Light pressure on chiral sculptured thin films and the circular Bragg phenomenon

For a circularly polarized plane wave incident normally on a chiral sculptured thin film (STF), we determined that light pressure per unit thickness on the film is high when the incident plane wave and the chiral STF are co-handed, the loss factor in the chiral STF is as low as possible, the wavelength is either the center-wavelength of the Bragg regime or close to it, and the ratio of the film thickness normalized by the period is moderately high.     

On the dependence of circular Bragg phenomenon of noble metals helicoidally periodic sculptured thin films on visible and IR wavelengths

The concept of local homogenization in the visible and infra-red frequencies is used by estimating the permittivity dyadics of noble metals (Cu, Ag, and Au) in the form of thin film helicoidal bianisotroptic media (TFHBM). Despite the fact that the absorption transitions of dielectric to metal (percolation threshold) in metallic TFHBMs occur at long wavelengths at lower volumetric fraction of metallic particles, at these wavelengths the transition from dielectric to metal in composite relative permittivity scalar occurs at higher volumetric fraction of metallic particles. The latter is responsible for the increase of circular Bragg phenomenon.     

Theory of electrically controlled exhibition of circular Bragg phenomenon by an obliquely excited structurally chiral material - Part 1: Axial dc electric field

The boundary-value problem of the reflection and transmission of a plane wave due to a slab of an electro-optic structurally chiral material (SCM) is formulated in terms of a 4×4 matrix ordinary differential equation. The SCM slab can be locally endowed with one of 20 classes of point group symmetry, and is subjected to a dc voltage across its thickness (i.e., parallel to the axis of non-homogeneity of the SCM). The enhancement (and, in some cases, the production) of the circular Bragg phenomenon (CBP) by the application of the dc voltage has either switching or circular-polarization-rejection applications in optics. The twin possibilities of thinner filters and electrical manipulation of the CBP, depending on the local crystallographic class as well as the constitutive parameters of the SCM, emerge.     

On gyrotropic chiral sculptured thin films for magneto-optics

The planewave response of a chiral sculptured thin film with gyrotropic properties is examined, under the assumption of axial excitation. Although the reflectances and the transmittances are definitely affected by gyrotropy, the circular Bragg phenomenon is still exhibited. Most importantly, structural chirality and gyrotropy contribute cooperatively to transmission optical rotation.     

Theory of electrically controlled exhibition of circular Bragg phenomenon by an obliquely excited structurally chiral material – Part 2: Arbitrary dc electric field

Numerical examination of the solution of the boundary-value problem of the reflection and transmission of a plane wave due to a slab of an electro-optic structurally chiral material (SCM) indicates that the exhibition of the circular Bragg phenomenon by the SCM can be controlled not only by the sign and the magnitude of a dc electric field but also by its orientation in relation to axis of helicoidal non-homogeneity of the SCM. Thereby, the possibility of electrical control of circular-polarization filters has been extended.     

Spectral responses of gyrotropic chiral sculptured thin films to obliquely incident plane waves

Gyrotropic chiral sculptured thin films (STFs) exhibit optical activity due to their structural chirality, local anisotropy, and magneto-optic gyrotropy. We adapted two algorithms for nongyrotopic chiral STFs to investigate the circular-polarization-sensitivity of gyrotropic chiral STFs to incident plane waves. The impacts of gyrotropy and oblique angles of incidence on the reflectances and the transmittances were examined, and several conclusions drawn. In particular, we found that the incorporation of gyrotropy results in a blueshift of the Bragg regime.     

Optical spectra of graded nanostructured TiO2 chiral sculptured thin films

The optical properties of graded chiral sculptured TiO₂ thin films in axial and non-axial excited states are calculated using the rigorous coupled wave analysis method (RCWA) in conjunction with the Bruggeman homogenization formalism. The filtering frequency and polarization selectivity of these graded nanostructured TiO₂ sculptured thin films showed dependence on both structural and deposition parameters. The results achieved are consistent with the experimental data [K. M. Krause and M. J. Bret, Adv. Funct. Mater. 18 (2008) 3111].     

