Short Wavelength Intersubband Transitions Research Articles

Passively mode-locked Tm,Ho:YAG laser at 2 µm based on saturable absorption of intersubband transitions in quantum wells

We report the first demonstration of a solid state laser passively mode-locked through the saturable absorption of short-wavelength intersubband transitions in doped quantum wells: a continuous wave Ti:sapphire laser end-pumped Tm,Ho:YAG laser at the center wavelength of 2.091 mum utilizing intersubband transitions in narrow In(0.53)Ga(0.47)As/Al(0.53)As(0.47)Sb quantum wells. Stable passive mode-locking operation with maximum average output power of up to 160 mW for 2.9 W of the absorbed pump power could last for hours without external interruption and a mode-locked pulse with duration of 60 ps at repetition rate of 106.5 MHz was generated.

Strain effects on the intersubband transitions in GaN/AlN multiple quantum wells grown by low-temperature metal organic vapor phase epitaxy with AlGaN interlayer

The strain in low-temperature-grown GaN/AlN multiple quantum wells (MQWs) have been tailored by inserting an AlGaN interlayer between an AlN template and the MQWs, for the purpose of intersubband transition (ISBT) at shorter wavelength (1.52 μm) with smaller full-width at half-maximum (FWHM) (113 meV). The strain in GaN wells, ISBT wavelength and its FWHM were dependent on Al-content in the AlGaN interlayer. The compressive strain in GaN wells shifted ISBT to shorter wavelengths and narrowed absorption peaks. The interlayer with an appropriate Al-content has been proved to be mandatory for achieving strong and short-wavelength ISBT by metal organic vapor phase epitaxy-grown GaN/AlN MQWs.

Strain-induced interface instability in GaN∕AlN multiple quantum wells

It is shown that in GaN∕AlN multiple quantum wells (MQWs), strain is a critical parameter for achieving short-wavelength intersubband transitions (ISBTs). This is investigated by comparing GaN∕AlN MQWs grown by metal organic vapor phase epitaxy on either AlN or GaN templates. The GaN∕AlN interface is found to be unstable when pseudomorphically strained onto GaN, in agreement with theory. This effect deeply affects the quantum well potential profile leading to a strong redshift of the ISBT energies.

Short-wavelength intersubband transitions down to 1.6 μm in ZnSe/BeTe type-II superlattices

We report photoinduced electron intersubband absorption in ZnSe/BeTe type-II superlattices. The wavelength of the intersubband transition as short as 1.6 μm, covering the 1.55 μm optical communication wavelengths within its absorption band width (∼250 nm), is achieved in the ZnSe/BeTe SLs with 4.5 ML-thick ZnSe layers. The intensity in photoinduced intersubband absorption increases sublinearly with pump intensity, reflecting the characteristic recombination processes of electron-hole pairs in a heterostructure with type-II band alignment.

Pump and probe measurement of intersubband relaxation time in short-wavelength intersubband transition

Population relaxation dynamics of the short-wavelength (∼2.5 μm) intersubband transition (ISB-T) in narrow InGaAs/AlAs quantum wells is investigated. Femtosecond (∼100 fs) pump and probe measurement yields a relaxation time of ∼2.7 ps, which is as fast as those observed for 10–5 μm ISB-T (0.7–1.5 ps, respectively). The ISB relaxation time increases more rapidly than that predicted from the usual intersubband optical-phonon scattering model as the ISB-T energy increases. Our theoretical calculation, which takes into account the intrasubband energy relaxation process, agrees well with the experimental results. The intrasubband energy relaxation process is found to be important in the short-wavelength region.

Relaxation Time of Short Wavelength Intersubband Transition in InGaAs/AlAs Quantum Wells

Linear and nonlinear light absorption characteristics of short-wavelength (near-infrared) intersubband transition in InGaAs/AlAs quantum wells are investigated for estimation of the intersubband relaxation time. Linear absorption measurements reveal that the intersubband absorption width is dominated by the inhomogeneous broadening caused by well-width fluctuations. Nonlinear absorption measurement results are analyzed by focusing on the inhomogeneous broadening and probe pulse width, which reveals that the intersubband energy relaxation time is as short as 1–10 ps. The observed relaxation time is of the same order as those of long wavelength (far-infrared) intersubband transitions. This result is expected to be useful for the ultrafast application of short-wavelength intersubband transition.

Absorption magnitude and phase relaxation time in short wavelength intersubband transitions in InGaAs/AlAs quantum wells on GaAs substrates

Short wavelength (∼2.5 μm) intersubband transitions in n-doped InGaA/AlAs quantum wells are investigated in concern with the absorption magnitude and phase relaxation time. Intersubband absorption magnitudes increase with increasing Si doping density, however, they are much smaller than expected from the doping density. Theoretical calculation reveals that only 30% of the doped electrons remain in the well layer. The other electrons leak to the AlAs barrier layer (X minimum) and do not contribute to intersubband transition. Absorption saturation characteristics indicate that the phase relaxation rates are about 10–30 ps −1. In contrast, the calculation of Coulomb scattering considering only the well electrons show that phase relaxation rate is about 200 ps −1. This discrepancy is found to be corrected by taking into account the screening effect by the electrons leaked to the barrier layer. The re-calculated scattering time is about 50 ps −1 which almost agree with the experimental value. The carrier leakage is shown to have an important effect not only on the absorption magnitude but also on the phase relaxation of the intersubband transition.

Enhancement of Absorption Magnitude of Short-Wavelength Intersubband Transition in InGaAs/AlAs Quantum Wells

Reduction of absorption magnitude with shortening of the intersubband transition wavelength is observed in InGaAs/AlAs quantum wells on GaAs substrates. Theoretical calculation of the carrier distribution shows that the reduction is caused by carrier leakage to the X minimum of the AlAs barrier from the Γ minimum of the InGaAs well. In order to regain the absorption magnitude, we undertake to suppress the carrier leakage by reducing the density of states in the AlAs X minimum and by increasing the energy spacing between Γ and X minima. The optimizations lead to the enhancement of the absorption magnitude of more than fourfold, and a peak absorption coefficient of 14000 cm-1 is obtained with an optimized structure. The results obtained here are considered very useful for device applications of the short-wavelength intersubband transitions.

Investigation of short wavelength intersubband transitions in InGaAs/AlAs quantum wells on GaAs substrate

InGaAs/AlAs quantum wells on GaAs substrates were studied for near-infrared intersubband transitions. The relationships between the properties of the intersubband absorption and the structural/growth conditions of the quantum wells were quantitatively investigated by using theoretical calculations based on a k-p perturbation method. The total intersubband absorption magnitude is shown to be closely related to the leakage of free carriers from the Γ minimum of the well to the X minimum of the barrier. This work provides important information for design of devices utilizing short wavelength intersubband transitions.

Short wavelength intersubband transitions in InGaAs/AlGaAs quantum wells grown on GaAs

We have demonstrated intersubband transition energies as high as 580 meV (2.1 μm wavelength) in InGaAs/AlGaAs quantum wells (QWs) grown on GaAs substrates. The well width dependence of intersubband transition energies in both In0.5Ga0.5As/Al0.45Ga0.55As QWs and In0.5Ga0.5As/AlAs QWs have been determined. Good agreement of the intersubband transition energies to a single band effective mass model with band nonparabolicity included is found. Experimental studies of buffer and QW growth parameters for optimized intersubband absorption have also been performed.