Bragg Reflection Spectra Research Articles

Numerical analysis of Bragg reflection spectra of GaAs/AlGaAs super-layers depending on hydrostatic pressure

Numerical analysis of Bragg reflection spectra of GaAs/AlGaAs super-layers depending on hydrostatic pressure

A compact design of a modified Bragg grating filter based on a metal-insulator-metal waveguide for filtering and temperature sensing applications

In this work, we presented a numerical study on the modified Bragg grating structure based on a metal-insulator-metal (MIM) waveguide (WG) that can be simultaneously employed as an optical filter and temperature sensor. The device optimization is performed by utilizing the 2D-finite element method. The standard Bragg grating structure is composed of a periodic arrangement of nanoblocks (NBs) decorated in the center of the MIM WG resulting in the formation of a broadband Bragg reflection bandwidth. However, in the proposed study, the NBs are displaced to one side of the MIM WG creating a different optical path with unequal indices on both sides of the grating, as a result, a narrow MZI dip is obtained along with the Bragg reflection spectrum. To functionalize the MZI dip for temperature sensing application, the MIM WG slot is filled with a temperature-sensitive polydimethylsiloxane (PDMS) polymer. Consequently, the modified Bragg grating structure can be simultaneously employed as a bandstop filter (bandwidth~200 nm) and temperature sensor with an attractive sensitivity of 0.47 nm/°C.

A method for monitoring bolt torque in a composite connection using an embedded fiber Bragg grating sensor

This work presents a new method for monitoring the preload torque in a composite bolted connection using an embedded fiber Bragg grating sensor. A unique washer was designed to impose a specified nonuniform strain field across the grating, causing distortion in the reflected optical spectrum. Using the full-width at half maximum bandwidth of the Bragg reflection spectrum as the preload-sensitive feature, it is shown that this feature increases monotonically—and quite linearly—with increasing applied bolt torque. It is also demonstrated that, although distorted, the spectral structure of the sensor is maintained such that it is simultaneously able to function in its typical use as a uniaxial strain sensor, thus essentially creating a dual-purpose sensor. The computational design approach is validated with a prototype experiment.

Optical anisotropy of cubic photonic crystals under conditions of multiple-mode light propagation

Bragg reflection spectra of light are studied for opal-like photonic crystals made of polystyrene spheres. A resonant enhancement of reflectivity is observed in cross-polarization configuration of the analyzer and polarizer when varying the azimuthal orientation of a sample in respect to the incidence plane. The cross-polarization effect takes place at oblique incidence of light on the lateral (111) crystal plane with the plane of incidence being non-perpendicular to the inclined (11-1) crystal plane. The effect is shown to be due to the multiple Bragg diffraction of light when the resonant Bragg conditions are fulfilled at a certain angle of incidence and azimuth for the lateral and inclined crystal planes simultaneously.

Fabrication of titania inverse opals by multi-cycle dip-infiltration for optical sensing

We have demonstrated a low-cost method to fabricate TiO2 inverse opal photonic crystals with high refractive index skeleton. The TiO2 inverse opal films were fabricated from a polystyrene opal template by multi-cycle dip-infiltration-coating method. The properties of the TiO2 inverse opal films were characterized by scanning electron microscopy and Bragg reflection spectroscopy. The reflection spectroscopic measurements of the TiO2 inverse opal films were compared with theories of photonic band calculations and Bragg law. The agreement between experiment and theory indicates that we can precisely predict the refractive index of the infiltrated liquid sample in the TiO2 inverse opal films from the measurement results. The red-shift of the peak wavelength in the Bragg reflection spectra for both alcohol mixtures and aqueous sucrose solutions of increasing refractive index was observed and respective refractive index sensitivities of 296 and 286nm/RIU (refractive index unit) were achieved. As the fabrication of the TiO2 inverse opal films and reflection spectroscopic measurement are fairly easy, the TiO2 inverse opal films have potential applications in optical sensing.

Dynamical Diffraction of Light from 1D Photonic Crystals with Sinusoidal Profile of Permittivity

Bragg reflection and transmission spectra of the 1D photonic crystals characterized by a spatially sinusoidal profile of permittivity are studied as a function of the crystal-plate thickness. Applicability of the dynamical theory of diffraction in describing such spectra is considered. In the framework of the dynamical theory, we (i) calculated and analysed the reflection and transmission spectra for oblique incidence of polarized light, (ii) computed the spectra making use of the transfer matrix technique, and (iii) compared quantitatively the results of the two approaches. As a result, the analytical dynamical theory of diffraction is found to be correct in calculating the Bragg spectra in the vicinity of single photonic band-gap when the plate thickness is equal to the integer number of the spatial periods, or the permittivity is symmetric about the middle plane of the structure.

