emitted during the second regeneration.

The CDPF vehicle, Figure 2b, was characterized by high particle emission in the first minutes of the DPF regeneration. The condensable fraction reached a maximum mass loading of 150 µg/m 3 of bisulfate, ammonium, sulfuric acid and organic just after the regeneration start.

The pie chart in Figure 2b shows the relative weight percentage of each volatile species : bisulfate 52%, organics 29%, sulfuric acid 11% and ammonium 8%, averaged during 600 seconds. The organic fraction was relatively stable for the first 800 seconds, then slowly decreases for almost 800 seconds, at the end the engine was turned off. No black carbon measurements were available for this vehicle. The second regeneration of the CDPF vehicle is presented in Figure S1b: a first small burst of particles enriched in nitrate and organics is observed for the first two minutes, followed by the emission of high loading of sulfur containing particles and in minor extent organics. The sharp increase of temperature up to 620 °C involving a temperature overshoot can tentatively explain the initial burst of particles seen in Figure 1Sb. Particle were also emitted after the post-injection end, but for a relatively short time.

For both vehicles acidic particles were measured during the DPFs regeneration. Average mass spectra for the condensable fraction of particles and the corresponding mass spectra of the organic fraction only, are shown in Figures S3 andS4 (m/z 67, 79, 81, 95, 107, 109,..) typical of cycloalkanes. [START_REF] Tobias | Chemical Analysis of Diesel Engine Nanoparticles Using a Nano-DMA/Thermal Desorption Particle Beam Mass Spectrometer[END_REF][START_REF] Mclafferty | Interpretation of Mass Spectra[END_REF] Lower intensity is observed for the ion fragments C 6 H 5 C n H +2n (m/z 77, 91, 105, 119 . . . ) typical of aromatics. Reference spectra of the nebulized lubricant oil for the FBC-DPF and CDPF vehicles, and the mass spectrum of the nebulized fuel, used for both vehicles, are presented in the figures S5a, 5b and S6, SI document. By comparison of all these spectra it can be concluded that the organic fraction emitted during DPF regeneration resembled rather the fuel post-injection than lubricant oil.

Table S1 shows the elemental composition (ICP-MS analysis) of the lubricant oils. Sulfur content ranged between 0.14 and 0.12 weight % for FBC-DPF and CDPF vehicles, respectively. Only molybdenum was found in substantial higher amount in the CDPF lubricant oil.

Figures 3a and3b show the evolution of the UFPs size distribution and concentration between 4-35 nm measured at the DPFs outlet of both vehicles, as well as the DPF upstream temperature. As the post-injection started, a gradual increase in UFPs concentration, with mobility diameter around 7-15 nm, was observed for the FBC-DPF vehicle (Figure 3a), whereas for the CDPF vehicle (Figure 3b), a rapid increase of particle concentration with mobility diameter of 20-30 nm was observed. In both cases, ultrafine particle emission was observed as long as the active regeneration process took place. This duration, more or less 8-9 minutes, is likely related to the post-injection strategy, which relies on the estimated DPF soot load, traffic conditions met by the vehicle and fuel savings strategy.

In Figure 4 we further investigated the nature of the emitted particles combining aerosol mass spectrometry and TEM techniques. AMS chemically resolved mass normalized particle Time-of-Flight (pToF) size distributions for the FBC-DPF are shown in Figure 4a, 4b and4c.

Figure 4a at the very beginning of the DPF regeneration catches a burst of nucleating particles with aerodynamic vacuum diameter of 25 nm (D va ) composed by bisulfate, ammonium, sulfuric acid and an organic fraction. A second and broader mode between 100-800 nm dominated by the organic fraction is also observed. Transmission of particles below 70 nm is relatively poor in the AMS due to particle losses in the aerodynamic lens. [START_REF] Drewnick | A new time-of-flight aerosol mass spectrometer (TOF-AMS)-Instrument description and first field deployment[END_REF] Nevertheless, the instrument was able to monitor the presence of nucleating particles with aerodynamic diameter of 25 nm. Figure 4b shows the particle size distribution 240 seconds after the regeneration start, the nucleation mode is still present together with the broad mode enriched in bisulfate and organic material. A TEM image of the same time period, figure 4d, shows the appearance of soot in form of fractals and agglomerates, and the presence of droplets of condensable material ranging from 15-20 nm to 100 nm, probably partly adsorbed onto the soot particles. The last period of the DPF regeneration is shown in the figures 4c, 4e and 4f. The AMS pTof plot shows the broad particle mode composed by bisulfate, organic material and some sulfuric acid. TEM images (figure 4e and4f) show an increasing amount of BC, consistent with the observations presented in figure 2a, and still the presence of small liquid droplets. were observed. Figure 5b shows a bimodal distribution, the persistent nucleation mode and an accumulation mode mainly composed by organic material and sulfate, partly adsorbed onto soot particles. This was confirmed by Figure 5d where the both liquid droplets and fractal soot covered by liquid droplets were observed. The liquid fraction easily evaporated under the TEM electron beam, as can be seen in Figure 5e. Soot was observed only after few minutes from the start of the regeneration, probably when the soot-cake was removed. No big soot agglomerates were observed during the regeneration. Figure 5c shows the particles chemical composition and size distribution during post-regeneration, 900 seconds after the beginning of the DPF regeneration.

The nucleating particles observed during both DPFs regeneration were composed of condensable material mainly by sulfuric acid and bisulfate/sulfate which was partially neutralized by ammonia and an additional organic condensable fraction (as said before from the post-injection). The presence of traces of ammonia, even at pptv level, has been shown to enhance nucleation rate by orders of magnitude via ternary nucleation process. [START_REF] Coffman | A preliminary study of the effect of ammonia on particle nucleation in the marine boundary layer[END_REF][START_REF] Korhonen | Ternanry nucleation of H2SO4, NH3 and H2O in the atmosphere[END_REF] Our results on the chemical and morphological nature of the nucleating particles complete and support the observations of Arnold and co-workers, where gaseous sulfuric acid (GSA) was identified as key precursor in the UFPs formation for engines equipped with DPF. Furthermore, the authors modeled particle size distribution using the measured GSA concentrations and showed that the observed particle size distribution could be reproduced only introducing condensable organic material, [START_REF] Arnold | First Online Measurements of Sulfuric Acid Gas in Modern Heavy-Duty Diesel Engine Exhaust: Implications for Nanoparticle Formation[END_REF] which was actually observed in our measurements. TEM and AMS analysis of the UFPs emitted during Euro5 DPF regeneration are consistent with previous works suggesting that at high temperatures, sulfur from fuel as well from lubricating oil previously stored on the catalyst surface, is released as SO 2 , converted to SO 3 in the DOC, forming then sulfuric acid and/or bisulfate/sulfate. [START_REF] Herner | Effect of advanced aftertreatment for PM and NO x reduction on heavy-duty diesel engine ultrafine particle emissions[END_REF][START_REF] Bergmann | On-road and laboratory investigation of low-level PM emissions of a modern diesel particulate filter equipped diesel passenger car[END_REF][START_REF] Bikas | Regulated and Non-Regulated Pollutants Emitted during the Regeneration of a Diesel Particulate Filter[END_REF][START_REF] Giechaskiel | Particle Measurement Programme (PMP): Particle Size and Number Emissions Before, During and After Regeneration Events of a Euro 4 DPF Equipped Light-Duty Diesel Vehicle[END_REF][START_REF] Guo | Performance of a catalyzed diesel particulate filter system during soot accumulation and regeneration[END_REF][START_REF] Cauda | Secondary nanoparticle emissions during diesel particulate trap regeneration[END_REF][START_REF] Arnold | First Online Measurements of Sulfuric Acid Gas in Modern Heavy-Duty Diesel Engine Exhaust: Implications for Nanoparticle Formation[END_REF][START_REF] Liang | Effects of diesel fuel sulfur level on performance of a continuously regenerating diesel particulate filter and a catalyzed particulate filter[END_REF][START_REF] Maricq | Chemical characterization of particulate emissions from diesel engines: A review[END_REF][START_REF] Swanson | Evaluation of thermal denuder and catalytic stripper methods for solid particle measurements[END_REF] The major conclusions drawn from the present investigations are as follows. The active regeneration of Euro5 vehicles produced significant emissions of the particles both in terms of total number and total mass concentration, reaching up to 400-500 µg/m 3 of BC and 80-150 µg/m 3 of semi-volatile material (inorganic and organic). Ultrafine particles (below 25-30 nm) consisted of small droplets of sulfuric acid and ammonium bisulfate/sulfate and, to a minor extent, of organic material, most likely coming from the post-injection. Desulfation of the catalyst was identified as the key process in the formation of UFPs as their generation ended as soon the temperature in the after-treatment devices was reduced. Soot particles were observed in the accumulation mode and bigger agglomerates, this latter for the FBC-DPF vehicle only. Particles in the accumulation mode were emitted few minutes after the start of the regeneration process, once the soot-cake was removed causing reduced DPF filtration efficiency. The particle emission profiles and size distribution for the FBC-DPF and the CDPF vehicles were quite different and can be explained by different factors : different exhaust after-treatment design and regeneration strategies but also vehicles characteristics (such as engine, catalysts technologies, volumes and locations, actuators, different mileages), and should not be related to the after-treatment technology only. Nevertheless it can be already concluded that in order to minimize high particle concentration, regeneration strategies should avoid full removal of the soot cake. This work, based on an original analytical approach, allowed a better assessment of particle emission during regeneration of two Euro 5 light-duty vehicles equipped with the most commonly used after treatment systems. The results provide new information on morphology, size distribution, chemical composition and dynamics of particle emission during DPF regeneration.

