Sensitivity and noise in THz electro-optic upconversion radiometers.

This paper presents a study of noise in room-temperature THz radiometers that use THz-to-optical upconversion followed by optical detection of thermal radiation. Despite some undesired upconverted thermal noise, no noise is intrinsically introduced by efficient electro-optic modulation via a sum-frequency-generation process in high quality factor (Q) whispering-gallery mode (WGM) resonators. However, coherent and incoherent optical detection results in fundamentally different noise characteristics.

Near-UV electroluminescence in unipolar-doped, bipolar-tunneling GaN/AlN heterostructures.

Cross-gap light emission is reported in n-type unipolar GaN/AlN double-barrier heterostructure diodes at room temperature. Three different designs were grown on semi-insulating bulk GaN substrates using molecular beam epitaxy (MBE). All samples displayed a single electroluminescent spectral peak at 360 nm with full-width at half-maximum (FWHM) values no greater than 16 nm and an external quantum efficiency (EQE) of ≈0.

Terahertz vibrational signature of bacterial spores arising from nanostructure decorated endospore surface.

This theoretical effort is the first to explore the possible hypothesis that terahertz optical activity of Bacillus spores arises from normal vibrational modes of spore coat subcomponents in the terahertz frequency range. Bacterial strains like Bacillus and Clostridium form spores with a hardened coating made of peptidoglycan to protect its genetic material in harsh conditions. In recent years, electron microscopy and atomic force microscopy has revealed that bacterial spore surfaces are decorated with nanocylinders and honeycomb nanostructures.

Broadband impedance match to two-dimensional materials in the terahertz domain.

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  = 377 Ω). Near this point, the interaction is maximized, and the conductor absorbs strongly.

High extinction ratio terahertz wire-grid polarizers with connecting bridges on quartz substrates.

A terahertz (THz) wire-grid polarizer with metallic bridges on a quartz substrate was simulated, fabricated, and tested. The device functions as a wide-band polarizer to incident THz radiation. In addition, the metallic bridges permit the device to function as a transparent electrode when a DC bias is applied to it.

THz and mm-Wave Sensing of Corneal Tissue Water Content: Electromagnetic Modeling and Analysis.

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).

THz and mm-Wave Sensing of Corneal Tissue Water Content: Sensing and Imaging Results.

A pulsed terahertz (THz) imaging system and millimeter-wave reflectometer were used to acquire images and point measurements, respectively, of five rabbit cornea . These imaging results are the first ever produced of 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.

AC conductivity parameters of graphene derived from THz etalon transmittance.

THz frequency-domain transmittance measurements were carried out on chemical-vapor-deposited (CVD) graphene films transferred to high-resistivity silicon substrates, and packaged as back-gated graphene field effect transistors (G-FETs). The graphene AC conductivity σ(ω), both real and imaginary parts, is determined between 0.2 and 1.

Tunable millimeter and sub-millimeter spectral response of textile metamaterial via resonant states.

We report on a new textile metamaterial created by adding metal wires directly into the polymer yarn. Split-ring resonator-like extended states are created. Simulations revealed that the extended states can be easily tuned via the geometry.

Narrow terahertz attenuation signatures in Bacillus thuringiensis.

Terahertz absorption signatures from culture-cultivated Bacillus thuringiensis were measured with a THz photomixing spectrometer operating from 400 to 1200 GHz. We observe two distinct signatures centered at ∼955 and 1015 GHz, and attribute them to the optically coupled particle vibrational resonance (surface phonon-polariton) of Bacillus spores. This demonstrates the potential of the THz attenuation signatures as "fingerprints" for label-free biomolecular detection.

THz Medical Imaging: Hydration Sensing.

The application of THz to medical imaging is experiencing a surge in both interest and federal funding. A brief overview of the field is provided along with promising and emerging applications and ongoing research. THz imaging phenomenology is discussed and tradeoffs are identified.

Terahertz sensing in corneal tissues.

