Guilhem Madiot

Non-Hermitian photonics provides a framework to engineer the gain and loss of optical modes in open systems, enabling the control of their spectral and dynamical properties. In particular, the ability to dynamically tune modal losses offers a route to implement functionalities traditionally relying on cavity Q-factor modulation, such as Q-switching, within nanophotonic platforms. Here, we demonstrate the generation of short optical pulses in a pair of phase-coupled photonic crystal nanolasers by exploiting non-Hermitian coupling. Two waveguide-coupled nanocavities are operated below their individual lasing thresholds and subjected to asymmetric optical pumping such that a transient carrier-induced detuning modifies the interference conditions between them. This dynamically controls the gain and loss of collective modes. Upon crossing a resonance condition, this leads to the rapid release of stored carrier energy as an optical pulse. A rate-equation model captures the interplay between carrier dynamics and modal coupling and reproduces the observed behavior. Experiments performed on an indium phosphide platform show pulse generation from cavities that do not lase efficiently on their own in continuous-wave operation with temporal characteristics governed by carrier dynamics. These results indicate that non-Hermitian coupling can be used to control the effective cavity losses over time, providing a route to pulse generation in integrated photonic systems.

We experimentally demonstrate the direct time-domain characterization of photonic-crystal nanolasers at telecom wavelengths using a nonlinear optical gating technique based on four-wave mixing. This approach enables the temporal characterization of the ultrafast emission dynamics under short-pulse excitation with a picosecond time resolution. When a weak continuous-wave component is added to the pulsed pump, the emission becomes less sensitive to spontaneous-emission noise, resulting in a significantly reduced buildup time. The difference between purely pulsed and hybrid excitation regimes points to the influence of pulse-to-pulse timing fluctuations. To elucidate this effect, we perform Langevin-based simulations that reproduce the experimentally observed broadening and confirm that time jitter, originating from spontaneous emission noise near threshold, dominates the temporal dispersion. These results establish four-wave mixing gating as a powerful method to probe nanolaser dynamics with picosecond precision.

Modulation cohérente dans des résonateurs couplés électro-optomécaniques à cristal photonique : dynamique de Floquet et chaos: Les système nanomécaniques permettent d’explorer les relations physiques fondamentales entre les propriétés élastiques, thermiques and électromagnétiques des solides. Ils sont de surcroît souvent sujets à de fortes nonlinéarités – du fait de leurs dimensions nanométriques – ce qui les rend intéressants pour étudier des concepts fondamentaux tels que la synchronisation ou le chaos. Ces systèmes nanomécaniques peuvent être mis en interaction avec une cavité optiques ou couplés à des actuateurs électrostatiques. Ces deux approches sont étudiées dans le cadre de l’électro-optomécanique. Dans ce travail de thèse nous mettons à profit la versatilité des cristaux photoniques pour étudier la dynamique non linéaire optique et mécanique induite par la modulation cohérente de l’excitation de systèmes électromécaniques ou optomécaniques.Dans un premier temps nous utilisons une plateforme nanophotonique combinant une membrane d’InP suspendue au-dessus d’un guide de silicium intégré. La membrane comprend un cristal photonique bidimensionnel comprenant plusieurs cavités-défauts couplées entre elles par champ évanescent. Ces cavités constituent une molécule photonique dont les modes propres électromagnétiques peuvent être sondés par un laser, permettant ainsi d’accéder aux spectre de bruit mécanique de la membrane. L’utilisation d’une modulation cohérente du champ en entrée, nous démontrons le transfert du motif spectral depuis le domaine optique vers le domaine mécanique. La présence de nonlinéarités thermo-optiques dans le système mène à une désymétrization du spectre de bruit mécanique. L’expérience est décrite théoriquement par une approche de Floquet. Finalement, en se plaçant dans un régime de bistabilité thermo-optique, nous démontrons l’amplification d’un signal de faible amplitude dans un mode photonique par résonance vibrationnelle.Dans une seconde partie, nous étudions deux membranes à cristal photonique couplées entre elles mécaniquement. Le système est actué par un dispositif électro-capacitif et sondé par lecture optomécanique. Sous excitation suffisamment faible, le système peut être efficacement calibré au travers d’un modèle linéaire. Les fortes nonlinéarités mécaniques du système se manifestent lorsqu’une excitation plus forte est utilisée, ce qui est modélisé par un modèle impliquant deux oscillateurs de Duffing couplés et forcés. Cette fois l’utilisation d’une modulation cohérente de l’excitation induit une dynamique de route vers le chaos par doublement de périodes. L’excitation simultanée des deux modes normaux mécaniques dans leur régime non linéaire leur permet de se coupler de telle sortes que leur synchronisation en régime chaotique peut être étudiée. Le chaos pouvant être exploiter pour générer des nombres aléatoires, cette synchronisation chaotique bichromatique pourrait servir à développer de nouveaux protocoles de communication multi-spectrale.En perspective, ce travail ouvre la voie à l'étude de la dynamique collective dans de plus larges réseaux de systèmes optomécaniques.

