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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 Institut für Physikalische
Hochtechnologie, D-07445 Jena, Germany
Synthesis and properties of mesoporous films
Introduction to the new project
High Q resonators structures have been
investigated with FDTD simulations (nslab1.8,
nsubstrate1.14). 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.
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.
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 n1.14.
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.
9 nm

• Quasi - symmetric structures show the following
  properties
• vertical symmetry allows cassification of modes
  by slab mirror plane in even and odd (definition
  of parity)
• 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
• no etching of substrate is required due to high
  vertical index contrast
• strong vertical confinement of light results in
  high Q factors

• Basic properties of SBA-15F
• Low refractive index n 1.13 ... 1.18
• Very flat lt?z2gt ? (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

Q factor of the 2D - PC cavity as a function of
number of surrounding layers calculated from
simulated transmission spectra
Quasisymmetrical E - filed distribution in 2d -
PC cavity in TE - Polarization (FDTD) (ncore1.8,
nsubstrate1.14, a500nm, r150nm, tcore1.5mm)
Preliminary investigations with mesoporous silica
as air - like substrate for 2d - PC slab
waveguides
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.
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
n1.13 to n1.18.
To test the mechanical properties and the etching
behavior we fabricated a 2d - PC in BCB
(Bencocyclobutene, n1.55) 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.
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.
Square lattice of holes in BCB/SBA-15F
(ncore1.55, nsubstrate?1.14, a500nm, r150nm,
tcore1.5mm)
Theoretical investigation of 2D - PC waveguides
with very low index substrate
Example for the influence of different parameters
on the pre-reaction. The viscosity determines the
film thickness in the next synthesis step.
First trimming of PC transmission properties by
changing the refractive index
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 n1.8 (polymers
with n1.9 available, ??/??7).
Project aims
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 (n1.56 ? n1.51). The substrate
below the core was Teflon with a refractive index
of n1.3 .

• Theory, simulations (TUHH)
• Bulk photonic crystal slab properties
• Influence of symmetrization and vertical
  confinement on radiation losses
• Straight and bent defect waveguides
• Linear and point defect resonators
• Realization, experimental characterization
  (TUHH)
• Thin film formation of moderate refractive index
  materials on mesoporous substrate
• Electron beam lithography and ion etching
  parameters filling and sample preparation
• Transmission spectra- propagation and coupling
  losses
• High Q defect states

• Mesoporous films (Mülheim)
• Reliability of synthesis
• Existence region and optimization of film
  properties
• Detailed understanding of the film structure
• Influence of synthesis parameters on optical
  properties
• Exploration of the mechanical, dielectric and
  electrical properties
• Comparison to alternative mesoporous films

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.
Bandstructure of hexagonal lattice with high
vertical index contrast (by R. Iliew, FSU
Jena) (ncore1.8, nsubstrate1.13, a600nm,
r200nm, tcore0.7mm, tetching0.7mm)
Bandstructure of hexagonal lattice with low
vertical index contrast (ncore1.55,
nsubstrate1.3, a650nm, r150nm, tcore1.5mm,
tetching1.6mm)
Transmission of the 2D - PC structure on the
dielectric edge (TE - polarization). Different
colors refer to different applied UV dose.
SEM picture square lattice in P(MMA/DR-1) -
Teflon (ncore1.55, nsubstrate1.3, a500nm,
r150nm, tcore1.5mm, tetching3.0mm)
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.
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
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).