Selective Pyroelectric Detection of Millimetre Waves Using Ultra-Thin Metasurface Absorbers.

Sensing infrared radiation is done inexpensively with pyroelectric detectors that generate a temporary voltage when they are heated by the incident infrared radiation. Unfortunately the performance of these detectors deteriorates for longer wavelengths, leaving the detection of, for instance, millimetre-wave radiation to expensive approaches. We propose here a simple and effective method to enhance pyroelectric detection of the millimetre-wave radiation by combining a compact commercial infrared pyro-sensor with a metasurface-enabled ultra-thin absorber, which provides spectrally- and polarization-discriminated response and is 136 times thinner than the operating wavelength. It is demonstrated that, due to the small thickness and therefore the thermal capacity of the absorber, the detector keeps the high response speed and sensitivity to millimetre waves as the original infrared pyro-sensor does against the regime of infrared detection. An in-depth electromagnetic analysis of the ultra-thin resonant absorbers along with their complex characterization by a BWO-spectroscopy technique is presented. Built upon this initial study, integrated metasurface absorber pyroelectric sensors are implemented and tested experimentally, showing high sensitivity and very fast response to millimetre-wave radiation. The proposed approach paves the way for creating highly-efficient inexpensive compact sensors for spectro-polarimetric applications in the millimetre-wave and terahertz bands

Two dimensional Bragg structures (modeling and experimental testing selective properties)

The analysis of electrodynamic properties of two-dimensional (2D) Bragg resonators of planar and coaxial geometry realizing 2D distributed feedback was carried out in the frame of the coupled-mode approach and in 3D simulations taken into account. High selectivity of above structures is demonstrated for large Fresnel parameters. Formation of the high-Q eigenmodes in the vicinity of the Bragg resonance frequency has been demonstrated and explained by peculiarities of the dispersion characteristics of normal waves. Parameters of the resonator when the Q-factor of the fundamental mode exceeds the Q-factor of all other modes is described. The results of 3D simulations of excitation of the fundamental mode by an external e.m. pulse using the 3D code Microwave Studio are presented. Results of the theoretical analysis corroborated by the data obtained in "cold" microwave measurements

Production of powerful spatially coherent radiation in planar and coaxial fem exploiting two-dimensional distributed feedback

Two-dimensional distributed feedback is an effective method of producing ultrahigh-power spatially coherent radiation from an active medium, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. This paper describes the progress in the investigations of planar and coaxial free-electron masers (FEMs) based on a novel feedback mechanism. The theoretical analysis of these FEM schemes was conducted in the frame of the coupled-wave approach and 3-D simulations and agrees well with the experimental data obtained in ldquocoldrdquo and ldquohotrdquo tests. As a result, the effective transverse (azimuthal) mode selection has been demonstrated under a transverse size of about 20-25 wavelengths, and narrow-frequency multimegawatt microwave pulses have been generated in the Ka- and W-bands