New Route to Three‐Dimensional Photonic Bandgap Materials: Silicon Double Inversion of Polymer Templates

Abstract

In the quest for three-dimensional photonic crystals [1,2] with omnidirectional photonic bandgaps (PBGs), pioneering "topdown" and "bottom-up" approaches have been explored. The former have led to excellent quality photonic crystals; however, fabrication efforts and costs are high. [3][4][5] The latter, using colloidal crystals as templates, provide photonic crystals; however, their crystal structure is limited to face-centered cubic and quality is compromised by defects. [6][7][8] More recent approaches founded on holographic laser lithography [9,10] and direct laser writing in polymer photoresists [11][12][13][14][15][16] have turned out to fulfill most of the necessary requirements [17] for largescale fabrication of three-dimensional (3D) photonic crystals, facilitating straightforward incorporation of functional defects like waveguides and resonators.[18] Unfortunately, a major obstacle must be overcome before the way is clear to photonic bandgap materials; namely, polymer templates have insufficient refractive-index contrast to open a complete photonic bandgap. In this report, we overcome these problems by employing a novel, versatile, scalable, and cost-effective silicon double-inversion method to synthesize the first Si replica of a polymeric photonic crystal. This method can be applied to any photoresist template produced by direct-laser writing (DLW), holographic laser lithography, or combinations thereof. It also incorporates a template-independent step to fine-tune the filling fraction of the high-index material, thereby allowing larger PBGs to be obtained than with the template alone. The procedure exclusively comprises straightforward, inexpensive, industry-compatible steps, perfectly suitable for mass production of complex and functionalized photonic-crystal-based devices.The silicon double inversion is exemplified using the familiar "woodpile" (or log pile) structure to allow direct comparison with "top-down" photonic crystals reported previously. [3][4][5] The Si woodpile, obtained following the steps shown schematically in Figures 1a-f and detailed hereafter, is 24 layers thick and has a calculated PBG of 8.6 %.We begin with a polymer woodpile fabricated in the commercially available negative photoresist EPON SU-8. In brief, 120 fs pulses at 800 nm wavelength, derived from a regeneratively amplified Ti:sapphire laser, are focused into the photoresist using a high numerical aperture (NA= 1.4) oil-immersion microscope objective. Due to the high intensities in the focal volume, two-photon absorption initiates the polymerization process, resulting in exposed ellipsoidal volume elements (voxels) with a minimum lateral diameter down to 150 nm and a ratio of 2.7 between axial and lateral dimensions after development. Such voxels form the basic building blocks for all 3D structures that can be fabricated by sequentially scanning the laser focus through the resist using a computer-controlled 3D piezo scanning stage, synchronized with the laser pulses. Functional defects like resonators and waveguides can...

Introducing Defects in 3D Photonic Crystals: State of the Art

“…[55,56] In principle, direct writing involves nothing more than converting a 3D computer aided design into a target material. Because the required dimensions are on the micrometer scale, traditional fabrication techniques, such as rapid prototyping, are not suitable and new approaches have been required.…”

“…[90,91] An equally significant issue is that the refractive indices of common photoresists are too low to open a cPBG, regardless of the structure, thus, replicating the structures with high-refractive-index materials is essential. Advances in templating silicon with polymeric structures [55,56] indicate possible routes to create structures with enhanced refractive-index contrast. Also, very recently, Summers and coworkers [92] demonstrated the use of atomic layer deposition to replicate the structure of a holographic PC into TiO 2 , a highrefractive-index material which is transparent in the visible region.…”

“…[59,78,79] Similar to robotic ink writing, the PhC is often fabricated in a polymer, and thus a similar replication approach is necessary to create a high-refractive-index contrast structure. [56] Alternatively, there are certain highrefractive-index chalcogenide glasses such as As 2 S 3 that undergo solubility changes upon exposure to light and are therefore amenable to direct laser writing. [83] To date, there are no reports of optically active defects incorporated within TPP PhCs, however the process is straightforward.…”

“…Because the required dimensions are on the micrometer scale, traditional fabrication techniques, such as rapid prototyping, are not suitable and new approaches have been required. The most promising to date appear to be robotic ink writing [55,57,58] and laser writing by two-photon polymerization (TPP) [56,59] (Fig. 7).…”

Polarization Stop Bands in Chiral Polymeric Three‐Dimensional Photonic Crystals

“…[17] The effects would, however, be even larger if the polymer structure was replicated via the recently introduced silicon double-inversion procedure. [18] …”