Incident Polarization Direction Research Articles

Electrically controllable Fresnel lens in 90° twisted nematic liquid crystals.

This study presents a theoretical analysis and experimental demonstration of an electrically controllable Fresnel lens in a 90° twisted nematic liquid crystal cell. The cell gap was chosen to satisfy the Gooch-Tarry conditions, and therefore, the polarization rotation effect was valid regardless of the incident polarization direction. The polarization sensitivity of the diffraction efficiency of the 90° twisted nematic Fresnel lens was dependent on the applied voltage regime. Theoretical calculations effectively explain the experimental results.

Single Anisotropic Plasmonic Nanoparticle Three-Dimensional Orientation Determination Based on Fano-Like Resonance and Universal 3D Orientation-Dependent Scattering Trait

Single-nanoparticle orientation determination plays a vital role in studying complex nanoscale motion. In the present paper, we systematically investigate the three-dimensional (3D) orientation-dependent far- and near-field optical properties of single (Au core)-(dielectric shell) nanorice and gold nanorod. It shows that the scattering spectrum of the single anisotropic nanoparticle with arbitrary orientation is a linear superposition of a set of basic scattered spectra. And the scattering spectra of the single nanorice show a polarization-dependent Fano-like resonance which can be well described by a model with two coupled oscillators. Furthermore, by means of coordinate transformation, a universal analytic formula for description single nanoparticle 3D orientation- and polarization- dependent scattering intensity is proposed. Then we develop a new method to detect the 3D orientation of single nanoparticle, that is, by fitting the scattering intensity under different incident polarization directions based on our analytic formula and the 3D orientations could be resolved explicitly. Both of the experimental and numerical simulation data can be well described by our analytical formula. Our method for single particle 3D orientation determination has high precision only with subdegree uncertainty. In addition, based on the Fano resonances caused by the efficient coupling between the dielectric elliptical shell and the gold nanoellipsoid core, we found that the 3D orientation of single nanorice could be confirmed from either the transverse or longitudinal plasmon mode polarization-dependent scattering trait, while it is almost impossible for single gold nanorod based on the transverse plasmon mode. It is worth noting that the transverse plasmon mode of the nanorice is mostly insensitive to the aspect ratio then it allows nanorices with different lengths to be 3D orientation sensors without altering the incident laser wavelength.

Polarized Raman spectroscopy with differing angles of laser incidence on single-layer graphene

Chemical vapor deposition (CVD)-grown single-layer graphene samples, transferred onto a transmission electron microscope (TEM) grid and onto a quartz plate, were studied using polarized Raman spectroscopy with differing angles of laser incidence (θ). Two different polarization configurations are used. In an in-plane configuration, the polarization direction of both incident and scattered light is parallel to the graphene plane. In an out-of-plane configuration, the angle between the polarization vector and the graphene plane is the same as the angle of laser incidence (θ). The normalized Raman intensity of the G-band measured in the out-of-plane configuration, with respect to that in the in-plane configuration, was analyzed as a function of θ. The normalized Raman intensity showed approximately cos2θ-dependence up to θ = 70°, which can be explained by the fact that only the electric field component of the incident and the scattered photon in the out-of-plane configuration projected onto the graphene plane can contribute to the Raman scattering process because of the perfect confinement of the electrons to the graphene plane.

Polarization-dependent elliptical crater morphologies formed on a silicon surface by single-shot femtosecond laser ablation.

Formation of the elliptical-shaped craters on a silicon surface is investigated comprehensively using a single shot of a femtosecond laser. It is observed that the ablation craters are elongated along the major axis of the polarization direction, while their orientation is parallel to the polarization direction. The ablation area grows and the morphology of the craters evolves from an ellipse to nearly a circle with increasing fluence. The underlying physical mechanism is revealed through numerical simulations that are based on the finite-difference time-domain technique. It is suggested that the initially formed craters or surface defects lead to the redistribution of the electric field on the silicon surface, which plays a crucial role in the creation of the elliptical-shaped craters. In addition, the field intensity becomes enhanced along the incident laser polarization direction, which determines the elliptical crater orientations.

Theoretical and Experimental Demonstrations of a Dual-Band Metamaterial Absorber at Mid-Infrared

We present the design, fabrication, and physical interpretation of an infrared dual-band metamaterial absorber. The unit cell of the metamaterial absorber consists of a cross resonator ringed by four split-ring resonators spaced a distance above a gold ground plane with a dielectric layer of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The absorber shows two absorption peaks of 90.3% and 88.4% at 4.17 μm and 4.86 μm, respectively. The multireflection interference theory is applied to explain the absorption mechanism with the gold ground plane described by Drude model, and the theoretical calculation agrees well with the simulations and experiments. In addition, the designed absorber is insensitive to incident angle and polarization direction in a broad range, which are highly favorable features in practical applications.