We present a detailed study of Brillouin light scattering (BLS) spectroscopy applied to phononic waveguides embedded within optomechanical crystal structures. By comparing numerical simulations with experimental spectra, we validate the capability of BLS to probe GHz mechanical modes, including acoustic slow modes at 6.7 GHz, in silicon-on-insulator platforms at room temperature. Our results highlight the potential of BLS to characterize mechanical dispersion, guided modes, and acoustic bandgaps in cavity optomechanics, where photons and phonons are colocalized. This work provides critical insights into the feasibility of leveraging hypersonic phonons for in-chip interconnects and integrated broadband communication systems.

Tailoring the losses of optical systems within the frame of non-Hermitian physics has appeared very fruitful in the past few years. In particular, the description of exceptional points (EPs) with coupled resonators has become widespread. The on-chip realization of these functionalities is crucial for integrated nanophotonics but requires fine control techniques of the nanodevice properties. Here, we demonstrate pump-controlled directional emission of two coupled nanolasers that distantly interact via an integrated waveguide. This coupling scheme unusually enables both frequency and loss couplings between two cavities, which can be advantageously exploited to reach EPs by either detuning the cavities or controlling the gain of nanolasers. The system can be readily reconfigured from bidirectional to unidirectional emission by adjusting the pump power.

Exceptional points (EPs) attract lots of attention due to the richness of the phenomenology associated to their presence in the complex eigenspectrum of coupled non-Hermitian systems. Here we provide both a coupled mode theory analysis and an experimental investigation of two nanolasers interacting through a channel-mediated coupling. We demonstrate the transition from Parity-Time (PT) symmetric to PT-broken regime using a thermo-optic control over the laser frequency detuning

Tailoring the losses of optical systems within the frame of non-Hermitian physics has appeared very fruitful in the last few years. In particular, the description of exceptional points (EPs) with coupled resonators have become widespread. The on-chip realization of these functionalities is highly significant for integrated nanophotonics, but requires fine control techniques of the nanodevice properties. Here, we demonstrate pump-controlled directional emission of two coupled nanolasers that distantly interact via an integrated waveguide. This coupling scheme unusually enables both frequency- and loss-couplings between two cavities, which can be advantageously exploited to reach EPs by either detuning the cavities or controlling the gain of nanolasers. The system can be readily reconfigured from bidirectional to unidirectional emission by adjusting the pump power.

The interferometric control of dissipation in a two-port system is a fruitful concept enabling the enhancement or cancellation of the input amplitudes as a function of their relative phases. Here, beyond the canonical configuration of Coherent Perfect Absorption (CPA), we apply this concept to two simultaneously excited strongly-coupled nanoscale electromechanical resonators submitted to independently controlled phase-shifted excitations. Both subsystems are read simultaneously by optical means allowing us to completely reconstruct the signature of coherent annihilation or amplification on both quadrature. We evidence that the mechanical modes amplitude can be enhanced or inhibited with respect to the case of single port excitation while phase experiences strong variations with the excitation imbalance and phase difference. Meanwhile, phase singularities with opposite topological charges are observed for mechanical normal modes. Close to the phase singularity, we demonstrate that the input of a weak phase modulation induces a large, pure phase modulation of the normal mode. These experimental demonstrations are fully modelled via the mechanical dynamical equations of our system. The interferometric control may open avenues for low-power amplitude controlled phase modulation schemes and vice-versa for potential switches and logical gates.

Chip-scale multimode optomechanical systems have unique benefits for sensing, metrology, and quantum technologies relative to their single-mode counterparts. Slot-mode optomechanical crystals enable sideband resolution and large optomechanical couplings of a single optical cavity to two microwave-frequency mechanical modes. Still, previous implementations have been limited to nanobeam geometries, whose effective quantum cooperativity at ultralow temperatures is limited by their low thermal conductance. In this work, we design and experimentally demonstrate a two-dimensional mechanical–optical–mechanical (MOM) platform that dispersively couples a slow-light slot-guided photonic-crystal waveguide mode and two slow-sound ∼ 7 GHz phononic wire modes localized in physically distinct regions. We first demonstrate optomechanical interactions in long waveguide sections, unveiling acoustic group velocities below 800 m/s, and then move on to mode-gap adiabatic heterostructure cavities with a tailored mechanical frequency difference. Through optomechanical spectroscopy, we demonstrate optical quality factors Q ∼ 105, vacuum optomechanical coupling rates, go/2π, of 1.5 MHz, and dynamical back-action effects beyond the single-mode picture. At a larger power and adequate laser-cavity detuning, we demonstrate regenerative optomechanical oscillations involving a single mechanical mode, extending to both mechanical modes through modulation of the input laser drive at their frequency difference. This work constitutes an important advance toward engineering MOM systems with nearly degenerate mechanical modes as part of hybrid multipartite quantum systems.

Phonons offer the possibility to connect the microwave and optical domains while being efficiently transduced with electronic and optical signals. Here, we present a multimodal optomechanical platform, consisting of a mechanical-optical-mechanical resonator configuration. The mechanical modes, with frequencies at 265 MHz and 6.8 GHz, can be simultaneously excited into a phonon lasing regime as supported by a stability analysis of the system. Both the megahertz and gigahertz modes enter a self-sustained oscillation regime, leading to the intermodulation of two frequency combs in the optical field. We characterize this platform experimentally, demonstrating previously unexplored dynamical regimes. These results suggest the possibility to control multiple mechanical degrees of freedom via a single optical mode, with implications in gigahertz phononic devices, signal processing, and optical comb sensing applications.