Proceedings Published 2006 by the American Chemical Society

Two-Photon Polymerization of Embedded Features within Photonic Crystals Stephanie A. Pruzinsky, Florencio García-Santamaría, and Paul V. Braun Department of Materials Science and Engineering, Beckman Institute for Advanced Science and Technology, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

INTRODUCTION Many applications have been proposed for materials that possess

a complete photonic bandgap (PBG) (e.g. low-threshold lasers, low-loss waveguides, and on-chip circuitry). The majority of these applications require the controlled incorporation of defects into otherwise periodic structures with domain sizes on the order of the wavelength of light.

One widely explored technique for the fabrication of three dimensional (3D) PBG materials begins with the self-assembly of microspheres into colloidal crystals1 which can then be used as templates for PBG materials.2 A critical limitation to the utility of self-assembled photonic crystals for the realization of PBG-based applications was the lack of an inherent method to controllably incorporate defect structures. We successfully demonstrated the use of multi-photon polymerization to generate such embedded features within colloidal crystals,3 extending the viability of self-assembled photonic crystals for PBG-based applications.

Two-photon polymerization (TPP) is a high-resolution three-dimensional (3D) free-form fabrication technique that has been used to define a variety of structures, including microchannels,4-6 cantilevers,7-9 and photonic crystals.8,10 TPP begins with the two-photon excitation (TPE) of a dye molecule with then directly or indirectly initiates polymerization. TPE only occurs within a small volume centered at the focal point of a tightly-focused laser beam which can be translated through photoresist to define 3D features. Sub-diffraction limited feature generation is possible due to the TPP threshold, enabling features with possible lateral dimensions of 100 nm.11

Here, we use a modulated beam rastering approach to write 3D embedded features within self-assembled photonic crystals and then invert those samples in silicon. Engineering defects into these structures is interesting as silicon-air inverse opals have the potential to exhibit a complete PBG.12

EXPERIMENTAL TPP and confocal imaging were performed on a laser scanning

confocal microscope (Leica, DMIRBE microscope and SP-2 scanhead) coupled for multi-photon operation with an electro-optic modulator and Ti:sapphire laser (Spectra Physics Tsunami, ~60 fs pulses, 82 MHz repetition rate) operated at a wavelength of 780 nm. The estimated pulse-width at the objective was ≤ 100 fs. All experiments were carried out using a 63x oil-immersion objective (Leica) with a numerical aperture of 1.32. The synthesis and characterization of the two-photon properties of the primary dye used in this study, tris[4-(7-benzothiazol-2-yl-9,9-diethylfluoren-2-yl)phenyl]amine (AF-350), was described elsewhere13 and was donated by the U.S. Air Force Research Laboratory. The monomer used was trimethylolpropane triacrylate (TMPTA) (Aldrich) from which the inhibitor (100 ppm hydroquinone monomethyl ether) was removed prior to use with inhibitor removal beads (Aldrich). Approximately 0.1 w/w solutions of AF-350 in TMPTA were used. For fluorescence confocal imaging in colloidal crystals, a 10-5 M solution of rhodamine 6G (Acros) was made in a 67 % glycerol (Acros) and 33 % water mixture for optimal index matching with the colloids. Colloidal crystals were assembled via controlled drying14 or in ultra-sonicated flow cells,15,16 with some modifications as described elsewhere.17 A literature procedure for the infiltration of silicon into colloidal crystals was used.18

RESULTS AND DISCUSSION A laser scanning confocal microscope outfitted with an electro-

optic modulator (EOM) is used to carry out our modulated beam rastering approach to TPP. Here, a beam is rastered throughout a particular volume and during the scan the EOM modulates the laser power between a high and low setting in order to selectively polymerize a particular region of interest. In this fashion 3D features can be rapidly fabricated within self-assembled photonic crystals (Fig. 1).3,19

