Terahertz gain and collective fluctuation modes in microcavity 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 probe beam.[2] The THz probe excites new angular momentum components, and mixes coherently with the laser frequency to excite the fluctuation spectrum. The fluctuations modes, which include collective modes (related to, but distinct from, Higgs modes[3]), lead to THz gain, with possible technological applications.

[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 preparation for publication.)