  Figure S4a and S4b, is dominated by the ion fragments C n H +2n+1 (m/z 29, 43, 57, 71, 85, 99 . . . ), typical of normal and branched alkanes and CnH +2n-1 (m/z 27, 41, 55, 69, 83, 97, 111 . . . ) and C n H +2n-3

  Chemically resolved particle size distribution from AMS measurements of the CDPF vehicle are shown in Figure 5a. A nucleation mode was observed for the first two minutes of DPF regeneration owning an average aerodynamic vacuum diameter of 28nm. The chemical composition indicated the presence of ammonium bisulfate, sulfuric acid, and organic matter. Figures 5d shows a TEM image sampled at the beginning of the DPF regeneration, many small droplets (liquid particles) owing a diameter of about 20 nm (geometric diameter)

Figure 1 .

 1 Figure 1. Schematic of the experimental set-up. 260x152mm (78 x 78 DPI)

Figure 2 .Figure 3 .

 23 Figure 2. Time evolution and pie charts of the particle chemical composition during the first regeneration for the (a) FBC-DPF and (b) CDPF vehicle, respectively. Temperature upstream DPF and methane emission for the two regenerations are also presented. 154x120mm (150 x 150 DPI)

Figure 4 .

 4 Figure 4. Particle size distribution and morphology during the regeneration of the FBC-DPF vehicle : chemically resolved particle aerodynamic vacuum diameter taken AMS data sampled PM collected in the period (a) 0 -60 s; (b) 180 -300 s; (c) 300 -540 s; TEM images collected in the period : (d) 180-300 ; (e) and (f) 300-540 s. 231x144mm (96 x 96 DPI)

Figure 5 .

 5 Figure 5. Particle size distribution and morphology during the regeneration of the CDPF vehicle : chemically resolved particle aerodynamic vacuum diameter taken AMS data sampled PM collected in the period (a) 0 -120 s; (b) 240 -360 s; (c) 900 -1020 s; TEM images samples collected in the period : (d) 0 -120 s ; (e) and (f) 240 -360 s. 238x143mm (96 x 96 DPI)

ACS Paragon Plus EnvironmentEnvironmental Science & Technology

Acknowledgements

The authors would like to thank the ADEME agency for financing support in the framework of the research project CAPPNOR (ADEME conventions 1166C0084 CORTEA program). The authors would also like to thank the Electron Microscopy facility at IRCELYON. ACS Paragon Plus Environment Environmental Science & Technology ACS Paragon Plus Environment Environmental Science & Technology ACS Paragon Plus Environment Environmental Science & Technology ACS Paragon Plus Environment Environmental Science & Technology

Supporting Information

Time series and pie chart relative to the second DPF regeneration for both vehicles, BC time profiles for three DPF regenerations (FBC-DPF vehicle), average mass spectra of the semi-volatile and organic fraction of the particles emitted during the first regeneration, mass spectra of the lubricating oil and the TAE fuel used for both vehicles.


Robin Pierron

Sylvain Lecler

Julien Zelgowski

Pierre Pfeiffer

Frédéric Mermet

Joël Fontaine

Etching of

Keywords: Micro-machining, Laser etching, Metals, Photonic jet, Optical fiber

Introduction

Direct laser etching at sub-wavelength scale remains a challenging issue. Numerical simulations have proven the feasibility of light concentration in the near field with a full width at half maximum (FWHM) smaller than the diffraction limit (λ/2) remaining smaller than λ over a distance of few wavelengths depending on the conditions [START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF][START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF]; the intensity can be as high as two hundred times the one of the incident wave [START_REF] Ounnas | Single and dual photonic jets and corresponding backscattering enhancement with tipped waveguides: Direct observation at microwave frequencies[END_REF][START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond near-infrared laser[END_REF]. This phenomenon is called "Photonic Jet" (PJ). It can be obtained at the output of dielectric cylinders [START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF], spherical particles [START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF], square particles [START_REF] Pacheco-Pena | Multifrequency focusing and wide angular scanning of terajets[END_REF] or planar waveguides [START_REF] Ounnas | Single and dual photonic jets and corresponding backscattering enhancement with tipped waveguides: Direct observation at microwave frequencies[END_REF].

It has been shown that PJ can be used to downsize laser etching. Microspheres are deposited randomly directly on the sample [START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond near-infrared laser[END_REF], or periodically organized in a monolayer [START_REF] Grojo | Monitoring photonic nanojets from microsphere arrays by femtosecond laser ablation of thin films[END_REF]. Experimental demonstrations have been performed at different wavelengths and pulse durations [START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond near-infrared laser[END_REF][START_REF] Münzer | Local field enhancement effects for nanostructuring of surfaces[END_REF][START_REF] Huang | Pulsed laser-assisted surface structuring with optical near-field enhanced effects[END_REF][START_REF] Guo | Near-field laser parallel nanofabrication of arbitrary-shaped patterns[END_REF][START_REF] Wu | A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars[END_REF]. However, the particles are not easy to manipulate and they are polluted after the first etching due to their contact with the sample. One solution is to trap the sphere by an optical tweezer [START_REF] Mcleod | Subwavelength direct-write nanopatterning using optically trapped microspheres[END_REF]; an another one that we propose, is to achieve the photonic jet using an optical fiber with a shaped tip, without spheres [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF].

In our case, the shaped tip has a given length and a shape described thanks to a rational Bézier curve having a weight w [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF]. Several tip shapes have been investigated: from triangular (w = 0) to quasi elliptical (w 0.7).