Strong dipole-quadrupole coupling and Fano resonance in H-like metallic nanostructures.

Under certain conditions of the incident light polarization direction a Fano resonance arises in small gold nanorods arranged in a H-like configuration. This stems from the coupling between a bright dipole plasmon mode excited in the horizontal rod and dark quadrupole plasmon modes in both vertical rods. We investigate these surface plasmon modes, and analyze the dependence of the Fano resonance on the geometry parameters such as rod size and interparticle separation, and refractive index of embedding medium. To describe the degree of this energy transfer, we introduce a new parameter: the Fano resonance efficiency. We calculate absorption cross-sections for visible and NIR spectrum in each element of the structure, and near-field distributions at different wavelengths. We show that Fano resonance in small H-like structures exhibits high sensitivity with respect to the refractive index of the host medium, outperforming the values for larger plasmonic structures based on nanorods already investigated.

Numerical study of mode excitation in a partially illuminated silver rod.

Using electrodynamics theory, we numerically demonstrated that when the incident light polarization direction is parallel to a silver rod axis, the modes that are forbidden in a fully illuminated silver rod can be excited when the rod is partially illuminated. The modes are excited due to the symmetry breaking of the rod when it is partially illuminated. They are characterized by the even number of nodes in the excited surface plasmons and quadrupole pattern of the scattered light in the space. These modes are different from the allowed modes in a fully illuminated rod, which shows an odd number of nodes in the excited surface plasmons and a dipole pattern of the scattered light. The conclusion was further supported by the calculation results when the illuminated spot was moved from the rod end to its center and when the illuminated length was varied. We also demonstrated dark modes when the incident light polarization is perpendicular to the rod axis. Contrary to the hypothesis that a dark mode will be an efficient waveguide mode, the dark mode shows very poor waveguide applications when the incident polarization is perpendicular to the rod axis.

Effect of the shapes of nanostructures on the light absorption in organic thin films

We theoretically investigate the enhancement of optical absorption in organic thin films by embedding the periodic metallic nanostructures (NSs) inside the thin films. For the case study, one of the organic solar cell materials copper phthalocyanine (CuPc) has been used. Finite Different Time Domain technique has been used to illustrate the role of the several different shapes of the metallic NSs. It has been found that the NSs can result in broadband optical absorption enhancement. A study on the position of the metallic particles in the organic thin film and the effect of particle dimension on absorption has also been performed. A significant enhancement in the absorption in the wavelength range 350–550 nm has been observed. Furthermore, the investigation has been performed on the effect of the light source incident angle and polarization direction on the absorption in the organic thin films. The best incident angle is found at 75°.

Crystal direction dependence of quantum confinement effects of two-dimensional Si layers fabricated on silicon-on-quartz substrates: modulation of phonon spectra and energy band structures

We experimentally studied the crystal direction dependence of phonon confinement effects (PCEs) and bandgap (EG) modulation of a two-dimensional (2D) Si layer fabricated on (100) silicon-on-quartz (SOQ) wafers without a handle Si substrate. For the first time, by polarization Raman spectroscopy, in the case of Raman intensity spectra in the asymmetrical broadening region owing to the PCEs in the 2D Si layer, we demonstrated that the incident laser polarization direction dependence of the Raman intensity deviates from the Raman selection rule. However, a photoluminescence (PL) method shows that the EG expansion is isotropic in the 2D Si layer. On the other hand, the reflectivity of the 2D Si layer in UV region is also modulated. The reflectivity property modulation is possibly attributable to the energy band modulation in the 2D Si layer.

Fano Resonances Induced by Strong Interactions Between Dipole and Multipole Plasmons in T-Shaped Nanorod Dimer

A simple T-shaped plasmonic nanostructure composed of two perpendicular coupled nanorods is proposed to produce strong Fano resonances. By the near-field coupling between the “bright” dipole and “dark” quadrupole plasmons of the nanorods, a deep Fano dip is formed in the extinction spectrum, which can be well fitted by the Fano interference model. The effects of the geometry parameters including nanorod length, coupling gap size, and coupling location to the Fano resonances are analyzed in detail, and a very high refractive index sensitivity is achieved by the Fano resonance. Also by adjusting the incident polarization direction, double Fano resonances can be formed by the interferences of the dipole, quadrupole, and hexapole plasmons. The proposed nanorod dimer structure is agile, and a trimer which supports double Fano resonances can be easily formed by introducing a third perpendicular coupled nanorod. The proposed T-shaped nanorod dimer structure may have applications in the fields of biological sensing and plasmon-induced transparency.

