Free-standing Wire Grids Research Articles

Fabrication of large-scale freestanding THz wire grid polarizers by femtosecond laser micromachining

High-performance THz polarizers, as a quasi-optical device to manipulate THz waves, are highly demanded for THz spectroscopy and imaging applications. In this work, we have employed the femtosecond laser micro-machining technique to fabricate freestanding THz wire grid polarizers with two kinds of reinforced line structures which weaken the effects of gravity sagging and entanglement of long free wires, thus enabling large-scale processing. The effect of the reinforced lines on the polarizers were simulated by the Finite-Difference Time-Domain (FDTD) method, and the performances of the polarizer were characterized by THz time-domain spectroscopy and compared with that of a commercial wire gird polarizer. The calculated transmittance is in good agreement with the FDTD results. The degree of polarization and the extinction ratio of the polarizer with discontinuous reinforced lines are better than those of a commercial wire grid polarizer.

飞秒激光微加工制备大尺寸无衬底太赫兹线栅偏振器

飞秒激光微加工制备大尺寸无衬底太赫兹线栅偏振器

Transmission and Reflection Characterization of Polarizing Beam Splitters at Submillimeter Wavelengths

This paper presents a comparison of the transmission and reflection performance of different free-standing wire grids and photolithographic polarizers. This study has been carried out in the frame of the development work of the submillimeter-wave instrument (SWI) as part of the JUpiter and ICy moons Explorer mission of the European Space Agency. SWI is a passive heterodyne radiometer that is operating in the frequency bands from 530 to 625 GHz and from 1080 to 1275 GHz. The optical coupling network of SWI includes a polarizing beam splitter to separate the signal into the two channels. Simulations of the transmittance and reflectance in copolarization and in cross polarization have been performed at different incidence angles and polarizations to support the selection of the optimum beam splitter for SWI. The nominal and actual design parameters of the investigated polarizing beam splitters, measured with a confocal laser microscope, have been considered in the simulations. Measurements of the transmission and reflection losses in co- and in cross polarization have been performed at 636.5 GHz to validate these numerical calculations. The very low losses of the beam splitters in copolarization were accurately measured by means of a free-space cavity resonator setup. These measurements are in very good agreement with the simulations and radiometric measurements of the transmission loss which were performed with a breadboard model of the SWI receiver unit at 590.0 GHz. The lowest transmission losses among the studied beam splitters are observed for a free-standing wire grid with a single plane of gold-coated wires. The highest reflectance in copolarization and the lowest levels of cross-polarization leakage are determined for the photolithographic polarizers.

High Fill-Factor Substrate-Based Wire-Grid Polarizers With High Extinction Ratios

Low-cost, substrate-based, millimeter-wave-to-THz wire-grid polarizers have been fabricated on crystalline-quartz substrates using planar processing techniques. The polarizers achieve high extinction ratios (at least 60 dB) with a single layer, while maintaining low insertion loss (a few decibels) by taking advantage of previously under-utilized “spoof” surface-plasmon effects. Full-wave finite-element simulations done with High Frequency Structure Simulator and experiments both show that metal fill-factors upwards of 90% greatly improve polarizer extinction ratio. The extinction ratio of high fill-factor polarizers exceeds that of a commercial free-standing wire grid by up to 20 dB at normal incidence.

Demonstration of an Octave-Bandwidth Negligible-Loss Metamaterial Horn Antenna for Satellite Applications

This paper reports on the detailed design and experimental demonstration of a metamaterial-enabled low-sidelobe horn antenna (metahorn) based on principles similar to those of earlier soft horn antennas. The target application is a linearly polarized feed horn in the super-extended C-band (3.4-6.725 GHz) for communication satellite reflector antennas. The paper describes the detailed design and manufacturing of the -plane metamaterial liner (metaliner) based on a freestanding wire grid without the need for a dielectric substrate material. The measured copolarized and cross-polarized antenna patterns from the feed horn demonstrate over an octave pattern bandwidth with negligible loss. The results show similar bandwidth with lower sidelobes and backlobes than those of the trifurcated horn that is currently used as the standard C-band feed for single linear polarization. This demonstration shows promise for lightweight metamaterial horns to replace heavy and expensive C-band corrugated horns.

Optical Properties of Plasmonic Crystal at Terahertz Frequencies and the Advanced Application

We introduce the recent topics concerning the 1D- and 2D- plasmonic crystals, i.e., a wire grid polarizer and a metal mesh device at terahertz frequencies. We show the remarkable increase in polarization characteristics on a freestanding double-layered wire grid polarizer. We also show the performance of metal mesh devices for biosensing application.

Fabrication and Characterization of Large Free-Standing Polarizer Grids for Millimeter Waves

Construction of large aperture free-standing metal wire grids is demonstrated for the lower end of the millimeter wave spectral region. For the constructed two grids the co-polarized and cross-polarized components of transmitted power were measured at 45° oblique incidence. The measurements were performed as a function of wire orientation angle and in more detail at selected angles. The results are in good agreement with the theoretical results presented in the literature. In order to save time and costs the construction apparatus was simplified from those reported previously by other authors. It was shown that for this frequency range the grid characteristics are not degraded when such an apparatus is applied. One of the constructed grids will be used in a calibration system of the Helsinki University of Technology (HUT) polarimetric radiometer.

