Three‐Dimensional Optical Lithography for Photonic Microstructures

Abstract

Microcavities[1] and waveguides [2] operating within the optical bandgap of a photonic crystal [3,4] have the potential to create integrated optical devices capable of all-optical signal processing. [5] To achieve this degree of control over visible or near-infrared light is a materials-engineering challenge requiring precise local modification of wavelength-scale microstructure.In this communication we demonstrate a rapid and flexible technique for optical fabrication by creating a device embedded in, and in registration with, a 3D photonic crystal. We use holographic lithography to define the underlying periodic microstructure in a single exposure, [6] and direct twophoton laser writing [7][8][9] to create localized structural defects.An intermediate latent image of the photonic crystal is used to align the two exposure processes. Optical devices based on waveguides and resonators embedded within a photonic crystal have the potential to integrate high-frequency optical-signal-processing functions;[5] use of 3D photonic crystals can, in principle, eliminate the radiative losses that plague 2D photonic-crystal devices. Many elegant techniques for 3D photonic-crystal fabrication have been demonstrated, but not all lend themselves to the controlled introduction of the structural 'defects' from which devices operating within the photonic bandgap are constructed. [1,2,5] Engineered defects within opal-based photonic crystals are generally made by surface modification [10][11][12] followed by overgrowth, [12] leading to essentially 2D device layouts, although 3D structures created by multiphoton polymerization of infiltrated resin have been demonstrated. [13][14][15] Autocloning, electrochemical etching, and wafer fusion also restrict defect geometries. [16][17][18][19] The most flexible approach uses planar electron-beam lithography to build structures layer by layer; defects can be incorporated at any depth [20,21] but the overall thickness is limited by the processing time and by the build up of thermal stresses. Two new optical-lithography techniques are intrinsically well adapted to 3D photonic-crystal device fabrication. The fastest of these-holographic lithography-uses a 3D interference pattern, created at the intersection of four laser beams, to define periodic microstructure within a photoresist in a single step; [6] exposure to a single, few-nanosecond Q-switched laser pulse is sufficient. This is a flexible method for fabricating polymeric photonic-crystal templates for the creation of structures with higher refractive-index contrast.[22] (3D structures can also be defined by an interference pattern formed by illumination through a 2D phase mask, [23] although the design and fabrication of photonic-crystal devices by this method has not yet been demonstrated.) Alternatively, 3D microstructures can be made by exploiting the quadratic intensity dependence of two-photon absorption. [7][8][9] Two-photon photoexcitation by an infrared writing beam is effectively confined to a small focal volume where the intensi...

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

“…Second, addition of optically active defects into the photoresist prior to development via laser direct writing is possible. The direct writing of features in holographic PhCs has now been demonstrated in both 2D [87] and 3D [88] structures (Fig. 8).…”

“…[132] When performed in conjunction with in situ fluorescence confocal imaging, it is possible to pinpoint the location of the TPP features with respect to the PhC lattice. [88] An important consideration for any cPBG application is the ability to convert the PhC to a high-refractive-index structure that exhibits a cPBG. Generally, this is accomplished for colloidal PhCs through infiltration with a high-index material such as Si at an elevated temperature, followed by removal of the silica, resulting in an inverse opal structure.…”

“…As described in Section 3.2.1, TPP can also be used to define features in PhCs formed by multibeam holography. [88] TPP is an appealing method for writing defects within selfassembled or holographic PhCs for a number of reasons. TPP is a flexible technique capable of writing isolated, embedded, 3D features throughout the bulk of a crystal (on the order of 150 lm deep).…”

Multi-Beam Interference Advances and Applications: Nano-Electronics, Photonic Crystals, Metamaterials, Subwavelength Structures, Optical Trapping, and Biomedical Structures

“…Similarly, Figure 6(b) demonstrates a multi-step procedure to create more complex composite patterns. Others have combined MBIL with various lithographic techniques including other mask-based techniques (e.g., proximity or contact lithography) [159,173], electron-beam lithography [81,174,175], electron-beam-induced deposition [176], focused ion-beam lithography [155,177], direct laser writing [77,150,178,179], atomic force microscopy nano-indentation [180], and multi-photon polymerization [181][182][183]. For example, interference lithography has been combined with optical contact lithography to fabricate triple-gate metal-oxide-semiconductor field effect transistors [161].…”

Photonic crystals with defect structures fabricated through a combination of holographic lithography and two-photon lithography

“…Two-photon ablation or polymerization was used to create defects in PhC template in SU-8, which were created through holographic lithography, by three research groups in 2005, 18 2006, 19 and 2008. 20 Sun et al 18 demonstrated the hybrid holographic and direct writing method in a two-dimensionally periodic structure while Scrimgeour et al 19 described the creation of defect structures inside 3D PhCs. The other group 20 fabricated and developed polymer-air PhCs in SU-8 via holographic lithography and the PhCs were infiltrated with trimethylol propane triacrylate and a two-photon sensitive photoinitiator for the direct writing of defects via an ultrafast laser.…”