tmp7D76

Non-polarizing single layer inorganic and double layer organic-inorganic
one-dimensional guided mode resonance filters
Muhammad Rizwan Saleem
a,b
, Seppo Honkanen
a
, and Jari Turunen
a

a
University of Eastern Finland,

Department of Physics and Mathematics, P. O. Box 111, FI-80101
Joensuu, Finland.
b
National University of Sciences and Technology (NUST), School of Chemical and Materials
Engineering (SCME), Sector H-12, Islamabad, Pakistan.

* Corresponding author: (Tel) +358 50 442 3469, (Fax) +358 13 251 2721
E-mail: [email protected], [email protected]
Abstract
Guided mode resonance (GMRF) phenomena occurs when the evanescent orders of a diffraction grating are
coupled to the waveguide modes and propagate out at given optical parameters such as wavelength, angle, and
state of polarization of incident light. The outcoupling field from a waveguide is, in general, polarization
sensitive. Polarization insensitive 1D subwavelength grating structures with high diffraction efficiency at
normal and oblique incidence are required, for example, in optical communications where output light may
possess any polarization state. This means that an s- or p-polarized input optical field, which generally couples
TE- or TM-modes in the waveguide under different resonance conditions, can be tuned at one resonance by
selecting suitable grating parameters, regardless of the input polarization state. All of the polarization insensitive
devices fabricated to date either employing a method which is not cost-effective or simple enough to some
extent. In this work, we report the design and fabrication of two types of non-polarizing binary-structured one-
dimensional (1D) GMRF at normal incidence. A single layer binary-profile TiO
2
resonant grating (grating-I) is
fabricated by Atomic layer deposition (ALD), electron beam lithography (EBL) and reactive ion etching (RIE),
which demonstrates almost perfect non-polarizing filtering effect with 1D grating under normal incidence. A
double layer rectangular-profile polycarbonate-TiO
2
1D GMR grating (grating-II) is fabricated by nanoimprint
lithography (NIL) and ALD which also shows good non-polarizing property and the potential of cost-effective
mass fabrication of such functional devices.
1. Introduction
Polarization insensitive diffraction gratings are highly desirable for optical communications because of the
unknown polarization state of the light emerging from optical fibers in dense-wavelength-division-multiplexing
system. Guided mode resonance filters GMRFs can be employed as the polarization insensitive gratings which
can couple TE- or TM-modes with either s- or p-polarized input optical fields. GMRF effect occurs when the
evanescent diffraction orders coupled into the waveguide mode and propagate in waveguide layer at appropriate
optical parameters such as wavelength, angle, and state of polarization of incident light. All of the polarization
independent devices fabricated to date pass through a process which is not cost-effective to some extent. For
example, total internal reflection polarization independent gratings have been fabricated in single dielectric
material with low insertion and polarization dependent loss
1
. Similarly high efficiency polarization independent
designed and fabricated gratings in fused silica for wideband transmission are used in chirped pulse
applications
2
. Narrowband polarization independent 1D guided mode resonance filters GMRFs are designed at
conical incidence for simultaneous excitation of two modes, the spectral position and width are controlled by
Advanced Fabrication Technologies for Micro/Nano Optics and Photonics VI, edited by Georg von Freymann,
Winston V. Schoenfeld, Raymond C. Rumpf, Proc. of SPIE Vol. 8613, 86130C · © 2013 SPIE
CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2001692
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grating duty cycles and depth respectively for TE and TM modes
3
. Similarly, narrowband reflection filters as
the resonance grating filters showing polarization independence behavior by considering orthogonal incidence
plane parallel to grating grooves with the result that a small change in the conical angle can lead to a large drop
in the efficiency and distortion of lineshape of the resonance peak
4
. High diffraction efficiency GaAs
polarization independent gratings are designed for signal processing, optical computing applications and in- out-
coupling devices
5
. The design of one dimensional metal grating as polarization independent wavelength filter
can be achieved by tuning the geometrical parameters of the grating structure, for example, the linewidth of the
transmission peak is changed by varying the aspect ratio of the structure
6
. In general, a linearly polarized input
optical field can couple both TE and TM modes in the waveguide under different resonance conditions which
can be tuned at the same resonance condition by choosing appropriate grating parameters, regardless of the
input polarization state. Sub-wavelength one-dimensional with high diffraction efficiency and low polarization-
dependence GMRFs at normal incidence are highly desirable
7
.
Atomic layer deposition is a process that produces very smooth, conformal, high index material coatings with
precise film thickness without any deposition inadequacy such as line of sight over corrugated structures. The
deposition process proceeds in sequence of precursor cycles in terms of monolayer formation over the surface of
substrate through surface reactions of the injected precursor materials. For TiO
2
waveguides, the requirements
of uniform film thickness are important since the guided mode losses its coupling power while propagation
along the grating because of change of out of plane coupling effects due to thickness changes. In this way the
nature of guided mode, in general, becomes a leaky mode with a corresponding complex propagation constant.
In this paper we report on the design and fabrication of polarization independent single layer inorganic (grating
I) and double layer organic-inorganic (grating II) at normal incidence using Fourier Modal Method (FMM)
based on the rigorous coupled wave analysis. By selecting suitable parameters of the designed gratings I and II,
the resonances for TE- and TM-polarized light occur at the same wavelength. The complete fabrication of two
types of gratings with experimentally measured optical responses is presented. Fabrication of polarization-
independent grating I consist of several steps which increase the cost of the process, whereas a low cost
polarization-independent filter is demonstrated which is highly desirable in a number of applications, for
example, an add-drop optical filter in a communication system. High efficiency GMRFs fabricated in
thermoplastics after nanoimprint lithography are of much more interest to employ as the non-polarizing filters
with the aim of low cost and a simplified fabrication process
8
. Figures 1(a) and (b) show the ideal profiles of
GMR gratings of type-I and type-II, respectively.

