Soichiro Tsujino

Enhancement of intersubband transition energies in GaAs quantum wells by Si delta doping of high concentration

Intersubband light absorption measurements have been performed on a series of GaAs/AlAs quantum wells with heavy delta doping of Si atoms in the range 9.0×1011–6.5×1012 cm−2. The increase of intersubband transition energy by as much as 38 meV has been observed, and attributed to the deepening of the V-shaped potential by the Si layer and to the depolarization effects.

Control of electron population by intersubband optical excitation in potential‐inserted double quantum well structures

A remarkable change in photoluminescence spectra was observed when CO2 laser light was incident to induce an intersubband transition in a novel asymmetric double quantum well structure where the second subbands of both wells are resonantly coupled, while the ground subbands are not. It is found that the observed change originates from the CO2‐laser‐induced transfer of electrons from one well to the other, demonstrating the controllability of electron population by means of infrared light.

Delayed luminescence induced by intersubband optical excitation in a charge transfer double quantum well structure

Photoluminescence processes in a novel GaAs double quantum well (DQW) structure were studied and found to be controlled by a 10 μm mid-infrared (MIR) light. In this DQW structure, photogenerated electron-hole pairs are normally separated into different wells and their radiative recombination is inhibited. However, when a MIR light pulse is supplied to induce an electron intersubband transition, electrons are efficiently transferred to the hole-rich well, resulting in a significant enhancement of luminescence. By time shifting the MIR pulse with respect to the light pulse for interband excitation, we demonstrated the generation of a delayed photoluminescence. Device potentials of this MIR to near-infrared conversion are discussed.

Saturation of Intersubband Absorption by Real-Space Transfer in Modulation Doped Single GaAs–AlAs Quantum Well

Intersubband absorption spectrum was investigated with intense infrared light in a modulation doped GaAs quantum well (QW) of 10 nm thickness with 6 nm AlAs barriers. When the excitation intensity was 9.8 kW/cm, the absorption was bleached to half of its weak field value. The corresponding effective intersubband relaxation time t was 14 ps, which is much larger than the intersubband LO phonon emission time. We also observed that the peak position energy Ep shifts toward lower photon energy as the excitation intensity is increased, however, the amount of the shift was about a factor four smaller than the value predicted from the depolarization effect. The observed large t and the small shift of Ep indicates an efficient real-space transfer of the excited electrons. The escape of electrons was directly detected as a photocurrent.

Transient negative photoconductance in a charge transfer double quantum well under optical intersubband excitation

Abstract Recently, it was shown that an electron–hole radiative recombination is induced by a mid-infrared light exciting an intersubband transition in a charge transfer double quantum well (CTDQW). This recombination was attributed to an upstream transfer of electrons from an electron-rich well to a hole-rich well. In this study, we investigated the electrical response of a CTDQW under intersubband optical excitation, and found that a positive photocurrent, opposite in sign and proportional to the applied electric field, accompanies the intersubband-transition-induced luminescence (ITIL) signal. A negative photocurrent component was also observed and attributed to heating processes. This work brings a further evidence of the ITIL process and shows that an important proportion of the carriers are consumed by the transfer of electrons.

Effect of intense intersubband optical excitation on the electron distribution in GaAs/AlAs quantum wells

Hot electrons were generated in n-doped GaAs/AlAs multiple quantum wells by optical intersubband excitation and their energy distributions were studied at 10 K. By measuring photoluminescence (PL) spectra, the hot-electron temperature T el was quantitatively evaluated, taking into account such effects as the degenerate distribution of electrons, impurity-induced level broadening, and the distribution of holes. It was found that, when the intensity P of the excitation was 30 kW cm-2, i.e. a few percent of the onset intensity for absorption saturation, T el increased from 10 K to 120 K while the lattice temperature remained unchanged. When P was between 10 W cm-2 and 30 kW cm-2, the rise of T el was well explained by the balance between the photon energy absorption and the energy loss due to the LO phonon emission inside the ground subband.

Control of Electron Population by Intersubband Optical Excitation in a novel Asymmetric Double Quantum Well Structure

A novel scheme of electron pumping in asymmetric double quantum well (QW) structure is studied, in which an intersubband excitation induces an interwell tunneling via the second subbands, making electrons transfer from one quantum well (QW-A) to the other (QW-B). We have designed and fabricated a novel asymmetric double quantum well structures suitable to this scheme, and demonstrated the electron transfer by measuring photoluminescence (PL) spectra of both QWs while electrons in QW-A are excited by infrared light from CO2 laser. We have found that PL from QW-B is remarkably enhanced while PL from QW-A is reduced, demonstrating the electron transfer by the intersubband excitation. We discuss requirements in order to achieve more efficient modulation.