Gate-modulated reflectance spectroscopy for detecting excitonic states in two-dimensional semiconductors*

Due to the reduced dielectric screening arising from two-dimensional (2D) structure, exciton binding energies in 2D semiconductors, such as monolayer transition metal dichalcogenides (TMDs), can be one to two orders of magnitude higher than that in conventional semiconductors, enabling the formation of excitons even at room temperature. Photoluminescence (PL) spectroscopy has been the primary method to study exciton physics in 2D TMDs. However, observing excitonic Rydberg states, which is of importance in understanding strong Coulomb interaction in 2D systems and underlying many-body physics, is difficult using PL spectroscopy. Additionally, PL spectroscopy only detects radiative recombinations that compete with nonradiative recombinations. In contrast, absorption or reflection spectroscopy is independent of the exciton relaxation and recombination processes and is suitable for observing not only ground states (1s) but also higher-energy excited states (such as 2s) of 2D TMDs that are inaccessible by PL spectroscopy.

Here, we have applied an advanced reflectance spectroscopy method, gate-modulated reflectance (GMDR) spectroscopy, which selectively detects signals that respond to carrier density modulation, to probe excitonic states, particularly higher-energy excited states, in 2D TMDs. The 2s states of exciton and trion were identified in a monolayer WS₂ sample, in which only ground states were observable in standard reflectance spectroscopy at cryogenic temperature. The peaks in the 2s energy region were fitted well using the transfer matrix method (TMM) for spectral analysis, assuming the coexistence of 2s exciton (X^2s) and trion (T^2s). Our work has shown that GMDR spectroscopy is a sensitive method to explore exciton physics in 2D TMDs, leading to a further application for investigating exotic excited states, such as moiré excitons in 2D moiré superlattice.