Prithu Roy

Clémence Badie

Jean-Benoît Claude

Aleksandr Barulin

Antonin Moreau

Julien Lumeau

Marco Abbarchi

Lionel Santinacci

Jérôme Wenger
email: jerome.wenger@fresnel.fr

Preventing Corrosion of Aluminum Metal with Nanometer-Thick Films of Al 2 O 3 Capped with TiO 2 for Ultraviolet Plasmonics

Keywords: aluminum, corrosion, atomic layer deposition, plasmonics, ultraviolet UV, zero-mode waveguide

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Figure for Table of Content

Introduction

Ultraviolet plasmonics is a promising new range of research to push nano-optics forward and take maximum advantage of the strong molecular absorption in the 200-300 nm range. [1][2][3][4] Applications involve surface-enhanced Raman spectroscopy, [5][6][7][START_REF] Tian | Aluminum Nanocrystals: A Sustainable Substrate for Quantitative SERS-Based DNA Detection[END_REF][START_REF] Ponzellini | Metallic Nanoporous Aluminum-Magnesium Alloy for UV-Enhanced Spectroscopy[END_REF][START_REF] Wang | Improving Aluminum Ultraviolet Plasmonic Activity through a 1 Nm Ta-C Film[END_REF] protein autofluorescence detection, [START_REF] Jha | Enhancement of the Intrinsic Fluorescence of Adenine Using Aluminum Nanoparticle Arrays[END_REF][START_REF] Chowdhury | Aluminum Nanoparticles as Substrates for Metal-Enhanced Fluorescence in the Ultraviolet for the Label-Free Detection of Biomolecules[END_REF][START_REF] Ray | Enhanced Fluorescence of Proteins and Label-Free Bioassays Using Aluminum Nanostructures[END_REF][START_REF] Jiao | UV Fluorescence Lifetime Modification by Aluminum Nanoapertures[END_REF][START_REF] Barulin | Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides[END_REF][START_REF] Barulin | Ultraviolet Photostability Improvement for Autofluorescence Correlation Spectroscopy on Label-Free Proteins[END_REF] and catalysis. [START_REF] Hao | Aluminum Plasmonic Photocatalysis[END_REF][START_REF] Zhou | Aluminum Nanocrystals as a Plasmonic Photocatalyst for Hydrogen Dissociation[END_REF][START_REF] Wang | Endothermic Reaction at Room Temperature Enabled by Deep-Ultraviolet Plasmons[END_REF][START_REF] Ito | Photocatalytic Activity of Ga2O3 Supported on Al2O3 for Water Splitting and CO2 Reduction[END_REF] Working in the UV range imposes to rethink the choice of classical metals used for plasmonics, as gold and silver have interband transitions and strong losses below 400 nm. [START_REF] Sanz | UV Plasmonic Behavior of Various Metal Nanoparticles in the Nearand Far-Field Regimes: Geometry and Substrate Effects[END_REF][START_REF] Mcmahon | Plasmonics in the Ultraviolet with the Poor Metals Al, Ga[END_REF] So far, aluminum is best material choice for UV plasmonics, as it features relatively low losses, has a low intrinsic cost and is CMOS compatible. [START_REF] Knight | Aluminum for Plasmonics[END_REF][START_REF] Desantis | Walking the Walk: A Giant Step toward Sustainable Plasmonics[END_REF][START_REF] Martin | Fabrication of Aluminium Nanostructures for Plasmonics[END_REF] Another interest of aluminum is that its plasmonic optical response covers a very broad spectral range from the deep UV up to the infrared, enabling multicolor applications in spectroscopy, [START_REF] Knight | Aluminum Plasmonic Nanoantennas[END_REF][START_REF] Eizner | Aluminum Nanoantenna Complexes for Strong Coupling between Excitons and Localized Surface Plasmons[END_REF] fluorescence sensing, [START_REF] Baibakov | Extending Single-Molecule Förster Resonance Energy Transfer (FRET) Range beyond 10 Nanometers in Zero-Mode Waveguides[END_REF][START_REF] Siddique | Aluminum Metasurface with Hybrid Multipolar Plasmons for 1000-Fold Broadband Visible Fluorescence Enhancement and Multiplexed Biosensing[END_REF] light harvesting [START_REF] Zhou | 3D Self-Assembly of Aluminium Nanoparticles for Plasmon-Enhanced Solar Desalination[END_REF][START_REF] Lee | Aluminum Nanoarrays for Plasmon-Enhanced Light Harvesting[END_REF] and optical color filters. [START_REF] Zheng | Color-Selective and CMOS-Compatible Photodetection Based on Aluminum Plasmonics[END_REF][START_REF] Olson | Full-Color Aluminum Plasmonic Pixels[END_REF] While aluminum has many advantages, a major drawback concerns its limited stability in presence of water. [START_REF] Lee | Aluminum Nanoarrays for Plasmon-Enhanced Light Harvesting[END_REF][START_REF] Vargel | Corrosion of Aluminium[END_REF][START_REF] Xhanari | Organic Corrosion Inhibitors for Aluminium and Its Alloys in Acid Solutions: A Review[END_REF][START_REF] Renard | Polydopamine-Stabilized Aluminum Nanocrystals: Aqueous Stability and Benzo[a]Pyrene Detection[END_REF][START_REF] Renard | UV-Resonant Al Nanocrystals: Synthesis, Silica Coating, and Broadband Photothermal Response[END_REF] Bulk aluminum is pretty reactive and will dissolve quickly while reacting with water. [START_REF] Vargel | Corrosion of Aluminium[END_REF] Although the alumina layer naturally present at the surface partially protects the metal against water corrosion, the oxide layer exhibits cracks at the junction between metal grains where pitting corrosion can occur. [START_REF] Yang | Liquid-Like, Self-Healing Aluminum Oxide during Deformation at Room Temperature[END_REF][START_REF] Nguyen | Atomic-Scale Insights into the Oxidation of Aluminum[END_REF] Therefore, corrosion will limit the long-term durability of aluminum-based devices. [START_REF] Zhang | Long-Term Stability of Plasmonic Resonances Sustained by Evaporated Aluminum Nanostructures[END_REF] Owing to the large surface to volume ratio of plasmonic nanostructures, the corrosion phenomenon becomes even more restrictive at the nanoscale. [START_REF] Renard | Polydopamine-Stabilized Aluminum Nanocrystals: Aqueous Stability and Benzo[a]Pyrene Detection[END_REF][START_REF] Renard | UV-Resonant Al Nanocrystals: Synthesis, Silica Coating, and Broadband Photothermal Response[END_REF] In the context of ultraviolet plasmonics, the situation is further degraded as UV light has been shown to further accelerate the corrosion process. 41- 43 UV illumination has two main effects: (i) while increasing the growth of aluminum oxide, it also increases the porosity and the presence of cracks where pitting corrosion occurs, [START_REF] Deng | Influence of UV Light Irradiation on the Corrosion Behavior of Electrodeposited Ni and Cu Nanocrystalline Foils[END_REF][START_REF] Chinnam | Aluminium Oxide Prepared by UV/Ozone Exposure for Low-Voltage Organic Thin-Film Transistors[END_REF] and (ii) UV irradiation of water promotes the formation of radicals which dramatically accelerate the dissolution of alumina and aluminum. [START_REF] Barulin | Preventing Aluminum Photocorrosion for Ultraviolet Plasmonics[END_REF][START_REF] Nikogosyan | Two-Photon Ionization and Dissociation of Liquid Water by Powerful Laser UV Radiation[END_REF][START_REF] Crowell | Multiphoton Ionization of Liquid Water with 3.0-5.0 EV Photons[END_REF][START_REF] Attri | Generation Mechanism of Hydroxyl Radical Species and Its Lifetime Prediction during the Plasma-Initiated Ultraviolet (UV) Photolysis[END_REF] Various approaches have been explored to protect aluminum against corrosion, [START_REF] Vargel | Corrosion of Aluminium[END_REF][START_REF] Xhanari | Organic Corrosion Inhibitors for Aluminium and Its Alloys in Acid Solutions: A Review[END_REF] for instance by passivating the aluminum surface with polyvinylphosphonic acid and polydopamine layers. [START_REF] Barulin | Preventing Aluminum Photocorrosion for Ultraviolet Plasmonics[END_REF][START_REF] Korlach | Selective Aluminum Passivation for Targeted Immobilization of Single DNA Polymerase Molecules in Zero-Mode Waveguide Nanostructures[END_REF][START_REF] Lee | Mussel-Inspired Surface Chemistry for Multifunctional Coatings[END_REF] However, as we show in the Supporting Information Fig. S1 and S2, the use of organic molecules is limited in the context of UV plasmonics due to the direct photodamage on the organic polymers by the UV light and the photogeneration of reactive radicals. Atomic layer deposition (ALD) and plasmaenhanced chemical vapor deposition (PECVD) of thin inert layers of metal oxides offer powerful alternative approaches for corrosion protection owing to their ability to deposit dense layers of oxide materials with controlled nanometer thickness and conformal coating at the nanoscale. [START_REF] George | Atomic Layer Deposition: An Overview[END_REF] Alumina (Al2O3), [START_REF] Abdulagatov | 2O3 and TiO2 Atomic Layer Deposition on Copper for Water Corrosion Resistance[END_REF][START_REF] Potts | Ultra-Thin Aluminium Oxide Films Deposited by Plasma-Enhanced Atomic Layer Deposition for Corrosion Protection[END_REF][START_REF] Spajić | The Effect of Surface Preparation on the Protective Properties of Al2O3 and HfO2 Thin Films Deposited on Cp-Titanium by Atomic Layer Deposition[END_REF] titania (TiO2) [START_REF] Liu | Improved Corrosion Resistance and Antibacterial Properties of Composite Arch-Wires by N-Doped TiO2 Coating[END_REF][START_REF] Fusco | Investigation of the Corrosion Behavior of Atomic Layer Deposited Al2O3/TiO2 Nanolaminate Thin Films on Copper in 0.1 M NaCl[END_REF][START_REF] Dufond | Electrochemical Stability of N-Si Photoanodes Protected by TiO2 Thin Layers Grown by Atomic Layer Deposition[END_REF] and hafnia (HfO2) [START_REF] Li | Comparison of Chemical Stability and Corrosion Resistance of Group IV Metal Oxide Films Formed by Thermal and Plasma-Enhanced Atomic Layer Deposition[END_REF][START_REF] Staišiūnas | Anticorrosion Performance of Hafnium Oxide Ultrathin Films on AZ31 Magnesium Alloy[END_REF] are commonly used to protect flat metal surfaces.

