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.
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
Coherent terahertz radar systems, using CO<sub>2</sub> laser-pumped molecular lasers have been used during the past decade for
radar scale modeling applications, as well as proof-of-principle demonstrations of remote detection of concealed
weapons. The presentation will consider the potential for replacement of molecular laser sources by quantum cascade
lasers. While the temporal and spatial characteristics of current THz QCLs limit their applicability, rapid progress is
being made in resolving these issues. Specifications for satisfying the requirements of coherent short-range THz radars
will be reviewed and the feasibility of incorporating existing QCL devices into such systems will be described.
As short range, ground based, surveillance systems operating at terahertz frequencies continue to evolve,
increasing attention is being directed towards the behavior of dielectric materials at terahertz frequencies as well as the
behavior of optical components used to control terahertz radiation. This work provides an overview of several terahertz
optical components such as frequency selective filters, laser output couplers, artificial dielectrics, and electromagnetic
absorbers. In addition, a database was established that contains terahertz properties of common materials that have been
largely unexplored in this region of the spectrum. The database consists of transmittance and reflectance spectra of a
variety of materials measured using Fourier transform infrared spectroscopy techniques from 175 GHz - 2 THz. In
addition, ultra-stable, CO<sub>2</sub> optically pumped, far-infrared gas lasers were used to collect fixed-frequency transmittance
data at 326 GHz, 584 GHz, and 1.04 THz. A Gunn oscillator was used for measurements at 94 GHz.