Metal-mesh achromatic half-wave plate for use at submillimeter wavelengths

A metal-mesh achromatic half-wave plate (HWP) has been designed, manufactured, and tested for potential use in millimeter and submillimeter astronomical instruments. The prototype device presented here is based on a 12-grid Shatrow [ IEEE Trans. Antennas Propag. 43, 109 (1995)] recipe to operate over the frequency range of 120–180 GHz . Transmission line modeling and finite-element analysis [Ansoft HFSS website: http://www.ansoft.com/hfss/ ] were used to optimize the design geometrical parameters in terms of the device transmission, reflection, absorption, phase-shift, and cross-polarization as a function of frequency. The resulting prototype device was constructed and characterized using incoherent radiation from a polarizing Fourier transform spectrometer to explore its frequency and polarization behavior. These measurements are shown to be in excellent agreement with the models. Lists of the achieved HWP performance characteristics are reported

Effects of quasi-optical components on feed-horn co- and cross-polarisation radiation patterns

Astrophysical experiments dedicated to the study of the Cosmic Microwave Background are in needs of very welldefined beam shape in order to get an accurate re-construction of the anisotropies power spectrum. These beams have tobe carefully designed, but also properly characterised. Some of these instruments will be located in a cryostat for whichfilters and windows are necessary, and the effects of these additional optical components on the beam shape need to betaken into account. We present here, measurements of such effects on the co- and cross-polarisation radiation patterns ofcorrugated horns using a Vector Network Analyse

Dog bone triplet metamaterial wave plate

Metamaterials are artificially made sub-wavelength structures arranged in periodic arrays. They can be designed to interact with electromagnetic radiation in many different and interesting ways such as allowing radiation to experience a negative refractive index (NRI). We have used this technique to design and build a quasi-optical Half Wave Plate (HWP} that exhibits a large birefringence by virtue of having a positive refractive index in one axis and a NRI in the other. Previous implementations of such NRI-HWP have been narrow band (~1-3%) due to the inherent reliance on needing a resonance to create the NRI region. We manufacture a W-band prototype of a novel HWP that uses the Pancharatnam method to extend the bandwidth (up to more than twice) of a usual NRI-HWP. Our simulated and experimentally obtained results despite their differences show that a broadening of a flat region of the phase difference is possible even with the initially steep gradient for a single plate

Dielectrically embedded flat mesh lens for millimeter waves applications

A flat lens based on subwavelength periodic metal meshes has been developed using photolithographic techniques. These mesh grids are stacked at specific distances and embedded in polypropylene. A code was developed to optimize more than 1000 transmission line circuits required to vary the device phase shift across the lens flat surface, mimicking the behavior of a classical lens. A W-band mesh-lens prototype was successfully manufactured and its RF performance characterized using a vector network analyzer coupled to corrugated horn antennas. Co-polarization far-field beam patterns were measured and compared with finite-element method models. The excellent agreement between data and simulations validated our designing tools and manufacturing procedures. This mesh lens is a low-loss, robust, light, and compact device that has many potential applications including millimeter wave quasi-optical systems for future cosmic microwave background polarization instruments

Achromatic half-wave plate for submillimeter instruments in cosmic microwave background astronomy: experimental characterization

An achromatic half-wave plate (HWP) to be used in millimeter cosmic microwave background (CMB) polarization experiments has been designed, manufactured, and tested. The design is based on the 5-plates Pancharatnam recipe and it works in the frequency range 85–185 GHz . A model has been used to predict the transmission, reflection, absorption, and phase shift as a function of frequency. The HWP has been tested by using coherent radiation from a back-wave oscillator to investigate its modulation efficiency and with incoherent radiation from a polarizing Fourier transform spectrometer (FTS) to explore its frequency behavior. The FTS measurements have been fitted with an optical performance model which is in excellent agreement with the data. A detailed analysis of the data also allows a precise determination of the HWP fast and slow axes in the frequency band of operation. A list of the HWP performance characteristics is reported including estimates of its cross polarization

Spectral definition of the ArTeMiS instrument

ArTeMiS is a sub-millimetre camera to be operated, on the Atacama Pathfinder Experiment Telescope (APEX). The ultimate goal is to observe simultaneously in three atmospheric spectral windows in the region of 200, 350 and 450 microns. We present the filtering scheme, which includes the cryostat window, thermal rejection elements, band separation and spectral isolation, which has been adopted for this instrument. This was achieved using a combination of scattering, Yoshinaga filters, organic dyes and Ulrich type embedded metallic mesh devices. Design of the quasi-optical mesh components has been developed by modelling with an in-house developed code. For the band separating dichroics, which are used with an incidence angle of 35 deg, further modelling has been performed with HFSS (Ansoft). Spectral characterization of the components for the 350 and 450 bands have been performed with a Martin-Puplett Polarizing Fourier Transform Spectrometer. While for the first commissioning and observation campaign, one spectral band only was operational (350 microns), we report on the design of the 200, 350 and 450 micron bands

Development of millimetre-wave quasi-optical devices based on the mesh technology

Photolithographic techniques have been used for decades to manufacture mesh filters at millimetre and sub-millimetre wavelengths. These frequency selective devices, based on metal grids with sub-wavelength periodic structures, have found many applications, especially in astronomical instrumentation. Given the modeling and manufacturing accuracies achieved recently, the same mesh technology has been adopted to develop more complex devices based on anisotropic and inhomogeneous surfaces. It is also now possible to design more exotic devices such as those required to manipulate the Orbital Angular Momentum (OAM) of the radiation

Recovering the frequency dependent modulation function of the achromatic half-wave plate for POL-2: the SCUBA-2 polarimeter

A spectro-polarimetric method is presented to allow the recovery of the frequency dependent polarization modulation function for an achromatic half-wave plate. We show how the non ideal nature of the modulator can be characterized for removal of both instrument effects and variations related to the source spectral index