Time-resolved cathodoluminescence: multiscale characterization of photovoltaic materials
A unique cathodoluminescence (CL) tool has been installed at C2N in late 2015 with state-of-the-art capabilities (only three similar setups exist worldwide). Its basic principle is the following (see the figure): a material is excited with an electron beam in a scanning electron microscope (SEM), providing a spatial resolution of 10nm. Secondary electrons (SE), emitted photons (cathodoluminescence, CL) and even electron-beam-induced current (EBIC) are collected and recorded simultaneously in order to form 2D maps. For each spatial position, CL spectra provide information on the luminescence efficiency, band structure and defects. In our tool, laser-controlled bunches of electrons can also be used for excitation instead of a steady-state excitation beam, resulting in time-resolved CL measurements (TRCL) that provide valuable information on carrier dynamics and lifetime. Our CL/TRCL setup has state-of-the-art specifications and is extremely versatile: wide ranges of wavelengths (200nm-1600nm) and temperatures (10K-350K), time-resolved measurements (temporal resolution 10ps). In addition, its very high collection efficiency on a wide field of view is perfectly adapted to CL and TRCL mapping of a wide variety of photovoltaic (PV) materials.
This project is focused on polycrystalline semiconductor materials (CdTe, CIGS, perovskites) for PV in the framework of internal and collaborative projects. The goal of the internship is to perform multiscale CL/TRCL mapping in order to investigate the impact of (large-scale) inhomogeneities, of localized defects (at the nanoscale), of the size and shape of grains, and of the passivation of grain boundaries. The results will be analyzed and correlated to the macroscopic properties and efficiency of PV devices.
The candidate will be first trained on the CL/TRCL tool. Then, she/he will use this technique to perform and analyze multiscale CL/TRCL mapping of selected series of samples, with the goal to develop new methods to reveal the dynamics of carriers (lifetime, diffusion length, recombination velocities at interfaces...) and correlate these properties to the functional parameters of solar cells. In this context, she/he will work with several members of the sunlit team (C2N) and in close collaboration with the “Institut photovoltaïque d’Ile-de-France” (IPVF). A PhD position on CL/TRCL characterization of photovoltaic nanomaterials may be opened in 2019.
(in french) Theoretical study of thermal and thermoelectric properties of polytype Ge nanowires by using ab-initio Monte Carlo Simulation
As the Fourier heat equation does not rigorously describe the thermal transport at the nanoscale, we have developed a unique home-made Monte Carlo simulator based on the Boltzmann’s transport equation for phonons. An internship position is available in the COMputationnal electronICS group and aims to investigate theoretically the nanoscale heat transfer in Nanowires.
Theoretical study of thermoelectric properties beyond the linear response of Single Electron Transistor
Specific properties of nanostructures have generated a recent revival of interest in thermoelectric devices . Thanks to their delta-like density of states, devices based on quantum dots are expected to exhibit high Seebeck coefficient, nearly zero electronic thermal conductance and ultra-low phononic thermal conductance if embedded in an oxide matrix . Due to single-electron tunneling across discrete levels in the Quantum Dot (QD), such devices are likely to behave as quasi-ideal energy filters giving rise to incomparable thermoelectric properties, i.e. with an efficiency very close to the ideal Carnot efficiency. An internship position dedicated to the simulation of such device is available in the COMputational electronICS group belonging to Center of Nanosciences and Nanostructures.
Manybody physics with light in semiconductor microcavities
Quantum simulation, first discussed by Richard Feynman, is an emerging experimental research field which aims at understanding the eigenstates of quantum systems with many interacting particles. When the number of particles increases, the Hilbert space size diverges and the eigenstates are impossible to calculate with a classical computer. Richard Feynman proposed to learn about these eigenstates by realizing the experiment with a well-controlled artificial quantum system. While the most advanced platforms are cold atoms, trapped ions or superconducting loops, a very promising approach is to use photons in microcavities to realize such manybody quantum states. One advantage would be that multi photon entanglement could be imprinted on photons leaking out of the system, thus realizing a new source of quantum light.
Our group at C2N has developed a unique expertise of designing lattices of coupled microcavities and pioneered the emulation of different Hamiltonians with these lattices. We have realized the first topological laser with a 1D lattice, we are exploring Dirac physics with 2D honeycomb lattices, to name just a few examples.
To induce photon-photon interactions and progress toward the simulation of manybody physics, we mix the cavity photons with electronic excitations, named excitons, created in quantum wells located in the cavity. The resulting exciton-photon state, named cavity polaritons, shows significant interactions which have allowed demonstrating many fascinating properties such as superfluidity of light.
