Acoustic waves extend exciton lifetimes

The energy of a light-induced surface acoustic wave in a solid can be stored as energy and subsequently converted back into photoluminescence. The storage time is orders-of-magnitude greater than the natural recombination lifetime of direct-bandgap semiconductors. Using elliptically polarized surface acoustic waves (SAWs), researchers at the University of Munich (Munich, Germany) have split optical excitons into electron-hole pairs that propagate over millimeters at the speed of sound prior to controlled recombination (photoluminescence).

In general, direct-bandgap semiconductors such as gallium arsenide (GaAs) that feature strong interband optical transitions have very short radiative lifetimes, whereas the indirect-bandgap semiconductors that feature long radiative lifetimes have only weak interband absorption. The work of the Munich researchers may provide a method to achieve long radiative lifetimes in GaAs semiconductors, opening the door for inducing precise optical delays in optoelectronic devices.

Acoustic-wave polarization

The devices consist of indium gallium arsenide (InGaAs) quantum wells grown on a GaAs substrate and covered by a GaAs cap layer; the active regions are etched into 2.5 × 0.3-mm mesas. At either end of the mesa lies an interdigital transducer designed to operate at a center frequency of 840 MHz. During the experiments, the devices were cryostatically held at 4.2 K.

By pumping the samples with a pulsed diode laser operating at 780 nm, the researchers generated optical excitons in the material. In the absence of SAWs, the pump pulses thus led to photoluminescence at the pump site. As the power of the applied SAW rose, however, the photoluminescence intensity in the sample dropped until it was completely quenched.

At sufficiently high powers, the acoustic wave (traveling at 2865 m/s) generated by one of the transducers creates a piezoelectric field in the material. This lateral electric field polarizes the excitons into spatially separated electron-hole pairs that are stored in the conduction and valence bands. Once the electron-hole pairs are screened from the lateral potential, at some spatial separation from the original pump pulse and after a time delay, they recombine to generate photons.

Radiative recombination

In the Munich devices, a semitransparent nickel-chromium layer, beginning 1 mm from the pump region, screened the lateral piezoelectric field of the SAW to trigger recombination. In the actual experiment, a 200-ns SAW pulse was launched to reach the pump region at the same time as the diode-laser pulse (see figure). The piezoelectric field ionized the excitons and trapped them in the moving lateral potential wells of the SAW. When the acoustic wave reached the screened region, after 350 ns, the electron-hole pairs recombined and generated photoluminescence. By increasing the distance from the pump region to the screening layer, the group was able to achieve time delays on the order of several microseconds.

The group also induced recombination by using both transducers to create a standing wave pattern between SAWs of identical wavelength and amplitude. If the powers of the two waves are unequal, the higher-power SAW polarizes and transports the electron-hole pair. When the powers become equal, the standing-wave pattern emerges and radiative recombination occurs.

The dual-acoustic-wave approach to recombination opens the door to devices in which the optical storage time can be selected by a set time delay between a pair of SAWs. Potentially, this technique could allow designers to develop devices with optical-delay, beam-steering, multiplexing, and demultiplexing capabilities on a single chip.