However, the performance of these coatings has never been tested in the newly developing context of UV plasmonics, i. e. in the presence of UV irradiation and the photogenerated radical species, and while keeping the constraint of nanoscale features for the plasmonic nanodevices.

Here we assess the UV photocorrosion protection performance of nanometer-thick layers of various metal oxides deposited by ALD or PECVD on top of aluminum nanoapertures. Individual circular nanoapertures of 65 nm diameter milled in a 100 nm thick aluminum film form a simple and reproducible platform to check the nanostructure photostability in presence of water and UV irradiation. [START_REF] Barulin | Preventing Aluminum Photocorrosion for Ultraviolet Plasmonics[END_REF][START_REF] Baibakov | Zero-Mode Waveguides Can Be Made Better: Fluorescence Enhancement with Rectangular Aluminum Nanoapertures from the Visible to the Deep Ultraviolet[END_REF] Beyond the choice of the best material for the nanometer capping layer, we also discuss the influence of other experimental conditions like the laser wavelength, its operation mode or the pH of the solution. As soon as water is present, aluminum UV plasmonics will face the photocorrosion problem. Therefore finding the optimal protection method at the nanoscale is crucial for the future development of aluminum plasmonics and its applications in chemistry and molecular biology. As additional advantage of our approach, our results are easy to reproduce and do not involve complex or uncommon materials or procedures.

Experimental Section

Nanoaperture fabrication. A 100 nm-thick layer of aluminum is deposited on top of cleaned quartz coverslips using electron-beam evaporation (Bühler Syrus Pro 710). The deposition parameters (chamber pressure 10 -6 mbar, deposition rate 10 nm/s) are set to minimize the amount of oxides into the bulk aluminum layer. [START_REF] Knight | Aluminum for Plasmonics[END_REF][START_REF] Mcpeak | Plasmonic Films Can Easily Be Better: Rules and Recipes[END_REF] Nanoapertures with 65 nm diameter are then milled using gallium-based focused ion beam (FEI dual beam DB235 Strata, voltage 30 kV, ion current 10 pA). [START_REF] Mao | Effect of Ga Implantation and Hole Geometry on Light Transmission through Nanohole Arrays in Al and Mg[END_REF] ALD deposition. The Al2O3, HfO2, TiO2 thin films were grown by ALD in a Fiji 200 reactor (Veeco/Cambridge Nanotech) operating with Ar as gas carrier. These materials are commonly used for ALD, [START_REF] Abdulagatov | 2O3 and TiO2 Atomic Layer Deposition on Copper for Water Corrosion Resistance[END_REF][START_REF] Potts | Ultra-Thin Aluminium Oxide Films Deposited by Plasma-Enhanced Atomic Layer Deposition for Corrosion Protection[END_REF][START_REF] Spajić | The Effect of Surface Preparation on the Protective Properties of Al2O3 and HfO2 Thin Films Deposited on Cp-Titanium by Atomic Layer Deposition[END_REF][START_REF] Liu | Improved Corrosion Resistance and Antibacterial Properties of Composite Arch-Wires by N-Doped TiO2 Coating[END_REF][START_REF] Fusco | Investigation of the Corrosion Behavior of Atomic Layer Deposited Al2O3/TiO2 Nanolaminate Thin Films on Copper in 0.1 M NaCl[END_REF][START_REF] Dufond | Electrochemical Stability of N-Si Photoanodes Protected by TiO2 Thin Layers Grown by Atomic Layer Deposition[END_REF][START_REF] Li | Comparison of Chemical Stability and Corrosion Resistance of Group IV Metal Oxide Films Formed by Thermal and Plasma-Enhanced Atomic Layer Deposition[END_REF][START_REF] Staišiūnas | Anticorrosion Performance of Hafnium Oxide Ultrathin Films on AZ31 Magnesium Alloy[END_REF] ensuring that our results can be easily reproduced by others. However, their application for aluminum plasmonics has never been reported so far. The reaction chamber was maintained at 150°C

The deposition conditions have been set according to previous work. [START_REF] Assaud | Atomic Layer Deposition of TiN/Al2O3/TiN Nanolaminates for Capacitor Applications[END_REF][START_REF] Assaud | Atomic Layer Deposition of HfO2 for Integration into Three-Dimensional Metal-Insulator-Metal Devices[END_REF][START_REF] Dufond | Quantifying the Extent of Ligand Incorporation and the Effect on Properties of TiO2 Thin Films Grown by Atomic Layer Deposition Using an Alkoxide or an Alkylamide[END_REF] Al2O3 was grown from Trimethylaluminium (TMA from Strem Chemicals, 98%) and deionized water ( = 18.2 MΩ•cm) that were stored in dedicated canisters at room temperature (RT). The ALD cycle consisted of sequential pulse and purge of TMA and H2O, alternatively. The pulse and purge durations were 0.06:10 s for both precursors. HfO2 layers were deposited from Tetrakis(ethylmethylamino)hafnium (TEMAH from Strem chemicals, 99.999%) and O2 (Linde Electronics, 99.996%) by plasma-enhanced ALD (PE-ALD). The TEMAH was stored at 115°C. The PE-ALD cycle consisted of sequential pulse and purge of TEMAH and O2, alternatively. The pulse and purge durations were 1:5 s and 6:5 s for TEMAH and O2, respectively. Plasma-assistance was used during the O2 injection using a ICP remote plasma set at P = 300 W. TiO2 films were grown from Tetrakis(dimethylamino)titanium (TDMAT, Strem chemicals, 99%) and deionized water by thermal ALD. The TDMAT was heated at 85°C and water was left at RT. The ALD cycle consisted of sequential pulse, exposure and purge of both TDMAT and H2O, alternatively. The pulse, exposure and purge durations were 1:7:15 s for the Ti precursor and 0.2:7:15 s for water.

PECVD deposition. PECVD treatment was performed using a PlasmaPro NGP80 from Oxford Instruments. After a pumping step of 20 minutes at 300°C, the SiO2 layer is deposited at the same temperature using two percursors: 5% SiH4/N2 and N2O with flows of 160 and 710 sccm respectively.

The chamber pressure is 1000 mTorr and high-frequency power of 20W. The deposition rate of silica is 1 nm/s. For the UV/ozone post-PECVD treatment, we use a Novascan PSD-UV cleaner with a 100 W mercury lamp. and 10 mM DABCO (1,4-Diazabicyclo[2.2.2]octane) antifading agent as previously described. [START_REF] Barulin | Ultraviolet Photostability Improvement for Autofluorescence Correlation Spectroscopy on Label-Free Proteins[END_REF] Each FCS trace is recorded for 100 s integration time. Fitting of the correlation functions is performed with a standard Brownian diffusion model: 15

G(τ) = 〈𝐹(𝑡)𝐹(𝑡+𝜏)〉 〈𝐹(𝑡)〉 2 -1 = 1 N mol (1 - B F PMT ) 2 1 (1+ τ τ d )•(1+ τ κ 2 τ d ) 0.5 (1)
B/FPMT is the ratio of background counts B to the total detected fluorescence signal FPMT, Nmol is the number of detected molecules, τd is the diffusion, and κ is the aspect ratio of the detection volume which is kept at a constant value of 1 according to our earlier works [START_REF] Barulin | Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides[END_REF][START_REF] Baibakov | Zero-Mode Waveguides Can Be Made Better: Fluorescence Enhancement with Rectangular Aluminum Nanoapertures from the Visible to the Deep Ultraviolet[END_REF][START_REF] Wenger | Nanoaperture-Enhanced Signal-to-Noise Ratio in Fluorescence Correlation Spectroscopy[END_REF] . At 5 µW excitation power, the background B amounts to 1.2 kcounts/s while it increases to 2.7 kcounts/s at 20 µW. the background stems mostly from the GODCAT fluorescence, and it saturates at high power.

