Excitation of electrically biased photoconductive switches with femtosecond optical pulses is a well established method
of generating wide bandwidth (0.5 to 2.5 THz) pulses of terahertz frequency radiation. This method of pulse generation
draws energy from the bias power supply to accelerate optically injected charge carriers, giving rise to a current pulse
that can radiate into free space; nevertheless, the output power is limited by both poor coupling of the radiation out of the
substrate and by charge carrier screening. By optimizing the electrode structures and illumination geometries it may be
possible to address both of these limitations. To explore this idea, we have experimentally studied illumination with
coherent array of two sources spaced by less than a wavelength and operated near saturation in a semi-insulating GaAs
source. By illuminating with an array of N=2 spots, we demonstrate a doubling in output terahertz power with no
increase in input optical power. This result is consistent with results that have been shown for optical excitation that has
been stretched along the anode using cylindrical lenses; however, the array of two spots permits steering the beam by
adjusting the relative time of arrival of the two exciting pulses. Experimental evidence of this beam steering is presented.
With proper electrode design, this approach may ultimately enable an N-fold increase in output power with an array of N
spots, the formation of narrow beams, and the adjustment of beam direction by control of the relative time of arrival of
the exciting optical pulses.
We present experimental results for an uncooled imaging focal plane array technology that consists of a
polymer/metal/polymer layered membrane suspended over a micro-fabricated array of cavities. The device operation
is Golay-like (heating of air in the cavity causes a detectable deflection of the membrane proportional to incident
EM power), but potentially offers both greater sensitivity and more read-out options (optical or electrical) than a
traditional Golay cell through tailoring of the membrane properties. The membrane is formed from a layer-by-layer
deposition of polymer with one or more monolayers of gold nanoparticles (or other metal) that help control the
membrane's elasticity and deformation-dependent optical reflectivity/electrical conductivity. Baseline capabilities of
the device have been established through optical measurements of membrane deflection due to incident mm-wave
radiation modulated at 30 Hz (corresponding to a video refresh rate). The device demonstrates an NEP of
300 nW/√Hz at 105 GHz for a 19-layer membrane (9 poly/1 Au/9 poly) suspended over an array of 80 μm diameter
cavities (depth = 100 μm) etched in a 500 μm thick substrate of Si. Calculations of membrane sensitivity show that
this NEP could be reduced to ~ 100 pW/√Hz with enlarged cavity diameters on the order of 600 μm.
A new class of phosphorescent nanoparticles has been developed that use halogen-containing polymers and copolymers to encapsulate phosphorescent molecules. Their strong phosphorescence of long lifetime and large Stoke shift are not subject to oxygen quenching under ambient conditions due to the low oxygen permeability of the encapsulation matrix. The cross-linked phosphorescent particles are very stable and easily re-suspendable in aqueous media with surface functional groups to allow covalent tagging of biological recognition molecules such as antibodies. The conjugates can be used to provide very sensitive detection of analytes through time-resolved phosphorescence measurements. In addition to their applications for solution-based biological assays, those particles have also been demonstrated to be very useful for dry-chemistry-based time-resolved luminescent lateral flow assays.
Although existing night vision equipment provides a significant improvement in target detection in low light conditions,
there are several limitations that limit their effectiveness. Focus is a significant problem for night vision equipment due
to the low f-number optics required to obtain sufficient sensitivity as well as the dynamic nature of night vision
applications, which requires frequent focus adjustments. The Georgia Tech Research Institute has developed a prototype
next-generation night vision device called the Improved Night Vision Demonstrator (INVD) in order to address these
shortfalls. This paper will describe the design of the INVD system as well as an analysis of its performance.
Holograms have been utilized to authenticate financial instruments and high value products for many years. The security provided by embossed holograms is limited by their low surface relief, typically 0.25 micron, which makes them susceptible to counterfeiting: stripping the hologram from the substrate exposes the complete holographic microstructure which can be easily used to create counterfeit tooling. A large improvement in counterfeit deterrence can be gained by the use of high precision non-holographic micro-optics and microstructures having a surface relief greater than a few microns. An unlimited range of distinctive optical effects can be obtained from micro-optic systems. Many of the possible optical effects, such as optical interactions between discrete elements, cannot be effectively simulated by any other means, including holography. We present descriptions of five Visual Physics document authentication micro-optic systems that provide sophisticated optical effects: Virtual Image, BackLite, Encloak, Optical Black, and Structural Color . Visual Physics document authentication micro-optics impose an additional level of counterfeit deterrence because the production of polymer films incorporating these microstructures requires unconventional manufacturing methods; conventional holographic reproduction processes, typical of hologram counterfeiting operations, are inadequate to faithfully reproduce the details and the function of these micro-optic elements. We have developed mastering, tooling, and high precision/high speed manufacturing processes that can faithfully replicate these complex surface relief micro-optics at low cost.
We describe experimental measurements of the scattering properties of two conducting surfaces with 1D roughness. The surfaces have been fabricated in photoresist and have been characterized with a stylus that is small compared to the surface correlation length. In studies of diffuse scatter, we present measurements of the four unique elements of the Stokes matrix. Backscattering enhancement and associated polarization effects are observed for the rougher surface while behavior consistent with tangent plane models is seen for the smoother surface. The polarization-dependence of the coherent scatter is also investigated, and comparisons are made with the results calculated for a flat surface. Finally, we briefly present results for the angular correlation functions of intensity, where the coherent effects that produce backscattering enhancement are more directly observed.
We discuss measurements of the infrared scattering properties of one- and two-dimensional conducting randomly rough surfaces. The surfaces are fabricated in photoresist and are checked with a stylus profilometer to verify that the surface statistics agree with the desired results. For surfaces that have steep slopes and lateral scale sizes comparable to the illumination wavelength, we observe strongly enhanced backscattering toward the source. These observations are shown to be strongly dependent on polarization. In the case of a one- dimensional surface, four distinct quantities appear in the Stokes scattering matrix, and examples of measurements of these quantities are presented. For the case of a two- dimensionally rough surface it is discussed that, even if the incident field is purely linearly polarized, the scattered light consists of both polarized and randomly polarized components. In the backscattering region, the polarized component contains linear, elliptical, and even nearly circular polarization states at various field angles. These data are interpreted and are consistent with the statistical isotropy of the surface.