Luminescence upconversion in GaAs quantum wells

Photoluminescence upconversion removes energy from the medium. It has been applied extensively for laser cooling of atomic vapors and more recently also to condensed matter, such as dye solutions and glasses. The application to semiconductors has also been proposed, but so far, clear evidence is missing. We present a detailed experimental study of photoluminescence upconversion in GaAs quantum wells. We study the conversion efficiency over a wide temperature range as a function of the laser detuning. The best results are obtained when the laser detuning is comparable to the thermal energy, $2k_{B}T$.

Figure 1

(Color online) Schematic representation of phonon-assisted photoluminescence upconversion in semiconductor QWs. The full horizontal lines represent the ground state of the corresponding band, the dashed lines intermediate virtual states.

Figure 2

(Color online) PL Spectrum of the sample at $T=4.3\text{K}$. The sample was excited by a laser diode with a wavelength of 638 nm, corresponding to a photon energy of 1.94 eV. The inset of the figure shows schematically the structure of the sample. The surface is on the right hand side.

Figure 3

(Color online) PL spectra of the 11.7 nm QW at $T=14.5\text{K}$ (mean PL energy: 1.5482 eV). The laser energy was varied from 1.5465–1.5501 eV.

Figure 4

(Color online) Anti-Stokes PL spectra of the 11.7 nm QW. The PL peaks at 1.5457, 1.5468, and 1.5482 eV correspond to bound excitons, trions, and free excitons. The inset shows the integrated and normalized PL intensities of the free exciton as a function of temperature. An exponential fit of the data indicates a doubling of the PL intensity every 5.3 K.

Figure 5

(Color online) Anti-Stokes PL spectra at various temperatures. New PL lines appear on the anti-Stokes side when the thermal energy exceeds the laser detuning $\Delta E$ for the corresponding QWs.

Figure 6

(Color online) AS-PL of various QWs from 180–293 K. Due to line broadening at these temperatures the PL of different QWs overlaps. At $T=190\text{K}$, we also show a decomposition of the PL spectrum into contributions from the different QWs (Lorentz fits) as well as their sum (red solid line which is almost indistinguishable from the experimental spectrum).

Figure 7

(Color online) PLE and PL spectra of the 11.7 nm QW at $T=14.5\text{K}$.

Figure 8

(Color online) Integrated and normalized hh PL intensity for the 11.7 nm QW at various temperatures as a function of the laser detuning $\Delta E=E_{\text{laser}}-E_{\text{PL}}$.

Figure 9

(Color online) Integrated and normalized intensity of anti-stokes PL as a function of $\Delta E/k_{B}T$ for the 11.7 nm QW at various temperatures. The data are displaced vertically to avoid overlap.

Figure 10

(Color online) Normalized PL intensity of light hole as a function of laser detuning $\Delta E$ at $T=54$ and $T=73\text{K}$ for the 11.7 nm QW. The peak at $-10.7\text{meV}$ corresponds to the heavy-hole resonance. The dashed blue lines indicate the corresponding PL spectra at 54 K.

Figure 11

(Color online) Relative cooling rate (integrated AS-PL multiplied by the detuning $\left|\Delta E\right|$) as a function of $\Delta E/k_{B}T$ for three different temperatures. The blue curve indicates the phonon density as a function of energy.