Results and Discussion

We assess the corrosion resistance of a single nanoaperture of 65 nm diameter milled into a 100 nm thick aluminum layer, which serves as a generic platform for UV aluminum plasmonics. [START_REF] Barulin | Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides[END_REF][START_REF] Baibakov | Zero-Mode Waveguides Can Be Made Better: Fluorescence Enhancement with Rectangular Aluminum Nanoapertures from the Visible to the Deep Ultraviolet[END_REF] The nanoaperture is filled with a water solution and is exposed to a focused UV laser spot of increasing power (Fig. 1a). To monitor the occurrence of photocorrosion, we simultaneously record the transmission through the nanoaperture. This transmission is highly sensitive to small changes in the aperture diameter, allowing to monitor the photocorrosion evolution in situ and in real time.

In the absence of any extra protective layer, we find that the nanoaperture quickly corrodes even at a low UV power of 10 µW within a few tens of seconds (Fig. 1b,c). The aluminum surrounding the aperture is largely dissolved, leading to the emergence of a porous region at the boundary of the untouched aluminum film. On the contrary, when a 5 nm-thick Al2O3 layer followed by a 5 nm-thick TiO2 capping layer are deposited by ALD, the nanoaperture withstands a 10× higher laser fluence for an extended time period (Fig. 1b). This demonstrates the interest for appropriate photocorrosion protection by covering the aluminum nanostructure with a nanometer-thick films of metal oxides. It also shows that direct laser-induced photodamage is not present in our observations (with our pulsed laser at 266 nm, we estimate that the photodamage threshold occurs at 150 µW average power). 66 To assess the photocorrosion resistance of a sample, we introduce the damage threshold power 𝑃 𝑡ℎ as the maximum laser power for which the transmission signal is stable for at least 90 s. For optical powers higher than 𝑃 𝑡ℎ , the aperture will corrode within less than 90 s, leading to a rapid increase in the transmission signal (as in Fig. 1c for instance). We use this definition of 𝑃 𝑡ℎ as a benchmark to assess the influence of the oxide material deposited by ALD and PECVD (Fig. 2a) and other experimental parameters (Fig. 2b-e). All the aluminum nanoapertures follow the same treatment before the protective layer deposition process. They all feature the same typical 3-4 nm natural aluminum oxide layer. To enable a straightforward comparison between the different materials, we use the same thickness of 10 nm for all our experiments. Larger thicknesses could further improve the corrosion resistance as it was previously reported in the context of flat metal surfaces. [START_REF] Potts | Ultra-Thin Aluminium Oxide Films Deposited by Plasma-Enhanced Atomic Layer Deposition for Corrosion Protection[END_REF][START_REF] Fusco | Investigation of the Corrosion Behavior of Atomic Layer Deposited Al2O3/TiO2 Nanolaminate Thin Films on Copper in 0.1 M NaCl[END_REF][START_REF] Li | Comparison of Chemical Stability and Corrosion Resistance of Group IV Metal Oxide Films Formed by Thermal and Plasma-Enhanced Atomic Layer Deposition[END_REF][START_REF] Staišiūnas | Anticorrosion Performance of Hafnium Oxide Ultrathin Films on AZ31 Magnesium Alloy[END_REF] However, thicknesses above 15 nm are not appropriate for plasmonic applications which require nanoscale features and short distances to the metal film so as to maximize the electromagnetic field enhancement. 1 On the lower side, we found that a 5 nm thickness did not provide a sufficient improvement in the photocorrosion protection. Therefore, we selected a constant 10 nm as a tradeoff between corrosion protection and plasmonic performance. As compared to an unprotected aluminum nanoaperture, [START_REF] Barulin | Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides[END_REF] the presence of the 10 nm protective layer did not significantly alter the plasmonic performance: the experimental fluorescence enhancement factor recorded on p-terphenyl molecules inside a 65 nm aluminum aperture was reduced by less than 10% in presence of a supplementary 10 nm-thick metal oxide layer. While all the different extra layers show an improvement of the photocorrosion resistance as compared to the raw aluminum aperture (Fig. 2a), there are significant differences among the materials. Al2O3, TiO2 and HfO2 and their combinations were deposited by ALD while SiO2 was deposited by PECVD (see Methods section for details). The 10 nm-thick Al2O3 layer offers the lowest improvement in the protection resistance among the materials tested. The cracks and residual porosity of this material together with its limited resistance to water dissolution explain this behaviour. [START_REF] Yang | Liquid-Like, Self-Healing Aluminum Oxide during Deformation at Room Temperature[END_REF][START_REF] Nguyen | Atomic-Scale Insights into the Oxidation of Aluminum[END_REF][START_REF] Abdulagatov | 2O3 and TiO2 Atomic Layer Deposition on Copper for Water Corrosion Resistance[END_REF] The other oxides SiO2, TiO2 and HfO2 feature a better protection performance with a higher threshold power. The best overall protection is brought by the combination of 5 nm Al2O3 layer followed by a 5 nm TiO2 capping layer. In this case, our experiments are limited by the direct laser damage of the aluminum structure at 150 µW. [START_REF] Lee | Pulsed Laser-induced Damage Threshold of Thin Aluminum Films on Quartz: Experimental and Theoretical Studies[END_REF] Our observation of the best performance for the Al2O3 layer with a TiO2 capping goes in line with the earlier works which found that these two layers provided the best barrier to protect copper against water corrosion. 51,55 Surprisingly, the combination of 5 nm Al2O3 layer followed by a 5 nm HfO2 layer does not provide an improved corrosion resistance despite the good performance of the 10 nm HfO2 layer alone. This might be due to the limited 5 nm thickness and the residual porosity of the HfO2 layer which may not be enough to fully protect against pitting corrosion in the Al2O3 layer cracks. Beyond the material choice and its thickness, there are many other experimental parameters to explore. In Fig. 2b we assess the influence of an additional exposure of the oxide layer to UV mercury lamp in a UV/ozone cleaner. During the UV/ozone treatment with a mercury lamp (100W, emission wavelengths 185 nm and 254 nm), the oxygen present in the air reacts to form ozone, which in turn improves the corrosion and dissolution resistance of SiO2. [START_REF] Fink | Silicon Oxidation by Ozone[END_REF][START_REF] Kajihara | UV-VUV Laser Induced Phenomena in SiO2 Glass[END_REF][START_REF] Crundwell | On the Mechanism of the Dissolution of Quartz and Silica in Aqueous Solutions[END_REF] One hour of UV/ozone treatment significantly improves the corrosion resistance of the SiO2 layer from 50 to 120 µW. On the contrary, we found that while UV/ozone has no noticeable effect on the resistance performance of the hafnia and titania layers, it degrades the resistance of the alumina layer. For Al2O3, the UV/ozone exposure increases the density of available or unoccupied aluminum sites, which will increase the dissolution rate. [START_REF] Burleigh | Photo-Corrosion of Different Metals during Long-Term Exposure to Ultraviolet Light[END_REF][START_REF] Crundwell | The Mechanism of Dissolution of Minerals in Acidic and Alkaline Solutions: Part III. Application to Oxide, Hydroxide and Sulfide Minerals[END_REF] The pH of the solution also plays a critical role in the photocorrosion (Fig. 2c), as a lower pH reduces the dissolution rate. [START_REF] Barulin | Preventing Aluminum Photocorrosion for Ultraviolet Plasmonics[END_REF][START_REF] Crundwell | On the Mechanism of the Dissolution of Quartz and Silica in Aqueous Solutions[END_REF] Switching from pH 7 to pH 4 significantly increases the corrosion threshold power for the different materials tested. For the combined Al2O3 + TiO2 layer, the performance is limited by the direct laser photodamage of aluminum at 150 µW, but it is remarkable that this layer protects efficiently the aluminum even at pH 7 condition.

