Observation of Polarization Vortices in Momentum Space

Abstract

The vortex, a fundamental topological excitation featuring the in-plane winding of a vector field, is important in various areas such as fluid dynamics, liquid crystals, and superconductors. Although commonly existing in nature, vortices were observed exclusively in real space. Here, we experimentally observed momentum-space vortices as the winding of far-field polarization vectors in the first Brillouin zone of periodic plasmonic structures. Using homemade polarization-resolved momentum-space imaging spectroscopy, we mapped out the dispersion, lifetime, and polarization of all radiative states at the visible wavelengths. The momentum-space vortices were experimentally identified by their winding patterns in the polarization-resolved isofrequency contours and their diverging radiative quality factors. Such polarization vortices can exist robustly on any periodic systems of vectorial fields, while they are not captured by the existing topological band theory developed for scalar fields. Our work provides a new way for designing high-Q plasmonic resonances, generating vector beams, and studying topological photonics in the momentum space.

Nonradiating photonics with resonant dielectric nanostructures

“…Recently, similar analysis was done for periodic chains of dielectric rods [35] and spheres [73]. Later, polarization vortices of BICs were observed experimentally [74][75][76]. Based on these findings, topologically enabled ultra-high-Q resonances were proposed [71] arising when multiple BIC merge in the momentum space, with the measured factor of 5 × 10 5 .…”

Bound States in the Continuum through Environmental Design

“…In practice however, systems supporting BICs from the first category are difficult to experimentally realize due to the high degree of fine-tuning required. Thus, much of the current excitement surrounding BICs has focused on systems which feature symmetry-protected and accidental BICs; moreover, these BICs have been shown to possess topological protection that guarantees their existence under perturbations to the system [20,[46][47][48][49][50].Traditionally, the appearance of accidental BICs is understood in terms of modal interference [22,45,51], with two or more resonances of the device destructively interfering in the system's radiation channels and resulting in a bound mode spatially localized to the device. This interpretation emphasizes how tuning the device's parameters changes the spatial profile of its resonances to realize this modal interference, while considering the available radiation channels in the surrounding environment as fixed.…”

“…In practice however, systems supporting BICs from the first category are difficult to experimentally realize due to the high degree of fine-tuning required. Thus, much of the current excitement surrounding BICs has focused on systems which feature symmetry-protected and accidental BICs; moreover, these BICs have been shown to possess topological protection that guarantees their existence under perturbations to the system [20,[46][47][48][49][50].…”

Bound states with complex frequencies near the continuum on lossy periodic structures

“…A particularly simple structure supporting BICs is a slab waveguide with anisotropic core and substrate [11,12]. Many recent works are concerned with BICs on periodic structures, including two-dimensional (2D) structures with one periodic direction [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], threedimensional (3D) biperiodic structures [28][29][30][31][32][33][34][35][36][37], and 3D rotationally symmetric structures [38,39]. In these studies, the periodic structures are sandwiched between or surrounded by homogeneous media, the BICs are guided Bloch modes above the lightline, and the radiative waves are propagating plane waves in the homogeneous media.…”