Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: $\mathbb{Z}_{2}$ Topological Index for Continuous Photonic Materials   Electronic topological insulators are one of the breakthroughs of the 21st
century condensed matter physics. So far, the search for a light counterpart of
an electronic topological insulator has remained elusive. This is due to the
fundamentally different natures of light and matter and the different spins of
photons and electrons. Here, it is shown that the theory of electronic
topological insulators has a genuine analogue in the context of light wave
propagation in time-reversal invariant continuous materials. We introduce a
Gauge invariant $Z_2$ index that depends on the global properties of the
photonic band structure and is robust to any continuous weak variation of the
material parameters that preserves the time-reversal invariance. A nontrivial
$Z_2$ index has two possible causes: (i) the lack of smoothness of the
pseudo-Hamiltonian in the ${\bf{k}} \to \infty$ limit, and (ii) the
entanglement between positive and negative frequency eigenmode branches. In
particular, it is proven that electric-type plasmas and magnetic-type plasmas
are topologically inequivalent for a fixed wave polarization. We propose a
bulk-edge correspondence that links the number of edge modes with the
topological invariants of two continuous bulk materials, and present detailed
numerical examples that illustrate the application of the theory.

22
The categories of this paper are: chao-dyn
36
The categories of this paper are: cmp-lg
14
The categories of this paper are: math-ph
15
The categories of this paper are: nucl-ex
10
The categories of this paper are: q-bio
31
The categories of this paper are: ao-sci
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
26
The categories of this paper are: bayes-an
21
The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys