3-D Crystals Created for the Visible Spectrum
OXFORD, UK -- One of the main challenges the photonics industry faces is the development of a three-dimensional crystal for the visible spectrum that will act on photons much as silicon devices act on electrons. Such structures exhibit a periodicity similar to the wavelength of light used in the device: Under the right circumstances, the interference between scattered waves prohibits the passage of a range of frequencies, creating a photonic bandgap similar to the electronic bandgap in a traditional semiconductor. Researchers have employed standard lithography techniques to produce two-dimensional crystals, but the construction of three-dimensional crystals poses a number of difficulties.
A group led by Andrew Turberfield and Bob Denning in the physics and chemistry departments at the University of Oxford reported in the March 2 issue of Nature that it has overcome the problems using 3-D holographic lithography. The scientists created photonic crystals 30 µm thick, with a submicron periodicity, making them useful for the visible region. These structures are truly three-dimensional, with between 14 and 80 layers. The process could have many applications including the development of integrated optical and optoelectronic devices two orders of magnitude smaller than current designs, Turberfield said.
The researchers started by illuminating a photoresist using four non-coplanar laser beams. They controlled the lattice spacing of the crystal by adjusting the beam angles. The intensities and polarizations are carefully adjusted, too, to control the shape of the interference pattern within each unit cell. Where the photoresist (based on the Epon-SU8) is highly exposed, it becomes insoluble. The unexposed polymer is washed away, leaving a high-contrast structure of air gaps and cross-linked polymer with a refractive index of approximately 1.6.
Researchers at the University of Oxford have constructed photonic crystals using holographic lithography. They generated a diamondlike polymeric photonic crystal (a) by exposing a 10-µm film of photoresist to a face-centered cubic laser interference pattern. The top surface is a (111) plane; the film was fractured along the (111) cleavage planes. Close-ups of (b) (111) and (c) (111) surfaces show the three-dimensionality of the crystals.
The laser source is a frequency-tripled Nd:YAG from Spectra-Physics Lasers Ltd. of Hemel, Hempstead, UK. It produces 6-ns pulses by Q-switching, delivering around 100 mJ/cm2. This is sufficient to expose the resist before the chemical changes affect the absorption or the refraction of the beams. So far the group has formed a face-centered cubic lattice with a constant of 922 nm and a body-centered cubic lattice with a constant of 794 nm, both from the same laser wavelength of 355 nm.
This process has many advantages over other methods so far developed. "Unlike rival technologies, this process is cheap, rapid and potentially scalable to volume production," Turberfield said.
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