Degeneracy lifting due to thermal fluctuations around the frustration point between anticlinic antiferroelectric SmCA*and synclinic ferroelectric SmC*

In the binary mixture phase diagram of MC881 and MC452, the borderline between anticlinic antiferroelectric SmC(A)(*) and synclinic ferroelectric SmC(*) becomes apparently parallel to the temperature ordinate axis at the critical concentration r(c). The free energy difference between SmC(A)(*) and SmC^{*} is extremely small in a wide temperature range near r(c). In such circumstances, by observing Bragg reflection spectra due to the director helical structure and electric-field-induced birefringence, we have observed the continuous change from SmC(A)(*) to SmC(*) for r</~r(c) and the gradual growth of the full-pitch band over 5 °C or more in the apparent SmC(*) temperature region for r>/~r(c). These intriguing phenomena have been explained, successfully at least in the high-temperature region, by a thermal equilibrium between the synclinic and anticlinic orderings and the resulting Boltzmann distribution for the ratio between them; the thermal equilibrium is considered to be attained in a nonuniform defect-assisted way through solitary waves moving around dynamically. We have also discussed qualitatively an important role played by the effective long-range interlayer interactions in the low-temperature region.

Bragg reflection of light from silicon-based two-dimensional photonic crystals under multiwave-diffraction conditions

Mechanisms of the formation of the Bragg reflection spectra of light from two-dimensional photonic crystals have been studied in terms of an approach based on dynamic diffraction theory. The spectra have been calculated for a structure with a hexagonal silicon-based lattice. The calculated reflection contours are compared with the energy spectra of the eigenmodes of a spatially limited photonic crystal. It is demonstrated for the first time that it is possible to observe multiwave diffraction effects in the optical spectra of two-dimensional photonic-crystal structures.

Secondary Emission From Synthetic Opal Infiltrated by Colloidal Gold and Glycine

A comparison of the secondary emission (photoluminescence) and Bragg reflection spectra of photonic crystals (PC), namely, synthetic opals, opals infiltrated by colloidal gold, glycine, and a complex of colloidal gold with glycine is performed. The infiltration of colloidal gold and a complex of colloidal gold with glycine into the pores of PC causes a short-wavelength shift (about 5–15 nm) of the Bragg reflection and increases the intensity of this band by 1.5–3 times. In photoluminescence, the infiltration of PC by colloidal gold and colloidal gold with glycine suppresses the PC emission band near 375–450 nm and enhances the shoulder of the stop-zone band of PC in the region of 470–510 nm. The shape of the observed PC emission band connected with defects in synthetic opal is determined by the type of infiltrates and the excitation wavelength. Possiblemechanisms of the effects are discussed.

Resonant multiple diffraction of light in 3D opal‐like photonic crystals

AbstractPropagation of electromagnetic waves in spatially confined three‐dimensional (3D) photonic crystals (PhCs) of high dielectric contrast is studied for the case when the regime of the multiple Bragg diffraction (MBD) is of importance. The Bragg reflection spectra of opal‐like PhCs made up of polystyrene spheres are measured. Detailed comparison of the eigenmode energy spectrum with the measured and calculated reflectance contours is performed. The complicated structure of the Bragg reflection spectra is shown to be due to the additional eigenmodes excited resonantly by diffraction of light from the crystal planes inclined to the lateral ones.

Resonant acousto-optics in the terahertz range: TO-phonon polaritons driven by an ultrasonic wave

The resonant acousto-optic effect is studied both analytically and numerically in the terahertz range where the transverse-optical (TO) phonons play the role of a mediator which strongly couples the ultrasound and light fields. A propagating acoustic wave interacts with the TO phonons via anharmonic channels and opens band gaps in the TO-phonon polariton energy dispersion that results in pronounced Bragg scattering and reflection of the incoming light. The separation in frequency of different Bragg replicas, which is at the heart of acousto-optics, allows us to study the resonant acousto-optic effect in the most simple and efficient geometry of collinear propagation of electromagnetic and ultrasonic waves. The acoustically induced energy gaps, Bragg reflection spectra, and the spatial distribution of the electric field and polarization are calculated for CuCl parameters, in a wide range of frequencies and intensities of the pumping acoustic wave. Our results show drastic changes in terahertz spectra of semiconductor crystals that opens the way for efficient and accessible manipulation of their infrared properties, by tuning the parameters of the acoustic wave.

