Direction Of Light Propagation Research Articles

Alignment of crystal orientations of the multi-domain photonic crystals in Parides sesostris wing scales

It is known that the wing scales of the emerald-patched cattleheart butterfly, Parides sesostris, contain gyroid-type photonic crystals, which produce a green structural colour. However, the photonic crystal is not a single crystal that spreads over the entire scale, but it is separated into many small domains with different crystal orientations. As a photonic crystal generally has band gaps at different frequencies depending on the direction of light propagation, it seems mysterious that the scale is observed to be uniformly green under an optical microscope despite the multi-domain structure. In this study, we have carefully investigated the structure of the wing scale and discovered that the crystal orientations of different domains are not perfectly random, but there is a preferred crystal orientation that is aligned along the surface normal of the scale. This finding suggests that there is an additional factor during the developmental process of the microstructure that regulates the crystal orientation.

Effect of anisotropic lattice deformation on the Kerr coefficient of polymer-stabilized blue-phase liquid crystals

We investigate the effect of anisotropic lattice deformation on the Kerr coefficient of polymer-stabilized blue-phase liquid crystals (PSBP-LCs). PSBPs with orthorhombic and tetragonal symmetry were prepared by polymer-stabilizing a blue-phase liquid crystal under electrostriction. Both orthorhombic and tetragonal PSBPs showed smaller Kerr coefficients than the cubic PSBP, despite an increase in the unit cell volume caused by the elongation of the lattice along the direction of light propagation. Our results indicate that the Kerr coefficient of PSBPs is not determined simply by the volume of the unit lattice but by the lattice size perpendicular to the direction of light propagation.

Lambertian light trapping in thin crystalline macroporous Si layers

AbstractLambertian light trapping is a benchmark for efficient light trapping. In this Letter we experimentally quantify the degree of Lambertian light trapping in a macroporous silicon layer. The optical absorption of the effective (26.7 ± 5.5) µm thick sample with randomly arranged pores yields a photogeneration corresponding to a maximum current density of (40.8 ± 0.4) mA cm–2 and thus achieves a fraction of 0.985 ± 0.012 of the current density expected from a Lambertian light trapping scheme. The measured spectrum of the escape reflectance is well described with an analytic model assuming a complete randomization of the directions of light propagation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced Electro-Optic Modulation Performance in Optical Buffers by Slowing Light in Optimized Photonic Crystal Slab Structures

Slow light holds the key to advanced optical bu ering and time-domain optical signal processing technologies. Photonic crystal based optical bu ers are particularly attractive due to their nanoscale size, room temperature operation, and enhanced eld dependent nonlinear response associated with the presence of slow light. In this study, the slow light and electro-optic modulation characteristics of a line-defect Si photonic crystal slab with triangular arrangement of holes lled with an electro-optic polymer (n = 1.6) are investigated by three-dimensional plane-wave expansion and nite-di erence time-domain methods. The rst rows adjacent to the line-defect are shifted gradually in the direction of light propagation and a slow light region with a high group index below the light-line is obtained for a shifting amount between 0.22a and 0.27a. For the photonic crystal con guration with 0.22a shifted rows, under modulated voltage change, the average group index is found to be decreasing with an increase in the bandwidth. The results show that the low group velocity supports a large delay time in a small modulated voltage variation. A linear change of group index with modulated voltage is obtained and the modulation sensitivity of central wavelength is obtained as 9.45 nm/V for a delay line length of 0.5 mm. Almost the same bu er capacity and bit length are found which provides the control of delay time exibly while keeping the bu er capacity and the bit length almost unchanged.

Acoustic beam steering by light refraction: illustration with directivity patterns of a tilted volume photoacoustic source.

The symmetry of a thermoelastic source resulting from laser absorption can be broken when the direction of light propagation in an elastic half-space is inclined relatively to the surface. This leads to an asymmetry of the directivity patterns of both compressional and shear acoustic waves. In contrast to classical surface acoustic sources, the tunable volume source allows one to take advantage of the mode conversion at the surface to control the directivity of specific modes. Physical interpretations of the evolution of the directivity patterns with the increasing light angle of incidence and of the relations between the preferential directions of compressional- and shear-wave emission are proposed. In order to compare calculated directivity patterns with measurements of normal displacement amplitudes performed on plates, a procedure is proposed to transform the directivity patterns into pseudo-directivity patterns representative of the experimental conditions. The comparison of the theoretical with measured pseudo-directivity patterns demonstrates the ability to enhance bulk-wave amplitudes and to steer specific bulk acoustic modes by adequately tuning light refraction.

