ENS?05 Paris, France, 14-16 December 2005
TUNABLE INTEGRATED-OPTICS NANOSCALED DEVICES BASED ON MAGNETIC
PHOTONIC CRYSTALS
M. Vasiliev1, V.I. Belotelov2, K.E. Alameh1, R. Jeffery1, V.A. Kotov3, A.K. Zvezdin3
1 - Centre for MicroPhotonic Systems, Electron Science Research Institute
Edith Cowan University, Joondalup, WA, 6027, Australia.
2 - M.V. Lomonosov Moscow State University, 119992, Leninskie gori, Moscow, Russia
3 - A.M. Prokhorov General Physics Institute, RAS, 119991, Vavilov St., 38, Moscow, Russia
ABSTRACT
Magnetooptical properties of magnetic photonic crystals
have been investigated in the view of their possible
applications for the modern integrated-optics devices. A
?transfer matrices? formalism was expanded for the case
of oblique light incidence on the periodic nanoscaled
magnetic multilayered systems. Several new effects such
as the Faraday effect dependence on the incidence angle
and the tunability of the bandgap defect modes spectral
location by external magnetic fields were found. Several
possibilities of one-dimensional magnetic photonic
crystals applications for the optical devices are discussed.
Initial steps towards the practical implementation of the
proposed devices are reported.
1. INTRODUCTION
For the last decade, photonic crystal (PC) materials have
been the subject of intense theoretical and experimental
studies, which is largely due to their outstanding optical
properties that are very promising for the numerous
integrated-optics applications [1,2]. Photonic crystals are
nanostructured materials with one-, two-, or three-
dimensional periodicity in their dielectric constant. One of
the most prominent features of PCs is the presence of a
photonic band-gap ? the spectral range for which light
propagation through the photonic material is completely
prohibited.
Investigation of PCs with paramagnetic or ferromagnetic
constituents - magnetic photonic crystals (MPC) is of
prime importance nowadays. Magnetic materials placed
periodically in the PC system lead to several new optical
phenomena and give rise to the substantial enhancement
of conventional magnetooptical (Faraday, Kerr, Voight)
effects [3-5]. This makes MPC even more promising
materials for the development of modern nanoscaled
optical devices.
One dimensional (1D) MPC usually represents alternating
magnetic and nonmagnetic layers of a quarter-wavelength
thickness [4,5]. However, some structural defects in that
periodicity can produce additional transmission and
Faraday rotation resonances inside the photonic band-gap.
The examples of two-dimensional MPCs are periodical
hole arrays in the transparent dielectric matrix filled with
a magnetic medium. Systems of periodically located
spheres with voids occupied by some magnetic fluids are
common examples of three-dimensional MPCs [6].
This work is devoted to the investigation of 1D MPCs in
the near-infrared wavelength range. We describe a
theoretical approach for the calculation of their optical
properties and consider several possible applications for
the modern nanoscaled optical devices. First steps of their
experimental implementation are also presented.
2. THEORY
There are several different approaches applicable for the
calculation of the multilayered systems optical properties
[5,7,8]. Among them, the 4?4 transfer matrices method
was proven to be an efficient and intuitively clear
technique [5,9]. That is why in this work we adopt this
approach for the modeling of MPCs.
The essence of the transfer matrices method applied for
the simulation of 1D MPCs is as follows: Let us consider
a 1D MPC placed between substrate of dielectric
Fig.1. The geometry of the problem. The MPC structures of
(NM)2(MN)3(NM)2 with two defects is shown.
z
y
?
Rk
r
Substrate Superstrate
M. Vasiliev, V.I. Belotelov, K.E. Alameh, R. Jeffery, V.A. Kotov, A.K. Zvezdin
Tunable INTEGRATED-OPTICS NANOSCALED DEVICES BASED ON MPC
constants a? and superstrate of dielectric constant b?
