Confocal shift interferometry of coherent emission from trapped dipolar excitons

Abstract

We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.In 1962 a Bose-Einstein-condensation of excitons was predicted by Blatt 1 and Moskalenko 2 . Dipolar excitons, where the composing electrons and holes are located in adjacent quantum wells of a semiconductor heterostructure 3,4 , are particularly suited to probe such a condensate. Due to the reduced overlap of the electron and hole wave functions, the lifetime of these quasi-particles is strongly increased. This enables them to cool down to the lattice temperature and to relax to their quantum mechanical ground state. To reach a Bose-Einstein condensate, a high particle density and low temperatures are a prerequisite, as demonstrated for related quasi-particles, namely polaritons 5 . For dipolar excitons, a condensation has been recently reported 6-8 . In particular, shiftinterferometry was used to study the spatial coherence of the excitonic emission which is considered as an indication of a condensed excitonic phase 6,[8][9][10] . Yet, there has been a controversial discussion about the impact of a limited spatial resolution on the ability to resolve the spatial coherence of the excitonic emission 11,12 . Here, we introduce a fiber-based confocal interferometer which operates down to cryogenic temperatures. We demonstrate that the approach allows measuring the coherence variation of the photoluminescence (PL) of trapped, dipolar excitons with a high spatial resolution. We extract a coherence length on the micrometer scale which is consistent with earlier reports 8 . However, we also observe that this length does not exceed the point spread function of our confocal set-up with two objectives even at the lowest temperature of 250 mK. In turn, we interpret the data such that a temporal coherence dominates the excitonic emission in our samples. We begin with the interferometer. It comprises two confocal objectives which can be positioned individually by piezo-positioners [Fig.

Light–matter interaction in transition metal dichalcogenides and their heterostructures

“…A BEC is a coherent state of bosons in the same ground state. BEC of excitons in solid state systems is a subject of intense research thus far concentrated on low-dimensional heterostructures embedded in conventional three-dimensional crystals studied below 4K [172][173][174][175] . It has been predicted by numerical investigations together with scaling arguments that hetero-bilayers prepared from SC-TMDs with an insulating interlayer prepared from atomically thin hexagonal boron nitride sheets (hBN) could result in a degenerate Bose gas with a superfluid phase with record high temperature approaching 100 K [61] .…”

Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

“…IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electronhole wave functions resulting in dense IX ensembles that are thermalized to the lattice temperature. Besides IX ensembles in III-V HS [5][6][7][8][9][10][11][12][13][14][15], hetero-bilayers prepared from semiconducting transition metal dichalcogenides (TMDs) exhibit superior potential for studying interacting IX ensembles [3,[16][17][18][19][20][21][22][23][24][25] with intriguing spin-and valley-properties [26][27][28]. At the same time, TMDs are truly two-dimensional (2D) crystals coupled to each other or to substrates by van der Waals forces.…”

Temporal coherence of spatially indirect excitons across Bose–Einstein condensation: the role of free carriers

“…We note that the interference visibility follows a monoexponential decay d t -| | e t c , at every bath temperature T b below 2.36 K. According to the Wiener-Kintchine theorem [22], the photoluminescence spectrum then consists of a single Lorentzian. This shows that we study a homogeneously broadened gas which constitutes a major improvement compared to previous works [23][24][25][26][27][28].…”