Temperature dependence of active photonic band gap in bragg-spaced quantum wells

A novel all-optical polarization switch of active photonic band gap structure based on non-resonant optical Stark effect bragg-spaced quantum wells was investigated and it could be compatible with the optical communication system. The theory is based on InGaAsP/InP Bragg-spaced quantum wells (BSQWs). Mainly through the design of the InGaAsP well layer component and InP barrier thickness to make the quantum-period cycle meet the bragg condition and the bragg frequency is equal to re-hole exciton resonance frequency. When a spectrally narrow control pulse is tuned within the forbidden gap, such BSQWs have been shown to exhibit large optical nonlinearities and ps recovery times, which can form T hz switch. However, the exciton binding energy of InGaAsP will be automatically separate at room temperature, so the effect of all-optical polarization switching of active photonic band gap bragg structure quantum wells can only be studied at low temperature. By a large number of experiments, we tested part of the material parameters of BSQWs in the temperature range 10-300K. On this basis, the InGaAsP and InP refractive index changes with wavelength, InP thermal expansion coefficient are studied and a relationship equation is established. Experimental results show that the bragg reflection spectra with temperature mainly is effected by InP refractive index changes with temperature. Our theoretical study and experiment are an instruction as a reference in the designs and experiments of future practical optical switches.

Three-dimensional tunable colloidal photonic crystal self-assembled on the end face of an optical fiber

High-quality three-dimensional nanostructure colloidal crystal-fiber structure is obtained by the modified vertical deposition method. The morphology of the colloidal crystal is examined by the scanning electron microscope, which illustrates that the (111) plane is parallel to the substrate of the fiber end face. The optical characterization of the colloidal crystal is also analyzed with an all-fiber network system. Reflection spectra show the existence of photonic band gap, which is located at 845 nm. As the liquid refractive index filled in the voids of the sample increases, the tunable wavelength of reflected light which is predicted by Bragg's equation with considering the effect of photonic band gap shows a good agreement with experimental results. Also, as the refractive index is changed because of different concentrations of solution, the colloidal photonic crystal-fiber system can also be distinguished by recording the shift of the maximum of Bragg reflection spectra.

Numerical simulation of the forbidden Bragg reflection spectra observed in ZnO

Thermal motion induced (TMI) scattering is a unique probe of changes in electronic stateswith atomic displacements in crystals. We show that it provides a novel approach toextract atomic correlation functions. Using numerical calculations, we are able toreproduce the temperature-dependent energy spectrum of the 115 ‘forbidden’ Braggreflection in ZnO. Our previous experimental studies showed that the intensitygrowth of such reflections over a wide range of temperatures is accompanied by adramatic change in the resonant spectral lineshape. This is the result of the interplaybetween the temperature-independent (TI) and temperature-dependent TMIcontributions. Here, we confirm that the TI part of the resonant structure factor canbe associated with the dipole–quadrupole contribution to the structure factorand show that the temperature-dependent part arises from the zinc and oxygenvibrations, which provide additional temperature-dependent dipole–dipole tensorcomponents to the structure factor. By fitting the experimental data at varioustemperatures we have determined the temperature dependences of autocorrelation⟨ux2(Zn)⟩ andcorrelation ⟨ux(O)ux(Zn)⟩ functions.

Simulations of wave propagation and disorder in 3D non-close-packed colloidal photonic crystals with low refractive index contrast

The plane-wave expansion method (PWEM), the multiple-scattering method (MSM) and the 3D finite-difference time-domain method (FDTD) are applied for simulations of propagation of electromagnetic waves through 3D colloidal photonic crystals. The system investigated is not a "usual" artificial opal with close-packed fcc lattice but a dilute bcc structure which occurs due to long-range repulsive interaction between electrically charged colloidal particles during the growth process. The basic optical properties of non-close-packed colloidal PhCs are explored by examining the band structure and reflection spectra for a bcc lattice of silica spheres in an aqueous medium. Finite size effects and correspondence between the Bragg model, band structure and reflection spectra are discussed. The effects of size, positional and missing-spheres disorder are investigated. In addition, by analyzing the results of experimental work we show that the fabricated structures have reduced plane-to-plane distance probably due to the effect of gravity during growth.

Transformation of electromagnetically induced transparency into enhanced absorption with a standing-wave coupling field in an Rb vapor cell

We present the transformation of electromagnetically induced transparency (EIT) into narrow enhanced absorption with an on-resonant standing-wave coupling field in the 5S(1/2)-5P(1/2) transition of the Lambda-type system of (87)Rb atoms. When a coupling laser field was changed from a travelling-wave to a standing-wave that was made by adding a counter-propagating L(C) laser, the transmittance spectrum of the L(P) laser transformed the typical EIT into dramatically enhanced absorption, and a Bragg reflection signal was generated by the periodic modulation of atomic absorption. The reflected probe laser corresponding to a Bragg reflection was measured to be approximately 11.5% of the power of the incident probe laser. We analyzed the enhanced absorption signal and Bragg reflection spectrum as a function of the power and frequency detuning of the coupling laser.

