The mid-infrared (mid-IR) spectral range (λ in [3 – 12] μm) has experienced an extremely fast gain in technological maturity in the past decades, owing to the development of semiconductor laser sources (quantum and interband cascade lasers) that evolved from laboratory objects to commercially available devices. However, besides lasers and detectors, mid-IR optoelectronic components run far behind their visible and near-IR counterparts and many functionalities are not easily available, hampering the development of mid-IR systems. In particular, optical phase and amplitude modulation are extremely important signal processing functions that are only scarcely available in the MIR.

In the near-IR, photonic integration represented a major breakthrough in device performances and functionalities, but also cost and scalability. Following this path, it is clear that a part of the future of mid-IR photonics lies in photonic chips allowing fast and power efficient complex photonic functions. This integration strategy is currently actively pursued on both group IV (Si and SiGe) and III-V (GaAs/AlGaAs and InGaAs/InP) semiconductors. Recently, there has been demonstrations of free-space [1] and integrated [2] mid-IR modulators. In this context, our group has identified the strong potential of the acousto-optic interaction to develop novel non-linear mid-IR integrated photonic devices, from phase modulators to magnetic field-free optical isolators. As an example, we recently demonstrated the first integrated acousto-optic phase modulator operating at a long wavelength of 8 µm.

These devices couple elastic waves, in the form of surface acoustic waves (SAWs) electrically generated from electrodes through the piezoelectric effect, to optical waves. They use the unique combination of properties of III-V heterostructures that offer both mid-IR transparency and piezoelectricity. They require state-of-the-art microstructuration to confine together the acoustic and optical fields in order to enhance their interaction.

The goal of this internship is to develop and study new devices geometry, allowing phase modulation but also potentially on-chip optical routing, a pre-requisite for optical isolation. The internship will be mostly experimental, involving the use and development of two existing setups. The first one is a mid-IR integrated photonic bench allowing to characterize the operation of the devices. The second one is a heterodyne interferometer, that allows to image the SAW-related vibration (amplitude and phase) of the sample surface to characterize the acoustic properties of the devices.

[1] Malerba et al., Appl. Phys. Lett., 125, 041101 (2024)                                              [2] Huertas-Pedroche et al., Optics Express, 33, 42408 (2025)

followed PhD

The Institute:

The Centre for Nanosciences and Nanotechnology (C2N) is a joint research unit between the CNRS and Université Paris-Saclay. The center develops research in the fields of material science, nanophotonics, nanoelectronics, and nanotechnologies, covering all the range from fundamental to applied sciences. The PhD thesis offered here will take place in the Sunlit Team where we develop innovative approaches for solar cells, aiming at high efficiencies, cost reduction and reliability of the photovoltaic devices.

Scientific project:

III-V epitaxy is normally performed on monocrystalline substrates a few hundred micrometers thick, while the active thin-film device itself is usually only a few micrometers. During the fabrication process, the substrate is either etched or retained in the structure without serving any functional purpose. In this project, we aim to detach the thin film from the substrate. This will yield semiconducting membranes that are altogether flexible, lightweight, integrable, and high-performance (both optically and electronically). These membranes have applications in numerous fields, including silicon photonics, flexible devices, and high-efficiency solar cells. Furthermore, this approach offers the opportunity to recycle the monocrystalline substrates, which are both expensive and made from critical materials, thereby reducing material consumption 100-fold.

Our strategy is to deposit a graphene layer onto the substrate before performing the epitaxy, as shown in attached file. We have demonstrated that graphene enables the fabrication of high-quality monocrystalline material while allowing its exfoliation. The general purpose of the internship is to develop this method, which requires both exploring the fundamental physical phenomena occurring at the graphene surface during material synthesis and defining practical methodologies and quality assessment protocols for each process step. Without restrictions, we aim to fabricate ultrathin solar cells.

The intern will work on the development of the process as well as on the characterization of the obtained structures and intermediate products. He / she will propose further developments of the techniques already existing at the laboratory, as well as suggest the exploration of new methods. He / she will propose models to explain the observed phenomena, and design experiments for their validation. To complete those tasks, the intern will use his own knowledge as well as the scientific literature. The intern will take advantage of a unique collection of fabrication and characterization methods (XPS, TEM, SEM, luminescence) available in partner laboratories. This environment will provide various opportunities to tackle this project challenge and gain experience.

