FOREWORDIn recent years, the field of Terahertz (THz) science and technology has entered a completely new phase of unprecedented expansion that is generating every growing levels of broad-based international attention. Indeed, the plethora of activities that have arisen recently in both the technology and scientific arenas associated with the THz frequency domain -i.e., usually defined as the portion of the submillimeter-wavelength electromagnetic (EM) spectrum between approximately 1 millimeter (300 GHz) and 100 micrometers (3 THz) -suggest that the field might be attempting to undergo a dramatic transition that could lead to long-awaited payoffs in a number of application areas. The inherent advantages and potential payoffs of the THz regime for military and security relevant applications have long stood as an important driver of interest in this science and technology area. For example, this extremely expansive and spectrally unique portion of the EM spectrum was initially of high interest for such applications as space-based communications, upper atmospheric sensing and communications, and potentially for short-range terrestrial communications and non-intrusive package screening. However, the very rapid growth in more recent years is arguably most closely linked to the potential payoffs of THz sensing and imaging (THz-S&I) for an array of military and security applications. These applications include the spectroscopic-based detection identification and characterization of chemical and biological (CB) agents and materials, remote and standoff early-warning for CB warfare threats, and video-rate imaging of concealed weapons and explosives, just to name a few. In addition, these same THz-S&I capabilities have a close synergy and dual-use potential for private-sector application areas as biological science, medical diagnostics, pharmaceutical characterization and security screening.While recent developments in the THz field offers promise that a broad spectrum of commercial and scientific payoffs may be on the horizon, there still remain significant technological challenges that will need to be resolved. In particular, the refinement of THz systems is needed to enable very near-term payoffs such as security screening and the detailed characterization of materials such as explosives and pharmaceuticals; a significant advancement is needed in THz source and detector technology to enable medium-range applications such as remoteMandoff detection and identification of biological and chemical agents; and, new breakthroughs in sensor architectures and probing techniques will be required for enabling far-future applications such as detailed spectroscopic characterizations of biological molecules and nanoscale systems. This special issue presents some of the leading fundamental research efforts that working towards the realization of practical THz-S&I capabilities for military and security applications. Specifically, the papers that follow span an array of pertinent THz Science

Abstract-There is a n increasing demand for compact watt-level CO-herent sources in the millimeter-and submillimeter-wave region. The approach that we have taken to satisfy this need is to fabricate twodimensional grids loaded with oscillators and multipliers for quasi-optical coherent spatial combining of the outputs of large numbers of lowpower devices. This was first demonstrated through the successful fabrication of monolithic arrays with 2000 Schottky diodes. Watt-level power outputs were obtained in doubling to 66 GHz. In addition, a simple transmission-line model was verified with a quasi-optical reflectometer that measured the array impedance. This multiplier array work is being extended to novel tripler configurations employing blocking barrier devices. The technique has also been extended to oscillator configurations where the grid structure is loaded with negative-resistance devices. This was first demonstrated using Gunn devices. More recently, a 25-element MESFET grid oscillating at 10 GHz exhibited power combining and self-locking. Currently, this approach is being extended to a 100-element monolithic array of Gunn diodes. This same approach should be applicable to planar vacuum electron devices such as the submillimeter-wave BWO and vacuum FET.

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Articles you may be interested inEffect of growth oxygen pressure on anisotropic-strain-induced phase separation in epitaxial La0.67Ca0.33MnO3/NdGaO3 (001) films J. Appl. Phys. 113, 203701 (2013); 10.1063/1.4807293High field transport and magnetic properties of RECo(P/As)O AIP Conf.

Abstract-The dependence of the I-V characteristic of the negative differential resistance (NDR) devices on the power level and frequency of the rf input signal has been theoretically analyzed with a modified large-and small-signal nonlinear circuit analysis program [1,2]. The NDR devices we used in this work include both the tunnel diode