Francis Granger
email: francis.granger@cea.fr

Raj Saransh

Gosain

Gilles Nogues

Edith Bellet-Amalric

Joël Cibert

David Ferrand

Kuntheak Kheng
email: kuntheak.kheng@cea.fr

Brightness and purity of a room-temperature single-photon source in the blue-green range

We discuss a promising solid-state system that emits single photons at room temperature in the blue-green range, making it an attractive candidate for quantum communications in free space and underwater. The active element is a core-shell ZnSe tapered nanowire embedding a single CdSe quantum dot grown by molecular beam epitaxy. A patterned substrate enables a comprehensive study of a single nanowire using various methods.

Our source shows potential for achieving a total brightness of 0.17 photon per pulse and anti-bunching with g (2) (0) < 0.3 within a restricted spectral window. Additionally, we analyze the impact of charged excitons on the

Quantum communication is a rapidly growing field with enormous potential for secure data transmission. One major challenge in this field is the development of single-photon sources (SPS) that can effectively generate on-demand single photons for use in quantum information systems. The main advantage of using solid-state systems such as quantum dots (QDs) is that they present a high purity of the emitted single-photons, making them ideal for use as flying qubits in quantum key distribution protocols 1 . In contrast to classical bits, flying qubits cannot be cloned, making it impossible for eavesdroppers to intercept the transmission secretly.

Many single-photon sources are based on III-V grown QDs and exhibit spectrally narrow lines in near infra-red when

operating at cryogenic temperatures, with impressive performances in terms of brightness and purity [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF][START_REF] Claudon | A highly efficient single-photon source based on a quantum dot in a photonic nanowire[END_REF] . However, the need for cryogenic temperatures is still a major drawback for practically implementing these emitters in quantum information systems, despite efforts toward elevated temperatures [START_REF] Laferriére | Position-Controlled Telecom Single Photon Emitters Operating at Elevated Temperatures[END_REF][START_REF] Zhi | Integrated room temperature single-photon source for quantum key distribution[END_REF][START_REF] Murtaza | Efficient room-temperature molecular single-photon sources for quantum key distribution[END_REF] . Nitride semiconductors have shown roomtemperature single-photon emission at different wavelengths [START_REF] Zhou | Room temperature solid-state quantum emitters in the telecom range[END_REF][START_REF] Deshpande | Electrically pumped single-photon emission at room temperature from a single ingan/gan quantum dot[END_REF][START_REF] Chen | Room temperature triggered single photon emission from selfassembled gan/aln quantum dot in nanowire[END_REF] . CdSe QDs offer an alternative solution, as they are operational at 300K [START_REF] Bounouar | Ultrafast Room Temperature Single-Photon Source from Nanowire-Quantum Dots[END_REF][START_REF] Morozov | Purifying single photon emission from giant shell CdSe/CdS quantum dots at room temperature[END_REF] . They emit in the blue-green range, thus allowing for free-space long-distance communication in seawater and air [START_REF] Li | Proof-of-principle demonstration of quantum key distribution with seawater channel: towards space-to-underwater quantum communication[END_REF] . This makes them an attractive alternative for real-world implementation of SPS in quantum information systems. Efficient light extraction from QDs is a critical issue towards high emission rates. A good way to achieve this is to embed a QD in a nanowire (NW) [START_REF] Claudon | A highly efficient single-photon source based on a quantum dot in a photonic nanowire[END_REF][START_REF] Friedler | Solid-state single photon sources: the nanowire antenna[END_REF][START_REF] Reimer | Bright single-photon sources in bottom-up tailored nanowires[END_REF] which acts as a single-mode waveguide channeling the photons emitted by the QD. Furthermore, a tapered NW improves the control of the emission properties. The tapered shape adiabatically expands the guided mode and reduces the divergence angle, increasing the collection efficiency [START_REF] Claudon | A highly efficient single-photon source based on a quantum dot in a photonic nanowire[END_REF][START_REF] Reimer | Bright single-photon sources in bottom-up tailored nanowires[END_REF][START_REF] Gregersen | Controlling the emission profile of a nanowire with a conical taper[END_REF][START_REF] Jaffal | InAs quantum dot in a needlelike tapered InP nanowire: a telecom band single photon source monolithically grown on silicon[END_REF] .

We report here on a SPS based on a semiconductor QD-tapered NW, with purity g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) < 0.3, emitting in the bluegreen range and operating at room temperature. Our study focuses on a unique QD-NW submitted to a whole range of characterizations, although other emitters on the same sample exhibit similar properties.

We have used molecular beam epitaxy (MBE) to grow, at 350 • C on a GaAs (111)B substrate, the ZnSe core embedding a CdSe QD near its top (Fig. 1(a)). This was followed by the growth at 320 • C of a tapered ZnSe shell. Growth conditions are detailed in Refs. [START_REF] Saransh | The onset of tapering in the early stage of growth of a nanowire[END_REF] and [START_REF] Saransh | Room temperature single-photon source based on semiconductor quantum-dot nanowire for integrated photonics[END_REF] . The epiready substrate was patterned before growth by combining laser lithography with chemical etching, enabling the NW position to be marked and allowing several observations on the same NW by switching from one experimental setup to another.

