Abstract
Abstract We propose and theoretically and numerically investigate integrated diffraction gratings for the Bloch surface wave (BSW) platform, which have subwavelength or near-subwavelength period. We demonstrate that, in the oblique incidence geometry of a transverse-electric polarized BSW and with a properly chosen band gap configuration of the photonic crystal supporting the surface waves, the proposed structures operate in the scattering-free regime, when the energy of the incident BSW is divided between the reflected and transmitted BSWs with the same polarization corresponding to the propagating diffraction orders of the grating, and not scattered away from the propagation surface. In this regime, the studied integrated gratings support high-Q resonances and bound states in the continuum not only in the subwavelength case when only the specular (zeroth) diffraction orders propagate, but also in the case when non-evanescent zeroth and −1st diffraction orders satisfy the so-called Littrow mounting condition. The proposed integrated gratings on the BSW platform can be used as efficient narrowband spatial or spectral filters operating in reflection, or as BSW beam splitters or deflectors operating in transmission. The obtained results may find application in two-dimensional photonic circuits for steering the BSW propagation.
Highlights
Resonant effects play a crucial role in photonics, since they enable creating photonic structures with unique optical properties that can be used in sophisticated devices for optical filtering, sensing and switching, among other applications [1]
In the oblique incidence geometry of a transverse-electric polarized Bloch surface wave (BSW) and with a properly chosen band gap configuration of the photonic crystal supporting the surface waves, the proposed structures operate in the scatteringfree regime, when the energy of the incident BSW is divided between the reflected and transmitted BSWs with the same polarization corresponding to the propagating diffraction orders of the grating, and not scattered away from the propagation surface
Along with the fundamental importance, BICs are of great practical interest, since in real photonic structures, due to finiteness, fabrication imperfections, and material absorption, the BICs collapse to resonances with finite, but potentially very high quality factor useful for numerous applications
Summary
Resonant effects play a crucial role in photonics, since they enable creating photonic structures with unique optical properties that can be used in sophisticated devices for optical filtering, sensing and switching, among other applications [1]. Due to the fact that the structure is not invariant to translation in the direction of the z axis, each of the diffraction orders will, in the most general case, “contain” a reflected or transmitted BSW having the same polarization as the incident one (TE-polarization for all the examples considered below), and a “cross-polarized” (TM-polarized) BSW as well as non-guided radiation scattered out of the propagation surface to the superstrate (plane waves in a homogeneous dielectric with the refractive index n0) and to the substrate (“bulk” modes of a one-dimensional photonic crystal). Let us assume that the integrated grating is subwavelength, i.e., that only the zeroth reflected and transmitted diffraction orders having the x wavevector component equal to the wavevector component of the incident wave kx,0 = k0neff,TE sin θ are propagating In this case, total suppression of the “parasitic” waves is possible if the following conditions are fulfilled.
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