D.W. van der Weide

Microwave imaging via space-time beamforming: experimental investigation of tumor detection in multilayer breast phantoms

Microwave imaging via space-time (MIST) beamforming has been proposed recently for detecting small malignant breast tumors. In this paper, we extend the previously presented two-dimensional space-time beamformer design to three-dimensional (3-D), and demonstrate its efficacy using experimental data obtained with a multilayer breast phantom. The breast phantom consists of a homogeneous normal breast tissue simulant covered by a thin layer of skin simulant. A small synthetic malignant tumor is embedded in the breast phantom. We have developed several tumor simulants that yield the range of dielectric contrasts between normal and malignant tissue that are expected in clinical scenarios. A microwave sensor comprised of a synthetic planar array of compact ultrawide-band (UWB) antennas is immersed in a coupling medium above the breast tissue phantom. At each position in the array, the antenna transmits a synthetically generated pulse (1-11 GHz) into the phantom. The received backscatter signals are processed by a data-adaptive algorithm that removes the artifact caused by antenna reverberation and backscatter from the skin-breast interface, followed by 3-D space-time beamforming to image backscattered energy as a function of location. Our investigation includes a numerical (finite difference time domain) and experimental study of the UWB antenna performance in the immersion medium, as well as a study of the influence of malignant-to-normal breast tissue dielectric contrast on dynamic range requirements and tumor detectability. This paper represents the first experimental demonstration of 3-D MIST beamforming in multilayer breast phantoms with malignant-to-normal dielectric contrasts down to 1.5 : 1 for a 4-mm synthetic tumor.

Numerical and experimental investigation of an ultrawideband ridged pyramidal horn antenna with curved launching plane for pulse radiation

We report the numerical analysis and experimental characterization of an ultrawideband (UWB) antenna designed for radiating short microwave pulses. The antenna consists of a pyramidal horn, a ridge, and a curved launching plane terminated with resistors. The pyramidal horn is connected to the outer conductor of the coaxial feed and serves as the ground plane. The curved launching plane is connected to the central conductor of the coaxial feed. Detailed three-dimensional finite-difference time-domain (FDTD) simulations have been conducted to assist with the characterization of the antenna. FDTD results are compared with experimental data and are shown to be in good agreement. We demonstrate that the antenna exhibits a very low voltage standing wave ratio (/spl les/1.5) over a wide frequency range from 1 to 11 GHz and a very high fidelity (/spl ges/0.92). The spatial distribution of radiated energy is characterized both in the time domain, using transient field observations at various angles, as well as in the frequency domain, using single-frequency far-field radiation patterns. We conclude that this antenna offers high-fidelity transmission and reception of ultrashort microwave pulses with minimal distortion.

A compact branch-line coupler using discontinuous microstrip lines

We introduce a branch-line coupler using discontinuous microstrip lines whose size is significantly reduced relative to the standard design. We manipulate the reactive characteristics of discontinuities in its microstrip lines to achieve a physical size reduction of almost 60% with comparable performance.

Controllable valley splitting in silicon quantum devices

Silicon has many attractive properties for quantum computing, and the quantum-dot architecture is appealing because of its controllability and scalability. However, the multiple valleys in the silicon conduction band are potentially a serious source of decoherence for spin-based quantum-dot qubits. Only when a large energy splits these valleys do we obtain well-defined and long-lived spin states appropriate for quantum computing. Here, we show that the small valley splittings observed in previous experiments on Si–SiGe heterostructures result from atomic steps at the quantum-well interface. Lateral confinement in a quantum point contact limits the electron wavefunctions to several steps, and enhances the valley splitting substantially, up to 1.5 meV. The combination of electrostatic and magnetic confinement produces a valley splitting larger than the spin splitting, which is controllable over a wide range. These results improve the outlook for realizing spin qubits with long coherence times in silicon-based devices.