The vertical NW depicted in this article was selected from its far-field emission diagram recorded using Fourier imaging [START_REF] Bulgarini | Nanowire Waveguides Launching Single Photons in a Gaussian Mode for Ideal Fiber Coupling[END_REF] . To this purpose, we use a high numerical aperture objective (N A = 0.72) along with a cw excitation laser at 405 nm. Then, we performed micro-photoluminescence measurements (µ-PL) as well as time-resolved photoluminescence (TRPL) in a second setup with non-resonant pulsed excitation from a titanium-sapphire laser frequency-doubled to 440 nm (repetition time T0 = 13.1 ns, frequency frep = 76 MHz). Light is collected through a N A = 0.55 objective and sent to a spectrometer equipped with a 600 grooves/mm grating (input slit width 0.2 mm). In order to identify the involved transitions, complementary measurements were performed at cryogenic temperature using a 1800 grooves/mm grating and a slit width of 0.05 mm. A Hanbury Brown and Twiss (HBT) setup composed of two fast avalanche photodiodes (APD) allowed us to characterize the purity of the SPS through correlation measurements. Finally, the NW was observed under a Scanning Electron Microscope (SEM), as displayed in Fig. 1 previous studies on NWs with a small taper angle [START_REF] Gregersen | Controlling the emission profile of a nanowire with a conical taper[END_REF][START_REF] Jaffal | InAs quantum dot in a needlelike tapered InP nanowire: a telecom band single photon source monolithically grown on silicon[END_REF] . Note that the upward emission of a CdSe QD without the thick tapered ZnSe shell is very weak due to the dielectric screening present in nanowires with small diameters [START_REF] Mathieu Jeannin | Enhanced Photon Extraction from a Nanowire Quantum Dot Using a Bottom-Up Photonic Shell[END_REF] . The tapered shell ensures an adiabatic coupling between the guided mode and free space, leading to a collection efficiency within N A = 0.72 calculated to be close to unity. Measurements in Fig. 1(c) show that 85% of the collected light falls within N A = 0.55. We note that light emitted towards the substrate is lost, leading to a theoretical maximum source efficiency close to 50%. Additionally, the microscope objective with N A = 0.55 shows a transmission of 90% at the wavelength of interest 21 . Thus, an ideal SPS made of a QD inserted in the present NW would provide a photon flux around 0.38 photon per pulse at the output of this objective.

The QD-NW room temperature µ-PL spectra are shown in Fig. 2 for values of the pulsed excitation power P ranging from 0.2 to 6 µW. We observe two peaks at 582.5 nm (L1) and 586.5 nm (L2). It can be seen qualitatively that the L1 intensity grows linearly with P , while L2 grows at a faster rate. At P = 6 µW, the QD starts to saturate. Using the setup calibration explained in Supplement 1, the number of photons collected out of the microscope objective for L1 and L2

together yield to a promising value of 0.28 photon per pulse. This is discussed later on.

The onset of saturation around P = 6 µW was confirmed by further measurements up to 20 µW. However, these measurements at high-power caused an evolution of the spectrum, as seen in the inset of Fig. 3(a) for P = 5.3 µW. The L1 intensity decreased significantly and L2 to a lesser extent. After this evolution, L1 + L2 signals yield to 0.09 photon per pulse, a value which remained constant for weeks after this initial decrease.

L 1 L 2 Figure 3 shows the TRPL signal of the QD-NW emission within a spectral window of 585 ± 0.5 nm (blue arrow labeled 1 in the inset). The red solid line in Fig. 3(a) is the convolution of the response function of the setup (a Gaussian with a FWHM of 60 ps [START_REF]TCSPC Performance of the id100-50 detector[END_REF] ), and a sum of three exponential functions added to a constant baseline:

I(t) = 3 ∑ i=1 Ai exp(- t τi ) + B (1)
The main component is a decay with a short lifetime τ2 = 0.74 ns. There is also a weak, slow component, A3 exp(-t τ 3 ) with τ3 = 3 ns and A3/A2 < 0.06. The term A1 exp(-t τ 1 ), introduced to describe a population rise of the emitting state, was found to be hidden by the time-resolution of the APD [START_REF]TCSPC Performance of the id100-50 detector[END_REF] .

The purity of the SPS was determined using a HBT autocorrelation measurement. The coincidence histogram for the same conditions as in Fig. 3(a) and the same spectral window (1 nm centered on the blue arrow) is shown in Fig. 3(b). Under pulsed excitation, a series of correlation peaks appear, with equal spacing given by the laser repetition The solid red line is a fit using Eq. 1. The inset displays the ţ-PL spectrum at 5.3 µW, with arrows indicating the central positions of spectral bandwidths (1 nm) used for time-resolved measurements: 585 ± 0.5 nm (blue arrow 1), 586.2 ± 0.5 nm (green arrow 2), and 588 ± 0.5 nm (red arrow 3). (b) Second-order intensity correlation of the QD emission under 5 µW pulsed excitation. The symbols show the histogram of delay times between photons detected on the start-stop APDs, averaged over seven channels. The value g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) = 0.3 has been calculated by removing a constant baseline and computing the ratio of the central peak area (dark grey) to the lateral ones (light grey). period T0. The peak widths agree with the lifetime τ2. As τ2 ≪ T0, the purity is computed using the ratio of the central peak area to the lateral ones. At 585.0 nm, our system exhibits a second-order correlation at zero delays g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) = 0.3, below the threshold of 0.5 considered as a signature of single-photon emission.

It is difficult to identify the nature of L1 and L2 from the spectra at room temperature, but this can be done by a comparison with the PL spectrum at 6K shown in Fig. 4(c). Sharp zero-phonon lines lie on top of broad acousticphonon sidebands. Polarization-resolved PL spectra (see Supplement 1) allow us to observe a fine structure splitting with opposite signs for two of those lines, therefore attributed to the neutral exciton X and biexciton XX. This agrees with the linear and quadratic behavior observed in Fig. 2. The most intense line in Fig. 4(c) shows no fine structure. Hence, it is attributed to a charged exciton CX. A similar spectral arrangement of the X, CX, and XX lines has been observed previously on similar samples [START_REF] Saransh | Quantitative analysis of the blue-green single-photon emission from a quantum dot in a thick tapered nanowire[END_REF] . The CX line was attributed to the positively charged exciton X + in Ref. [START_REF] Bounouar | Photon correlations on a room temperature semi-conductor single photon emitter[END_REF] , and the lines with energies smaller than XX were attributed to the charged biexciton CXX in Refs. [START_REF] Bounouar | Photon correlations on a room temperature semi-conductor single photon emitter[END_REF][START_REF] Akimov | Fine structure of the trion triplet state in a single self-assembled semiconductor quantum dot[END_REF] . Finally, the small line at 560.8 nm in Fig. 4(c) was attributed to the negatively charged exciton X -in [START_REF] Mathieu Jeannin | Enhanced Photon Extraction from a Nanowire Quantum Dot Using a Bottom-Up Photonic Shell[END_REF] . We note a large splitting of 5.66 nm (22.4 meV) between the X and XX lines.

