PhD defense

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    Optical nonlinearities in III-V-on-Si active nanostructures

    C2N - Centre de Nanosciences et de Nanotechnologies, , Palaiseau

    Andrea Demarchi

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Hybrid integrated micro- and nanolasers enable efficient light emission at chip scale with reduced power consumption and footprint. Their unique properties make them good candidates for overcoming the limitations of copper-based interconnects. Together with this, the exploitation of their optical nonlinearities has been shown to be effective in the demonstration of nonlinear activation functions in the context of optical neuromorphic computing.
    The PhD work regards the design, fabrication and characterisation of InP-on-Si microdisk laser diodes, together with the modelling and characterisation of the latter in the injection locking regime for the implementation as an all-optical activation function. The design is based on numerical simulations to study the electro-optical properties of the laser. In particular, with respect to previous demonstrations, a novel electrical pumping scheme is proposed. The modelling of the device shows the possibility of achieving sub-mW operation in such a configuration, considering a disk of 3.75 μm radius. The fabrication of the devices follows, based on the adhesive bonding of the III-V and SOI sections. Experimental characterisation of the device under DC electrical pumping reports sub-mW operation for the 3.75 μm radius microdisk (390 μA of threshold current at 1.06 V of applied bias). The last part of the thesis is devoted to the study of the nonlinear operation of the microdisk in the injection-locked regime. After the theoretical description of the injected system, two different characterisations are reported at 10 kHz and 2 GHz of modulation, respectively. A saturating, sigmoid and bistable response with μW-scale switching powers is reported in the first case. A clipped GeLU, sigmoid and inverse ELU response are reported in the second case.

    Figure : (left) scheme of the III-V on SOI microdisk laser diode. (right) SEM image of the device.

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    Multimodal one-dimensional photonic crystal cavities

    C2N - Centre de Nanosciences et de Nanotechnologies, , Palaiseau

    Loredana Maria Massaro

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Integrated photonics research focuses its efforts in developing miniaturised and efficient devices for applications in quantum information, metrology and medicine. In this context, photonic crystal cavities occupy a leading position in the integration of photonic structures owning small footprint and tailorable spectral properties. Special attention in recent years is given to photonic crystal cavities consisting of III-V semiconductor materials, as their nonlinear properties are an asset for further development in complex circuitry of coherent states of light through nonlinear interactions.
    In this thesis, we present the design, fabrication and experimental characterisation of one-dimensional photonic crystal cavities integrated on silicon on insulator. First, we detail the design of our cavities which is based on the gentle confinement of the field. We show that our design technique allows the design of multimodal as well as single-mode photonic crystal cavities working at telecom window. The introduced design technique is versatile and easily tailorable to devise cavities of different materials, as InGaP, Si, or InP. Then, we detail the fabrication of III-V photonic structures heterogeneously integrated on silicon on insulator. From the adhesive bonding of the two levels to the final integrated device, fabrication process flow is reported and main challenges encountered commented. Fabrication of silicon-based cavities is also reported. Experimental characterisation of the cavities is conduced by measuring the transmission of the samples. We present how to tailor the spectral properties of the cavities through nanometric variation of their geometrical parameters, and comment on some peculiarities of multimodal coupling.

    Figure : One-dimensional InGaP photonic crystal cavities integrated on SOI

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    Monte Carlo simulations of phonon transport in nanostructures based on ab-initio methods

    C2N - Centre de Nanosciences et de Nanotechnologies, , Palaiseau

    Junbum Park

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    As the trend towards miniaturization of electronic devices continues, understanding heat transport at the nanoscale becomes increasingly crucial for developing energy-efficient and reliable systems. Conventional Fourier's law fails to capture the complex dynamics of phonon-based heat transport in such miniaturized devices. In this thesis, we study the phonon transport within nanostructures employing stochastic Monte Carlo methods. The accuracy of the simulations is enhanced by utilizing full band material description derived from ab-initio calculations based on density functional theory (DFT) without reliance on empirical parameters. We focus on examining alternative materials, such as GaAs, and 2D materials, like graphene, h-BN, and TMDCs, each selected for their unique thermal properties. This work not only offers substantial contributions to the field of thermal transport in nanostructures but also opens new pathways for the design and application of advanced materials in electronics.

