Beam focusing in reflections from flat subwavelength diffraction gratings

Abstract

We predict that narrow beams, reflecting from flat subwavelength diffraction gratings, can focus. The effect is shown for the beams of electromagnetic radiation; however, it should be observable for beams of waves of arbitrary nature (microwaves, surface plasmons, and acoustic and mechanical waves). We present analytical estimations of the focusing performance obtained by multiple scattering calculations and demonstrate the focusing effect numerically for an optical system (reflections from an array of dielectric cylinders), using the finite-difference time-domain calculations.

Near-Infrared and Optical Beam Steering and Frequency Splitting in Air-Holes-in-Silicon Inverse Photonic Crystals

“…As a result, it has been found that the termination of the photonic crystal structure can be designed to provide desired surface wave dispersion properties, − while an additional corrugation can undertake the coupling of the dark surface states to outgoing radiation. The mechanism has been employed, among others, to sustain the collimation of free-space traveling beams, , to produce and control near-field focusing effects, , and to decrease the π angular spread, or 2π for a solid angle in 3D, of a forward-propagating beam that exits a subwavelength photonic crystal waveguide operating in the microwave and optical regime. − Moreover, in the case of the microwave regime and the corresponding high dielectric rods in air photonic crystals, it has been shown both theoretically and experimentally that the angle of the beam’s directionality can be tuned by properly designing the terminating corrugations. , The frequency-dependent bend of the beams provides steering and frequency splitting capabilities, and consequently the manipulation of the dielectric, ohmic-loss-free, surface waves may be used in a variety of applications involving free-space coupling, as for example in demultiplexer components for optical communications, optical spectroscopy, and sensor applications. − …”

Flat focusing mirrors

“…Such near field focusing is principally different from far field lensing such as by conventional lens, by engineered subwavelength gratings or by metasurface thin lens -all with optical axis. We consider several configurations of such flat focusing mirrors: 1) Bragg-like mirrors, consisting of specially designed dielectric multilayers (longitudinally modulated one-dimensional (1D) structures) [1][2][3]; 2) periodically modulated surfaces on a subwavelength scale (transversally modulated 1D and two-dimensional (2D) structures) [4][5][6]. In the latter case the transverse invariance is broken at the modulation period, but is present on larger space scales.…”

Flat focusing in reflection from a chirped dielectric mirror with a defect layer

“…The flat mirror effect has also been demonstrated in reflections from two-dimensionally modulated (2-D) sub-wavelength dielectric gratings. 9,10 There are, in fact, other arrangements for focusing a beam in reflection with engineered grating structures. 11,12 However, the latter cases pose the optical axes and the lateral invariant is absent.…”