Room‐Temperature Active Modulation of Valley Dynamics in a Monolayer Semiconductor through Chiral Purcell Effects

Abstract

of free-electron spins. Such valley pseudospins feature robust spin-valley locking and direct coupling to photon spins. [3,4] As a result, excitons in monolayer semiconductors possess additional valley degrees of freedom that are directly addressable through external means, making them promising carriers for information storage and processing in valleytronic devices. [1,2,5,6] Application of valley degree of freedom in optoelectronic devices requires the capability to access and manipulate the valley behaviors. The optical selection rule (i.e., selective coupling to photons with σ + and σ − circular polarization at K and K′ valleys, respectively) in monolayer semiconductors enables the direct addressing of a specific valley by excitation using circularly polarized lasers at cryogenic temperature. The valley information can then be characterized by substantial handedness in far-field photoluminescence (PL) due to the spin conservation during recombination of valley excitons. [7,8] The incorporation of monolayer semiconductors in optical cavities with polarization-dependent responses to an incident laser can further enhance the asymmetric excitation of valley excitons and the resulting PL handedness. [9][10][11] However, the phonon-assisted intervalley scattering accelerates dramatically when temperature is increased, resulting in volatile valley states and significantly reduced handedness of far-field PL at room temperature. [8,12,13] Recent efforts have shown that near-field couplings between optical cavities and valley spins are promising in solving the cryogenic-temperature limitation and enabling room-temperature control of valley dynamics. Specifically, spin-orbit couplings between valley-polarized exciton recombination and circularly polarized surface plasmons can spatially separate the valley spins into opposite inplane directions. [14][15][16] The formation of exciton-polaritons via strong coupling between valley excitons and optical cavities can also prevent the valley polarization from complete vanishing upon enhanced intervalley scattering. [17][18][19][20][21][22] Such valley-optical cavity hybrid systems exhibited room-temperature far-field PL of maintained handedness, which would benefit the application of valley degree of freedom. [19,22] However, modulation of valley behaviors at room temperature is still limited by the requirement of precise spatial and spectral overlap between excitons and optical cavities in current approaches. In addition, it remains unclear how to distinguish the effects of spindependent excitation and near-field-controlled relaxation on Spin-dependent contrasting phenomena at K and K′ valleys in monolayer semiconductors have led to addressable valley degree of freedom, which is the cornerstone for emerging valleytronic applications in information storage and processing. Tunable and active modulation of valley dynamics in a monolayer WSe 2 is demonstrated at room temperature through controllable chiral Purcell effects in plasmonic chiral metamaterials. The strong spindependent...

Steering valley-polarized emission of monolayer MoS 2 sandwiched in plasmonic antennas

“…Generally, a faster radiative decay rate will lead to a higher polarization helicity. The exciton transition from excited states to ground states can be affected strongly by the antenna through the Purcell effect (37), which can contribute to the increasing of valley polarization. The chiral plasmonic antenna modulates the excitons in the K or K′ valley with different decay rates and antenna efficiencies.…”

A Single Chiral Nanoparticle Induced Valley Polarization Enhancement

“…[ 10,11 ] It is crucial to increase the contrast of valley‐dependent photoluminescence (PL) to build practical valleytronics devices for quantum information processing. [ 12–17 ] Yet, obtaining a clear valley contrast at room temperature (RT) is challenging, due to its severe contrast reduction by phonon‐assisted scattering processes. Consequently, the majority of the valley polarization experiments are limited to cryogenic temperature.…”

“…The utilization of chiral nanostructured surfaces is suggested as promising candidates, which include plasmonic and dielectric chiral metasurface arrays as well as gold moiré chiral patterns. [ 14,19–21 ] These chiral nanostructures could efficiently enhance the degree of circular polarization up to 43% at room temperature with monolayer, [ 22 ] yet the attempts so far required cumbersome and sophisticated nanofabrication protocols to make nanostructured arrays.…”

Routing valley exciton emission of a WS2 monolayer via delocalized Bloch modes of in-plane inversion-symmetry-broken photonic crystal slabs

“…One feasible way is to selectively excite valleys by utilizing different external stimuli such as optical and electric fields [14][15][16][17][18] , while the usually required low-temperature environment makes it difficult for practical applications. Due to the powerful ability of manipulating light, nanostructures [19][20][21] are also proposed to separate valleys [22][23][24][25][26][27][28][29][30][31][32] . For example, based on either the transverse spin momentum of surface plasmons 27,28 or the variable geometric phase of metasurfaces 31 , valley separation was reported to be achieved in the near or far field at room temperature.…”

“…For example, based on either the transverse spin momentum of surface plasmons 27,28 or the variable geometric phase of metasurfaces 31 , valley separation was reported to be achieved in the near or far field at room temperature. However, both the intrinsic loss of metal materials and the localized spatial distribution of resonant modes of nanoantennas limit efficient valley separation, leading to a low degree of valley polarization [24][25][26][27][28][29][30] . As a counterpart of metasurfaces, photonic crystals (PhCs) eliminate all these disadvantages due to delocalized photonic Bloch modes and low-intrinsic-loss dielectric constituents.…”