Saturation Of Intersubband Absorption Research Articles

Mono-cycle terahertz pulses from intersubband shift currents in asymmetric semiconductor quantum wells

Both second-order frequency mixing in nonlinear optical media and photoconductive antennas have provided terahertz (THz) transients in a wide parameter range. Here, we demonstrate a novel type of ultrafast nonlinear optical response in asymmetric semiconductor quantum wells, originating from electron shift currents. Resonant intersubband excitation by a femtosecond mid-infrared pulse induces a transient spatial shift of electronic charge, which leads to the emission of a mono-cycle THz pulse. This mechanism is characterized and separated from conventional difference frequency mixing by nonlinear two-dimensional THz spectroscopy. The amplitude of the THz electric field emitted by the shift current reaches several percent of the mid-infrared driving field, significantly higher than in difference frequency mixing. Changes in time structure of the THz pulse upon nonlinear saturation of intersubband absorption allow for generating tailored THz transients. The present concept can be implemented in highly compact devices driven by mid-infrared pulses at megahertz repetition rates to provide versatile THz pulses for spectroscopy and optoelectronics.

Subpicosecond saturation of intersubband absorption in (CdS∕ZnSe)∕BeTe quantum-well waveguides at telecommunication wavelength

Ultrafast all-optical switching at an optical communication wavelength has been investigated by utilizing an intersubband transition (ISBT) of II–VI-based multiple quantum wells (MQWs) fabricated in high-mesa waveguide devices. The waveguide structure consists of a CdS∕ZnSe∕BeTe MQW core layer and two top and bottom ZnMgBeSe quaternary cladding layers grown by molecular beam epitaxy on a (001) GaAs substrate. A marked increase in waveguide transmittance was observed only for transverse-magnetic-polarized subpicosecond pulse with increasing incident pulse energy at λ=1.57μm, indicative of the ISBT absorption saturation. The pulse energy necessary for a 10dB transmittance increase is as low as 13.3pJ for a waveguide device with 2.7μm mesa, and the saturation pulse energy can be even further reduced by employing a narrower mesa structure. Ultrafast gate switching within a time window of 0.56ps was also demonstrated with pump pulse at λ=1.57μm and probe pulse at λ=1.63μm in this waveguide device.

Spin relaxation times of two-dimensional holes from spin sensitive bleaching of intersubband absorption

We present spin relaxation times of two-dimensional holes obtained by spin sensitive bleaching of the absorption of infrared radiation in p-type GaAs/AlGaAs quantum wells (QWs). It is shown that the saturation of intersubband absorption of circularly polarized radiation is mainly controlled by the spin relaxation time of the holes. The saturation behavior has been determined for different QW widths and in a wide range of temperatures with the result that the saturation intensity substantially decreases with narrowing of the QWs. Spin relaxation times are derived from the measured saturation intensities by making use of calculated (linear) absorption coefficients for direct intersubband transitions. It is shown that spin relaxation is due to the D’yakonov–Perel’ mechanism governed by hole–hole scattering. The problem of selection rules is addressed.

Hole spin-relaxation in quantum wells from saturation of inter-subband absorption

Spin-sensitive saturation of absorption of infrared radiation has been investigated in p-type GaAs QWs. It is shown that the absorption saturation of circularly polarized radiation is mostly controlled by the spin relaxation time of the holes. The saturation behavior has been investigated for different QW widths and in dependence on the temperature with the result that the saturation intensity substantially decreases with smaller QW width. Spin relaxation times were experimentally obtained by making use of calculated (linear) absorption coefficients for inter-subband transitions. The question of selection rules for intersubband transitions between hole subbands is addressed.

Polarization-dependent intersubband absorption saturation and its effect on polarization selection in vertical cavity surface-emitting lasers

A mechanism of nonlinear dichroism of net gain in vertical-cavity surface-emitting lasers based on polarization-dependent saturation of intersubband absorption in p-contact layers is proposed and analyzed. Microscopic perturbative calculations are used to obtain approximate analytical expressions for the magnitude of the effect. It is shown that the proposed mechanism can play a significant part in polarization switching and bistability in vertical-cavity lasers.

