Periodic Variation Of Refractive Index Research Articles

Spherical colloidal photonic pigments with saturated structural colors through temperature-controlled polymer nanoparticle assembly

Structural color is based on the reflection of light by materials with a periodic variation of refractive index in the length scale of the wavelength of visible light. Unlike conventional dyes and pigments, there is no absorption of radiation, and therefore structural pigments are impervious to fading by photobleaching. Also, because structural color is mainly dependent on structure, not composition, it can be obtained from materials with low environmental impact and toxicity. Structural pigments with colors covering the full visible spectrum can therefore be obtained from the same colloidal materials, by tuning only their size and the spatial arrangement of individual nanoparticles.Here, we show that using polymer nanoparticles with low size dispersity, and controlling the kinetics and the inter-particle interactions during the assembly process, one can obtain spherical non-iridescent photonic pigment. Using polymer nanoparticles with diameters in the range of 170 nm–330 nm, we obtain structural pigments with bright colors spanning the full visible spectrum, with different hue and saturation. The optical properties of these pigments are directly related to the arrangement of the polymer nanoparticles within the spherical structural pigment. This arrangement, controlled by the surface charge of the nanoparticles and the rate of water evaporation from the template droplets containing the nanoparticles, changes from an ordered face-centered cubic packing at 50 °C, to an almost amorphous packing at 65 °C. Our spherical photonic pigments reflect a plethora of vibrant colors with low angle dependence, with excellent prospects for used as dispersible pigments in the development of environmentally friendly coatings with non-fading color.

Tuning the band structures and electromagnetic density of modes in fused Silica slab by acoustic waves

The band structures and electromagnetic density of modes in fused Silica slab perturbed by acoustic waves have been modelled and studied. The strain caused by the acoustic wave modulates the refractive index of the fused Silica slab. This leads to the periodic variation of refractive index in the slab along propagation direction similar to one dimensional photonic crystal. The band structures, spectral transmittance and electromagnetic density of modes in induced one dimensional photonic crystal have been calculated using transfer matrix method and Wigner time phase approach. A wide photonic band gaps with enhanced magnitude of density of modes at the edges are observed for the photonic crystal induced by longitudinal acoustic wave. It is found that the photonic band gaps are shifted to higher frequencies with the increase in angle of incidence for both polarized electromagnetic waves. The photonic bandgap and density of mode are tuned by varying the acoustic frequencies. Also, the increase in acoustic amplitude leads to enhanced density of modes at the edges of photonic band gaps.

Cellulose-Based Reflective Liquid Crystal Films as Optical Filters and Solar Gain Regulators

Many promising approaches for designing interactions of synthetic materials with light involve solid optical monocrystals and nanofabricated photonic crystal structures with spatially periodic variations of refractive index. Although their high costs limit current technological applications, remarkably, such photonic and optically anisotropic materials have also evolved throughout nature and enable narrow or broad-band spectral reflection of light. Here we use self-assembly of biomaterial cellulose nanocrystals to obtain three-layer films with helicoidal and nematic-like organization of the cellulose nanoparticles, which mimics naturally occurring polarization-insensitive reflectors found in the wings of Plusiotis resplendens beetles. These films were characterized with polarized optical microscopy and circular dichroism spectrometry, as well as scanning and transmission electron microscopies. These films exhibit high reflectivity tunable within the visible and near-infrared regions of the optical spectrum and may find applications ranging from color filters to smart cloth designs and in solar-gain-regulating building technologies.

Tough and Variable-Band-Gap Photonic Hydrogel Displaying Programmable Angle-Dependent Colors.

One-dimensional photonic crystals or multilayer films produce colors that change depending on viewing and light illumination angles because of the periodic refractive index variation in alternating layers that satisfy Bragg’s law. Recently, we have developed multilayered photonic hydrogels of two distinct bulk geometries that possess an alternating structure of a rigid polymeric lamellar bilayer and a ductile polyacrylamide (PAAm) matrix. In this paper, we focus on fabrication of composite gels with variable photonic band gaps by controlling the PAAm layer thickness. We report programmable angle-dependent and angle-independent structural colors produced by composite hydrogels, which is achieved by varying bulk and internal geometries. In the sheet geometry, where the lamellae are aligned parallel to the sheet surface, the photonic gel sheet exhibits strong angle-dependent colors. On the other hand, when lamellae are coaxially aligned in a cylindrical geometry, the gel rod exhibits an angle-independent color, in sharp contrast with the gel sheet. Rocking curves have been constructed to justify the diverse angle-dependent behavior of various geometries. Despite varying the bulk geometry, the tunable photonic gels exhibit strong mechanical performances and toughness. The distinct angle dependence of these tough photonic materials with variable band gaps could benefit light modulation in displays and sensor technologies.

