We present experiments on the luminescence of excitons confined in a potential trap at sub-Kelvin temperatures after nanosecond pulsed laser excitation. Analysis of the experimental results with a rate model shows that the so-called Auger decay of yellow excitons, which in previous studies led to a rapid decay of the excitons at high densities and thus prevented reaching the critical density for Bose-Einstein condensation (BEC), is greatly reduced for paraexcitons. We demonstrate that exciton numbers well above 10 10 can be collected in a potential trap, albeit at temperatures in the 10 K range. During their lifetime of about 500 ns the paraexcitons cool down to the temperature of the He bath. This opens up the possibility to observe a BEC of paraexcitons provided that the bath temperature can be reduced to below 100 mK.(1) with 0 being the average oscillator frequency of the trapping potential and ζ the Riemann zeta function, decreases much faster with temperature, T , than in free space.Due to their unique properties, the excitons of the so-called yellow series in the semiconductor cuprous oxide (Cu 2 O) are still considered one of several promising candidates for excitonic BEC (for reviews see [4][5][6]). This is related to the large binding energy of 150 meV, which shifts the Mott density to 10 19 cm −3 at cryogenic temperatures [7,8]. Made up of doubly degenerate valence and conduction bands, the ground state of this series splits into the triply degenerate orthoexciton and the nondegenerate paraexciton, which is the energetically lowest exciton state, = 12.1 meV below the orthoexciton states. Due to the positive parity of the bands, the orthoexciton is only weakly optically allowed (quadrupole transition with oscillator

We present experiments on the luminescence of excitons confined in a potential trap at milli-Kelvin bath temperatures under continuous-wave (cw) excitation. They reveal several distinct features like a kink in the dependence of the total integrated luminescence intensity on excitation laser power and a bimodal distribution of the spatially resolved luminescence. Furthermore, we discuss the present state of the theoretical description of Bose-Einstein condensation of excitons with respect to signatures of a condensate in the luminescence. The comparison of the experimental data with theoretical results with respect to the spatially resolved as well as the integrated luminescence intensity shows the necessity of taking into account a Bose-Einstein condensed excitonic phase in order to understand the behaviour of the trapped excitons.

The band structure of Cu(2)O is calculated using density functional theory in the generalized gradient approximation. By taking spin-orbit coupling into account the split between the Γ(7)(+) and the Γ(8)(+) valence band states is obtained as 128 meV. The highest valence band shows a noticeable nonparabolicity close to the Γ point. This is important for the quantitative description of excitons in this material, which is considered to be the best candidate for the confirmation that Bose-Einstein condensation also occurs in excitonic systems.

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The presence of strong spin–orbit coupling in the valence band and weak spin‐splitting in the conduction band result in the lowest energy exciton in WX2 (X = S, Se) being spin forbidden and optically dark. Because of their long lifetimes, dark excitons are highly attractive for quantum optics and optoelectronic applications. To date, studying dark excitonic emissions is limited to cryogenic temperatures or requires very complex experimental configurations to observe them at room temperature (RT). Here, the radiative decay of dark exciton related emission in WSe2 monolayers is studied using both conventional and tip‐enhanced photoluminescence (TEPL) at RT. Monolayer WSe2 flakes are sandwiched between noble metal substrates and polydimethylsiloxane nanopatches providing a strong local electrostatic out‐of‐plane dipole moment with respect to the 2D plane resulting in the observation of dark excitonic emission at RT. The spatial distribution of this dark exciton related emission is studied by TEPL with a spatial resolution of <10 nm confirming the confinement of these excitons within the polydimethylsiloxane nanopatches. The tip‐enhanced Raman scattering (TERS) investigation excludes any local strain induced effects and reveals a direct correlation between dark excitons and defects in WSe2. Finally, removal of the nanopatches led to the recovery of bright excitonic emission in WSe2.