Exciton-correlated hole tunneling in mixed-type GaAs quantum wells

We report an experimental study on the tunneling of holes in a mixed-type GaAs quantum-well structure, in which an electron-hole bilayer forms through optical carrier injection. In this bilayer system, an interlayer correlation between holes and excitons can strongly enhance the hole tunneling rate, opening a new avenue for optical control of transport processes in semiconductors. Interlayer hole tunneling is characterized with respect to these excitonic correlations, as well as carrier densities, temperature, and magnetic field, providing valuable insights on the tunneling process.

Figure 1

(a) Band-edge profile of a MTQW. A HeNe laser (1.96 eV) excites electrons from the valence band (VB) to the conduction band (CB) of the NW. The electrons relax to the lower energy WW via an intermediary state in the $X$ valley of the barrier, while the holes remain trapped in the NW. (b) Schematic illustrating interlayer Coulomb correlations between NW holes and WW excitons. These correlations can lead to accelerated hole tunneling.

Figure 2

Experimental configuration for tunneling measurements. A continuous-wave infrared (cw IR) diode laser serves as a probe to monitor sample transmission at the HH exciton resonance, while a HeNe laser, gated by an acousto-optic modulator (AOM), periodically injects electrons to the WW from the NW. For studies involving exciton injection, a third beam (a cw Ti:sapphire laser) was added at the LH exciton resonance. PMT: photomultiplier tube.

Figure 3

(a) Absorption spectrum showing wide-well HH and LH exciton resonances (solid lines) at 10 K. A continuous-wave HeNe laser with an intensity of 0.1 mW/${\mathrm{cm}}^2$ injects an electron gas, which bleaches the exciton resonances (dashed lines). (b) Time-resolved optical transmission at the HH exciton line in response to a 1-ms pulsed HeNe excitation of 5 mW/${\mathrm{cm}}^2$. Decay is well described by a double-exponential function (dashed line).

Figure 4

(a) Extracted hole tunneling rates as a function of electron density, as controlled by the peak intensity of a 1-ms HeNe laser pulse. (b) Dependence of the hole tunneling rates on magnetic field. A 1-ms HeNe laser pulse with a peak intensity of 4 mW/${\mathrm{cm}}^2$ was used.

Figure 5

Hole tunneling rate (fast component) on a logarithmic scale as a function of pump intensity. At high exciton densities, the hole tunneling rate is accelerated. This effect is more pronounced at lower temperatures. Inset: Time-resolved transmission at the heavy-hole exciton resonance of the WW in the presence of a HeNe laser pulse 1 ms in duration and 5 mW/${\mathrm{cm}}^2$ in peak intensity. Hole tunneling is accelerated in the presence of a cw pump beam with an intensity of 0.2 W/${\mathrm{cm}}^2$ at the LH exciton (green line) relative to without exciton pumping (blue line). Vertical scale is logarithmic.

Figure 6

Time-resolved transmission at the heavy-hole exciton resonance of the WW at 10 K, in response to a 1-ms HeNe pulse with a peak intensity of 4 mW/${\mathrm{cm}}^2$. Experiment performed at 10 K in the presence (green lines) and absence (blue lines) of a 100 mW/${\mathrm{cm}}^2$ exciton injection beam tuned to the LH exciton resonance in various perpendicular magnetic fields. The tunneling rate enhancement evident in Fig. 5 is not observed at these magnetic fields.

Figure 7

Calculated subband dispersion for narrow (dashed lines) and wide (solid lines) wells. Red lines indicate LH bands; other lines indicate HH. Tunneling occurs where the ground NW state and the fifth excited WW HH state overlap. The gray band indicates the energetic difference made by monolayer fluctuations in the NW.