The useful compromise between resolution and penetration power of the submillimeter or terahertz (THz) spectral region has long made it attractive for a variety of imaging applications. However, many of the demonstrations of imaging in this spectral region have used strategically oriented targets, especially favorable concealment materials, proximate imaging geometries, etc. This paper reports the results of studies aimed at better understanding the phenomenology of targets, the impact of this phenomenology on various active and passive imaging strategies, and most importantly, the development of imaging strategies that do not require the aforementioned special circumstances. Particular attention is paid to the relationship between active and passive images, especially with respect to how they interact with the illumination-and detector-mode structures of various imaging scenarios. It is concluded that the very large dynamic range that can be obtained with active single-mode systems (including focal-plane arrays) can be used in system designs to overcome the deleterious effects that result from the dominance of specular reflections in single-mode active systems as well as to strategically orient targets to obtain recognition. This will aid in the development of a much more robust and generally useful imaging technology in this spectral region.
We have performed, to our knowledge, the first direct measurement of plasma growth during the process of supercontinuum generation in a bulk material. We describe experimental and numerical results for sapphire and fused silica. 02000 Optical Society of America OCIS codes: (320.71 10) Ultrafast Nonlinear Optics; (320 2250) Femtosecond Phenomena. SummaryWhen a sufficiently energetic short laser pulse propagates through a medium, an explosive increase in bandwidth known as supercontinuum generation (SCG) can occur [l]. Several different physical processes contribute to this phenomenon, including the nonlinear Kerr effect, Raman scattering, ionization and dispersion [ 1-31. The interaction between these processes is complex; describing even qualitative features in the SCG spectrum generally requires treating the breakdown of the slowly varying envelope approximation and the interplay between the spatial and temporal evolution of the pulse. Yet the effort to understand SCG is worthwhile, as it has found wide application in such diverse areas as communications, precision measurement, generating tunable short pulse radiation, and even in clinical diagnostics.We recently introduced a pump-probe technique that directly measures the non-instantaneous contribution to SCG [4]. It allows the direct determination of the relevant Raman resonances and their coherence times without need for numerical simulation. By performing a numerical simulation, the strength of the Raman contribution relative to the instantaneous response can also be determined [SI.In this work, we describe a different approach that allows direct measurement of the plasma formed as an intense pulse undergoes SCG during its propagation through a solid medium, in this case, sapphire and fused silica. To our knowledge, this is the first such measurement performed. Plasma effects are important in the evolution of SCG, and our data serve to greatly constrain model calculations. We use a short pulse Ti:Sapphire system producing 0.5 mJ, 50 fs pulses at a 1 kHz repetition rate; we then split our pulses for use in a pumpprobe experiment. The pump is an 800 run pulse; the cross-polarized 400 nm probe is derived from the second harmonic of the short pulse. The energies of both pulses vary, but the pump is of order several @/pulse while the probe is well under 1 @/pulse. As the pump begins to broaden, it also ionizes the surrounding medium via multi-photon ionization and avalanche ionization [3,6]. The resulting plasma reflects the probe by varying degrees when the probe is delayed with respect to the pump. We measure this retro-reflection as a function of the delay between the two pulses. We observe a sharp rise in plasma density on the leading edge of the pump, but with a rise time somewhat longer than that of the pump. The plasma decay occurs over several picoseconds, depending on excitation conditions. We compare our results to a numerical model that accounts for all the major processes in play. References 2. 3. 4. . . R. R. Alfano(ed.), The Supercontinuum Laser Sou...
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