     

    Figure :

    - Left grahp : Sketch of the simulated 100 nm 2D h-BN/graphene lateral heterostructure with specific positions (A to F) marked for thermal transient analysis. Positions A, B, C, D, E, and F are located at 2, 25, 48, 52, 75, and 98 nm, respectively, along the x-axis within the structure. Red/blue faces for hot/cold thermostats with TH = 400 K, TC = 300 K, respectively.
    - Right graph : Transient heat flux density at 6 points in 100 nm h-BN/graphene lateral heterostructure, measured from 0 to 200 ps. The rainbow color grade intuitively represents the varying heat flux densities according to their respective positions : red for position A, orange for B, yellow for C, green for D, blue for E, and navy for F.

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    C2N - Centre de Nanosciences et de Nanotechnologies, , Palaiseau

    Federico Monti

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    PhC nanolasers are receiving increasing attention due to their unique capacity to manipulate and confine light at a very small scale. Their small footprint and low thresholds make them ideal candidates for realizing optical interconnects, thus addressing the increasing demands for data transmission speed and power consumption. Moreover, their singular geometry enables the control of their spontaneous emission properties. This reveals the unique nature of PhC nanolasers from a fundamental perspective, highlighting their potential to serve as candidates for novel research in light-matter interaction. Despite these advantages, the current limitations in near-infrared detection capabilities impede the comprehensive characterization of their emission and dynamical properties, leaving a crucial gap in our understanding of these devices.
    This thesis presents a time-gating detection technique based on four wave mixing, developed to measure the ultra-fast response of 1D nanolasers. By studying the interplay between nonlinearities and dispersion, a high sensitivity level of a few photons and a resolution of 2 picoseconds has been achieved. Further improvements in sensitivity, down to detection of less than one photon, are predicted by employing higher gate powers. This approach can be applied to the study of photon statistics and the quantum nature of materials.
    The profiles of 1D nanolasers exhibit a rapid onset of emission and long decay, consistent with a β factor of 0.12 and a photon lifetime of 20 picoseconds.
    A novel approach to obtaining the values of these two parameters controlling laser dynamics has been developed: they have been directly retrieved from the ultra-fast response of nanolasers, instead of solely relying on steady-state measurements such as the S curve, which, in many cases, can lead to inaccurate estimations due to the interdependence of these parameters. The dynamical response of 1D nanolasers is compatible with a maximum modulation speed of approximately 30 GHz, which fulfills the requirement for low-threshold ultracompact laser sources for photonic integrated circuits and optical communications.
    The high sensitivity and resolution of the technique permitted the first measurement of an adiabatic wavelength conversion of photons with a wavelength shift as large as 1.2 nanometers. This demonstrates the potential of the technique in studying ultra-fast dynamics at near-infrared wavelengths.

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    Developing a slow-mode nanophotonic platform for strong interaction between cold Rb atoms and guided photons

    C2N - Centre de Nanosciences et de Nanotechnologies, , Palaiseau

    Sukanya Mahapatra

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    Light-matter interaction at the single-quanta level is the keystone of quantum information science. However, single quanta are generally weakly interacting and enhancing this coupling has been the driving force for a large community and the development of the cavity quantum electrodynamics (CQED), where single atoms and single photons can be strongly coupled via a high-finesse cavity. Very recently, integrated photonic nanostructures appeared as a promising avenue of tailoring light-matter interaction by engineering the emitter environment. Modern nanofabrication techniques have enabled the design of solid-state systems with embedded emitters, such as quantum dots in photonic crystal waveguides or nanocavities with high-quality factors, leading to Quantum Nanophotonics. In this context, we explore the waveguide QED approach by trapping atoms close to photonic crystal waveguides exhibiting slowly propagating modes, reaching strong interaction without a cavity. 

    Figure : Suspended slow mode Half W1 photonic crystal waveguide fabricated in GaInP during the thesis. 

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    (in french) Croissance par épitaxie par jets moléculaires d'hétérostructures BiSb/MnGa pour l'étude de la conversion d'un courant de charge en courant de spin

    Auditorium de Thales R&T, , Palaiseau

    Diana She

    Centre de Nanosciences et de Nanotechnologies, C2N, Palaiseau

    PhD defense

    figure : STEM-HAADF cross section of the BiSb(9 nm)/MnGa(10 nm)//GaAs(001) heterostructure