Figure 1. Schematic diagrams of the GMR (a) grating I and (b) grating II.

2. Design of non-polarizing waveguide gratings
All the sub-wavelength grating structures are designed, simulated and rigorously computed by FMM, also called
rigorous coupled wave analysis. During analysis it was found that structure depth has a significant influence on
the resonance position for both polarizations, for example, an increase in groove height (depth) shifts the
resonance peak towards shorter wavelength for TE-polarization and towards longer wavelength for TM-
polarization. One can tune both polarizations at single resonance wavelength since the grating height becomes a
limiting parameter after a certain value that can also cause constraints on the replicating structures. An increase
in the refractive index of the structure shifts both the TE- and TM-peaks towards longer wavelengths. The most
important and significant design parameter that facilitates the tuning of both TE- and TM-peaks at a single
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0 . z






wavelength is the linewidth w (duty cycles) of the considered structure. It is important to mention that this
parameter dependence attracts little consideration as compared to grating height h. Grating I showed a strong
dependence on duty cycles whereas grating II possess relatively weak dependence. If one considers constant
grating height and grating period for a particular polarization, the corresponding changes in linewidths alter the
number of linewidth modes, energy at which resonance occurs, and the field distribution inside the material.
This describes the stronger dependence of grating I on duty cycles than grating II due to contribution of high
index TiO
2
material.
The GMRFs are based on the principle of interference of two propagating modes in the grating region for both
polarizations TE and TM
9
. The resulting outcoupling wave depends on the phase between the two interfering
waves
10
. In the design of non-polarizing GMRFs, an adjustment of the structural parameters can result in a fine
tuning of the dispersion relations of TE and TM excited leaky guided modes in the grating layer. As a result,
there exist a situation where both polarizations have the same propagation constant at the cross point of the
dispersion curves of TE- and TM-modes at normal incidence. In this design, subwavelength (d < λ) grating
structures are considered to allow only the propagation of zeroth transmitted diffraction order at the resonance.
Since the resonance wavelength λ
r
is related to the grating period d, one can tune GMR phenomena at any
desired wavelength by selecting a period d < λ
r
/n
sub
11
. Finally, the non-polarizing effect is obtained by
optimizing the structural parameters which require engineering of the dispersion relations of TE- and TM-leaky
guided modes so that the simultaneous excitation of both modes occurs at normal incidence. One possible set of
realizable grating parameters taking into account the feasibility of fabrication: for grating I, structure period
d=540 nm, linewidth w=401.5 nm, and structure height h=199 nm; while for the grating II, structure period
d=540 nm, linewidth w=200 nm, structure height h=145 nm, and TiO
2
thickness t=60 nm. The simulated spectra
of the zeroth orders of the two gratings are shown in Fig. 2. It is evident that the non-polarizing effect occurs at
a wavelength λ=850 nm for both gratings.


Figure 2. Simulation results of specular reflectance at normal incidence showing a resonance wavelength λ
r
=850 nm at the
designed values of a period d, a linewidth w, and a ridge height h for both TE- and TM-modes for (a) grating I and (b)
grating II.

