Infiltrated Crystal Offers Tunable Bandgap
Researchers in Katsumi Yoshino's group at Osaka University in Suita, Japan, have demonstrated photonic bandgap tunability in synthetic opal infiltrated with liquid crystal. They also have produced a tunable inverse opal, promising applications in low-threshold tunable lasers, prisms and polarizers.
The application of a voltage to nematic liquid crystal in the voids of synthetic opal enables electrical tuning of the structure's photonic bandgap. Courtesy of Osaka University.
The researchers constructed the thin-film opal by introducing a suspension of 300-nm-diameter silica spheres between two glass plates that they separated by 12 µm and coated with indium tin oxide. When they applied a voltage to the face sheets, the 4-pentyl-4'-cyanobiphenyl liquid crystal in the interconnected tetrahedral and octahedral voids oriented parallel to the field, which decreased the average refractive index of the liquid crystal and shifted the overall reflectance of the photonic crystal.
They measured the reflectance of the film with a Shimadzu spectrometer and determined that an applied 160 V shifted the reflectance peak from 726.5 to 721 nm, approximately two-thirds of the shift that would be expected if all the liquid crystal molecules oriented to the electric field. Because the volume of each void was on the order of 0.005 µm3, they attributed this to the influence of surface effects on the liquid crystal.
By monitoring the transmitted intensity of a 680-nm diode laser, the scientists found that the change in reflectance occurred in approximately 10 µs, faster than the response time of the liquid crystal in a conventional twisted nematic cell. They suspect that the nonspherical shape of the voids enhanced the local electric fields within.
Finally, when they removed the electric field, the reflectance of the photonic crystal did not return to its original value until the crystal was heated. They also believe that this optical memory effect is a surface phenomenon, in which the nematic liquid crystal anchors to the walls of the voids.
Tuning in inverse opal
As in the pioneering work of Kurt Busch and Sajeev John at the University of Toronto, the researchers also have investigated the electric tuning of the photonic bandgap in an inverse opal infiltrated with liquid crystal. Masanori Ozaki, an associate professor at the university and member of the research team, reported that this structure demonstrates a larger peak shift and a clear, discontinuous threshold.
With electric tunability of the bandgap, applications are closer to realization. "If we can control the position and/or width of the photonic bandgap, for example, we can actively control the laser action or wavelength tuning of the emission," Ozaki said.
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