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International audience; Due to the strong evolution of data transmission worldwide, silicon photonics needs to provide low power consuming and ultra-fast modulators. Pockels effect is known to answer both these demands. However, silicon is a centrosymmetric crystal, which makes it inadequate to use Pockels effect. Nonetheless, by straining a silicon waveguide, it is possible to unlock second order nonlinear optics effects, enabling electro-optic modulation through Pockels effect. In our work, we experimentally demonstrated a high-speed strain-induced Pockels effect based electro-optic modulation in a Mach-Zehnder silicon modulator. We also present a complete analysis of Pockels, Kerr, and free carriers effects.

International audience; We present recent results on high-speed waveguide p-in photodetectors with lateral hetero-structured Silicon-Germanium-Silicon (Si-Ge-Si) junctions monolithically integrated on Silicon-on-Insulator substrates. Optical photodetectors leverage a unique integration strategy, combining butt-waveguide-coupling and lateral Si-Ge-Si p-in hetero-junctions. Fabrication is then easier, more robust and fully compatible with available Si-foundry processes. Under a low-voltage operation, 1 µm wide devices have dark currents of at most 150 nA, high responsivity of 1.2 A/W, and-3dB cutoff frequency of 12 GHz. Furthermore, an errorless detection is experimentally achieved for a conventional 10 Gbps data rate, with power sensitivity down to-13.9 dBm with a bit-error-rate (BER) of 10-9. Moreover, under avalanche operation, a 0.8 µm wide p-in diode offers attractive improvements in opto-electrical performances. In particular, initial results show that device responsivity is enhanced from 0.42 A/W up to 2.79 A/W thanks to an avalanche multiplication gain of 6.7. The cutoff frequency remains larger than 17 GHz with a gain of 10.6, resulting in a gain-bandwidth product greater than 180 GHz. These promising results also yield error-free communication at 10-9 with 28 Gbps signal, providing power sensitivities down to-12.7 dBm at 10-9 BER. These results make hetero-structured p-in photodetectors appealing choice for high-bit-rate systems in integrated silicon nanophotonics.

International audience; Owing to its low-cost, high-yield, and dense integration ability, silicon nanophotonics is a good candidate to tackle the needs of exponentially growing communications in data centers, high-performance computers, and cloud services. Moreover, a number of nanophotonic functions are now available on a single chip, as they take advantages of silicon-foundry process maturity and epitaxial germanium integration. Optical photodetectors are key building blocks in the library of group-IV components and their performances are quite successful nowadays. In particular, silicon-germanium waveguide-integrated photodetectors are fixing new standards for next generation of on-chip interconnects in terms of compactness, speed, power consumption and cost. Indeed, conventional pin photodetectors yield good responsivities (~1 A/W in a 1.5 µm wavelength range), high bandwidths (~50 GHz), and dark currents well below 1 µA. Despite recent advances, their optical power sensitivities remain rather modest and speeds are limited to 25 Gbps only, however. Compensating for the insufficient photodetector sensitivity requires higher transmitter output powers and therefore higher energy consumption. Additional energy savings can be obtained by eliminating receiver electronics. Alternatively, an appealing approach is to exploit device structures with an internal multiplication gain to lower even more the power budget and improve energy efficiency of chip-based optical links. In this work, we report on waveguide-integrated photodetectors with lateral silicon-germanium-silicon heterojunctions. Here, we present avalanche photodetectors fabricated on 200-mm silicon-on-insulator wafers using complementary metal-oxide-semiconductor-compatible processes. Devices operate in the low-gain-regime to facilitate high-speed link operations at 1.55 µm wavelengths. An error-free signal detection was achieved at 28 Gbps, with power sensitivity of-11 dBm for 10-9 bit-error-rate, which is a relevant link rate for emerging chip-scale optical interconnects.

International audience; We report on the first demonstration of high-speed optical modulation based on Pockels effect in strained silicon waveguides. Bandwidths higher than 20GHz and low insertion loss have been achieved at a wavelength of 1550nm.

