H. C. Liu

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.

Quantum dot infrared photodetectors

Abstract We discuss key issues related to quantum dot infrared photodetectors. These are the normal incidence response, the dark current, and the responsivity and detectivity. It is argued that the present devices have not fully demonstrated the potential advantages. The dominant infrared response in devices so far is polarized in the growth direction. The observed dark currents are several orders of magnitude higher than those for quantum well photodetectors; while ideally they should be lower. The areas that need improvements are pointed out.

Dark current in quantum well infrared photodetectors

Modeling the dark current in quantum well infrared photodetectors has been a topic of much recent research, but the implications of many of the underlying assumptions have not been clarified. We attempt to justify one such model and to provide physical insight for its success. We compare the dark current expression with experiments on several samples, and show that the model provides a good approximation for a wide range of device parameters including barrier thicknesses from 250 to 700 A and number of wells from 4 to 32.

Tunnel current in quantum dot infrared photodetectors

Infrared photodetectors have been fabricated based on InAs/GaAs self-assembled quantum dot (QD) layers, with various QD densities and doping levels. Dark currents have been measured as a function of applied bias and temperature. They show a clear activation energy, which decreases as the QD shell filling increases. Its absolute value and dependence on applied bias indicate that electrons tunnel from QD levels into the wetting layer of the next period. Resonant structures in the current–voltage curve and in its first derivative confirm the tunneling through the GaAs barrier. Negative differential resistances are observed in highly doped samples at low temperature.

Terahertz Quantum Well Photodetectors

The terahertz (THz) part of the electromagnetic spectrum promises a wide range of new and novel, some may be disruptive, applications. However, the development of technologies in the THz spectrum or the very far infrared region has been slow mainly because of the lack of convenient detectors and lasers. There are a few competing new approaches for better detectors, and here, we concentrate on one based on quantum wells. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1-10 THz). Quantum well, barrier, and doping parameters are optimized in terms of operating temperature, absorption, and detectivity. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance. These devices are suited for a variety of applications, especially in conjunction with the newly developed THz quantum cascade lasers. Examples include THz sensing and imaging and free space communication.

Recent Progress in Terahertz Quantum-Well Photodetectors

Terahertz quantum-well photodetectors (QWPs) represent a new and emerging photon-type detector in a terahertz region. Recent progress in the development of terahertz QWPs is reviewed. We first discuss the many-particle effects on the accurate design of terahertz QWPs. Second, three types of light couplers for terahertz QWPs are introduced. At resonant coupling frequencies, the polarization of light field is effectively changed by the light couplers to fulfill the selection rule of intersubband transition. Meanwhile, the electric field intensities in the active multiquantum-well region of terahertz QWPs are enhanced. The performance of terahertz QWPs with these light couplers is improved significantly. Finally, terahertz-QWP-based wireless communication and imaging are demonstrated.

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.

PC2/CPE-mediated pro-protein processing in tumor cells and its differentiated cells or tissues

n Abstractn n Pro-protein convertase-2 (PC2) and carboxypeptidase-E (CPE) proteins are two major members of the pro-protein convertases that involve in the maturation of protein precursor. By using PC2 activity, immunocytochemistry (ICC) and Western blot method, PC2, CPE and preproNPY protein expression levels were compared among mature retina tissue, RGC-5 cells and its differentiated cells, or brain cortex tissue, NS20Y tumor cells and its differentiated cells, or mature breast tissue, breast tumor cell RM1 and breast adenocarcinoma tissue. The experimental results indicated that the differentiated cells or tissues had higher or highest PC2 activity. In the comparative experiments, more PC2 protein expression in the mature tissues and more CPE and preproNPY protein expression in the tumor cells or tumor tissue were observed, but no expression of preproNPY protein was observed in the mature tissues. Compared with NS20Y or RGC-5 undifferentiated cells, its differentiated cells showed less proPC2, more proCPE and more preproNPY protein expressions. The results demonstrated that the mature tissues showed stronger PC2/CPE-mediated pro-protein processing ability than the tumor cells or tissue. The results also showed that the artificial differentiation of RGC-5 or NS20Y cells was different from maturation of its corresponding normal tissue.n n

Dark current and noise analyses of quantum dot infrared photodetectors

Because the dark current and the noise of quantum dot infrared photodetectors (QDIPs) can bring about a degradation in their performance, they have attracted more and more attention in recent years. In this paper, an algorithm used to evaluate the dark current of the QDIP is proposed, which is based on the algorithm including the common contribution of the microscale and the nanoscale electron transport. Namely, by accounting for the dependence of the drift velocity on the applied electric field, we greatly enhance the accuracy of the dark current calculation compared with that in the previous algorithm. This proposed algorithm is further used to estimate the noise current of QDIP, and the calculated results show a good agreement with the published data.