S. D. Lin

Multicolor infrared detection realized with two distinct superlattices separated by a blocking barrier

A multicolor infrared photodetector was realized with two superlattices separated by a blocking barrier. The photoresponse is switchable between 7.5–12 and 6–8.5 μm by the bias polarity, and is also tunable by the bias magnitude in each wavelength regime. In addition, our detector exhibits advantages including little temperature dependence of the spectral response and the same order of responsivity in the two wavelength regimes. The measured peak responsivities in the two regimes are 117 mA/W at 9.8 μm under 1 V and 129 mA/V at 7.4 μm under −0.8 V, respectively. Also, the detectivities are comparable with the conventional multistack detector. The zero background peak detectivities are 2.3×1010 cm Hz0.5/W at 50 K and 9.8 μm under 0.7 V, and 8.7×1010 cm Hz0.5/W at 70 K and 7.4 μm under −0.7 V.

2–3 μm mid infrared light sources using InGaAs/GaAsSb “W” type quantum wells on InP substrates

The structure of a “W” type QW used in this work is composed of symmetric InGaAs/GaAsSb/InGaAs layers, which are sandwiched between two InAlAs barrier layers, lattice matched to InP. The barrier layers provide a strong quantum confinement to enhance the electron-hole wave function overlap and hence the optical matrix element. The band alignment of a represented In0.53Ga0.47As latticematched to InP/GaAs0.4Sb0.6 0.7% compressive strain/In0.52Al0.48As “W” structure is shown in Fig. 1a .I n

Observation of the negative differential capacitance in Schottky diodes with InAs quantum dots near room temperature

The negative differential capacitance (NDC) of Schottky diodes with the layers of InAs quantum dots (QDs) has been clearly observed near room temperature. A simple model involving two zero-dimensional quantum states is proposed to explain the NDC behavior. The simulation results show that the NDC is caused by the fast charging-discharging process in the second states of QDs.

Frequency dependence of negative differential capacitance in Schottky diodes with InAs quantum dots

The frequency dependence of negative differential capacitance (NDC) in Schottky diodes with InAs quantum dots (QDs) is studied. The measured peak capacitances of NDC decay rapidly as the testing frequencies are higher than a few kilohertz. A kinetic model considering the testing signal is proposed and the capture rates of QDs are extracted. The simulation result is quantitatively consistent with the experimental data when the charging effect in QDs is included.

Room temperature negative differential capacitance in self-assembled quantum dots

The negative differential capacitance (NDC) of Schottky diodes with layers of InAs quantum dots (QDs) has been clearly observed at room temperature. The frequency dependence of the NDC is investigated. The measured peak capacitances of NDC decay rapidly at the testing frequencies higher than a few kilohertz. A kinetic model considering the testing signal is proposed and the capture rates of QDs are extracted. The simulation result is quantitatively consistent with the experimental data when the charging effect in QDs is included.

Self-assembled GaAs antiwires in In0.53Ga0.47As matrix on (100) InP substrates

The growth of GaAs antiwires in the In0.53Ga0.47As matrix on InP substrate has been investigated. The periodic, wire-like structure was obtained when a proper amount of GaAs was deposited. The grown antiwires have a height about 1.2–2.0 nm and a period about 23 nm. Using an In0.53Ga0.47As/In0.52Al0.48As modulation-doped structure, the effect of the GaAs antiwires on the two-dimensional electron gas mobility was investigated. For the sample with antiwires near the two-dimensional channel, a significant anisotropy in low temperature mobility was observed.

Effects of two-photon absorption on terahertz radiation generated by femtosecond-laser excited photoconductive antennas

Terahertz (THz) radiation can be generated more efficiently from a low-temperature-grown GaAs (LT-GaAs) photoconductive (PC) antenna by considering the two-photon absorption (TPA) induced photo-carrier in the photoconductor. A rate-equation-based approach using the Drude-Lorentz model taking into account the band-diagram of LT-GaAs is used for the theoretical analysis. The use of transform-limited pulses at the PC antenna is critical experimentally. Previously unnoticed THz pulse features and anomalously increasing THz radiation power rather than saturation were observed. These are in good agreement with the theoretical predictions. The interplay of intensity dependence and dynamics of generation of photoexcited carriers by single-photon absorption and TPA for THz emission is discussed.

Wide spectral range confocal microscope based on endlessly single-mode fiber.

We report an endlessly single mode, fiber-optic confocal microscope, based on a large mode area photonic crystal fiber. The microscope confines a very broad spectral range of excitation and emission wavelengths to a single spatial mode in the fiber. Single-mode operation over an optical octave is feasible. At a magnification of 10 and λ = 900 nm, its resolution was measured to be 1.0 μm (lateral) and 2.5 μm (axial). The microscopes use is demonstrated by imaging single photons emitted by individual InAs quantum dots in a pillar microcavity.

“Embedded Emitters”: Direct bandgap Ge nanodots within SiO2

Microdisk-arrays of vertically stacked 30–70 nm Ge nanodots embedded within SiO2 were fabricated using thermal oxidation of Si0.75Ge0.25 abacus-shaped pillars and followed by post-annealing in oxygen-deficient conditions. The Ge nanodots are subjected to increasing quantum-confinement and tensile-strain by reducing dot size. We show that considerable quantum-confinement and tensile-strain can be generated within 30 nm Ge nanodots embedded in SiO2, as evidenced by large Raman red shifts for the Ge-Ge phonon lines in comparison to that for bulk Ge. These large quantum-confinement and tensile-strain facilitate direct-bandgap photoluminescence experimentally observed for the Ge nanodots, and are consistent with the strain-split photoluminescence transitions to the light-hole (LH) and heavy-hole (HH) valence bands at 0.83 eV and 0.88 eV, respectively. Time-resolved photoluminescence measurements conducted from 10–100 K show temperature-insensitive carrier lifetimes of 2.7 ns and 5 ns for the HH and LH valence-...

Chirped-pulse control of carrier dynamics in low-temperature grown GaAs

In this work, using chirped pulse excitation pump-probe technique, we present the results of a study of carrier dynamics modulation in low-temperature-grown GaAs. Increasing carrier relaxation times as varying the excitation chirped conditions from negative to positive is demonstrated and explained by spectral dependence of carrier dynamics in Shockley-Read Hall model. We also indicate that chirped pulse has the feasibility of controlling carrier dynamics in ultrafast optical devices.