We analyze the physical meaning of exceptional points (EPs), which are the spectral degeneracies of the non-Hermitian optical systems. The situation is complicated by the existence of two definitions of the scattering matrix giving different positions for the EPs. Taking a PT-symmetric multilayer structure as a specific example, we give the unambiguous formulation of the scattering matrix and find the “true” EPs where the symmetry breaking occurs. However, the EPs given by another definition of the scattering matrix are not totally meaningless being associated with coherent perfect absorption and lasing.

1.
L.
Feng
,
R.
El-Ganainy
, and
L.
Ge
,
Nat. Photon.
11
,
752
(
2017
).
2.
R.
El-Ganainy
,
K. G.
Makris
,
M.
Khajavikhan
,
Z. H.
Musslimani
,
S.
Rotter
, and
D. N.
Christodoulides
,
Nat. Phys.
13
,
11
(
2018
).
3.
S. K.
Özdemir
,
S.
Rotter
,
F.
Nori
, and
L.
Yang
,
Nat. Mater.
18
,
783
(
2019
).
4.
D. V.
Novitsky
,
A.
Karabchevsky
,
A. V.
Lavrinenko
,
A. S.
Shalin
, and
A. V.
Novitsky
,
Phys. Rev. B
98
,
125102
(
2018
).
5.
A. U.
Hassan
,
B.
Zhen
,
M.
Soljačić
,
M.
Khajavikhan
, and
D. N.
Christodoulides
,
Phys. Rev. Lett.
118
,
093002
(
2017
).
6.
W.
Chen
,
S. K.
Özdemir
,
G.
Zhao
,
J.
Wiersig
, and
L.
Yang
,
Nature
548
,
192
(
2017
).
7.
H.
Hodaei
,
A. U.
Hassan
,
S.
Wittek
,
H.
Garcia-Gracia
,
R.
El-Ganainy
,
D. N.
Christodoulides
, and
M.
Khajavikhan
,
Nature
548
,
187
(
2017
).
8.
L.
Feng
,
Z. J.
Wong
,
R.-M.
Ma
,
Y.
Wang
, and
X.
Zhang
,
Science
346
,
972
(
2014
).
9.
H.
Hodaei
,
M.-A.
Miri
,
M.
Heinrich
,
D. N.
Christodoulides
, and
M.
Khajavikhan
,
Science
346
,
975
(
2014
).
10.
Z. J.
Wong
,
Y.-L.
Xu
,
J.
Kim
,
K.
O’Brien
,
Y.
Wang
,
L.
Feng
, and
X.
Zhang
,
Nat. Photon.
10
,
796
(
2016
).
11.
L.
Ge
,
Y. D.
Chong
, and
A. D.
Stone
,
Phys. Rev. A
85
,
023802
(
2012
).
12.
D. V.
Novitsky
,
J. Opt.
21
,
085101
(
2019
).
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