ABSTRACT
The quasi-optical modulation of linear polarization at millimeter and sub-millimeter wavelengths can be achieved by using rotating half-wave plates (HWPs) in front of polarization-sensitive detectors. Large operational bandwidths are required when the same device is meant to work simultaneously across different frequency bands. Previous realizations of half-wave plates, ranging from birefringent multi-plates to mesh-based devices, have achieved bandwidths of the order of 100%. Here we present the design and experimental characterization of a reflective HWP able to work across bandwidths of the order of 150%. The working principle of the novel device is completely different from any previous realization, and it is based on the different phase-shift experienced by two orthogonal polarizations reflecting, respectively, off an electric conductor and an artificial magnetic conductor.
ABSTRACT
Electromagnetic metamaterials, made from arrangements of subwavelength-sized structures, can be used to manipulate radiation. Designing metamaterials that have a positive refractive index along one axis and a negative refractive index along the orthogonal axis can result in birefringences, Δn>1. The effect can be used to create wave plates with subwavelength thicknesses. Previous attempts at making wave plates in this way have resulted in very narrow usable bandwidths. In this paper, we use the Pancharatnam method to increase the usable bandwidth. A combination of finite element method and transmission line models was used to optimize the final design. Experimental results are compared with the modeled data.
ABSTRACT
Initial three-dimensional phase and intensity measurements of a 100 GHz l=±1 orbital angular momentum (OAM) vortex are presented. The vortex was generated by illuminating a polypropylene spiral phase plate. Measurements were taken with a three-dimensional field scanner operating in the W-band (75-100 GHz). Early analysis shows splitting of the OAM phase dislocation at the vortex center, resulting in a complex inner vortex intensity pattern.
ABSTRACT
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.
ABSTRACT
q-plates are quasi-optical devices specifically designed to generate and detect the orbital angular momentum states of the light. It is possible to produce q-plates working at millimeter wavelengths by using a well-known and cheap manufacturing technique. The technique consists of creating inhomogeneous, artificial birefringent materials by machining grooves with specific geometries into normal dielectric materials. In this work, a q-plate working around 100 GHz has been designed, manufactured, and tested using a vector network analyzer. The experimental data validate the modeled intensity and phase for the transformation of an incident Gaussian beam.
ABSTRACT
We explore a free-space polarization modulator in which a variable phase is introduced between the right- and left-handed circular polarization components and used to rotate the linear polarization of the outgoing beam relative to that of the incoming beam. In this device, the polarization states are separated by a circular polarizer that consists of a quarter-wave plate in combination with a wire grid. A movable mirror is positioned behind and parallel to the circular polarizer. As the polarizer-mirror distance is changed, an incident linear polarization will be rotated through an angle that is proportional to the introduced phase delay. We demonstrate a prototype device that modulates Stokes Q and U over a 20% bandwidth, from 77 to 94 GHz.