At room temperature, the zero-phonon lines are vanishingly small, and the spectra only show broad phonon sidebands. The spectra for P = 6 µW in Fig. 2 and the inset of Fig. 3(a) are fitted and displayed in Fig. 4 while CXX, composed of several lines, has a larger width. Assuming that the trapping of electron-hole pairs does not depend on the presence of charges in the QD, the amplitude of each line is set so that the ratios of intensities obey

IXX /IX = ICXX /ICX .
The fit in Fig. 4(b) results in a X, CX, and XX linewidth equal to 5.2 nm (19 meV), and 7.2 nm (26 meV) for CXX. The common biexciton/exciton intensity ratio is 0.45, illustrating that we are not fully at saturation at P = 6 µW. Although X is dominant in Fig. 4 (a), the contribution of CX is sizable with ICX /(ICX + IX ) = 0.40. The lines X and CX together yield 0.17 photon per pulse.

The ratios IXX /IX = ICXX /ICX = 0.28 in Fig. 4 (b) indicate that either the spectrum (after the intensity decrease) is further away from saturation, or that non-radiative processes reduce the X emission intensity. Additionally, the ratio ICX /(ICX + IX ) = 0.82 shows that CX dominates X. In Fig. 4 (b), the X and CX lines together contribute for 0.06 photon per pulse. Figure 3(a) displays the decay at 585.0 nm, where the fit shows that the strongest contribution is due to the CX line, with a small overlap with CXX and X lines. The corresponding decay curve (Fig 3(a)) exhibits a dominant exponential decay with a sub-nanosecond lifetime (τ2 = 0.74 ns), a value slightly larger than the lifetime reported in Ref. [START_REF] Saransh | Room temperature single-photon source based on semiconductor quantum-dot nanowire for integrated photonics[END_REF] . Moreover, the fast rise is in good agreement with an excitation power far from saturation.

We also investigated the impact of the spectral window position on the emitter's purity. At 585.0 nm (blue arrow in the inset of Fig. 3(a)), the zero-delay correlation peak is g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) = 0.3. We measure g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) = 0.4 close to L2 peak center (green arrow labeled as 2 in the inset) and g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) = 0.6 on the red side of L2 (red arrow labeled as 3). See Supplement 1 for additional data. Each of these 1nm-broad portions of the spectrum yields to approximately 0.02 photon per pulse. We discuss below how higher values can be achieved. Shifting from spectral window 1 to 3, the overlap between CX and CXX increases, enhancing a correlation through the biexciton-exciton cascade in the charged QD. Coming back to spectral window 1, there is still some CX-CXX overlap at 585.0 nm but we cannot rule out the possibility of a cross radiative cascade resulting from a fast charge trapping forming the positively charged exciton X + h -→ X + , as illustrated in Fig. 4(d). We may expect to increase the purity even more (g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) < 0.3) by blue-shifting the spectral window below 585 nm. This would make it possible to identify the role of the cross-cascade. However, the intensity diminishes drastically as we shift the spectral window to lower wavelengths. There is an unavoidable tradeoff between purity which requires a narrow spectral window, and brightness, where a broad spectral bandwidth with high intensity is needed.

In conclusion, we demonstrate a promising triggered SPS operational at room temperature, with large X-XX splitting of 5.66 nm (22.4 meV), enabling us to obtain a high single-photon purity with g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) < 0.3. We identify neutralizing the quantum dot as a highly promising way to enhance the performance of the source. Adding the full intensity of the X and CX lines, we obtain a SPS brightness ranging from 0.17 photon per pulse initially (i.e. an emission rate of 13 MHz with a 76 MHz excitation rate) to 0.06 (i.e. 4.5 MHz) after intensity decrease. The observed subnanosecond decay time would allow a much higher repetition rate towards the GHz range. Further improvements are necessary to address the observed charging effect. Additional non-resonant laser excitation may be used to fill the traps surrounding the QD. Alternatively, modifying the growth conditions [START_REF] Mathieu Jeannin | Enhanced Photon Extraction from a Nanowire Quantum Dot Using a Bottom-Up Photonic Shell[END_REF] and surface passivation of the NW may also effectively mitigate surface states from serving as traps for charge carriers. Lower temperatures down to 220K (achieved using a Peltier cooler) can be used to reduce the linewidths and hence the overlap between lines undergoing a radiative cascade [START_REF] Laferriére | Position-Controlled Telecom Single Photon Emitters Operating at Elevated Temperatures[END_REF] .

We did carry out cryogenic measurements on the studied nanowire and checked that the value of g [START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) is below a few percent (see Supplement 1). Intermediate temperature measurements are underway to understand the phenomena better, and further optimize the SPS performances. The efficient coupling between the guided mode in the NW and free-space paves the way for a deterministic integration of the SPS into a photonic device [START_REF] Mnaymneh | On-Chip Integration of Single Photon Sources via Evanescent Coupling of Tapered Nanowires to SiN Waveguides[END_REF] .

Brightness and purity

Experimental setup

The setup used for micro-PL and TRPL measurements presented in the article (at 6K and 300K) is displayed in Figure S1 (a). The sample was positioned on a continuous flow cold-finger cryostat, which can cool the sample down to 5-6 K for cryogenic measurements. The cryostat is mounted on a X-Y linear stage, enabling high precision and smooth motion control. The excitation is provided by a frequency doubled pulsed Ti:Sapphire laser at 440 nm. This results in pulses (< 2 ps) with a repetition frequency of frep = 76 MHz that excite the NW along its axis. A N A = 0.55 M Plan apo SL 100X Mitutoyo microscope objective is used to focus the excitation laser on a selected NW-QD and to collect the QD emitted light. Emission spectra are recorded using a Horiba Jobin Yvon HR-460 spectrometer and a DU920P-BEX2-DD Andor CCD camera. Second-order correlation measurements are performed on an HBT setup equipped with two ID Quantique single-photon detectors (ID100-50) mounted on the side exit of the spectrometer. The correlations are computed using a SPC-130 Becker and Hickl time-correlated photon counting module (TCSPC).