The generation rate of radicals by UV light in water depends on the illumination wavelength. [START_REF] Nikogosyan | Two-Photon Ionization and Dissociation of Liquid Water by Powerful Laser UV Radiation[END_REF] Changing the laser wavelength from 266 to 295 nm improves the corrosion resistance by above twofold (Fig. 2d), which goes in line with the change in the water photolysis rate reported previously. [START_REF] Nikogosyan | Two-Photon Ionization and Dissociation of Liquid Water by Powerful Laser UV Radiation[END_REF] Lastly, we also tested the operation mode of the 266 nm laser, switching from the pulsed 70 ps 80 MHz repetition rate to a continuous wave (CW) mode. With the same average power, the picosecond pulses can induce multi-photon dissociation and ionization process in water, [START_REF] Barulin | Preventing Aluminum Photocorrosion for Ultraviolet Plasmonics[END_REF][START_REF] Nikogosyan | Two-Photon Ionization and Dissociation of Liquid Water by Powerful Laser UV Radiation[END_REF] while the CW laser only leads to one-photon absorption processes. Our observations show an improved corrosion resistance while using the CW laser as compared to the picosecond laser (Fig. 2e). However, the UV photocorrosion resistance is not infinitely high for the CW laser, indicating that one-photon absorption also participate to initiate the photocorrosion in addition to multiphoton processes. We use the photocorrosion strategy outlined in Fig. 2 to improve the detection of label-free streptavidin proteins (Fig. 3). Each streptavidin tetramer contains 24 natural tryptophan residues which are fluorescent in the ultraviolet range and can be detected using single aluminum nanoapertures combined with a UV confocal optical microscope. [START_REF] Barulin | Deep Ultraviolet Plasmonic Enhancement of Single Protein Autofluorescence in Zero-Mode Waveguides[END_REF][START_REF] Barulin | Ultraviolet Photostability Improvement for Autofluorescence Correlation Spectroscopy on Label-Free Proteins[END_REF] However, in the absence of additional protective layer, the UV photocorrosion of aluminum limits the maximum usable power to a few microwatts only. With the 295 nm laser, the maximum power is 5 µW to avoid damaging the nanoaperture (Fig. 2d). In this condition, fluorescence correlation spectroscopy (FCS) can still be recorded, but the curve is very noisy and FCS parameters such as the number of proteins, their brightness or diffusion time cannot be reliably extracted (Fig. 3b,c). At higher powers with an untreated sample, the aperture quickly deteriorates as shown in Fig. 1c, making the experiment impossible.

Thanks to a 10 nm SiO2 layer with UV/ozone treatment, the corrosion resistance of the aluminum sample is significantly improved so that the laser power can be increased to 20 µW and no photocorrosion is observed (Fig. 3a). This higher excitation power yields a larger fluorescence brightness, which directly improves the quality of the FCS data and reduces the noise (Fig. 3b,c). [START_REF] Wenger | Nanoaperture-Enhanced Signal-to-Noise Ratio in Fluorescence Correlation Spectroscopy[END_REF] This set of data demonstrates that the application of the corrosion protection strategies derived from Fig. 2 enable the detection of label-free proteins using an aluminum plasmonic nanostructure in the UV.

As aluminum is necessary for UV plasmonics and water is ubiquitous for biochemistry and molecular biology applications, the corrosion protection will be central to many other applications. Additionally, we show in the Supporting Information Fig. S3 that the photocorrosion protection developed here for UV plasmonics is also useful to protect the aluminum structures in presence of a corrosive buffer solution as often used for protein denaturation studies. The approaches used here can thus be extended to other uses of aluminum plasmonics, they are not restricted only to UV.

Conclusions

The enhanced photocorrosion rate of aluminum while exposed to UV light is a major bottleneck limiting the applications of aluminum UV plasmonics. Here, we have explored how nanometer-thick layers of metal oxides can help in protecting the aluminum surface against the UV photocorrosion. The combination of a 5 nm Al2O3 layer covered by a 5 nm TiO2 capping provided the best resistance performance, both in acidic and neutral pH. Using a 10 nm SiO2 capping with a 1 hour UV/ozone post-treatment is a good alternative. The ease of reproducibility and the use of common materials are additional advantages of our approach to further generalize the use of aluminum in plasmonics. In addition to finding the best nanomaterial for the protective capping, the photocorrosion resistance can be tuned by setting the experimental conditions to pH 4, using a CW laser and a higher illumination wavelength around 300 nm. Moreover, the 5 nm Al2O3 layer with 5 nm TiO2 capping also provides an excellent protection against long-term exposure to corrosive chloride solutions. This enables protein denaturation studies and other biophysical works using aluminum nanostructures for an extended time period while reusing the same aluminum nanostructures. By bringing new data on the aluminum photocorrosion process as well as efficient nanomaterial solutions, this work makes a significant advance to enable the future development of UV plasmonic applications in chemistry and biology.

S1. Polyvinylphosphonic acid and polydopamine as photocorrosion protection

In an earlier work, some of the present authors used a surface passivation based on the organic molecules polyvinylphosphonic acid (PVPA) and polydopamine (PDA) to protect against UV photocorrosion. 1 Figure S1 compares the results of this past approach with the current results achieved using a nanometer-thick conformal oxide layer deposited by ALD or PECVD.

The corrosion threshold powers previously published in 1 cannot be directly compared to the present data because of two major changes in our experimental setup. First, the transmission of the microscope objective has been modified significantly. Previously, we used a Zeiss Ultrafluar 0.6NA whose transmission got degraded over time (Supporting Information Fig. S2). We have solved this issue here using a LOMO 0.8NA objective featuring with 3x larger transmission. Second, in our previous work, we mainly investigated apertures of large 160 nm diameter featuring lower plasmonic enhancement and hence less photoinduced corrosion. The case of 65 nm diameter apertures is more challenging, but it also corresponds to the main application for UV plasmonics (Fig. 3).

To easily compare the protections brought by PVPA and PDA polymers with those achieved using nanometer-thick conformal oxide layers, we have performed a new set of experiments using the same LOMO objective and the same 65 nm diameter aperture (Fig. S1). The organic polymer layers have been deposited using the protocols described in 1 which build on earlier reports for PVPA 2 and PDA. 3,4 Using these protocols, the PVPA layer thickness is about 5 nm, 2 while the PDA layer is 15 nm thick. 3 Figure S1 summarizes our results regarding the photocorrosion stability. Using the 266 nm pulsed laser and a pH 4 buffer, the PVPA-coated aperture withstands up to 18 µW laser power without showing clear signs of photocorrosion. For PDA, the corrosion threshold power moves to 30 µW. At pH 7, the corrosion resistance is reduced to 8 µW for PVPA and 9 µW for PDA. Comparing these levels with the conformal oxide layers deposited by ALD and PECVD, it appears that the photocorrosion protection offered by organic polymers is clearly overwhelmed. The new approach developed here using nanometer-thick films of Al2O3 capped with TiO2 provides a better solution. Altogether, this supplementary data clarifies the positioning of our work respective to the state-of-the-art and clearly motivates the nanomaterials deposition by ALD and PECVD in the context of UV plasmonics.

S2. Ageing of UV microscope objective

Figure S2. Comparison between the 266 nm transmission of the LOMO objective used in this work and the ZEISS ultrafluar objective used in our previous study. 1 While the ZEISS objective transmission tend to degrade over time, the LOMO objective appears to remain free of this effect. A possible explanation for this phenomenon is the degradation of silicone glues used in the ZEISS objective upon UV irradiation. 5 While comparing between the present experiments and some of our past results, the transmission of the microscope objective has increased by 3 times, leading to a higher UV intensity and thus a faster photocorrosion rate.

  Experimental setup. Our home built confocal microscope features three different ultraviolet laser sources: a pulsed picosecond 266 nm laser (Picoquant LDH-P-FA-266, 70 ps pulse duration, 80 MHz repetition rate), a pulsed picosecond 295 nm laser (Picoquant VisUV-295-590, 70 ps pulse duration, 80MHz repetition rate) and a CW 266 nm laser (CryLas FQCW266-10-C). The lasers are used separately from each other. The laser beams are expanded to reach a 5 mm diameter before the LOMO 58x 0.8 NA water immersion microscope objective. The spot size at the focus of the objective has a diameter of 500 nm. To monitor the occurrence of corrosion, we measure the increase in the transmission in the 310-410 nm spectral band through the nanoaperture using a microLED illumination (Zeiss 423053-9071-000). A 50 μm pinhole is conjugated to the microscope focus for spatial filtering and ensuring only the region of interest around the nanoaperture is monitored. A photomultiplier tube (Picoquant PMA 175) with a photon counting module (Picoquant Picoharp 300) records the transmitted intensity after spectral filtering to select the 310 -410 nm range (Semrock FF01-300/LP-25 and FF01-375/110-25).

  kDa, 24 tryptophan residues) is dissolved in pH-7 Hepes buffer (100mM NaCl, 25mM Hepes, 0.5 v/v% Tween 20). The streptavidin concentration is set to 10 µM and controlled by a spectrophotometer (Tecan Spark 10M). To improve the UV photostability, our measurement buffer includes GODCAT oxygen scavenger (10 wt% D-glucose, 0.83 μM catalase and 0.3 μM glucose oxidase)

Figure 1 .

 1 Figure 1. (a) Single aperture of 65 nm diameter milled in a 100 nm thick aluminum film. The aperture is illuminated by a focused ultraviolet laser beam, and a water solution fills the aperture and the upper medium. The natural aluminum oxide (alumina) layer covers the aluminum bulk, but this layer is not fully dense or homogeneous on the surface, leaving defect sites for aluminum corrosion. The nanometer-thick extra oxide layer efficiently protects the aluminum surface. (b) Example of scanning electron microscopy images of a single aperture without and with a protective ALD layer (here 5 nm of Al2O3 followed by 5 nm of TiO2 as capping). In the absence of protection, corrosion is clearly visible around the central aperture. (c) Transmitted intensity time trace for different nanoaperture samples and UV laser power. The stability of the time trace indicates corrosion resistance.