The challenge we propose now is to increase interactions to enter the strong quantum regime with single photon non-linearities. The work will start with the development and characterization of novel active materials, based on coupled quantum wells, which are expected to give rise to much stronger interactions. These interactions will be measured by detailed low temperature spectroscopy and photon correlations. The smoking gun evidence for the quantum regime will be the measure of single photon emission. Then we will implement manybody Hamiltonians of increasing complexity building larger and larger lattices, and we will probe their quantum properties.
The work will be essentially experimental, with low temperature optical spectroscopy on microcavities. The PhD student will participate to the processing of the samples, profiting from the unique technological environment that will be able in the new C2N clean room. This work is part of a large international collaboration and the PhD student will directly interact with scientists from other laboratories and particularly with theoreticians.
Frequency-tunable vertical external cavity terahertz quantum cascade laser
The goal is to develop a frequency tunable THz Quantum Cascade Vertical External Cavity Surface Emitting Laser (QC-VECSEL). It will consist of an active medium made of an electrically pumped microstructured semiconductor/metal heterostructure, providing a reflective gain thanks to THz intersubband transitions, and an external semi-transparent mirror. It will be frequency tunable thanks to the use of an external mirror made of a blazed grating, that will provide a narrow band adjustable feedback. Beside this, the device will have significantly improved beam-quality, which is a limiting property in standard edge emitting QCLs, but can be achieved by design in a VECSEL configuration.
Analog computing with brain-inspired micropillar lasers
Artificial neural networks, which are at the heart of recent progress in analog computation and machine learning, are becoming increasingly important for our future digital societies. We propose to study coupled spiking photonic nodes in order to implement simple photonic artificial neural networks. Each node is materialized by a micropillar laser with integrated saturable absorber, whose neuromimetic properties have already been explored in the team.
In neurons, information is coded with spikes (electrical pulses) which are excited in an all-or-none fashion provided input stimuli to the neuron soma exceed a given threshold. This generic property is called excitability and has been demonstrated in micropillar lasers. Though, the optical spikes emitted by these latter are more than one millions times shorter in duration than biological action potentials. Hence, photonic neurons could in principle be interesting to build ultrafast artificial neural networks. The computing capability of optical neurons are enforced by the property of temporal summation also already demonstrated by us in micropillar lasers, and which provides universal computation capability.
The internship will consist in participating to the research lead in the group on the implementation of brain-inspired, photonic analog computing. The work consists majorily of optical experiments, with modelling and technology in the C2N clean-room.
Pulse train interaction and control in a microcavity laser with delayed optical feedback S. Terrien, B. Krauskopf, N. G. Broderick, R. Braive, G. Beaudoin, I. Sagnes, S. Barbay, Opt. Lett. 43, 3013 (2018)
Spike latency and response properties of an excitable micropillar laser F. Selmi, R. Braive, G. Beaudoin, I. Sagnes, R. Kuszelewicz, T. Erneux, S. Barbay, Phys. Rev. E 94, 042219 (2016)
Temporal summation in a neuromimetic micropillar laser F. Selmi, R. Braive, G. Beaudoin, I. Sagnes, R. Kuszelewicz, S. Barbay, Opt. Lett. 40, 5690 (2015)
Relative Refractory Period in an Excitable Semiconductor Laser F. Selmi, R. Braive, G. Beaudoin, I. Sagnes, R. Kuszelewicz, S. Barbay, Phys. Rev. Lett. 112, 183902 (2014)
F. Selmi, Thèse de doctorat, Réponse excitable et propriétés neuromimétiques de micropiliers lasers à absorbant saturable, 2015.
website : https://toniq.c2n.universite-paris-saclay.fr/fr/activites/smila/neuromimetic-photonics/
Hybrid nanostructures for nonlinear parametric processes
The control of light propagation at the nanoscale is one of the major subjects of present research. By enabling the confinement of the light in volumes as small as few cubic half-wavelength, photonic nanostructures allowed the demonstration of very interesting devices such as efficient single photon sources, low threshold nanolasers and low energy activation all optical gates.
Since a few years, our team at LPN has been particularly interested in exploiting the enhancement of the light-matter interaction using photonic crystals in order to obtain large nonlinear effects with reduced powers. The nonlinear effects under study, such as Kerr effect or second harmonic generation, enable a range of possibilities such as the control of light by light, light amplification or the generation of new frequencies.