Observation of wavelength-dependent shift in Brewster angle in 3D photonic crystals

The interaction of polarized light with photonic crystals exhibits unique features due to its sub-wavelength nature on the surface and the periodic variation of refractive index in the depth of the crystals. Here, we present a detailed study of polarization anisotropy in light scattering associated with three-dimensional photonic crystals with face centered cubic symmetry over a broad range of wavelength and angle. The polarization anisotropy leads to a shift in the conventional Brewster angle defined for a planar interface with certain refractive index. The observed shift in Brewster angle depends strongly on the index contrast and lattice constant. Polarization-dependent stop gap measurements are performed on photonic crystals with different index contrasts and lattice constants. These measurements indicate unique stop gap branching at high-symmetry points in the Brillouin zone of the photonic crystals. The inherited stop gap branching is observed for TE polarization whereas it is suppressed for TM polarization as a consequence of the Brewster effect. Our results have consequences in the scattering of polarized light from plasmonic structures and dielectric meta-surfaces and are also useful in applications such as nanoscale polarization splitters and lasers.

Controllable lasing behavior enabled by compound dielectric waveguide grating structures.

The photonic density of states (PDOS) is one of the key physical quantities governing the lasing behavior for photonic band-edge lasers. The PDOS is conventionally altered by exploiting the high-Q band-edge mode within a device, which is typically achieved by increasing the contrast of periodic refractive index variation (Δn) or increasing the periodic number of the photonic crystals. In this paper, we propose a different approach to achieve a high-Q band edge mode within an active compound dielectric waveguide grating (CWDG). We demonstrate that the lasing threshold and intensity can be flexibly tuned by changing the filling factors of the CWDG. This design can effectively improve the performance of electrically pumped photonic band-edge lasers.

POLICRYPS-based electrically switchable Bragg reflector.

The formation and characterization of a switchable volume reflective element fabricated from a polymer liquid crystal (LC) polymer slice (POLICRYPS) structure by holographic photopolymerization at high temperature (65 °C) using a photosensitive/nematic liquid crystal prepolymer mixture is reported. The submicron Bragg structure formed consists of periodic continuous polymeric walls separated by periodic LC channels. The phase separated NLC self-aligns in a homeotropic alignment between the polymer walls as indicated by polarizing optical microscopy analysis (Maltese cross). The resulting periodic grating structure results in a Bragg reflection notch upon illumination with white light due to the periodic variation in refractive index. Electro-optical experiments realized through in-plane electrodes and temperature experiments confirm that the multilayer structure acts as a Bragg mirror whose reflection efficiency can be controlled by either a small (~3V/µm) electric field or temperature.

All-optically controlled one-dimensional photonic crystal of AlGaN film via photorefractive effect

In this work, we report theoretical design of one-dimensional photonic crystal of Al0.4Ga0.6N material, in which the photonic band is formed and controlled all-optically via photorefractive effect. The simulation results show that the photorefractively periodic variation of refractive index, enhanced by the strong polarization field in AlGaN material, gives evident photonic band gap, and the band feature is a function of the incidence direction of illumination and signal light. Moreover, the response time of photonic band writing and erasing can be greatly reduced by applying high electron mobility transistor structure, and the work frequency is expected to enter GHz range. The results demonstrate a simple, high efficiency, high speed, and flexible way of realizing photonic crystal all-optically.

Tuning of zero group velocity dispersion in infiltrated vertical silicon slot waveguides

In this work the design of Si / hybrid waveguides which contain a vertical infiltrated slot is studied. The case of slots infiltrated with a χ³ nonlinear material of relatively high refractive index (e.g. chalcogenide glasses) is specifically discussed. An optimized waveguide geometry with periodic refractive index modulation, a nonlinear figure of merit > 1 and minimum effective mode cross section is presented. Introducing a periodic refractive index variation along the waveguide allows the adjustment of the group velocity dispersion (GVD). Choosing the period accordingly, the phase matching condition for degenerate four wave mixing (GVD = 0) can be fulfilled at virtually any desired frequency and independently from the fixed optimized waveguide cross section.