For the Fourier imaging technique, a second setup (Figure S1 (b)) is used, which extends the confocal microscopy setup with an additional lens (Fourier lens in Figure S1 (b)). The lens is used to project the back focal plane of the objective (image plane) onto a CCD camera, enabling Fourier imaging of the emitter. In the present 2-lens configuration, given that the Fourier lens (FL) and convergence lens (CL) have long focal lengths of 400 mm and 250 mm, we do not expect a significant impact on the Fourier plane image quality compared to a 4f lens configuration.

Fine-structure splitting

A fine-structure is observed with opposite signs for the lines assigned to X and XX as shown in Figure S3. The central line does not show fine structure splitting, hence it is attributed to a charged exciton CX. Figure S3: Zoomed view of the three lines for two orthogonal linear polarizations. In that case, the excitation power is 10.5 µW with a 405 nm CW laser. The green curve shows the subtraction between the two spectra.

Low-temperature second order measurements

Additionally, low-temperature autocorrelation measurements were performed on the studied nanowire and are presented in Figure S4 below.

  (b).

Figure 1 :

 1 Figure 1: (a) Schematic of the core-shell tapered NW. (b) Tilted (45 • ) SEM image of the studied NW (scale bar 1 µm) alongside another NW that is laid down. (c) Radiation pattern of the same QD-NW, under 14 µW cw excitation at 405 nm. The white dashed line represents the numerical aperture N A = 0.72 of the microscope objective used for Fourier imaging, and the gray circle is the numerical aperture N A = 0.55 of the microscope used for spectra and TRPL measurements. The cross-sections (blue and green curves) are averaged over 5 pixels.

Figure 2 :

 2 Figure 2: Power-dependent spectra of the QD-NW of Fig. 1(b).

Figure 3 :

 3 Figure 3: Room temperature time-resolved measurements on the same NW as in Fig. 1(b). (a) TRPL decay at 585 ± 0.5 nm (P = 5.3 µW).The solid red line is a fit using Eq. 1. The inset displays the ţ-PL spectrum at 5.3 µW, with arrows indicating the central positions of spectral bandwidths (1 nm) used for time-resolved measurements: 585 ± 0.5 nm (blue arrow 1), 586.2 ± 0.5 nm (green arrow 2), and 588 ± 0.5 nm (red arrow 3). (b) Second-order intensity correlation of the QD emission under 5 µW pulsed excitation. The symbols show the histogram of delay times between photons detected on the start-stop APDs, averaged over seven channels. The value g[START_REF] Senellart | High-performance semiconductor quantum-dot single-photon sources[END_REF] (0) = 0.3 has been calculated by removing a constant baseline and computing the ratio of the central peak area (dark grey) to the lateral ones (light grey).

Figure 4 :

 4 Figure4: PL measurements on the same NW as in Fig.1(b). (a) Spectrum under 6 µW excitation at room temperature. The signal is fitted (magenta curve) with a sum of four Voigt functions, resulting in the blue, green, yellow and red filled-lines. The same method is applied for the spectrum after intensity decrease in (b). (c) Spectrum performed at 6K, under 5 µW pulsed excitation, with grating 1800 grooves/mm, slits 0.05 mm. The wavelength range was shifted to match the X lines with the 300K spectrum. In all the spectra, constant baselines have been removed (electronic noise). (d) Schematic diagram of the mechanisms possibly involved in the luminescence process: the simple radiative cascade (neutral or charged) and a possible cross-radiative cascade, involving CX resulting for the transfer of an extra charge.

Figure S1 :

 S1 Figure S1: Schematics of the spectroscopy setups. (a) TRPL setup. BS: beam-splitter; (b) Setup used for Fourier imaging technique. FL: Fourier Lens; CL: Collection Lens. (c) Schematic of the setup calibration. OD: Optical Density; P: Polarizer

  ) PL Intensity (a.u.)

Figure

  Figure S4: (a) PL spectrum of the QD-NW taken at 6K under 2 µW excitation power. (b) Second order correlation measurement performed on the line marked with a blue arrow in (a) (1-nm broad window).

Table 1 : Values of parameters used in Figure S2 (a) and (c)

 1

	Window central position (nm) Contribution	Ai	τi (ns)
	586.2	Rise (i = 1)	-2.76	0.12
		Fast decay (i = 2)	39.85	0.81
		Slow decay (i = 3) 4.16	3.07
	588.0	Rise (i = 1)	0
		Fast decay (i = 2)	13.93	0.64
		Slow decay (i = 3) 2.41	1.65

ACKNOWLEDGMENTS

The substrate has been patterned by lithography at the NanoFab cleanroom facility of Institut Néel, Grenoble. Thanks are due to Yann Genuist for his participation to the growth of samples. The authors thank Jean-Michel Gérard for fruitful discussions.

DATA AVAILABILITY STATEMENT

Data may be obtained from the authors upon reasonable request.

SUPPLEMENTAL DOCUMENT

See Supplement 1 for supporting content.

FUNDING

We acknowledge funding from the Laboratoire d 'excellence LANEF in Grenoble, ANR-10-LABX-51-01, and CEA-PE Bottom-Up QPhotonics.

DISCLOSURES

The authors declare no conflicts of interest.

