Chiral Photonic Structures Research Articles

Tuning Q-Factor and Perfect Absorption Using Coupled Tamm States on Polarization-Preserving Metasurface

The circular polarization of light flips its handedness after a conventional metallic mirror reflection. Therefore, a polarization-preserving metasurface is a crucially important element in a series of chiral photonic structures. They include tunable cholesteric LCs and anisotropic photonic crystals. Chiral structures are rich in interfacial localized modes including Tamm states. In this report, coupled modes formed as a result of the interaction between two chiral optical Tamm states or a chiral optical Tamm state and a chiral Tamm plasmon polariton are analytically and numerically investigated. It is shown that the effective control of coupled modes can be carried out by changing the pitch of the cholesteric and the angle between the optical axis of the cholesteric and the polarization-preserving anisotropic mirror. The influence of the metasurface period on the spectral characteristics of coupled modes is investigated. The possibility of realizing a bound state in the continuum of the Friedrich–Wintgen type, resulting from the destructive interference of coupled modes, which leads to the collapse of the resonance line corresponding to the chiral optical Tamm state, has been demonstrated.

Circularly Polarized Luminescence from Chiral Photonic Crystals of Caesium Lead Halide Perovskites

AbstractThe generation and detection of circularly polarized light (CPL) are of importance in many modern technologies such as digital communications, machine vision, bio‐imaging, and 3D displays. In this study, inspired by the fact that various species of plants and animals can efficiently produce and/or perceive CPL with chiral photonic crystal structures, efforts are made to fabricate luminescent chiral photonic crystals with caesium lead halide perovskites, a class of solution‐processable semiconductors that have excellent luminescence properties and readily tunable electronic band gaps. Coupling between the emission frequency of the perovskite and the cavity resonance frequency of the photonic crystal structure can be established at any desired point in the visible region, giving rise to CPL output with a dissymmetry factor up to −0.34. These results suggest a new approach to chiral light‐emitting materials and may contribute to the development of semiconductors with ordered microstructures.

Achieving maximum scattering circular dichroism through the excitation of anapole states within chiral Mie nanospheres

The chirality-dependent asymmetric light-matter interaction can be enhanced using various artificial photonic structures as well as engineered incident field. Compared with chiral photonic structures, the engineered optical field is less explored and recently recognized as a new regime in tailoring light-matter interaction. In this work, we demonstrate that weakly chiral spheres can exhibit the maximum scattering circular dichroism (CD) by tailoring the incident field to construct chirality-sensitive anapole states. By considering the chiral terms radiated from Mie nanospheres as perturbation, a multiscattering model is established to predict the chiroptical Mie scattering response of these nanospheres. This model provides a specific guideline to tailor the incident field to realize the upper limit of the achievable scattering CD.

Machine Learning‐Based Optimization of Chiral Photonic Nanostructures: Evolution‐ and Neural Network‐Based Designs

Chiral photonics opens new pathways to manipulate light–matter interactions and tailor the optical response of metasurfaces and ‐materials by nanostructuring nontrivial patterns. Chirality of matter, such as that of molecules, and light, which in the simplest case is given by the handedness of circular polarization, have attracted much attention for applications in chemistry, nanophotonics and optical information processing. The design of chiral photonic structures using two machine learning methods, the evolutionary algorithm, and neural network approach, for rapid and efficient optimization of optical properties for dielectric metasurfaces, is reported. The design recipes obtained for visible light in the range of transition‐metal dichalcogenide exciton resonances show a frequency‐dependent modification in the reflected light's degree of circular polarization, that is represented by the difference between left‐ and right‐circularly polarized intensity. Our results suggest the facile fabrication and characterization of optical nanopatterned reflectors for chirality‐sensitive light–matter coupling scenarios employing tungsten disulfide as possible active material with features such as valley Hall effect and optical valley coherence.

Anisotropic Iridescence and Polarization Patterns in a Direct Ink Written Chiral Photonic Polymer.

The iridescence of structural color and its polarization characteristics originate from the nanoscale organization of materials. A major challenge in materials science is generating the bright, lustrous hues seen in nature through nanoscale engineering, while simultaneously controlling interaction of the material with different light polarizations. In this work, a suitable chiral nematic liquid crystal elastomer ink is synthesized for direct ink writing, which self-assembles into a chiral photonic structure. Tuning the writing direction and speed leads to the programmed formation of a slanted photonic axis, which exhibits atypical iridescence and polarization selectivity. After crosslinking, a freely programmable, chiroptical photonic polymer material is obtained. The strongly perspective-dependent appearance of the material can function as specialized anticounterfeit markers, as optical elements in decorative iridescent coatings, or, as demonstrated here, in optically based signaling features.

Ultra‐Broadband Tunable Bragg–Berry Optical Vortex Generators of a Circularly Symmetric Chiroptic Structure

AbstractNew optical–photonic devices with the capability to manipulate, modify, and even tailor the light characteristics dynamically, such as wave surface, play an increasingly important role in the development of modern and future optics. Optical vortex with spiral wave surfaces is a special type of light with intrinsic orbital angular momentum (OAM). Thus, this vortex can be applied to the development of advanced optics, such as optical tweezers, ultra‐resolution microscope, optical communication, and quantum technology. This study demonstrates an optical device known as Bragg–Berry optical vortex generator (BBOVG) with a circularly symmetric chiral photonic structure using an easy‐to‐implement method by filling a ferroelectric liquid crystal‐doped cholesteric liquid crystal (FLC‐CLC) material into an indium‐tin‐oxide‐coated cell pre‐treated with photoalignment along the azimuthal direction. The BBOVG devices can be operated by dynamically tuning the photonic bandgaps of the opposite‐handed FLC‐CLCs throughout the full‐visible region (400–700 nm) in a low voltage range (≤ 3.0 V) to obtain reflective optical vortices with opposite OAM signs in a full‐visible range near room temperature. The present devices, which can transform geometric phase and have ultra‐wideband manipulability and high operability at room temperature, provide a good example in demonstrating key devices that fit the use in future optical/photonic applications.

