Nonlinear Optics at Excited States of Exciton Polaritons in Two-Dimensional Atomic Crystals

Abstract

Exciton polaritons (EPs) are partial-light partialmatter quasiparticles in semiconductors demonstrating striking quantum phenomena such as Bose−Einstein condensation and single-photon nonlinearity. In these phenomena, the governing process is the EP relaxation into the ground states upon excitation, where various mechanisms are extensively investigated with thermodynamic limits. However, the relaxation process becomes drastically different and could significantly advance the understanding of EP dynamics for these quantum phenomena, when excited states of EPs are involved. Here, for the first time, we observe nonlinear optical responses at the EP excited states in a monolayer tungsten disulfide (WS 2 ) microcavity, including dark excited states and dynamically metastable upper polariton bands. The nonlinear optics leads to unique emissions of ground states with prominent valley degree of freedom (DOF) via an anomalous relaxation process, which is applicable to a wide range of semiconductors from monolayer transition metal dichalcogenides (TMDs) to emerging halide perovskites. This work promises possible approaches to challenging experiments such as valley polariton condensation. Moreover, it also constructs a valleydependent solid-state three-level system for terahertz photonics and stimulated Raman adiabatic passage.

Black Phosphorus@Laser‐Engraved Graphene Heterostructure‐Based Temperature–Strain Hybridized Sensor for Electronic‐Skin Applications

“…To the best of our knowledge, the strain sensitivity of the BP@LEG strain sensor is outstanding compared to the state‐of‐the‐art strain sensors. [ 4,6,7,10,12,16,42 ] ( Table 2 and Figure S12b, Supporting Information). The performance of the BP@LEG hybrid sensor was subsequently examined for values of strain ranging from 2 to 20%, as indicated in Figure 5d.…”

“…[ 8,9 ] However, the integration of individual single‐parameter sensors into one substrate involves sophisticated and expensive fabrication that limits widespread commercial application. [ 7 ] Alternatively, multiparameter sensors that transduce each stimulus into separate signals can overcome the limitations of the aforementioned approach. Recently, dual‐parameter temperature‐ and pressure‐sensing capabilities were successfully realized in a consolidated device using independent thermoelectric and piezoresistive effects for each stimulus.…”

“…Recently, dual‐parameter temperature‐ and pressure‐sensing capabilities were successfully realized in a consolidated device using independent thermoelectric and piezoresistive effects for each stimulus. [ 7 ] A sensor with different transduction principles for each stimulus may have the ability to simultaneously perceive and differentiate multiple stimuli, but the problem of crosstalk between multiple stimuli is critical. However, a single functional material on a common substrate that can detect multiple stimuli is of profound importance in satisfying the requirements of advanced e‐skin applications.…”

Engineering photonic environments for two-dimensional materials

“…(D) The schematic for the preserved valley polarization in strongly coupled exciton polaritons [138]. (E) The schematic for the established nonlinear optics to probe the dynamics of excited states of exciton polaritons [143]. (F) The optical valley hall effect could separate the propagations of exciton polaritons into two valley polarized paths as shown in the data via the nonlinear optical excitation [144].…”

“…In the FP cavity configuration, the valley DoF was proved to be well preserved for the strongly coupled EPs and could be even robust at RT ( Figure 3D [138]) by various groups [135,[138][139][140][141], while this DoF may vanish in excitons at RT for increased intervalley scattering. By optimizing the FP cavity structure, the underlying CQED could be eventually resolved [142] and the nonlinear optics could be established ( Figure 3E [143]) to significantly change the valley dynamics of EP excited states and ground states [143,144]. With controlled valley dynamics by nonlinear optics, a prominent optical valley Hall effect was also observed ( Figure 3F) [144] by taking advantage of the FP cavity configuration.…”