Room Temperature Terahertz Electroabsorption Modulation by Excitons in Monolayer Transition Metal Dichalcogenides

Abstract

The interaction between off-resonant laser pulses and excitons in monolayer transition metal dichalcogenides is attracting increasing interest as a route for the valley-selective coherent control of the exciton properties. Here, we extend the classification of the known off-resonant phenomena by unveiling the impact of a strong THz field on the excitonic resonances of monolayer MoS2. We observe that the THz pump pulse causes a selective modification of the coherence lifetime of the excitons, while keeping their oscillator strength and peak energy unchanged. We rationalize these results theoretically by invoking a hitherto unobserved manifestation of the Franz–Keldysh effect on an exciton resonance. As the modulation depth of the optical absorption reaches values as large as 0.05 dB/nm at room temperature, our findings open the way to the use of semiconducting transition metal dichalcogenides as compact and efficient platforms for high-speed electroabsorption devices.

Efficient ionization of two dimensional excitons by intense single cycle terahertz pulses

“…The shift is well explained by the exciton Stark effect, 17,[28][29][30] while the broadening is commonly attributed to the reduced exciton lifetime due to field-induced exciton ionization. 26,31,32 However, detailed interpretations of recent electroabsorption experiments on both monolayer MoS 2 33 and carbon nanotubes 34 reveal that exciton ionization is not the dominating contribution to this broadening. The authors of Refs.…”

“…The authors of Refs. 33,34 base their arguments on the fact that the measured broadening is proportional to the square of the electric field strength, which is not predicted by exciton ionization. 18,20 The apparent contradiction is resolved by noting that the contribution to the broadening by exciton ionization alone 16,18,20 is much lower than the field-induced phonon contribution.…”

“…18,20 The apparent contradiction is resolved by noting that the contribution to the broadening by exciton ionization alone 16,18,20 is much lower than the field-induced phonon contribution. 33 As a result, it is very difficult to measure exciton ionization in electroabsorption experiments.…”

“…To study exciton ionization induced by THz pulses, a time-dependent field term is included in the Wannier equation. We have obtained a trace of the experimental THz pulse used in Ref., 33 and the shape of the pulse used throughout the present paper is based on this experimental pulse. For performing computations, we assume that only the excitonic ground state is occupied initially, and then let the wave function ψ(t) evolve in time.…”

“…Here, the black circles represent the experimental THz pulse received from the authors of Ref. 33 To ensure easy reproducibility of the results, we introduce a fitted pulse with a known functional form. This pulse is represented by the red line in Fig.…”