Efficient and reliable on-chip optical amplifiers and light sources would enable versatile integration of various active functionalities on the silicon platform. Although lasing on silicon has been demonstrated with semiconductors by using methods such as wafer bonding or molecular beam epitaxy, cost-effective mass production methods for CMOS-compatible active devices are still lacking. Here, we report ultra-high on-chip optical gain in erbium-based hybrid slot waveguides with a monolithic, CMOS-compatible and scalable atomic-layer deposition process. The unique layer-by-layer nature of atomic-layer deposition enables atomic scale engineering of the gain layer properties and straightforward integration with silicon integrated waveguides. We demonstrate up to 20.1 ± 7.31 dB/cm and at least 52.4 ± 13.8 dB/cm net modal and material gain per unit length, respectively, the highest performance achieved from erbium-based planar waveguides integrated on silicon. Our results show significant advances towards efficient on-chip amplification, opening a route to large-scale integration of various active functionalities on silicon., Realizing efficient on-chip amplification in silicon is challenging due to either non-compatible integration or small gain per unit length of the amplifier material. Here, the authors report ultra-high on-chip optical gain in erbium-based hybrid silicon nitride slot waveguides with a monolithic, CMOS-compatible and scalable atomic-layer deposition process.

International audience; En déformant de façon hétérogène la maille cristalline du silicium, un effet Pockels a été démontré pour des fréquences au-delà de 20GHz. Ce travail ouvre la voie vers le développement de nouvelles générations de composants photoniques Si.

International audience; Here we experimentally demonstrate two-octave spanning supercontinuum generation in nitrogen-rich silicon nitride waveguides fabricated through backend CMOS compatible processes. Experimental results are in good agreement with our numerical calculations.

International audience; Silicon photonics is being considered as the future photonic platform for low power consumption optical communications. Numerous demonstrations of optoelectronic devices have been reported including plasma dispersion effect based optical modulators, germanium photodetectors or III-V laser on silicon (Si) platform. Nevertheless, the strong evolution of silicon photonics towards very low power consumption circuits call upon new strategies to be developed, especially for power-hungry components such as optical modulators. One strategy is to use Pockels effect in Si waveguides. However, bulk Si is a centrosymmetric semiconductor, which cannot exhibit any second order optical nonlinearities. Nonetheless, under a strain gradient, generated by depositing a stressed layer, typically by silicon nitride (SiN)on Si waveguides, this restriction vanishes. Over the years, many attempts to characterize the second order nonlinear susceptibility tensor through Pockels effect have been performed. However, the semiconductor nature of Si, hinders correct analysis. Indeed, carriers in Si, at the Si/SiN interface and in SiN have a screening effect when performing electro-optic modulation. This yields to the fact that the first works in the field largely overestimate the second order nonlinear susceptibility. On the other hand, it is also possible to characterize the second order susceptibility tensor through second harmonic generation (SHG). This way, we are able to highlight a second order nonlinearity effect, yet without applying an external field which would suffer from the screening effect from the Si/SiN interface. In this study we explore the design of SHG devices using dispersion engineered strained silicon waveguides. Modal methods exploiting both the phase-matching and thequasi-phase-matching approach taking into account the overlaps between strain gradient profiles and optical modes. Recent experimental result on SHG in strained silicon waveguides will also be discussed.

International audience; Silicon photonics is being considered as the future photonic platform for low power consumption optical communications. However, Silicon is a centrosymmetric semiconductor, which cannot exhibit any second order optical nonlinearities, like second harmonic generation nor the linear electro-optic effect (i.e. Pockels effect). Nonetheless, by means of strain gradients, generated by depositing a stressed layer (typically SiN) on silicon waveguides, this restriction vanishe. Hence, for years, many attempts on characterizing the second order nonlinear susceptibility tensor through Pockels effect have been performed. However, due to the semiconductor nature of silicon, its analysis has been wrongly carried out. Indeed, carriers in Si, at the Si/SiN interface and in SiN have a screening effect when performing electro-optic modulation, which have led to overestimations of the second order nonlinear susceptibility and eventually rose a controversy on the real existence of Pockels effect in strained silicon waveguides. Here, we report on unambiguous experimental characterization of Pockels effect in the microwave domain, by taking advantage of the inherent limitation of carrier effect in high frequency range. Recent results on high-speed measurements will be presented and discussed. Both charge effects and Pockels effect induced under an electric field will be also analysed.