Figure 2 .

 2 Figure 2. Corrosion protection strategies and influence of experimental parameters. Throughout this figure, the upper line indicates the pH, laser wavelength and laser operation mode (pulsed or continuous). (a) Threshold power leading to corrosion in 90 s as a function of different oxides deposited on top of the aluminum surface. Al2O3, TiO2 and HfO2 were deposited by ALD while SiO2 was deposited by PECVD. (b) Influence of supplementary UV/ozone exposure on different samples. (c) Influence of the solution pH, a more acidic pH enables a better corrosion protection. The Al2O3+TiO2 case is limited by the available laser power here and the occurrence of direct laser-induced damage. (d) Influence of the laser wavelength. (e) Influence of the illumination mode: pulsed or continuous (CW).

Figure 3 .

 3 Figure 3. Application of the corrosion protection of single aluminum nanoapertures for the ultraviolet autofluorescence detection of label-free streptavidin proteins. (a) Ultraviolet (310-410 nm) autofluorescence time traces for a single 65 nm diameter aperture filled with a 10 µM solution of streptavidin at pH 7. The fluorescence stems from the 24 tryptophan aminoacid residues present in each streptavidin tetramer. (b) Temporal correlation of the time traces in (a), the thick black line is a numerical fit. Thanks to the higher 20 µW excitation power enabled by the extra SiO2 protection layer, the noise is significantly reduced. Fitting the FCS data yields Nmol = 26 ± 2 at 5 µW and Nmol = 25 ± 1 at

Figure S1 .

 S1 Figure S1. Comparison between metal oxide layers and organic polymers as UV photocorrosion for aluminum. (a,b) Transmitted intensity time trace for different 65 nm nanoaperture samples and UV laser power. The stability of the time trace indicates corrosion resistance while the sudden exponential growth in transmission is the sign of corrosion occurrence. In (a) the aluminum surface is covered by 5 nm PVPA and in (b) with 15 nm PDA. (c,d) Comparison of the threshold powers leading to corrosion in 90 s as a function of different layers deposited on top of the 65 nm aluminum nanoaperture for pH 4 (c) and pH 7 (d).

Funding sources

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 723241) and from the Excellence Initiative of Aix-Marseille University -A*MIDEX, a French "Investissements d'Avenir" program. We acknowledge the NanoTecMat platform of the IM2NP institute of Marseille.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Polyvinylphosphonic acid and polydopamine as photocorrosion protection, ageing of UV microscope objective, aluminum corrosion protection in presence of chloride buffer

Conflict of Interest

The authors declare no competing financial interest.

Supporting Information for

Preventing Corrosion of Aluminum Metal with Nanometer-Thick Films of Al2O3

Capped with TiO2 for Ultraviolet Plasmonics

Prithu Roy, 1 Clémence Badie, 2 Jean-Benoît Claude, 1 Aleksandr Barulin, 1 Antonin Moreau, 1 Julien Lumeau, 1 Marco Abbarchi, 3 Lionel Santinacci, 2 Jérôme Wenger

S3. Aluminum corrosion protection in presence of chloride buffer

The protection strategies developed here using ALD for UV plasmonics can also turn useful to protect the aluminum structures in presence of a corrosive buffer solution. Biophysical studies of protein unfolding often use high molar concentrations of guanidinium chloride (GdmCl) to serve as a denaturant for proteins. 6,7 Because of the high chloride ions content of GdmCl, this compound is highly corrosive for aluminum. Here, we test our different ALD layers to record their ability to protect the aluminum layer against the long-term corrosion by GdmCl. The samples are immersed in a de-ionized water solution with 6 M of GdmCl and 500 mM of NaCl at pH 7, which is representative of the highest concentrations used to denaturate proteins and peptides. 6,7 Without any corrosion protection, the aluminum film starts to be corroded within a few hours and becomes significantly damaged after 5 days of immersion (Fig. S3). Using the combined 5 nm Al2O3 + 5nm TiO2 or 5 nm Al2O3 + 5nm HfO2 layers significantly improves the corrosion resistance up to 3 weeks immersion into the GdmCl solution.

Remarkably, these combined layers feature much less defects than the layer containing only SiO2. This set of results illustrates another interest of ALD to protect aluminum against corrosion and enable biophysical studies in harsh environments containing a high chloride concentration of several molar.

Supplementary references


Laila Dakroub

Tatyana Sinyakova

D Cubaynes

A K Pandey

M Pittman

E Baynard

Charles Bourassin-Bouchet

Olivier Guilbaud

A Klisnick

Temporal characterization of a plasma-based seeded XUV laser using a laser-dressed photoionization technique

Keywords: seeded XUV-laser, photoelectron spectroscopy, temporal characterization, laser-dressed photoionization, sidebands

INTRODUCTION

Plasma-based XUV lasers operated in the seeded mode [START_REF] Wang | High-Brightness Injection-Seeded Soft-X-Ray-Laser Amplifier Using a Solid Target[END_REF] have led to significant improvements of their beam properties, in particular in terms of coherence or wavefront. On the other hand numerical simulations suggest that a better control of the seeding conditions is a promising way to achieve ultra-short, ultra-intense amplified pulses, below the picosecond timescale limit imposed by the amplifier spectral width [START_REF] Larroche | Two-dimensional Maxwell-Bloch simulation of quasi-π-pulse amplification in a seeded XUV laser[END_REF] . Progress towards this goal requires that reliable temporal diagnostics, able to characterize the XUV pulse temporal profile with femtosecond resolution, possibly in a single-shot, become available. The development of such temporal diagnostics is also a current challenge for other high-brightness, ultrashort XUV sources, namely free-electron lasers [START_REF] Düsterer | Development of experimental techniques for the characterization of ultrashort photon pulses of extreme ultraviolet free-electron lasers[END_REF] and high-order harmonics generated from gas or from plasma mirrors [START_REF] Orfanos | Attosecond pulse metrology[END_REF] . Among the different methods which have been proposed or tested, those based on the photoionization of atoms in a gas by the XUV pulse in the presence of a laser-dressing field, have become the most extensively explored. In the presence of the dressing field the distribution of kinetic energy of the released photoelectrons is modulated in a way that is controlled, in particular, by the relative magnitude of the dressing field period Td with respect to the duration tXUV of the XUV pulse [START_REF] Radcliffe | Atomic photoionization in combined intense XUV free-electron and infrared laser fields[END_REF] . If tXUV << Td the photoline induced by the XUV photons is either shifted or broadened (streaked), depending on the relative timing between the XUV pulse and the temporal phase of the dressing field. Temporal information about the XUV pulse can be retrieved in a single-shot from the streaked spectrum. The streaking technique has been successfully demonstrated with XUV free-electron lasers, using a THz dressing pulse [START_REF] Azima | Direct measurement of the pulse duration and frequency chirp of seeded XUV free electron laser pulses[END_REF] . If tXUV >> Td satellite peaks called sidebands appear on each side of the main photoline, when the XUV pulse and the dressing pulse are overlapped in time. Each sideband corresponds to the absorption or stimulated emission of one or more dressing photons and is separated from the next one by exactly the dressing photon energy. By measuring the variation of the sidebands intensity while varying the delay between the XUV pulse and the dressing pulse, a cross-correlation signal is obtained from which the temporal profile of the XUV pulse can be retrieved, if the profile of the dressing pulse is known. The sideband technique has been used to measure the duration of XUV 7 and X-ray 8 free-electron laser (XFEL) pulses, as well as of high-order harmonic (HH) radiation generated from gas [START_REF] Bouhal | Cross-Correlation Measurement of Femtosecond Noncollinear High-Order Harmonics[END_REF] , using an infrared (IR) dressing field in the femtosecond range. The generation of sidebands is also at the heart of more elaborate techniques such as RABBIT [START_REF] Paul | Observation of a train of attosecond pulses from high harmonic generation[END_REF] , or FROG-CRAB [START_REF] Goulielmakis | Single-cycle nonlinear optics[END_REF] , with which temporal characterization in the attosecond range has been demonstrated.

The main goal of our work is to implement the technique of laser-dressed photoionization in both the sideband (IR dressing) and the streaking (THz dressing) regimes, for the temporal characterization of seeded XUV lasers. A home-made Velocity-Map-Imaging Spectrometer (VMIS), which provides photoelectron spectra with angular resolution, is the central element of the diagnostic [START_REF] Bourassin-Bouchet | Single-shot temporal characterization of ultrashort XUV pulses", X-ray Lasers and Coherent X-ray sources: Development and Applications XIII[END_REF] .

