Terahertz spectroscopy of transition-metal dichalcogenide monolayer lasers

In a semiconductor microcavity laser, coherence forms spontaneously among the inverted populations of electrons (e) and holes (h), and the emitted photons. One method of describing the coherent e-h states is the BCS approach, adapted from the Bardeen-Cooper-Schrieffer theory of superconductivity.[1] The spectral properties of the BCS state include the formation of conduction and valence intra-band BCS gaps. The direct measurement of the BCS gap can be challenging in high-Q cavities. However, far detuned from the order parameter, the BCS gaps can be measured with a terahertz (THz) probe beam.[2] The THz probe excites new angular momentum components, and mixes coherently with the laser field to excite the fluctuation spectrum.[3] The fluctuation spectrum provides insight into the quantum states of the transition-metal dichalcogenide (TMD) monolayers.



[1] J. Hu et al., Phys. Rev. X 11, 011018 (2021).



[2] M. Spotnitz, N. Kwong, and R. Binder, Phys. Rev. B 104, 115305 (2021).



[3] M. Spotnitz, N. Kwong, and R. Binder, Collective fluctuation modes induced by terahertz radiation in a polariton laser (in review).