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Quasi Symmetric Moderate Refractive Index Photonic Crystal
Waveguides on Mesoporous Substrates
Markus Schmidt, Gunnar Böttger, Christian Liguda, Dion Klunder and Manfred Eich
Technische Universität Hamburg-Harburg, Materialien der Mikroelektronik und Optik, D-21071 Hamburg, Germany
Frank Marlow and Ursula Wilczok
Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim an der Ruhr, Germany
Hans-Georg Meyer and Uwe Hübner
www.tuhh.de/et7
Institut für Physikalische Hochtechnologie, D-07445 Jena, Germany
The main subject of the proposed project is the realization of a „air - bridge“ structure
on a solid substrate. The key idea is to use a substrate whose refractive index is so
low that nearly symmetrically conditions can be achieved which we call quasi
symmetry. By using such a kind of waveguide setup also the vertical index contrast is
strongly increased which results in lower radiation losses specially in high Q cavities.
Deep etching into the waveguide substrate is not requiered any longer. The material
we focused on is mesoporous silica (SBA-15F) which has good optical and
mechanical properties and a refractive index of n=1.14.
High Q resonators structures have been investigated with FDTD simulations (nslab=1.8, nsubstrate=1.14). A
strong confinement of the electromagnetic field resulting in a high Q factor can be achieved by using a
material with low refractive index without etching the substrate. The high Q factors are necessary to
observe defect tuning of a 2D - PC cavity by the Pockels effects.
• „air - bridge“ - structure can easily be
fabricated retaining a solid substrate (no
underetching)
• high refractive index contrast between cladding
and core increases frequencies of light line
• direction independent stop gap in hexagonal
lattice with moderate refractive index core
2D - PC
Very low index substrate
Schematic view of the architecture of the
mesoporous films. The ordered hexagonal
arrangement of channels can be proven by
X-ray scattering. A disordered domain
structure is assumed, based on the findings
on similar systems and the current x-ray
investigations.
3D - simulation
(nslab=1.8; nsubstrate=1.14)
2D - simulation (n=1.8)
Q(10)=2140
1000
9 nm
Basic properties of SBA-15F:
100
5
Quasisymmetrical E - filed distribution in 2d - PC cavity in
TE - Polarization (FDTD)
(ncore=1.8, nsubstrate=1.14, a=500nm, r=150nm, tcore=1.5mm)
6
7
L [1]
8
9
10
Q factor of the 2D - PC cavity as a function of
number of surrounding layers calculated from
simulated transmission spectra
Preliminary investigations with mesoporous silica as „air - like“ substrate for 2d - PC
slab waveguides
First experiments: Slightly higher intrinsic waveguide losses (few dB/cm) as compared to substrates of
higher n (like Teflon or SiO2) have been measured. The refractive index was measured by prism coupling
and is located in the range of n=1.13 to n=1.18.
• no etching of substrate is required due to high
vertical index contrast
waveguide
In this project, we will investigate the influence of fabrication parameters for these substrates
on the optical properties. The aim is the full understanding of the synthesis process and the
fabrication of substrates with high homogeneity and large sizes.
Q(10) = 3070
Quasi - symmetric structures show
the following properties:
• vertical symmetry allows cassification of modes
by slab mirror plane in even and odd (definition
of parity)
Synthesis and properties of mesoporous films
Q [1]
Introduction to the new project
To test the mechanical properties and the
etching behavior we fabricated a 2d - PC in
BCB (Bencocyclobutene, n=1.55) with SBA-15F
as substrate. The SEM shows that the polymer is
not penetrating the SBA - 15 layer. A high
vertical index contrast is achieved.
• strong vertical confinement of light results in
high Q factors
symmetric mode profile
We focus our research onto materials with moderate refractive index. The advantages of this class
of materials are low optical losses in the regime of optical communication wavelengths and an easy
fabrication of thin films (spin - coating). The moderate index reduces Rayleigth scattering from
defects and surface roughness and also decreases the coupling mismatch from a single - mode fibre
to the PC structure.
• Low refractive index
n = 1.13 ... 1.18
• Very flat
<z2>  (2 nm)2
• Sufficiently hard
HB  30 (like Al)
• Transparent
SBA-3 (a similar material):  0.1 mm-1
• Stable (Solvents, temperature, time)
Status:
• Refractive index:
n = 1.14
• Size: up to 25 x 30 mm2
• Thickness:
200 … 700 nm
Synthesis control:
The synthesis of the mesoporous films consists of educt mixing, pre-reaction, film drawing and
calcination. The reaction is a sol-gel process combined with a micelle-forming template. The name
of the special synthesis system is SBA-15F (invented in 1996 by Stucky et al.) Every synthesis step
has decisive influence on the film properties.