In this paper we report on the current progress in the development of our project. We have recently performed an experiment at the LASERIX facility (Université Paris-Saclay, France), aimed to characterize the temporal profile of a seeded Ne-like Ti XUV laser emitting at 32.6 nm 13 , using the laser-dressed photoionization technique in the sideband regime with an IR dressing pulse. A measurement of the pulse duration of the seeded Ne-like Ti XUV laser was previously reported by Wang et al. [START_REF] Wang | Measurement of 1-ps soft-x-ray laser pulses from an injection-seeded plasma amplifier[END_REF] several years ago, using an ultrafast X-ray streak camera. After deconvolution from the measured instrumental resolution of 1.5 ps, a pulse duration of 1.1 +/-0.47 ps was inferred. Compared to streak cameras, the laser-dressed photoionization technique has the potential to provide a better temporal resolution (of the order of the duration of the dressing pulse, ~100 fs), a higher dynamic range (strongly limited by space-charge effects in the case of streak cameras) and to allow the determination of the absolute timing between the HH seed and the amplified output pulse. To the best of our knowledge no implementation of this technique with plasma-based XUV lasers has been reported in the literature until now. The specific features of these sources in comparison with XFELs or HH lead to additional challenges in the experimental implementation, as discussed at the end of in this paper.

In section 2 we recall the principle of the measurement and we present numerical simulations of the dressed photoelectron spectra, using a model based on the Strong Field Approximation (SFA) [START_REF] Kazansky | Angle-resolved electron spectra in short-pulse two-color XUV+IR photoionization of atoms[END_REF] . In section 3 we describe the experimental setup, which was implemented at LASERIX to ensure the temporal and spatial overlap of the XUV laser beam and the IR dressing beam in the interaction zone of the VMIS. In section 4 we present a preliminary analysis of our experimental data, showing the effect of the dressing field at a particular IR-XUV delay. Conclusions and directions for future work are summarized in section 5.

EXPERIMENTAL METHOD AND NUMERICAL SIMULATIONS

For the purpose of our project we have built a custom velocity-map imaging spectrometer [START_REF] Bourassin-Bouchet | Single-shot temporal characterization of ultrashort XUV pulses", X-ray Lasers and Coherent X-ray sources: Development and Applications XIII[END_REF] , with a thick-lens configuration including 11 electrodes, following the design proposed by Kling et al. [START_REF] Kling | Thick-lens velocity-map imaging spectrometer with high resolution for high-energy charged particles[END_REF] . The principle of the VMI spectrometer consists in projecting with an intense electrostatic field the expanding sphere of photoelectrons, produced by the interaction of the XUV beam with a target gas, onto a position-sensitive detector: a microchannel plate coupled to a phosphor screen, then imaged on a CCD camera. Following image processing with an Abel-inversion procedure, the detected spectrum gives access to the distribution of kinetic energy of the photoelectrons with an angular resolution around the symmetry axis (polarization axis or beam propagation axis). Figure 2 shows how the angular distribution of photoelectrons is modulated when an IR dressing pulse is synchronized in time and overlapped in space with the XUV ionizing pulse. The numerical simulations were performed using a model based on the Strong Field Approximation 15 . The IR pulse, with a focused intensity of 2.10 12 W/cm 2 , is linearly polarized with its polarization axis parallel to the vertical py direction. Up to four sideband peaks are visible on each side of the photoline, corresponding to the simultaneous absorption or stimulated emission of up to 4 IR photons. The sideband signal is maximum along the direction of polarization and is totally absent in the perpendicular direction. The number of sidebands and their relative intensity with respect to the main photoline are controlled by several parameters, among which the intensity of the IR dressing pulse, as well as its temporal delay and its duration relative to the XUV photoionizing pulse. Until now, the knowledge about the detailed temporal structure of seeded XUV lasers is provided by numerical simulations, based on a Maxwell-Bloch description of the amplified intensity 2 . Figure 3 shows an example of such a calculated profile. It is typically composed of three components with distinct timescales. The seeded amplified pulse follows the (unamplified) HH seed by a delay of the order of the dipole relaxation time td (~1 ps) [START_REF] Kumar-Pandey | Towards subpicosecond pulses from solid target plasma based seeded soft X-ray laser[END_REF] . The pulse then exhibits few ultrashort (~100 fs) oscillatory structures related to Rabi oscillations of the two atomic states interacting coherently with the resonant electromagnetic field. This oscillatory part is followed by a coherent wake with a timescale of the order of td. The overall seeded amplified pulse is surrounded by a longer ASE background, with a duration, related to the gain lifetime, typically of the order of 5-10 ps. This ASE background pulse is difficult to avoid in seeded XUV lasers, while its level should be kept as small as possible in our measurement. For the numerical simulations of photoelectron spectra presented above the XUV ionizing pulse (hn = 38 eV) was modeled by a short 30 fs Gaussian pulse representing the coherent seeded part, surrounded by a broader 300fs Gaussian pulse, representing the ASE background, with a 1:1 energy repartition in each pulse. The duration of the IR (l = 800 nm) dressing pulse was 15 fs. Durations shorter than the actual ones were used in order to reduce the computing time in our calculations. The simulation results presented here should thus be mainly considered as a support for a qualitative discussion.

The left image in

Figure 4.a shows a spectrogram, i.e. the variation of the dressed photoelectron spectrum along the direction of polarization (i.e q = 0° and 180° in Fig 2 .a), when changing the delay between the XUV pulse and the IR pulse (same parameters as above) from -150 fs to +150 fs. The 15 fs IR pulse overlaps the 30fs XUV peak at zero delay. The number and intensity of the sidebands decrease on both sides of the zero delay, as the overlap between the XUV and dressing pulses decreases. Figure 4.b shows that by adding the intensity of all apparent sidebands for each delay, the resulting cross-correlation signal is close to the input XUV pulse profile.

EXPERIMENTAL SET-UP

The experiment was carried out at the LASERIX facility, hosted at IJC-Lab, Université Paris-Saclay. Figure 5 shows the experimental setup. The seeded XUV laser beam (in pink) was generated in the grazing-incidence, transient pumping scheme by focusing on a titanium slab target (length 3mm) a sequence of 2 laser pulses: a 10ns, 400 mJ pulse generated from a frequency-doubled Q-switched Nd YAG laser (l = 532 nm), followed after a delay of 4 ns by a 4 ps, 1.2 J pulse generated from a Ti:Sa laser (l = 815 nm). The second pulse is focused on the target by a 500 mm spherical mirror used off-axis, leading to a grazing incidence pumping angle of 22° at the target. For seeded operation using H25, the HH seed pulse is generated by focusing an auxiliary 40 fs, 4 mJ infrared beam of in a 10 mm long Argon filled gas-cell. A more detailed description of the experimental conditions of the source is given in Ref. [17].

The XUV laser beam was transported and focused at the position of interaction in the VMIS using a 500 mm spherical multilayer (Mo:Si) mirror followed by a multilayer planar mirror, both used at near-normal incidence. The position of the spherical mirror was adjusted so as to produce a ~10x magnified image of the XUV laser source to the center of the VMIS positioned at a distance of 5 m. The IR dressing beam (in orange) was focused at the same position with a f = 3m spherical silica lens and reflected with a 45° mirror to follow the path of the XUV beam with a small (~8 mrad) angle. A variable aperture placed in front of the lens was used to adjust the energy/intensity of the dressing pulse.

The precise superposition in space and overlap in time of the XUV pulse and the IR dressing pulse are critical requirements of the experiment. The spatial superposition was controlled with a ~30µm accuracy by sending both beams with a retractable mirror to an XUV CCD positioned at the same pathlength than the VMIS interaction zone (see Fig. 5). This camera was also used to determine the size of the focal spots in the VMIs interaction zone: typically 300µmx500µm for the XUV beam, and ~600µm diameter for the IR dressing beam. The temporal overlap was adjusted by observing interference fringes between the IR dressing pulse and the IR pulse used for the generation of the HH seed. These interference fringes were observed on a CCD camera placed along the propagation axis at the exit of the VMIS. From this position of temporal overlap, corresponding to delay ∆td = 0, the time of arrival of the dressing pulse in the VMIS interaction zone was varied with a delay line. During the experiment the duration of the dressing pulse was varied between 50 fs (optimal compression) and 1 ps by changing the distance between the gratings of the compressor positioned before the delay line in the IR beam path. As the generation of sidebands is also strongly dependent on the dressing intensity in the interaction region, several independent diagnostics were used to measure this parameter, as a function of the diameter of the beam (controlled with the variable aperture). In particular, we used the VMIS in the ion spatial mode, which allows to obtain a spatially resolved image of the ions produced in the interaction zone on the detector. By varying the diameter of the beam aperture we were able to assess the level of intensity above which ions were actually produced by the IR beam alone, as a result of multiphoton above-threshold-ionization (ATI) processes. Such ATI processes require a focused intensity of the order of 18 10 13 W/cm 2 . Figure 6 shows typical images obtained while increasing the diameter of the IR beam aperture. In each case the pulse energy was measured with a joulemeter and the size of the focal spot was inferred from the image taken with the XUV CCD. One can see that the trace of ATI processes appears at the position of the argon gas jet (which is also the position of beam focus) when the focused intensity reaches 1.9 10 13 W/cm 2 . For a larger intensity (bottom image) a weaker trace of ATI processes is also observed along the beam path, as the intensity is large enough to induce ATI in the residual gas close to the focus position. For the dressing measurements presented in section 4, the focused IR intensity was reduced to below 10 13 W/cm 2 .

