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Terahertz (THz) imaging of corneal tissue water content (CTWC) is a proposed method for early, accurate detection and study of corneal diseases. Despite promising results from ex vivo and in vivo cornea studies, interpretation of the reflectivity data is confounded by the contact between corneal tissue and rigid dielectric window used to flatten the imaging field. This work develops a novel imaging system and image reconstruction methods specifically for nearly spherical targets such as human cornea. A prototype system was constructed using a 650 GHz multiplier source and Schottky diode detector. Resolution and imaging field strength measurement from characterization targets correlate well with those predicted by the quasioptical theory and physical optics analysis. Imaging experiments with corneal phantoms and ex vivo corneas demonstrate the hydration sensitivity of the imaging system and reliable measurement of CTWC. We present successful acquisition of non-contact THz images of in vivo human cornea, and discuss strategies for optimizing the imaging system design for clinical use.

The coupling of an electromagnetic plane wave to a thin conductor depends on the sheet conductance of the material: a poor conductor interacts weakly with the incoming light, allowing the majority of the radiation to pass; a good conductor also does not absorb, reflecting the wave almost entirely. For suspended films, the transition from transmitter to reflector occurs when the sheet resistance is approximately the characteristic impedance of free space (Z 0 = 377 Ω). Near this point, the interaction is maximized, and the conductor absorbs strongly. Here we show that monolayer graphene, a tunable conductor, can be electrically modified to reach this transition, thereby achieving the maximum absorptive coupling across a broad range of frequencies in terahertz (THz) band. This property to be transparent or absorbing of an electromagnetic wave based on tunable electronic properties (rather than geometric structure) is expected to have numerous applications in mm wave and THz components and systems.

A pulsed terahertz (THz) imaging system and millimeter-wave reflectometer were used to acquire images and point measurements, respectively, of five rabbit cornea in vivo. These imaging results are the first ever produced of in vivo cornea. A modified version of a standard protocol using a gentle stream of air and a Mylar window was employed to slightly dehydrate healthy cornea. The sensor data and companion central corneal thickness (CCT) measurements were acquired every 10-15 min over the course of two hours using ultrasound pachymmetry.. Statistically significant positive correlations were established between CCT measurements and millimeter wave reflectivity. Local shifts in reflectivity contrast were observed in the THz imagery; however, the THz reflectivity did not display a significant correlation with thickness in the region probed by the 100 GHz and CCT measurements. This is explained in part by a thickness sensitivity at least 10× higher in the mm-wave than the THz systems. Stratified media and effective media modeling suggest that the protocol perturbed the thickness and not the corneal tissue water content (CTWC). To further explore possible etalon effects, an additional rabbit was euthanized and millimeter wave measurements were obtained during death induced edema. These observations represent the first time that the uncoupled sensing of CTWC and CCT have been achieved in vivo.

Terahertz (THz) spectral properties of human cornea are explored as a function of central corneal thickness (CCT) and corneal water content, and the clinical utility of THz-based corneal water content sensing is discussed. Three candidate corneal tissue water content (CTWC) perturbations, based on corneal physiology, are investigated that affect the axial water distribution and total thickness. The THz frequency reflectivity properties of the three CTWC perturbations were simulated and explored with varying system center frequency and bandwidths (Q-factors). The modeling showed that at effective optical path lengths on the order of a wavelength the cornea presents a lossy etalon bordered by air at the anterior and the aqueous humor at the posterior. The simulated standing wave peak-to-valley ratio is pronounced at lower frequencies and its effect on acquired data can be modulated by adjusting the bandwidth of the sensing system. These observations are supported with experimental spectroscopic data. The results suggest that a priori knowledge of corneal thickness can be utilized for accurate assessments of corneal tissue water content. The physiologic variation of corneal thickness with respect to the wavelengths spanned by the THz band is extremely limited compared to all other structures in the body making CTWC sensing unique amongst all proposed applications of THz medical imaging.

We have designed, simulated, fabricated, and tested an ultra-wideband (UWB) multifunctional communication and radar system utilizing a single shared transmitting antenna aperture. Two surface acoustic wave bandpass chirp filters were used to modulate the radar and communications pulses, generating linear frequency modulation waveforms with opposite slope factors. The system operates at a center frequency of 750 MHz with 500 MHz of instantaneous bandwidth. The measured range resolution is 63 cm (25 in) using targets with a radar cross section of 2.7 [m.sup.2]. The probability of detection was measured to be 99%, and the probability of false alarm was 7% with the communication and radar systems operating simultaneously. The bit error rate for simultaneous communication at 1 Mb/s, and radar at 150 kHz pulse repetition frequency and 1.5-ns pulsewidth is 2e-3. Our UWB multifunctional system demonstrates the ability to simultaneously interrogate the environment and communicate through a shared transmitting antenna aperture, while realizing a simple system architecture with low output power and not employing time-division multiplexing. Index Terms--Chirp, communications, radar, RF.

