Topological charge engineering in lasing bound states in continuum

Abstract

Recently, optical bound states in continuum in various passive photonic crystals have been identified and similar structures incorporated with optical gain have been reported to exhibit lasing. However, no explicit control over the type of lasing BIC has been reported. In this work, we utilize all four fundamental BICs related to the lowest energy Γ-point of a square photonic crystal lattice. We identify the associated topological charges from experimentally obtained dispersions, finite element method simulations, as well as from spherical decomposition method based on the microscopic polarization currents in the photonic crystal plane. By tailoring the periodicity and the hole diameter of the photonic crystal slab, we selectively bring each of the four BIC resonances to a wavelength regime, where fluorescent IR702 molecules overlaid with the photonic crystal provide sufficient gain for the onset of lasing. We experimentally analyze all four observed lasing BICs by imaging their far-field polarization vortices and their associated topological charges. The results correspond excellently with the transmission results as well as the simulation results in the absence of gain. Finally, we experimentally present a case where the lasing signal reveals the coexistence of two BICs with opposite topological charges, resulting in a unique polarization pattern. We believe our results enable tailoring the properties, such as polarization winding and topological charge of BICs, by a priori design and thus pave the way for a more general utilization of their appealing properties.

Bound states in the continuum in periodic structures with structural disorder

“…In the last two decades, BICs have been observed in many photonic structures such as photonic crystals, metasurfaces, [2][3][4], chains of coupled resonators [5][6][7][8], waveguides [9][10][11][12][13] and even single subwavelength resonators supporting quasi-BIC in a form of supercavity modes [14][15][16]. A divergent radiative quality (Q) factor, strong spatial localization, and drastic enhancement of the incident field make BICs very prospective for many applications including lasers [17][18][19][20][21][22][23][24][25], optical filters [26][27][28], biological and chemical sensors [29][30][31][32][33], and non-linear photonics [34][35][36][37][38][39][40][41][42][43].…”