Setup calibration and emission line brightness

In this paper, we define the brightness of an emission line (B line ) as the average photon number transmitted by the microscope objective for each laser excitation pulse (i.e the averaged photon number per pulse measured at the position 2 of Figure S1.a). Note that the brightness is sometimes defined in the literature as the photon number collected by the microscope objective (i.e the averaged photon number per pulse measured at the position 1 of Figure S1.a). At the QD emission wavelength, about 10% of QD emission is lost in the microscope objective and B line represents an effective brightness that can be used for practical implementations. For a pulsed laser excitation with the repetition frequency frep, B line is directly related to the line emission rate γ line measured at the position 2:

For the measurements done with the set-up shown in Figure S1.(a), we compute the line emission rate γ line from the integrated count rate (γ CCD ) of the CCD spectra recorded under pulsed excitation : γ CCD = ϵγ line where ϵ is the set-up collection efficiency.

To evaluate ϵ at the QD emission wavelength (580 to 590 nm), we use an attenuated 532 nm cw laser diode (see Figure S1.c). The value of ϵ was deduced from the ratio of the laser diode integrated CCD spectra and the laser power measured in front of the optical density (OD=4). At this particular wavelength, we observe that ϵ does not depend on the incident polarization. The influence of the injection beam splitter, the spectrometer entrance slit and of the spot size was taken into account. The transmission of the 50/50 injection beam splitter was checked at the laser diode wavelength. Please note that the CCD camera was used with an internal gain of 10 (one CCD count corresponding to 10 detected photons). In our setup, ϵ was evaluated to be approximately 9.2 × 10 -3 using a 0.2 mm entrance slit. Additional TRPL data are presented in Figure S2, for two spectral window positions (centered at 586.2 nm and 588 nm). Table 1 shows the parameters used to fit the decay time curves shown in Figure S2 (a) and (c) insets.

ADDITIONAL DATA

Room-temperature TRPL measurements


David Dorchies

Olivier Delaigue

Guillaume Thirel

airGRiwrm: an extension of the airGR R-package for handling Integrated Water Resources Management modeling

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

## STDIZ_04 LOUVE_19 VITRY_25 CHALO_21 ## [1,] 0.7 0.4 0.9 0.6 ## [2,] 0.0 0.0 0.0 0.0 ## [3,] 0.0 0.0 0.0 0.0 ## [4,] 0.0 0.0 0.0 0.0 ## [5,] 0.0 0.1 0.0 0.0 ## [6,] 6
We use the classical functions and procedures of airGR to build objects embedding the whole network data...

Processing with a network of SD models

Simulation run and outputs

The GRIwrmOutputsModel has an attribute "Qm3s" containing a data.frame with simulated flows for all the nodes in m 3 /s.

Qm3s <-attr(OM, "Qm3s")

A dedicated plot function displays simulated flow time series for all the nodes. Naturalised vs influenced flows

  airGRiwrm R package an extension of the airGR R-package for handling Integrated Water Resources Management modeling David Dorchies 1 , Olivier Delaigue 2 , and Guillaume Thirel 2 1. G-EAU, Univ Montpellier, AgroParisTech, CIRAD, IRD, INRAE, Institut Agro, Montpellier, France 2. Université Paris-Saclay, INRAE, HYCAR research unit, Hydrology Research Group, It extends airGR package functions for handling a network of semi-distributed GR models instead of a single global GR model All required information for running a semi-distributed model network are set in a formated data.frame called a GRiwrm object which can be displayed as a scheme of the network. NA RunModel_GR4J 6291.55 Hydroclimatic input data handling Inputs are represented by matrices with one named column by sub-basin instead of vectors as in airGR head(P, 10)

#

  CreateInputsModel.GRiwrm: Treating sub-basin STDIZ_04... ## Treating sub-basin LOUVE_19... ## CreateInputsModel.GRiwrm: Treating sub-basin VITRY_25... ## CreateInputsModel.GRiwrm: Treating sub-basin CHALO_21... # Calibration period selection I_Run <-366:length(DatesR) Plot downstream sub-basin # with airGR::plot.OutputsModel plot(OM[[4]], Q[I_Run, 4])

  Scheme of the real network

A supervisor provides simulation outputs during simulation to controllers that execute a logic of control which apply regulated flows in the network.

The user can write control logic representing withdrawal restriction, reservoir operations... with a complex algorithm For complete documentation https://airgriwrm.g-eau.fr

Package installation

The package will be submitted on CRAN soon.

In the meantime, to install the package*:

install.packages("devtools") ## required remotes::install_gitlab("in-wop/airgriwrm", host = "gitlab.irstea.fr")

* It also the priori installation of 'Rtools' on windows platforms for compiling airGR Fortran's code / 18


Charlie Demené

Jérôme Mairesse

Jérôme Baranger

Mickael Tanter

Olivier Baud
email: olivierfrancois.baud@hcuge.ch

Charlie Demene

Prof Olivier

Ultrafast Doppler for Neonatal Brain Imaging

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction to Ultrafast Doppler imaging

