This paper presents a computational procedure to simulate the time-domain behavior of photoconductive antennas made of semiconductor and metal materials. Physical modeling of semiconductor devices at terahertz regime can be achieved by applying joint electronic and electromagnetic procedures, e.g., solving a coupled system of equations inferred from Poisson's drift-diffusion and Maxwell's equations. A set of discrete equations are derived by applying a combined finite-difference methodology for the previous steady-state and the finite-difference time-domain procedure for the transient regime. The results for the radiated electric field at broadside direction show good agreement with the experimental results previously reported in the literature.

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length, and the bistability can be controlled by the cavity loss. However, the width of the bistable region is relatively constant over a range of the cavity loss. The bistability can be used to widely tune the ring laser across the entire L-band. The output optical SNRs are 20-dB better than those in a linear cavity laser. These results may increase the usefulness of the dual-wavelength bistable phenomenon in practical applications. INTRODUCTIONBreast cancer is one of the most common types of cancer and a major cause of death among women. However, nearly 70% of the cases can be cured if detected in time.Up to now, X-ray mammography seems to be the preferred method of diagnosis, but this technique presents important limitations [1,2]. The use of confocal microwave imaging (CMI) is a novel alternative [3] that appears to be a promising technique to detect malignant tumors at an early stage (as small as 2 mm). Its working principle is based on the dielectric contrast between the malignant-tumor tissues and healthy/benign-tumor ones. In CMI, the breast is illuminated with low-power ultra-wideband pulses generated by an array of antennas located close to it. The scattered signals are collected at the antennas and, after some signal processing, the tumors are identified and discriminated from clutter. The antennas, which should be small, must have ultra-wideband performance operating in the appropriate range of frequencies in order to achieve adequate resolution and penetration of the signals into the biological tissues. Resistively loaded bow-tie antennas [3], straight thin-wire dipole antennas [4], genetically designed V antennas [5], and so on, are some examples of antennas used in previous detection systems. These antenna systems, which are placed in a fixed position on top of the breast, consist of a single radiating element or a planar array of them, each of which is individually excited, and the field scattered by the breast is recorded in this same element. A signal post-processing algorithm serves to locate the tumor's position.In order to be able to increase the number of signals to postprocess, it is desirable to be able to excite each element and record the scattered field at all the elements of the array. For this purpose, the mutual coupling between the antennas must be minimized up to negligible levels so as not to prejudice the detection process. In this paper, a planar array of antennas is designed and tested with this goal in mind.A 2 ϫ 2 planar array of resistively loaded bow-tie antennas is printed on top of a lossy substrate that can freely rotate around the breast (Fig. 1). The conductivity losses of the substrate permits us to minimize the coupling between the antennas and enhance their broadband characteristics. The rotation of this layout at four different positions around the breast provides us with a significantly increased amount of information to be processed, in comparison with previous designs. This paper is organized as follows: sections 2 and 3 describe the geometry of the proposed ...