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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. [ABSTRACT FROM PUBLISHER]

An analysis of dark current mechanisms has been performed on high-operating-temperature (up to 330 K) split-off (SO) band p+-GaAs/AlGaAs heterojunction infrared detectors (3-5 µm). In contrast to conventional 1-D current models due to carrier transport based on tunneling and/or thermionic emission mechanisms, a 2-D electrical model is used to explain nonuniformity degradation of zero-bias differential resistance (RoA) with temperatures as measured on SO detectors. The 2-D characteristic of carrier transport could have the limitation on high-temperature performances of detectors and, hence, needs optimizing. A theoretical model shows that this 2-D effect can be reduced by structural modifications such as using smaller mesa sizes, higher doping of the p+-GaAs layer, and a higher potential barrier that prospectively provides better electrical uniformity for SO detectors working at high temperatures. [ABSTRACT FROM AUTHOR]

We study experimentally and numerically the polarization sensitivity of quantum well infrared photodetectors coupled to a diffraction grid. The polarization extinction ratio of such system is determined by two factors: polarization sensitivity of the diffraction grid and the intrinsic polarization sensitivity of the photodetector itself. The combined effect of these factors result in nonmonotonic dependence of the polarization extinction ratio on the parameters of the diffraction grid. By varying the grid parameters, i.e., increasing the height and tuning the grid period, a maximum value for the polarization extinction ratio can be achieved. Both front side and back side illuminations of the photodetector are studied. The strongest polarization sensitivity is achieved under front side illumination. [ABSTRACT FROM AUTHOR]

The thermal dynamic behaviors of terahertz quantum cascade lasers are investigated using a spectrally matching terahertz quantum-well photodetector. The THz output power of a lasing device under a pulse excitation is found to decrease with time, which is attributed to the increase of active-region temperature. The measurements show that the duration of output lasing pulse ranges from a few microseconds up to 77 μs, depending on device bias and heat-sink temperature. Atheoretical analysis based on a two-dimensional heat diffusion model is presented. The model calculates lasing quenching time based on an "average active-region temperature" criterion as well as a "reserve of gain" criterion. The best fit for quenching time is found when the vertical thermal conductivity in the quantum well region is two orders of magnitude lower than that of substrate. The measured lasing quenching time is in very good agreement with theoretical modeling calculated using both criteria. [ABSTRACT FROM AUTHOR]

We present a systematic study on a set of n-type GaAs-AIGaAs quantum-well infrared photodetectors (QWIPs) with varying Si doping density in the wells. It is revealed that the increase in doping density enhances proportionally the absorption efficiency and responsivity while increasing exponentially the dark current and hence the dark current noise. We experimentally confirm the theoretically predicted optimum conditions for background-limited infrared performance temperature and detector-noise-limited detectivity. It is suggested that, to achieve the optimal QWIP performance, the doping density in the wells should be determined according to application and the desired operating temperature. We point out that a simulation is highly recommended to achieve the best possible performance since the choice of doping may not be obvious. As shown here, an optimized doping for temperature is actually the worst for detectivity for the particular set of samples. [ABSTRACT FROM AUTHOR]

This paper investigates the heat conduction behavior of a terahertz (THz) quantum-cascade laser (QCL) active region by measuring its temperature using in-situ microprobe band-to-band photoluminescence (PL) technique. The heat resistance of different regions in QCL structure is derived from the temperature measurement. Experimental results show that thinning the substrate from 300 μm thick to 140 μm lowers thermal resistance of the device by 21%, which helps achieving continuous-wave operation. A thermodynamic differential equation was numerically solved and the temperature profiles and thermal behavior of various regions within actively biased QCL devices under various conditions were obtained. The simulation confirms the measured results that the substrate accounts for 59% of the total device thermal resistance. [ABSTRACT FROM AUTHOR]