(Fig.1). The coordinate system is chosen in such a way
that z-axis is perpendicular to the interfaces of MPC
layers. Visible or near-infrared electromagnetic radiation
is incident at the angle ?. During its propagation through
the structure, multiple reflections and refractions take
place producing a relatively complex distribution of the
electromagnetic field amplitudes inside MPC. Due to the
periodicity of the MPC system, the propagating field is
represented by Bloch waves. Nevertheless, for further
analysis in the framework of transfer matrices, it is much
more convenient to consider the electromagnetic field in
each MPC layer as a superposition of the so-called
?proper modes?, which in the 1D case are four waves
propagating independently of each other in the two
opposite directions and having the states of polarization
which are preserved during propagation.
The main purpose of the transfer matrices method is to
relate the electromagnetic fields within the substrate and
the superstrate. In order to accomplish this, one needs to
describe fully the proper modes propagation throughout
the entire system. That is why three basic matrices are
introduced for each layer. These are proper modes column
vector An, propagation matrix Pn, and dynamic matrix nD .
The column vector An gives the electric field amplitudes
of proper modes at the exit boundary of the n-th layer, the
propagation matrix Pn represents the phase shifts of the
proper modes throughout the n-th layer, and the dynamic
matrix nD relates the amplitudes of the proper modes in
adjacent layers.
The boundary conditions for the tangential components of
electric and magnetic fields at the boundary between n-th
and )1( +n -th layers lead to the following equation for the
amplitudes of proper modes in the superstrate and
substrate media:
00
1
111
11
11 ADDPDDPDDA NNNNN
???
++ = K . (1)
Once proper modes amplitudes before and after MPC are
related, one can find the electric field amplitudes for the
reflected and transmitted radiation, which is necessary for
the optical and magnetooptical properties calculation. For
example, provided that p-polarization is incident, the
Faraday rotation angle is determined by
?
?
?
?
?
?
?
?
=? 2
?-1
?Re2atan
2
1 , (2)
where ps EE=? , sE , pE are proper modes amplitudes
of the transmitted light.
One of the advantages of the transfer matrices technique is
its universality, which enables the optical properties of
both periodic and non-periodic multilayered structures to
be accurately modeled.
Thus, the most important point in transfer matrices
approach is to find proper modes amplitudes in each layer.
They can be determined from the Fresnel equation:
EEnnEn rrrrr ??)(2 =? (3)
where ?kcn
rr
= , k
r
is the wave vector, and ?? is the
dielectric tensor of the medium. Since the tangential
component of wave vector is continuous at all interfaces,
then ?? sinayn = . The component nz is determined
from the condition of solvability of the equation (3) with
respect to the electric field components.
There are infinite number of the proper modes bases for
the electromagnetic waves in non-magnetic layers, but
usually s- and p- plane polarizations are chosen. The unit
vectors of electric and magnetic fields for s- and p-
polarizations of light with 0>zk are given by
( ) ??
?
?
???
? ?==
??
yz
ps
nnee ,,0,0,0,1 rr
(4)
For magnetic layers the situation is different due to the
presence of the gyration vector g. In general, they are to
be characterized in terms of the permittivity and
permeability tensors ??
and ?? , but at the visible and
near-infrared wavelength range the latter can be
considered as the unit tensor.
The permittivity tensor of an optically isotropic magnetic
medium magnetized along the Z-axis has the following
non-zero components [10]:
igigmmmmm =?==== (m)21)(12)(33)(22)(11 , , ?????? (5)
It is assumed here that the second-order magnetooptical
effects are negligibly small for the considered materials.
Using (4) and (5), one can get the following expression
for the longitudinal wavenumber nz in a magnetic layer
magnetized along Z-axis
( ) ??
?
?
???
? ???=
?
m
y
ym
m
z
ngnn
??
2
22)( 1
. (6)
The proper modes electric field components are
determined from the Fresnel equation (3):
( ) ,, ,2
)(
,,2)(, ?
?
??
?
? ?=?
?=
y
my
m
zy
zy
m
mx
En nnEE
n
igE
?? (7)
where ( )2)(2)( mzym nnn ?? += .
