The term 'photonics' describes a technology whereby data transmission and processing occurs largely or entirely by means of photons. Photonic crystals are microstructured materials in which the dielectric constant is periodically modulated on a length scale comparable to the desired wavelength of operation. Multiple interference between waves scattered from each unit cell of the structure may open a 'photonic bandgap'--a range of frequencies, analogous to the electronic bandgap of a semiconductor, within which no propagating electromagnetic modes exist. Numerous device principles that exploit this property have been identified. Considerable progress has now been made in constructing two-dimensional structures using conventional lithography, but the fabrication of three-dimensional photonic crystal structures for the visible spectrum remains a considerable challenge. Here we describe a technique--three-dimensional holographic lithography--that is well suited to the production of three-dimensional structures with sub-micrometre periodicity. With this technique we have made microperiodic polymeric structures, and we have used these as templates to create complementary structures with higher refractive-index contrast.

The demonstration of a practical technology for 3D optical microfabrication is a vital step in the development of photonic-crystal-based optical signal processing.[1] However, the extension of the optical methods that dominate integrated electronic circuit fabrication to three dimensions is a formidable materials-processing challenge: such a process must be capable not only of sub-micrometer pattern definition in three dimensions, but also of the transfer of this pattern into a homogeneous dielectric with an appropriately high refractive index. In a companion paper, [2] we show that two optical methods, holographic lithography [3] and direct two-photon laser writing, [4][5][6] can be combined to create a rapid and flexible method for the definition of photonic crystal device structures in photoresist. In this communication, we report a further essential step towards the creation of devices operating within a full photonic bandgap: we have used atomic layer deposition (ALD), itself an established semiconductor processing technique, to create high-index TiO 2 inverted replicas of holographically defined photonic crystals, followed by removal of the polymeric template by plasma etching. A range of techniques for 3D optical lithography has been demonstrated. A 3D photonic crystal structure can be written by holographic lithography, [3] which makes use of a periodic interference pattern generated by a multiple-beam interferometer to expose a thick layer of photoresist. 3D microstructures, both periodic and aperiodic, can also be generated by point-by-point exposure of the resist by two-photon absorption at a laser focus. [4][5][6][7] Two-photon laser writing is a serial process; point-by-point fabrication of a 3D photonic crystal is necessarily slower than holographic lithography, which is capable of defining the entire periodic structure in a single laser pulse.[3] The two techniques are complementary: two-photon laser writing can be used to modify a holographic exposure.[8]We have shown that, by imaging the distribution of photochemical change induced by holographic exposure, it is possible to align a subsequent two-photon exposure with the 3D photonic crystal lattice to achieve the precise registration that is required of a device structure embedded in a 3D photonic crystal. [2] This hybrid technique is rapid and flexible, but the polymeric resists used for 3D microfabrication have refractive indices n in the range 1.4-1.6, which is too low for most device applications. Devices based on waveguides and microcavities embedded within a photonic crystal [1] are designed to operate at frequencies within a complete (omnidirectional) photonic bandgap in order to suppress radiative loss; [9] to create a complete photonic bandgap, even in an optimized air-dielectric structure, a refractive contrast of at least 1.9 is necessary.

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...

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