Tapered photonic crystal fibers (PCFs), similar to tapered conventional fibers, can be either adiabatic of nonadiabatic. Here, we examine the adiabaticity of the PCF taper and obtain three-dimensional plots showing the evolution of power flow inside the tapered PCF. Also, we analyze different taper shapes, such as the linear taper, raised-cosine taper, and the modified exponential taper. Other factors, such as the taper length and the number of air hole rings are also investigated.

A finite element-based, rigorous full-vectorial modal solution approach has been developed to calculate the effective index of the fundamental space filling mode, the cut-off condition of the fundamental and the second guided modes to identify single mode operation ranges for a photonic crystal fiber design. Furthermore, structural asymmetry has been introduced in the model to maximise the modal birefringence to create a design for polarization maintaining photonic crystal fiber.

In this paper we report the fabrication of microstructured optical fibers (MOFs) metallic metamaterials using a bottom-up processing technique for surface enhanced Raman scattering (SERS) applications. The inner walls of the silica-based holey optical fiber have been modified by depositing granular films of Ag nanoparticles from its organometallic precursor at high pressure condition. The resulting fibers demonstrate strong SERS effect when analyte molecules are infiltrated within the MOF due to large electromagnetic field enhancement and long interaction length. The chemically modified MOFs with 3D patterning represent an exciting platform technology for next generation SERS sensors and plasmonic in-fiber integrated devices.

Long-period gratings (LPGs) have been fabricated in endlessly single mode photonic crystal fibers (ESM-PCFs) utilizing focused CO₂ laser irradiation and residual stress relaxation technique. The responses of the ESM-PCF-LPGs to external refractive index and applied bending curvature have been investigated. As compared with the conventional fiber-based LPGs inscribed under the same condition, the ESM-PCF-LPGs exhibit higher sensitivity to external refractive index change and macro-bending, making them attractive candidates for chemical and biological sensing applications.

Photonic crystal fiber (PCF) has been studied intensively in the past decade owing to its potential in fiber optic communication and sensing applications. Many research interests have been attracted to the long period gratings (LPGs) and LPG based devices in PCFs in recent years. Because of the microstructured air holes, the effective modal index shows strong wavelength dependence, which will result some anomalous properties in such gratings distinguished with conventional fiber. However, the mode coupling characteristics have not been investigated in details and the systematic comparison with the single mode fiber (SMF) based LPGs have not been analyzed yet. Moreover, because of the existence of the air holes in the cladding, we can include different aqueous solutions in such holey structure, such as pure water or sugar solutions. Meanwhile, the refractive index of these inclusions can be easily tuned by changing the temperatures, which will result in an evolution of the cladding modes. So some interesting coupling effects can be expected between the core and cladding modes. In this paper, the evolution of mode coupling with the structure change is discussed firstly. Then the attention is focused to investigate the PCF based mechanical LPGs with aqueous solution inclusions. The shift of resonance wavelength with the solution brix and the temperature is evaluated both theoretically and experimentally. This grating device offers the unique advantages of being tunable, removable, reconfigurable and strain-stable. These properties guarantee it to be utilized as a stable candidate for temperature and refractive index sensing.

Colloidal crystalline arrays are three dimensionally periodic lattices of self-assembled monodisperse colloidal spheres. These periodic structures have been actively explored as functional components in fabricating new types of diffractive devices such as optical filters and chemical sensors, mechanical sensors, and photonic bandgap structures. We have demonstrated the synthesis of silica or polystyrene spheres uniformly coated with titania nanosheets and the fabrication of these spheres into close-packed colloidal crystalline arrays. We have also reported on the optical properties and microstructures of the colloidal crystalline array estimated by angle-resolved reflection spectra measurements. The titania nanosheets were synthesized by delamination of layered titanate crystallites. The titania nanosheets coated spheres were prepared by the LBL (Layer-By-Layer) assembly coating process, which consisted of alternately laminating cationic polyelectrolyte and anionic titania nanosheets on monodisperse silica or polystyrene spheres. The close-packed colloidal crystalline array was fabricated in the glass cell by drying process of the aqueous dispersion of the spheres. The Bragg diffraction peak of the colloidal crystalline array shifted to longer wavelengths with increasing thickness of titania nanosheets layers. Angle-resolved reflection spectra measurements showed that this red shift was caused by increasing the mean effective refractive index n_eff of this crystalline lattice without changing interplanar spacing d₁₁₁ with increasing thickness of titania nanosheets layers. Since a wide range of coated colloids of different size, composition, and optical properties can be prepared via the LBL coating, the current work suggests new possibilities for the creation of advanced colloidal crystalline arrays with tunable optical properties.