Development of an Interferometer with Interchangeable Polarization and Amplitude Divisions for the 1000-10 cm−1 Region

An interferometer for spectroscopic measurements from approximately 1000 cm−1 into the far-infrared region employing a single beamsplitter is developed. It consists of a free-standing wire grid beamsplitter and two roof-top mirrors. When the beamsplitter grids are oriented at 45° to the roof axes of the mirrors and the input light is polarized at 45° to the beamsplitter grids, the interferometer functions as a Martin-Puplett type with polarization division from approximately 600 cm−1 to about 10 cm−1. Alternately, when the beamsplitter grids are parallel to the roof axes of the mirrors, and the input light is unpolarized the interferometer functions as a Michelson type with amplitude division, for spectral measurements in the 1000-100 cm−1 region.

Far infrared polarizing grids for use at cryogenic temperatures

A technique is presented for constructing free-standing wire grids for use as far-IR polarizers. The grids are reliable under repeated cycling between room temperature and 1.5 K.

Characteristic curves for the specroscopic performance of free-standing wire grids at millimeter and submillimeter wavelengths

Characteristic curves for the specroscopic performance of free-standing wire grids at millimeter and submillimeter wavelengths

Wide band measurement of power transmission coefficients and polarizing efficiency of free standing wire grids in the far infrared

In this paper we describe wide band measurements of power transmission coefficients and the degree of polarization of a set of free-standing wire grids in the far infrared. The measurements have been performed in two different spectral regions, with the grid parameter d/g ranging between 0.4 and 0.003. The results are compared with theoretical predictions and the accuracy of the experimental method is fully discussed.

Transmission coefficients of free-standing wire grids in the far infrared: A theoretical approach for easy computation

In this paper we present a method for the calculation of the power transmission coefficients of free-standing wire grids in case of normally incident radiation. The method, based on the transmission line analogy, although not as general as other theoretical approaches already discussed in the literature, has the great advantage of being suitable for easy and quick numerical computation. Comparison with other authors allows us to state that the accuracy of the present method is quite acceptable, provided that the wavelength of incident radiation is at least five times greater than the diameter of the wires.

Large Aperture, Freestanding Wire Grid Polarizers For The Far-Infrared: Fabrication, Characterization, And Testing

A method is presented for the routine fabrication and testing of freestanding wire grids with large apertures for the Cosmic Back-ground Explorer (COBE) Far Infrared Absolute Spectrophotometer (FIRAS) experiment. Design criteria and performance specifications are discussed. Test and characterization results are also included that indicate that these wire grids are quite rugged and can withstand the rigors of qualification for spaceflight use.

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

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.

Submillimetre wave measurements of optical constants of water at various temperatures

A polarization interferometer employing free standing wire grid polarizer and beamsplitter and reflection dispersive Fourier transform spectrometry are used to determine the optical constants of water in the region 5–220cm −1. The liquid cell uses a single crystal semiconductor or quartz window. The complex evaluation method first determines the optical constants of the window material and then takes into account all the interface contributions in terms of ‘complex insertion loss’ formalism and thus determines the precise values of optical constants of liquids. Power absorption coefficient and refractive index data of hyperpure silicon, and power absorption coefficient, refractive index and real and imaginary parts of complex relative permittivity data of liquid water at 4. 30 and 57°C are presented and discussed.

High-precision dielectric measurements on liquids and solids at millimeter and submillimeter wavelengths

A brief outline is given of the improvements which have been made in the measurement techniques available for the determination of the complex relative permittivity of liquids and low-loss solids in the frequency range v = 100 GHz to 18 THz. These improvements involve very stable interferometric systems with either dielectric beam splitters for the upper frequency end or else free standing wire grid beam splitters for the lower frequency end, novel cell designs, the use of sensitive detectors, and of new computational procedures. The results can be checked by independent spot frequency measurements using HCN and H 2 O lasers coupled to an improved Mach-Zehnder interferometer. The optical properties of the polymeric (polyethylene, TPX, polypropylene, PTFE) and single crystal (quartz, silicon germanium) windows now assume an increased importance and we have determined refractive index n (v) and power absorption coefficient α(v) to much improved precision. With our present technique we can now determine the complex permittivity of very heavily absorbing liquids such as primary alcohols and water as far as 6 THz and beyond in favorable cases as functions of frequency over a wide temperature range.

New techniques for dispersive Fourier transform spectrometry of liquids

Dispersive Fourier transform spectrometry (DFTS) techniques are described for measuring the optical constants of liquids. Free standing wire grid polarizing and melinex beam splitters are used to cover a wide frequency range and link low and high frequency FTS data with microwave and mid infrared data respectively. Both power absorption coefficient α and refractive index n are computed as continuous functions of frequency from the same dispersive measurements and good agreement has been achieved with the power absorption coefficient spectra obtained using conventional FTS. The use of windowed liquid cells enables amplitude reflection measurements to be carried out and this not only solves the liquid vapour problem, but also permits a very thin layer (10 μm) of absorbing liquid to be used. Data are presented and discussed for water, aliphatic alcohols, halobenzenes and halogen-substituted methanes.