International audience

International audience

International audience; With the fast growing demand of data, current chip-scale communication systems based on electrical links suffer rate limitations and high power consumptions to address these new requirements. In this context, Silicon Photonics has proven to be a viable alternative by replacing electronic links with optical ones while taking advantage of the well-established CMOS foundries techniques to reduce fabrication costs. However, silicon, in spite of being an excellent material to guide light, its centrosymmetry prevents second order nonlinear effects to exist, such as Pockels effect an electro-optic effect extensively used in high speed and low power consumption data transmission. Nevertheless, straining silicon by means of stressed thin films allows breaking the crystal symmetry and eventually enhancing Pockels effect. However the semiconductor nature of silicon makes the analysis of Pockels effect a challenging task because free carriers have a direct impact, through plasma dispersion effect, on its efficiency, which in turn complicates the estimation of the second order susceptibility necessary for further optimizations. However, this analysis is more relaxed working in high-speed regime because of the frequency limitation of free carriers-based modulation. In this work, we report experimental results on the modulation characteristics based on Mach-Zehnder interferometers strained by silicon nitride. We demonstrated high speed Pockels-based optical modulation up to 25 GHz in the C-band.

International audience

International audience

International audience

International audience; We report on the first third order nonlinear experimental characterization of Ge-rich Si 1-x Ge x waveguides, with Germanium concentrations x ranging from 0.7 to 0.9. The experimental values will be compared with theoretical models. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near-and mid-IR wavelength ranges.

International audience; We report the first third order nonlinear characterization of Ge-rich Si 1-x Ge x waveguides, with Germanium concentrations ranging from 0.7 to 0.9. A bi-directional top hat D-Scan method was used to determine the waveguide nonlinear parameters and to deduce the Kerr nonlinear refractive index and the two-photon absorption coefficient at the wavelength of 1.58 µm.

International audience; Silicon photonics is expected to allow the implementation of a wide range of applications including sensing, datacom and security with the potential to leverage the already existing CMOS fabrication facilities for cost-effective and large production. Among the building blocks to develop, optical modulator is considered as a key device for integrated circuits. The main physical effect to perform optical modulation in silicon is based on plasma dispersion effect. However the maximum modulation speed achievable with this approach is limited by the mobility and recombination time of the carriers. Furthermore, these types of modulators face some limitations in terms of power consumption, chirp and loss that may compromise their future integration in integrated circuits. The purpose of this work is to explore an alternative approach, based on the use of the Pockels effect. Such an effect, commonly used in Lithium-Niobate, allows high speed and low power consumption optical modulation. Unfortunately silicon is a centro-symmetric crystal leading to a vanishing of the second order nonlinear coefficient, i.e. no Pockels effect. To overcome this limitation, it is possible to break the crystal symmetry by straining the silicon lattice. The new approach developed here is to generate strain by the epitaxial growth of crystalline functional oxides, which exhibit more appropriate strain-induced characteristics in silicon than the use of silicon nitride with lower defect concentration at the Si/oxide interface. Indeed, these functional oxides are deposited by epitaxial growth at high temperature on silicon with pulsed-laser deposition. The induced strain due to lattice parameter mismatch and the difference in the thermal expansion coefficients between oxides and silicon are strong. Furthermore, these functional oxides exhibit very interesting multiferroicity and piezoelectricity properties that paves the way to a new route to implement silicon photonic circuits with unprecedented functionalities. Last results on the integration of high quality oxides on silicon waveguides will be presented as well as a full characterization of the strain-induced property changes including X-ray diffraction, AFM-SEM microscopies, and Raman spectroscopy. The first experiments of light propagation in hybrid oxides on silicon photonic structures will be also reported.

International audience; Silicon photonics is being considered as the future photonic platform for low power consumption optical communications. However, silicon is a centrosymmetric crystal, i.e. silicon doesn't have Pockels effect. Nevertheless, breaking the crystal symmetry of silicon can be used to overcome this limitation. This crystal modification is achieved by depositing a SiN high-stress overlayer. In this work, we present recent developments on the subject taking into account parasitic effects including plasma dispersion effect and fixed charge effect under an electric field. We theoretically and experimentally investigated Pockels effect in silicon waveguides and last results will be presented.