The year 1999 was a major turning point for Ultrasound imaging with the invention of Ultrafast Imaging [START_REF] Sandrin | Time-Resolved Pulsed Elastography with Ultrafast Ultrasonic Imaging[END_REF]. By reaching frame-rates up to several thousands of images per second, it enabled to image natural phenomena, like shear wave propagation [START_REF] Bercoff | Supersonic shear imaging: a new technique for soft tissue elasticity mapping[END_REF][START_REF] Sandrin | Shear modulus imaging with 2-D transient elastography[END_REF][START_REF] Tanter | Quantitative Assessment of Breast Lesion Viscoelasticity: Initial Clinical Results Using Supersonic Shear Imaging[END_REF], that were previously invisible to conventional techniques limited to a frame-rate of about 100 Hz. Ultrafast Ultrasound Imaging relies on a new paradigm for ultrasonic emissions : instead of using focused beams to build line-by-line images as it is done in conventional ultrasound imaging (Figure 1 A), plane wave are emitted through the tissues to build image-by-image (Figure 1 B) ultrasonic movies at an ultrafast framerate (up to several tens of kHz) [START_REF] Montaldo | Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography[END_REF]. This ultrasonic emission strategy enables to acquire a considerably larger amount of information on the propagation medium compared to conventional ultrasound (Figure 1), and was found of special interest and effectiveness for detecting red blood cell motion with higher sensitivity and quantification capabilities [START_REF] Bercoff | Ultrafast compound Doppler imaging: providing full blood flow characterization[END_REF]. This combination of high spatiotemporal resolution and very high sensitivity is the key of the Ultrafast Doppler (UfD, Ultrafast Imaging applied to blood flow imaging) performance [START_REF] Demene | Spatiotemporal clutter filtering of ultrafast ultrasound data highly increases Doppler and fUltrasound sensitivity[END_REF]. These potentialities has been applied in various clinical applications ranging from cardiac [START_REF] Hansen | Intra-Operative Vector Flow Imaging Using Ultrasound of the Ascending Aorta among 40 Patients with Normal, Stenotic and Replaced Aortic Valves[END_REF][START_REF] Maresca | Noninvasive Imaging of the Coronary Vasculature Using Ultrafast Ultrasound[END_REF][START_REF] Osmanski | Ultrafast Doppler Imaging of Blood Flow Dynamics in the Myocardium[END_REF] to carotid [START_REF] Ekroll | Combined vector velocity and spectral Doppler imaging for improved imaging of complex blood flow in the carotid arteries[END_REF][START_REF] Lenge | Plane-wave transverse oscillation for high-frame-rate 2-D vector flow imaging[END_REF][START_REF] Provost | 3D ultrafast ultrasound imaging in vivo[END_REF], an even ophthalmologic imaging [START_REF] Urs | Ultrafast Ultrasound Imaging of Ocular Anatomy and Blood Flow[END_REF].

Among the organs of interest for Ultrafast Doppler imaging, the brain is probably the one showing the most stunning possibilities for this new imaging modality. In a seminal publication introducing functional Ultrasound (fUltrasound), it was shown that Ultrafast Doppler could be used in trepanned rats for imaging the cerebral vasculature with a unprecedented sensitivity (Figure 1C), and that the technique was able to detect the small local cerebral blood volume changes in the barrel cortex when stimulating one of the rat whiskers [START_REF] Macé | Functional ultrasound imaging of the brain[END_REF]. In the same paper, spatiotemporal dynamics of local hemodynamic changes related to the propagation of an epileptic seizure were captured via Ultrafast Doppler and showed cortical spreading depression waves at a time and space scale no other modality was able to describe before (Figure 1D). This was the entry point of fUltrasound in the neuroimaging field, with a large scope of preclinical applications in both anesthetized and awake animals (Figure 1E): studies has been conducted on the rat olfactory tract (B. F. Osmanski et al., 2014), visual system [START_REF] Gesnik | 3D functional ultrasound imaging of the cerebral visual system in rodents[END_REF], spatial representation [START_REF] Sieu | EEG and functional ultrasound imaging in mobile rats[END_REF], visual system of the pigeon [START_REF] Rau | 3D functional ultrasound imaging of the visual system in the pigeon brain[END_REF], auditory system of the ferret [START_REF] Demené | Functional Ultrasound Imaging of the thalamo-cortical auditory tract in awake ferrets using ultrafast Doppler imaging[END_REF]. In most of these animal studies a thinned skull surgery or a trepanation was needed to enable the penetration of the ultrasound waves, and the first in-human proof of concept [START_REF] Imbault | Intraoperative Functional Ultrasound Imaging of Human Brain Activity[END_REF] used a similar approach to circumvent the obstacle of the thick human skull (Figure 1F). Imbault et al. showed that they were able to precisely and in-depth map the functional areas implicated in motor and somatosensory tasks on the imaged portion of the brain, thus giving valuable insight to the surgeon for tumor removal. (left). For Doppler imaging, this process has to be repeated several times in order to identify moving scatterers (namely the red blood cells) before electronically moving to another location in space. The image is then build line sets by line sets, resulting in fragmented information across space and time, as depicted on the graph on the right. B. In ultrafast ultrasound imaging, plane waves are sent into the propagation medium, and the echoes backscattered from all scatterers are collected at once. The ultrasonic movie is therefore build images by images, resulting in far more information. C. Rat cerebral vascular network imaged via Ultrafast Doppler. D. Propagation of epileptic seizure imaged in fUltrasound using Ultrafast Doppler (C & D adapted from [START_REF] Macé | Functional ultrasound imaging of the brain[END_REF]). E. Implantation of an ultrasonic probe for fUltrasound on an awake and freely moving rat head. Adapted from [START_REF] Sieu | EEG and functional ultrasound imaging in mobile rats[END_REF]. [START_REF] Imbault | Intraoperative Functional Ultrasound Imaging of Human Brain Activity[END_REF]).

F. Proof of concept of fUltrasound imaging on humans during open-skull surgery, the probe is directly applied on the brain. G. Ultrafast Doppler signal changes in the sensory cortex (top) and adjacent area (bottom) during stimulation of the lips of the patient, the map on the right shows the correlation of this signal with the stimulation pattern. (F & G adapted from

Ultrafast Doppler for Imaging the Neonatal Brain

Ultrafast Doppler and fUltrasound takes advantage of the fontanels, anatomical windows specific to the neonates that give a natural way of circumventing the problem of the skull penetration for ultrasound, and that boosted the translation to clinics for brain imaging in neonates with very promising perspectives.

Ultrafast Doppler enabled three major advances for neonate cerebral imaging: imaging of usually undetected blood vessels, gathering imaging and quantification in a single ultrasonic modality, and functional imaging of the brain based on the monitoring of local subtle hemodynamics changes.

