Exploration of suitable pumping schemes for Bose-Einstein condensates of vacuum-ultraviolet photons

In recent years, Bose-Einstein condensates of photons have emerged as a well-established experimental platform. Typically, visible-spectral-range photon condensates are realized using wavelength-sized optical microresonators filled with a liquid dye solution. Within these microresonators, photons are repeatedly absorbed and emitted by the dye molecules, leading to a thermally equilibrated energy distribution. Such a BEC of photons constitutes a source of coherent and monochromatic light and, unlike traditional lasers, does not require population inversion. This distinction makes the creation of such condensates in the vacuum-ultraviolet (VUV) spectral regime (100 – 200 nm) particularly intriguing, as the development of lasers in this wavelength range is challenging due to the need for intense pumping sources. We propose to adapt the scheme for photon condensation to the VUV regime. For this endeavor, our candidate to replace the liquid dye solution used in the visible spectral range are dense mixtures of xenon, serving as the optically active constituent, and another noble gas, acting as a buffer gas. We plan to achieve a thermalization of VUV photons via repeated absorption and emission cycles between the quasimolecular states associated with the (atomic) 5p⁶ and 5p⁵6s levels. Recent work has focused on the exploration of possible pumping schemes for such a vacuum-ultraviolet photon condensate. Among others, we here present excitation spectra of xenon-helium and xenon-krypton mixtures, driving two-photon transitions from the 5p⁶ ground state to the 5p⁵6p excited states with UV-light near 250 nm wavelength. This approach appears particularly suitable, due to the commercial availability of laser sources in this wavelength range. In our measurements, particular attention is devoted to the influence of both admixture conditions as well as constituent partial pressures, with total sample pressures as high as 90 bar.