Pluronic123
Si(OR)4 + 2 H2O
SiO2 + 4 ROH
Square lattice of holes in BCB/SBA-15F
(ncore=1.55,
nsubstrate1.14,
a=500nm,
r=150nm, tcore=1.5mm)
Theoretical investigation of 2D - PC waveguides with very low index substrate
The two diagrams show bandstructure calculations of hexagonal lattice in the material system which is
now in use with low vertical index contrast (left) and the new system with high vertical contrast (right).
The low refractive index of the new substrate pushes up the frequencies of the light line,which increases
the accessible bandwidth for ideally lossless Bloch modes and defect modes. A direction independent
stop gaps appears when the waveguide core is made of a material with an index of n=1.8 (polymers
with n=1.9 available, /7%).
240
280
PC structures with high wavelength resolution for optical communications would require sub - nm
fabrication tolerances. Therefore trimming and tuning schemes are inevitable for PC technology.
Trimming of the transmission characteristics of a 2D -PC structure was investigated by bleaching the
waveguide core which consists of P(MMA/DR-1) (polymethylmethacrylate, Disperse Red 1). ncore changes
its refractive index irreversibly under UV illumination (n=1.56  n=1.51). The substrate below the core
was Teflon with a refractive index of n=1.3 .
1,0
tBleach=0min
tBleach=20min
n [THz]
120
transmission [a.u.]
200
160
n [THz]
First trimming of PC transmission properties by changing the refractive index
240
200
160
120
80
80
40
0
even
odd
G
even
odd
40
K
M
G
k - vector
Bandstructure of hexagonal lattice with low vertical
index contrast
(ncore=1.55,
nsubstrate=1.3,
a=650nm,
r=150nm,
tcore=1.5mm, tetching=1.6mm)
0
Example for the influence of different parameters on the pre-reaction. The viscosity determines the film
thickness in the next synthesis step.
G
M
K
tBleach=80min
0,5
tBleach=100min
tBleach=120min
1300
k - vector
We acknowledge support from CST, Darmstadt, Germany with their finite integration FDTD
software (Mafia and Microwave Studio). PW simulations were done with the MPB-package (MIT).
tBleach=60min
0,0
G
Bandstructure of hexagonal lattice with high vertical
index contrast (by R. Iliew, FSU Jena)
(ncore=1.8,
nsubstrate=1.13,
a=600nm,
r=200nm,
tcore=0.7mm, tetching=0.7mm)
tBleach=40min
SEM picture square lattice in P(MMA/DR-1) - Teflon
(ncore=1.55, nsubstrate=1.3, a=500nm, r=150nm,
tcore=1.5mm, tetching=3.0mm)
1350
1400
1450
1500
l [nm]
Transmission of the 2D - PC structure on the dielectric
edge (TE - polarization). Different colors refer to
different applied UV dose.
Because of the field concentration in the dielectric, a large shift of 40nm of the dielectric edge is
observed. Next step in our project will be the investigation of trimming the defect frequency of a 2d -PC
cavity. Therefore a strong confinement in the slab waveguide is absolutely necessary to reach high Q
values which can only be realized with the low index substrate.
Project aims:
• Theory, simulations (TUHH):
• Mesoporous films (Mülheim):
• Bulk photonic crystal slab properties
• Reliability of synthesis
• Influence of symmetrization and vertical
• Existence region and optimization of
confinement on radiation losses
film properties
• Straight and bent defect waveguides
• Detailed understanding of the film
• Linear and point defect resonators
structure
• Realization, experimental characterization (TUHH):
• Influence of synthesis parameters on
• Thin film formation of moderate refractive index
optical properties
materials on mesoporous substrate
• Exploration of the mechanical,
• Electron beam lithography and ion etching
dielectric and electrical properties
parameters – filling and sample preparation
• Comparison to alternative
• Transmission spectra- propagation and coupling
mesoporous films
losses
• High Q defect states
Summary
The quasi - symmetric 2d - PC structure is a very promising concept to combine the
advantages of a moderate refractive index PC waveguide core materials (less
Rayleigh scattering) with very low index substrates. The „air-bridge“ structure with
solid substrate is relatively easy to realize and mechanically stable. Due to strong
vertical confinement deep etching is no longer required. Radiation losses to substrate
are suppressed, hence high Q - structures seem feasible.
Collaboration within DFG - Schwerpunkt:
W. Freude, Universität Karlsruhe; K. Busch, Universität Karlsruhe; H. Bartelt, IPHT - Jena;
F. Lederer, Universität Jena; V. Sandoghar, ETH - Zürich; H. Föll, Universität Kiel