J. E. Bjarnason

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.

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.

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.

Attenuation contrast between biomolecular and inorganic materials at terahertz frequencies

Wideband photomixing spectroscopy is used in the present work to contrast the transmission spectra of macromolecules commonly found in biomaterials such as potato starch, wheat flour and cornstarch, and proteins (Cytoplex™), and micromolecules such as sucrose, and inorganic materials such as sodium bicarbonate, and calcium sulfate. Powdered samples were measured at 0.1–0.5THz frequencies. A significant difference in attenuation is found between these samples. At 300GHz starch shows an absorption coefficient of ∼6cm−1 whereas Cytoplex shows 1–3cm−1, while inorganic micromolecules have ∼1cm−1. The absorption in starch increases rapidly with frequency tending to follow a power law α=fn with n typically between 1.5 and 2.0. In contrast, protein materials display a slower dependence on frequency with n between 1.0 and 1.5, and simple molecules show the least n among all three categories. The difference between these ubiquitous macromolecular and micromolecular materials is explained in terms of water content a...

Room temperature, THz photomixing sweep oscillator and its application to spectroscopic transmission through organic materials

An all-solid-state continuous-wave sweep oscillator has been developed that provides high-resolution, transmission measurements between ∼30 GHz and 3 THz. It is based on difference-frequency generation between two cw frequency-offset lasers driving an ultrafast photoconductive mixer (photomixer). The output power around 100 GHz is approximately 10 μW, falling to about 1 μW around 1 THz and 0.1 μW around 3 THz. The sweep oscillator is used with two types of detectors: a hot electron bolometer for high-sensitivity measurements of weak absorption features below 1 THz, and a room temperature Golay cell for coarse measurements anywhere between 30 GHz and 3 THz. The sweep oscillator facilitates the rapid characterization of a broad variety of materials including inorganic solids, biological materials, liquids, and gases with far greater resolution (∼10 MHz), frequency accuracy (∼0.1 GHz), and spectral density (∼1 μW/MHz) than competitive wideband instruments such as Fourier-transform or time-domain spectrometer...

0.15–3.72THz absorption of aqueous salts and saline solutions

It has recently been suggested that near-field terahertz ionic contrast microscopy can be employed to image subtle changes in ionic concentrations arising from neuronal activity. To do so, however, requires that solvated ions exhibit significant absorbance at terahertz frequencies. The authors have investigated this issue and find that, at room temperature, the molar extinctions of both sodium chloride and guanidine hydrochloride are approximately two orders of magnitude below some previous measurements and are, therefore, too low to support the proposed imaging application.

Sensitivity measurement and analysis of an ErAs:GaAs coherent photomixing transceiver

A frequency domain transceiver is implemented with a pair of 780-nm-based ErAs:GaAs photomixers, one serving as the transmitter and the other as the receiver. The transceiver is all solid state, operated at room temperature, and has a fast response. The receiver detects the amplitude of the incoming electric field coherently and the sensitivity is limited by 1∕f mechanisms which are excited by a photovoltage that is always present in the receiver photomixer. This letter reports the first measurements and calculations of the signal-to-noise (S/N) ratio in the transceiver. Measurement yields a differential terahertz power-to-current sensitivity of 4mA∕W and noise equivalent power of 0.1pW∕Hz. The maximum S/N ratio was 58dB∕rtHz around 86GHz which is 17dB lower than our theoretical estimate. The discrepancy is attributed to radiative coupling losses and nonoptimal laser spatial overlap.

Analysis of Pulsed THz Imaging Using Optical Character Recognition

Using a reflection-based pulsed THz imaging system built upon our ErAs:GaAs photoconductive switch and a gated receiver, we quantify image quality at different detection bands (centered at 100, 400, and 600 GHz). Zero-bias Schottky diode detectors mounted in various waveguide sizes are used to tune the operational frequency bands of the imaging system, while the rest of the imaging system remains unchanged. The image quality is quantified by applying an optical character recognition (OCR) algorithm on THz images of 8-by-10 mm copper letters on a fiberglass substrate. Using the OCR success rate as a metric, we see a fivefold improvement in image quality from a 400 GHz to a 600 GHz imaging system, while our 100 GHz images do not produce any correct OCR results. In a comparison experiment performed at 600 GHz, the image signal-to-noise ratio (SNR) is degraded by placing increasing numbers of denim sheets (5.4 dB decrease in signal per layer) into the beam path. We find that the OCR success rate is roughly constant from one sheet to four sheets of denim (33-25 dB SNR) and then drops off sharply starting at five denim sheets.

Comparison of the THz absorption feature in lactose to related saccharides

Solid-state organic compounds such as &agr;-lactose-monohydrate and biotin have been shown to have narrow and intense THz absorption features at room temperature. Interest in lineshapes in the THz region is justified not only for practical reasons, since they are of crucial importance to spectroscopy-based identification of materials, but also because of the information the line-widths contain about the solid-state physics of the materials. The line-width of THz absorption features (generally from lattice vibrations) in solids is excepted to be inversely proportional to the scattering time of optical phonons. The line-width of absorption features might thus have implications on the solid-state physics of the material, in particular, the interaction of phonons and the phonon density of states. We use a continuous wave THz photomixing system to obtain a high resolution spectrum of &agr;-lactose-mohohydrate and analyze two of its lowest-frequency absorption lines. For comparison we measure the transmission spectra of 5 chemically related saccharides: melecitose, trehalose, maltose, cellobiose, and raffinose. Since &agr;-lactose-monohydrate has a stronger and narrower absorption feature than any of its related saccharides, this comparison study is an important step in understanding the mechanism of THz radiation absorption by organic solids and what line-widths to expect in THz spectroscopy.