Abstract
Transitions metal dichalcogenides (TMDs) are direct gap semiconductors in the monolayer (ML) limit with fascinating optical and spin-valley properties. The strong optical absorption of up to 20% for a single ML is governed by excitons, electron-hole pairs bound by Coulomb attraction. Excited exciton states in and monolayers have so far been elusive because of their low oscillator strength and strong inhomogeneous broadening. Here, we show that encapsulation in hexagonal boron nitride results in an emission line width of the exciton below 1.5 meV and 3 meV in our and monolayer samples, respectively. This allows us to investigate the excited exciton states by photoluminescence upconversion spectroscopy for both monolayer materials. The excitation laser is tuned into resonance with the transition, and we observe emission of excited exciton states up to 200 meV above the laser energy. We demonstrate bias control of the efficiency of this nonlinear optical process. We discuss the origin of the upconversion effect. Our model calculations suggest an exciton-exciton (Auger) scattering mechanism specific to TMD MLs involving an excited conduction band, thus generating high-energy excitons with small wave vectors. The optical transitions are further investigated by white light reflectivity, photoluminescence excitation, and resonant Raman scattering, confirming their origin as excited excitonic states in monolayer thin semiconductors.
- Received 11 May 2018
- Revised 16 July 2018
DOI:https://doi.org/10.1103/PhysRevX.8.031073
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
When thinned to sheets just one atom thick, transition metal dichalcogenides behave as semiconductors, offering potential applications in a wide range of thin, flexible optics and electronics. Their strong absorption of visible light is due to excitons, bound pairs of electrons, and positively charged holes. At high densities, excitons can scatter off one another, leading to the annihilation of one exciton as a second exciton absorbs its energy and enters a higher energy state (a process known as upconversion). Here, we show experimentally and theoretically that these types of interactions are qualitatively different in transition metal dichalcogenides when compared to conventional semiconductors.
First, we develop a theory that goes beyond usual analyses of excitons to show that exciton upconversion is an efficient, resonant process. This opens exciting new possibilities that we explore in our experiments. We use upconversion photoluminescence spectroscopy to study the excited exciton states in and monolayers, which have so far not been accessible in conventional samples using standard techniques. These experiments reveal many firsts, including electrical control of exciton upconversion in , identification of particular exciton excited states, and scattering between excitons in .
These nonlinear optical effects give insights into exciton-exciton interactions, relevant physical processes at the heart of population inversion, and other density-dependent phenomena for devices such as lasers.