Surface phonons of the Si(111)-(7×7) reconstruction observed by Raman spectroscopy

We have studied the surface phonon modes of the reconstructed Si(111)-($7\ifmmode\times\else\texttimes\fi{}7$) surface by polarized Raman spectroscopy. Six surface vibration modes are observed in the frequency range between 62.5 and 420.0 cm${}^{\ensuremath{-}1}$. The mode frequencies agree very well with reported calculation results. This enables their attribution to calculated eigenmodes, whose elongation patterns are dominated by specific atomic sites: the two most characteristic novel fingerprints of the ($7\ifmmode\times\else\texttimes\fi{}7$) reconstruction are sharp Raman peaks from localized adatom vibrations, located at 250.9 cm${}^{\ensuremath{-}1}$, and collective vibrations of the adatoms and first- and second-layer atoms, located at 420.0 cm${}^{\ensuremath{-}1}$. While the sharp localized adatom vibration peak is a substantial refinement of an earlier broad spectral structure from electron energy-loss spectroscopy, no spectroscopic features were reported before in the collective-vibration frequency region. Furthermore, we observe in-plane wagging vibrations in the range from 110 to 140 cm${}^{\ensuremath{-}1}$, and finally the backfolded acoustic Rayleigh wave at 62.5 cm${}^{\ensuremath{-}1}$, which coincides with helium atom scattering data. Moreover, the Raman peak intensities of the surface phonons show a mode-specific dependence on the polarization directions of incident and scattered light. From this polarization dependence the relevant symmetry components in the Raman scattering process (${A}_{1}$ and/or $E$ symmetry) are deduced for each mode.

Chemical imaging of live fibroblasts by SERS effective nanofilm.

Reliable and strong surface enhanced Raman scattering (SERS) signatures of intracellular compartments in live NIH3T3 fibroblasts are collected in real time by means of SERS active thin nanofilm (30 nm) on colloidal silica (1.5 μm). Nanofilm is composed of preformed silver nanoparticles in the matrix of polyacrylic acid, protecting against heating (37 °C) in water, or culture medium or phosphate buffered saline aqueous solution. The SERS enhancement factors (EFs) of the order 10(8) allow single biomolecule detection in the native environment of a single live cell. Primary and secondary SERS hot spots of nanofilm are responsible for such high EFs. A slow SERS EF intensity decay occurs over a broader distance of micron silica with nanofilm, not achievable in a common core-shell model (silver nanoparticle coated with a thin silica layer). Extensive local field EFs and SERS EFs are mainly delivered by prolate silver nanoparticles ("rugby-like" shape). This is achieved if an incident field is polarized along the z-axis and the direction of incident polarization and main axis (z) are perpendicular to each other, not observable in water or on gold.

Polarizing and focusing properties of reflective fresnel zone plate

Polarizing and focusing properties of binary Fresnel zone plate (FZP) designed for x-ray was investigated experimentally in visible range for wavelengths 405 nm, 445 nm, and 633 nm. It was shown that peripheral FZP zones rotate the direction of polarization of linearly-polarized incident light on angle equal to doubled angle between incident polarization and relief direction. Illuminated by red and blue linearly polarized light FZP forms 3 focal spots, located on distances that decrease with wavelength decreasing. Minimal focal spot on distance of 105 um above the surface was elliptical and has maximal intensity 9 times larger than the intensity of incident light, and half maximum diameter of diffractive limit of our optical system (2 of wavelength). Peripheral part of FZP forms diffractive limited focal point on distance of 2.8 um that has diameter 9 of wavelength.

From scattering regime to strong localization: a statistical analysis of the near-field intensity on random gold films

Random metallic films have very specific optical properties due to disorder. Strong localization of electromagnetic fields can be observed due to plasmons. This localization strongly depends on excitation conditions, and different optical regimes can exist. To characterize these regimes, we investigated the spatial intensity correlation functions using near-field scanning optical microscopy for different excitation wavelengths and different incident polarization states. The transition between a weak scattering regime where no plasmon resonances are excited and a regime where strong plasmon resonances occur has clearly been observed. In the strong plasmonic regime, by varying the incident polarization direction, the shape of the correlation function has been correlated to the intensity enhancement which raises the possibility to control the intensity localization using the incident polarization state.