Optical properties of mesoporous three-dimensional photonic crystals (3D PCs) based on thin opal films were found to be extremely sensitive to a humidity of the surrounding air. It was manifested that the internal structure of a single SiO₂ sphere together with the net of voids between them in a thin opal film acts as a sponge for wet steams. Our experimental data have shown that hydrophilic internal structure of a mesoporous film sponges up (and lose) water (dry or wet steams) that influences dielectric permittivity, the latter causes significant changes in transmission spectra. High sensitivity, quick response and possibility of contactless measurements makes sensors based on optical effects in mesoporous PCs to be very promising. It concerns not only humidity sensors, but also sensors of various gases, temperature, deformation and other environmental impacts.

This paper describes an elastic colloidal crystal, soft opal, with tunable stop band and its potential application for mechanical strain sensing. A poly-dimethylsiloxane, PDMS, rubber sheet was coated with a thin layer of the soft opal. In the layer, polystyrene, PS, submicron particles were cubic-closely packed, CCP, and among the particles filled with PDMS elastomer. We design the array of CCP (111) planes selectively diffracts light in visible wavelength. The wavelength, i.e. photonic stop band, was reversibly tuned by tensile strain. As the PDMS sheet was stretched in horizontal direction, the film becomes compress in vertical direction. As a result, the lattice distance of CCP (111) planes decreased and the stop band shifted to shorter wavelengths. As released the mechanical strain on the PDMS sheet, the stop band completely returned to the initial wavelength. The PDMS sheet also displays reversible change of the structural color during elastic deformation. The potential application is a simple stress sensor enables visual judgment of the intensity of tensioning.

Novel nonlinear optical effects - photonic flame effect (PFE)^1,2 and stimulated globular scattering (SGS)³ - have been discovered. SGS was observed both in forward and backward direction. Pure opal crystal, consisting of the close-packed SiO₂ globules with diameter 200 nm, and crystal with pores, filled with molecular liquid, have been studied. Two Stokes components, shifted from the exciting light frequency by 0.4 - 0.6 cm^-1, have been observed in SGS. Photonic flame effect consisted in the appearance of the few seconds' duration emission in blue-green spectral range under 20 ns ruby laser pulse excitation.

In this work, we investigated the possibility of application of magnetophotonic crystals to the optical magnetic field sensor. The structure of 1D-MPC was (Ta₂O₅/SiO₂)⁵/Bi:YIG/ (Ta₂O₅/SiO₂)⁵ (magnetic material as a defect layer between two Bragg reflectors) on a fused quarts substrate using RF magnetron sputtering apparatus. We used Bismuth substituted yttrium iron garnet (Bi:YIG) polycrystal film as a defect layer, because Bi:YIG is well known as the magnetic material with effective MO properties, even if it is polycrystal. Due to specially designed structure, the localized mode appeared at the wavelength of 880 nm, which is tunable by the thickness of multi layers or defect layer. At the wavelength of localized mode, Faraday rotation was shown large enhancement of 1.5°, that is fifty times larger than for single Bi:YIG polycrystal film of the same thickness.

We proposed flexible bandwidth control for a two-dimensional (2D) photonic crystal coupled-cavity waveguide. The 2D waveguide is designed to operate in single-mode. The bandwidth not only determines the operating frequency range of the waveguide, but also affects the group velocity of the guided modes much. Researches in enlargement and precise controlling of bandwidth are of great importance for waveguide structure design based on photonic crystals. Moreover, to keep the signal pulse shape along a single-mode waveguide, minimized group velocity within a wide bandwidth is required for the design. In our previous studies we have demonstrated controlling the upper and lower cut off frequencies of the guided band both independently and simultaneously. In this work, large bandwidth-tuning for a single-mode guided band with fixed center frequency is realized by changing two configuration parameters, namely defect radius and defect width. Plane wave expansion method is utilized for calculation. The largest bandwidth tuning range up to 50.7% of photonic bandgap (PBG) is achieved for normalized center frequency at 0.377. Furthermore, for different bandwidths, we investigate the relations of group velocities and wave vectors, which are crucial to engineer the group velocity dispersion in the waveguide. Our results demonstrate the possibility of large bandwidth tuning while single-mode operation is maintained, which could be extended to photonic crystal slab waveguide with some modifications. We believe this work will contribute to the design of integrated optical devices based on photonic crystal waveguides, such as multiplexers and de-multiplexers which can make use of the flexible bandwidth control capabilities.

We present exact analytical solutions to the ideal one-dimensional planar photonic crystal waveguides (1-D PPCWs) that consist of a central core and infinite number of cladding layers symmetrically placed around the core. We show that these exact solutions allow one to distinguish clearly between light guidance due to total internal reflection and light guidance due to the photonic crystal effect. We also compare structures with finite and infinite cladding layers and provide results for the propagation characteristics and the modal field distributions.