A cryogenic apparatus for coupling two-dimensional materials to a confocal multimode optical cavity.

Two-dimensional van der Waals materials exhibit a variety of correlated electron phases, and optical driving offers a promising route toward manipulating them. For example, cavity-enhanced, continuous-wave (CW) Raman excitation has been suggested as a way to coherently and superradiantly populate phonons or charge density waves via material excitons. A steady-state phonon population may be sustained with sufficiently strong electron-phonon coupling to drive a novel collective response. We describe an apparatus built to meet the requirements of such an experimental program: Namely, an ultrahigh-vacuum system housing a length-tunable confocal Fabry-Pérot cavity with an intracavity sample, both cryogenically cooled and stabilized against vibrations. A four-axis nanopositioner aligns the sample and supports electrical leads intended for future sample carrier density modulation and transport measurements. Transmission through the multimode cavity enables in situ sample imaging for alignment; the sample is a monolayer transition metal dichalcogenide in this work. Operating near the confocal geometry concentrates the optical field into a localized supermode that is expected to substantially enhance light-matter coupling. This enhancement is preserved despite the millimeter-scale cavity length, which provides room for sample alignment and exchange.