J. Martyn Chamberlain

Optical properties of tissue measured using terahertz pulsed imaging.

The first demonstrations of terahertz imaging in biomedicine were made several years ago, but few data are available on the optical properties of human tissue at terahertz frequencies. A catalogue of these properties has been established to estimate variability and determine the practicality of proposed medical applications in terms of penetration depth, image contrast and reflection at boundaries. A pulsed terahertz imaging system with a useful bandwidth 0.5-2.5 THz was used. Local ethical committee approval was obtained. Transmission measurements were made through tissue slices of thickness 0.08 to 1 mm, including tooth enamel and dentine, cortical bone, skin, adipose tissue and striated muscle. The mean and standard deviation for refractive index and linear attenuation coefficient, both broadband and as a function of frequency, were calculated. The measurements were used in simple models of the transmission, reflection and propagation of terahertz radiation in potential medical applications. Refractive indices ranged from 1.5 ± 0.5 for adipose tissue to 3.06 ± 0.09 for tooth enamel. Significant differences (P < 0.05) were found between the broadband refractive indices of a number of tissues. Terahertz radiation is strongly absorbed in tissue so reflection imaging, which has lower penetration requirements than transmission, shows promise for dental or dermatological applications.

Applications of terahertz (THz) technology to medical imaging

An imaging system has been developed based on pulses of Terahertz (THz) radiation generated and detected using all- optical effects accessed by irradiating semiconductors with ultrafast pulses of visible laser light. This technique, commonly referred to as T-Ray Imaging or THz Pulse Imaging (TPI), holds enormous promise for certain aspects of medical imaging. We have conducted an initial survey of possible medical applications of TPI and demonstrated that TPI images show good contrast between different animal tissue types. Moreover, the diagnostic power of TPI has been elicidated by the spectra available at each pixel in the image, which are markedly different for the different tissue types. This suggests that the spectral information inherent in TPI might be used to identify the type of soft and hard tissue at each pixel in an image and provide other diagnostic information not afforded by conventional imagin techniques. Preliminary TPI studies of pork skin show that 3D tomographic imaging of the skin surface and thickness is possible, and data from experiments on models of the human dermis are presented which demonstrate that different constituents of skin have different refractive indices. Lastly, we present the first THz image of human tissue, namely an extracted tooth. The time of flight of THz pulses through the tooth allows the thickness of the enamel to be determined, and is used to create an image showing the enamel and dentine regions. Absorption of THz pulses in the tooth allows the pulp cavity region to be identified. Initial evidence strongly suggests that TPI my be used to provide valuable diagnostic information pertaining to the enamel, dentine, and the pump cavity.

Micromachined components for terahertz circuits and systems

This paper discusses the need for micromachined components in very high frequency (terahertz) electronic circuits. It is shown that, with the use of micromachining, conventional rectangular rectangular technology can be scaled down to the dimensions required for these high frequency circuits. Furthermore, the fabrication techniques are capable of producing the high quality surface necessary for low loss components. Fabrication processes are presented together with the results of electrical characterisation.

Use of novel photoresists in the production of submillimeter-wave integrated circuits

A new technique is reported for micro-machining millimeter and submillimeter-wave rectangular waveguide components using an advanced thick film UV photoresist known as EPONTM SU-8. The recent introduction of this resist has been of great interest to the millimeter-wave and terahertz micro-machining communities, as it is capable of producing features up to 1 mm in height with very high aspect ratios in only a single UV exposure. It therefore represents a possible low-cost alternative to the LIGA process. S-parameter measurements on the new rectangular waveguides show that they achieve lower loss than those produced using other on-chip fabrication techniques, they have highly accurate dimensions, are physically robust, and cheap and easy to manufacture.

Quasi-optical characterization of waveguides at frequencies above 100 GHz

We analyze the precision of a quasi-optical null-balance bridge reflectometer in measuring waveguide characteristic impedance and attenuation using a one-port de-embedding after taking into account errors due to imperfect coupling of two fundamental Gaussian beam. In order to determine the desired precision, we present in-waveguide measurements of characteristic impedance and attenuation for a WR-8 adjustable precision short in the 75-110 GHz frequency range using a Hewlett-Packard HP 8510 vector network analyzer.

