3D photonics

Photonic crystals — optical analogues of electronic semiconductors — hold great promise for manipulating and processing light signals in future photonic devices. A new 3D microassembly technique could greatly aid the development of photonic crystals for such applications.

6 February 2003

Ed Gerstner

Scanning electron micrograph of one of the authors' 3D photonic crystal structures. Multiple plates are patterned using standard lithographic and etching techniques, and then assembled into a stack using fine mechanical probes.

The tremendous progress in electronics that has occurred over the past 50 years has, to a large extent, been made possible by the unique electronic bandgap structure of semiconducting crystals such as silicon. Consequently, there has been great interest in the development of optical analogues — known as photonic crystals — to electronic semiconductors. Photonic crystals offer the possibility to achieve the same control over light as can be achieved over electrons, but in practice they are notoriously difficult to design and construct. In Nature Materials this month, Kanna Aoki and colleagues demonstrate a novel fabrication method that makes it possible to assemble three-dimensional photonic crystal structures at the microscopic scale.

Photonic crystals are periodic structures formed at micro- and submicrometre scales that influence the propagation of light waves in a manner similar to that in which atomic crystals influence the propagation of electron waves. The main goal of much research into photonic crystals is the formation of a photonic bandgap. The simplest of photonic bandgap structures is the fibre Bragg grating, in which periodic variations in refractive index introduced along an optical fibre restrict the propagation of certain wavelengths within the fibre. Such bandgap behaviour has now also been demonstrated in two dimensions in a variety of planar waveguide structures. But achieving the formation of a photonic bandgap in three dimensions has so far proved elusive.

The main obstacle to making photonic crystals that exhibit true 3D photonic bandgap behaviour is the difficulty in constructing 3D periodic structures with the precision and control required to avoid physical defects that degrade their optical properties. In an attempt to overcome this, Aoki and colleagues have taken an approach that minimizes the chances of such defects being introduced. They start by fabricating a series of 2D grid structures from indium phosphide by conventional lithographic and etching techniques. These grid plates are then stacked on top of each other using fine mechanical manipulator probes, and then aligned and secured using polystyrene microspheres that act as microscopic rivets when placed in alignment holes situated in each of the plates (see figure).

By patterning all the fine structures of their photonic crystal in a single lithographic step, the authors significantly improve their control over the fabrication process. This enables them to achieve a spatial precision of less than 50 nm. Moreover, the flexibility of the microassembly approach allows them to actively engineer specific defects into their photonic crystal — analogous to the introduction of specific dopants into the atomic lattice of a semiconductor.

Although their present structures do not yet achieve a full 3D bandgap, they do demonstrate a significant decrease in the transmission of infrared wavelengths in the range of 3–4.5 µm. More significantly, demonstration of the use of microassembly techniques to create such structures opens up new avenues for the development of not only true 3D photonic crystals, but a whole host of complex 3D micro- and nano-scale structures.