International audience; We report on controlling of the group velocity dispersion in silicon (Si) strip waveguides, with refractive index engineered sub-wavelength grating (SWG) metamaterials, covered by silicon nitride (Si3N4) layer. This waveguide geometry is utilized for breaking the centrosymmetric nature of silicon and opening the route for second-order nonlinearities in the silicon-on-insulator technology. We show by calculations that attractive waveguide dispersion profiles over a broad spectral range can be flexibly tailored by proper control of transversal dimensions and appropriate refractive index engineering. Both normal and anomalous dispersion regimes can be obtained, being one important parameter for various nonlinear applications in photonics. Waveguide dispersion is an essential characteristic of nanophotonic structures and careful control of this property is of critical importance, playing significant role in development of photonic components using nonlinear processes [1]. Substantial efforts have been put forward in the last years, elaborating various approaches to control the group velocity dispersion, both theoretically and experimentally [2-7]. This particularly includes changing the transversal dimensions [2], utilizing of slot-assisted geometries [3], using conformal dielectric overlayers [4], or adding thin material layers [5,6]. Fig.

Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated, mainly driven by absorption spectroscopy applications. Germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is very promising for the demonstration of efficient non-linear optics based active devices. Si1−xGex alloys have been recently studied due to their ability to fine-tune the bandgap and refractive index. The material nonlinearities are very sensitive to any modification of the energy bands, so Si1−xGex alloys are particularly interesting for nonlinear device engineering. We report on the first third order nonlinear experimental characterization of Ge-rich Si1−xGex waveguides, with Ge concentrations x ranging from 0.7 to 0.9. The characterization performed at 1580 nm is compared with theoretical models and a discussion about the prediction of the nonlinear properties in the mid-IR is introduced. These results will provide helpful insights to assist the design of nonlinear integrated optical based devices in both the near- and mid-IR wavelength ranges.

International audience; Silicon photonics is expected to allow the implementation of a wide range of applications including sensing, datacom and security with the potential to leverage the already existing CMOS fabrication facilities for cost-effective and large production. Among the building blocks to develop, optical modulator is considered as a key device for integrated circuits. The main physical effect to perform optical modulation in silicon is based on plasma dispersion effect. However the maximum modulation speed achievable with this approach is limited by the mobility and recombination time of the carriers. Furthermore, these types of modulators face some limitations in terms of power consumption, chirp and loss that may compromise their future integration in circuits. Our work focuses on the integration of Yttria-Stabilized Zirconia (YSZ) functional oxide on sapphire substrates and on silicon. The common aim for both integrations is to induce nonlinear optical effects at the wavelength around 1550nm, thanks to strain tuning in full oxide-or in hybrid oxide/Si-photonic structures. Indeed, we explore an alternative approach, based on the use of the Pockels effect for optical modulation [1]. Such an effect, commonly used in Lithium-Niobate, allows high speed and low power consumption optical modulation [2]. Unfortunately silicon is a centro-symmetric crystal leading to a vanishing of the second order nonlinear coefficient, i.e. no Pockels effect. To overcome this limitation, it is possible to break the crystal symmetry by straining the silicon lattice [3]. The new approach developed here is to generate strain by the epitaxial growth of crystalline functional oxides, which exhibit more appropriate strain-induced characteristics than the use of silicon nitride with lower defect concentration at the Si/oxide interface. Indeed, these functional oxides are deposited by epitaxial growth at high temperature with pulsed-laser deposition [4], [5]. The induced strain due to lattice parameter mismatch and the difference in the thermal expansion coefficients between oxides and silicon are strong. Furthermore, functional oxides exhibit very interesting multiferroicity and piezoelectricity properties that pave the way to a new route to implement silicon photonic circuits with unprecedented functionalities. Our study includes:-Epitaxial growth of YSZ functional oxide by pulsed-laser deposition and material characterizations with several complementary techniques such as X-ray diffraction, atomic-force (AFM) and electronic microscopies (SEM and TEM).-Fabrication processes with electronic lithography and different etching technics which allow us to fabricate photonic structures such as gratings coupler, strip or sub-wavelength waveguides on full oxide-and hybrid oxide/Si chips and their optical characterisation around 1550 nm.-Micro-Raman spectroscopy used to provide information about strain in silicon or YSZ structures throughout the processes [6]. Last results about full oxide-and hybrid oxide/Si-photonic structures will be presented and discussed.

