We describe a monolithically integrated THz transceiver consisting of a Schottky diode embedded into a THz
quantum cascade laser (QCL) waveguide. Besides functioning as a heterodyne receiver for externally incident
radiation, the device is a useful tool for characterizing the performance and dynamics of the QCL. Here we
present an overview of the device, demonstrate receiver operation, and present laser dynamics measurements
especially related to feedback of the QCL's emission due to retroreflections.
Integration of THz quantum cascade lasers (QCLs) with single-mode 75 μm x 37 μm rectangular waveguide components, including horn antennas, couplers, and bends, for operation at 3 THz has been designed and fabricated using thick gold micromachining. Measurements on the isolated waveguide components exhibit fairly low loss and integration with THz QCLs has been demonstrated. This technology offers the potential for realizing miniature integrated systems operating in the 3 THz frequency range.
Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental
technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or
photonic properties at terahertz frequencies. This results in formidable challenges for creating the components needed
for generating, detecting, and manipulating THz waves. Considering the promising applications of THz radiation, it is
important overcome such material limitations by searching for new materials, or by constructing artificial materials with
a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties
that derive from their sub-wavelength structure. The potential of metamaterials for THz radiation originates from a
resonant electromagnetic response which can be tailored for specific applications. Metamaterials thus offer a route
towards helping to fill the so-called "THz gap". In this work we discuss novel planar THz metamaterials. Importantly,
the dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials.
We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted
in the creation of efficient THz switches and modulators of potential importance for advancing numerous real world THz
Tunable electromagnetic metamaterials can be designed through the incorporation of semiconducting materials.
We present theory, simulation, and experimental results of metamaterials operating at terahertz frequencies.
Specific emphasis is placed on the demonstration of external control of planar arrays of metamaterials patterned
on semiconducting substrates with terahertz time domain spectroscopy used to characterize device performance.
Dynamical control is achieved via photoexcitation of free carriers in the substrate. Active control is achieved by
creating a Schottkey diode, which enables modulation of THz Transmission by 50 percent, an order of magnitude
improvement over existing devices. Because of the universality of metamaterial response over many decades of
frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly
play a key role in future demonstrations of novel high-performance devices.
A split-grating-gate detector design has been implemented in an effort to combine the tunability of the basic grating-gate detector with the high responsivity observed in these detectors when approaching the pinchoff regime. The redesign of the gates by itself offers several orders of magnitude improvement in resonant responsivity. Further improvements are gained by placing the detector element on a thermally isolating membrane in order to increase the effects of lattice heating on the device response.
Grating gated field effect transistors (FETs) are potentially important as electronically tunable terahertz detectors with spectral bandwidths of the order of 50 GHz. Their utility depends on being able to 1) use the intrinsic high speed in a heterodyne mixer or 2) sacrifice speed for sufficient sensitivity to be an effective incoherent detector. In its present form the grating gated FET will support IF frequencies up to ~10 GHz, an acceptable bandwidth for most heterodyne applications. By separating the resonant plasmon absorption from the responsivity mechanism, it appears that a tuned, narrow terahertz spectral band bolometer can be fabricated with NEP ~ 10<sup>-11</sup> watts/√Hz and response times of the order of 30 msecs, useful in a passive multispectral terahertz imaging system.
Small volume high-T<SUB>c</SUB> super-conducting YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7</SUB> (YBCO) thin films are used as low power, very wide bandwidth mixers in the frequency range of 75 GHz to 2.5 THz. The YBCO films are patterned into lattice-cooled hot-electron bolometers (HEB) coupled to an integrated Au thin-film antenna and transmission line. Near 77 K, these mixers have responsivity as high as 780 V/W using only 8 nW of local oscillator (LO) power at 585 GHz. The responsivity can be shown to be truly bolometric. Direct heterodyne and homodyne down-conversion mixing using local-oscillator frequencies of 75 GHz and 585 GHz show overall conversion gains of -35 dB, which includes a -18 dB coupling loss, using only approximately 1 (mu) W of LO power. The gain bandwidth shows a simple Lorentzian roll-off with -3 dB point of 5 to 8 GHz. The large gain bandwidth and small power requirements make these high-T<SUB>c</SUB> superconducting mixers an attractive alternative to existing Schottky diode and conventional superconducting receiver technologies.
The spatial distribution of supercurrent in high-T<SUB>c</SUB> Josephson junction devices has been studied extensively using field modulation measurements of the critical current and microwave absorption. The devices are edge junctions composed of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7</SUB>-YBa<SUB>2</SUB>Co<SUB>0.21</SUB>Cu<SUB>2.79</SUB>O<SUB>7</SUB>-YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7</SUB>. The l<SUB>c</SUB>(H) patterns allow a quantitative Fourier transform analysis to obtain a self-consistent spatial supercurrent density distribution, J<SUB>s</SUB>(x). These junctions are found to be more homogeneous than in most other high-T<SUB>c</SUB> Josephson junctions reported to date.