Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of Unidirectional Bound States in the Continuum Enabled by
  Topological Defects   Unidirectional radiation is important for a variety of optoelectronic
applications. Many unidirectional emitters exist, but they all rely on the use
of materials or structures that forbid outgoing waves, i.e. mirrors. Here, we
theoretically propose and experimentally demonstrate a class of resonances in
photonic crystal slabs, which only radiate towards a single side with no mirror
placed on the other side - we call them ``unidirectional bound states in the
continuum". These resonances are found to emerge when a pair of half-integer
topological charges in the polarization field bounce into each other in the
momentum space. We experimentally demonstrate such resonances in the
telecommunication regime, where we achieve single-sided quality factor as high
as 1.6e5, equivalent to a radiation asymmetry ratio of 27.7 dB. Our work
represents a vivid example of applying topological principles to improve
optoelectronic devices. Possible applications of our work include grating
couplers, photoniccrystal surface-emitting lasers, and antennas for light
detection and ranging.

9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
22
The categories of this paper are: chao-dyn
35
The categories of this paper are: mtrl-th
1
The categories of this paper are: cs
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
4
The categories of this paper are: cond-mat
6
The categories of this paper are: astro-ph
10
The categories of this paper are: q-bio
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess