High quality produces higher-order stopgaps
A technique for fabricating polymer-resin photonic crystals (PCs) developed by researchers at the Swinburne University of Technology (Melbourne, Australia) can be carried out in a single step in less than...
A technique for fabricating polymer-resin photonic crystals (PCs) developed by researchers at the Swinburne University of Technology (Melbourne, Australia) can be carried out in a single step in less than an hour, requires no chemical post-processing, and results in PC features smooth enough that higher-order stopgaps are easily created.1, 2 Higher-order stopgaps, which correspond to higher-order Bragg scattering, have the advantage that they occur at wavelengths much shorter than fundamental stop gaps, allowing short-wavelength PCs to be created from relatively large features.
The PC substrate the researchers start with is an ultraviolet-curable optical cement made by Norland Products (Cranbury, NJ) that has been squeezed between two glass slides and then cured for two hours, resulting in a hard polymer-resin film with a refractive index of 1.56. Void-channel microstructures are created within the film by moving the focus of a 14-mW-average-power beam of 540-nm light (from a Ti:sapphire laser combined with a frequency-doubled optical parametric oscillator) through the material. The 200-fs pulses create microscopic explosions within the resin, producing smooth void channels surrounded by densified material. The researchers had to get the laser power and scanning speed just right: too little, and either nothing happened or the refractive index changed but no voids occurred; too much, and the microchannels became rough in shape.
Experimental "woodpile" void-channel PCs were fabricated (see figure). The PCs have fundamental stopgaps in the 4- to 8-µm range and a multitude of sizable higher-order stopgaps. The number of stopgaps can be changed by altering the ratio of the layer spacing to the in-plane channel spacing. The gap-to-midgap ratios are changed by altering the filling ratio of the structures by changing the channel diameter.
When scanned in a straight line at 300 µm/s, the laser focus created void channels in the polymer with a lateral dimension of 750 nm and an elliptical cross section. Woodpile structures 80 × 80 µm in size and 20 layers deep were created, with layer spacings ranging from 1.6 to 2.7 µm and in-plane channel spacings of 1.1 to 2.6 µm.
The researchers first examined the fundamental and first higher-order gaps, both theoretically and experimentally. Because the polymer does not transmit beyond 5.5 µm, PCs with a layer spacing of 1.6 µm were fabricated. The main gaps reduced transmission by up to 85%, with the higher-order gaps reducing transmission by up to 40%. In theoretically reproducing the experimental spectral transmission curve, the researchers discovered that theory showed a higher effective refractive index than the actual index of the polymer. They attribute this difference to regions of compressed polymer around the channels.
Higher-order gaps of up to third order were examined in void-microchannel PCs with layer spacings of from 2.4 to 2.7 µm. The fundamental gaps were designed to be located at 7.8 µm—in a region of high polymer absorption, which was no matter for testing of higher orders. The first two higher-order gaps repressed transmission at 3.8 µm by up to 75% and 2.6 µm by 65%. The third-order gap at 2.0 µm reduced transmission by a maximum of 30%. Larger layer spacings resulted in a slightly lower effective refractive index increase in this series of PCs. Measuring reflection as a function of wavelength for PCs with a 2.55-µm layer spacing and in-plane channel spacing varying from 1.2 to 2.55 µm revealed transitions from conventional Bragg reflection to stopgap total reflection.
The ease of creating short-wavelength PCs based on relatively large layer spacings gives the void-microchannel fabrication technique great potential for practical PC-based optoelectronic components at telecommunications wavelengths, say the researchers.
- M. J. Ventura et al., Appl. Physics Lett. 82 (11), 1649 (March 17, 2003).
- M. Straub et al., Physical Rev. Lett. 91(4), 043901 (July 21, 2003).