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The reflectance ( R ) and transmittance ( T ) of Si and GaAs wafers irradiated by a 6 ns pulsed, 532 nm laser have been studied for s- and p-polarized 250 GHz radiation as a function of laser fluence and time. The measurements were carried out using precision timing of the R and T signals, allowing an accurate determination of the absorptance ( A ) where A = 1 - R - T . Both wafers had a maximum reflectance above 90% for a laser fluence ≥ 8  mJ / cm 2 . Both also showed an absorptance peak of ~50% lasting ~2 ns during the risetime of the laser pulse. Experimental results were compared with a stratified medium theory using the Vogel model for the carrier lifetime and the Drude model for permittivity. Modeling showed that the large absorptance at the early part of the rise of the laser pulse was due to the creation of a lossy, low carrier density layer. For Si, the measured R ,   T and A were in very good agreement with theory on both the nanosecond time scale and the microsecond scale. For GaAs, the agreement was very good on the nanosecond scale but only qualitatively correct on the microsecond scale. These results may be useful for planning applications of laser driven semiconductor switches.

A reduced parameter model of fast laser-driven semiconductor switches of THz and mm-waves has been developed. The model predicts peak reflectivity and minimum transmissivity of switches, showing good agreement with experimental data, while requiring fewer inputs than published models. This simplification facilitated a systematic survey of laser parameters required for efficient switching. Laser energy density requirements are presented as a function of laser wavelength, laser pulse width, switched frequency, reflection angle, and semiconductor material (silicon or gallium arsenide). Analytical expressions have been derived to explain the dependence of laser requirements on switch parameters and to derive practical minima of required laser energy density. Diffusion is shown to quickly negate the shallow absorption advantage of laser wavelengths shorter than about 500 nm in silicon or 800 nm in gallium arsenide. Decreasing laser pulse width, to a derived limit, and switching S-polarized THz or mm-wave signals are shown to be means of lowering required laser energy. This is an especially useful result for devices operating at high power levels or THz frequencies, where extended switches are used in quasioptical systems.

The effect of reflection is studied experimentally and theoretically on a high-power 110 GHz gyrotron operating in the TE 22,6 mode in 3 μ s pulses at 96 kV, 40 A. The experimental setup allows variation of the reflected power from 0 to 33 % over a range of gyrotron operating conditions. The phase of the reflection is varied by translating the reflector along the axis. Operating at a higher efficiency point, at 4:40 T with 940 kW of output power, reflected power exceeding 11% causes a switch from operation in the TE 22,6 to simultaneous operation in the TE 22,6 and TE 21,6 modes with a large decrease of the total gyrotron output power. This switching effect is in good agreement with simulations using the MAGY code. Operating at a more stable point, 4:44 T with 580 kW of output power, when the reflection is increased, the output power remains in the TE 22,6 mode but it decreases monotonically with increasing reflection, dropping to 200 kW at 33% reflection. Furthermore, at a reflection above 22%, a power modulation at 25 to 30 MHz is observed, independent of the phase of the reflected wave. Such a modulated signal may be useful in spectroscopic and other applications.

A set of three apparatus enabling RF exposure of aerosolized pathogens at four chosen frequencies (2.8 GHz, 4.0 GHz, 5.6 GHz, and 7.5 GHz) has been designed, simulated, fabricated, and tested. Each apparatus was intended to operate at high power without leakage of RF into the local environment and to be compact enough to fit within biocontainment enclosures required for elevated biosafety levels. Predictions for the range of RF electric field exposure, represented by the complex electric field vector magnitude, that an aerosol stream would be expected to encounter while passing through the apparatus are calculated for each of the chosen operating frequencies.

We report experimental measurements of the threshold for multipactor discharges on dielectric surfaces at 110 GHz. Multipactor was studied in two geometries: electric field polarized parallel to or perpendicular to the sample surface. Measured multipactor thresholds ranged from 15 to 34  MV/m, more than 10 times higher than those found at conventional microwave frequencies. Measured thresholds were compared with prior data at lower frequencies, showing agreement with theoretical predictions that thresholds increase linearly with frequency. Measurements of the rf power dissipated in the multipactor show low dissipation (≤1%) for the parallel electric field case, but very strong dissipation for the perpendicular case, also in agreement with theoretical predictions. The agreement between experiment and theory over a wide range of frequencies provides a strong basis for the understanding of dielectric multipactor discharges.

A laser-driven semiconductor switch (LDSS) employing silicon (Si) and gallium arsenide (GaAs) wafers has been used to produce nanosecond-scale pulses from a 3 μ s, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532 nm laser, which produced 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with a 9 ns width and >70% reflectance. Under the same conditions, a single GaAs wafer yielded 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. Using two active wafers, pulses of variable length down to 3 ns duration were created. The switch was tested at incident 110 GHz RF power levels up to 600 kW. A 1-D model is presented that agrees well with the experimentally observed temporal pulse shapes obtained with a single Si wafer. The LDSS has many potential uses in high power millimeter-wave research, including testing of high-gradient accelerator structures., (Copyright © 2019 Author(s).)

Simple analytical formulae are presented for the design of linear tapers with very low mode conversion loss in overmoded corrugated waveguides. For tapers from waveguide radius a 2 to a 1 , with a 1 < a 2 , the optimal length of the taper is 3.198 a 1 a 2 /λ. Here, λ is the wavelength of radiation. The fractional loss of the HE 11 mode in an optimized taper is [Formula: see text]. These formulae are accurate when a 2 ≲ 2 a 1 . Slightly more complex formulae, accurate for a 2 ≤ 4 a 1 , are also presented in this paper. The loss in an overmoded corrugated linear taper is less than 1 % when a 2 ≤ 2.12 a 1 and less than 0.1 % when a 2 ≤ 1.53 a 1 . The present analytic results have been benchmarked against a rigorous mode matching code and have been found to be very accurate. The results for linear tapers are compared with the analogous expressions for parabolic tapers. Parabolic tapers may provide lower loss, but linear tapers with moderate values of a 2 / a 1 may be attractive because of their simplicity of fabrication.

Directed assembly of nanostructures within temporary and recyclable self-assembled scaffolds is emerging as an attractive method for the synthesis of nanomaterials with programmed properties. Understanding interactions of building blocks with amphiphilic scaffolds is critical for rational design of new nanostructures and nanodevices. Here we examine loading of hydrophobic monomers into bilayers with different curvatures. Time-resolved loading was studied by high performance liquid chromatography and dynamic light scattering. Despite differences in initial bilayer geometry, loading rates and maximum bilayer capacity are the same for liposomes with radii ranging from 25 to 100 nm. When using divinylbenzene (DVB) and dimyristoylphosphatidylcholine (DMPC), monomer/lipid loading ratio of 1.2 was achieved within 12 h. While accommodation of a large amount of monomers is likely to be accompanied with significant changes in bilayer structure, all liposomes in this study including those with smallest size and higher bilayer curvature retain encapsulated content and show no evidence of fusion during monomer loading. These results contribute to our understanding of interactions between hydrophobic molecules and lipid bilayers and expand the scope of the directed assembly method.