Chiral mirror and optical resonator designs for circularly polarized light: suppression of cross-polarized reflectances and transmittances

A left-handed chiral sculptured thin film (STF) that reflects strongly at the wavelength of the circular Bragg resonance tends to partially convert the handedness of incident LCP (left-circularly-polarized) light to RCP (right-circularly-polarized). We show that the cross-polarized component of the reflected RCP beam can be eliminated by interference with an additional RCP beam that is reflected at the interface of an isotropic cover and an AR (antireflecting) layer. For best results the refractive index and thickness of the AR layer need to accommodate a phase change on reflection that occurs at the chiral film. Effective suppression of the reflectances R_RR, R_RL, R_LR and the transmittances T_RL, T_LR can be achieved by sandwiching the chiral reflector between such amplitude and phase-matched AR coatings. Co-polarized chiral reflectors of this type may form efficient handed optical resonators. For LCP light the optical properties of such a handed resonator are formally the same as the properties of the isotropic passive or active Fabry–Perot resonators, but the handed resonator is transparent to RCP light.     

Maxwell stress distributions in chiral sculptured thin films due to obliquely incident plane waves

When a circularly polarized plane wave is obliquely incident on a chiral sculptured thin film (STF), the Maxwell stress dyadic exhibits a decreasing periodic variation across the thickness of the film. Normal and tangential surface tractions exist on the two faces of the chiral STF, as well as a net normal pressure across the film. These stresses are affected by the incidence angle of light, and are maximized when (i) the incident plane wave and the chiral STF are co-handed, (ii) the wavelength falls within a regime called the Bragg regime, (iii) the ratio of film thickness to the structural period of the chiral STF reaches a saturation value, and (iv) the deviation from normal incidence is small.     

Comparative study between dispersive and non-dispersive dielectric permittivity in spectral remittances of chiral sculptured zirconia thin films

The transmission and reflection spectra from a right-handed chiral sculptured zirconia thin film are calculated using the piecewise homogeneity approximation method and the Bruggeman homogenization formalism using both dispersive and non-dispersive functions for axial and non-axial states. The comparison of spectral results shows that the dispersion of the dielectric function has a considerable effect on the results. In axial excitation of cross-polarized reflectances and co-polarized transmittances the dispersion effect becomes more pronounced at wavelengths further away from the homogenization wavelength. This is also true in the case of non-axial excitation of circular transmittances, while there are considerable differences for cross-polarized reflectances where (wavelength) the first Bragg peak occurs. At wavelengths in the vicinity of the homogenization wavelength the dispersion effect of the dielectric function in R_RR becomes more significant.     

Axially excited, tightly interlaced, chiral sculptured thin films for polarization-universal bandgaps

A tightly interlaced chiral sculptured thin film (STF), whose unit cell comprises one structural period each of two otherwise identical chiral STFs but of opposite structural handedness, exhibits a polarization-universal bandgap on axial excitation. This bandgap lies wholly inside the Bragg regime of either constituent chiral STF.     

Planewave remittances of an axially excited chiral sculptured thin film with gain

Reflectances and transmittances of chiral sculptured thin films (STFs) with gain qualitatively differ from those of lossless and lossy chiral STFs, chiefly at the edges of the Bragg regime, for normally incident plane waves. The difference is explained in terms of the photonic density of states. A chiral STF with a central twist defect exhibits a reflection hole if there is no gain and emits narrowband radiation if there is gain, provided the thickness is not large. This narrowband feature diminishes as the thickness increases, and is replaced by an ultranarrowband transmission hole only if the chiral STF has neither gain nor loss.     

Simultaneous propagation of two Dyakonov-Tamm waves guided by the planar interface created in a chiral sculptured thin film by a sudden change of vapor flux direction

While growing a chiral sculptured thin film by physical vapor deposition, a sudden change in the direction of the vapor flux creates a plane of discontinuity. In effect, this is a planar interface between two different structurally chiral materials. Theoretical analysis shows that more than one Dyakonov-Tamm waves, with different phase speeds and degrees of localization to this interface, may propagate in different directions guided by the interface.     

Multi-band giant circular dichroism based on conjugated bilayer twisted-semicircle nanostructure at optical frequency

Plasmonic circular dichroism (CD) effect has been drawn great attention increasingly for its wide application in the fields of bio-sensing, biological detection, pharmaceuticals, and analytical chemistry. In this paper, we propose a chiral metasurface (CMS) to achieve strong multi-band CD effect at optical frequency. The designed CMS is composed of a periodic array of conjugated bilayer twisted-semicircle nanostructures. The numerical simulation results show that the CMS can produce strong multi-band CD effect due to the different coupling resonance modes under the excitations of left-handed circular polarization (LCP) light and right-handed circular polarization (RCP) light. It is shown that the chiral-selective absorption peaks can reach 89.4% and 95% for LCP light, 79% and 78.2% for RCP light, and the maximum CD is about 0.69 and −0.61 at 198.75 THz and 352.25 THz, 0.69 and −0.54 at 291.75 THz and 402.25 THz, respectively. The mechanism of the giant CD effect of the CSM has been revealed by analyzing the coupling mode of electric dipoles on the top and bottom layer through surface current distributions. Furthermore, the geometric parameter dependences of CD effect in the proposed CMS have been also studied numerically. The present results will guide the design of plasmonic chiral nanostructures for enhancing the CD effect.     

