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...

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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.

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Ultrashort visible-light pulses create smooth-walled void microchannels in polymer, forming an infrared photonic crystal (drawing, above). The first four layers were imaged by a reflection confocal microscope (right, top to bottom); the apparent degradation of the deeper layers is solely an artifact of imaging through the upper layers.
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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.

REFERENCES

  1. M. J. Ventura et al., Appl. Physics Lett. 82 (11), 1649 (March 17, 2003).
  2. M. Straub et al., Physical Rev. Lett. 91(4), 043901 (July 21, 2003).

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