The light harvesting efficiency of dye-sensitized photoelectrodes was enhanced by coupling a TiO(2) photonic crystal layer to a conventional film of TiO(2) nanoparticles. In addition to acting as a dielectric mirror, the inverse opal photonic crystal caused a significant change in dye absorbance which depended on the position of the stop band. Absorbance was suppressed at wavelengths shorter than the stop band maximum and was enhanced at longer wavelengths. This effect arises from the slow group velocity of light in the vicinity of the stop band, and the consequent localization of light intensity in the voids (to the blue) or in the dye-sensitized TiO(2) (to the red) portions of the photonic crystal. By coupling a photonic crystal to a film of TiO(2) nanoparticles, the short circuit photocurrent efficiency across the visible spectrum (400-750 nm) could be increased by about 26%, relative to an ordinary dye-sensitized nanocrystalline TiO(2) photoelectrode.

No abstract

Manipulation of light is in strong demand in information technologies. Among the wide range of linear and nonlinear optical devices that have been used, growing attention has been paid to photonic crystals that possess a periodic modulation of dielectric function. Among many photonic bandgap (PBG) structures, liquid crystals with periodic structures are very attractive as self-assembled photonic crystals, leading to optical devices such as dye lasers. Here we report a new hetero-PBG structure consisting of an anisotropic nematic layer sandwiched between two cholesteric liquid-crystal layers with different helical pitches. We optically visualized the dispersion relation of this structure, displaying the optical diode performance: that is, the non-reciprocal transmission of circular polarized light at the photonic-bandgap regions. Transmittance spectra with circularly polarized light also reveal the diode performance, which is well simulated in calculations that include an electro-tunable diode effect. Lasing action was also confirmed to show the diode effect with a particular directionality.

A cholesteric liquid crystal (CLC) is a self-assembled photonic crystal formed by rodlike molecules, including chiral molecules, that arrange themselves in a helical fashion. The CLC has a single photonic bandgap and an associated one-colour reflection band for circularly polarized light with the same handedness as the CLC helix (selective reflection). These optical characteristics, particularly the circular polarization of the reflected light, are attractive for applications in reflective colour displays without using a backlight, for use as polarizers or colour filters and for mirrorless lasing. Recently, we showed by numerical simulation that simultaneous multicolour reflection is possible by introducing fibonaccian phase defects. Here, we design and fabricate a CLC system consisting of thin isotropic films and of polymeric CLC films, and demonstrate experimentally simultaneous red, green and blue reflections (multiple photonic bandgaps) using the single-pitched polymeric CLC films. The experimental reflection spectra are well simulated by calculations. The presented system can extend applications of CLCs to a wide-band region and could give rise to new photonic devices, in which white or multicolour light is manipulated.

Lasing via a phase retardation defect mode is reported for the first time. The Figure shows the far‐field pattern of laser emission realized by inserting an anisotropic nematic liquid crystal (LC) defect layer between polymer cholesteric LC layers. The system mimics the cuticle of Plusiotis resplendens, a beetle, using the birefringence of the anisotropic layer to achieve reflectance greater than the 50 % provided by simple cholesteric LC photonic bandgaps.

This letter describes a simple synthetic approach to fabricate two-dimensional midinfrared CdSe photonic crystals ͑PC͒ by electrodeposition of CdSe in a polymer template defined using interference lithography. Characterization of the transmission spectra of CdSe PCs with a hexagonal array of 1.3 m diameter and 2.7 m pitch air voids showed a well-defined drop in transmission at 4.23 m. The drop in transmission increased with incident angle, reaching a maximum of approximately 2.6 dB at 40°relative to the surface normal. This two-step synthetic approach can be used to incorporate photonic crystals onto arbitrary substrates for integration into future advanced optical circuits.

The authors have demonstrated increased light outcoupling from organic light-emitting diodes (OLEDs) based on nanoimprinted amorphous fluoropolymer, poly[perfluoro(4-vinyloxy-l-butene)] (PPFVB). Because PPFVB has a low refractive index and a high transmittance over wide wavelength range, the first-order Bragg diffracted light shows high directionality and decreased waveguide absorption. These characteristics provide an advantage for small- or medium-sized OLEDs, which are mainly viewed from the normal direction.

Filling-factor tunable titanium dioxide replicas of colloidal crystals were fabricated by using a liquid phase deposition (LPD) technique. Ammonium hexafluorotitanate [(NH4)2TiF6] was used as a precursor, which was converted slowly to titanium dioxide by hydrolysis with boric acid. After the oxide was formed in voids of latex colloidal crystals, the latex was removed by calcination. By applying the LPD process to the replica again, the filling factor of the replica could be increased in a controllable fashion. With 203 nm template spheres, the peak shift in the ∼490 nm stop band depended linearly on the deposition time for the first 20 min, at a rate of 4.4 nm/min. Thus the position of the Bragg reflection peak could be adjusted precisely by controlling the filling factor of the replica structure.