Chiral Photonic Structures Research Articles

A novel mode converter based on a 2D dielectrically chiral photonic structure

Abstract We designed a novel two-dimensional periodic chiral waveguide structure, which can achieve complete TE-TM mode conversion. We study the effect of the relevant parameters of structure on the conversion efficiency and confirm that in a certain range, the mode conversion has a rather high efficiency.

Spontaneous Emission of a Two-Level Atom in Chiral Photonic Structures

The spontaneous emission spectrum of a two-level atom in a one-dimensional (1D) anisotropic photonic crystal with symmetry of double helix is investigated using Laplace transform. Properties of spectrum of the radiation modes are discussed in detail under different values of detuning of atomic resonant frequency from a band-edge. Since spontaneous emission is proportional to the density of states (DOS), it is suppressed within the reflection band, corresponding to vanishing of the DOS. However, it is enhanced at the band edge. Therefore, spontaneous emission spectrum of the system is influenced at the edge of the stop band rather than at its centre.

Iridescent Chiral Nematic Cellulose Nanocrystal/Polymer Composites Assembled in Organic Solvents.

We describe an approach to prepare polymer composites with chiral nematic photonic structures through the self-assembly of cellulose nanocrystal (CNC) dispersions in organic solvents. Contrary to previous reports, we demonstrate that dispersions of neutralized sulfated CNCs in polar organic media readily form lyotropic chiral nematic liquid crystalline phases. We have investigated the effect of the neutralizing base on the CNC self-assembly, observing chiral nematic ordering for all counterions studied. The self-assembly of the organic CNC dispersions can be exploited to prepare iridescent polymeric composites simply by casting the CNC dispersion with a suitable polymer soluble in the organic solvent. Photonic properties of the composite films can be easily controlled by either varying the ratio of CNCs to polymer or adding salts.

Self-induced spin-polarized carrier source in active photonic device with artificial optical chirality

We demonstrate a scheme of active photonic devices which can spontaneously polarize the carrier spin in active regions. Rather than externally breaking the time-reversal symmetry of carrier spin states such as applying magnetic fields, only the optical chirality is required in this setup. Spin-polarized carriers are generated by interactions between carriers and self-initiated circularly polarized photons without the spin-selective optical pumping/electrical injection and breaking of the reciprocity. The device requires a cavity which supports modes with only one circular polarization and can be implemented with the vertical-cavity type of arrangements for distributed Bragg reflectors and artificial chiral photonic structures.

Experimental Investigation of the Polarization Plane Rotation of Light in Cholesteric Liquid-Crystalline Film with an Anisotropic Defect Layer

This paper experimentally demonstrated that in the cholesteric liquid-crystalline films it is possible to induce defect, which can be controlled by external electric field. Three cases of induced defects were examined and the main result of the experiment is that the rotation of polarization plane has maximum value when light first propagates through chiral liquid-crystalline photonic structure and then falls to the anisotropic layer. We have also supporting calculations, showing an agreement between experimental results and theoretical calculations. We present a new liquid crystal device configuration that performs as a tunable linear polarizer for both polarized and unpolarized lights.

Bistability, multistability, and nonreciprocity in a chiral photonic bandgap structure with nonlinear defect

The effects of the bistability, multistability, and nonreciprocity in a finite photonic bandgap structure of chiral and achiral layers with an asymmetrically placed nonlinear defect are investigated. The effect of the chirality parameter and angle of incidence on the wave multistable transmission and reflection is studied.

Two‐photon absorption circular‐linear dichroism on axial enantiomers

We report on the experimental-theoretical analysis of degenerated two-photon absorption circular-linear dichroism (2PA-CLD) in (S)-(-)-1,1'-bi(2-naphthol) and (R)-(+)-1,1'-bi(2-naphthol) dissolved in THF. 2PA-CLD reveals similarities and differences between optical polarization states for the different enantiomers. These results unveil the potential applications of 2PA-CLD for the study and identification of chiral molecules with active and pivotal role in biological systems and the systematic design of chiral photonic structures.

Polarization properties of symmetrical and asymmetrical nonreciprocal chiral photonic bandgap structure with defect

The polarization properties of perfectly periodical and defective one-dimensional photonic bandgap structures with nonreciprocal chiral (bi-isotropic) layers are studied. The method of solution is based on the 2×2 block-representation transfer-matrix formulation. Numerical simulations are carried out for different types of structures (symmetrical or asymmetrical) in order to reveal the dependence of the reflection and transmission coefficients on frequency, chirality, nonreciprocity parameters, and angle of wave incidence.

Tunable circularly polarized lasing emission in reflection distributed feedback dye lasers

A tunable circularly polarized distributed feedback (DFB) laser based on reflection grating configuration was realized in a solution of 4- (dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) doped methanol. Chiral photonic DFB structures with tunable photonic bandgap were generated by controlling the state of polarization of the two interfering pump beams. The dual-peak lasing emission spectrum indicates that the periodic chiral DFB grating is affected by a combination of modulations of gain and refractive index.