PRELIMINARY RESULTS

During the experiment, carried out in November-December 2020, series of VMIS images were taken while varying several parameters of the IR dressing pulse: intensity, duration, and delay with respect to the time of HH injection. The data are still under analysis. In this section we show the first results of a preliminary analysis of the series of images taken for a duration of the dressing pulse enlarged to 900 fs. Such a long dressing pulse was used to increase the proportion of photoelectrons, which could "see" the dressing field, over those released before and after the dressing pulse over the duration of the ASE background. Other series were taken with shorter IR pulse durations, and at different dressing intensities.

Figure 7 (left) presents a raw photoelectron spectrum obtained from the VMIS, which shows a single photoline at Ec = 22.24 eV, corresponding to the 3p ionization of argon by the seeded XUV laser photons at 38 eV. The image is an average of ten individual images, each one being obtained from the accumulation of ten laser shots. For these the dressing pulse was also present in the interaction zone with an intensity of 2.4 10 12 W/cm 2 . In the center of Fig. 7, the same image is shown in a polar representation. As already explained in section 2, the effect of the laser dressing pulse is expected to be maximal along the polarization axis, i.e. at q = 0° and 180°. The right image in Fig. 7 shows lineouts of the signal taken along the horizontal direction at q = 180° for two different values of the XUV-IR delay (∆td = 600 fs and 900 fs). We remind that ∆td = 0 corresponds to the IR dressing pulse synchronized with the IR pulse that generates the HH seed, at the position of the interaction zone in the VMIS. A lineout corresponding to an averaged image obtained from shots with the XUV pulse only (no IR dressing pulse) is shown for comparison. For each lineout the signal was first normalized to the radius-averaged signal at q = +/-90° (i.e. in the direction perpendicular to the polarization axis). The black arrow indicates the position of the first sideband at Ec= 22.24 + 1.55 = 23.79 eV. One can see that for the two delays shown, the signal with the IR dressing pulse is slightly higher at this position and correlatively weaker at the position of the main photoline. The resolution of the VMIS is not sufficient to fully resolve the sideband peak, so the sideband peaks is blended with the main photoline. A similar wing feature is also observed for lineouts taken at q = 180°. We believe that this feature is related to the effect of IR dressing. The fact that this effect is observed for ∆td = 600 fs and 900 fs and not at shorter or longer delays would be consistent with the delay between the HH seed and the amplified coherent pulse predicted from the simulations (see Figure 3). Again we emphasize that these results are still preliminary and have to be confirmed by a more complete treatment/analysis of the series of experimental data. The angular resolution in the experimental data provide more robustness in the reliability of their interpretation, as any effect of the IR laser dressing pulse should be observed in a similar way along the IR polarization axis (q = 0°, 180°), while being absent in the perpendicular direction (q = +/-90°).

CONCLUSION

In conclusion, we have reported on the first implementation of a velocity-map imaging spectrometer to characterize the temporal structure of a seeded XUV laser using the technique of laser-dressed photoionization in the sideband regime. This technique was already demonstrated with other ultrashort XUV sources, namely gas high-order harmonics and XUV free-electron lasers. The principle of the method was investigated using numerical simulations in the strong field approximation. The experimental implementation of the diagnostic with seeded XUV lasers is found to be challenging, because the observation of the sideband peaks induced by the IR dressing pulse is partially hindered by several source features, in particular the presence of a long ASE background pulse, or the contribution of incoherent thermal plasma emission. A preliminary analysis of the experimental data recently obtained at the LASERIX facility suggests that the effect of the dressing pulse is observed in the photoelectron spectra at specific delays between the XUV pulse and the IR dressing pulse. A more detailed and complete treatment of the series of data is in progress.

Fig. 1 .

 1 a shows a calculated VMI spectrum, in which the photoline induced by XUV laser photons at 38 eV in argon (Ip = 15.76 eV, including both 3p1/2 and 3p3/2 components) appears as a circle. The radius of the circle is given by the modulus of the photoelectron momentum |𝑝 ⃗| = %𝑝 ! " + 𝑝 # " = %2𝑚 $ . 𝐸 % , where me is the electron mass and Ec = hn -Ip is the kinetic energy of the photoelectron released from an atom with ionization potential Ip by photons of energy hn. The spectrum can be transformed to a polar representation shown in the right image of Fig. 1.a, where the vertical axis is the angle q.

Figure 1 .

 1 b shows the angularly integrated spectrum.

Figure 1 .

 1 Figure 1. (a) Simulated photoelectron angular distribution, showing the single photoline corresponding to 5p ionization in argon with 38eV photons. Left: cartesian plot. Right: polar plot. (b) Angularly integrated spectrum showing the main photoline.

  Figure2shows how the angular distribution of photoelectrons is modulated when an IR dressing pulse is synchronized in time and overlapped in space with the XUV ionizing pulse. The numerical simulations were performed using a model based on the Strong Field Approximation 15 . The IR pulse, with a focused intensity of 2.10 12 W/cm 2 , is linearly polarized with its polarization axis parallel to the vertical py direction. Up to four sideband peaks are visible on each side of the photoline, corresponding to the simultaneous absorption or stimulated emission of up to 4 IR photons. The sideband signal is maximum along the direction of polarization and is totally absent in the perpendicular direction. Figure2.b shows the angularly integrated spectrum, in which four sideband peaks are apparent. It is important to note, by comparing Fig.1.b and Fig.2.b (while noticing that their vertical scale are different), that the presence of the sideband peaks leads to a corresponding depletion of the main photoline intensity, since the number of photoelectrons remains constant.

Figure 2 .

 2 Figure 2. (a) Simulated photoelectron angular distribution, showing the sideband structures induced by an IR dressing pulse, synchronized with the XUV ionizing pulse, with intensity 2.10 12 W/cm 2 and linear polarization in the vertical (py) direction. Left: cartesian plot. Right: polar plot. (b) Angularly integrated spectrum showing the sideband peaks on each side of the main photoline.

Figure 2 .

 2 Figure2.a illustrates the potential benefit of using a VMI spectrometer for our temporal diagnostic, rather than a magnetic bottle or a time-of-flight spectrometer, with which the potentially rich information contained in the angular distribution of the photoelectrons is not accessible. A magnetic bottle spectrometer would lead to an angularly integrated spectrum, such as shown in Fig.2.b. On the other hand a time-of-flight spectrometer would only detect a small portion (a few degrees) of the full angular distribution, at a given angle.

Figure 3 .

 3 Figure 3. Calculated temporal structure of a seeded XUV laser, showing 3 components with different timescales (see text).

Figure 4 .

 4 Figure 4. (a) Calculated spectrogram showing the evolution of the dressed photoelectron spectrum along the IR polarization direction, while varying the delay between the XUV and the IR dressing pulse by steps of 15 fs. (b) Integrated intensity of sidebands signal for the successive delay steps (red circles) compared to the input pulse profile (blue).

Figure 5 .

 5 Figure 5. Experimental setup showing the seeded XUV laser generation chamber (right), as well as the XUV and IR beamlines used to transport and focus the beams to the VMI spectrometer (left). Additional diagnostics were implemented in order to control the spatial superposition and temporal overlap of the two pulses in the interaction region (see text).

Figure 6 .

 6 Figure 6. Spatial imaging of the argon ions produced in the VMIS by the dressing IR beam alone. The presence of ions at the position of the argon gas jet indicates that the focused intensity is large enough to induce multiphoton ATI processes.

Figure 7 .

 7 Figure 7. Left: Raw photoelectron spectrum (average of 10 images, each one obtained by accumulating 10 shots) corresponding to the 5p ionization of argon by 38 eV photons. The red arrow indicates the direction of polarization of the IR dressing pulse (duration 900 fs, intensity 2.6 10 12 W/cm 2 ). Center: Same spectrum in polar representation. Right: Lineouts of the signal at q = 180° (as shown by the yellow dotted line in the center image) for two different XUV-IR delays ∆td and for the case without IR dressing pulse. The black vertical arrow indicates the position of the first sideband peak at Ec= 23.79 eV.

ACKNOWLEDGEMENTS

This project has received funding from Labex PALM (ANR-10-LABX-0039-PALM), LIDEX OPT2X, Sesame PULSE-X, Laserlab-Europe (EU-H2020 871124). The technical support of engineers and technicians, in particular J. Guigand, C. Charrière, N. Tournier at ISMO, and J. Demailly at IJC-Lab, is gratefully acknowledged.


