Mechanism of the optical unidirectional transmission in metal
subwavelength grating with different surfaces
$
Chen Zhao
a
, Yun-Song Zhou
a,
n
, Huai-Yu Wang
b,
nn
, Hai Wang
a
, Li-Ming Zhao
a
a
Center for Theoretical Physics, Department of Physics, Capital Normal University, Beijing 100048, China
b
Department of Physics, Tsinghua University, Beijing 100084, China
article info
Article history:
Received 22 June 2013
Received in revised form
15 November 2013
Accepted 22 November 2013
Available online 6 December 2013
Keywords:
Unidirectional transmission
Reciprocity
Thin films
Grating
abstract
The mechanism of the unidirectional transmission in metal subwavelength gratings with different
surfaces is investigated theoretically. This kind of unidirectional transmission belongs to the extra-
ordinary optical transmission assisted by the surface plasmon polariton (SPP). The SPP wave comes
through the multi-reflection process and finally transforms into the transmission wave. The difference of
amplitude transmissivities and reflectivities at different surfaces in opposite incident directions results in
the unidirectional transmission. A division method is proposed to calculate the parameters of subwave-
length gratings with asymmetric surfaces.
& 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction
Optical unidirectional (OUD) devices are crucial in optical
circuits just as diodes in electric circuits. They play a significant
role in optical information processing and are named as optical
diodes. Some kinds of systems have been proposed in order to
implement OUD transmission such as nonlinear materials [1–3],
photonic crystal heterostructures [4–6], anisotropic liquid crystals
[7], magneto-optic (MO) materials [8], MO-metal film compound
structures [9–11 ] and MO photonic crystals [12,13]. The OUD
phenomenon has been observed experimentally in the nonlinear
material, photonic crystal heterostructure, and anisotropic liquid
crystal systems. Recently, a kind of simpler and more effective
OUD device was proposed [14–16] in which an optical OUD
phenomenon has been observed. This new kind of OUD device
was composed of subwavelength structures, such as hole or
slit arrays, cut into the metal film with two different surfaces.
We refer this kind of OUD devices to as mid-plane asymmetry
structure in this paper because the mid-plane asymmetry struc-
ture is the unique characteristic of these devices. Compared with
the previous ones, a mid-plane asymmetry structure possesses
three advantages: there is no restriction that the incident light
must be polarized in mid-plane asymmetry structures with hole
array (but not the mid-plane asymmetry structures with slit
array); it is available for normal incident light; it is structurally
simpler and easier to fabricate. However, the mechanism of the
OUD phenomenon in the mid-plane asymmetry structure has not
been quite clear till now in spite of that numerical simulations and
experimental work have been carried out [14–16]. For instance, in
studying the OUD transmission in mid-plane asymmetry structure
in Ref. [15], the dual-metal grating in a normal direction was
employed and the non-reciprocity was purported. But the authors
of Ref. [16] did not believe that the zero-order nonreciprocity
could not be achieved in this structure. We agree with them. In
fact, in Ref. [15], the phase difference between the lights radiated
from the nearest-neighbor slits in the subwavelength grating is
just
π
, and the normally transmission light is not the zero-order
transmission. So the OUD transmission in the studied structure did
not mean the non-reciprocity. It should be convinced that, in the
mid-plane asymmetry metal structure, the reciprocity was valid
and any OUD transmission was attributed to the higher order
transmission. However, the above discussion is from the view
angle of diffraction theory, what happened inside the mid-plane
asymmetry structure was still to be explained from a
microscopic view.
In this paper, we disclose the physical mechanism of the OUD
transmission in the mid-plane asymmetry structures. A division
method is proposed to calculate the OUD parameters. The results
calculated by this method are in good agreement with those
obtained from the finite-difference time-domain (FDTD) method.
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/optcom
Optics Communications
0030-4018/$ - see front matter & 2013 The Authors. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.optcom.2013.11.045
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This is an open-access article distributed under the terms of the Creative
Commons Attribution-NonCommercial-No Derivative Works License, which per-
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n
Corresponding author.
nn
Corresponding author.
E-mail addresses: 263zys@263.net (Y.-S. Zhou),
wanghuaiyu@mail.tsinghua.edu.cn (H.-Y. Wang).
Optics Communications 316 (2014) 17–21