E. R. Brown

Millimeter-wave, terahertz, and mid-infrared transmissionthrough common clothing

This letter reports electromagnetic transmission measurements through cloth samples from eight types of fabrics common in garments and baggage. The transmission at millimeter-wave and terahertz frequencies was measured with a custom ErAs:GaAs tunable photomixing spectrometer. The IR transmission between 3 and 8μm was measured with a Fourier-transform infrared spectrometer. All samples were usefully transparent at millimeter-wave frequencies (up to 300GHz) based on a 3dB criterion, but became progressively opaque at higher frequencies in a highly sample-dependent manner. This is explained by the samples becoming “optically dense” in the THz region, so that the transmission becomes exponentially dependent on sample thickness. The attenuation in the IR region is very high (⩾25dB) except in two samples (rayon and nylon), whose exceptional transparency (e.g., −12dB in nylon) is attributed to pores intrinsic to the material.

THz Medical Imaging: in vivo 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. A THz medical imaging system, operating at ~525 GHz center frequency with ~125 GHz of response normalized bandwidth is introduced and details regarding principles of operation are provided. Two promising medical applications of THz imaging are presented: skin burns and cornea. For burns, images of second degree, partial thickness burns were obtained in rat models in vivo over an 8 hour period. These images clearly show the formation and progression of edema in and around the burn wound area. For cornea, experimental data measuring the hydration of ex vivo porcine cornea under drying is presented demonstrating utility in ophthalmologic applications.

ErAs:GaAs photomixer with two-decade tunability and 12μW peak output power

This letter reports the fabrication and demonstration of an ErAs:GaAs interdigitated photomixer as a tunable THz source ranging from ∼20GHzto∼2THz, with 12μW maximum power typically around ∼90GHz. Each photomixer is coupled to a composite dipole-spiral planar antenna that emits a Gaussian-type beam into free space. The beam switches from dipole to spiral antenna behavior as the frequency increases. A distributed Bragg reflector is embedded in the device beneath the photomixer to increase its external quantum efficiency. The photomixer has a 900A thick silicon nitride coating which serves as an antireflection and passivation layer, and also improves the reliability and heat tolerance of the device.

MEMS and Si micromachined circuits for high-frequency applications

RF micromachining and microelectromechanical structure (MEMS) technology promise to provide an innovative approach in the development of effective and low-cost circuits and systems. This technology is expected to have significant application in the development of low-cost antenna arrays and reconfigurable apertures, due to its potential to support novel systems architectures. This paper presents a brief history and the state-of-the-art in the development of RF MEMS devices, with primary emphasis on switches and Si-micromachined circuit components for use in high-performance high-density on-wafer packaged circuits.

A 280-GHz Schottky Diode Detector in 130-nm Digital CMOS

A 2×2 array of Schottky-barrier diode detectors with an on-chip patch antenna and a preamplifier is fabricated in a 130-nm logic CMOS process. Each detector cell can detect the 25-kHz modulated 280-GHz radiation signal with a measured responsivity and noise equivalent power (NEP) of 21kV/W and 360pW/ √Hz, respectively. At 4-MHz modulation frequency, NEP should be about 40pW/ √Hz. At supply voltage of 1.2V, the detector consumes 1.6mW. By utilizing the detector, a millimeter-wave image is constructed, demonstrating its potential application in millimeter-wave and THz imaging.

On the strong and narrow absorption signature in lactose at 0.53THz

The authors report on the room-temperature characterization of a narrow absorption signature in an organic molecular solid (α-lactose monohydrate) centered around 530GHz and having a full width at half maximum as small as 23GHz. Three different spectrometric techniques were applied, two of them (tunable photomixing and tunable frequency multiplication) having a high enough resolution (⪡100MHz) to accurately curve fit the signature and determine that it is much better described by a Lorentzian than a Gaussian function. The Lorentzian model provides an estimate of 14ps for the damping time associated with this signature.

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 20.4%, 14.7%, 11.7%, 9.6%, and 7.4% at 0.2, 0.4, 0.6, 0.8, and 1 THz, respectively. Comparison of nine corneas hydrated from 79.1% to 91.5% concentration by mass demonstrated an approximately linear relationship between THz reflectivity and water concentration, with a monotonically decreasing slope as the frequency increases. The THz-corneal tissue interaction is simulated with a Bruggeman model with excellent agreement. THz applications to corneal dystrophy, graft rejection, and refractive surgery are examined from the context of these measurements.

Ultra-Wideband Multifunctional Communications/Radar System

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 m2. 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.

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. Based on these factors, a series of higher frequency filters has been designed, and simulation results are presented.

Efficient terahertz emission from ballistic transport enhanced n-i-p-n-i-p superlattice photomixers

The authors report on photomixing terahertz sources that overcome the transit time versus RC-time trade-off and allow for independent optimization of both of them, using a n-i-p-n-i-p superlattice. Furthermore, they take advantage of ballistic transport for reduced transit times. Apart from more favorable material parameters, In(Al)GaAs photomixers benefit from the advanced telecommunication laser technology around 1.55μm as compared to GaAs. In such devices, a terahertz-power output of 1μW has been achieved at 0.4THz at a photocurrent of 3.8mA. A comparison between corresponding GaAs- and InGaAs-based n-i-p-n-i-p photomixers reveals an improvement of performance by at least an order of magnitude for the latter one.