Polarization-dependent optical switch and its application for integrated optical circuit devices

An optical switching mechanism that uses chiral photonic band-gap structures is proposed. The switching is carried out by shifting the band-gap of the configuration. The component employs selective reflection in cholesteric layers and phase-shifting variance in retardation plates. The prototype has been designed and its parameters have been investigated. The spectral response of the component is calculated with respect to the input of orthogonal linear polarizations which are shown to be either efficiently transported to the forward and the backward directions within different photonic band-gaps. Two designs for stop-band tuning can be considered in the modulation functionalities, and thus a tunable dual-state polarization switching for mutually orthogonal polarized lights could be realized. Related applications of this component as a variable in-line polarization router and combiner for optical communication systems are emphasized.

Polarization-dependent filters based on chiral photonic structures with defects

A one-dimensional photonic device to perform optical multiple-wavelength filtering for light of a particular circular polarization state is presented and modelled. It is shown that the transmission spectrum of a stacked structure consisting of two cholesteric films with the same pitch gradient in the helix can be tailored by incorporating the defects of pitch jump and phase jump in the middle of the structure. The introduction of the defects results in the exhibition of multiple-band-pass characteristics inside the photonic band gap when incident circularly polarized light has the same handedness as the cholesterics. Some critical parameters are analysed: the thickness of the double-composite film, co-polarized/cross-polarized transmittance and reflectance, and the twist angle. For light of specified circular polarization, the shift of the filter in transmitted wavelength can be tuned linearly by adjusting the amount of the phase jump. The proposed chiral filter may be useful for multichannel lightwave systems. Some rules for the practical implementation of a filter with polarization sensitivity are given.

Twist defect in chiral photonic structures with spatially varying pitch

The properties of photonic defect modes in a chiral photonic structure were investigated using the finite element method. By stacking two cholesteric liquid crystal (ChLC) films, the defect mode due to the introduction of a twist defect was considered in both cases of chiral structures with constant pitch and spatially varying pitch. Two types of linear pitch gradients for achieving a broadband reflection were analysed, and the number of chiral pitches required for establishing the stop band was simulated. The effect of a finite sample thickness on the energy density distribution of the defect mode and on the required polarization of the incident light to excite the defect mode was studied. In both cases of constant pitch and spatially varying pitch, an unusual crossover behaviour in reflection at the defect resonance wavelength of a single circularly polarized mode appears when the structure thickness increases beyond a specific value. The energy distribution inside the sample also reveals the unusual distribution. Two different resonance wavelengths can be created by a twist defect in the ChLC composite film with linearly varying pitch, while only one resonance wavelength can be created in the identical film with constant pitch.

Broad Stopband Characteristic of Dielectric Chiral Photonic Crystals

In this paper, a novel chiral photonic crystal structure is presented. The formula of reflection coefficient of multi-layer chiral media is applied to dielectric-chiral photonic crystal structure, which is composed of thin chiral layers sandwiched by conventional media. To compare with previous literature, we consider the dielectric structure with alternate glass and GaAs layers. The power reflectance as a function of wavelength for this photonic crystal structure has been calculated. The results are in good agreement with that of Reference. However, our method is simpler. From these graphs, it is found that 100% reflectance is only in finite wavelength ranges, and reflection bandwidth is also small for conventional photonic crystal structure. For chiral photonic crystal, the results show that the chiral photonic band gap (PBG) structure gives nearly 100% reflections in the near-infrared region in addition to some parts of the visible region of the wavelengths. Therefore, it can be used as a broadband reflector and filter.

One-dimensional chiral photonic band gap structure analyzed by non-symmetric transmission-line method

In this paper, the formula of reflection coefficient of multi-layer chiral media is derived by non-symmetric transmission-line method. Then, it is applied to one-dimensional (1-D) chiral photonic crystal structure, which is composed of thin chiral layers sandwiched by air. The results show that it is difficult to obtain photonic band gap for general dielectric when the contrast of two media refractive indices is not large, and the reflection coefficient is small. However, for chiral photonic crystal, although the refractive index of chiral layer is small, the wave spectrum contains forbidden zones and the reflection coefficient from such a structure is found to be almost equal to 1, i.e., the wave is almost totally reflected through adjusting chiral parameter. Therefore it is easier to obtain an ideal photonic band gap.

Supermodes of chiral photonic filters with combined twist and layer defects.

We consider the circularly polarized localized modes of chiral photonic structures with combined central twist and isotropic layer defects. The general filter is shown to suffer from anomalous remittance and saturation of linewidth as the thickness of the structure is increased. However, by choosing parameters that phase match the elements of the round-trip matrix of the isotropic layer defect, we demonstrate the existence of supermodes that maintain exceptional purity of polarization state and exponential decrease in linewidth as the thickness is increased.

Twist defect in chiral photonic structures.

We demonstrate that twisting one part of a chiral photonic structure about its helical axis produces a single circularly polarized localized mode that gives rise to an anomalous crossover in propagation. Up to a crossover thickness, this defect results in a peak in transmission and exponential scaling of the linewidth for a circularly polarized wave with the same handedness as structure. Above the crossover, however, the linewidth saturates and the defect mode can be excited only by the oppositely polarized wave, resulting in a peak in reflection instead of transmission.

All-optical switching with a nonlinear chiral photonic bandgap structure

A pump–probe optical switching mechanism that uses a nonlinear chiral photonic bandgap structure is proposed. The switching is achieved by shifting the bandgap of the structure; the chirality permits the use of the same frequency for the pump and the probe beam. The pump beam is never transmitted, so there is no discrimination problem. A new extension of the finite-difference time-domain technique is developed to simulate the propagation of an electromagnetic wave in a Kerr nonlinear chiral medium.