Profile:

    The candidate must possess solid knowledges in material physics, characterization, and fabrication processes in a clean room environment. He / she must show good project management skills, for the development of technological procedures involving numerous parameters. He / she will be able to work independently and suggest innovative solutions to reach the project objectives. Collaborative work being at the core of the program, communication skills are required for team working as well as regular presentation of work progress in internal meetings.

This internship can be followed by a PhD.

Starting: 02/2027 (adjustable)

Duration: 6 months

Laboratory: Center for Nanosciences and Nanotechnologies (C2N)

Address: 10 Boulevard Thomas Gobert, 91120 Palaiseau, France

Supervisor(s): Amaury Delamarre, Stéphane Collin

Phone number: 01 70 27 04 80

Send application to: amaury.delamarre@cnrs.fr

followed PhD

Context

In the past years, photovoltaics (PV) became one of the cheapest sources of energy. 97% of commercial solar cells are made of silicon, and their lab-scale record efficiencies of 28.2 % are now close to the theoretical limit (29.4 %). Yet, expectations of both the society and the PV industry are still high, and most of the research efforts are now dedicated to pushing forward the efficiency of solar cells. Silicon-based tandem devices are the most-regarded solutions for next-generation photovoltaics. To keep low costs and preserve the silicon bottom cell, low-temperature deposition is mandatory for the top cell. Current options are polycrystalline, high-bandgap semiconductors like hybrid perovskites and inorganic Cu(In,Ga)(S,Se)₂ -CIGS- or CdTe thin films, but they are still limited by both efficiency and/or stability issues that are hardly explained by current models. Further developments require a better understanding of the properties and limitations of low-cost thin-film materials. In particular, it is necessary to differentiate the properties of grain interiors and grain boundaries, and the macroscopic fluctuations that may occur in semiconductor alloys.

Scientific project

The goal of this project is twofold. From the one side combining CathodoLuminescence (CL) and PhotoLuminescence (PL) techniques would provide a multi-scale analysis tool for elementary processes and properties of bulk materials (doping levels, diffusion length, carrier lifetime, defect levels and densities...) and surfaces (surface recombination velocity, density of defects, surface charges…). The host team has been using CL extensively to study - down to the nanometer scale - some of the aforementioned systems such as Cd(Se)Te cells (Selenium diffusion/passivation^1,2,3, lifetime increasing²), GaAs thin-films and nanowires (doping assessment^4,5), or hybrid perovskite (degradation⁶). On the other side, given this experimental background, we aim at pushing our analysis a step further by calibrating our tool in absolute terms, i.e. extracting the number of CL photons emitted by the sample.  Another long-term objective is to use advanced computational methods for correlative data analysis, and simulation to build a realistic model of thin-film solar cells using the measured quantities.

This internship will first focus on the unique CL tool available at C2N. Its basic principle is the following: a material is excited with an electron beam in a scanning electron microscope (SEM), providing a spatial resolution of 10 nm. Secondary electrons are collected to form an SEM image, and emitted photons (CL) are collected simultaneously to acquire an hyperspectral image (luminescence spectrum at each point of the map). Time-resolved CL (TRCL) is also available to measure the luminescence decay after a pulsed excitation.

The candidate will be first trained on the unique, extremely powerful CL/TRCL tool. Then, she/he will use this technique to perform and analyze multiscale CL/TRCL mapping of polycrystalline thin films, with the goal to develop new methods to reveal the dynamics of carriers and correlate these properties to the functional parameters of solar cells.

References

¹ Frouin B. et  al., APL Materials 12, 031135 (2024)

² Bidaud et al., Sol. En. Mat. & Sol. Cells 303, 114390 (2026)

³ Ablekim T. et a, Solar RRL 5, 2100173 (2021)

⁴ Chen H.-L. et al., Phys. Rev. Applied 15, 024006 (2021)

⁵ Chen H.-L. et al., Phys. Rev. Applied 15, 024007 (2021)

⁶ Mejaouri S. et al., Small Methods 8, 2300901 (2024)

The institute

The candidate will work with several members of the sunlit team (C2N) and in close collaboration with the Institut photovoltaïque d’Ile-de-France (IPVF).