In this paper, a systematic study of the metal etching using a near-infrared (NIR) nanosecond pulsed photonic jet at the end of shaped fiber tips is presented. Several optical fibers with shaped tips have been experimented. The influence of the core size and the tip shape on the working distance, on the etching size and on the energy balance is discussed. The selected fiber tip has been tested on silicon, stainless steel and titanium. The required injected power is discussed in relation with the known ablation threshold. Finally, the influence of the number of pulses on the ablation depth is studied at the minimum etching size.

Experimental details

The experimental setup consists in a NIR pulsed laser (VGEN ISP 1-40-30) commonly used for industrial laser processing, a focusing lens (achromatic doublet Thorlabs AC127-019-C), a XYZ stage with FC or SMA connectors, an optical fiber on a motorized Z-axis stage and a sample target placed perpendicularly to the shaped fiber tip on a motorized X-Y stage. The experiments are carried out in ambient atmosphere conditions. The laser works at a central wavelength of 1064 nm, a pulse duration of 100 ns and a repetition rate of 35 kHz. The setup is controlled by LabView.

Three different step-index silica fibers were used: (1) a single mode 20/125 Large Mode Area (LMA) with a numerical aperture (NA) of 0.08; (2) a multimode 50/125, NA = 0.22;

(3) a multimode 100/140, NA = 0.22. The distance between the fiber tip and the sample is controlled by a camera with a telecentric objective (5x magnification). The optical fiber tip shape is described using a Bezier curve with a weight w chosen to give a PJ with a FWHM of 1 µm, located a few tens micrometers from the tip.

Three types of materials were tested: monocrystalline silicon (wafer) with a passive layer and an initial roughness (R a ) of 2 ± 1 nm, stainless steel (R a = 187 ± 14 nm) and titanium (R a = 423 ± 20 nm). No additional mechanical polishing was performed leaving relatively significant striations on the stainless steel and titanium samples. For the characterization after laser ablations, two different optical methods have been used: an optical microscope with a lateral resolution of 610 nm (50x focusing objective Zeiss, NA = 0.55) and a Zygo NewView 7200 profilometer with an axial resolution of 10 nm and a lateral resolution of 550 nm (50x Mirau objective, NA = 0.55). The power at laser and fiber outputs has been measured with a calorimeter Ophir Vega with a 3A-P cell.

Results and discussion

Influence of the fiber core diameter

Theoretically, it is possible to achieve PJ having a FWHM smaller than a half wavelength [START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF][START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF]. However previous studies have shown that, in this case, the PJ intensity maximum was just on the tip end [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF], which was not a convenient situation. Therefore, the first step has been to study the influence of the core diameter size of the fiber on the PJ position in order to select a fiber achieving a PJ with a minimum FWHM around 1 µm, at a distance from the tip large enough to avoid any disturbance. The fiber tips have been achieved by the LovaLite company thanks to thermoforming. They are shown in Figure 1. The tests have been performed on the wafer of silicon at the minimum power to ablate. The number of pulses was 35 for each PJ etching.

The fiber tip parameters and the results are summarized in Table 1. The PJ distance is the distance between the end of the tip and the maximum intensity in the PJ. It is the working distance in our etching process. As expected, the diameters of PJ etching marks (PJ mark ) are around 1.0 ± 0.6 µm whatever the core diameter, which corroborates our numerical simulations. We can deduce that the size of PJ marks does not depend on fiber core diameter but on the shaped fiber tip (L tip , w).

Besides, the results show that to get a PJ with a FWHM of 1 µm, the corresponding PJ distance must be of the same order than the fiber core diameter (relative deviation less 30%). In order to avoid the possible deposition of ablated material on the fiber tip, the fiber of 100 µm core was selected; its working distance is 97 ± 2 µm.

Influence of the optical fiber on the minimum required power

The 1 µm etching can be achieved independently on the fiber core diameter, however the minimum required injected power is not the same. For example, the results (cf. Table 1) show that the pulse energy (E pulse ) injected in the 100 microns core fiber and necessary to alter the material is five times higher than the one injected in the 50 microns core diameter. Using a shaped optical fiber tip, the photonic jet is due to the fundamental mode. The higher groups of modes are off-axis focused on larger surfaces, giving lower power densities, insufficient to etch. The number of mode groups increases with the size of the fiber core [START_REF] Snyder | Optical waveguide theory[END_REF]. The injected energy is then spread in these groups and the intensity in the fundamental mode becomes lower. Therefore, a singlemode fiber seems to have an interest. The 20/125 LMA fiber is singlemode. However, because of the smaller core diameter, the injection rate is four times smaller than for the two other fibers (cf. Table 1). Thus, to select the fiber core diameter, a compromise must be found between the working distance, the injection condition and the required etching power.

In the following part, etchings on stainless steel and titanium with a 100/140 fiber have been studied. With this fiber, the injection efficiency is higher than the one of the LMA fiber and the working distance is larger than the one of the 50/125 fiber. On the two metals, etching marks around 1.1 µm have been successful achieved without polluting the fiber tip. The results (cf. Table 2) show that a similar minimum energy per pulse (E pulse ) is required to etch the two materials. The PJ mark diameters (PJ mark ) have the same size order, almost 1.2 µm on titanium and stainless steel (cf. Figure 2).

For comparison, the theoretical pulse energy required to reach the ablation threshold with a singlemode fiber (E pulse SM) has been computed. In this case all the energy is assumed to be concentrated on the etched surface (values Table 2). Under our operating conditions (1064 nm, 100 ns, 35 kHz), a threshold fluence of ∼4.5 J/cm 2 for titanium [START_REF] Vladoiu | Nanopulsed ablation rate of metals dependence on the laser fluence and wavelength in atmospheric air[END_REF] and ∼4.0 J/cm 2 for stainless steel have been considered. The required energy with the 100 µm fiber is almost 100 times larger than the one considered in a singlemode. This factor is in accordance with the estimation of the mode groups number M=46 (M = 2π/λ • N A • a/ √ 2 where a is the fiber radius), taking into account that knowing the exact energy distribution on the guided modes is difficult. Table 2: For stainless steel and titanium: PJ marks diameters (PJ mark ), minimum pulse energy injected to begin to etch (E pulse ) and theoretical energy by pulse injected to etch if all the energy would be concentrated on the etched surface. 100/140 fiber with tip numerically designed to achieve PJ with FWHM of 1 µm. 35 pulses were used for each PJ etching.

Influence of the number of pulses on the ablation depth

Finally, the relation between the number of pulses and the ablation depth has been studied for the parameters giving the minimum etching size. The experiments have been performed with the 100/140 fiber on the silicon wafer and measured with the Zygo profilometer. The pulse energy was constant at 34 ± 4 µJ corresponding to the minimum injected energy required to ablate. In order to validate the repeatability, three series of etchings were produced for each value of pulse number.

Figure 3 shows the ablation depth versus the number of laser pulses for the silicon wafer. The ablation depth increases significantly during the first 3500 laser pulses. Then, the depth reaches a maximum value around 700 nm. The fiber tip is not polluted and can still achieve new etchings. The computed photonic jet field depth reaches a few tenths of microns, which can not be the explanation. We suppose that the etched matter is probably redeposited because of the low energy of each pulse.