International audience; Silicon photonics is being considered as the future photonic platform, mainly for the reduction of photonic system costs and the increase of the number of functionalities on the same integrated chip by combining photonics and electronics. However, silicon is a centrosymmetric crystal, which inhibits Pockels effect: a second order nonlinear effect which allows for light modulation at speeds that are not limited by carriers and driven at very low power consumption. Nevertheless, this limitation can be overcome by straining the crystal lattice and breaking the crystal symmetry of silicon. This crystal modification is achieved by depositing a SiN high-stress overlayer. Over the last few years, several researches have been performed exploring Pockels effect in strained silicon. In this work, we present recent developments on the subject taking into account parasitic effects including plasma dispersion effect and fixed charge effect under an electric field. We experimentally demonstrate Pockels effect in silicon waveguides strained by a SiN overlayer deposited by PECVD as a function of the wavelength and the waveguide width. Recent results on high-speed measurements have been performed to well dissociate Pockels effect and plasma dispersion effect.

International audience; The hybrid integration of oxides on silicon or sapphire is promising to bring other properties (such as multiferroicity or piezoelectricity) that can be combined and tuned at the nanoscale to develop new devices. Moreover, their epitaxial growth and difference of thermal expansion induce strain that can be used to break the symmetry and reveal second-order nonlinear effect in silicon. This work is mainly devoted to optimization of the growth of functional oxides such as YSZ (Yttria-Stabilized Zirconia) on sapphire and silicon, in order to control the crystalline quality, the control of strain distribution and the interfaces YSZ/Si. Epitaxial growth of YSZ thin films Preliminary optical and strain measurements on YSZ on silicon Conclusions Perspectives POPSTAR In the case of YSZ, the yttrium content can be used to tune the lattice mismatch with silicon and thus the epitaxial strain. The growth of YSZ on sapphire substrate allows measuring optical properties of this oxide without silicon interface effects. Functional oxides are deposited by epitaxial growth (pulsed-laser deposition) on silicon and so naturally induce strong strain due to large lattice parameter mismatch. Furthermore the difference of thermal expansion coefficients between YSZ and silicon is 5 times higher than Si 3 N 4 Si YSZ S i O 2 SEM image of silicon strip waveguide strained with epitaxial YSZ thin film Pulsed-laser deposition (PLD) Red parameters have been tested :  Sapphire : oxygen pressure [3] and surface state of the substrate are the most important parameters which lead to the growth of (002) or (111)YSZ on sapphire C-cut. Even if we have seen only (002)YSZ on sapphire R-cut, the quality is lower than on C-cut.  Silicon : oxygen pressure is the key parameter to grow epitaxially YSZ on silicon and to reduce the silica thickness at the interface. Map of Si raman peak [2] Wolf, " Micro-Raman Spectroscopy to Study Local Mechanical Stress in Silicon Integrated Circuits. " Semicond. Sci. Technol. 1996 • Shift of Si Raman peak : Waveguide strained by YSZ about 500 MPa [2]. • Weak transmission of 30 dB/cm. Need to improve the quality of the growth and control YSZ/Si interface. Transmission measurement at 1,55 µm Sapphire structure • Efficiently control and understand the chemical and thermal treatment of the substrates • Fabricate optical structures of YSZ on sapphire including waveguides • Optimize the growth quality of YSZ on silicon to avoid formation of silica at interface and increase the strain generation • Study the strain on different structures of silicon with different process of YSZ growth in PLD • YSZ-strained Si waveguides :-From First measurements, transmission in YSZ-strained Si waveguides is quite low, showing that the quality of the waveguides has to be further improved.-Si waveguides have been elaborated and strained thanks to the difference of thermal expansion of YSZ thin films and silicon substrate (strain evidenced by Raman measurements). • Epitaxial growth on sapphire :-The growth of YSZ thin films on sapphire has been optimized by studying the effects of all the PLD parameters.-Oxygen pressure and state surface of the substrate are the key to control YSZ growth orientation and films quality.-Optimized films surface exhibits a very low roughness (below 1nm) with atomic steps (related to the ones of the substrate).-Films grown on C-cut substrates have significantly better crystalline quality than on R-cut substrates. Only one orientation of YSZ film (002) can be stabilized with substrate treatment.-2-1

