Compared to Millimeter waves THz frequencies can provide an almost magnitude higher resolution, which enables detection of small threats at larger distances. Here we present an active THz standoff imaging system based on commercially available BWO sources and a thermal detector. At a excitation frequency of 720GHz an almost diffraction limited resolution was achieved of about 1.6mm at 1.2m distance to the target. With signal to noise ratio of 35dB detection of metal and ceramic weapons covered by different clothing is demonstrated. A detailed system analysis is done, which led to limiting factors of the proposed system.

The authors present a concept of a scanning near-field optical microscope for second harmonic imaging. The microscope is based on uncoated silicon atomic force microscope tips, which provide high intensity transmission in the midinfrared spectral range. An approximately three times larger contrast was found for the second harmonic compared to the linear signal. By using a lithographically designed sample of an array of gold nanoparticles the capability of imaging structures much smaller than the fundamental and second harmonic wavelength is demonstrated.

We investigate the formation dynamics of self-assembled polyelectrolyte multilayers on glass substrates by in situ and ex situ second harmonic generation (SHG) measurements and atomic force microscopy (AFM). The time dependence of the SHG signal during the adsorption process is attributed to a time dependent surface potential of the polyelectrolyte film. The dynamics can be quantitatively understood using a random sequential adsorption (RSA) model for the buildup of a film consisting of polyelectrolyte disks with polydisperse sizes. Differences between wet and dry films are also investigated.

It is now known that plasmon oscillations supported by nanostructured metal thin films of fractal morphology, can result in large local fields and strong enhancement of optical phenomena, for example Raman scattering. The localized plasmons, acting like nano-antennas, can concentrate very large electromagnetic energy in nanometer- sized areas, hot spots, and provide particularly strong enhancement of optical responses, in a very broad spectral range. Our new experimental results show up position dependence of the hot spots on the polarization state of the light. Moreover as expected from recent theoretical predictions, on this kind of thin percolating films, there is a dramatic enhancement of the second harmonic generation (2(omega) ) out of the specular directions. This unusual diffuse SHG could be connected to possible chirality of the percolating metallic films, which is expected to manifest itself as change in the hot-spot distribution for the left and right circularly polarized incident light.

Summary form only given. In random metal-dielectric films where the metal coverage of the dielectric substrate is close to the percolation threshold, disorder-induced localization of plasmons occurs, resulting in huge fluctuations of local fields. One of the most interesting, theoretical predictions for such percolation films is that nonlinear light scattering at the nth frequency harmonic n/spl omega/ of an incident beam can be significantly enhanced and is characterized by a broad, nearly isotropic angular distribution. According to theory, this effect, which was denoted in as percolation-enhanced nonlinear scattering (PENS), is caused by the huge local-field fluctuations associated with the localized plasmons. Here we experimentally verify the existence of the PENS effect by measurements of the second harmonic generation from semicontinuous and continuous gold films.