Berry Phase for Evolution of Spinning Particle along Geodesic Arcs on a Sphere of Directions

We demonstrated the noncyclic Berry phase based on the geodesic rule proposed by Samuel and Bhandari using two dynamical phase-free systems in a spin-redirection evolution. In the atom interferometer with a noncyclic rotating magnetic field, the quasi-geodesic arc was realized by a straight line connecting the initial and the final points. The phase shifts were measured for noncyclic evolutions and cyclic evolutions connected by several quasi-geodesic arcs. The other system to observe the noncyclic Berry phase for change in the propagation direction of light was a single mode fiber wounded on an auxiliary cylinder to realize strictly back along the real geodesic arc joining the final to the initial point. Theses observed phase shifts were in good agreement with the theoretical values of the noncyclic Berry phase based on the geodesic rule.

Dye sensitized solar cells as optically random photovoltaic media

In order to enhance optical absorption, light trapping by multiple scattering is commonly achieved in dye sensitized solar cells by adding particles of a different sort. Herein we propose a theoretical method to find the structural parameters (particle number density and size) that optimize the conversion efficiency of electrodes of different thicknesses containing spherical inclusions of diverse composition. Our work provides a theoretical framework in which the response of solar cells containing diffuse scattering particles can be rationalized. Optical simulations are performed by combining a Monte Carlo approach with Mie theory, in which the angular distribution of scattered light is accounted for. Several types of scattering centers, such as anatase, gold and silver particles, as well as cavities, are considered and their effect compared. Estimates of photovoltaic performance, insight into the physical mechanisms responsible for the observed enhancements, and guidelines to improve the cell design are provided. We discuss the results in terms of light transport in weakly disordered optical media and find that the observed variations between the optimum scattering configurations attained for different electrode thicknesses can be understood as the result of the randomization of the light propagation direction at different depths within the active layer. A primary conclusion of our study is that photovoltaic performance is optimised when the scattering properties of the film are adjusted so that the distance over which incident photons are randomized is comparable to the thickness of the film. This simple relationship could also be used as a design rule to attain the optimum optical design in other photovoltaic materials.

Nanogradient optical coatings

Basic optical coatings with refractive-index gradient along the light propagation direction, structures of gradient coatings, and technology of their manufacturing are considered. Issues relating to longitudinal-nanogradient coatings are discussed.

Pulse propagation dynamics in the presence of a continuous-wave field

We present theoretical results for the propagation dynamics of an electromagnetic field pulse through rubidium vapor, while another field, a continuous-wave electromagnetic field, is present. The frequencies of both electromagnetic fields are resonant with the transition between the ground and excited state hyperfine levels of Rb, Fg → Fe = Fg ± 1. Detuning from resonance is done by the magnetic field oriented along the light propagation direction (Hanle configuration). When both the electromagnetic fields are simultaneously interacting with Rb atoms, either electromagnetically induced transparency or absorption is induced. Propagation dynamics was obtained solving the set of Maxwell–Bloch equations for the interacting atoms with two electromagnetic fields. Motivated by recent results (Brazhnikov et al 2011 Eur. Phys. J. D 63 315–25; Brazhnikov et al 2010 JETP Lett. 91 625–9; Kou et al 2011 Phys. Rev. A 84 063807), we have analyzed the influence of experimental parameters, laser polarization, and mutual phases between lasers, which can lead to optical switching, i.e. the transformation from electromagnetically induced absorption to transparency and vice versa.

A Uniform Magnetic Field Produced by Rectangular Coils<i></i>

To produce a better uniform magnetic field along the direction of light propagation in studying magnetometer, a pair of rectangular coils are used instead of traditional Helmholtz coils. In this work, the magnetic homogeneity in the center of the rectangular coils is calculated and discussed theoretically. We measured the magnetic field distribution along the light propagation direction of the coils in experiment proving that the field has the relative fluctuation less than 1% in the range of 30cm along the direction.

