The device applications of plasmonic systems such as graphene and two dimensional electron gases (2DEGs) in III-V
heterostructures include terahertz detectors, mixers, oscillators and modulators. These two dimensional (2D) plasmonic
systems are not only well-suited for device integration, but also enable the broad tunability of underdamped plasma
excitations via an applied electric field. We present demonstrations of the coherent coupling of multiple voltage tuned
GaAs/AlGaAs 2D plasmonic resonators under terahertz irradiation. By utilizing a plasmonic homodyne mixing
mechanism to downconvert the near field of plasma waves to a DC signal, we directly detect the spectrum of coupled
plasmonic micro-resonator structures at cryogenic temperatures. The 2DEG in the studied devices can be interpreted as
a plasmonic waveguide where multiple gate terminals control the 2DEG kinetic inductance. When the gate tuning of the
2DEG is spatially periodic, a one-dimensional finite plasmonic crystal forms. This results in a subwavelength structure,
much like a metamaterial element, that nonetheless Bragg scatters plasma waves from a repeated crystal unit cell. A
50% in situ tuning of the plasmonic crystal band edges is observed. By introducing gate-controlled defects or simply
terminating the lattice, localized states arise in the plasmonic crystal. Inherent asymmetries at the finite crystal
boundaries produce an induced transparency-like phenomenon due to the coupling of defect modes and crystal surface
states known as Tamm states. The demonstrated active control of coupled plasmonic resonators opens previously
unexplored avenues for sensitive direct and heterodyne THz detection, planar metamaterials, and slow-light devices.
Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3, are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately ½ electron per surface unit cell at each GdTiO3/SrTiO3 interface. Insights into charge distribution, the influence of the electrostatic boundary conditions, and strong correlation effects will be presented.
The two-dimensional plasma resonance excited in the channel of a field effect transistor has recently been utilized as
the frequency-selective absorber in a monolithic far infrared plasmonic cavity detector. In this article we discuss the
relevant parameters pertaining to engineering the plasmonic cavity and an integrated detection element as
constituent elements of a resonant far infrared detector. The spectra of low-order plasmon modes in 18 μm and 34
μm long two-dimensional plasmonic cavities with 4 μm period grating gates have been measured. When the length
of the plasma cavity is significantly larger than the gate length or period, the cavity length rather than grating period
defines the plasmon wavevector. Electronic noise sources are considered; random telegraph noise is suggested as a
dominant noise source when the device is operated as a highly resistive bolometric detector.
Voltage-tunable plasmon resonances in a InGaAs/InP high electron mobility transistor (HEMT) are reported. The gate
contact consisted of a 0.5 micron period metal grating formed by electron-beam lithography. Narrow-band resonant
absorption of THz radiation was observed in transmission in the range 10 - 50 cm-1. The resonance frequency red-shifts
with increasing negative gate bias as expected. Photo-response to a tunable far-IR laser is reported. The device may
have application in high-frame-rate THz array detectors for spectral imaging with real-time chemical analysis.
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.
Biopolymers such as proteins, DNA and RNA fold into large, macromolecular chiral structures. As charged macromolecules, they absorb strongly in the terahertz due to large-scale collective vibrational modes; as chiral objects, this absorption should be coupled with significant circular dichroism. Terahertz circular dichroism (TCD) is potentially important as a biospecific sensor, unobscured by spectral features related to abiological material. We have constructed atomistic simulations and elastic continuum models of TCD. These models estimate the magnitude of the TCD and the relation between TCD spectroscopic signatures (zero crossings) and the structure, charge distribution and mechanical properties of biomaterials. A broad band TCD spectrometer based on a polarizing interferometer is developed to explore TCD in biomolecules in aqueous solution. Preliminary results on TCD in lysozyme in water at several terahertz frequencies is presented.
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.
We have investigated the terahertz photoresponse of a single semiconductor quantum dot, electrostatically defined by a sharp conducing Atomic Force Microscope tip in contact with a resonant tunneling diode structure. The quantum dot is excited by radiation from a Free Electron Laser in experiments both at room temperature and at cryogenic temperatures. Pronounced resonant tunneling features and classical rectification at frequencies from 0.3 to 3THz are observed in the I-V curves of these devices. These results demonstrate a novel approach to achieving terahertz excitation and studying transport in quantum dots.
We have explored near-infrared (NIR)--far-infrared (FIR) two-color optical experiments in quantum-confined semiconductor systems, using NIR radiation from a tunable cw Ti:Sapphire laser and intense and coherent FIR radiation from the UCSB Free-Electron Lasers. In this paper two recent experiments are discussed, both of which provide new insight into the internal structure and dynamics of confined excitons: (1) We have observed for the first time FIR internal transitions associated with the direct exciton in GaAs/AlGaAs quantum wells. The spectrum of excitations is enriched by the complexities of the valence band and differ significantly from simple reduced-mass, hydrogenic models. We provide a critical test of detailed calculations including the valence-band mixing of Bauer and Ando. (2) We have discovered resonant nonlinear optical mixing of NIR and FIR radiation, which results in strong near-bandgap emission lines, or optical sidebands. The sidebands appear when optically-created excitons are driven strongly by intense FIR fields. The frequencies of the sidebands are (omega) NIR +/- 2n(omega) FIR, where (omega) NIR is the interband exciton-creation frequency, (omega) FIR is the frequency of the driving field, and n is an integer. The intensity of the sidebands exhibits pronounced resonances as a function of applied magnetic field, which are well- explained in terms of virtual transitions between magnetically-tuned energy levels in the excitons.
We have measured the broad band terahertz response of state of the art InGaAs/AlAs and InAs/AlSb resonant tunneling diodes from 180 GHz to 3.6 THz using the free-electron lasers at UCSB. A tungsten whisker antenna in a conventional probe station is used to couple the far- infrared radiation into the device. Normalizing the resonant tunneling response with the off- resonant response allows us to circumvent the much slower RC time constant of the device and consequently enables a measurement of the relaxation time due to the quantum inductance.
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