A wafer fused GaP/GaAs waveguide was developed for THz QCLs to achieve high confinement factor benefiting from its lower refractive index in THz regime. The modal simulation of several waveguide structures using COMSOL showed an increase of confinement factor up to 2 as compared to regular waveguide; however it also resulted in high losses. Experimental results showed good electric characteristics but poor optical performance, which is mainly due to the degradation of crystal quality after high temperature process, confirmed by stress analysis and XRD. Therefore, a low temperature fusion process is necessary to fabricate GaP/GaAs THz waveguide.
This paper introduces the continuously tunable THz radiation through sideband generation of a free running and solidnitrogen- cooled THz quantum cascade laser. The 2.324 THz QCL operating in a single longitudinal mode (SLM) in continuous-wave (cw) was mixed with a swept synthesized microwave signal by a THz Schottky-diode-balanced mixer. Through sideband generation, two frequency branches were observed at low and high frequency, characterized with a Fourier-transform spectrometer. At low frequency, the sideband generates frequencies from -50 GHz to +50 GHz. At high frequency, it generates sideband frequencies from 70 GHz to 115 GHz. The total ±100 GHz tuning range can be further expanded with higher frequency millimeter wave amplifier/multiplier source. The sideband generates total 1 μW of output power at both upper and lower frequency with 200 μW of driven power from the THz QCL, showing a power conversion efficiency of 5 × 10<sup>-3</sup>. The demonstration of this SM, continuously tunable THz source enables its applications where SM, spatially coherent beam is required.
Operational temperature increase of CW THz QCLs to 77 K has enabled us to employ solid nitrogen (SN2) as the
cryogen. A roughing pump was used to solidify liquid nitrogen and when the residual vapor pressure in the nitrogen
reservoir reached the pumping system's minimum pressure the temperature equilibrated and remained constant until
all the nitrogen sublimated. The hold time compared to liquid helium has thereby increased approximately 70-fold,
and at a greatly reduced cost. The milliwatt CW QCL was at a temperature of approximately 60 K, dissipating 5 W
of electrical power. To measure the long-term frequency, current, and temperature stability, we heterodyned the
free-running 2.31 THz QCL with a CO<sub>2</sub> pumped far-infrared gas laser line in methanol (2.314 THz) in a corner-cube
Schottky diode and recorded the IF frequency, current and temperature. Under these conditions the performance
characteristics of the QCL, which will be reported, exceeded that of a device mounted in a mechanical cryocooler.
THz semi-insulating surface plasmon waveguide QCL's based on the bound-to-continuum design have been developed
with 2.31 THz output powers of ~0.5 milliwatt from a single facet, input powers of ~5 watts, and threshold current
densities of 117 A/cm<sup>2</sup> operating continuous wave at 77K. These results were achieved by depositing alloy metal on both
contact layers, only annealing the bottom metal layers, and thinning the substrate thickness to ~170 μm to assure good
heat dissipation. The structure was based on a previously published 2.83 THz design that was scaled to emit at 2.31 THz.
The demonstration of this high temperature, high power laser with low input power enables its use in compact, coherent
THz transceivers for heterodyne detection with liquid nitrogen cooling.
In this paper, a high operating temperature (HOT) broadband InAs/GaAs quantum dot
(QD) infrared photodetector (QDIP) is reported. The QDIP covers a wide detection
spectrum range from 3 μm to 10 μm. A large photoresponsivity of 12.0 A/W at a low
bias voltage of 0.15V and a high peak specific photodetectivity D* of 1.2×10<sup>8</sup> cmHz<sup>1/2</sup>/W
are obtained at a high operating temperature of 298 K.
We report a longwave infrared quantum dot infrared photodetector working at room temperature (RT) (298K). A
photoresponsivity and photodetectivity of 0.02A/W and 9.0x10<sup>6</sup> cmHz<sup>1/2</sup>/W was achieved at 298K with a low bias
voltage of -0.1V. The RT QDIP avoids bulk and heavy cryogenic cooling systems and thus enables the development
of ultra-compact IR sensing and imaging systems.
We report a voltage-tunable multispectral quantum dot infrared photodetector with
integrated carbon-nanotube based flexible electronics. Such integrated photodetection
and flexible electronics would not only enhance the detectors functionalities, but also
reduce the time delay by performing image processing locally, making it promising for
adaptive multi-spectral photodetection and sensing.
A coherent transceiver using a THz quantum cascade laser as the transmitter and an optically pumped molecular laser as
the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire
high-resolution images of fully illuminated targets, including scale models. Phase stability of the received signal,
sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by
frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the
target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric
attenuation at 2.408 THz (2.0 dB/m at 40% RH), the coherence length of the QCL transmitter will allow coherent
imaging over distances up to several hundred meters. In contrast to non-coherent heterodyne detection, coherent
imaging allows signal integration over time intervals considerably longer than the reciprocal of the source, or signal
bandwidth, with consequent improvement in the signal-to-noise ratio. Image data obtained with the system will be
We demonstrate room temperature electroluminescence from intersublevel transitions in self-assembled InAs quantum
dots in GaAs/AlGaAs heterostructures. The quantum dot devices are grown on GaAs substrates in a Varian Gen II
molecular beam epitaxy system. The device structure is designed specifically to inject carriers into excited conduction
band states in the dots and force an optical transition between the excited and ground states of the dots. A downstream
filter is designed to selectively extract carriers from the dot ground states. Electroluminescence measurements were
made by Fourier Transform Infrared Spectroscopy in amplitude modulation step scan mode. Current-Voltage
measurements of the devices are also reported. In addition, both single period and multi-period devices are grown,
fabricated, characterized, and compared to each other. Finally, we discuss the use of plasmonic output couplers for these
devices, and discuss the unique emission observed when the quantum dot layer sits in the near field of the plasmonic top
We report a longwave infrared quantum dot infrared photodetector working at room
temperature (RT) (298K). A high photoresponsivity and photodetectivity of 0.02A/W and
9.0x10<sup>6</sup> cmHz<sup>1/2</sup>/W were achieved at 298K with a low bias voltage of -0.1V. The RT
QDIP avoids bulk and heavy cryogenic cooling systems and thus enables the
development of ultra-compact IR sensing and imaging systems.