B Dagens

A Labrousse

R Brenot

B Lavigne

M Renaud

SOA-based devices for all-optical signal processing

SOA-based devices for all-optical signal processing

Introduction

Development of all-optical and wavelength domain multiplexing (WDM) networks requires numerous and diversified optical processing functions like wavelength conversion, regeneration, add&drop, time domain multiplexing or logic functions. A fast non-linear element is necessary to realize such functions. The SOA (Semiconductor Optical Amplifier) is a particularly good candidate thanks to its compactness, its low consuming operation, its thermal stability and its non-linear properties : with reasonably moderate input optical and electrical powers, SOA can induce interaction between signals thanks to FWM (Four Wave Mixing), XGM (Cross Gain Modulation), or XPM (Cross Phase Modulation) [START_REF] Kelly | OFC'01[END_REF]2]. FWM is intrinsically fast and can simultaneously act on several WDM channels, but requires a quite complex pumping scheme to ensure polarization independence and a large wavelength span operation. The XGM scheme can be polarization independent, operate on a wide wavelength span and have a large input power dynamic range (IPDR). Anyway, because of a strong induced chirp and a low output extinction ratio, the cross-gain modulated signal needs to be corrected before a transmission or a cascade, for example thanks to a fiber Bragg grating [3]. Finally, the XPM is usually used with an interferometric structure in order to extract an amplitude modulated signal. For this reason, XPM devices present the highest non-linearity degree, made not only of the non-linear SOA but also of the non-linear interferometer : this makes very functional and flexible structures with the particularity to be able to re-distribute the noise and thus to regenerate the signals.

In this context we will focus on SOA-based interferometric devices. The principles and the achievable functions with such structures will be first presented, with a particular interest for regenerative devices. Then, we will present different SOA-based interferometer integration schemes and their capability for an easy insertion in real systems.

SOA-based interferometers: principles and functions

The particularity of the SOA-based devices using an interference scheme is their capacity to regenerate the signals thanks to their sinusoidal transfer function. In these devices, the regeneration is often based on wavelength conversion. In these cases, an optical clock can be introduced and become the converted signal to realize complete (3R) signal regeneration (Reamplifying, Reshaping, Retiming). In the semiconductor ultrafast nonlinear interferometer (UNI) [4], the probe signal (clock signal) is split into two orthogonally polarized pulses, which are separated by several picosecond (typically twice the data pulse width and lower than the bit duration) before to go through a SOA whose gain and index are modulated by the data signal. The delay between both probe pulses is then reversed and the two components recombined before passing through a polarizer. Cascaded 40 Gb/s 3R data regeneration has been shown with this device, demonstrating 2000 km transmission in a recirculating loop with 100 km spaced regenerators. In a quite different scheme, the DISC (delayed-interference signal-wavelength converter) [5], and, in integrated configuration, the SOA-DI (SOA with delayed interference) [6] are made of an SOA followed by an asymmetric two branches interferometer: the converted signal (probe) is phase modulated in the SOA and goes through the interferometer which extracts the amplitude modulation. The carrier induced relaxation time of the SOA is masked thanks to the temporal delay between the two interferometer branches and the tunable phase shift. Such structures have been implemented with a clock as the probe signal for 3R regeneration at 84 Gb/s [5] and 40 Gb/s [6] but with cascadability issue still to be demonstrated. In a third configuration, 3R optical regeneration was successfully demonstrated at 40 Gb/s with monolithically integrated SOA-based Mach-Zehnder interferometers (SOA-MZI) used in differential mode : the basic layout of this regenerator, made of two interferometers, is shown in Figure 1(a). In each SOA-MZI, the conversion SOA's are directly placed in the interferometer arms. The 3R regeneration is realized thanks to the interferometers high non-linearity, the conversion to the clock and the amplification. A penalty-free cascade of more than 100 regenerators has been obtained as shown in Figure 1(b) [START_REF] Lavigne | ECOC'01[END_REF]. As mentioned before, these regenerators are firstly performant wavelength converters. Other regenerator schemes have been proposed without wavelength conversion. Loss-free 3R regeneration has been realized thanks to synchronous modulation in an SOA-MZI [START_REF] Leclerc | OFC'01[END_REF]: an error-free transmission over 10,000km with 2 23 -1 PRBS sequence has been obtained. SOA-MZI used in pass-through mode [START_REF] Wolfson | OFC'99[END_REF] and MMI-SOA's (Multimode interferometer-SOA's) [START_REF] Nielsen | OFC'02[END_REF] are other structures proposed for 2R regeneration, without cascade demonstration yet.

Anyway, in numerous cases the high bit rate operation is based on the definition of switching windows, thanks to different combinations of differential mode and short pulses width, masking the possible too slow response of the SOA's. As the SOA-MZI can present at least three independent inputs in the interferometer, the differential mode can be directly used to realize high bit rate functions like demultiplexing or add&drop : for example, error free 336 Gb/s toward 10.5 GHz all-optical demultiplexing has been successfully demonstrated with an hybrid integrated symmetric SOA-MZI [START_REF] Nakamura | OFC'02[END_REF]. Without differential mode, logic functions like the XOR function can be realized by injecting separately both data streams to be compared in each branch of the MZI: both streams are converted on the same probe, as shown at 20 Gb/s [12].

SOA-based integrated interferometers: technologies suitable for real optical systems

Among the XPM based devices mentioned above, the SOA-MZI is particularly attractive for optical processing thanks to its functionality and its integrability. It has been extensively studied in different integration schemes including hybrid MZI [START_REF] Sato | ECOC'01[END_REF][START_REF] Maxwell | ECOC'02[END_REF], active-passive MZI [START_REF] Dulk | [END_REF][START_REF] Labrousse | OAA'01[END_REF] and all-active device [17]. Anyway, its insertion in real systems will be successful only if several characteristics are simultaneously obtained : reduced cost (short fabrication time, high reproducibility and low power consumption during operation) ; compactness ; stability ; speed (operation at least at 40 Gb/s) ; use simplicity (by avoiding the differential mode and by limiting the tunable SOA number) ; tolerance to input signal fluctuations (high input power dynamic range (IPDR) and low polarization dependence) ; quality of the output signal. The hybridization concept as used for PLC (planar lightwave circuit) is particularly attractive due to its capability to integrate different functions on the same platform, and so to achieve very compact sub-systems. Up to now, such a SOA-MZI [START_REF] Sato | ECOC'01[END_REF] has shown 10 Gb/s low consuming wavelength conversion and 40Gb/s wavelength conversion in differential mode [START_REF] Maxwell | ECOC'02[END_REF]. Anyway, it requires a quite complicated technological process whose risk increases with the number of functions to be integrated. In opposition, monolithically integrated all-active devices are fabricated with the simple and reproducible technological process of the standard SOA [17]. Thanks to the input signal SOA's, an IPDR higher than 15 dB [17] has been shown at 10

Gb/s and good performance at 40 Gb/s in differential mode [START_REF] Wolfson | APCC/OECC'99[END_REF]. Anyway the price of the technological simplicity is the use complexity due to the differential mode and to the tuning of at least two "not functional" input SOA's. Up to now, monolithically integrated active-passive devices with a butt-joint based technology have been also extensively fabricated, following different schemes (butt-joint with buried passive waveguide [START_REF] Dulk | [END_REF], butt-joint with deep ridge passive waveguides [START_REF] Labrousse | OAA'01[END_REF]), with up to 40 Gb/s wavelength conversion, exclusively in differential mode [START_REF] Dulk | [END_REF]. Anyway, the fabrication of butt-joint based devices is a long and difficult process including at least three epitaxial steps [START_REF] Labrousse | OAA'01[END_REF] with finally similar performance as the all-active device.

For these reasons, a new active-passive device has been recently fabricated, associating the realization simplicity and the high IPDR of the all-active one, with the active-passive device operation simplicity added to the standard mode operation simplicity at 40 Gb/s : thanks to an evanescent coupling between the active and the passive waveguides, the 4-SOA's MZI (Figure 2) has been realized with the single SOA technology. A long and highly confined conversion SOA design has been chosen to shorten the response time and achieve 40 Gb/s standard mode operation on the C-band [START_REF] Dagens | ECOC'02[END_REF]: a low penalty and floor free BER has been simultaneously measured on the 4 tributaries (obtained after demultiplexing), as well as an 8dB IPDR by controlling the input SOA current. This device belongs at the same time to a new component generation, flexible and modular, and to a mature technology, simple and reproducible. It is particularly promising for high bit rate future all-optical 3R regenerators.

Conclusion

Among SOA based devices for optical processing, XPM based interferometers are particularly attractive due to their functionality on one hand and their regenerative properties on the other hand. Different technological approaches have been presented to evaluate their suitability for insertion in system : while the hybrid integration presents the main advantage to allow integration of diversified functions for subsystem implementation, monolithically integrated active-passive evanescent coupling SOA-MZI assembles the interests of the technology simplicity and maturity, the design modularity and the use simplicity including standard mode 40 Gb/s operation.

Fig. 1 .

 1 Fig. 1.SOA-MZI based optical 3R regenerator for 40Gb/s operation (a). Sensitivity evolution during cascade measurements (b)

10 Fig. 2 .

 102 Fig. 2. (a) Scheme of the active-passive 4-SOA's evanescent coupling MZI. (b) Standard mode 40 Gb/s wavelength conversion on the four tributaries. (c) IPDR at 40 Gb/s and corresponding control current.