Figure 1. Fluorescence confocal micrographs of cross-sections from 3D TPP features written within a colloidal crystal. Four features were defined—one is a 2D double-bend structure and three are 3D triple-bend structures with varying bend angles in the horizontal plane. At the top is a vertical cross-section through the 2D double-bend structure. The lower cross-sections show the: (a) vertical components of all four features extending ~10 µm from the substrate into the crystal, (b) horizontal components containing 0°, 30°, 60°, or 90° double bends, and (c) additional vertical components extending ~10 µm further into the colloidal crystal. Since our initial proof of principle work,3 the critical parameters for TPP and damage specific to our modulated beam rastering setup were determined and TPP response diagrams were constructed.19 These diagrams map the polymerization and damage thresholds, enabling visualization of the polymerization window (Fig. 2). These diagrams enabled the identification of a more efficient initiator (AF-350)13 than we previously used,3,4 now enabling the reliable fabrication of high resolution, three-dimensional embedded defects within self-assembled photonic crystals (Fig. 1). TPP response diagrams were also used to compare the behavior of the same solution within or outside a colloidal crystal in order to confirm their similar TPP response (Fig. 2). The similar resolution for operation within or outside a colloidal crystal was confirmed via electron microscopy.19

Figure 2. TPP response diagram for 0.1 wt.-% AF-350 in TMPTA (a) in bulk monomer solution and (b) within a colloidal crystal. The polymerization threshold was calculated to best fit the data, while the damage threshold was simply drawn to guide the eye.

The poly(TMPTA) does not lose weight upon heating up to about 300 °C, enabling post-processing at these temperatures. Here, the

Proceedings Published 2006 by the American Chemical Society

TPP feature occupies the interstitial space of the colloidal crystal in order to block off those regions from infiltration with high index material. In this fashion, high resolution 3D defects can be incorporated within high index contrast photonic crystals (Fig. 3).

Figure 3. Schematic of the summarized experimental scheme: (a) grow colloidal crystal, (b) write two-photon polymerized features, (c) infiltrate with high index material, and (d) remove silica colloids and polymer features.

We previously obtained selenium inverse opals containing embedded TPP features via melt imbibing.20 More recently, we have explored the replication of these structures in silicon via low temperature chemical vapor deposition. Silicon has a sufficiently high refractive index to realize a complete 3D PBG in the inverse face-centered cubic (FCC) replicas obtained after removal of the original colloidal template. After removal of the poly(TMPTA), the result is a silicon inverse FCC structure18 with embedded air defects (Fig. 4). The fabrication and characterization of optically interesting features within these structures is currently in progress.

Figure 4. Linear air defects defined via TPP in a colloidal crystal which was then infiltrated with silicon via CVD. After removal of polymer and silica, the resulting structure is air defects in a silicon inverse opal.

CONCLUSIONS

Modulated beam rastering was used to write 3D embedded features within self-assembled photonic crystals. TPP response diagrams were presented as a means to visualize the TPP window and compare between the TPP response within versus outside of a colloidal crystal. Finally, colloidal crystals with embedded TPP features were inverted in silicon via low temperature chemical vapor deposition. As silicon-air inverse opals have the potential to exhibit a complete PBG, incorporating defects within these materials could enable the realization of PBG-based applications.

ACKNOWLEDGEMENTS This material is based upon work supported by the U. S. Army

Research Laboratory and the U. S. Army Research Office grant DAAD19-03-1-0227, the National Science Foundation grant DMR 00-71645, and the U.S. Department of Energy, Division of Materials Sciences grant DEFG02-91ER45439, through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign (UIUC). This work was carried out in part in the Beckman Institute Microscopy Suite, UIUC, the Micro and Nanotechnology

Laboratory, UIUC, and the Center for Microanalysis of Materials, UIUC, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439. We gratefully acknowledge Dr. L.-S. Tan (U.S. Air Force Research Laboratory) for providing the two-photon sensitive dyes, K. Garsha for advice and assistance with confocal microscopy, and Hal Romans for experimental assistance.

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