Websites: https://sunlit-team.eu , https://www.c2n.universite-paris-saclay.fr/en , https://www.ipvf.fr

Description of the CL setup and recent publications: https://sunlit-team.eu/resources/cl-and-trcl-tool/

Profile

Student in M2 with a solid knowledge in semiconductor physics and optics.

Possibility to continue with a PhD grant in 2027.

=> Send CV and motivation letter to stephane.collin@cnrs.fr and  stefano.pirotta@c2n.upsaclay.fr.

Starting date: 02/2027 (adjustable).

followed PhD

Context

Photovoltaics (PV) are a cornerstone of the global energy transition. Today, silicon-based solar cells dominate the market (97%), achieving average conversion efficiencies of 23% and records exceeding 28.2%, typically using 150 µm-thick wafers. The next generation of solar cells could achieve a seemingly paradoxical goal: higher efficiency with less material. For example, tandem architectures could push efficiencies beyond 30% and even up to 45%. Ultrathin solar cells (10x thinner than conventional ones) offer a transformative solution for material savings and reduced carbon footprint, lightweight and flexible applications, and are compatible with tandem solar cells for even higher performance.

However, their performance has been limited by insufficient light-trapping to compensate for the reduced material volume. Our team has already achieved a major milestone: in 2019, we demonstrated a 19.9%-efficient ultrathin solar cell with only 200 nm of GaAs, using a nanostructured back mirror that leverages multi-resonant absorption (Nature Energy, 2019). We’ve also published a comprehensive review on ultrathin solar cells (Nature Energy, 2020) and derived theoretical upper bounds for light trapping (PRX Energy, 2026), opening new perspectives for ultrathin solar cells.

Scientific project

This internship builds on these results to develop and demonstrate practical solutions for light-trapping in ultrathin solar cells. It will combine clean-room work and optical simulation, and it will explore the potential of correlated-disorder nanostructures for light-trapping using low-cost, self-assembly processes, aiming at their integration in single-junction silicon solar cells and tandems.

After a training to clean-room security and processes, the candidate will develop new fabrication processes in clean room in order to study and compare two emerging concepts: (1) multi-resonant absorption using perfectly ordered structures (arrays) fabricated by nanoimprint lithography, and (2) directional scattering using structures with correlated disorder, fabricated by self-assembled nanopatterning techniques such as polymer-blend and colloidal lithography. The performances will be analyzed by optical spectroscopy (reflection, transmission, absorption measurements) and optical simulation. The best performing nanopatterns will be integrated into thin and ultrathin silicon solar cells.

The team

This project is a joint effort between C2N (SUNLIT Team) and UMR IPVF, both located on the Paris-Saclay campus (Palaiseau). It is part of the ERC Synergy UltiMatePV project, in collaboration with Glunz's group at Fraunhofer ISE and Ballif's group at EPFL. More information on the SUNLIT Team: https://sunlit-team.eu

Profile: We are looking for a high-level student in M2 with a solid knowledge in semiconductor physics and optics.

Possibility to continue with a PhD grant on ultrathin, high-efficiency tandem solar cells in 2027.

Send CV and motivation letter to: stephane.collin@cnrs.fr

Starting date: 02/2027 (adjustable).

References:

[1] H.-L. Chen et al., A 19.9%-efficient ultrathin solar cell based on a 205-nm-thick GaAs absorber and a silver nanostructured back mirror. Nature Energy 4, 761-767, 2019.

[2] I. Massiot, A. Cattoni, S. Collin. Progress and prospects for ultrathin solar cells. Nature Energy 5, 959-972, 2020.

[3] S. Collin and M. Giteau, Upper Bounds on Broadband Absorption and Application to Solar Cells. PRX Energy 5, 023006 (2026).

[4] S. Collin et al., Advances in solar photovoltaic technologies and the role of photonics, Photoniques 128, 41-49 (2024).

followed PhD