It is important to note here that because of the relatively
small values of the gyration g ( 24 1010~ ?? ?g ), the
condition gmm >>?? cos is satisfied for almost all angles
m? , where m? is the average angle between the Z-axis and
proper mode wave vectors inside the magnetic medium.
Taking these conditions into account, expressions (6) and
can be simplified substantially, giving
M. Vasiliev, V.I. Belotelov, K.E. Alameh, R. Jeffery, V.A. Kotov, A.K. Zvezdin
Tunable INTEGRATED-OPTICS NANOSCALED DEVICES BASED ON MPC
m
mm
m
z
gn
??? 2cos
)( ?=
? . (8)
It can be shown that for the case of oblique incidence, the
proper modes inside the magnetic medium are two
elliptically polarized waves with the electric field vector
circumscribing cone surfaces with the elliptical basis and
opening angle very close to 180?, so the cone surfaces are
very close to planes. Consequently, to describe the
Faraday effect for the oblique incidence one can ignore
slight non-orthogonality of the electric field and the wave
vector and consider two mutually perpendicular p- and s-
directions, in analogy with the conventional terminology.
The Faraday effect is determined by the value of the phase
shift between two orthogonal elliptical polarizations,
which is proportional to k? , which, taking into account
(8), is given by
( )
m
zz
gnnk
??
pi?pi
0
0 =?=? ?+ . (9)
where 0? is the incident light wavelength in the vacuum.
That is why for relatively small medium magnetization the
Faraday rotation can be assumed independent of the
incidence angle and, hence, light ellipticity can be
neglected. Consequently, the change in Faraday rotation
with respect to the incidence angle is mainly due to the
redistribution of electromagnetic wave energy among
magnetic and nonmagnetic layers of MPC.
Once the proper modes for magnetic and nonmagnetic
constituents are found, it is possible to calculate transfer
matrices An, Pn, nD and obtain transmittance, reflectance
and Faraday rotation spectra using (1) and (2).
3. RESULTS AND DISCUSSION
Due to the magnetooptical effects, MPC structures can
be used as tunable optical nano-devices. This can be
accomplished by two approaches, namely, (i) the presence
of the Faraday effect inside MPCs allows for the
substantial polarization rotation, which depends on
whether s- or p-polarised wave is incident; and (ii)
magnetooptical light-medium interaction leads to the
changes in s- and p-waves transmittance spectra.
During the modelling of the MPCs optical properties,
we have found that they are considerably more influenced
by the magnetic field at the wavelengths of bandgap
defect modes, rather than near the band-gap edges. For
this reason, the process of engineering nano-devices based
on MPC involves the optimisation of quasi-periodic
multilayer structures and careful placement of structural
defects, which can generate bandgap modes with desired
properties. We have developed efficient C++/Windows-
based algorithms capable of assisting in the MPC design
process [11]. In the remaining parts of this paper, we
present our on-going work on the development of several
novel nano-level optical devices for applications in the
telecommunications industry, which are based on the
unique properties of MPC structures optimised for
operation in the oblique incidence geometry. The features
of the oblique-incidence MPC operation we found most
remarkable are the tunability of the overall spectral region
of operation achieved by controlling the angle of
incidence, and the tunability of the spectral locations of
bandgap defect modes resonances achieved by changing
the magnitude of the applied external magnetic field.
3.1. WDM demultiplexers with transmission loss
equalisation
The ability to tune the operational wavelength of the
MPC by varying the angle of incidence opens up a
possibility of designing demultiplexers with magneto-
optic equalization of transmission loss. A passive or
reconfigurable (based on a spatial light modulator
utilizing liquid crystal on silicon (LCOS) technology ? an
Opto-VLSI processor) diffraction grating can be used to
generate an appropriate (peak transmission-tuned) array of
incidence angles for the light carrying several WDM
signal channels [12]. An Opto-VLSI processor is an array
of liquid crystal (LC) cells whose crystallographic
orientations are independently addressed by a Very-
Large-Scale-Integrated (VLSI) circuit to create a
reconfigurable, reflective, holographic diffraction grating
plate. Application of voltage between the electrodes of the
VLSI circuit induces a phase hologram in the LC layer,
resulting in optical beam steering and/or beam shaping.
