PhD student (M / F) on parametric sources of photon pairs in the visible domain in nitride...

Automatic translation from French:
Description of tasks :
The thesis will take place at the Center for Nanosciences and Nanotechnologies - C2N (CNRS / Université Paris-Saclay) in close collaboration with Thales R&T, located a few hundred meters from each other, The student will be required to perform modeling in order to design the non-classical photon source. It will thus be able, through the parameter of commensurability of the two periods of the photonic crystal, to modify the number of resonances in play ranging from the canonical system with three resonances to a comb having a large number of modes for the generation of entangled quantum states multi-. photons [20].
The student will carry out his structures in the clean rooms of C2N. Various steps will be necessary such as the transfer of the large gap material onto a photonic integrated circuit, electronic lithographs, chemical or plasma etchings.
Finally, he will characterize the sources, mainly in the nearby Thales research center, first in a classical regime (quality factors, alignment of resonances, etc.) and secondly in the quantum regime (measurement of correlations,…).

Automatic translation from French:
Introduction:
In recent years, the integration of quantum optics on photonic integrated circuits (PICs) has become an essential step in dealing with the growing complexity of quantum experiments, thereby making it possible to respond to the problems of stability, reproducibility, robustness and ease of implementation. A remarkable demonstration [1] of multidimensional entanglement was thus carried out very recently in an integrated photonic circuit comprising more than 500 components including non-classical light sources.
The generation of unconventional or "compressed" light is an essential requirement for photonic architectures with continuous variables (CV), for quantum simulation and quantum computation [2,3] but also to overcome the "quantum limit" of detection. , which is critical under low flow constraints (imaging, delicate biological tissues, limited power budget, etc.). Distributed quantum detection (DKS) is also based on unclassical light, which exploits several distributed measurements to improve sensitivity [4]. Today there is fierce competition to demonstrate highly efficient, miniaturized and easily integrated unconventional light sources on a chip, in order to be able to transpose today's optical table experiments into easily usable components.
Operating below the threshold, Optical Parametric Oscillators (OPO) generate unconventional light through which fluctuations in the electromagnetic field are manipulated. Micro-frequency OPOs and combs appeared in the context of integrated optics about ten years ago and are based on micro-discs or ring resonators [5]. While the nonlinear parametric interaction in ring resonators is commonly used for the generation of photon pairs in silicon photonic circuits, the strong nonlinear absorption penalizes squeezing and prevents parametric oscillations. Therefore, the use of alternative materials such as silicon nitride [6, 7, 8, 9] has proved necessary.
However, silicon nitride, in the amorphous phase, offers a relatively modest index contrast compared to silicon, which severely limits the potential for miniaturization (the radii of the rings used vary between 120 and 200 μm [10]). Certain alloys of III-V semiconductors (in crystalline phase) allow much stronger confinement thanks to a higher refractive index. For example, the generation of frequency penalties ("combs") has been demonstrated in AlGaAs rings of about 10 µm in radius [11] with threshold powers of around 40 µW [12], in the spectral domain. telecom.
In order to be compatible with quantum systems operating in the visible domain (essentially to allow interaction with atoms), these parametric sources must use materials whose linear and non-linear losses are low in this spectral domain. The family of nitrides (GaN and AlN and their alloys) is perfectly transparent in the visible range and the nonlinear losses are low because the electronic band gap is very wide. On the other hand, the index contrast is clearly greater than that of SiN, which allows significant confinement.
For example, a team at Yale demonstrated a series of impressive results in parametric interaction from high Q resonators (> 500,000) in visible AlN [13,14,15]
Within the framework of this thesis we wish to exploit a mechanism for the confinement of light alternative to total internal reflection, namely the Bragg reflection in "photonic crystals". Generally speaking, resonators are much smaller, which favors nonlinear interaction, but they also offer a very large number of degrees of freedom for doing containment engineering.
Very recently, the C2N and Thales teams demonstrated a new class of miniature OPOs based on a triple resonant photonic crystal cavity [16]. This demonstration is based on the use of a “bichromatic” nanostructured resonator [17] in which the confinement potential of the photons is made harmonic, allowing a comb of equi-spaced frequencies to be obtained with a high quality factor. A thermal effect tunability has been exploited here to finely adjust the resonance frequencies in order to satisfy the energy conservation condition. The power required to reach the oscillation threshold has been considerably reduced thanks to a very low interaction volume. The material used here was InGaP, a transparent alloy up to a wavelength of around 650 nm, which is therefore not optimal for replicating these results in the visible range.

Description of objectives:
The aim of the thesis is the demonstration of an ultra-miniaturized and very energy efficient parametric photon source for quantum technologies (sensor, quantum computing, communication). To do this, we will rely on the new concept of photonic crystal resonators with harmonic potential such as developed in [16] which makes it possible to further reduce the modal volume compared to ring resonators. In order to avoid multi-photon absorption mechanisms in the near infrared (NIR) range and to extend the operation of the source to the visible range, we will rely on “large gap” materials and more particularly nitride materials (GaN , AlN,…). The teams involved already have extensive experience with large gap materials and particularly nitride materials such as GaN [18] and AlN [19]. The material will be transferred to a silicon or silicon nitride (SiN) circuit in order to integrate the source into other functions and perform more complex functions. Although the spacing between modes can be controlled by the design of the structure, a tuning mechanism is necessary in order to counteract the effect of disorder (imperfections) introduced during manufacture. Besides the thermal effect, it is possible to exploit the piezoelectric properties of nitride materials (GaN, ALN) to generate a local stress suitable for finely tuning the resonances of the resonator in order to satisfy the condition of conservation of energy.