Besides, the etched mark FWHM as a function of the number of pulses have been measured (cf. Figure 4). The FWHM is constant and measures almost 0.8 ± 0.6 µm independently of the number of pulses, which was not expected. Generally the etched mark diameters have a logarithmic dependence with the numbers of pulses [START_REF] Mannion | The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air[END_REF]. The fact that the etching depth does not increase anymore can be a reason. Moreover, as illustred in Figure 5, we see that the complexity of etched marks profiles increases with the number of pulses probably due to the higher-order modes contribution. Higher pulse energies may be required for deeper etchings but likely with

Conclusions

An experimental study of the influence of the fiber core diameter and the tip shape, on the working distance and the minimum required energy to etch has been performed on a silicon wafer with a nanosecond infrared laser (35 laser pulses, 1064 nm, 100 ns). On this basis, we showed that a compromise between the working distance, the injection rate and the lost energy in the higher guided modes led to choose the 100/140 shaped tip fiber. The photonic jet maximum intensity is located at almost 100 µm of fiber tip. With this fiber, etching diameters of around 1.1 µm have been demonstrated on silicon, stainless steel and titanium. On silicon with a pulse energy as small as 34 ± 4 µJ, after a first regime where the depth increases with the pulse number, a maximum is reached at around 700 nm (< 1µm). It is probably due to a material redeposition owing to the low pulse energy, suggesting that a higher pulse energy is required for

Figure 1 :

 1 Figure 1: Shaped optical fiber tips designed to achieve PJ with a FWHM of 1 µm. (a) LMA 20/125 fiber (b) 50/125 fiber and (c) 100/140 fiber. Fiber L tip [µm] w Injection [%] PJ distance [µm] PJ mark [µm] E pulse [µJ] 20/125 5 ± 1 0.4 9 ± 3 15 ± 2 1.0 ± 0.6 16 ± 5 50/125 17 ± 1 0.5 38 ± 4 51 ± 2 0.8 ± 0.6 7 ± 1 100/140 74 ± 1 1.0 46 ± 5 97 ± 2 1.2 ± 0.6 34 ± 4

Figure 2 :

 2 Figure 2: Etched marks with a diameter of around 0.8 ± 0.6 µm on the stainless steel. 100/140 fiber with tip numerically designed to achieve the PJ with FWHM of 1 µm. Ablations were done at 3 ± 1 µJ pulse energy corresponding to the minimum to etch. 35 pulses were used for each PJ etching. Metal PJ mark [µm] E pulse [µJ] E pulse SM [µJ] Stainless steel 0.8 ± 0.6 3 ± 1 0.020 ± 0.001 Titanium 1.4 ± 0.6 4 ± 1 0.069 ± 0.004

Figure 3 :

 3 Figure 3: The ablation depth [nm] as a function of the number of laser pulses. 100/140 fiber with tip numerically designed to achieve the PJ with FWHM of 1 µm. Ablations were done at 34 ± 4 µJ pulse energy corresponding to the minimum to etch.

Figure 4 :

 4 Figure 4: The etched mark FWHM [µm] as a function of the number of laser pulses. 100/140 fiber with tip numerically designed to achieve the PJ with FWHM of 1 µm. Ablations were done at 34 ± 4 µJ pulse energy corresponding to the minimum to etch.

Figure 5 :

 5 Figure 5: Examples of ablation profiles on silicon for (a) 1050 pulses and (b) 17500 pulses. 100/140 fiber. Ablations were done at 34 ± 4 µJ.

Table 1 :

 1 Dimensions of the optical fiber; tip parameters numerically designed to achieve PJ with FWHM of 1 µm; corresponding minimum pulse energy injected to begin to etch silicon; associated PJ distances and mark diameters. 35 pulses were used for each PJ etching.

			5	38 ± 4	51 ± 2	0.8 ± 0.6	7 ± 1
	100/140	74 ± 1	1.0	46 ± 5	97 ± 2	1.2 ± 0.6	34 ± 4

† ICube Laboratory, CNRS, University of Strasbourg, France ‡ IREPA LASER -Carnot MICA Institute, Pôle API, 67412 Illkirch, France * Corresponding author: sylvain.lecler@unistra.fr

Acknowledgements

We would like to thank Audrey Leong-Hoï (ICube Laboratory, France) for the measures of roughness of metal samples with the Leiz-Linnik microscope developed at ICube (50x objective, NA = 0.85). We acknowledge also the financial support from Conectus Alsace, a member of SATT network.


Muriel Ney

Mônica Macedo-Rouet

Sandrine Charles

Geneviève Lallich-Boidin

* Lbbe

SIG 7 -Learning and Instruction with computers Are well-designed Web sites efficient for learning mathematics at the undergraduate level?

Keywords: Web sites, educational design, mathematics, undergraduate level

We conducted an experiment in order to examine the effectiveness of an educational Web site, which provides resources to undergraduate students in mathematics. The participants' task was to solve 18 questions about differential equations. Students in the paper condition outperformed those in Web site conditions. There was no difference between the original and an improved Web site version. Moreover, students in all conditions preferred working with the paper resource. Practical and theoretical implications are discussed.

Introduction

The use of electronic technologies for teaching and learning at the university has increased tremendously in recent years. But what do we know about their pedagogical effectiveness? The first literature reviews indicated that the benefits of educational technologies were quite limited. Recent studies, however, show that it is possible to optimize learning through the use of appropriate educational design (e.g., [START_REF] Mayer | The promise of multimedia learning: using the same instructional design methods across different media[END_REF][START_REF] Van Merrienboer | Redirecting learners' attention during training: effects on cognitive load, transfer test performance and training efficiency[END_REF].

Our research investigates the efficacy of a set of Web-based resources for undergraduate mathematics learning. We address the following questions: can students efficiently search mathematical information in a Web site? Are they motivated by this resource? Is it possible to increase usability with design guidelines? Based on the notion that usability improves the ease of use of a Web site, we used a set of ergonomic guidelines [START_REF] Scapin | Ergonomic criteria for evaluating the ergonomic quality of interactive systems[END_REF] to evaluate and modify an educational Web site used at the undergraduate level in a French university. Our hypothesis was that the revised version would improve learning outcomes.

Method

Our experiment consisted of a learning session. Students were asked to solve a number of questions on the topic of differential equations and look for related information on a Web site. Participants were 122 undergraduate students in life sciences (75% female; mean age=18.8). They completed a pre-test about their experience with computers and the Web, initial knowledge of the study topic, and socio-demographics.

Materials were a questionnaire with 18 questions on differential equations and a set of documents (6000 words). For each question, there was one page in the set of documents containing relevant information (target-page). These materials were presented in one of four formats: (G1) original Web site; (G2) modified, design, Web site; (G3) paper; (G4) control (questions only). Students' learning outcomes were measured through the number of correct answers and perception of usability and satisfaction. We also measured participant's workload (with an adapted version of the NASA-TLX), time spent in each question and number of pages visited.

Students were assigned to one of the four experimental conditions. They worked individually for 1 1/2 hours. At the end of the session, students worked with their tutor on a related subject, and finally were dismissed.

Results

We found that groups differed in their learning outcomes (p<.001). Post-hoc analysis showed that participants in G3 correctly solved more questions than any other group. The control group solved less questions than any other group, and G1 and G2 did not differ among each other. The effect of group explained 36% of the variance.

The time spent to solve the questions, the number of pages visited, and the mental workload were not different in G1 and G2. Thus, the modified Web site was not more efficient for learning. However, time spent was correlated with learning outcomes (r=.39, p<.01), with the number of target-pages found (r=.36, p<.01), and with Web experience (r=-.38, p<.01). Only a trend in the expected direction existed for the correlation between mental workload and learning outcomes (r=-.10, p>.05).

Students' perceived the menu (table of contents) as "simple and easy to use" to a greater extent in the G3 (94.4%) than in G2 (63.2%) and G1 (51.2%) (p<.05). While no student felt "tired to read on paper ", 42.1% of the students in G2 and 37.2% in G1 felt "tired to read on screen" (p<.05). Finally, more students were satisfied with the aesthetics of materials in G1 (69.8%), than in G2 (50%) and G3 (55.6%) (p<.05). Overall, students declared that they prefer to use the paper rather than the Web site to study, mainly if they had been assigned to the paper condition in the experiment (48.8% in G1, 52.6% in G2, 61.1% in G3).