Monitoring of Colloidal Crystal Formation of Highly Charged Polystyrene and pH Sensing

Colloidal crystals have been given a lot of attention in the scientific arena because of their versatility. Here we present a drop and slide technique that was adopted to fabricate a 3-D colloidal crystal array from an aqueous solution of monodispersed, highly charged polystyrene spheres. Bragg reflectance spectra were obtained for the array of colloidal crystal at 50% volume fraction of the colloids as a function of time. The time-resolved Bragg reflection spectra observed show a reflection peak maximum gradually moving towards higher energy state during the colloidal crystallization process. The path taken as the wavelength blue-shifts during drying has been hypothesized to be highly dependent on not only the ordering, but also the shape of the liquid-air interface. The study also highlights the crystallization process during drying. The pH effect on reflectance spectra was also studied on the crystal array by introducing various pH solutions into the system. The results of the pH investigation show that the reflectance spectra are blue-shifted as the pH decreases.

Reflective spectrum broadened by nano-particle-network in chiral liquid crystals

Based on the analytical expression of the electromagnetic field solution of a helical symmetric dielectric material, the relationship between spectral reflectivity and birefringence of a chiral liquid crystal and the blue shift of the Bragg reflection in the condition of oblique incidence are presented in this paper. The theoretical results indicated that: 1) If birefringence (Δn) of the liquid crystal is greater than 0.2 and the thickness of the liquid crystal layer reaches 3 μm, the liquid crystal layer will reflect more than 90% of the incident light; 2) To reflect the whole visible spectrum by Bragg reflection, Δn of the liquid crystals in plane alignment state should exceed 0.6; 3) When the incident beam inclines to 60° from the normal direction, the blue shift of the reflective spectrum will reach to 100 nm. On the other hand, since the Δn of the commercial chiral liquid crystals is not larger than 0.2, to get the entire visible reflective spectrum, it needs to introduce a polymer network into the liquid crystals and make a sagging structure on the surface of substrates. The contribution of the network is to establish random anchorage that makes the pitch varied, hence broadening the Bragg reflection spectra. The random distribution of the sagging structure on the surface substrate is used to induce random screw axes of the chiral liquid crystal, which not only causes a blue shift of Bragg reflection but also further stretches the reflection spectra. Experiments demonstrated that the Bragg reflection spectrum could be broadened from 80 nm to more than 120 nm, and the contrast reaches 4:1 by introducing both polymer network and sagging structure on the substrate surface cell.

Surface modified latex particles: Synthesis and self-assembling into photonic crystals

The capability of monodisperse carboxylated particles of poly(methyl methacrylate) and of styrene copolymers with glycidyl methacrylate or with methacrylic acid for self-assembling into thin films exhibiting properties of photonic crystals was studied in relation to their polymer nature, carboxyl group distribution, and to the compositions of the dispersion medium. These particles were capable of self-assembling from their dispersions and forming close-packed three-dimensional ordered arrays on glass slides. The poly(styrene- co-methacrylic acid) particles revealed to be the best choice as the building blocks for photonic crystals formation. To ensure the appropriate surface structure of the particles and narrow particle size distribution in the submicron range the effects of reaction mixture pH, chain-transfer agents, and initiator concentration were investigated subsequently. The Bragg reflection spectra from the (1 1 1) planes of their ordered structures showed a predominant high-reflectance peak corresponding to photonic band gap caused by 3D periodicity of refractive index. The experimental spectra obtained for polarized light at different angles of incidence were well simulated in the theoretical calculations.

Effect of annealing on parameters of synthetic opal

We report on the effect of high temperature annealing on the reflection spectra of syntheticopals. The analysis of conditions for simultaneous diffraction on the (111) planesparallel and inclined to the sample surface has shown that both annealed andunannealed opals are compressed along the growth [111] axis and the shape of theSiO2 balls forming the opals’ close packed structure can be described as spheroidal.The structure parameters were evaluated from the analysis of the angulardependences of the peak positions in the Bragg reflection spectra of unfilled andglycerol-filled samples. The major effect of annealing is due to the sintering(interpenetration) of the structural elements of opals. The maximum temperature of1050 °C leads to a 10-fold increase in the degree of spheroid sintering. As a result, the interspheroidspacing decreases by over 10%, while the filling factor increases from 0.75 to 0.96 togetherwith the effective dielectric constant of the opal as a whole (from 1.74 to 2.08). Sinteringtakes place not only between spheroids, but also inside spheroids between theα-SiO2 nanoparticles constituting them. This results in a noticeable(by ∼7%) increase in the dielectric constant of opal spheroids.