Coupling of metasurfaces to intersubband transitions (ISTs) in semiconductor quantum wells (QWs) has been extensively studied for various applications ranging from generating giant nonlinear optical response to designing tunable metasurfaces for applications such as ultrafast spatial optical modulators and voltage tunable filters. In this work, we experimentally demonstrate a fundamentally new approach of actively controlling the coupling of ISTs in QWs to a metasurface for voltage tuning its optical response. Unlike previous approaches, we use voltage-controlled quantum tunneling to control the carrier concentration in the QWs for turning on/off the ISTs. We design a multi-quantum well structure consisting of four undoped InGaAs wells with AlInAs barriers grown on top of a highly doped InGaAs layer that acts as an electron reservoir. The heterostructure is optimized such that the first IST in all the wells is at 11µm. A complementary gold metasurface with dipole resonances at 11µm is fabricated on top of the QW structure. We designed the heterostructure such that by applying a bias of 1V, the energy bands of all the QWs get aligned simultaneously, leading to the occupation of the ground state of all the QWs via quantum tunneling of the electrons from the electron reservoir. The ISTs which were turned off due to negligible electron density gets turned on at 1V, and this leads to coupling between the ISTs and the dipoles resonances of the metasurface. The voltage induced coupling leads to reflectance modulation which we confirmed experimentally by rapid scan double modulation FTIR measurements.
The temporal intensity profile of pulse(s) from passively Q-switched and passively Q-switched mode locked (QSML) solid-state lasers is known to be dependent on cavity length. In this work, the pulse width, modulation depth, and beat frequencies of a Nd:Cr:GSGG laser using a Cr+4:YAG passive Q-switch are investigated as function cavity length. Measured temporal widths are linearly correlated with cavity length but generally 3-5 ns larger than theoretical predictions. Some cavity lengths exhibit pulse profiles with no modulation while other lengths exhibit complete amplitude modulation. The observed beat frequencies at certain cavity lengths cannot be accounted for with passively QSML models in which the pulse train repetition rate is τRT-1, τRT= round-trip time. They can be explained, however, by including coupled cavity mode-locking effects. A theoretical model developed for a two section coupled cavity semiconductor laser is adapted to a solid-state laser to interpret measured beat frequencies. We also numerically evaluate the temporal criterion required to achieve temporally smooth Q-switched pulses, versus cavity length and pump rate. We show that in flash lamp pumped systems, the difference in buildup time between longitudinal modes is largely dependent on the pump rate. In applications where short pulse delay is important, the pumping rate may limit the ability to achieve temporally smooth pulses in passively Q-switched lasers. Simulations support trends in experimental data. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
The quantum cascade laser (QCL) is currently the only solid-state source of coherent THz radiation capable of
delivering more than 1 mW of average power at frequencies above
~ 2 THz. This power level combined with
very good intrinsic frequency definition characteristics make QCLs an extremely appealing solid-state solution
as compact sources for THz applications. I will present results on integrating QCLs with passive rectangular
waveguides for guiding and controlling the radiation emitted by the QCLs and on the performance of a THz
integrated circuit combining a THz QCL with a Schottky diode mixer to form a heterodyne receiver/transceiver.
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.
The terahertz (THz) region has been shown to have considerable application potential for spectroscopic imaging,
nondestructive imaging through nonpolar, nonmetallic materials and imaging of biological materials. These applications
have all been possible due to the recent progress in THz sources, detectors and measurement techniques. However, only
moderate progress has been made in developing passive and active devices to control and manipulate THz radiation,
which can enhance current imaging capabilities. One promising approach for implementing passive and active devices at
THz frequencies are metamaterials - composite materials designed to have specific electromagnetic properties not found
in naturally occurring materials. The most common implementation utilizes a metallic resonant particle periodically
distributed in an insulator matrix where the periodicity is significantly smaller than the wavelength of operation. We
have designed and implemented three metamaterial based devices with potential applications to THz imaging. We
present an electrically-driven active metamaterial which operates as an external modulator for a ~2.8 THz CW quantum
cascade laser. We obtained a modulation depth of ~60%. We also demonstrate a polarization sensitive metamaterial
which can be used as a continuously variable attenuator or as a wave plate. The latter may be useful for the development
of THz phase contrast imaging.
Thick multi-layer metal stacking offers the potential for fabrication of rectangular waveguide components,
including horn antennas, couplers, and bends, for operation at terahertz frequencies, which are too small to machine
traditionally.