Discussion

We did not find evidence that a "well-designed" Web site can promote learning. Although we applied design guidelines to construct a modified version of an existing Web site, we did not succeed in demonstrating its superiority for reading and searching information. This result could be due to our type of task (differences in cognitive load may not appear in the context of an exercise, cf. Paas, 1992); to an effect of familiarity with the original Web site; or an inappropriate use of the guidelines. However, we observed significant differences in learning outcomes between the Web sites and a paper version of the same materials. Not only did the paper lead to higher scores but it also received the students' preference as a media for learning. Perhaps, as [START_REF] Martin | Printing and screen reading in the medical school curriculum: Gutenberg versus the Cathode Ray Tube[END_REF] write, the function of the computer is to give access to information, but it delivers information to students that are used to carry books and take notes on paper.

Our results have practical and theoretical implications. At the practical level, we suggest that institutions should consider the problems of delivering pedagogical information online and promote evaluations of their materials. At the theoretical level, our results sum up to studies showing the limitations of on screen legibility (eg. [START_REF] Mayes | Comprehension and workload differences for VDT and paper-based reading[END_REF] and contribute to theories of cognitive load (eg. [START_REF] Van Merrienboer | Redirecting learners' attention during training: effects on cognitive load, transfer test performance and training efficiency[END_REF]. In the perspective of developing new educational technologies, our results suggest that cognitive factors, but also individuals' perceptions, should be taken into account.


Robin Pierron
email: robin.pierron@unistra.fr

Julien Zelgowski

Pierre Pfeiffer

Joël Fontaine

Sylvain Lecler

Photonic jet: key role of injection for etchings with a shaped optical fiber tip

Keywords:

We demonstrate the key role of the laser injection into a multimode fiber to obtain a photonic jet (PJ). PJ, a high concentrated propagating beam with a full width at half maximum smaller than the diffraction limit, is here generated with a shaped optical fiber tip using a pulsed laser source (1064 nm, 100 ns, 35 kHz). Three optical injection systems of light are compared. For similar etched marks on silicon with diameters around 1 µm, we show that the required ablation energy is minimum when the injected light beam is close to the fundamental mode diameter of the fiber. Thus, we confirm experimentally that to obtain a PJ out of an optical fiber, light injection plays a role as important as that of the tip shape, and therefore the role of the fundamental mode in the process.

Direct laser etching at sub-wavelength scale remains a challenge. Numerical simulations have shown that the light can be concentrated in the near field with a full width at half maximum (FWHM) smaller than the diffraction limit [START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF][START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF] and with a power density significantly higher than the incident wave [START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF][START_REF] Heifetz | Photonic Nanojets[END_REF][START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond near-infrared laser[END_REF]. This high concentrated propagating beam is known as a photonic jet (PJ). It has been initially obtained at the output of dielectric particles: cylinders [START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF][START_REF] Itagi | Optics of photonic nanojets[END_REF], spherical particles [START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF][START_REF] Li | Optical analysis of nanoparticles via enhanced backscattering facilitated by 3-d photonic nanojets[END_REF] or square particles [START_REF] Pacheco-Peña | Terajets produced by dielectric cuboids[END_REF][START_REF] Liu | Photonic jets produced by dielectric micro cuboids[END_REF]. Experimental demonstrations have been performed at different pulse widths and wavelengths with microspheres deposited directly on the sample either randomly [START_REF] Münzer | Local field enhancement effects for nanostructuring of surfaces[END_REF][START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond near-infrared laser[END_REF] or periodically organized [START_REF] Wu | A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars[END_REF][START_REF] Guo | Near-field laser parallel nanofabrication of arbitrary-shaped patterns[END_REF][START_REF] Grojo | Monitoring Photonic Nanojets from Microsphere Arrays by Femtosecond Laser Ablation of Thin Films[END_REF]]. Yet, the spheres are not easy to manipulate and they are polluted after the first irradiation due to their contact with the sample. One solution is to trap the particles by an optical tweezer [START_REF] Mcleod | Subwavelength direct-write nanopatterning using optically trapped microspheres[END_REF]; but the technique is not easy to implement in an industrial process.

In the microwaves frequencies, the PJ emerging from a planar waveguide with a shaped tip has been demonstrated [START_REF] Ounnas | Single and dual photonic jets and corresponding backscattering enhancement with tipped waveguides: Direct observation at microwave frequencies[END_REF]. In the optical range, spheres put at the end of a core-etched fiber [START_REF] Aouani | Optical-fibermicrosphere for remote fluorescence correlation spectroscopy[END_REF] or a hollow-core photonic crystal fiber [START_REF] Ghenuche | Hollow-core photonic crystal fiber probe for remote fluorescence sensing with single molecule sensitivity[END_REF] have been used to improve the sensitivity in fluorescence correlation spectroscopy. Yet, the applications are limited to low energies otherwise the fiber end is burned or the spheres are ejected. Another solution, that we have developed, is to use an optical fiber with a shaped tip [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF][START_REF] Pierron | Etching of semiconductors and metals by the Photonic Jet with shaped optical fiber tips[END_REF].

In this letter, several laser-fiber injection systems have been experimented. The injected light power necessary to produce ablation on a substrate has been explored. The tests have been performed on silicon using a near-infrared nanosecond pulsed PJ at the end of a multimode shaped fiber tip. The role of the fundamental mode in the photonic jet phenomenon out of the optical fiber has been experimentally confirmed and the key role of injection in the process has been demonstrated.

The experimental setup consists in a laser source, a system of injection into the fiber and a 3-axis controlled etching platform. The laser is pulsed and emits at a central wavelength (λ) of 1064 nm, the pulses are 100 ns wide and the repetition rate is 35 kHz. The output beam has a 1/e 2 diameter (D laser ) of 6 mm and the beam quality factor M 2 is 1.3. The injection into the fiber system is achieved using an optical component focusing on a SMA connector whose position is controlled by a XYZ stage. The etching platform consists in an optical fiber on a motorized Z-axis stage and the sample is placed perpendicularly to the shaped fiber tip on a motorized XY stage. The experiments are carried out at ambient atmosphere conditions.

A multimode 100/140 step-index silica fiber was used with a numerical aperture (NA) of 0.22, a cladding refractive index (n g ) of 1.457 and a core refractive index (n c ) of 1.440. The sample-tip distance is measured and monitored using a camera with a telecentric objective (5x magnification). The setup is controlled by LabView.

The laser-to-fiber coupling efficiency (η) is defined as the ratio of the power at the fiber tip end and the power available at the laser output. A high sensitivity power detector (Ophir Vega with the 12A-P sensor) has been used for its determination.

Optical components with different focal lengths, numerical apertures and aberration corrections have been tested: an achromatic doublet with a focal length (f ) of 19 mm, a 5x objective microscope with f = 40 mm and an aspheric lens with f = 75 mm.

A monocrystalline silicon wafer with a passive layer has been used as test sample. The laser punctual ablations have been characterized with an optical microscope with a lateral resolution of 610 nm (Zeiss 50x focusing objective, NA = 0.55).