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.

Spectroscopy of intersubband transitions in Si–Si 1− xGe x quantum wells

Abstract We investigated intersubband transitions in the valence band of Si1−xGex quantum wells. After a brief review of the recent achievements related to intersubband transitions, we present an original technique, photo-induced IR absorption spectroscopy, to study intersubband transitions in both doped and undoped quantum wells. A comparison with photo-induced intersubband absorption in GaAs quantum wells as well as the variation of the intersubband absorption with Ge content, layer thickness and doping and the temperature dependence of photo-induced absorption are reported. The normal incidence IR modulation is presented in a second part. It is shown that the index variations associated with free carrier and intersubband absorptions significantly affect the IR modulation. The thermal stability of narrow Si–Si1−xGex quantum wells is investigated in a third section. The interdiffusion leads to a red shift of the intersubband transitions. It is shown that the local variation of the effective mass can enhance the normal incidence intersubband absorption. Finally, intersubband relaxation in doped quantum wells is investigated using a picosecond free electron laser. The subband lifetime is first deduced from the saturation of intersubband absorption vs. pump intensity. In a second step, the subband lifetime is measured directly by time-resolved pump and probe experiments. Both techniques demonstrate that the subband lifetime T1 is shorter than 1 ps.

Intersubband relaxation time in the valence band of Si/Si1−xGex quantum wells

We have investigated the saturation of intersubband absorption and the intersubband relaxation time in the valence band of Si/SiGe quantum wells. The quantum wells consist of 3 nm thick p-doped Si0.8Ge0.2 layers separated by 20 nm thick silicon barriers. At room temperature, these quantum wells exhibit intersubband absorption in p polarization (component of the electric field parallel to the growth axis), which peaks around 130 meV (≊10 μm). The saturation of intersubband absorption has been investigated using a picosecond free electron laser by two different techniques. The intersubband absorption is first analyzed as a function of the pump intensity. The saturation is found to occur around 90 MW/cm2, which yields a characteristic lifetime T1≊0.7 ps. In a second set of experiments, the lifetime is determined by time-resolved pump and probe experiments in cross polarization. The intersubband transition is pumped in p polarization and the saturated absorption is probed with a delayed beam in s polarization. An intersubband lifetime ≊0.4 ps is deduced by this technique. The saturation behavior is analyzed in terms of an inhomogeneously broadened two-level system.

Saturation of intersubband absorption and optical rectification in asymmetric quantum wells

The problem of saturation of intersubband absorption and optical rectification in asymmetric quantum wells is discussed theoretically using the density matrix formalism. The effects of the electron-electron interaction are taken into account. It is shown that near the resonance the spectral shapes of the nonlinear optical absorption coefficient and the nonlinear optical rectification coefficient are very similar. In the absence of the electron-electron interaction the spectral responses are nearly Lorentzian. The electron-electron interaction plays an important role in heavy doped systems. In structures with large spatial separation of the ground and the excited states wave functions, the depolarization effect is small and the absorption spectrum is affected mainly by the electrostatic Coulomb interaction. This interaction shifts the absorption peak to higher energy by an amount depending on the intensity of the incident light and leads to the significant distortion (asymmetry) of the line.

Optical saturation of intersubband absorption in semiconductor superlattices.

The third-order nonlinear intersubband absorption in superlattices is studied theoretically by means of a general formalism of the Kronig-Penney model given recently. It is shown that there is a general relation between the tunnel bandwidth and the minimum optical saturation intensity. The physical origin of this relation is analyzed.

Simulation of saturation and relaxation of intersubband absorption in doped quantum wells

We have simulated the saturation and relaxation of intersubband absorption in doped GaAs quantum wells, in which radiation excites electrons between the ground and higher subbands. Screened longitudinal optic (LO) phonon, hot LO phonon, acoustic phonon, intrasubband scattering, conduction-band nonparabolicity, and intervalley scattering were included in our simulation. We find that the change in absorption relaxes with a time constant of about 5 ps after pulsed excitation. Under continuous excitation the absorption saturates as a nearly homogeneously broadened system.