Juncheng Cao

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.

Strong nonlinear optical response of graphene in the terahertz regime

We demonstrate that within the model of massless Dirac fermions, graphene has a strong nonlinear optical response in the terahertz regime. It is found that the nonlinear contribution significantly alters both the single frequency and frequency tripled optical response at experimentally relevant field strengths. The optical activity of single layer graphene is significantly enhanced by nonlinear effects, and the frequency tripled response opens the gateway to photonic and optoelectronic device applications.

Terahertz generation and chaotic dynamics in GaN NDR diode

Inflection of carriers in the ? valley, prior to the usual intervalley scattering when the electric field is approximately less than 300 kV cm?1, can cause a negative differential velocity in zinc-blende (Zb) GaN. GaN n+nn+ oscillators based on this mechanism have a self-oscillating frequency in the terahertz (THz) range. The situation is completely different from the case of traditional Gunn-effect devices. We have carefully studied the electric domain dynamics and the self-oscillation frequency dependence on the applied dc field. When the diode is driven by a dc and an ac bias, a typical nonlinear dynamic system is constructed with the dc bias, the ac amplitude and the ac frequency as the control parameters. Different mode locking and chaotic modes show up when tuning the control parameters.

Many-body effects on terahertz quantum well detectors

Many-body effects have been investigated in the design of terahertz quantum well photon detectors. A large discrepancy between the theoretical and experimental photoresponse peak positions exist without considering the many-body interactions. The calculated results agree with the experimental data quantitatively with including the exchange-correlation and depolarization effects within the local density approximation. Our numerical results show that it is a must to consider the many-body interactions for designing the quantum well detectors for the terahertz region.

Transport through a strongly correlated quantum dot with Fano interference

We present the transport properties of a strongly correlated quantum dot attached to two leads with a side coupled non-interacting quantum dot. Transport properties are analyzed using the slave boson mean field theory which is reliable in the zero temperature and low bias regime. It is found that the transport properties are determined by the interplay of two fundamental physical phenomena,i.e. the Kondo effects and the Fano interference. The linear conductance will depart from the unitary limit and the zero bias anomaly will be suppressed in the presence of interdot coupling. The zero bias shot noise Fano factor increases with the interdot coupling and tends to the Poisson value. The shot noise Fano factor shows a non-monotonic behavior as a function of the interdot coupling for various side dot energy levels.

Monte Carlo simulation of extraction barrier width effects on terahertz quantum cascade lasers

The effect of extraction barrier width on the performance of a resonant-phonon terahertz quantum cascade laser is studied using the ensemble Monte Carlo method. The width of extraction barrier is varied from 48to33A with a stepsize of −3A. The calculated threshold current density increases monotonously from 436to1054A∕cm2 with decreasing extraction barrier width, while the peak gain shows nontrivial behavior. These findings agree well with an experimental study. The simulation results suggest an optimal extraction barrier width of 36A for peak gain and for the best temperature performance.

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.

Similarity Matching-Based Extensible Hand Gesture Recognition

This paper presents an accelerometer-based smart ring and a similarity matching-based extensible hand gesture recognition algorithm. Users can wear the ring to perform gestures in 2-D space. The accelerations of hand motions are collected by the three-axis accelerometer, which is integrated in the ring. We divide the gestures into two types, i.e., the basic gesture and the complex gesture, which can be decomposed into a basic gesture sequence. A segmentation algorithm is developed to identify individual gestures in a sequence automatically. To recognize the basic gesture, a simple but effective feature based on the average jerk is extracted. The recognized basic gesture is then encoded by a Johnson code. Finally, the complex gesture is recognized by comparing the similarity between the obtained basic gesture sequence and the stored templates. A library of eight basic gestures and 12 complex gestures is created, and the users can easily define and add their own gestures without pretraining. The model discussed in this paper achieves a basic gesture recognition rate of 98.9% and a complex gesture recognition rate of 97.2%. Compared with complete matching, the proposed algorithm based on similarity matching improves the complex gesture recognition rate ~12%. Experimental results have successfully validated the feasibility and effectiveness of the gesture decomposition and similarity matching-based gesture recognition algorithm.