Numerically, PJs with a FWHM smaller than a half wavelength can be achieved when the maximum PJ intensity is close to the particle [START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF][START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF] or close to the fiber tip end [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF], that is from zero to few wavelengths, which is not practical for an etching process. The tip has been designed to achieve a PJ with a minimum FWHM around 1 µm at a distance of 100 µm of the tip in order to avoid any disruption from redeposited melt material. Its shape is described thanks to a Bézier curve set by a base radius (a = 50 µm), a tip length (b = 63 µm) and a Bézier weight (w 0 = 1) [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF]. The tip (cf. Fig. 1) has been achieved by the LovaLite company using an electric discharge based thermoforming technique. Conventionally, the maximum laser-fiber coupling efficiency can be achieved if the numerical aperture of the focusing optical system (NA f oc ) is lower than the numerical aperture of the multimode fiber (NA). In the paraxial approximation, the NA f oc is given by:

NA f oc ≃ D laser 2f (1)
where D laser is the laser beam diameter at 1/e 2 and f is the focal length of the optical injection system. However, in our case, we have considered as optimum the case when the minimum injected power was required to ablate. To find this minimum, the PJ distance has been determined. It corresponds in our etching process to the distance between the tip end and the maximum PJ intensity. A 2D array of points has been etched (cf. Fig 2). Each point has been performed with 35 pulses. On each line the fiber-sample distance was constant, only the power changed; on a column the power was constant, only the distance between the tip and silicon wafer changed.

Different etching sizes appear. The PJ distance can be determined: that is the distance for which the smallest etching is achieved at the smallest power.

As expected, for the three different optical components, a working distance of 100 ± 2 µm has been found with the smallest etched marks (PJ ∅ ) around 1 µm. However different minimum energies (E p ) have been required to ablate (cf. Table 1).

Table 1: Minimum pulse energy (E p ) injected to begin to etch the silicon for different optical focusing systems: an achromatic doublet (f = 19 mm), a 5x objective microscope (f = 40 mm), an aspheric lens (f = 75 mm). Numerical aperture of focusing system NA f oc . Spot diameter at the focus point (D f oc ). Laser-fiber coupling efficiency (η). Etching diameters on silicon (PJ ∅ ) The results (cf. Table 1) show that the maximum laser-fiber coupling efficiency (η = 76%) is achieved when the NA f oc is close to the NA of the multimode fiber. However, in this case, the minimum energy required to etch (E p ) is not the smallest. The lowest energy required to etch is reached when the coupling efficiency is only 61%. Moreover, the results reported in Table 1 show that when D f oc = 12 µm the minimum energy required to ablate is higher than when D f oc = 23 µm. It can be explained by taking into account the intensity distribution in the fiber groups of modes. Indeed, with a shaped optical fiber the photonic jet is due to the fundamental mode; the higher-order modes are focused off-axis on larger ring-shape surfaces [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF]. The higher is NA f oc , the more energy is injected in the higher groups of modes and the lower is the power in the fundamental mode of the fiber.

f [mm] NA f oc D f oc [µm] η [%] PJ ∅ [µm] E p [
Smaller pulse energies are required to etch if the spot size at the focus point of the converging light matches with the fundamental mode diameter of the fiber. The diameter of the light at the focus point (D f oc ) for a laser beam is given by [START_REF] Hachfeld | Laser-beam quality and brightness in industrial applications[END_REF]:

D f oc = 4 π λf D laser M 2 (2)
The diameter of the fundamental mode (2w) for a step-index fiber has been given by Marcuse [START_REF] Marcuse | Loss analysis of single-mode fiber splices[END_REF] approximating the fundamental Gaussian mode profile by: w a = 0.65 + 1.619

V 3/2 + 2.879 V 6 (3)
where a is the radius of the fiber and V = a • 2π/λ • NA is the normalized frequency. In our case, the diameter of the fundamental mode (2w) is 65 µm. In this way, when the spot diameter at the focus point was close to the diameter of the fundamental mode, the required injected energy to ablate has been reduced by a factor of two. In order to evaluate the ratio of the injected energy really involved in the etching process, the theoretical pulse energy required to reach the silicon ablation threshold has been computed. In this case, all the energy was assumed to be concentrated on the etched surface as if only the fundamental mode was involved. Under our operating conditions (1064 nm, 100 ns, 35 kHz), a threshold fluence of around 10 J/cm 2 has been considered (a material heating depth L H of 20 µm determined from [START_REF] Meyer | Optical heating in semiconductors: Laser damage in ge, si, insb, and gaas[END_REF] has been used in [START_REF] Wang | Laser-induced damage threshold of silicon in millisecond, nanosecond, and picosecond regimes[END_REF] to evaluate it). If we consider the best case (f = 75 mm, PJ ∅ = 1.3 µm, E p = 17 µJ), the theoretical minimum required pulse energy is 0.13 µJ; just over 100 times smaller than the measured one. This factor may be in accordance with the estimation of the mode groups number M = 46 (M = V / √ 2 [START_REF] Marcuse | Principles of Optical Fiber Measurements[END_REF]), taking into account that knowing the exact energy distribution on the guided modes is difficult. Furthermore, in our process the optical fiber is bent due to its length (≃ 1.2 m), which introduces mode coupling.

The laser process is repeatable. The fiber tip is not disturbed by the recast material and can be easily manipulated. The required energy can be minimized by controlling the injection. For example, a QR code has been achieved with a set of micro dots with diameters around 1 µm etched using only 17 µJ per pulse (cf. Fig. 3). In the figure, a shading can be observed around some ablated dots. Closer the etching dots, larger the shading around. The dots are spatially closed (5 µm) and achieved in short time (travel speed of the motorized XY stages: 1 mm/s). It is a heat accumulation effect, due to the high-order modes of the fiber which are spread off-axis [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF] and do not contribute to the etching. This confirm the importance of the energy distribution on the modes inside the fiber. To conclude, three optical components for the injection of the laser into the fiber have been used with different numerical apertures and spot diameters. The tests have been performed with a 100/140 µm thermoformed fiber tip. At the lowest numerical aperture (NA f oc = 0.04), when the spot diameter was close to the fundamental mode diameter of the fiber, the required energy to ablate has been reduced by a factor of two. Therefore, the experiments corroborate that the photonic jet is mainly due to the fundamental mode [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF]. This also demonstrates that the required energy can be minimized by controlling the injection to achieve etched marks having diameter smaller than the wavelength with a shaped multimode fiber tip.

Funding SATT Conectus Alsace; LaserJet Project.

Figure 1 :

 1 Figure 1: Multimode 100/140 optical fiber with a shaped tip. Working distance of 100 ± 2 µm.

Figure 2 :

 2 Figure 2: Etchings on silicon depending on the sample-tip distance (Z) and the injected power (Ep). Aspheric lens (f = 75 mm). 35 pulses for each PJ etching.

Figure 3 :

 3 Figure 3: QR code matrix (110x110 µm) on silicon. Each etched mark has a diameter around 1.3 ± 0.6 µm. Pulse energy of 17 µJ. Aspheric lens (f = 75 mm). 35 pulses for each PJ etching.

Acknowledgment The authors would like to thank Gary Argaud for his technical help.


