The research work in HYPNOTIC was carried out along to 4 research axis namely: Photonic crystal nanolaser diodes, Coherently intercoupled active nanocavities, Hybrid flip-flops and memories and Nano optical parametric source.
Since the beginning of the project, our work has consisted in exploring novel nanophotonic configurations in order to achieve our goals. Design and modelling, clean room fabrication and optical characterisation were the tasks undertaken.
3 novel electro-optical configurations enabling the necessary efficient current injection preserving a high quality factor for the micro/nanocavities were investigated. They are namely the nanorib Photonic crystal (PhC) nanocavity, 2D PhC cavity based on asymmetric line defect waveguide and microdisks. These nanophotonic structures were made in thin InP based slabs incorporating quantum wells as the gain materials emitting in the telecom window. Continuous wave laser emission was demonstrated using these 3 configurations with low laser threshold at room temperature. Light is directly emitted in Silicon on Insulator photonic integrated circuits. The ultrafast dynamics of these lasers was assessed both theoretically and experimentally using sophisticated optical experiments specifically mounted.
The physics of the coherent coupling of active nanocavities through a Silicon waveguide was studied during the project. This system has the particularity that the coupling possesses a phase term which can be adjusted at will by changing the distance between the cavities. This yields to 2 coupled modes (bonding and antibonding) exhibiting splitting both in loss and energy, which depend on the coupling phase. We observed that loss splitting leads to collective emission of the cavities with a drastic reduction of the laser threshold compared to a standalone cavity. When each cavity is solely pumped, laser threshold is never reached. It is only through the onset of a collective mode that laser emission can be observed. It is to the best of our knowledge the first observation of collective emission of coupled nanocavities, which can be qualified as superradiance. The resulting collective lasing emission is also characterized by a spatial directionality that can switch depending on the position of the system relative to the exceptional points observed in these systems.
Nonlinear regimes of emission of the coupled cavities were also explored theoretically. Calculations predict bistable operation, self-pulsing or excitable regimes. Injection locking was investigated on microdisks laser diodes. 3 different regimes were obtained evaluated depending on the wavelength detuning between the injection and the laser diode: a saturating, a sigmoid and a bistable response. The measured switching powers were very low in the μW range. The switching dynamics was explored and enabled the demonstration of data processing at 2Gbaud rate.
During the project, we realised that the very same cavity design we used for our nanolasers were suitable for triply resonant nonlinear processes. In these cavities, photons are trapped in a parabolic potential to shape the field amplitude in the fundamental mode as a Gaussian. Interestingly, as in a harmonic oscillator, the higher order modes resonant frequencies are perfectly equispaced. It is ideal for Four Wave Mixing applications. We worked on hybrid structures where InGaP nanobeam cavities are integrated on SOI, InGaP allowing the mitigation of 2 photon absorption. We firstly showed that our cavities provide efficient stimulated Four Wave Mixing. By improving the Q factors of our cavities and by implementing a fine tuning of the frequency of consecutive modes exploiting thermal effects, we demonstrated the first semiconductor nano optical parametric oscillator. This technology was exploited for the generation of non-classical light. We showed that it is possible to generate photon pairs at a rate already matching state of the art results. Time energy entanglement was demonstrated as well as single photon heralded single photon sources.
The work carried out during HYPNOTIC led to more than 14 publications with, notably 1 article published in Nature photonics and 1 in Science Advances. 2 patents were also filed.
