Absorption imaging of trapped dark excitons in a bulk semiconductor is realized. 1s-2p transition in the mid-infrared is used to detect 1s paraexcitons. This technique is crucial to observe the Bose-Einstein condensate of excitons directly.
We demonstrate absorption imaging of 1s paraexcitons in a strain-induced trap potential in bulk semiconductor Cu2O. Using excitonic $1s\text{\ensuremath{-}}2p$ induced absorption in the midinfrared region, clear images of exciton ensembles are taken at liquid-helium temperatures. The observed spatial spread and redshift of the $1s\text{\ensuremath{-}}2p$ resonance prove that the images are of a thermalized gas of trapped 1s paraexcitons. With its high sensitivity, midinfrared absorption imaging is especially important for characterizing quantum degenerate paraexcitons including Bose-Einstein condensates at near 100 mK temperatures, as they do not luminesce because of the principle of momentum conservation.
Abstract An unambiguous observation of the Bose-Einstein condensation (BEC) of excitons in a photoexcited bulk semiconductor and elucidation of its inherent nature have been longstanding problems in condensed matter physics. Here, we observe the quantum phase transition and a Bose-Einstein condensate appearing in a trapped gas of 1s paraexcitons in bulk Cu2O below 400 mK, by directly visualizing the exciton cloud in real space using mid-infrared induced absorption imaging that we realized in a dilution refrigerator. Our study shows that the paraexciton condensate is undetectable by conventional luminescence spectroscopy. We find an unconventionally small condensate fraction of 0.016 with the spatial profile of the condensate well described by mean-field theory. Our discovery of this new type of BEC in the purely matter-like exciton system interacting with a cold phonon bath could pave the way for the classification of its long-range order, and for essential understanding of quantum statistical mechanics of non-equilibrium open systems.
Cooling of a gas of trapped paraexcitons in Cu2O to 100 mK, close to the lattice temperature, is reported. We show that the activation of a transverse acoustic scattering mechanism by the applied stress is responsible for the production of ultracold excitons. This low temperature is indispensable to achieving a stable and pure Bose-Einstein condensate of three-dimensional excitons in a strongly inelastic environment.