Jon B. Williams

Far‐infrared pump‐probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled‐quantum well

We report pump‐and‐probe measurements of the electron intersubband lifetime (T1) in an AlGaAs/GaAs heterostructure using a picosecond pulsed far‐infrared laser. The subband spacing (11 meV) is less than the optical‐phonon energy. Time‐resolved measurements yield intersubband lifetimes ranging from T1=1.1±0.2 ns to T1=0.4±0.1 ns depending on measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities ≤1 W/cm2. The steady‐state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission.

Saturation of THz-frequency intraband absorption in InAs/GaAs quantum dot molecules

We have investigated the far-infrared absorption in InAs/GaAs quantum dot molecules. The quantum dot molecules consist of two vertically coupled InAs self-assembled quantum dots separated by a GaAs barrier. The electronic coupling between the dot states results in an intraband absorption at THz frequencies. We show that this absorption can be bleached under high excitation intensity delivered by a free-electron laser. The saturation intensity is found to be on the order of 1 W cm−2. The electron relaxation time T1 is estimated from the saturation intensity. A lower limit for T1 of the order of 30 ps is deduced.

Time-resolved photoresponse of a gallium-doped germanium photoconductor using a variable pulse-width terahertz source

Picosecond to nanosecond-wide terahertz pulses are used to study the fast photoresponse of a gallium-doped germanium (Ge:Ga) photoconductor operating at 4.2 K. A recombination time of about 2 ns is observed in the time-resolved photoresponse. Laser-activated semiconductor reflection switches are used to “slice” the variable-width terahertz pulses from the quasicontinuous-wave output of a free-electron laser.

Terahertz-frequency electronic coupling in vertically coupled quantum dots

We have studied terahertz absorption of samples containing two layers of self-aligned, self-assembled InAs quantum dots separated by a thin GaAs barrier. The vertically coupled dots were charged with electrons by applying a voltage bias between a metal gate and a doped layer beneath the dots. For a positive gate bias corresponding to flatband conditions, an absorption peak was observed near 10 meV (2.4 THz). The absorption is attributed to the inhomogeneously broadened transition between the quantum mechanically split levels (bonding and antibonding states) in the vertically coupled quantum dots.

Quantum well-based tunable antenna-coupled intersubband terahertz (TACIT) detectors at 1.8-2.4 THz

TUnable Antenna-Coupled Intersubband Terahertz (TACIT) detectors use semiconductor quantum well heterostructures to offer tunable detection of light at few-Terahertz frequencies. TACIT detectors have been predicted to have background-limited sensitivity for a 300 K blackbody when operating in either a bolometric or non-bolometric mode. The speed of detection is expected to be 1 ns to less than 10 ps depending on the operating electron temperature and device dimensions. A planar metal antenna couples the incident Terahertz radiation from free space to the quantum well heterostructure. Electrons in the quantum well absorb the radiation, exciting them from the first to the second energy subband. The absorption frequency of the intersubband transition can be tuned by applying a voltage across the device. The quantum well heterostructure is designed so that the subbands have different electron mobilities. Absorption changes the relative number of electrons in each subband, and the effective mobility of the device changes. A current is applied to the active area of the quantum well, and the change in effective mobility is detected as a change in the in-plane resistance of the device. TACIT detectors are being fabricated. Modeling and experimental progress will be discussed.

Linewidth and dephasing of THz-frequency collective intersubband transitions in a GaAs/AlGaAs quantum well

Abstract Terahertz-frequency intersubband (ISB) transitions in semiconductor quantum wells are of interest due to the potential for making devices that operate at THz frequencies, and the influence of many-body interactions on the intersubband dynamics. We present measurements of the linear absorption linewidth of ISB transitions in a single 40 nm delta-doped GaAs/Al 0.3 Ga 0.7 As square quantum well, with a transition energy of order 10 meV (3 THz). Separate back- and front-gates allow independent control of charge density (0.1– 1×10 10 cm −2 ) and DC bias (−2.5– 0.5 mV / nm ). A picture of scattering of the intersubband plasmon into single-particle excitations qualitatively explains the DC bias dependence of the line-width data.

Characterization of photoconducting materials using variable-length picosecond terahertz pulses

A source of high-intensity, ultra-short terahertz pulses has been developed. The operation and performance of a terahertz pulse-slicing system for use with the UCSB free-electron lasers are discussed. Short pulses are sliced from the microsecond long output of the free-electron laser using laser-activated semiconductor switches; the pulse length may be freely varied from a few picoseconds up to four nanoseconds. The temporal response of a heavily compensated gallium-doped germanium photoconductor has been investigated. At low excitation intensity, a recombination time of 2 +/- 0.1 ns is found. At higher THz pulse powers non-exponential relaxation is observed; the data is well modeled using a rate equation approach and including impact- ionization impact-ionization effects due to the terahertz- heated free holes.

Terahertz-frequency intraband absorption in semiconductor quantum dot molecules

We have studied THz absorption of samples containing two layers of self-aligned, self-assembled InAs quantum dots separated by a thin GaAs barrier. The electronic population of the vertically-coupled dots is controlled by an applied bias between a metal gate and a doped layer beneath the dots. Under flat band conditions, an absorption peak is observed near 10 meV (2.4 THz). The absorption is attributed to the intersublevel transition between the quantum mechanically split bonding and antibonding levels in the quantum dot molecules. This absorption can be bleached under high excitation intensity delivered by a free-electron laser. The saturation intensity is found to be of order 1 W cm -2 . A lower limit for the relaxation time T 1 of the order of 30 ps is deduced from the saturation intensity.

Linewidth of THz intersubband transitions in GaAs/AlGaAs quantum wells

Terahertz-frequency intersubband transitions in semiconductor quantum wells are of interest due to the potential for making devices which operate at THz frequencies, and the importance of many body interactions on the intersubband dynamics. We present measurements of the linear absorption linewidth of ISB transitions in a single 40 nm delta-doped GaAs/Al0.3Ga0.7As square quantum well, with a transition energy of order 10 meV. Separate back- and front-gates allow independent control of charge density and DC bias. The absorption linewidth is proportional to the dephasing rate of the collective excitation. In order to examine the dephasing dynamics at THz frequencies, we have begun a detailed measurement of the ISB absorption versus charge density.