Fabricated Opto-VLSI devices are electronically
controlled, software-configured, polarisation independent,
cost effective because of the high-volume manufacturing
capability of VLSI and very reliable since beam steering
is achieved with no mechanically moving parts. Fig. 2
shows a typical layout of an Opto-VLSI processor. Also
shown is typical LC cell design. Usually Indium-Tin
Oxide (ITO) is used as the transparent electrode, and
evaporated aluminium is used as reflective electrode.
Opto-VLSI processors can generate stepped blazed
grating for optical beam steering, as well as multicasting
grating for arbitrary beam splitting, where the diffraction
orders are deliberately enhanced to generate an arbitrary
beam splitting profile [13].
The design of a WDM equaliser employing an Opto-
VLSI processor as an angular demultiplexor and a MPC is
shown schematically in Fig. 3(a). The light incident on the
MPC has to be polarized (in an arbitrary plane for the
case of near-normal incidence, since the s- and p-
polarization responses in both transmission and Faraday
rotation are then nearly identical). Fig. 3(b) shows typical
M. Vasiliev, V.I. Belotelov, K.E. Alameh, R. Jeffery, V.A. Kotov, A.K. Zvezdin
Tunable INTEGRATED-OPTICS NANOSCALED DEVICES BASED ON MPC
input and equalized WDM spectra.
Opto-VLSI Processor
LC Material
ITO
Aluminium
Mirror
Glass
Silicon Substrate
VLSI Layer
Cell Structure
Data Decoder
Ad
dr
ess
De
co
de
r
QWP
Fig. 2. Typical Opto-VLSI processor and an LC cell structure
design.
Another polarizer (analyzer) can be used in the optical
path after the MPC to achieve extinction when the plane
of polarization is rotated magnetooptically by 45 degrees
in a chosen direction. The transmitted intensity in each
channel can then be controlled independently by rotating
the required planes of polarization by ?45 degrees with
magnetic fields applied locally using an array of
integrated current-carrying coils (deposited onto MPC exit
surface) that encircle the propagation paths of light. The
crosstalk in the output will be very low due to the spatial
separation of the channel propagation paths and the
spectral sharpness of the MPC transmission resonances
which are tuned to their corresponding angles of
incidence. Due to the very low absorption of commonly
used magnetic layer materials (bismuth or cerium-
substituted YIG) in the 1.55 ?m optical
telecommunications window, it is possible to design MPC
structures possessing very sharp spectral resonances
having high transmission coupled with significantly
enhanced Faraday rotation. In this case, relatively small
magnetic fields can achieve large Faraday rotations within
narrow spectral windows, which can be tuned by changing
the incidence angle. For example, a MPC structure with a
design formula (NM)12(MN)1(NM)1(MN)12, in which SiO2-
layers are used for non-magnetic constituent ( 24.2=? ),
has a very sharp resonant behaviour in both transmission
and Faraday rotation near the wavelength of 1.55 ?m.
This quarter-wave stack structure is composed of 52
layers and has a thickness of 11.29 ?m. The spectral
response of this structure, its variation with the angle of
incidence and its optical response to the gyration are
shown in Fig. 4. Here, we assumed two types of the
surrounding media pairs, namely, (i) air and glass as
before ( 1=a? , 310.2=b? ), and (ii) the substrate and the
exit medium which are index-matched to the mean
refractive index of the structure ( 168.3== ba ?? ).
Diffraction grating
(passive or reconfigurable ? LCOS
technology)
MPC
Polariser
?1, ?1
?n, ?n
? (?i)
(a)
Input Output
(b)
Fig. 3. (a) Principal diagram of MPC-based WDM equalizer utilising
the oblique incidence geometry for localised control of channel
transmittance, (b) input and equalized WDM spectra.
