C. Y. Song

Terahertz quantum-well photodetector

The design and projected performance of quantum-well infrared photodetectors (QWIP) for the terahertz (1–10 THz) or the very-far-infrared region are presented together with our initial demonstration of a GaAs/AlGaAs QWIP working at photon energies below the optical phonons. We point out the problem with this initial device, discuss possible causes, and suggest areas of improvement.

Background-limited terahertz quantum-well photodetector

We report terahertz quantum-well photodetectors with background-limited infrared performance (BLIP). The device dark current characteristics were improved by employing thick barriers to reduce interwell tunneling. BLIP operations were observed for all samples (three in total) designed for different wavelengths. BLIP temperatures of 17, 13, and 12K were achieved for peak detection frequencies at 9.7THz (31μm), 5.4THz (56μm), and 3.2THz (93μm), respectively.

Mode-Locked Pulses from Mid-Infrared Quantum Cascade Lasers

In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.

High absorption (>90%) quantum-well infrared photodetectors

The intrinsic short carrier lifetime (∼5 ps) makes the quantum-well infrared photodetector (QWIP) well suited for high speed and high frequency applications. In such cases, since lasers are commonly used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature operation is desirable. Using GaAs/AlGaAs QWIPs, high absorption (∼100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.

GaAs/AlGaAs quantum-well photodetector for visible and middle infrared dual-band detection

We present experimental results on quantum-well photodetectors for visible and infrared dual-band detection. Large band gap top contacts were used on a standard GaAs/AlGaAs quantum-well infrared photodetector so that visible light could reach the quantum-well region and be absorbed via interband transitions. Two designs were investigated, using a high Al fraction AlGaAs and a short period GaAs/AlAs superlattice contact layer. The dual-band response spectral regions are 0.55–0.7 and 7–10 μm. Measured responsivities are about 0.7 A/W at 8.3 μm and 0.1 A/W at 0.63 μm under −6 V bias voltage.

Pixelless thermal imaging with integrated quantum-well infrared photodetector and light-emitting diode

A large pixelless thermal imager based on the epitaxial integration of a GaAs-AlGaAs quantum-well infrared photodetector and a GaAs light-emitting diode is demonstrated. The device transforms a mid-infrared (9 /spl mu/m) scene to a near-infrared (0.82 /spl mu/m) emission image with a temperature resolution of /spl sim/1 K, limited by the integration capacity of the near-visible camera.

Voltage tunable two-color InAs∕GaAs quantum dot infrared photodetector

We report a bias voltage tunable two-color InAs/GaAs quantum dot infrared photodetector working under the normal incidence infared irradiation. The two-color detection of our device is realized by combining a photovoltaic and a photoconductive response by bias voltage tuning. The photovoltaic response is attributed to the transition of electron from the ground state to a high continuum state. The photoconductive response arises from the transition of electron from the ground state to the wetting layer state through the barrier via Fowler-Nordheim tunneling evidenced by a broad feature of the photocurrent peak on the high energy side

Midinfrared intersubband absorption in ZnxCd1−xSe∕Znx′Cdy′Mg1−x′−y′Se multiple quantum well structures

The authors report the observation of intersubband absorption in ZnxCd(1−x)Se∕Znx′Cdy′Mg(1−x′−y′)Se multiple quantum wells. Lattice-matched samples were grown by molecular beam epitaxy on InP (001) substrates. Photoluminescence measurements indicate that the samples have excellent material quality. The peak absorption wavelengths measured by Fourier transform infrared spectroscopy are 3.99 and 5.35μm for two samples with ZnxCd(1−x)Se well widths of 28 and 42A, respectively. These values fall within the 3–5μm wavelength range, which is of interest for midinfrared intersubband devices, such as quantum cascade lasers and quantum well infrared photodetectors. Their experimental results fit well with theoretical predictions based on the envelope function approximation. The results indicate that these wide band gap II-VI materials are very promising for midinfrared intersubband device applications.

Metal-Grating-Coupled Terahertz Quantum-Well Photodetectors

Three terahertz (THz) GaAs/AlGaAs quantum-well photodetectors with different 1-D metal gratings are fabricated for front-incident detection of THz waves. Photocurrent spectra are acquired and compared with 45° incident facet samples (without grating), and peak responsivities are determined with a calibrated blackbody radiation source. The results show that these gratings can couple THz waves into detectors effectively, resulting in good detector responsivities. The modal method is employed to simulate the light coupling efficiency and the optimization conditions of the gratings.

Terahertz two-photon quantum well infrared photodetector

A two-photon detector based on intersubband transitions in GaAs/AlGaAs quantum wells operating in the Terahertz regime below the Reststrahlenband is reported. Resonantly enhanced optical nonlinearities enables sensitive quadratic detection at pJ pulse energies. We demonstrate its use in a quadratic autocorrelator for far-infrared picosecond pulses at around 7 THz.