Effects of frequency on image quality in terahertz-pulsed images

Terahertz imaging is an emerging modality, with potential for medical applications, using broadband sub-picosecond electromagnetic pulses in the range of frequencies between 100 GHz and 100 terahertz (THz). Images can be formed using parameters derived from the time domain, or at the range of frequencies in the Fourier domain. The choice of frequency at which to image may be an important factor for clinical applications. Image quality as a function of frequency was assessed for a terahertz pulsed imaging system by means of; (i) image noise measurements on a specially designed step wedge, and (ii) modulation transfer functions (MTF) derived from a range of spatial frequency square wave patterns. It was found that frequencies with larger signal magnitude gave lower image noise, measured using relative standard deviation (standard deviation divided by mean) for uniform regions of interest of the step wedge image. MTF results were as expected, with higher THz frequency signals demonstrating a consistently higher MTF and higher spatial frequency limiting resolution than the lower THz frequencies. There is a trade-off between image noise and spatial resolution with image frequency. Higher frequencies exhibit better spatial resolution than lower frequencies, however the decrease in signal power results in a degradation of the image.

Highly nonlinear capacitance in quantum well/barrier heterostructures: application to harmonic multiplication at terahertz frequency

In this communication, we report on the design and the fabrication of quantum well barrier varactor structures with state of the art results in terms of capacitance ratio over a narrow voltage range. Basically, the fact to consider is a barrier cladded by two quantum wells with respect to a single barrier heterostructure. It has several consequences for the non linear character of the device. The capacitance mechanism is governed at low voltage by the electron population rates off the quantum well rather than the conventional depletion mode process. A true band gap capacitance engineering is here demonstrated with thee kinds of structures either in the InP material system with a InGaAs/InAs/AlAs heterostructure or in the GaAs material system with GaAs/InGaAs/AlAs pseudomorphic epilayers and lattice matched AlGaAs/GaAs/AlAs heterojunctions. Self- consistent simulations, based on the solution of Poisson and Schroedinger coupled equations system, were performed in order to calculate the electron wave function and the conduction band bending. High capacitance ratios can be predicted depending on material parameters and structure geometry. Test samples were then fabricated and rf tested. The devices very high capacitance ratios is excess of 5 to 1 over a 1 Volt range.

Fabrication and characterization of integrated waveguide for use at G-band

The fabrication of air-filled rectangular metal-pipe waveguide using a lithographically-based technique has recently been reported. This type of waveguide, together with other passive components such as antennas, couplers, mixers and filters may offer a realistic route to terahertz frequency integrated circuits in view of the compatibility of the fabrication technique with those of standard semiconductor processing. In this contribution, we report the fabrication of a range of waveguide components for operation at frequencies of up to 300 GHz. These measurements represent the highest frequency characterization study so far reported for a micromachined passive structure of this type and provide proof of TE10 propagation with the expected cut-off frequency. Numerous measurements have been taken using G-band (WR-F) guide which show an attenuation loss of approximately 0.6 dB per guide wavelength at 200 GHz. This low value of attneuation shows that these micromachined waveguide are viable components for use in integrated circuits at terahertz frequencies. The insertion loss repeatability (due to mismatch effects at the ports of the waveguides) has been measured and has been shown to be better than plus or minus 0.5 dB. Preliminary results are presented for J-band (WR-3) waveguide which clearly shows the cut off frequency.

Application of quantum barrier devices as microwave mixers

Experimental results together with computer simulations, provide evidence to support the premise that quantum barrier devices can indeed yield sufficient improvements over conventional Schottky diodes, as to warrant their further study, especially for low-power millimeter and sub-millimeter wave receivers and where local oscillator power is at a premium.

Investigation of doped multiple quantum well structures using modulation spectroscopy

We have studied the influence of silicon doping on the electric-field-induced Stark shifts of the subband states for AlGaAs/GaAs/AlGaAs multiple quantum well structures. The investigations were performed applying electroreflectance (ER), photoreflectance (PR), and for comparison photoluminescence (PL) spectroscopy. The analysis of the PR spectra yields the transition energies at approximately zero electric field, which are in good agreement with PL peaks. The shift of the Er ground state heavy hole transition with respect t the PR data and accompanying self-consistent calculations allow the determination of the field strength F at each gate voltage with high accuracy. On this bases, the other transitions can be identified by their expected shift. It will be shown that all transitions involving the first heavy hole subband show a strong increase of their signals with increasing field strength. The plot of F2 versus gate voltage for both samples fits a straight line, slope of which provides the corresponding concentrations of the ionized donors.