Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Nonreciprocal light transmission in parity-time-symmetric
  whispering-gallery microcavities   Optical systems combining balanced loss and gain profiles provide a unique
platform to implement classical analogues of quantum systems described by
non-Hermitian parity-time- (PT-) symmetric Hamiltonians and to originate new
synthetic materials with novel properties. To date, experimental works on
PT-symmetric optical systems have been limited to waveguides in which
resonances do not play a role. Here we report the first demonstration of
PT-symmetry breaking in optical resonator systems by using two directly coupled
on-chip optical whispering-gallery-mode (WGM) microtoroid silica resonators.
Gain in one of the resonators is provided by optically pumping Erbium (Er3+)
ions embedded in the silica matrix; the other resonator exhibits passive loss.
The coupling strength between the resonators is adjusted by using
nanopositioning stages to tune their distance. We have observed reciprocal
behavior of the PT-symmetric system in the linear regime, as well as a
transition to nonreciprocity in the PT symmetry-breaking phase transition due
to the significant enhancement of nonlinearity in the broken-symmetry phase.
Our results represent a significant advance towards a new generation of
synthetic optical systems enabling on-chip manipulation and control of light
propagation.

36
The categories of this paper are: cmp-lg
21
The categories of this paper are: adap-org
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
9
The categories of this paper are: nlin
23
The categories of this paper are: solv-int
26
The categories of this paper are: bayes-an
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas
31
The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
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The categories of this paper are: mtrl-th
16
The categories of this paper are: stat