Spatiotemporal continuum generation in polariton waveguides

Abstract

We demonstrate the generation of a spatiotemporal optical continuum in a highly nonlinear exciton–polariton waveguide using extremely low excitation powers (2-ps, 100-W peak power pulses) and a submillimeter device suitable for integrated optics applications. We observe contributions from several mechanisms over a range of powers and demonstrate that the strong light–matter coupling significantly modifies the physics involved in all of them. The experimental data are well understood in combination with theoretical modeling. The results are applicable to a wide range of systems with linear coupling between nonlinear oscillators and particularly to emerging polariton devices that incorporate materials, such as gallium nitride and transition metal dichalcogenide monolayers that exhibit large light–matter coupling at room temperature. These open the door to low-power experimental studies of spatiotemporal nonlinear optics in submillimeter waveguide devices.

Ultrafast-nonlinear ultraviolet pulse modulation in an AlInGaN polariton waveguide operating up to room temperature

“…We note that the spectral shape of the background scatter remains constant. As we will later confirm by comparison with simulations, the broadening of the waveguided light arises due to simultaneous nonlinear modulation of the pulse temporal and spatial ( y ) envelope 33 . The large spectral width and non-trivial y ( λ ) dependence imply an optical field with features that vary rapidly, on a timescale equal to the inverse of the spectral width, which can only be produced by sub-picosecond nonlinear dynamics.…”

“…Their exciton-like interactions lead to nonlinearity at least 1000 times larger than in weakly coupled semiconductors 28 , 29 . This has allowed observation of nonlinear phenomena such as parametric scattering 30 , superfluidity 31 , solitons 32 and optical continuum generation 33 as well as quantum light sources 7 , 34 . Achieving strong coupling nonlinearity requires exciton binding energy greater than the thermal energy k B T .…”

“…Strong modulation of picosecond optical pulses is achieved over a short 100 μm distance leading to spectral broadening up to 80 meV width. This broadening, which cannot occur in the linear regime, is the essential signature of pulse temporal envelope modulation 33 . We deduce an effective nonlinear refractive index > 10 −13 cm 2 W −1 , three orders of magnitude larger than those measured in other materials commonly used for ultrafast nonlinear optics in the UV (see Table 1 ).…”

Polariton relaxation and polariton nonlinearities in nonresonantly cw-pumped III-nitride slab waveguides

“…After the theoretical work [21] that pointed out the advantages of this configuration, an experimental confirmation of the possibility of amplification and polariton lasing in WGs [24], as well as a comparison of WG and planar cavity polariton lasers [34], were investigated under pulsed excitation using the ZnO platform. These works are complementary to those led on state-of-the-art GaAs WGs where numerous nonlinear optical features have been reported including darksoliton formation under cw excitation that exhibits enhanced polariton-polariton strength compared to experiments done with picosecond pulses [35] or modulational instabilities occurring in the pulsed regime like spatiotemporal continuum generation [36,37]. Let us note that all of those experiments take place in submillimeter long structures, which could be even further downscaled by applying longitudinal electric fields that increase even more the polariton-polariton interaction strength and thus nonlinear effects [25].…”

“…Finally, let us note that the observation of parametric instabilities in such a WG structure does correspond to a specific type of polariton-driven optical nonlinearities. Indeed, using different pumping conditions, e.g., under pulsed excitation, the balance between gain and nonlinear losses may also lead to the generation of solitonic features [35,36]. Thus, self-phase modulation, a precursor stage for the formation of optical solitons has recently been reported under resonant pulsed excitation up to room temperature in similar GaN/AlGaN WGs [48].…”