Optical Switching of a Birefringent Photonic Crystal

Abstract

There has been vast interest in the study of photonic crystals because of their unique abilities to control light propogation. [1,2] Photonic crystals have exhibited applications in displays, [3][4][5] optical wave-guides, [6,7] lasers, [8] sensors, [9][10][11] and so on. For most applications, photonic crystals with tunable or switchable optical properties are desired. Although there are a number of reports on the design and fabrication of tunable photonic crystals, [12][13][14] only a few works contribute to the construction of birefringent photonic crystals with optical tunability. [15,16] Liquid crystals are one of the important functional materials being derived to construct tunable photonic crystals with optical response to temperature, electric field, and photo-irradiation. Shifting the stop-band, changing the intensity of reflection, or even reversing the reflection peak at the stop-band have been realized. [17][18][19][20] In these experiments, the birefringence of a liquid crystal was only derived for light scattering. In this report, it is shown that it is possible to generate a birefringent photonic crystal by controlling the orientation of liquid crystal molecules. It will be shown that the transmission of light with polarization parallel and perpendicular to the liquid crystal molecule orientation exhibits different behaviors, both of which can be controlled by a control light. The photonic crystal used in the experiment was a stretched poly(methyl methacrylate) (PMMA) inverse opal infiltrated with a photo-responsive liquid crystal. The PMMA inverse opal was replicated from an opal template (with a thickness of 3-5 mm) composed of silica spheres (345 nm in diameter) and stretched by a drawing device. Figure 1 shows the scanning electron microscopy (SEM) images and reflection spectra of the PMMA inverse opal film with and without stretch. It can be seen that the inverse opal film before stretch is composed of spherical holes with a hexagonal arrangement. A 100% stretch of the film deformed the holes to an elliptical shape without destroying the structural order. The major axis of the spheroid was parallel to the stretch direction extended after stretch, while the axes perpendicular to the stretch direction shortened, which resulted in the decrease of the distance between the (111) faces parallel to the film surface. As a result, the stopband blue-shifted from 629 to 421 nm.The polarization-dependent reflection spectra of the stretched inverse opal after the infiltration of a liquid crystal are shown in Figure 2. The liquid crystal used for infiltration was a mixture of 4-pentyl-4 0 -cyanobiphenyl (5CB) and 4-butyl-4 0 -methoxyazobenzene (Azo-LC) with a volume ratio of 97: 3. When the polarization direction of light was parallel to the stretching axis, a reflective peak was observed at 592 nm. This peak gradually depressed by rotating the polarization direction of light and disappeared when the polarization direction of light was perpendicular to the elongation direction. The birefringent effect can a...

Tunable Colors in Opals and Inverse Opal Photonic Crystals

“…[ 48 ] The LC molecules tended to be oriented in the cavities along the stretched axis, and a clear birefringence effect was noted. Under visible light, a refl ection peak was present for polarized light in the direction parallel to the stretching direction but not for light perpendicular to the stretching direction.…”

Responsive photonische Kristalle

“…[152] Anders als bei der bipolaren Struktur ordneten sich die LC-Moleküle in der nematischen Phase entlang der Längsstreckung an. Beim Bestrahlen des Kristalls mit parallel polarisiertem Licht verursachte der große Unterschied der Brechungsindizes entlang der Längsstreckung eine Reflexion.…”

Color‐ and Reflectance‐Tunable Multiple Reflectors Assembled from Three Polymer Films

“…[1] Tuning of such PBGs in the visible region is attractive for active photonic applications in display, sensor, and lasing devices. [2][3][4][5][6][7][8][9] Here, we design and fabricate a tunable multiple PBGs system consisting of an anisotropic polymeric nematic liquid crystal (PNLC) film and two kinds of isotropic polymer film, and demonstrate tuning of reflectance and colors of the multiple PBGs in the whole visible region by changing the polarization direction of linearly polarized incident light. The only difference between the two tunable systems is the stacking sequence.…”

“…The spectral characteristics of the PBGs can be modulated by changing the refractive indices, thicknesses, and periodicities in given photonic crystals, which can be performed by external light, [2,3] electric [4,5] and magnetic [6] fields, and chemicals. [7][8][9] However, to obtain broad tunability covering the complete visible spectral range, exceptionally large changes such as enormous birefringence or swelling/de-swelling length are required.…”