The first aspect was illustrated in a paper published in 2014 [START_REF] Demené | Ultrafast Doppler reveals the mapping of cerebral vascular resistivity in neonates[END_REF] showing that Ultrafast Doppler could be translated to clinics in neonatology. Brain imaging through fontanels gives access to many structures of the brain including notably cortex, subplate, white matter and periventricular zones, and basal ganglia, and reveal microvessels down to the arteriole and venule scale, as illustrated in Figure 2A. For such images, a 6MHz probe is used, giving a good tradeoff between a 200 µm resolution and an up to 8 cm penetration. In coronal views, medullary vessels as well as thalamo-striate arteries are usually depicted (left image). In parasagittal planes, small cortical penetrating arteries are observed (center image), and in some cases the cortical watershed can clearly be identified (right image). UfD acquisitions keeping track of the blood flow complete information in every pixel, directional UfD images can be derived from the ultrafast cineloop, as shown in Figure 2B (left: the venous network underneath the ventricle is clearly depicted, middle: in parasagittal planes small thalamic and cortical arterioles and venules are easily discriminated, right: the circulation pattern in the Willis polygon is well defined). (RI) is between 0.8 and 1 (left and center image), whereas RI is lower in small lenticuliostriate arteries (~0.6) and very low (<0.3) in veins. Original data and images adapted from [START_REF] Demené | Ultrafast Doppler reveals the mapping of cerebral vascular resistivity in neonates[END_REF].

The second aspect, i.e. gathering imaging and quantification in the same modality, is illustrated in Figure 2C under the form of Resistivity Index mapping. As shown in Figure 1, in conventional focused imaging the number of temporal adjacent samples in each pixel is intrinsically limited by the need to electronically sweep the whole field of view with the ultrasonic beam, whereas in Ultrafast imaging this number is very large, added to the fact that samples in all pixels are synchronous. This enables the precise quantification of blood flow speeds based on efficient Fourier analysis in all pixels at the same time. The Resistivity Index [START_REF] Pourcelot | Diagnostic Ultrasound for Cerebral Vascular Deseases[END_REF] can then be calculated in every pixel based on the maximum and minimum speed along the cardiac cycle, and mapped on the whole field of view [START_REF] Demené | Ultrafast Doppler reveals the mapping of cerebral vascular resistivity in neonates[END_REF]. This approach could dramatically change the clinical practice, especially for neonates, not only because it provides much broader data on hemodynamics compared to the pulse wave single location measurement after color flow imaging scanning but also it opens the way to Resistivity Index monitoring across long time scales without operator-dependent measurements.

Functional Ultrasound in Human Neonates

Functional Ultrasound is the most recent and innovative aspect of the use of UfD in human neonate.

Using a miniaturized custom-made ultrasonic probe, maintained on the neonate anterior fontanel by a special headset (Figure 3 A), [START_REF] Demene | Functional ultrasound imaging of brain activity in human newborns[END_REF] showed that fUltrasound by UfD was a modality of high interest. First, they could differentiate two stages of sleep (quiet sleep and active sleep) based on the fluctuations of the UfD signal, and confirmed by a simultaneous 8-channel EEG recording.

Second, they could map cerebral blood volume fluctuations in neonates' brain during epileptic seizures. These latest findings were revealed in pathological cases of abnormal cortical development leading to uni-hemispheric seizures (due to ipsilateral lesions, e.g. hemimegalencephaly or cortical dysplasia) (Figure 3 B,D). UfD was able to show this confinement, with high fluctuation of the UfD signal in the pathologic hemisphere (Figure 3 C,E). Furthermore, the spatiotemporal resolution of the hemisphere. E. Relative changes in UfD signal during and after the seizure, the color represents the relative increase or decrease of the UfD signal in percentage. F.G. By analyzing the UfD signal during inter ictal period of electric spike activity, the author showed that propagation of vascular waves could be characterized in terms of time of arrival maps (F) and direction of propagation (G). Adapted from [START_REF] Demene | Functional ultrasound imaging of brain activity in human newborns[END_REF].

This first proof of concept that fUltrasound is feasible in human neonates opens a large field of applications for both preclinical and clinical imaging. Cerebral functional imaging with a common methodology and spatiotemporal resolution across both contexts (neonates and animal models) will be of utmost importance for developmental studies as well as pharmacological investigations.

Feasibility and relevance should also balanced with strict safety profile when implementing a new imaging modality, especially in very vulnerable neonates exposed to prolonged high intensity ultrasound scanning. Although routine conventional cerebral ultrasound scanning in infants is considered safe, Doppler imaging (especially for longer duration) here have been critically reviewed regarding the potential issues of prolonged high intensity scanning [START_REF] Ang | Prenatal exposure to ultrasound waves impacts neuronal migration in mice[END_REF] in the developing brain reported in rodents. In our clinical investigation, acoustic amplitudes and intensities remained below the Food and Drug Administration limitations for ultrasonic diagnostic imaging (FDA, 510 k, Track 3). Moreover, in order to be cautious, we chose a very restrictive approach as the Mechanical Index and the spatial-peak time-averaged intensity Ispta used in our investigation were kept to very low values (respectively MI = 0.85, Ispta = 43,5 mW/cm2) compared to the FDA accepted limits (respectively MI < 1.9 and Ispta < 720 mW/cm 2 ). Such lower acoustic values are possible without compromising the imaging quality and sensitivity thanks to the coherent recombination of backscattered signals from different transmissions and the better spatio-temporal spreading of acoustic energy in tissue when using plane ultrasonic waves instead of conventional line per line focused beams, thus reducing the amount of energy deposit.

Impact and perspectives in neonatal translational studies

It is worth noticing that the imaging modalities widely used in human imaging (fMRI, PET) are not the one offering the best spatio-temporal resolution for preclinical imaging, and this is even exacerbated for developmental studies in preclinical models of human diseases since brain of rodent pups are really small (Figure 4 A). Other techniques widely used for animal imaging (optical techniques, implanted electrodes, laser speckle imaging, etc) are too inconvenient or offering too poor penetration depth for human neonate imaging. fUltrasound using UfD fills this gap and offers a combination of penetration and spatiotemporal resolution unique for neonatal translational studies (Figure 4 A). Moreover, imaging through the fontanels or through the uncalcified skull of young rats and mice show similar ease of implementation. Proof of concepts of what can be achieved in the evaluation of pharmacologic effect of drugs using fUltrasound have already been done. In a method paper, (B.-F. Osmanski et al., 2014) showed that fUltrasound could be used as a tool to study cerebral connectivity using seed based resting state correlations between different areas of the rat brain. Those correlations gathered in so called "connectivity matrices" exhibited strong values of correlations on the anti-diagonal (interhemispheric connectivity) as well as on the adjacent diagonals (intra-hemispheric connectivity).