Robin Pierron
email: robin.pierron@unistra.fr

Pierre Pfeiffer

Grégoire Chabrol

Sylvain Lecler

Photonic jet: direct

Keywords: micro-peak machining self-organized micro-spikes, photonic jet, shaped optical fiber tip, multimode fiber, wettability

come

Introduction

Direct laser subwavelenght micromachining is currently a challeging topic. Photonic jets have already demonstrated the ability to reduce the laser etching size beyond the diffraction limit using micro-beads [START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond nearinfrared laser[END_REF][START_REF] Münzer | Local field enhancement effects for nanostructuring of surfaces[END_REF][START_REF] Wu | A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars[END_REF][START_REF] Guo | Nearfield laser parallel nanofabrication of arbitrary-shaped patterns[END_REF][START_REF] Grojo | Monitoring Photonic Nanojets from Microsphere Arrays by Femtosecond Laser Ablation of Thin Films[END_REF][START_REF] Mcleod | Subwavelength direct-write nanopatterning using optically trapped microspheres[END_REF] or more recently using shaped optical fiber tips [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF][START_REF] Pierron | Etching of semiconductors and metals by the photonic jet with shaped optical fiber tips[END_REF][START_REF] Pierron | Photonic jet: key role of injection for etchings with a shaped optical fiber tip[END_REF]. A Photonic jet (PJ) is a high concentrated propagative light beam with a full width at half maximum (FWHM) smaller than the diffraction limit [START_REF] Chen | Photonic nanojet enhancement of backscattering of light by nanoparticles: a potential novel visible-light ultramicroscopy technique[END_REF][START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF][START_REF] Itagi | Optics of photonic nanojets[END_REF][START_REF] Li | Optical analysis of nanoparticles via enhanced backscattering facilitated by 3-d photonic nanojets[END_REF]. The power density can be more than 200 times higher than the one of the incident wave [START_REF] Abdurrochman | Photonic jet breakthrough for direct laser microetching using nanosecond nearinfrared laser[END_REF][START_REF] Lecler | Properties of a three-dimensional photonic jet[END_REF][START_REF] Heifetz | Photonic Nanojets[END_REF]. To achieve PJ, shaped fiber tips are obviously easier to move than a microsphere and therefore, to implement in an industrial process. Moreover, the fiber tips have no contact with the processed surface and are not altered by the removed material [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF][START_REF] Pierron | Etching of semiconductors and metals by the photonic jet with shaped optical fiber tips[END_REF][START_REF] Pierron | Photonic jet: key role of injection for etchings with a shaped optical fiber tip[END_REF]. Until now, only sub-micro PJ etching was reported. Micro-peaks formation was reported for processes using femtosecond laser pulses on thin metal and silicon films [START_REF] Her | Microstructuring of silicon with femtosecond laser pulses[END_REF][START_REF] Ivanov | The mechanism of nanobump formation in femtosecond pulse laser nanostructuring of thin metal films[END_REF][START_REF] Kuznetsov | Nanostructuring of thin gold films by femtosecond lasers[END_REF][START_REF] Unger | Time-resolved studies of femtosecond-laser induced melt dynamics[END_REF][START_REF] Zhu | Silicon microstructuring using ultrashort laser pulses[END_REF][START_REF] Kuznetsov | Laser-induced jet formation and droplet ejection from thin metal films[END_REF]. Micro-peaks were also generated using nanosecond laser pulses on thin gold film [START_REF] Moening | Formation of high-aspect-ratio protrusions on gold films by localized pulsed laser irradiation[END_REF][START_REF] Moening | Focused ion beam and electron microscopy characterization of nanosharp tips and microbumps on silicon and metal thin films formed via localized single-pulse laser irradiation[END_REF], but never on silicon bulk.

In this paper, we report for the first time the possibility to achieve direct micro-peaks surface texturing using nanosecond pulses. Taking advantage of the photonic jet at the exist of a shaped optical fiber tip, peaks with a FWHM of around 1 µm, a height of almost an half micrometer and an apex radius of fews ten nanometers, were repeatability achieved on a silicon wafer. Surfaces with micro-peaks can have a wide range of applications. For example, A few millimeter squares surface with micro-spikes has shown an decrease of wettability.

Experiment details

The laser source is a commercial near-infrared pulsed laser (VGEN ISP 1-40-30) emitting at 1064 nm with pulses of 100 ns at a repetition rate of 35 kHz. The output beam has a diameter of 6 mm at 1/e 2 and a beam quality factor (M 2 ) of 1.3. This corresponds to a quasi-Gaussian beam profile. An achromatic doublet, with a focal length of 19 mm, couples the laser beam into the fiber, whose position is controlled by XYZ microstages.

The fiber system is a multimode step-index silica fiber with a core diameter of 100 µm and a cladding diameter of 140 µm. The numerical aperture (NA) is 0.22. The tip shape is numericaly described by a Bézier curve set by a base radius (a = 50 µm), a tip length (b = 63 µm) and a Bézier weight (w 0 = 1) [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF]. It has been designed to achieve photonic jet of around 1 µm at a distance of around 100 µm of the tip when excited by the fundamenal mode (Fig. 1 a). The numerical method is described in [START_REF] Zelgowski | Photonic jet subwavelength etching using a shaped optical fiber tip[END_REF][START_REF] Pierron | Etching of semiconductors and metals by the photonic jet with shaped optical fiber tips[END_REF][START_REF] Pierron | Photonic jet: key role of injection for etchings with a shaped optical fiber tip[END_REF]. The tip has been achieved by LovaLite using an electric-arc thermoforming technique [START_REF] Borsuk | Alternating current arc for lensing system and method of using same[END_REF]. The result has a base radius a slightly larger due the cladding: a 80 µm (Fig. 1 b). The shaped optical fiber tip is placed orthogonal to the sample. The fiber position is controlled by a Z-motorized stage and the sample position by XY-motorized stages. The distance between the tip and the sample is controlled by a camera with a 5x telecentric objective thanks to an image processing based on the reflectance of the tip into the sample. The laser, the motorized stages and the camera processing are controlled by LabView. The experiences are carried out at ambient atmosphere conditions. The power at the outputs of the laser and the fiber tip has been measured with a calorimeter Ophir Vega with a 12A-P sensor.

In the following we name working distance the distance between the tip end and the maximum of intensity of the photonic jet. Experimentally, when both the laser pulse energy and the distance between the tip and the sample vary, the working distance corresponds to the distance for which the smallest mark is achieved with the smallest energy. It is intrinsic to the fiber tip and has been determined experimentally in our previous work to be 100 ± 2 µm for the tip shown in Fig. 1 b) [START_REF] Pierron | Photonic jet: key role of injection for etchings with a shaped optical fiber tip[END_REF] not far from the 113 µm predicted by simulation (Fig. 1 a). Our experiments have shown that this working distance is enough to avoid the re-deposition of melt material on the fiber tip if material is ablated. This also allows ensure the tip integrity during the motion control: no shock and easier control due the 2 µm PJ positioning tolerance. From simulations, we can give some general rules: (1) Photonic jet with the same FWHM (Full Width at Half Maximum) can be obtained at larger working distance using larger optical fiber core. However, in this case more energy is required to achieve the same process. Namely, it is not so easy to couple energy in the fundamental mode. (2) For a given fiber radius, generally the working distance is larger increasing the Bézier weight (w 0 ) or decreasing the tip length (b). However, the photonic jet FWHM increases. Therefore this tip was a good compromise.

The sample is a monocrystalline silicon wafer with a passive layer (thickness of approximately 2 nm) and an initial roughness of 2 ± 1 nm. For each laser irradiation, 35 pulses have been used. Before and after the laser process, the sample was cleaned and dried with alcohol and dry air.

The micro-peaks have been characterized with two different methods: a white light interferometric microscope and an atomic force microscope (AFM). The interferometric system based on coherence scanning interferometry was a Zygo NewView 7200 profilometer with an axial resolution of 3 nm and a lateral resolution of 550 nm (50x Mirau objective, NA = 0.55). The AFM, a Park Systems XE-70 isolated inside an acoustic enclosure, worked in the non-contact mode. The field size used was 20x20 µm with a lateral resolution of 39 nm and a axial resolution of 2 nm.

The wettability of the flat and fakir's bed of nails-like textured sample was measured with a Krüss Drop Shape Analyser DSA25. Static contact angle were measured with around 10 µL distilled water droplets by the sessile drop method. A computation method based on the analysis of the droplet shape determined by the model of Laplace-Young was used.