This work introduces the potential application of terahertz (THz) sensing to the field of ophthalmology, where it is uniquely suited due to its nonionizing photon energy and high sensitivity to water content. Reflective THz imaging and spectrometry data are reported on ex-vivo porcine corneas prepared with uniform water concentrations using polyethylene glycol (PEG) solutions. At 79% water concentration by mass, the measured reflectivity of the cornea was 20.

Second-order bandpass terahertz filter achieved by multilayer complementary metamaterial structures.

We propose a multilayer complementary metamaterial structure fabricated on a crystal quartz substrate measuring between 100 and 700 GHz. The concept of a second-order terahertz bandpass filter is realized by this structure, and it offers a superior quality factor, steepness of skirts, and out-of-band rejection. Physical limitations on the quality factor and insertion loss have also been studied, including the skin depth of the metal and the optical phonon resonance in quartz.

Terahertz imaging of biological tissues.

A reflective THz imaging system sensitive to small variations in water concentrations has been developed. Biological tissues such as skin, eyes and teeth were imaged to ascertain the systems response to tissue hydration. Difference in water concentrations translated to contrast in the THz images.

Terahertz sensing of corneal hydration.

An indicator of ocular health is the hydrodyanmics of the cornea. Many corneal disorders deteriorate sight as they upset the normal hydrodynamics of the cornea. The mechanisms include the loss of endothelial pump function of corneal dystophies, swelling and immune response of corneal graft rejection, and inflammation and edema, which accompany trauma, burn, and irritation events.

Towards medical terahertz sensing of skin hydration.

Terahertz imaging has shown promise as a tool for noninvasive in-vivo detection of skin abnormalities, including skin cancer, burns, scars, and wounds due to its low non-ionizing photon energy and ability to penetrate clothing and gauze. This study examines whether low-level bulk differences in the water content between hyperhydrated and dehydrated skin can be detected using a scanning, reflective THz imaging system. Our results show an 8.

Tissue mimicking materials for dental ultrasound.

While acoustic tissue mimicking materials have been explored for a variety of soft and hard biological tissues, no dental hard tissue mimicking materials have been characterized. Tooth phantoms are necessary to better understand acoustic phenomenology within the tooth environment and to accelerate the advancement of dental ultrasound imaging systems. In this study, soda lime glass and dental composite were explored as surrogates for human enamel and dentin, respectively, in terms of compressional velocity, attenuation, and acoustic impedance.

Towards THz medical imaging; reflective imaging of animal tissues.

A reflective THz imaging system has been developed, and features a photoconductive switch and zero-bias Schottky diode detector. The system was used to image deli meats and can distinguish between muscle and adipose tissue based on water content. This capability is a step towards the development of THz medical imaging systems.

A flexible, conformal ultrasound array for medical imaging.

An ultrasound imaging system is under development that features a flexible transducer that can be wrapped conformally around curved surfaces of the body. The device is intended to deliver high image quality without the need for mechanical scanning, and will benefit medical personnel with limited ultrasound training. A 2 x 8 element piezoelectric transducer prototype and 15.

Ultrasound detection of submerged dental implants through soft tissue in a porcine model.

Statement Of Problem: Current methods of measuring soft tissue thickness over potential dental implant sites and locating submerged implants may be imprecise or invasive.

Purpose: The purpose of this study was to develop and demonstrate proof of the concept of a customized ultrasound imaging system in locating and measuring the depth of implants submerged beneath soft tissue.

Material And Methods: A complete ultrasound system, including a customized soft tissue-matched transducer, transceiver, and digital signal processing algorithms, was created for the specific application of detecting dental implants anchored in bone beneath soft tissue.

Portable X-band system for solution state dynamic nuclear polarization.

This paper concerns instrumental approaches to obtain large dynamic nuclear polarization (DNP) enhancements in a completely portable system. We show that at fields of 0.35 T under ambient conditions and at X-band frequencies, 1H enhancements of >100-fold can be achieved using nitroxide radical systems, which is near the theoretical maximum for 1H polarization using the Overhauser effect at this field.