Linear tunable phase shifter using a left-handed transmission line

We demonstrate a compact, linear, and low loss variation hybrid phase shifter using a left-handed (LH) transmission line. For frequencies from 4.3 to 5.6 GHz, this phase shifter gives a nearly linear phase variation with voltage, with a maximum deviation of /spl plusmn/7.5/spl deg/. Within this frequency range, the maximum insertion loss is 3.6 dB, and the minimum insertion loss is 1.8 dB over a continuously adjustable phase range of more than 125/spl deg/, while minimum return loss is only 10.2 dB. Furthermore, this phase shifter requires only one control line, and it consumes almost no power.

Microwave ablation with a triaxial antenna: results in ex vivo bovine liver

We apply a new triaxial antenna for microwave ablation procedures to an ex vivo bovine liver. The antenna consists of a coaxial monopole inserted through a biopsy needle positioned one quarter-wavelength from the antenna base. The insertion needle creates a triaxial structure, which enhances return loss more than 10 dB, maximizing energy transfer to the tissue while minimizing feed cable heating and invasiveness. Numerical electromagnetic and thermal simulations are used to optimize the antenna design and predict heating patterns. Numerical and ex vivo experimental results show that the lesion size depends strongly on ablation time and average input power, but not on peak power. Pulsing algorithms are also explored. We were able to measure a 3.8-cm lesion using 50 W for 7 min, which we believe to be the largest lesion reported thus far using a 17-gauge insertion needle.

Radar Cross-Section Analysis of Backscattering RFID Tags

In this letter, we present a graphical method based on Greens analysis of scattering antennas to optimize the load impedances for achieving maximum modulated radar cross-section (RCS) of backscattering radio-frequency identification (RFID) tags. A planar tag antenna specifically designed for working in the ultra-high-frequency band is used to experimentally validate the method. The measured RCS of the loaded tag antenna shows acceptable agreement with the theory and simulated results. This method is especially valuable to the design of semipassive RFID tags when the communication range is solely determined by the backscattering modulation efficiency of tags.

Gas-absorption spectroscopy with electronic terahertz techniques

In this paper, we present the first gas-absorption spectra measured with an all-electronic terahertz spectrometer. This instrument uses phase-locked microwave sources to drive GaAs nonlinear transmission lines that produce picosecond pulses, enabling measurement of broad-band spectra. By sweeping the fundamental excitation, however, the spectrometer can also measure single lines with hertz-level precision, a mode of operation not readily available with optoelectronic terahertz techniques. Since this system is based on integrated circuits, it could ultimately function as an inexpensive gas-sensing system, e.g., for vehicle emissions, an application for which we analyze the sensitivity of a prototypical system.

Potential for detection of explosive and biological hazards with electronic terahertz systems

The terahertz (THz) regime (0.1–10 THz) is rich with emerging possibilities in sensing, imaging and communications, with unique applications to screening for weapons, explosives and biohazards, imaging of concealed objects, water content and skin. Here we present initial surveys to evaluate the possibility of sensing plastic explosives and bacterial spores using field–deployable electronic THz techniques based on short–pulse generation and coherent detection using nonlinear transmission lines and diode sampling bridges. We also review the barriers and approaches to achieving greater sensing–at–a–distance (stand–off) capabilities for THz sensing systems. We have made several reflection measurements of metallic and non–metallic targets in our laboratory, and have observed high contrast relative to reflection from skin. In particular, we have taken small quantities of energetic materials such as plastic explosives and a variety of Bacillus spores, and measured them in transmission and in reflection using a broadband pulsed electronic THz reflectometer. The pattern of reflection versus frequency gives rise to signatures that are remarkably specific to the composition of the target, even though the targets morphology and position is varied. Although more work needs to be done to reduce the effects of standing waves through time–gating or attenuators, the possibility of mapping out this contrast for imaging and detection is very attractive.

Spectroscopic THz near-field microscope

We demonstrate a scattering-type scanning near-field optical microscope (s-SNOM) with broadband THz illumination. A cantilevered W tip is used in tapping AFM mode. The direct scattering spectrum is obtained and optimized by asynchronous optical sampling (ASOPS), while near-field scattering is observed by using a space-domain delay stage and harmonic demodulation of the detector signal. True near-field interaction is determined from the approach behavior of the tip to Au samples. Scattering spectra of differently doped Si are presented.