Quasi Symmetric Moderate Refractive Index Photonic Crystal Waveguides on Mesoporous Substrates

Markus Schmidt, Gunnar Böttger, Christian Liguda, Dion Klunder and Manfred EichTechnische Universität Hamburg-Harburg, Materialien der Mikroelektronik und Optik, D-21071 Hamburg, Germany

Frank Marlow and Ursula WilczokMax-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

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.

www.tuhh.de/et7

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).

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.

Introduction to the new project

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.

First trimming of PC transmission properties by changing the refractive index

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 .

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.

SEM picture square lattice in P(MMA/DR-1) - Teflon (ncore=1.55, nsubstrate=1.3, a=500nm, r=150nm, tcore=1.5m, tetching=3.0m)

Transmission of the 2D - PC structure on the dielectric edge (TE - polarization). Different colors refer to different applied UV dose.

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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)(ncore=1.8, nsubstrate=1.14, a=500nm, r=150nm, tcore=1.5m)

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.

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.

Square lattice of holes in BCB/SBA-15F (ncore=1.55, nsubstrate1.14, a=500nm, r=150nm, tcore=1.5m)

5 6 7 8 9 10100

1000

Q(10) = 3070

Q(10)=2140

3D - simulation (n

slab=1.8; n

substrate=1.14)

2D - simulation (n=1.8)

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Very low index substrate

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symmetric mode profile

2D - PC

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.

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.

• 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

Example for the influence of different parameters on the pre-reaction. The viscosity determines the film

thickness in the next synthesis step.

Status:• Refractive index:

n = 1.14• Size: up to 25 x 30 mm2

• Thickness: 200 … 700 nm

9 nm

Si(OR)4 + 2 H2O SiO2 + 4 ROHPluronic123

Synthesis and properties of mesoporous films

• 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

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%).

Bandstructure of hexagonal lattice with low vertical index contrast (ncore=1.55, nsubstrate=1.3, a=650nm, r=150nm, tcore=1.5m, tetching=1.6m)

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=0.7m, tetching=0.7m)

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Project aims:

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

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

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.

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.

Basic properties of SBA-15F:• Low refractive index

n = 1.13 ... 1.18• Very flat

<z2> (2 nm)2

• Sufficiently hardHB 30 (like Al)

• TransparentSBA-3 (a similar material): 0.1 mm-1

• Stable (Solvents, temperature, time)