The gyration required for achieving the 45 degrees of
rotation is g = 0.00035, which can be achieved at about
3.8 % of the saturation magnetisation for Ce:YIG.
The splitting of two opposite elliptical polarizations
resonances and its associated reduction in peak
transmission observed at high levels of induced gyration
limit the dynamic range of the device and introduce some
extra loss, however, in some equalizer applications these
drawbacks can be outweighed by the advantages of the
high-speed operation characteristic of the magneto-optic
devices. The response time required for switching of each
channel from a minimum to a maximum of transmitted
intensity can be as short as tens of nanoseconds.
The range of incidence angles (and, consequently the
number of channels that can be processed by a single
device of this type) can in principle be extended to large
angles if control of the polarization of incident light is
implemented. The wavelength-tuning curve (the
dependency of the MPC operational wavelength on the
angle of incidence) of the structure being considered is
shown in Fig.5. This dependency is highly nonlinear for
very small angles of incidence, but for moderate incidence
angles, it can be approximated by a linear function with a
slope of about 1 nm/deg for index-unmatched air and
glass surrounding media and 6.7 nm/deg for index-
matched surrounding media. The structure of the
dispersion grating to be used for demultiplexing the multi-
channel optical input must be optimized to closely match
M. Vasiliev, V.I. Belotelov, K.E. Alameh, R. Jeffery, V.A. Kotov, A.K. Zvezdin
Tunable INTEGRATED-OPTICS NANOSCALED DEVICES BASED ON MPC
its angular dispersion function with a selected section of
the wavelength-tuning curve of the MPC.
154850 154900 154950 155000 155050
0
20
40
60
80
100
(a)
g=0.00035
g=0
?=0 deg.?=4 deg.
g=0.00035
g=0
Wavelength, ?m
Po
we
r tr
an
sm
itta
nc
e,
%
1.54 1.54 1. 5 .55 1. 5
Wavelength (?m)
(a)
Po
we
r tr
an
sm
itta
nc
e (
%)
154850 154900 154950 155000 155050
-50
-40
-30
-20
-10
0
g=0.00035g=0.00035
g=0.00015g=0.00015
?=0 deg.?=4 deg.
(b)
Fa
rad
ay
ro
tat
ion
, d
eg
.
Wavelength, ?m
.5485 .549 .5495 . 5 1. ( )
Wavelength (?m)
Po
we
r tr
an
sm
itta
nc
e (
%)
Fa
rad
ay
ro
tat
ion
(d
eg
.)
Fig. 4. Transmittance and Faraday rotation spectra of the structure
(NM)12(MN)1(NM)1(MN)12 at normal and 4 degrees incidence with
various strengths of the applied magnetic field. (a) Averaged
(unpolarised) transmittance spectra at normal and 4? incidence and their
variation with gyration; (b) Faraday rotation spectra for p-component at
normal and 4? incidence at various gyrations. The spectral responses of
s- and p-components of polarization are almost identical at near normal
incidence. 1=a? , 310.2=b? .
An array of surface-deposited microcoils of 100 ?m
internal loop diameter and having either one or three turns
was designed, fabricated and tested for its suitability for
the proposed equalizer application. During our
experiments, a single-layer, 20-?m thick film of Bi-
substituted YIG was used to confirm the viability of
achieving sufficient levels of magnetisation within the
film for the operation of proposed device using a single-
turn microcoil. The geometry of microcoils and a
schematic of our experimental set-up are shown in Fig. 6.
A current pulser circuit was designed to output pulses of 2
?s duration at a repetition rate of 1 kHz which enabled
currents of up to 6 A to be used during testing. The
magneto-optic response was also studied using DC
currents of up to 0.3 A for field generation.
0 2 4 6 8 10 12 14
1500
1510
1520
1530
1540
1550
W
av
ele
ng
th,
nm
Incidence angle, deg.
W
av
ele
ng
th
(nm
)
Incidence angle (deg.)
