We fabricated a terahertz wire-grid polarizer consisting of a micrometer-pitch Al grating on a Si substrate by photolithography and wet etching. The ratio of TM and TE transmittances (extinction ratio) was over 35 dB at 0.5 THz. At the Brewster angle of the Si substrate, the polarization transmittance of a TM wave through the fabricated polarizer exceeded 95% and the extinction ratio was over 45 dB at approximately 1 THz. The fabricated polarizer has a higher extinction ratio than conventional free-standing terahertz wire-grid polarizers.

Terahertz wire-grid polarizers with micrometer-pitch Al gratings

Recent progress in magnetically confined plasma research has brought the goal of controlled thermonuclear fusion within reach. Simultaneously, the increased size, temperature, and density of these devices has resulted in rapid changes in diagnostic techniques. In this article, recent developments in instrumentation are reviewed together with trends for the future. The topics discussed include far‐infrared laser interferometry and polarimetry, ruby laser television Thomson scattering, excimer and Nd‐glass laser scattering, ion temperature determination via large‐α scattering with FIR and CO2 lasers, collective scattering, Schottky diode mixer technology, synchrotron radiation diagnostics and imaging, ion beam probes, x‐ray diagnostics and imaging, neutron diagnostics, resonance fluorescence scattering, ultraviolet diagnostics, and internal magnetic field measurement.

Instrumentation for magnetically confined fusion plasma diagnostics

Utilizing highly oriented multiwalled carbon nanotube aerogel sheets, we fabricated micrometer-thick freestanding carbon nanotube (CNT) polarizers. Simple winding of nanotube sheets on a U-shaped polyethylene reel enabled rapid and reliable polarizer fabrication, bypassing lithography or chemical etching processes. With the remarkable extinction ratio reaching ∼37 dB in the broad spectral range from 0.1 to 2.0 THz, combined with the extraordinary gravimetric mechanical strength of CNTs, and the dispersionless character of freestanding sheets, the commercialization prospects for our CNT terahertz polarizers appear attractive.

A Reel-Wound Carbon Nanotube Polarizer for Terahertz Frequencies

We investigate the polarization modulation properties of a variable-delay polarization modulator (VPM). The VPM modulates polarization via a variable separation between a polarizing grid and a parallel mirror. We find that in the limit where the wavelength is much larger than the diameter of the metal wires that comprise the grid, the phase delay derived from the geometric separation between the mirror and the grid is sufficient to characterize the device. However, outside of this range, additional parameters describing the polarizing grid geometry must be included to fully characterize the modulator response. In this paper, we report test results of a VPM at wavelengths of 350 μm and 3 mm. Electromagnetic simulations of wire grid polarizers were performed and are summarized using a simple circuit model that incorporates the loss and polarization properties of the device.

/pdf/properties-of-a-variable-delay-polarization-modulator-26bq5cn4lt.pdf

Properties of a variable-delay polarization modulator

The optical properties of freestanding and dielectrically backed inductive metal meshes are investigated in the far IR spectral regime. The most pronounced effect of the substrate is to cause a shift in the diffraction edge to longer wavelengths by a factor equal to the substrate index of refraction. Various theoretical models are reviewed and tested with the main result being that only the microwave based waveguide-array model successfully predicts the freestanding and dielectrically back optical features in and around the diffraction edge. A simplified version of this model is presented and shown to be adequate for describing the majority of the spectral features. The backed meshes are then characterized in terms of mirror quality for use in optically pumped far IR lasers.

Far-IR optical properties of freestanding and dielectrically backed metal meshes.

Free-standing grids wound from 5 μm diameter wire for spectroscopy at far-infrared wavelengths

Measurement of the spectrum of tokamak electron cyclotron emission on a submillisecond time-scale

The power- and phase-transmission spectra of free-standing wire grids at submillimeter wavelengths have been calculated for the case of normal incidence by a least-squares method. The calculations are compared with measurements made on grids wound from 10-μm-diameter tungsten wire with spacings in the 30–65-μm range. The measurements were made at ν˜ = 29.7 cm−1 (λ = 337 μm) by using an HCN laser and in the 20–450-cm−1 frequency range by a combination of conventional and dispersive Fourier-transform spectrometry. At frequencies below about 100 cm−1, excellent agreement is obtained between theory and experiment, indicating that this work should provide a suitable basis for the design of spectroscopic components, such as laser-coupling devices for use at submillimeter wavelengths. However, there are marked discrepancies between theory and experiment at frequencies near the Rayleigh wavelengths, where d/λ = 1, 2, etc., where d is the wire spacing. From detailed measurements of the periods of the grids, it is concluded that these discrepancies are primarily due to the effects of irregularities in the wire spacings.

Performance of free-standing grids wound from 10-μm-diameter tungsten wire at submillimeter wavelengths: computation and measurement

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Free-standing grids wound from 5 μm diameter wire for spectroscopy at far-infrared wavelengths

Measurement of the spectrum of tokamak electron cyclotron emission on a submillisecond time-scale

Performance of free-standing grids wound from 10-μm-diameter tungsten wire at submillimeter wavelengths: computation and measurement