Negative refraction and imaging properties of circular photonic crystals

The electromagnetic wave propagation in a two-dimensional (2D) circular photonic crystal (CPC) is investigated. The CPC structure is composed of air holes in the dielectric background material. The finite element method is used to study the optical and propagating properties of the CPC slab. Numerical simulations show that negative refraction and near-field imaging can appear in a 2D CPC slab. We also find that the high-symmetry CPC slab possesses an optics axis along the vertical direction intersecting the symmetric center. As a result, the CPC slab can exhibit an excellent imaging performance when a source is placed on the optical axis.     

Second-harmonic generation circular dichroism spectroscopy from tripod-like chiral molecular films

The second-harmonic generation (SHG) circular dichroism in the light of reflection from chiral films of tripod-like chiral molecules is investigated.The expressions of the second-harmonic generation circular dichroism are derived from our presented three-coupled-oscillator model for the tripod-like chiral molecules.Spectral dependence of the circular dichroism of SHG from film surface composed of tripod-like chiral molecules is simulated numerically and analysed.Influence of chiral parameters on the second-harmonic generation circular dichroism spectrum in chiral films is studied.The result shows that the second-harmonic generation circular dichroism is a sensitive method of detecting chirality compared with the ordinary circular dichroism in linear optics.All of our work indicates that the classical molecular models are very effective to explain the second-harmonic generation circular dichroism of chiral molecular system.The classical molecular model theory can give us a clear physical picture and brings us very instructive information about the link between the molecular configuration and the nonlinear processes.     

Modelling of transmission through a chiral slab using fractional curl operator

Transmission of electromagnetic plane wave through a slab of reciprocal chiral medium has been modelled using fractional curl operator. It is noted that when order of the fractional curl operator becomes zero, the equivalent situation may correspond to absence of the chiral slab. Variation of the order of fractional curl operator may explore situations which may be regarded as intermediate step of a situation dealing with no chiral slab and a situation dealing with a chiral slab. It is noted that real order of the fractional curl operator may model the optical rotation while complex order of the fractional curl operator may model optical rotation as well as circular dichroism.     

The effect of pressure on CD spectra of the chiral octahedral transition metal complexes Λ-, Δ- and (Λ,Δ){Cr[(+)(S)(S)Mebdtp]3}

Abstract

The effect of pressure on circular dichroism (CD) spectra of the octahedral chiral Λ-, Δ- and (Λ, Δ)trisO, O'-bis[(+)(S)-2-methylbutyl]dithiophosphate Cr(III) complexes, have been studied in the pressure range 0-2.5GPa. Results on polycrystalline samples dispersed in nujol show a pressure-induced A to A inversion of configuration at the metal center above 1.2 GPa. The high pressure stability of the A-form is explained in terms of most favoured crystal packing among different ligand conformations of the chiral complex.     

Tunable dual-band infrared chiral metamaterials based on double-layered asymmetric U-shape split ring resonators

We have numerically demonstrated chiral metamaterials based on double-layered asymmetric U-shape split ring resonators. Dual-band circular dichroism is found in the whole frequency regime from 50 to 250 THz by studying the transmission properties. Huge optical activity and the induced negative refractive index are obtained at resonance by calculating the optical activity and ellipticity of the transmitted E-fields. Chirality parameter and effective refractive index are retrieved to illustrate the tunable optical properties of the metamaterials. The underlying mechanisms for the observed circular dichroism are analyzed. These metamaterials would offer flexible electromagnetic applications in the infrared regime.     

倾斜L形手性结构的制备以及圆二色性

在单层聚苯乙烯小球模板上,制备了大面积倾斜L形手性结构.通过倾斜角沉积技术,在小球的一侧生长二氧化硅,在二氧化硅上面沉积金属层;在垂直方向沉积另一金属层,使两个金属层具有不同高度从而形成倾斜L形手性结构.研究发现,通过控制二氧化硅的厚度,可以实现倾斜L形手性结构的圆二色性的调控.数值模拟结果表明,倾斜L形手性结构的圆二色性机制符合Born-Kuhn模型理论.