3. Experiment
3.1. Fabrication of non-polarizing waveguide gratings
The fabrication process of grating I is shown in Fig. 3. The process starts with the cleaning of fused silica (FS)
sample of 1ʺ and 0.5 mm in thickness with isopropanol. A high index and amorphous TiO
2
film of ~200 nm is
grown on FS sample by atomic layer deposition process, using Beneq TFS 200-152 reactor using commonly
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W i g













known precursor materials TiCl
4
and H
2
O at a deposition temperature of 120 °C. After TiO
2
growth, the sample
was subjected to deposit a thin chromium layer of ~30 nm by electron beam evaporation at a vacuum level of
1.5 x 10
-6
mbar and deposition rate of 2 Å/s using Lebold L560 vacuum evaporator from Lebold Heraeus. After
Cr layer, sample was spin coated by positive electron beam resist ZEP 7000 22 at a spinning speed of 3000 rpm
for 60 s using Headway PWM101D from Headway research Ltd followed by soft baking at 180 °C for 180 s on
a hot plate. After all kind of coating layers on sample the grating patterns were written on an area of 49 mm
2
at a

Figure 3. Schematic diagram of the fabrication process of grating I.
scaled dose of 300 μC/cm
2
by Ebeam writer EBPG5000+ES HR from Vistec Lithography. The e-beam exposure
was followed by isopropanol for 30 s and finally rinsing with DI water and dry nitrogen blow. Dry chromium
etching was performed at low pressure (15 mtorr) process based on Cl
2
and O
2
gases together with inductively
coupled plasma (ICP) at 1500 watt by using Plasmalab 100 from Oxford Plasma Technology. The resist was
removed under O
2
plasma with O
2
flow of 20 sccm (standard cubic centimeter) at RF power 100 watt for 180 s
using March CS-1701 from Microtech-Chemitech AB. This process not only removes the resist and its
constituent ashes but also clean the sample thoroughly for next process. After resist removal, TiO
2
etching was
performed by SF
6
and Ar plasma using Plasmalab 80 from Oxford Plasma Technology. Subsequent to TiO
2

etching, sample was again cleaned for any particles by O
2
plasma under the same power and time using March
CS-1701. Final step was performed to remove Cr layer completely by wet Cr etching process, which was
achieved by a mixture of Ammonium cerium (IV) nitrate from Sigma-Aldrich, acetic acid and DI water for
sufficient time till the complete removal of Cr layer followed by rinsing with DI water and drying with nitrogen
blow. For the fabrication and replication of grating II, see [9]. The stamp is fabricated on Si-wafer by negative
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2 0 L P m
M a p = 1 7 f i W K F
E N T = 6 0 0 M /
w o - 6 m m
s B P . I R = I n L . n e 0 4 ] 0 M . 2 0 1 2
P h o t o M . = 8 8 8 6 T i m e : 1 1 : 2 2 4 4
S i g n a l F = 1 n L . n e D a b 2 8 M U 2 0 1 2
P h o t o N e . = 8 8 6 2 T i m e : 1 6 ' . 2 8 2 0

S g n S F = 1 n L e e e D a l e 2 6 M a r 2 0 1 2
P h o t o N o . . 8 4 3 6 T a m e ' . 1 2 0 6
2 0 0 3 8 3 E N T = 2 0 2 1 v s i g n a l A = I n L e n e 2 8 M n 2 0 1 2
P h o t o N o . 4 8 5 1 6 T i m e 1 ] % 0 1 M a p = 1 1 5 0 0 0 % - 0 0 0 = 3 m m


e-beam resist HSQ without any etching process and replication in thermoplastic is employed using Obducat
Eitre imprinter followed by TiO
2
amorphous, high index cover layer of thickness ~80 nm by ALD process.
3.2. Characterization of grating structures
The geometry of grating structures is characterized by scanning electron microscope (SEM LEO 1550 Gemini)
and is shown in Fig. 4. SEM picture of grating I shown in Fig. 4a exhibit a profile with a positive slope since the
etching ions undergo less degree of freedom with the increase of etching depth, as a result the etching depth
become narrow. The average etching depth of grating I is approximately h
1
= 200 nm which is the thickness of
TiO
2
film. Figure 4b shows the master stamp for replication in polycarbonate fabricated by HSQ resist on
silicon substrate. The stamp profile is approximately rectangular with a slight positive slope which is considered
as an ease for the replication process during de-adhesion of the stamp from polymer. The master stamp has an
average height h
2
= 147 nm. Figure 4c shows the front view of replicated profile in polycarbonate that is
obtained after cutting the sample of grating II in liquid nitrogen with an average height of ~145 nm. Figure 4d
shows the front cut view of the replicated profile in liquid nitrogen after TiO
2
cover layer of 80 nm.