This tool has been already used to study the drop of cortical connectivity in a rat model of intrauterine growth restriction (Figure 4 B) and to assess in vivo the consequences of fetal growth restriction on brain function [START_REF] Batista Novais | Transcriptomic regulations in oligodendroglial and microglial cells related to brain damage following fetal growth restriction: FGR and the Developing Brain[END_REF]. Using assessment of cerebral connectivity at day p28 between selected cerebral areas (in the cortex, hippocampus, thalamus and hypothalamus, see The impact of developing a new modality of brain imaging is high in the field of perinatal medicine. Indeed, prematurity and perinatal asphyxia could lead to neurocognitive deficits and subsequent handicaps, major concerns for public health because they affect infants and have lifelong consequences for their health and wellbeing. Strategies that screen high-risk neonates through novel markers of brain function and accurate brain monitoring, early after birth would have immediate and significant benefits in two major fields:

-Improving better knowledge on functional prognosis following perinatal brain injury, and about brain damage progression within the first year in high-risk population of neonates.

-Identifying sensitive markers of early brain damage in patients who could benefit from the neuroprotective strategies planned to be tested in near future clinical trials [START_REF] Parikh | Neuroprotective Strategies in Neonatal Brain Injury[END_REF].

Conclusion

fUltrasound has been introduced as a genuinely new modality in the field of neuroimaging preclinical models of central nervous system diseases, and clinical feasibility and relevance in human neonates has been recently demonstrated. This new tool should be ready to be introduced in the neonatal intensive care untis within 3-5 years to detect abnormal brain activity associated with injuries caused by perinatal adverse events such as perinatal asphyxia and prematurity. It could also be of crucial importance in eligibility criteria in clinical trials for drugs aimed at protecting the brain. fUltrasound is also a new technique equally important for neuroscientists who want to study normal brain development and the developmental origins of diseases such as autism and make much stronger predictions in the future.

Figure 1 .

 1 Figure 1. A. Conventional ultrasound imaging. The transducer array is used to emit focused waves in the propagation medium(left). For Doppler imaging, this process has to be repeated several times in order to identify moving scatterers (namely the red blood cells) before electronically moving to another location in space. The image is then build line sets by line sets, resulting in fragmented information across space and time, as depicted on the graph on the right. B. In ultrafast ultrasound imaging, plane waves are sent into the propagation medium, and the echoes backscattered from all scatterers are collected at once. The ultrasonic movie is therefore build images by images, resulting in far more information. C. Rat cerebral vascular network imaged via Ultrafast Doppler. D. Propagation of epileptic seizure imaged in fUltrasound using Ultrafast Doppler (C & D adapted from[START_REF] Macé | Functional ultrasound imaging of the brain[END_REF]). E. Implantation of an ultrasonic probe for fUltrasound on an awake and freely moving rat head. Adapted from[START_REF] Sieu | EEG and functional ultrasound imaging in mobile rats[END_REF]. F. Proof of concept of fUltrasound imaging on humans during open-skull surgery, the probe is directly applied on the brain. G. Ultrafast Doppler signal changes in the sensory cortex (top) and adjacent area (bottom) during stimulation of the lips of the patient, the map on the right shows the correlation of this signal with the stimulation pattern. (F & G adapted from[START_REF] Imbault | Intraoperative Functional Ultrasound Imaging of Human Brain Activity[END_REF]).

Figure 2 .

 2 Figure 2. A. So-called Power Doppler images obtained via Ultrafast Doppler trough the anterior fontanel. From left to right: coronal view, tilted parasagittal view, parasagittal view. B. Directional Power Doppler images, obtained by using the speed information (up or down) contained in each pixel. From left to right: medial sagittal view, parasagittal view, transtemporal transverse view (mastoid fontanel for imaging Willis polygon and cerebellum). C. Resistivity mapping based on Ultrafast Doppler acquisition. In big arteries the Resistivity Index

  imaging technique enabled them to observe and analyze waves of vascular activity propagating at speeds of the order of mm/s, during the inter ictal period characterized by spikes of electric activity (Figure 3 F,G).

Figure 3 .

 3 Figure 3. A. Schematics showing the disposition of the ultrasonic probe on the anterior fontanel. B. UfD image overlaid on the B-Mode ultrasonic image showing the cortical structures and position of the ventricles in a neonate with hemimegalencephaly (right side, ROI 1 and 2). The 3 ROIs are used for spatially averaging the UfD signal. C. Evolution in the Ultrafast Doppler signal in the 3 ROIs during an epileptic seizure taking place mostly in the right cerebral hemisphere. D. Simultaneous 8 channel EEG recording showing ictal activity in the right

Figure 4 B

 4 Figure 4 B), correlation matrices showed abnormally decreased inter an intra hemispheric connectivity, supported by conventional structural MR brain imaging and confirmed at the histological level using myelination quantification.

Figure 4 .

 4 Figure 4. A. Schematics depicting to scale the rat head and brain size at different ages, underlying the potentialities of fUltrasound for translational and developmental studies in preclinical settings compared to other modalities in terms of penetration and resolution (brain slice and pixel size are to scale). B. Use of fUltrasound to assess cerebral connectivity in an intra-uterine growth restriction model. The pregnant dam is fed with a low protein die (LPD)t, then the pups received an injection of interleukine IL1β to mimic post birth neuroinflammation, and fUS connectivity imaging is conducted on the young rat at post-natal age 28. C. The rat cerebral vasculature is subdivided in functional relevant areas whose spatially average UfD signal time course will be correlated. D. Matrices depicting these correlations for control rats (left) (N=3) and LPD+ IL1β rats (right) (N=3). (Original data).

Commenté [CD1]: Pas de pluriel sur un nom utilisé comme adjectif