Broad parameter optimization of polarization-diversity 2D grating couplers for silicon photonics

Polarization-diversity couplers, which are designed to couple the unknown polarization state of an optical fiber into the TE-polarized modes of integrated waveguides, are important for the development of practical all-optical circuits. We describe the use of a full 3D finite difference time domain (FDTD) calculation campaign to rigorously optimize the 2D photonic crystal grating that couples a single-mode telecom fiber to the silicon waveguides of a Silicon-on-Insulator (SOI) platform. With this approach we identify the unique optimum combination of etch-depth, hole-radius, and grating-pitch of the photonic crystal array for best performance at 1550 nm. The mean (polarization-averaged) coupling efficiency of 48% (-3.2dB) exceeds reported efficiencies of analogous couplers, and has only a marginal dependence on the polarization state of the input fiber (48 ± 3%). In addition, 3D-FDTD calculations are used to characterize the propagation direction, mode-profile, and polarization of light coupled from the fiber into the SOI slab. Such information is crucial for component design and goes beyond previously available results from existing approximations and simulations of 2D-grating coupler performance. Calculations of photonic mode dispersion in the grating coupler, by means of guided-mode expansion, indicate that the coupling is due to an optically active resonant guided mode in the photonic crystal array. This points towards a fast optimization scheme that enhances both the performance and the physical interpretation of 3D-FDTD simulations.

Particle sorting by optical radiation pressure with low energy density

This paper presents the development of a microfluidic device for particle sorting using optical radiation pressure with low energy density. The need for efficient particle manipulation in the microchannel has led to the recent development of a couple of advanced techniques of particle manipulation. An optical approach to sort particles in a microchannel by the optical radiation pressure can provide non-contact and remote handling technique of manipulating the particles. We utilized a two-dimensionally focused laser beam, namely light sheet, for a light source to enable the optical-based particle sorting with low energy density leading less damage to the sample and device. A microfluidic device in this study consisted of the channel structure made of SU-8 between silicon substrate and PDMS lid. Since SU-8 has higher refractive index than other materials, the total reflection of the optical wave occurs to serve the channel walls as a waveguide leading the 2D focused laser beam with less scattering. Particle migration to the direction of light propagation was verified without any damage in the chip under an irradiation of a 2 W laser beam, which had enough strong power to damage the polymer chip using a spot, namely 3D focused beam. Reasonable agreement of our experimental results with theoretical prediction was also confirmed.

Slow and stored light pulses in the presence of magnetic fields

We systematically studied slow light (SL) and stored light in the presence of a static magnetic field, either parallel to or transverse to the direction of light propagation. The theoretical calculations are in good agreement with the measured data, and paves the way for determining the information of optical density, coupling Rabi frequency, and ground state population distribution of a system. We also observed an unexpected double-peak profile in the output of SL and the retrieved signal of stored light under a large transverse magnetic field. Our work provides a better understanding in the studies of SL and storage of light.

Focused Ion Beam Nanoscale Patterned Transmission-Enhanced Fiber-Optic Tips

The angled and tapered antireflection and transmission-enhanced fiber-optic tips were designed, fabricated, and characterized based on nanoscale surface modification using the nano-precision focused ion beam (FIB). The developed optical fiber tips showed combined functions of antireflection and transmission enhancement for higher detection efficiency or signal-to-noise ratio because the FIB-milled nanoholes into the fiber tip endfaces could act as tapered air-filled nanopillars, changing the light propagation direction, enhancing the scattering effect, and allowing more photons to transmit along/near the glass/air interface to enhance the local evanescent field for sensing/detection.

Optical Isolator for TE Polarized Light Realized by Adhesive Bonding of Ce:YIG on Silicon-on-Insulator Waveguide Circuits

An optical isolator for transverse electric (TE) polarized light is demonstrated by adhesive bonding of a ferrimagnetic garnet die on top of a 380 nm thick silicon waveguide circuit. Polarization rotators are implemented in the arms of a nonreciprocal Mach-Zehnder interferometer to rotate the polarization to transverse magnetic in the nonreciprocal phase shifter regions. Calculation of the nonreciprocal phase shift (NRPS) as a function of bonding layer thickness experienced by the TM mode in the interferometer arms is presented, together with the simulation of the robustness of the polarization rotator. Experimentally, 32 dB isolation is measured at 1540.5 nm wavelength using a magnetic field transverse to the light propagation directions. This paves the way to the cointegration of laser diodes and optical isolators on a silicon photonics platform.

