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arXiv:1803.04067 · Nano Letters

Valley-Selective Exciton Bistability in a Suspended Monolayer Semiconductor

Hongchao Xie, Shengwei Jiang, Jie Shan, Kin Fai Mak

Published Apr 16, 2018• 15 citations
arXivHTMLPDF

Abstract

We demonstrate robust optical bistability, the phenomenon of two well-discriminated stable states depending upon the history of the optical input, in fully suspended monolayers of WSe 2 at low temperatures near the exciton resonance. Optical bistability has been achieved under continuous-wave optical excitation that is red-detuned from the exciton resonance at an intensity level of 10 3 W/cm 2 . The observed bistability is originated from a photothermal mechanism, which provides both optical nonlinearity and passive feedback, two essential elements for optical bistability. The low thermal conductance of suspended samples is primarily responsible for the low excitation intensities required for optical bistability. Under a finite out-of-plane magnetic field, the exciton bistability becomes helicity dependent due to the exciton valley Zeeman effect, which enables repeatable switching of the sample reflectance by light polarization. Our study has opened up exciting opportunities in controlling light with light, including its wavelength, power, and polarization, using monolayer semiconductors.

DOI
10.1021/acs.nanolett.8b00987

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We demonstrate robust optical bistability, the phenomenon of two well-discriminated stable states depending upon the history of the optical input, in fully suspended monolayers of WSe 2 at low temperatures near the exciton resonance. Optical bistability has been achieved under continuous-wave optical excitation that is red-detuned from the exciton resonance at an intensity level of 10 3 W/cm 2 . The observed bistability is originated from a photothermal mechanism, which provides both optical nonlinearity and passive feedback, two essential elements for optical bistability. The low thermal conductance of suspended samples is primarily responsible for the low excitation intensities required for optical bistability. Under a finite out-of-plane magnetic field, the exciton bistability becomes helicity dependent due to the exciton valley Zeeman effect, which enables repeatable switching of the sample reflectance by light polarization. Our study has opened up exciting opportunities in controlling light with light, including its wavelength, power, and polarization, using monolayer semiconductors.
We conducted a theoretical investigation of optical bistability based on the dipole-allowed transitions in a three-level ladder-type WSe 2 system. The system interacts with a laser beam through a unidirectional ring cavity. Our results demonstrate that efficient control of bistability can be achieved by tuning the laser frequency, decay rate, and cooperation parameters. We also discuss the influence of absorption and dispersion processes on the optical bistability behavior. The optical bistability exhibits absorptive and hybrid characteristics in small- and intermediate-output fields, respectively. This study can inform the strategic design of 2D materials for fast and low-threshold all-optical switches, leveraging their significant optical nonlinearities and exciton binding energies.
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The effect of specific features of the exciton and phonon spectra of layered semiconductors on the conditions of realization of optical bistability in the exciton absorption region has been theoretically investigated by the Green’s function method. By the example of the 2 H polytype of PbI 2 , it is shown that effective exciton scattering from low-energy bending-wave vibrations leads to a blue shift of the frequency range of bistability realization and its narrowing; expansion of the temperature range of bistability observation; and a shift of the hysteresis loop to higher intensities, accompanied by a decrease in its height and width. The possibility of observing polarization optical bistability, which is related to the dependence of the exciton energy spectrum on the propagation direction of a light wave in a crystal, is also substantiated.
Optical bistable devices containing GaAs/AlGaAs multiple quantum well (MQW) structures have shown great potential for performing fast low-power optical switching at room temperature.1 The presence of a much stronger exciton absorption feature in the MQW structures compared to bulk GaAs suggests far superior performance in the former. We report the observation of room-temperature bistability in bulk GaAs with input power <10 mW and intensities of a few kW/cm2. The intensities are consistent with the data of Miller et al.2 which show a saturation intensity of nearly 4 kW/cm2 in a MQW sample with a detuning of less than a halfwidth from the peak of the exciton resonance. Our detunings are always several halfwidths.
Abstract A joint effect of exciton–phonon and exciton–exciton interactions on optical properties of semiconductors is studied. In these substances the exciton density is shown to be a multivalued function of incident light intensity and the frequency and temperature conditions under which the optical bistability arises are specified. Investigation of nonlinear optical properties of semiconductors with account for the indirect phototransitions exhibits that both multistable optical states and the bistable ones distinguished by a complicated dynamics of the switching may be observed in such systems.
Abstract Optical bistability (OB) is a phenomenon that describes a light induced phase transition in the non-equilibrium state. It is vital for all-optical computing and communication, serving as the basis for switches, logic gates, transistors, and memory. By enabling two stable light states, OB allows storage and fast switching, making high-speed, energy-efficient data processing possible. In this work we study the phenomenon in a two-band semiconductor in an optical cavity. Starting from a many-body Hamiltonian of optically generated electrons, holes and light-semiconductor interaction and Coulomb electron–electron, hole–hole and electron–hole interactions, we obtain the optical Bloch equations, in the mean field approximation, where relaxation is included phenomenologically. We obtain expressions for particle density, excitonic pair density and output intensity as non-linear functions of input light intensity. All these quantities show bistable behaviour, with two stable phases, and hysteresis in the non-equilibrium state. Equilibrium is ensured by introducing damping terms for the particle density amplitude, excitonic density amplitude, and cavity photon amplitude phenomenologically. The electron–hole pairs increase with light intensity, shows bistable features and then disappears with further increase in the input light intensity. The bistable behaviour becomes pronounced as we pass from low Coulomb interaction strengths to high Coulomb interaction strengths between electrons and holes. For sub-bandgap excitations, the output light versus incident light shows a complex behaviour.