A spatially extended planar 75 GHz free-electron maser with a hybrid two-mirror resonator consisting of two-dimensional upstream and traditional one-dimensional downstream Bragg reflectors and driven by two parallel-sheet electron beams 0.8  MeV/1  kA has been elaborated. For the highly oversized interaction space (cross section 45×2.5 vacuum wavelengths), the two-dimensional distributed feedback allowed realization of stable narrow-band generation that includes synchronization of emission from both electron beams. As a result, spatially coherent radiation with the output power of 30-50 MW and a pulse duration of ∼100  ns was obtained in each channel.

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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 "cold" and "hot" 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.Index Terms-Free-electron maser (FEM), mode selection, powerful microwave radiation, 2-D distributed feedback.

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We review our recent results on development of passive quasi-optical selective devices based on metallized subwavelength microstructure arrays designed for controlling radiation beams at frequencies from a few tens GHz up to ten THz: filters, polarizers, metasurfaces, ET-metamaterial lenses. The methods of electromagnetic simulation, technological implementation and (sub)THz-characterization of microstructure devices, as well as their applications, are discussed.

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