Enhanced light absorption in thin-film solar cells with light propagation direction conversion

Enhancement of optical absorption in thin-film solar cells (TF-SCs) has been the long-lasting issue to achieve high efficiencies. Grating couplers have been studied for the conversion of incident light into guided modes propagating along TF-SCs to extend optical path for higher optical absorption. However the wavelength band for the efficient conversion remained relatively narrow and the overall improvement of TF-SC efficiencies has been limited. This paper demonstrates that the grating height design as well as the phase matching condition is important for the enhancement of optical absorption in TF-SCs with the calculation of short-circuit currents as a figure of merit for optimization. The influence of the light absorption coefficients and grating coupling strengths on the light absorption bandwidth is also discussed.

Adhesively bonded Ce:YIG/SOI integrated optical circulator

A classical 3-port optical circulator is demonstrated on the silicon-on-insulator (SOI) platform. A garnet die with a magneto-optical cerium-doped yttrium iron garnet (Ce:YIG) layer is bonded on top of a Mach-Zehnder interferometer circuit using a thin adhesive bonding layer. The power transmission between different ports is characterized in the presence of an external magnetic field, transversal to the light propagation direction. An isolation of 22 dB is measured at a wavelength of 1562 nm.

Optical manipulation of gold nanoparticles using an optical nanofiber

Gold nanoparticles are gaining increasing attention due to their biological and medical applications. In this letter, we experimentally demonstrate the optical manipulation of 250-nm-diameter gold nanoparticles along an optical nanofiber (550 nm in diameter) injected by an 808-nm laser light. The nanoparticles situated in the evanescent optical field are trapped by optical gradient force and move along the direction of light propagation due to optical scattering force. The velocities reach as high as 132 μm/s at an optical power of 80 mW.

The numerical study on large bandwidth photonic crystal waveguide with slow light phenomena

In this paper, two novel types of semi-slow light photonic crystal waveguide with large transmission bandwidth obtained by shifting the boundaries of a W1 waveguide in the direction of light propagation are presented. One includes air rings localized at only one side of the line defect and the other replaces the holes at each side of the waveguide by the uniform air rings which are constructed by the homocentric square dielectric rod inserted into the air holes. The structure produces unusual “n-type” transmission spectrum depending on the different parameters such as inner radius of air ring, dielectric constant of square dielectric rod, etc. It is shown that the transmission spectra of the two structures are completely different from each other. A versatile control of light propagation with large normalized bandwidth and slow light phenomena can be obtained using a unique geometrical parameter. Numerical simulation by the finite-difference time-domain (FDTD) method demonstrates the propagation of the broadband pulse.

Thermo-optical properties of uniaxial NaT(XO4)2 laser host crystals (where T = Y, La, Gd or Bi, and X = W or Mo)

Thermo-optic coefficients dn o/dT and dn e/dT were measured in tetragonal double tungstate and double molybdate crystals NaT(XO4)2 (where T = Y, La, Gd or Bi and X = W or Mo) by a laser beam deviation method in the spectral range 0.4–1.1 μm. Thermal expansion coefficients in the directions of a and c crystallographic axes were also measured. Analytical expressions for thermo-optic dispersion formulas were derived as series in 1/λ 2. All dn/dT values for NaT(XO4)2 crystals were found to be negative. Their absolute values satisfy the relation |dn e/dT| > |dn o/dT| for crystals without Bi and |dn o/dT| > |dn e/dT| for crystals with Bi. A clear tendency for dn/dT values to decrease with the increase of the volumetric thermal expansion coefficient α vol of the crystal was observed. This is related with dominant contribution of volumetric thermal expansion effect to the temperature dependence of the refractive index. Thermal coefficients of the optical path W = dn/dT + (n − 1)α T governing thermal lensing effect were calculated for different light propagation directions and polarizations as well as crystal athermal directions.