Theoretical and experimental studies of dielectric two-dimensional Bragg structures for development of spatially-extended heterolasers

Two-dimensional distributed feedback (2D DF) was originally proposed [1] for obtaining super-power coherent radiation in relativistic masers based on spatially-extended electron beams. In this case, the 2D DF mechanism is realized in a 2D-periodic metallic Bragg structures with doubly periodic corrugation, which due to the arising in them of the transverse wave-flaxes allow synchronization of radiation of wide electron beams and the establishment of a single-mode oscillation regime when their transverse dimensions are in orders of magnitude greater than the radiation wavelength. To date, the operability of the new feedback mechanism has been experimentally demonstrated in the FEMs, which were elaborated in the millimeter wavelength range (from Kaup to Wbands) under a record transverse size of the interaction space, reaching up to 50 wavelengths, and the output power level of ~ 50 100 MW [2, 3]. At the same time, high potential of the new feedback mechanism is not exhausted by microwave relativistic generators. In this aspect it should be noted that currently a 1D feedback mechanism, which is realized in "traditional" single-periodical Bragg structures based on the coupling and mutual scattering of two counter-propagating waves, is widely used in quantum DF lasers [4, 5]. However, the transverse sizes of such generators in the conditions of maintaining single-mode narrow-band oscillation are limited by several wavelengths, and further enhance of dimensions leads to a complication of the radiation spectrum and, thus, loss of its spatial coherence. One of the attractive ways to solve this problem in heterolasers is the use of novel 2D feedback mechanism (Fig. 1), which in this case can be realized in 2D Bragg structures of planar geometry with doubleperiodical modulation of the effective refractive index