In the proposed project, the candidate will focus her/his work on the study of second and third order nonlinear processes within semiconductor micro/nanostructures. The idea will be to use the unique dispersive properties of, e.g. photonic crystals, in order to obtain integrated parametric amplifier or frequency combs sources. As the use of only one type of material can be a limitation, heterogeneous integration of different materials will be implemented as a novel approach with the idea to exploit each class of material at its best.
The candidate will be involved in the modelling and the simulation of the structures under investigation, in the nanofabrication of the samples and in the sophisticated optical experiments necessary to observe the nonlinear behaviours.
The work can be pursued during a PhD thesis within the framework of the european Training Network H2020 - MOCCA.
Hybrid III-V semiconductor on SOI optoelectronic devices
Photonic devices play a crucial role in the domain of information and communication technology, due to their ability to bring efficient solutions to data transmission and processing.Tremendous development, through optical fibres backed by related devices and circuits composed of light sources, optical amplifiers, wavelength multiplexers, photodetectors, etc, have greatly revolutionized communication in general. As a necessary evolution, attention is now being directed to optical datacom and computercom with an emphasis on the conception of power efficient ultracompact optoelectronic components. In photonics, the challenges which we face today, swirls around providing together the necessary active and passive functionalities fully integrated into a chip. These functionalities are, among others, light emission and amplification, filtering, wavelength routing ((de)multiplexing), detection or switching. Because all of these functionalities have to comply with ultra-compactness and low-loss circuitry while maintaining low cost production in CMOS fabs, few materials can pretend to fit in. In this context, Silicon on insulator (SOI) photonics, enhanced by III-V semiconductors is a key technology combining the best of both materials leading to a highly versatile hybrid photonics platform which opens the way to large scale photonic integration. During the last years, in my research team, we have taken forward this domain to the nanophotonic world by demonstrating III-V on SOI active devices based on planar photonic crystals (PhCs) to address the issues of compactness and power efficiency. Indeed, planar PhCs which consist of wavelength scale arrangements of holes (typ. radius~100nm), drilled into a semiconductor thin slab enable a quasi-extensive control of the electromagnetic field confinement and propagation. They have demonstrated over the last decade their capacity to shelter extremely efficient nonlinear interactions exploitable for low threshold laser emission , all-optical switching , etc… The proposed project aims at building a new panel of hybrid III-V on SOI optical devices with performance beyond the state of the art in terms of footprint, power efficiency and speed, to meet the stringent requirements for on-chip optical interconnects. The targeted components will be electrically driven nanolaser, an optical amplifier and a memory.
The student will be required to focus on the design, fabrication, and experimental aspects withcontinuous feedbacks “loops” between all these different aspects. This multi-task work is possible with the remarkable facilities and resources of C2N.
The work can be pursued during a PhD thesis within the framework of the ERC project HYPNOTIC.
Direct band gap Ge-based micro-cavities emitters
Résumé / summary
The objective of the project is to explore the potential of merging direct band gap Ge-based material for developing new electro-optical application with group IV elements such as optical sources in the NIR and the MIR. During the training, the candidate will be involved in optical analysis of light emission from Ge-based micro-cavities. Depending on his motivation and on the lab schedules, the candidate will be also introduced to the clean room facilities of C2N to contribute to device processing using the micro-nano fabrication tools.
Research context :
One of the major challenges of photonics is to meet a full-CMOS compatible technology. The main missing element that would allow to fully exploit the technology of silicon photonics is the laser source. Unfortunately silicon and germanium, which are the main group-IV elements used in the microelectronics industry, are penalized for the emission of light because of the indirect nature of their electronic band structure. Currently the semiconductor laser sources are mainly made from III-V elements whose integration on silicon turns out to be complex, for reasons of chemical incompatibilities between III-V and IV-IV, and also of manufacturing cost. C2N research group have succeed to turn the band structure of germanium into direct band gap through the application of tensile mechanical stress or by alloying Ge with tin. This has led to recent demonstration of laser effect in Ge at low temperature. It is also possible to combine both strain and alloying to obtain direct band gap materials. It has been recently shown a laser emission regime in the GeSn operating at low temperature as well. It turns out that the alloy of Ge with tin and the application of tensile stress reduces the band gap of the materials and the operating wavelengths can extend from 2 μm to 5 μm in depending on the tin composition and the stress applied. At these wavelength CO2 and CH4 exhibit a high absorbtion signature. Applications for labs-on-chip spectroscopy for biosensing, gas detection and air monitoring are envisioned with the opportunity to integrate the full group IV photonic circuits in interconnected objects.