Fig. 5. MPC operational wavelength as a function of incidence angle for
the structure (NM)12(MN)1(NM)1(MN)12 sandwiched between (i) air
and glass ( 1=a? , 310.2=b? ) (solid line), or (ii) two identical
media index-matched to the mean refractive index of the structure
( 168.3== ba ?? ) (dashed line).
The investigations revealed that the saturation of
magnetisation in the film was reached at a coil current of
about 2.8 A, confirming the suitability of our system for
the proposed application with both DC and pulsed
currents, provided that the optimised MPC structure
requires less than about 10% of the saturation
magnetisation to achieve Faraday rotations approaching
45?.
(a)
(b)
Fig. 6. (a) Geometry of microcoils used for magnetic field generation;
(b) Schematic of the experimental set-up used for characterisation of
magneto-optic response and studies of magnetisation dynamics.
3.2. Tunable optical switches employing magnetic
control of bandgap defect modes.
Medium magnetization also affects the wavelength of the
transmittance peak, as shown in Fig. 7 where
M. Vasiliev, V.I. Belotelov, K.E. Alameh, R. Jeffery, V.A. Kotov, A.K. Zvezdin
Tunable INTEGRATED-OPTICS NANOSCALED DEVICES BASED ON MPC
transmittance spectra of the incident p-wave incident onto
the MPC of formula (NM)31(MN)18(NM)31 for ,0=g
,01.0=g ,03.0=g and 05.0=g are plotted. The
wavelength shift transmittance peak is proportional to the
medium gyration, and, consequently, to the medium
magnetization.
144400 144500 144600 144700 144800
0
20
40
60
80
Wavelength, ?m
Po
we
r tr
an
sm
itta
nc
e,
%
1. 4 1.445 1. 46 1.447 1.448
Wavelength (?m)
Po
we
r tr
an
sm
itta
nc
e (
%)
Fig. 7. Transmittance spectra of the p-polarization incident on the MPC
of formula (NM)31(MN)18(NM)31 at the 50? angle for four different
magnetic layers gyrations: g = 0 (solid line), g = 0.01 (dashed line), g =
0.03 (dash-dotted line), and g = 0.05 (dotted line).
1?
2?
0=g
1?
2?
02.0=g
1?
2?
05.0=g
MPC MPC MPC
Fig. 8. Schematic diagram of a tunable polarization-dependent optical
switch utilising magnetic control of the spectral localization of MPC
bandgap defect modes. Depending on the medium gyration, only one
polarization type (p-pol. and s-pol. are shown by arrows with circle-ends
and simple ends, respectively) of two incident wavelengths (shown by
solid and dotted lines) is transmitted for each of the shown gyrations.
The linear shift in the defect mode position has the same
nature as the magnetic field induced shift of the band gap
edges theoretically predicted and explained in [14]. It is
worth noticing that the peaks of different polarization (s-
polarization for the considered case) generally occur at
different wavelengths. That is why the effect described
here is of practical value because it can be utilized for
tunable optical devices such as polarizers and switches.
The latter, for example, being organized on the basis of
the suggested structures will enable to separate s- and p-
incident polarizations at the desired wavelength which can
be finely tuned by the magnetic field (Fig. 8).
While MPCs have been proposed for optical
polarizers, the output light ellipticity can significantly
limit the performance of MPC-based polarizers. In the
case of the discussed MPC scheme the ellipticity increases
with the gyration and it is about 1.5? for the Ce:YIG
saturation magnetization corresponding to 009.0=g .
Consequently, for the best performances, optimization of
the MPC structure is vital.
4. CONCLUSION
In this paper we considered magnetooptics of 1D MPCs
paying much attention to their possible applications for
the modern nanoscaled optics devices. For the theoretical
description we utilized a transfer matrices formalism
adopted to the case of oblique light incidence. Tunability
of the band-gap defect modes spectral location by external
magnetic fields was found. Several possibilities of one-
dimensional magnetic photonic crystals applications for
the optical switch and WDM demultiplexers with
transmission loss equalization were discussed. Initial steps
towards the practical implementation of the proposed
devices were reported.
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