Results and discussion

A 110x100 µm matrix with peaks every 5 µm has been achieved on silicon. In Fig. 2, the 3D view obtained by the interferometric microscope shows that micro-peak machining by PJ with an shaped optical fiber tip is a repeatable process. The measured mean height for 30 peaks was 354 ± 3 nm, the mean FWHM was 1 ± 0.6 µm and the maximum height was 590 ± 3 nm. Thus, peaks have a width larger than their height. Pay attention, due to the scales, the aspect ratio of the micro-peaks are different from what appears in Fig. 3.

The PJ micro-peaks were obtained with 35 pulses, that corresponded to our mimium controlale number of pulses. The laser source has a minium repetion rate of 35 kHz and its minium controllabled shot time is 1 ms. The energy per pulse was 30 µJ. Peak formation occurs only when the pulse energy is just under the ablation threshold (36 µJ with our experimental conditions [START_REF] Pierron | Photonic jet: key role of injection for etchings with a shaped optical fiber tip[END_REF]). For comparison purposes, the dimensions of a PJ ablation has been measured. An example of ablation with 35 pulses with a pulse energy of 36 µJ has been achieved (cf. Fig. 3 b)). The PJ ablation is a sub-micro ablation with a deep of 456 ± 3 nm and a FWHM of 0.9 ± 0.6 µm. An example of a PJ peak from the matrix is presented (cf. Fig. 3 a)). The PJ peak is a sub-micro peak with a height of 403 ± 3 nm and a FWHM of 1.3 ± 0.6 µm. Hence, the PJ peak has a FWHM with the same order magnitude (around 1 µm) as the FWHM of the PJ ablation, which is also the width of the PJ.

If a different distance between the tip and the sample is used, as illustrated in Fig. 3 c) with 110 µm, no more peaks are generated. The affect area (around 17 µm) has several maxima and minima and the highest maximum (around 120 nm) is not so high as the peaks and has a FWHM of 4 µm. This confirms the role of PJ in the peak generation.

In order to confirm the interferometer results, a micro-peak has been measured by AFM (cf. Fig. 4). A height of 335 ± 2 nm and a FWHM of 1.330 ± 0.040 µm have been measured (cf. Fig. 5). The micro-peak has a quasi-conical shape with an apex radius of 14 ± 2 nm. Thus, with a small deviation (inferior to 10 %), the AFM measurement confirms the interferometer profiles.

The physical mechanisms of the micro-peak formation are hypothetical. A similar phenomenon is the micro-peak formation on thin metal films by femtosecond lasers [START_REF] Ivanov | The mechanism of nanobump formation in femtosecond pulse laser nanostructuring of thin metal films[END_REF][START_REF] Kuznetsov | Nanostructuring of thin gold films by femtosecond lasers[END_REF][START_REF] Unger | Time-resolved studies of femtosecond-laser induced melt dynamics[END_REF][START_REF] Kuznetsov | Laser-induced jet formation and droplet ejection from thin metal films[END_REF]. The absorption processes are very different, however the properties of the PJ (characteristic sizes) allow interaction with a volume of matter similar to the case of femtosecond laser. In the two cases, the micro-peaks are generated by fluences just below the ablation threshold. Independently of the peaks, under nanosecond laser irradiation, melt dynamics is a dominant mechanism; femtosecond irradiation induces more complex dynamics including thermoplastic deformation [START_REF] Kuznetsov | Nanostructuring of thin gold films by femtosecond lasers[END_REF]. In silicon, as with femtosecond pulses, the formation mechanism of the micro-peaks could be apparently due to the hydrodynamic flow of molten material governed by surface-tension forces. The mechanism is due to the thermal expansion of the heated solid part of the material. This thermal extension induces stresses, which provide forces directly perpendicular to the solid-liquid interface. Theses forces push the melted material towards the center of the irradiated region. The peak formation would be arrested by the surface tensions forces and material solidification [START_REF] Kuznetsov | Nanostructuring of thin gold films by femtosecond lasers[END_REF][START_REF] Kuznetsov | Laser-induced jet formation and droplet ejection from thin metal films[END_REF].

An interest in microstructured surfaces with peaks has arisen thanks to the superhydrophobic effect [START_REF] Nayak | Superhydrophobic surfaces by replication of micro/nano-structures fabricated by ultrafast-laser-microtexturing[END_REF][START_REF] Hairaye | Functionalization of surfaces by ultrafast laser micro/nano structuring[END_REF]. As an application example, a 5 x 5 mm matrix with micro-peaks every 50 µm, called fakir's bed of nails surface, has been achieved on silicon. A wettability test has been carried on the non-textured surface and textured surface (cf. Fig. 6) with a drop of water. When the contact angle is below 90 degrees, the surface is considered hydrophilic. On the opposite, above 90 degrees the surface is hydrophobic [START_REF] Young | An Essay on the Cohesion of Fluids[END_REF][START_REF] Wenzel | Resistance of solid surfaces to wetting by water[END_REF][START_REF] Cassie | Wettability of porous surfaces[END_REF]. The contact angle of initial surface was 39.3 ± 0.9 degrees, whereas the one of the textured surface has increased to 42.8 ± 0.8 degrees. The silicon surface has been become less hydrophilic.

Conclusion

Direct micro-spikes machining by photonic jet using nanosecond laser pulses have been observed. Repeatable micro-peaks have been fabricated by 35 pulses of 100 nanoseconds at 1064 nm with a photonic jet generated in the vicinity of an shaped optical fiber tip. They are obtained on silicon with 30 µJ per pulses slightly under the ablation threshold (36 µJ). An hypothesis is that micro-peaks are formed due to hydrodynamic flow of molten material governed by surface-tension forces. This will be investigated. The potential of these micro-peaks for reducing the hydrophilicity of silicon has been illustrated.

Figure 1 :

 1 Figure 1: a) Simulation of the electrical field norm through and outside of the shaped silica fiber tip (a = 50 µm, b = 63 µm, w 0 = 1) excited by the fundamental mode of the fiber. b) Optical microscope view of the shaped fiber tip (a = 80 µm, b = 63 µm, w 0 = 1).

Figure 2 :

 2 Figure 2: 3D view with Zygo profilometer. a) 110x110 µm matrix on silicon with peaks every 5 µm; 35 pulses for each PJ peaks; pulse energy of 30 µJ -b) Zoom. Note that the unit lengths are not the same in the transverse plane and for height.

Figure 3 :

 3 Figure 3: Examples of profiles on silicon with the Zygo microscope: a) a micro-peak; 35 pulses; pulse energy of 30 µJ; tip-sample distance of 100 µm -b) an ablation; 35 pulses; pulse energy of 36 µJ; tip-sample distance of+ 100 µm -c) damage area without peak; pulse energy of 30 µJ; tip-sample distance of 110 µm. Note that the unit lengths are not the same in the x and y axes.

Figure 4 :

 4 Figure 4: 3D view with the AFM. Example of a micro-peaks on silicon. Height of 335 ± 2 nm, width 1.330 ± 0.040 µm, apex radius of 14 ± 2 nm -35 pulses -pulse energy of 30 µJ.

Figure 5 :

 5 Figure 5: Example of a micro-peak profile on silicon with the AFM. Height of 335 ± 2 nm, width of 1.330 ± 0.040 µm, apex radius of 14 ± 2 nm; 35 pulses; pulse energy of 30 µJ.

Figure 6 :

 6 Figure 6: A drop of water ( 10 µL) on: a) a non-textured silicon wafer; contact angle of 39.3 ± 0.9 degrees -b) the silicon wafer with micro-peaks, contact angle of 42.8 ± 0.8 degrees.

Acknowledgment The authors are grateful to Camille Hairaye (ICube Laboratory, France) for her technical assistance in the use of the characterization system of wettability.