A procedure is proposed for computing shot noise in resonant-tunneling structures with an arbitrary number of quantum wells.

The interaction of a high-power microwave beam with a face-centered-cubic photonic crystal is studied. A Sinus-6 high-power relativistic repetitively pulsed electron beam accelerator was used to drive a slow wave structure in vacuum to generate 450 MW at 9.65 GHz. It is shown that the photonic crystal, comprised of a periodic arrangement of air holes in a dielectric host, is capable of performing as a quasi-optical reflector similar to a metal. The crystals can be designed to operate as efficient frequency-selective reflectors over narrow frequency ranges. We propose that photonic crystals can have many applications in high-power microwave research.

We investigate the possibility of sensing opioid drugs, such as fentanyl, by their THz electromagnetic signatures. The methods include both computer modeling and experiments. Molecular dynamics simulations predict that fentanyl should display THz resonances, with several of them occurring below 1.0 THz; the lowest one is at around 0.337 THz (337 GHz). Spectroscopy measurements were conducted on oxycodone, which was used as a surrogate for fentanyl. They display vibrational absorption resonances between ∼1.4 and 1.6 THz. [ABSTRACT FROM AUTHOR]

Experimental studies conducted on GaAs/AlGaAs resonant tunneling diodes help examine the random-telegraph noise (RTN) and the power-spectra of low-frequency fluctuation in these diodes. A noise model based on experimental data explains the observed low-frequency fluctuations in the device. Experimental data suggest defect-induced hopping of electrons from the emitter to the quasi-bound state in the quantum well as the source for the low-frequency noise. Analysis of RTN data helps determine trap density and the capture cross section of the trap.

Minor dislocations due to lattice-mismatched InAs/AlSb resonant-tunneling diodes on GaAs substrates do not affect the resonant-tunneling process of the diodes at room temperature. The dislocations enhance the valley and peak current values in a bias polarity-independent manner, enabling the incorporation of the diodes in monolithic integrated GaAs substrate circuits.

A proposed heterojunction bipolar transistor/resonant tunneling diode (HBT/RTD) logic family is tested using SPICE simulation language. Results show that the proposed logic family provides high-speed switching capabilities with low static power dissipation. It also provides significant reductions in static power for high-speed digital logic applications. Further, the HBT/RTD logic and HBT-based ECL logic can be combined in the same integrated circuit.

A stationary-state method is employed to study the shot noise occuring in the first quasi-bound level of a double-barrier resonant-tunneling diode. An anlytical model characterizes the inhibition and enhancement of shot noise at the positive differential region and negative differential region, respectively. These are accounted for by the quantum-well charge storage in the first quasi-bound level. Experimental results verify the validity of the model.

An experimental study which employed a semiconfocal open cavity resonator for stabilizing a resonant tunneling diode waveguide oscillator at the frequency of103 GHz is presented. The approach led to an oscillator linewidth of around 10 kHz, a factor of 100 times narrower than unstabilized waveguide oscillators, due to the optimum quality factor of the open cavity. The resulting quasioptical oscillator thus becomes appropriate for operation in the submillimeter wavelength.

We report on the design and fabrication of ultrahigh current density GaN/AlN double barrier resonant tunneling diodes grown via rf-plasma assisted molecular-beam epitaxy. The device structure was grown on a metal-organic chemical vapor deposition GaN-on-sapphire template. The devices displayed repeatable room temperature negative differential resistance with peak tunneling current densities (Jp) between 637 and 930 kA/cm2. Analysis of temperature dependent measurements revealed the presence of severe self-heating effects, which allow strong phonon scattering that deteriorates the electron quantum transport. Finally, a qualitative comparison to the same structure grown on a low dislocation density freestanding GaN substrate has shown that sapphire-based templates are a feasible alternative. [ABSTRACT FROM AUTHOR]

Focuses on a study which examined the spectra of the responsively and noise equivalent power of liquid-helium-cooled indium antimonide detectors as a function of magnetic field. Details of the measurements using a Fourier transform spectrometer; Methodology of the study; Results and discussion.

Provides information on a study which discussed the measurement of the absorption coefficient of two high-purity n-InSb samples in a certain range using Fourier transform spectroscopy. Effect of the non-Drudian absorption on the performance of InSb hot-electron bolometers; Function of liquid-helium cooled n-type InSb; Factor that makes InSb a very sensitive bolometer; Way of determining the absorption coefficient.