International audience; Intensive researches are currently focused on the miniaturization of devices and on the combination of photonics and electronics in the same platform, in order to decrease the power consumption and to create novel functionalities. In this context, the hybrid integration of oxides on silicon is promising to bring other properties (such as multiferroicity or piezoelectricity) that can be combined and tuned at the nanoscale to develop new devices. However, such integration induces scientific and technological challenges to overcome. The aim of the project is to develop innovative silicon photonic devices by integrating complex epitaxial oxide thin films on silicon and to optimize their physical properties. This work is mainly focused on the optimization of the materials including the crystalline quality, the control of strain fields and the interfaces between oxide and silicon. The experimental study includes the growth of oxide layers on silicon and sapphire substrates by pulsed laser deposition and their characterization by several complementary techniques, such as X ray diffraction, AFM-SEM microscopies, and Raman spectroscopy. Furthermore, the first fabrication and characterization of optical waveguides will be presented and discussed.

International audience; Silicon-on-insulator (SOI) offers an unprecedented miniaturization capability and unique potential compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Specifically, the SOI platform with 220 nm thick Si layer is widely used for the implementation of state-of-the-art photonic circuits. However, recent studies suggest that thicker Si layers may be advantageous in terms of performance, and fabrication tolerances. Nevertheless, efficiently coupling light to and from SOI circuits with Si thickness of a few hundred nanometers is challenging regardless the specific value chosen. Grating couplers are a promising solution that provide robust alignment tolerances and allow wafer-scale device testing. The coupling efficiency is mainly limited by back reflections, mode mismatch between the radiated field and the optical fiber mode, and power loss due to radiation to the substrate. The back reflections and mode-mismatch limitations are typically alleviated by apodization techniques. We have recently shown that by interleaving the standard full (220 nm) and shallow etch (70 nm) trenches in a 220 nm thick silicon layer, power loss to the substrate can be reduced below 5%, independent of the bottom oxide thickness. Here we present a novel geometry relying on this blazing effect to minimize power loss to the substrate in grating couplers implemented on 300 nm thick Si-wires with two etch depths of 150 nm and 300 nm. We remove the previously used un-etched region between the full and shallow trenches, resulting in a step-like profile. This new configuration increases grating strength, halving its length, while maintaining directionality in excess of 90 %. This way, we can implement apodization techniques to yield an overlap between the radiated field and the fiber mode larger than 90% (~75% for non-apodized structure). We also implement a, single-step sub-wavelength transition region that ensures back-reflections below 5% in a 100 nm bandwidth. 5.

9 pags., 6 figs., 1 tab., High-performance optical beam splitters are of fundamental importance for the development of advanced silicon photonics integrated circuits. However, due to the high refractive index contrast of silicon-on-insulator platforms, state-of-the-art nanophotonic splitters are hampered by trade-offs in bandwidth, polarization dependence and sensitivity to fabrication errors. Here, we present a new strategy that exploits modal engineering in slotted waveguides to overcome these limitations, enabling ultra-broadband polarization-insensitive optical power splitters with relaxed fabrication tolerances. The proposed splitter design relies on a single-mode slot waveguide that is gradually transformed into two strip waveguides by a symmetric taper, yielding equal power splitting. Based on this concept, we experimentally demonstrate −3 ± 0.5 dB polarization-independent transmission for an unprecedented 390 nm bandwidth (1260–1650 nm), even in the presence of waveguide width deviations as large as ±25 nm., This work has been funded in part by the Spanish Ministry of Science, Innovation and Universities under grants TEC2015-71127-C2-1-R (FPI scholarship BES-2016-077798) and IJCI-2016-30484; and the Community of Madrid (S2013/MIT-2790). This project has received funding from the EMPIR program (JRP-i22 14IND13 Photind), co-financed by the participating countries and the European Union’s Horizon 2020 research and innovation program; and from the Horizon 2020 research and innovation program under the Marie Sklodowska- Curie Grant No. 734331. This work has been partially funded by the Agence National pour la Recherche, Project MIRSPEC (ANR-17-CE09-0041) and the H2020 European Research Council (ERC) (ERC POPSTAR 647342). The sample fabrication has been performed at the Plateforme de Micro-Nano-Technologie/C2N, which is partially funded by the “Conseil Général de l’Éssonne”. This work was partly supported by the French RENATECH network.