Diane C. Larrabee

Temperature dependence of intersubband transitions in InAs/AlSb quantum wells

We have carried out a systematic temperature-dependent study of intersubband absorption in InAs/AlSb quantum wells from 5 to 10 nm well width. The resonance energy redshifts with increasing temperature from 10 to 300 K, and the amount of redshift increases with decreasing well width. We have modeled the transitions using eight-band k⋅p theory combined with semiconductor Bloch equations, including the main many-body effects. Temperature is incorporated via band filling and nonparabolicity, and good agreement with experiment is achieved for the temperature dependence of the resonance.

Application of terahertz quantum-cascade lasers to semiconductor cyclotron resonance

Quantum-cascade lasers operating at 4.7, 3.5, and 2.3 THz have been used to achieve cyclotron resonance in InAs and InSb quantum wells from liquid-helium temperatures to room temperature. This represents one of the first spectroscopic applications of terahertz quantum-cascade lasers. Results show that these compact lasers are convenient and reliable sources with adequate power and stability for this type of far-infrared magneto-optical study of solids. Their compactness promises interesting future applications in solid-state spectroscopy.

Near-infrared intersubband transitions in delta-doped InAs/AlSb multi-quantum wells

Intersubband transitions (ISBTs) in narrow InAs∕AlSb multiple quantum wells (MQWs) were investigated for well widths, d, ranging from 5 nm down to 1.8 nm with 10, 20, or 60 periods. In order to observe a strong ISBT signal, a heavy silicon doping was made in each InAs quantum well. Delta doping was employed for the narrowest wells to prevent silicon incorporation into the AlSb barrier layers. As the well width decreased, the ISBT signal of the MQWs decreased. However, it persisted down to d=2.1nm with a sheet doping density in each quantum well of 9×1012cm−2 and 60 periods. The ISBT signal observed for d=2.1nm was peaked at an energy of 650 and 670 meV at 300 and 77 K, respectively. These are the highest energy values ever observed for ISBTs in InAs∕AlSb MQWs.

Intersubband transitions in narrow InAs/AlSb quantum wells

We have investigated intersubband transitions (ISBTs) in InAs/AlSb multiple quantum wells. In wells from 7 to 10 nm wide, the ISBT energy increases with decreasing well width and temperature. We do not observe photoluminescence (PL) from these wells. In wells from 2.4 to 6 nm wide, we observe PL but not ISBTs. We have calculated the band structure of these samples using an 8 band k.p theory including strain and many-body effects. We have modelled the dependence of the ISBT energy on well width and temperature. In addition, we have observed the effects on ISBTs of QW interface type and Si doping in the well.

Terahertz dynamics of photogenerated carriers in ferromagnetic InGaMnAs

We have measured the picosecond transient carrier response of InGaMnAs/InP by two-color pump–probe spectroscopy, using an intense near-infrared beam as the pump and a 52.5 μm (5.7 THz) beam as the probe. We observed strongly nonexponential decays, especially at high pump fluences and low temperatures, where a pronounced dip developed in the terahertz differential transmission. This dip disappeared at the highest fluence where a transmission plateau versus time delay was observed. Our band structure calculations suggest that this intriguing behavior may be due to carrier dynamics associated with Γ–L intervalley scattering that becomes efficient when ferromagnetism modifies the band structure.

Evaluation of interfaces in narrow InAs/AlSb quantum wells

InAs/AlSb quantum wells may be grown with two types of interfaces: InSb-like and AlAs-like. The interface type refers to the half-monolayer of the well material and half monolayer of barrier which are in contact. The type and quality of the quantum well interface is critical to the ISBT intensity and lineshape and, to a lesser extent, position. In addition to FTIR spectroscopy of the ISBT, we have performed transmission electron microscopy (TEM) to directly evaluate the quality of the interfaces at the atomic level. In order to evaluate the effects of interface type and quality on ISBT intensity, lineshape, and linewidth, we studied the TEM of a 10 nm QW sample with InSb-InSb interfaces and a 3 nm QW sample with InSb-AlAs interfaces.

Near infrared intersubband transitions in delta-doped InAs/AlSb multi-quantum wells

Recently, the development of a compact light source operating in terahertz (THz) frequency range has attracted great amount of interest. Our goal is to realize all optical light sources pumped by a compact near infrared diode laser. For this purpose, we investigate intersubband transitions (ISBTs) in InAs/AlSb multiple quantum wells (MQWs). The family of semiconductors with a lattice constant of around 6.1 /spl Aring/ (InAs, AlSb, and GaSb) has several advantageous features such as the large conduction band discontinuity of about 2.1 eV and small effective mass in InAs leading to strong ISBTs. Previously, we studied the ISBTs in unintentionally doped InAs/AlSb for relatively wide wells and reported that no ISBTs were observed for well width less than 5 nm. Ohtani et al. reported ISBTs can be observed in narrower (2.7 nm) InAs/AlSb MQWs. In order to observe ISBTs in near infrared region, we investigate heavily doped InAs/AlSb MQWs with the well width, d, less than 5 nm.

Picosecond time-resolved cyclotron resonance study of InSb quantum wells in a quantizing magnetic field

Using two-color pump-probe spectroscopy in a magnetic field, we have measured the time-resolved cyclotron resonance of photogenerated transient carriers in undoped and doped InSb quantum wells. We used an intense femtosecond pulse of near-infrared (NIR) radiation from a Ti:sapphire-based regenerative amplifier to create a large density of nonequilibrium carriers, which modifies the transmission of a delayed pulse of far-infrared (FIR) radiation from a free-electron laser. We monitored the dynamics of FIR transmission while varying the magnetic field and the time delay between the NIR and FIR pulses. Our data clearly show that the average electron cyclotron mass decreases as the electrons relax towards the band edge, as expected from the strong nonparabolicity of the InSb conduction band. Detailed lineshape analysis combined with Landau level calculations allowed us to determine the time evolution of the Fermi-Dirac distribution function of nonequilibrium. two-dimensional carriers in a quantizing magnetic field.