Two coupled-cavity systems, or « photonic dimers », are efficient test-beds for nonlinear dynamics in nanophotonics. I will focus on two evanescently coupled photonic crystal nanolasers, which can be engineered to experimentally access interesting bifurcation points. I will discuss our recent results on two emblematic bifurcations: spatial symmetry breaking (pitchfork) and asymmetric mode switching (Hopf). Importantly, the underlying nanophotonic structure enables a high degree of control, for instance in terms of intercavity coupling parameters. A second important aspect to be discussed is the role of noise in these relatively large spontaneous emission factor (β) lasers. The interplay between determinism and noise at bifurcation points results in a very rich photon statistics which, in the case of mode switching instabilities, leads to strong dips in the cross-correlation of mode fluctuations. These results open up interesting prospects in the study of few photon bifurcations in semiconductor laser devices.
Josephson dynamics, spontaneous symmetry breaking and quantum criticality are fascinating physical phenomena that can be realized today in coupled dissipative optical cavities with nonlinear interactions. Among the different experimental test-beds, photonic crystal coupled nanocavities operating in the laser regime are outstanding systems since nonlinearity, gain/dissipation and intercavity coupling can be judiciously tailored .
Complex photon statistics is inherent to the nature of nanolasers due to the presence of strong spontaneous emission noise. Yet, although most common scenarios emerge from quasi-dynamical equilibrium where the gain nearly compensates for losses, little is known about far-from-equilibrium statistics resulting, for instance, from a rapid variation of a parameter or "quench".
Our nanolasers are fabricated in suspended 2D InP-based Photonic Crystal membranes, and studied as a function of pump power and coupling strength. The modification of coupling strength is obtained by an original engineering procedure that allows us to tune the coupling strength between the nanocavities without affecting the nanolaser performance .
Under short (100 ps) pulse pumping, the strongly coupled laser nanocavity system exhibits two modes: a strong lasing mode, which has an anti-symmetric energy distribution, and a weak nonlasing one, possessing a symmetric energy distribution. We implement a simple experimental technique –single pulse energy detection scheme– that allows us to measure the statistical distributions of the photon number of both modes simultaneously. In particular, we analyze the photon number distributions of the weak one and link, using a mean field model, both the emergence of fat tails in the distributions and the superthermal nature of the emission through second order correlation (g2) measurements. We conclude that transient dynamics after quench, when projected onto the nonlasing mode, generically exhibit long-tailed superthermal light.
Such an optical quench mechanism is akin to the fast cooling of a suspension of Brownian particles under gravity, with the inverse temperature of the reservoir playing the role of the intracavity intensity. We show that passing through the lasing threshold corresponds to an abrupt decrease of the contribution of spontaneous emission —that plays the role of an effective temperature— during which the statistics of the nanolaser trajectories in phase space are dominated by nonlinear transport.
Probability density functions enabled the experimental quantification of the distance from thermal equilibrium –and hence the degree of residual order– via the thermodynamic entropy. This allowed us to further detect mixing of thermal states and coherent broken parity phases, which are beyond the simple Brownian particle description .
1. Hamel, P., et al., “Spontaneous mirror-symmetry breaking in coupled PhC nanolasers,” Nat. Phot., Vol. 9, 2015.
2. Marconi, M., et al., “Asymmetric mode scattering in strongly coupled photonic crystal nanolasers,” Optics Letters, Vol. 41, 5628, 2016.
3. Marconi, M., et al, “Quenched phases in strongly coupled dissipative optical cavities. ” arXiv preprint arXiv:1706.02993.
We report the formation of hybrid states of self-assembled PTCDI-C7 organic molecule excitons and surface plasmon polaritons (SPP).
Ten self-assembled monolayers with no host matrix are directly evaporated onto a gold thin film forming a ultra-dense and organized, 30nm thick layer. The π- π stacking among molecules leads to the formation of H-aggregates with alignment of molecular dipole moments along the local electric field vector. This collective excitations are known to give rise to a sharp excitonic peak in absorption with large oscillator strength, which are favorable properties for the observation of strong coupling.
Experimental wavevector-resolved reflectance spectra display an anticrossing, attesting the strong coupling regime with a Rabi splitting energy ΩR=102 meV at room temperature. By contrast, no anticrossing has been observed for PTCDI-C7 molecules evaporated in a different experimental condition and with a reduced local order. We interpret the observed strong coupling regime as resulting from the high degree of organization and the controlled molecular dipole orientation.
Under optical pumping, we observe an enhancement of the coupling efficiency between the molecular emission and the SPP mode. This observation is consistent with the small oscillator strength of the lowest Frenkel state of exciton and to the large Stokes shift of PTCDI-C7 molecules induced by H-aggregate stacking.
The use of ultra-dense layers of self-assembled molecules opens interesting perspectives for the control of the molecular dipole orientation at the nanoscale to maximize interaction with the SPP field and therefore the strong coupling strength.
The spontaneous breaking of mirror-symmetry in two coupled photonic crystal nanocavity-lasers is experimentally demonstrated. The inter-cavity evanescent coupling is tuned such that the nonlinear interaction –the carrier-induced nanolaser frequency shift– overcomes photon tunneling. This, together with the optimization of the nanocavity beaming, allows us to observe a spontaneous transition from a delocalized mode to two spatially localized states, in the form of a pitchfork bifurcation. Coexistence of these states is demonstrated through short pulse excitation. This kind of devices based on symmetry breaking could yield new types of flip-flop memories and nanolaser sources with strong photonic correlations.
We show both theoretically and experimentally that the lifetime of an active semiconductor photonic crystal nanocavity is enhanced thanks to the combination of two cooperative effects: slow light propagation based on coherent-population-oscillation effect and optical bistability. In particular we develop an analytical analysis enabling us to clearly show the physical mechanisms producing the enhancement of the cavity lifetime.
We demonstrate that the lifetime of a nanocavity can be enhanced by inserting a medium with a strong index
dispersion in the cavity. The strong dispersion is achieved through coherent population oscillations effect in the
quantum wells of a two-dimensional photonic crystal nanocavity. The initial cavity lifetime of ~3-6ps has been
extended to a maximum value of about 336 ps.
Temporal characteristics of band-edge photonic crystal are
precisely analyzed using a high-resolution up-conversion system. The
InGaAs/InP photonic crystal laser operates at room temperature at 1.55 μm
and turn on times ranging from 17ps to 30ps are measured.
We present continuous-wave laser operation at room temperature at 1.55 μm by optically pumping a photonic crystal
structure containing an InGaAs/InP quantum well active layer. The active layer is integrated onto a Silicon chip by
means of Au/In bonding technology. This metallic layer provides the reduction of heating by thermal dissipation into the
substrate, and increases the quality-factor by reducing the radiative losses.
In this manuscript we analyze the modal dynamics of multimode semiconductor quantum-well lasers. Modal switching is the dominant feature of the devices analyzed and it obeys a highly organized antiphase dynamics which leads to an almost constant total intensity output. For each active mode a regular switching at frequencies of
few MHz is observed. The activation order of the modes follows a well defined sequence starting from the lowest wavelength (bluest) mode to the highest wavelength (reddest) mode, then the sequence starts again from the bluest mode. Using a multimode theoretical model and a simpler phenomenological model we identify that four wave mixing is the dominant mechanism at the origin of the observed dynamics. The asymmetry of the susceptibility function of semiconductor materials allows to explain the optical frequency sequence.