Figure 4. SEM pictures of the fabricated GMR gratings. (a) Type-I grating on fused silica substrate. (b) HSQ master stamp
on Si substrate. (c) The replicated polycarbonate grating and (d) final double layer grating-II with a TiO
2
cover layer of 80
nm.
3.3. Optical characterization
The optical properties were characterized by a variable angle spectroscopic ellipsometer VASE by J. A.
Woollam Co. The ellipsometer was set to measure the spectral transmittance for both TE and TM modes at
normal incidence with a beam spot size of 3 mm and scanning wavelength step of 0.2 nm in the wavelength
range from 700 to 1000 nm.
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0 . 9 -
0 . 8 -

0 . 6
X : 8 4 6 2 . Y : 0 . 9 8 9 Y _ X 4 ^ _ G . Y 0 . 1 9 4
t
I 1
0 . 2 -
0 . 1 -
7 0 0 7 2 5 7 5 0 7 7 5 8 0 0
- T M T r a n s m i t t a n c e
- T E T r a n s m i t t a n c e
- - - T M R e f l e c t a n c e
- - - - T E R e f l e c t a n c e
i -

8 2 5 8 5 0 8 7 5 9 0 0 9 2 5 9 5 0 9 7 5 1 0 0 0
A [ u m ]

0 . 9
0 . 8
0 . 7
- - - T M R e f l e c t a n c e f s ,
- T E R e f l e c t a n c e
5
- T M T r a n s m i t t a n c e
- T E T r a n s m i t t a n c e
0 . 3 ; , .
0 . r
0 . 1 -
9 5 0 7 7 5



4. Results and discussion
Figure 5 shows the optical spectra of grating I with the full width at half maximum (FWHM) of 30 nm and 38
nm for TE and TM modes, respectively. Both the resonances appear at the same spectral positions with a slight
difference of 2.4 nm in peak resonance. The resonance of TE peak occurs completely in envelop of TM peak
which demonstrates an excellent non-polarizing characteristic of these devices. It has been observed both
theoretically and experimentally that duty cycles of the waveguide contribute an active role in shifting both the
peaks due to high index of TiO
2
material. The duty cycles of grating I show a strong dependence on the
structure depth. A slight variation in the duty cycles of finally fabricated structure results in the splitting of two
peaks from their single resonance position that has been calculate theoretically and experimentally. This fact
leads to a readjustment to the duty cycles of the structure which demonstrated perfect non-polarizing properties.
Figure 5 shows the optical spectra of grating II where FWHM of TE-peak is relatively longer than TM-peak,
however, TM-peak is completely located inside the envelop of TE-peak. The dependence of two peak splitting
shows major contribution on the structure depth inside polymer. The structure depth in polymer becomes a
limiting factor after achieving a certain height h
2
for which further manipulation of the structure height is
difficult due to aspect ratio. Alternatively, tuning of the duty cycles leads to simultaneous resonance of TE- and
TM-modes at the same wavelength with a slight difference of 1.4 nm. The final tuning of two resonance peaks
can be made after an adequate TiO
2
film thickness by ALD, which is an independent process regardless of the
fabrication process. The most distinctive feature of grating II is the simple fabrication and low-cost process.
Such gratings are used in a number of applications in terms of device encapsulation of nanophotonic flexible
components for optical communication system.

Figure 5. Measured transmittance/reflectance of the fabricated GMR gratings (a) type-I and (b) type-II, demonstrating the
non-polarizing properties.
5. Conclusion
We presented design, fabrication and characterization of two types of one dimensional guided mode resonance
gratings with simple binary geometries. The two types of fabricated gratings demonstrate the polarization-
insensitive GMRs effect under normal incidence. The type-I GMR grating consist of a simple single layer TiO
2

binary profile on the fused silica substrate that is fabricated by employing ALD, EBL, and RIE techniques with
almost perfect non-polarizing property. The type-II GMR grating is fabricated by nanoimprinting into the
polymer (polycarbonate) substrate followed by an over-coating of amorphous TiO
2
layer by ALD. Both types of
non-polarizing GMR gratings are realized first time in experiment so far, which show the potential of low-cost
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mass production for functional devices in practical applications, for example, as fluorescence biosensors and in
telecommunication.
Acknowledgement
We acknowledge the financial support by the Academy of Finland, the Strategic Funding Initiative TAILOR of
the University of Eastern Finland and Higher Education Commission (HEC), Pakistan.



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