Fig. 1: A novel setup for absorption imaging and appearance of the condensate.

a Experimental setup inside the cryogen-free dilution refrigerator, showing the optical paths of the pump laser (orange solid line), luminescence (yellow solid line), mid-infra-red probe light (blue solid line), and location of the sample material (red cube). The optical components (mirrors and mirror holders), thermal shields (blue and grey cylinders), windows, narrow bandpass filters, and a nitrogen-cooled HgCdTe photodiode (beige cylinder) are also labelled. We measured the spatially resolved differential transmission that was the absorption image using the photodiode set on a translation-motorized stage for scanning. The spatial resolution of the imaging system was 7.8 μm (FWHM). (upper right panel) Schematic illustration of the physical processes involved for paraexcitons in the sample: excitation, luminescence, and absorption. The excitation beam (orange solid line) propagated in the sample (red cube). A paraexciton (yellow sphere) consists of one electron (blue sphere) and one hole (red sphere). We detected paraexcitons by either luminescence (yellow shade) or the differential transmission of the probe light (blue shade). An objective lens set behind the sample collected luminescence from paraexcitons. The probe beam also propagated through the objective lens. We applied inhomogeneous stress using a lens set under the sample. b Emergence of the exciton condensate in the density distribution at P_pump = 1.6 mW when T_mix < 400 mK. Left (Right) panel shows the spatial distribution of the paraexciton density measured by induced absorption imaging at T_mix = 500 mK (100 mK). The vertical axis shows the paraexciton density, and the horizontal plane shows the position in the trap potential. The density of a local dense signal at T_mix = 100 mK exceeds the BEC transition density of 1.7 × 10¹⁵ cm⁻³.