Air-filled, TE10, rectangular waveguides for 3 THz operation were fabricated using two stacked electroplated
gold layers on both planar and non-planar substrates. The initial layer of lithography and electroplating defined 37
micrometer tall waveguide walls in both straight and meandering geometries. The second layer, processed on top of the
first, defined 33 micrometer thick waveguide lids. Release holes periodically spaced along the center of the lids
improved resist clearing from inside of the electroformed rectangular channels. Processing tests of hollow structures on
optically clear, lithium disilicate substrates allowed confirmation of resist removal by backside inspection.
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.
Terahertz detection using excitations of plasmon modes offers a high-speed, high resolution, and frequency-selective
alternative to existing technology. Plasmons in high mobility quantum well two-dimensional electron gas (2DEG)
systems can couple to radiation when either the channel carrier density, or the incident radiation, is spatially modulated
with appropriate periodicity. Grating-gated terahertz detectors having a voltage tunable frequency response have been
developed based on this principle. A continuous wave THz photomixer was used to characterize the resonant absorption
in such devices. At the fundamental 2DEG plasmon frequency, defined by the grating and the quantum well carrier
density, a 20% change in transmission was observed. As the resonance is tuned from the 'natural' plasmon frequency
through application of a gate bias, it shifts as expected, but the transmission change drops to only a few percent.
We have fabricated and characterized plasmonic terahertz detectors that integrate a voltage controlled planar barrier with
a grating gated GaAs/AlGaAs high electron mobility transistor. These detectors exhibit a narrowband, tunable
plasmonic response. Substantially increased responsivity is achieved by introducing an independently biased, narrow
gate that produces a lateral potential barrier adjacent to the drain when biased to pinch-off. DC electrical characterization
in conjunction with bias-dependent terahertz responsivity and time constant measurements indicate that a hot electron
bolometric effect is the dominant response mechanism over a broad range of experimental conditions. The temperature
dependence of the bolometric response is consistent with the energy relaxation time and absorption coefficient of a
2DEG. Rectification resulting from non-linear current-voltage characteristics also appears to contribute to the response.
Additionally, we have begun investigating the operation of this device with the full grating gate biased to pinch-off to
produce many detection elements in series.
Split grating-gate field effect transistors (FETs) detectors made from high mobility quantum well two-dimensional
electron gas material have been shown to exhibit greatly improved tunable resonant photoresponse compared to single
grating-gate detectors due to the formation of a 'diode-like' element by the split-gate structure. These detectors are
relatively large for FETs (1mm x 1mm area or larger) to match typical focused THz beam spot sizes. In the case where
the focused THz spot size is smaller than the detector area, we have found evidence, through positional scanning of the
detector element, that only a small portion of the detector is active. To further investigate this situation, detectors with
the same channel width (1mm), but various channel lengths, were fabricated and tested. The results indicate that indeed,
only a small portion of the split grating gated FET is active. This finding opens up the possibility for further
enhancement of detector sensitivity by increasing the active area.
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 ~ 10-11 watts/√Hz and response times of the order of 30 msecs, useful in a passive multispectral terahertz imaging system.
A semi-confocal etalon has been sued as a quasi-optical cavity to explore the dynamical conductance of Bloch- oscillating superlattices at terahertz frequencies. To maintain both DC and irradiated field uniformity and to maximize the coverage of the cavity mode with the devices of interest, the tunneling structures have been photolithographically fabricated into micro-sized mesa- isolated devices forming a quasi-optical square array interconnected by a metal grid with a period which is less than the wavelength in the semiconductor of the IR probe radiation. At a given bias on the device array and scanning the cavity through a resonance, the loss and reactance of the tunneling devices embedded in the array is measured by detecting a change in the position and line shape of the cavity resonance. Transmission measurements of the cavity loading by the biased quasi-optical arrays at frequencies from 250GHz to 3.0THz will be presented and compared to theoretical predictions.
We explored harmonic generation by Bloch oscillation in miniband superlattices driven by intense THz radiation from the UCSB free electron lasers, as a function of both THz intensity and applied DC bias. To accomplish this we integrated micrometers size superlattice mesas in a quasi-optical array which amplified a plane wave incident normal to the array and coupled it into the growth direction of the superlattice. We were able to successfully measure both second and third harmonic generation quantitatively. The harmonics are compared to a quasi-classical picture of Bloch oscillation.
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