Permeable nanoconfinement of hierarchical block copolymer volume gratings

A hierarchical structure of poly(ethylene oxide)-b-poly(e-caprolactone) (PEO-b-PCL) block copolymer (BCP) confined between crosslinked resin was patterned into Bragg volume gratings using a holographic polymerization (HP) process. The BCP formed a lamellar structure confined between the layers of the grating created by HP. The periods of the volume grating and the BCP were controlled to be ∼200 nm and 20 nm, respectively. These two different length scale layers were aligned parallel to one another yielding a polymeric film which exhibits distinct diffraction behavior due to a periodic refractive index variation. This system exhibits complex thermo-optical behavior during heating and cooling cycles with reversible changes in both the diffraction wavelength and efficiency induced by BCP melting and crystallization in the confined region. Transmission electron microscopy studies show reversible diffusion of PEO-b-PCL into and out of the crosslinked resin, indicating that the nanoconfinement imposed by the resin is soft and permeable for the BCP. The morphological changes in nanoconfinement with temperature account for the complex thermo-optical behavior of the grating and the system provides an interesting platform to investigate soft nanoconfinement of BCP materials.

The Role of the Thermal Oxide in GaAs-Based Photonic Bandgap Waveguide Microcavities

Recent work on optical microcavities is concisely reviewed in this article. Optical microcavities can be fabricated using III–V compound semiconductors by introducing a large periodic refractive-index variation along the axis of GaAs waveguides. Strong optical confinement in the microcavities is achieved by oxidizing the underlying III–V material and further optimized by preferentially etching away the oxide to form an air-bridge structure. The III–V thermal oxide is obtained from the wet thermal oxidation of high Al-content AlxGa1–xAs material. The critical role of the thermal oxide in the design and fabrication of the waveguide microcavities is outlined and the characterization of the properties of the thermal oxide salient to the fabrication of the devices is described. Finally, preliminary transmission data demonstrating the successful realization of these microcavities are presented.

Electrically switchable mirrors and optical components made from liquid-crystal gels

In liquid-crystal (LC) display devices, patterned electrodes are used to effect switching of molecular orientation within a pixel element, and thin layers of a material (typically a polymer) at the surfaces of the cell plates induce the liquid-crystal molecules to revert to their original orientation after the electric field is switched off1. Because of their periodic variation in refractive index, cholesteric LC phases (which have a helical variation in orientation) reflect light at a wavelength determined by the helical pitch2,3, and so can potentially be used in switchable optical devices such as shutters, reflectors and notch- and band-pass filters. But orientation layers are unable to restore the initial orientation in cholesteric phases. They can instead be given an internal ‘memory’ of their initial orientation by creating an anisotropic polymer network within the system using in situ photopolymerization4. Here we show that such crosslinked cholesteric gels can be used to produce fast electrically switchable reflectors with narrow- to broad-band widths (the latter having a silvery mirror-like appearance). By photopolymerizing in a patterned manner, we can make image recordings in the gels which become visible on application of an electric field. These patterned gels offer the prospect of making optical components such as lenses and gratings by holographic recording.

Crystal growth of strontium barium niobate BaxSr1− xNb2O6

Study of solid state reactions which occur during the synthesis of barium strontium niobate revealed the presence of a previously unrecorded monoclinic phase belonging to the P21/a space group in coexistence with the desired tetragonal phase. Under certain preparation conditions, formation of this phase can produce growth defects in the single crystal. The production of single crystals without defect requires use of raw materials which do not contain this monoclinic phase. Single crystals pulled using the Czochralski method contain striations; these periodic variations of refractive index were shown to be related to a heterogeneous distribution of small quantities of impurities such as calcium and potassium. The growth of single crystals free of striations can be obtained using very pure raw materials by carrying out the pull without rotation and by thermally insulating the upper part of the crucible. These single crystals were tested by the determination of the half wave voltage which was shown to be 85 V, and by the extinction ratio, which is on the order of 30. Experiments of light modulation were also carried out.