Gate-tunable trion switch for excitonic device applications

Abstract

Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favourable doping conditions can be created in a coherent manner using resonant excitation. In this work, we exploit these properties to demonstrate a gate controlled trion switch in a few-layer graphene/monolayer WS 2 /monolayer graphene vertical heterojunction. By using a high resolution spectral scan through a temperature controlled variation of the bandgap of the WS 2 sandwich layer, we obtain a gate voltage dependent vertical photocurrent strongly relying on the spectral position of the excitation, and the photocurrent maximizes when the excitation energy is resonant with the trion peak position. Further, the resonant photocurrent thus generated can be effectively controlled by a back gate voltage applied through the incomplete screening of the bottom monolayer graphene, and the photocurrent strongly correlates with the gate dependent trion intensity, while the non-resonant photocurrent exhibits only a weak gate dependenceunambiguously proving a trion driven photocurrent generation under resonance. We estimate a sub-100 fs switching time of the device. The findings are useful towards demonstration of ultra-fast excitonic devices in layered materials. Introduction:The monolayer semiconducting transition metal dichalcogenides (MoS 2 , WS 2 , MoSe 2 , and WSe 2 ) exhibit strongly bound two-dimensional excitons with a binding energy on the order of few hundreds of meV, making these ultra-thin monolayers an excellent test bed for excitonic manipulation even at room temperature [1][2][3][4][5]. The neutral excitons (X 0 ) show excellent valley polarization and valley coherence properties that can be readily probed through initialization by circularly and linearly polarized photons, respectively, followed by detection through a circular or linear analyzer [6][7][8][9][10][11]. However, controlling these excitonic states electrically remains a challenge due to the charge neutral nature of these excitons. In addition, transport of the exciton also remains challenging due to the ultra-fast radiative recombination of exciton [12][13][14][15][16] resulting from the high oscillator strength [17][18][19] -limiting the application of excitonic devices.Recently, this problem has been addressed by creating inter-layer exciton [20][21][22][23][24] to suppress the fast radiative decay, and exciton transport over several micrometer in the plane of the layered material has been demonstrated [4]. An external gate control has also been achieved by modulating the binding energy of the neutral exciton [4,5].In this regard, the charged exciton or trion (X − ) is promising since its intensity can be readily controlled electrically by modulating the doping density using a gate voltage. In addition, the trion, while being optically initiated, can be electrically detected in a spatially nonlocal manner through measuring a charge current [25]. The relatively longe...

All-optical control of high-purity trions in nanoscale waveguide

“…Exploiting the drift-dominant exciton flux and converting confined excitons to trions through the nanoscale strain gradient result in the efficient harvesting of excitonic quasiparticles in the form of trions. Unlike highly radiative trions in plasmonic cavity platforms, our highpurity trions exhibit their intrinsic temporal characteristics, leading to the high-efficiency photoconversion 36,37 . Meanwhile, generating trionic flux with the converted trion should be a pressing matter, as it opens a pathway toward manipulating excitonic/trionic flux efficiently at the nanoscale combined with existing plamonic MIM waveguide geometry [38][39][40] .…”

All-optical control of high-purity trions in nanoscale waveguide

“…Exploiting the drift-dominant exciton flux and converting confined excitons to trions through the nanoscale strain gradient result in the efficient harvesting of excitonic quasiparticles in the form of trions. Unlike highly radiative trions in plasmonic cavity platforms, our high-purity trions exhibit their intrinsic temporal characteristics, opening a pathway toward high-efficiency photoconversion [31,32] and new types of trion-based excitonic devices.…”

Probing biexciton in monolayer WS$_2$ through controlled many-body interaction

“…Due to strong confinement and relatively large carrier effective mass, monolayers of semiconducting transition metal dichalcogenides (TMDCs) exhibit light-matter interaction strongly driven by excitonic complexes, and are thus an excellent platform for exploring many-body physics at the two-dimension [1][2][3][4]. Experimental Observations of neutral bright and dark excitons (X 0 and X 0 D ) [1,[4][5][6][7][8], and other excitonic complexes such as positive or negatively changed trion (X ± ) [9][10][11][12][13], dark trion (X ± D ) [14][15][16], neutral biexciton (XX 0 ) [6] and charged biexciton(XX ± ) [6,17,18] have been widely reported. One recent report also explained electroluminescence from exciton and its complexes [19].…”