Revealing anisotropic strain in exfoliated graphene by polarized Raman spectroscopy

We report on a polarized Raman study on mechanically cleaved single-layer graphene films. Under a specific orientation of scattering measurement, the width and position of the G peak change with the incident polarization direction, while the integrated intensity of that is unaltered. This phenomenon is explained by a proposed mode in which the peak is contributed by a mixture of un-, compressive-, and tensile-strained G sub-modes. The compression and tension are both uniaxial and approximately perpendicular to each other. They are undesigned and located in either separated or overlapped sub-areas within the probed local region. Compared to the unstrained wavenumber of 1580 cm(-1), compression induces a blue shift while tension causes a red one. The sub-modes correlated with the light polarization through different relationships split the G peak into three sub-ones. We develop a method to quantitatively analyze the positions, widths, intensities, and polarization dependences of sub-peaks. This analysis quantitatively reveals local strain, which changes with the detected area of a graphene film. The method presented here can be extended to detect the strain distribution in the film and thus is a promising technology for graphene characterization.

Observation of strain effect on the suspended graphene by polarized Raman spectroscopy

We report the strain effect of suspended graphene prepared by micromechanical method. Under a fixed measurement orientation of scattered light, the position of the 2D peaks changes with incident polarization directions. This phenomenon is explained by a proposed mode in which the peak is effectively contributed by an unstrained and two uniaxial-strained sub-areas. The two axes are tensile strain. Compared to the unstrained sub-mode frequency of 2,672 cm−1, the tension causes a red shift. The 2D peak variation originates in that the three effective sub-modes correlate with the light polarization through different relations. We develop a method to quantitatively analyze the positions, intensities, and polarization dependences of the three sub-peaks. The analysis reflects the local strain, which changes with detected area of the graphene film. The measurement can be extended to detect the strain distribution of the film and, thus, is a promising technology on graphene characterization.

Effective Young’s Modulus of Bacterial and Microfibrillated Cellulose Fibrils in Fibrous Networks

The deformation micromechanics of bacterial cellulose (BC) and microfibrillated cellulose (MFC) networks have been investigated using Raman spectroscopy. The Raman spectra of both BC and MFC networks exhibit a band initially located at ≈ 1095 cm(-1). We have used the intensity of this band as a function of rotation angle of the specimens to study the cellulose fibril orientation in BC and MFC networks. We have also used the change in this peak's wavenumber position with applied tensile deformation to probe the stress-transfer behavior of these cellulosic materials. The intensity of this Raman band did not change significantly with rotation angle, indicating an in-plane 2D network of fibrils with uniform random orientation; conversely, a highly oriented flax fiber exhibited a marked change in intensity with rotation angle. Experimental data and theoretical analysis shows that the Raman band shift rate arising from deformation of networks under tension is dependent on the angles between the axis of fibrils, the strain axis, the incident laser polarization direction, and the back scattered polarization configurations. From this analysis, the effective moduli of single fibrils of BC and MFC in the networks were estimated to be in the ranges of 79-88 and 29-36 GPa, respectively. It is shown also that for the model to fit the data it is necessary to use a negative Poisson's ratio for MFC networks and BC networks. Discussion of this in-plane "auxetic" behavior is given.

Light scattering in colloids of diamond and graphite

Research results of light scattering in two polydisperse colloids are presented. Scattering of linearly polarized light by particles of diamond and graphite in water is considered. Cases of different polarization directions of incident and scattered light are researched. Angular dependencies of scattered light intensity and its depolarization degree are compared at chaotic and ordered particle orientation. The particle orientation was caused by the external electric field. The input of dipole and quadrupole scattering of particles at chaotic and ordered particle orientation in intensity and depolarization of scattered light by the colloids is discussed.

Plasmonic properties of silver cross-shape nanostructure

The extinction spectra and the electric field distributions of the cross-shaped nanostructures are calculated by the discrete dipole approximation method. Compared with the individual nanorod, the cross-shape nanostructure can generate high local electric fields at the lateral surface. Because of the electric field couplings between adjacent protruding parts, much enhanced electric fields always occur at the lateral surface of the cross-shape nanostructure, with the incident polarization direction varied. In addition, the effects of the structural parameters of the cross-shape nanostructures on their plasmonic properties are also investigated. These results would guide the preparation of the cross-shape nanostructures for their applications in surface enhanced Raman scattering.

Dynamic switching of the chiral beam on the spiral plasmonic bull’s eye structure [Invited

A polarization-dependent switchable plasmonic beaming structure composed of metallic hole surrounded by double spiral dielectric gratings is proposed. The main mechanism of the proposed structure is based on the angular momentum change of surface plasmon caused by the spiral geometry. On- and off-states of the proposed device are determined by the condition whether the rotating direction of incident polarization is the same as or opposite of the direction of the spiral rotations. Qualitative analytical expressions of the switching mechanisms and full-vectorial numerical results are presented.