Photonic crystal formed by the imaginary part of the refractive index

Titania Photonic Crystals with Precise Photonic Band Gap Position via Anodizing with Voltage versus Optical Path Length Modulation

Photonic crystals based on titanium oxide are promising for optoelectronic applications, for example as components of solar cells and photodetectors. These materials attract great research attention because of the high refractive index of TiO₂. One of the promising routes to prepare photonic crystals based on titanium oxide is titanium anodizing at periodically changing voltage or current. However, precise control of the photonic band gap position in anodic titania films is a challenge. To solve this problem, systematic data on the effective refractive index of the porous anodic titanium oxide are required. In this research, we determine quantitatively the dependence of the effective refractive index of porous anodic titanium oxide on the anodizing regime and develop a model which allows one to predict and, therefore, control photonic band gap position in the visible spectrum range with an accuracy better than 98.5%. The prospects of anodic titania photonic crystals implementation as refractive index sensors are demonstrated.

Vivid colours in hyperuniform complex-index photonic structures by resonant interference of photonic band gaps and optical band gaps

Hyperuniform photonic structures (HPSs) have been doped with complex index materials to increase their reflectivity and colour expression range. HPSs synthesized using dielectric SiO₂ nanoparticles have been mixed with a small amount of dopant nanoparticles (c_d ≤ 1%) having a complex refractive index. Various dyes including Sudan I, Sudan Blue II, Alizarin yellow GG, Bromocresol purple and polydopamine (PDA) are used as dopants. Large reflectivity enhancements of HPSs (∼100%) are observed by resonant interference of photonic band gaps (PBGs) and optical band gaps (OBGs). Reflectivity enhancements are observed only when PBGs of HPSs match with OBGs of dopants. The colour expression range of HPS increases by 600% by doping with melanine-like PDA nanoparticles, which have the imaginary part of the refractive index in whole visible range.

Hyperuniform photonic structures (HPSs) have been doped with complex index materials to increase their reflectivity and colour expression range.

High-resolution ghost imaging experiments with cosh-Gaussian modulated incoherent sources

We experimentally demonstrate that ghost imaging (GI) quality can be significantly improved by modulating the incoherent Gaussian sources with hyperbolic cosine functions. In our experiments, a source with desired cosh-Gaussian intensity distribution can be generated by a programmable spatial light modulator, and is used in the GI setup to obtain high quality ghost images. The influences of the hyperbolic cosine parameter on the point-spread function and the GI quality have been investigated experimentally and interpreted theoretically. Our experimental results show that the resolution of ghost images can be obviously enhanced with cosh-Gaussian modulated incoherent sources. The kind of source-shaping technique proposed in this paper is a high-efficiency method to improve the GI quality and may be very useful for the GI applications.

New approach for multilayered microstructures fabrication based on a water-soluble backing substrate

We demonstrate a new approach for woodpile microstructure fabrication. The method involves the use of polyvinyl alcohol (PVA) as a sacrificial substrate for the transfer of SU-8 films to prepatterned structures. The surface activated PVA substrate allows good wettability of SU-8 film and its solubility in water eliminates the need of delaminating SU-8 structures from the substrate. This makes the fabrication process much simplified and we successfully demonstrate eight-layer stacking of gratings. Fourier transform infrared spectra of single-layer and four-layer grating structures show a broader transmission dip spectrum compared to their film counterparts, indicating their potential use as broadband terahertz (THz) absorbers.

Diffractive properties of imaginary-part photonic crystal slab

The diffraction spectra of Imaginary-Part Photonic Crystal (IPPC) slabs are analyzed by using Scattering-Matrix Method. By investigating the thickness dependence of the diffraction, we find the remarkable red shift of central wavelength of diffraction spectrum, which obviously distinguishes from the phenomenon of spectral hole. We observe that diffraction efficiency can be enhanced more than twentyfold by optimizing the geometry parameters. These imply that the diffraction spectra of the IPPC slab can be controlled at will and used to guide the design to achieve useful nanoscale devices.

Light propagation in a resonantly absorbing waveguide array

Light propagation behavior in a resonantly absorbing waveguide array is analyzed. Both a Lorentzian line shape and an inhomogeneous broadened absorbing line shape are considered, with their imaginary and real part of the refractive index determined by a Kramers-Kronig relationship. The diffracted wave is shown to have the frequency spectra determined by the material absorption, dispersion as well as the waveguide structure. An interesting phenomenon is that a spectral hole is produced and becomes deeper in the diffraction spectrum as the thickness of the resonantly absorbing waveguide array increases. The experimental measurements conducted in a waveguide array are found to be in good agreement with the numerical results.