Transmission and reflection of metallic mesh in the far infrared

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.

Modeling parameters for the spectral behavior of infrared frequency-selective surfaces

Comparisons of experiment and theory are presented for transmission spectra over the range 2-15 mum of a set of frequency-selective surfaces consisting of arrays of simple dipole patches of aluminum on or in silicon. The arrays are fabricated by direct-write electron-beam lithography. Important parameters controlling the spectral shape are identified, such as dipole length, spacing, resistance, and dielectric surroundings. The separate influence of these variables is exhibited. Encouraging agreement between simple model calculations and the measurements is found.

Polarization characteristics of wire mesh at 119 microm

Polarization characteristics of the transmitted beam through the various sizes of metal wire grid meshes are examined experimentally at 118.8 microm. When these results were compared to Chen's waveguide theory, we obtained excellent agreement. In particular, when the incident beam polarization is neither parallel (TM) nor perpendicular (TE) to the wire grid axis, a linearly polarized incident beam can be made elliptic due to the phase difference between the TE and TM modes.

Performance of metal meshes as a function of incidence angle

An inductive mesh was measured for transmission as a function of frequency, incidence angle, and polarization. The experimental data agree well with Chen's waveguide theory for meshes as long as an adequate number of modes are included in the calculations. To simplify calculations of mesh transmission a lumped circuit model has been developed to fit Chen's theory. This model predicts transmission for the two major polarizations and a variety of angles to within 1%.

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

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.

A study of an array of square openings

The diffraction theory for the transmission of electromagnetic fields through a grid in a dielectric is derived and experimentally checked in the visible wavelength region. In the far infrared, an approximate mathematical representation is found to describe the grids in dielectrics. The empirical equations in the far infrared are combined with the electrical circuit transmission line analog for inductive grids. Detailed structure of the transmittance vs wavenumber curves are presented.

Far infrared airborne spectroscopy

Equipment and techniques employed in an airborne far ir (80-400 micro) spectral measurement of the solar brightness temperature are described, with particular attention paid to the scanning Michelson interferometer and the radiometric calibration. The airborne performance of several telescopic guiding systems, a liquid helium cooled bolometer, and golay cells are discussed, summarizing experience gained on twenty-five jet aircraft flights.

Metal mesh interference filters for the far infrared

Far ir Fabry-Perot type interference filters consisting of two parallel, closely spaced metal meshes have been reported by others. Using improved techniques, filters of this type have been fabricated with larger finesse and peak transmittance. Measured transmittance curves, obtained with the Aerospace lamellar grating interferometer have led to transmittance peaks whose sharpness appears to be limited principally by the beam convergence angle within the instrument (8 degrees ). From the measured peaks, finesses as large as 100 are obtained. Available mesh materials permit the construction of filters for any frequency from 10 cm(-1) (or lower) to at least 200 cm(-1). These will require absorbing components to eliminate unwanted harmonics.