A heterojunction interfacial workfunction internal photoemission (HEIWIP) terahertz detector with ∼1 x 10[sup18] cm[sup-3] n-type doped GaAs emitters in a multilayer GaAs/Al[sub0 13]Ga[sub0.87]As heterostructure is presented. The detection mechanism is based on free carrier absorption with a broad response extending to ∼5.26 THz (57 μm), corresponding to an effective workfunction of ∼21.8 meV, which is much smaller than the offset expected for an Al fraction of x = 0.13 at a 1 x 10[sup18] cm[sup-3] doping. This is attributed to a reduction of the conduction band offset by interface dipole formation between the accumulated negative charges at the interface states and migrated positively charged donors in the barrier. The device has a peak responsivity of 0.32 A/W at ∼26 μm at 5 K. It is demonstrated that the dopant migration-induced interface dipole effect can be used to extend the zero response threshold frequency (f[subo]) of n-type HEIWIP detectors. [ABSTRACT FROM AUTHOR]

We report a hybrid organic-inorganic optical upconverter that converts input 1.5-μm infrared light to output visible light. The device was made by direct tandem integration of an inorganic InGaAs/InP photodetector with an organic light- emitting diode (OLED). Incoming 1.5-μm light optical radiation is absorbed by the semiconductor p-i-n detector. The resultant photocurrent drives the OLED to emit visible light at ∼520 nm in wavelength. The ratio of the photocurrent-induced versus dark- current-induced visible light emission was measured to be over 100 at a device bias of 14 V at room temperature. Devices with different configurations of the hole transport layer (HTL) were fabricated and tested. It was found that the devices with an HTL of C60/CuPc/NPB exhibited a lowest turn-on voltage (∼3.8 V). The results show that the interfacial structure at the inorganic-organic interface plays a vital role in enabling the integration of the hybrid organic-inorganic devices. [ABSTRACT FROM AUTHOR]

Generation of terahertz (THz), and sub-THz, coherent waves has been demonstrated at room temperature in GaAs-AIGaAs asymmetric quantum wells by mixing two CO2 laser beams. Two overlapping regimes were studied: the double resonance regime where three states are involved and the optical rectification which relies on two states only. The measured conversion efficiency of ~ 10-7 W-1 is in reasonable agreement with theoretical predictions. In addition, this technique of THz generation provides a model system to study the emission profile from an ensemble of radiating dipoles. For instance, backward emission and strong diffraction effects have been observed. [ABSTRACT FROM AUTHOR]

In this paper, results are reported showing response enhancement in GaAs-AIGaAs IR detectors using a doped substrate to increase reflection, enhancing the resonant cavity effect. Responsivity for heterojunction interfacial workfunction detectors grown on semi-insulating (SI) and doped substrates are compared. For a device grown on an SI substrate, a 9-μm resonance peak had a response of 1.5 mA/W while a similar device on an n-doped substrate showed 12 mA/W. Also, the difference between response under forward and reverse bias (3 versus 12 mA/W) for the sample grown on the doped substrate, as well as calculated results confirm that the increased response is due to the resonant enhancement. An optimized design for a 15-μm peak (24 μm 0 response threshold) detector grown on a doped substrate could expect a peak response of 4 A/W with a 50% quantum efficiency and D*; ∼ 2 × 1010 Jones at the background limited temperature of 50 K. [ABSTRACT FROM AUTHOR]

Active mode locking in broadband quantum cascade (QC) lasers with a repetition rate of about 14.3 GHz has been achieved through the modulation of the laser bias current. At low driving currents, the active mode locking in broadband QC lasers resembles the active mode locking in single-wavelength QC lasers, while at high driving currents, the mode locking properties are governed by the broad spectral gain of these lasers. At high bias currents, the active modulation excites Fabry--Perot modes across the entire gain spectrum from 6.7 to 7.4 μm, with clear evidence of mode locking. The spectral width of the optical gain in the broadband QC lasers exceeds 2 THz and indicates the potential for generating subpicosecond pulses. [ABSTRACT FROM AUTHOR]

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of ∼12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width. [ABSTRACT FROM AUTHOR]

Presents a study that investigated the performance of GaAs multilayer homojunction interfacial workfunction internal photoemission far-infrared (FIR) detector as a function of emitter layer concentration. Experimental methods; Dark current characteristics; Conclusion.

A transmission imaging system based on a terahertz quantum-cascade laser and a spectrally-matched quantum-well photodetector is presented. Terahertz imaging of paper money is demonstrated. An imaging resolution of 0.5 mm is achieved. This experiment demonstrates that the laser and the photodetector are potentially useful for imaging applications. [ABSTRACT FROM PUBLISHER]

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 ^\circ 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. [ABSTRACT FROM AUTHOR]