Trapping of Cold Excitons in Quantum Well Structures with Laser Light

Optical trapping and manipulation of neutral particles has led to a variety of experiments from stretching DNA-molecules to trapping and cooling of neutral atoms. An exciting recent outgrowth of the technique is an experimental implementation of atom Bose-Einstein condensation. In this Letter, we propose and demonstrate laser-induced trapping for a new system—a gas of excitons in quantum well structures. We report on the trapping of a highly degenerate Bose gas of excitons in laser-induced traps.

Figure 1

Images of laser-induced trapping of excitons. (a) Energy band diagram of the CQW structure; $e$is electron;$h$, hole. (b) Image of the PL signal in $E\mathrm{\text{−}}x$ coordinates. (c), (d), (e) Experimental $x\mathrm{\text{−}}y$ plots of the PL intensity from indirect excitons created by 532 nm cw laser excitation in a $30\mu \mathrm{m}$ diameter ring on the CQW sample. For (c), (d), (e) the excitation powers are $P_{\mathrm{ex}}=10$, 35, $100\mu \mathrm{W}$ and for (b) $P_{\mathrm{ex}}=75\mu \mathrm{W}$. Sample temperature $T_{\mathrm{b}}=1.4\mathrm{K}$.

Figure 2

Spatial profiles of the PL intensity and energy for the excitons in the laser-induced trap. (a) Measured PL intensity, (b) calculated PL intensity, (c) measured energy position of the PL line, and (d) calculated exciton concentration against radius $r_{\parallel }$ for four optical excitation powers $P_{\mathrm{ex}}$. The vertical axes of (c) and (d) cover the same range due to the relation $\delta E=4\pi e^2{n}_{2\mathrm{D}}d/{\epsilon }_{\mathrm{b}}$, where $d=12\mathrm{nm}$ for our sample. The ring-shaped profile of the laser excitation is shown by the thin dotted lines in (a) and (b). Sample temperature $T_{\mathrm{b}}=1.4\mathrm{K}$.

Figure 3

The calculated parameters of excitons in the laser-induced traps. (a) The radial dependence of the exciton temperature $T$ for optical excitation power of $1000\mu \mathrm{W}$ (solid black line) and the lattice temperature $T_{\mathrm{b}}=1.4\mathrm{K}$ [dotted line (blue online)]. (b) The occupation number $N_{E=0}$ for our samples against radius $r_{\parallel }$ for the same four optical excitation powers as in Fig. 2. (c) Vector plot showing the calculated exciton velocities $v$ for excitation power of $250\mu \mathrm{W}$. The length of the outermost vectors is $1.3\times {10}^6\mathrm{cm}/\mathrm{s}$. The dotted line (red online) shows the maximum calculated exciton concentration and the solid line (red online) shows the maximum of incident laser power used in calculation.