Electro-optic effect of graded-pitch chiral photonic structures under oblique illumination

We studied theoretically optical spectra when circularly polarized light obliquely impinges on a slab of a chiral photonic structure with linearly and uniformly spatial varying pitch. The material that we considered locally has a $\overline{4}2m$ point-group symmetry and presents Pockels effect, hence, we controlled optical spectra by a low frequency (DC) electric field applied along the nonhomogeneity axis. The spectra display a Bragg-type broadband, where the edge wavelengths of the photonic band are expressed in terms of the medium parameters and the external electric-field magnitude. This allows us to select the region of the electromagnetic spectrum, where the band could become as broad as we choose. We studied three samples with iridescent, silver, and golden colors due to their reflection properties. We observed an enhancement as well as a broadening of the optical band by the application of the electric field. We found that, under certain conditions, when the optical band is practically absent, a very broad band could be created (comparable to the whole visible spectrum) when a DC electric field is applied. This is the result of electro-optic contributions in the permittivity tensor elements, which give rise to a tremendous increase in both the rotatory power and dichroism. Therefore, these media could be used as electrically controlled broadband frequency and polarization filters. Moreover, we observed the usual blueshift of the band as the incidence angle of light increases and an asymmetry in the reflection band amplitude as the incidence angle of the light increases, depending on whether the pitch increases or decreases and the pitch gradient; which endows the sample with different reflection colors for different gradients and incidence angles of the light. The asymmetry in the reflection band vanishes as the magnitude of the electric field increases.

Eigenmodes in a photonic structure with a torsion-deformed nematic liquid crystal exposed to a magnetic field.

The polarized components of transmission spectrum of a multilayered photonic structure containing a nematic liquid crystal under the magnetic-field-induced torsion deformation have been investigated. Since the planarly twisted director structure of the nematic is unrelated to a decrease in its optical anisotropy, the control of the polarization and spectral characteristics of the eigenmodes is based on the occurrence of a geometric phase, whose contribution gradually increases upon nematic deformation. Along with the previously observed blue shift of the cavity o modes, the anomalous shift of the cavity e modes to the long-wavelength region (red shift) of the transmission spectrum has been detected. It has been shown that the countershift of the modes narrows the intermode interval. Remarkably, the extremal approaching of e and o modes in magnetic field H_{ex} occurs below the avoided crossing phenomenon observed in the stronger equalization field H_{eq} (H_{ex}<H_{eq}). The experimental results have been interpreted by the numerical simulation of the transmission spectra of the chiral photonic structure using the transfer matrix method.

Study of polarization effects in phase-controlled multi-beam interference lithography towards the realization of sub-micron photonic structures

In this study, we theoretically demonstrate the effect of polarization in the realization of the submicron woodpile and chiral helix photonic structures formed through multi-beams interference in an umbrella geometry using a phase-only spatial light modulator (SLM). The polarization states of the interfering beams play a decisive role in the formation of submicron features when the requisite high interference angle is imparted through reflective type multi-mirror arrangements. Leaving aside the scalar model for intensity distribution, we estimate the actual scenario by taking into account the contributing vector electric fields in such SLM based multi-mirror set-up. Comparing with different interference angles, we recognize the lower limit in structural periodicities obtainable without deforming the desired photonic structures. Numerical calculations of the transmittance response of such polarization induced structures are also compared in contrast to their analogous scalar counterpart. Moreover, this study is extended by conceptualizing the introduction of different polarization converter that can be used to manipulate the outcomes towards achieving desired structural features.

Method for single-shot fabrication of chiral woodpile photonic structures using phase-controlled interference lithography.

In this report, we propose a large-area, scalable and reconfigurable single-shot optical fabrication method using phase-controlled interference lithography (PCIL) to realize submicrometer chiral woodpile photonic structures. This proposed technique involves a 3 + 3 double-cone geometry with beams originated from a computed phase mask displayed on a single spatial light modulator. Simulation studies show the filtering response of such structures for linearly polarized plane wave illumination, with structural features tunable through a single parameter of interference angle. Further, these single chiral woodpile structures show dual chirality on illumination with both right circularly and left circularly polarized light through simulation. Experimentally fabricated patterns on photoresist show resemblance to the desired chiral woodpile structures.

Water-Stable Flexible Nanocellulose Chiral Nematic Films through Acid Vapor Cross-Linked Glutaraldehyde for Chiral Nematic Templating.

Cellulose nanocrystals (CNCs) have drawn considerable attention for their use in optical and sensor applications due to their appealing properties of chiral nematic photonic structures. However, the flexibility and water instability of neat CNC chiral nematic films are questionable and compromise their outstanding properties. We propose a room-temperature process for fabricating flexible, water-stable chiral nematic CNC films. Aqueous glutaraldehyde (GA) was first mixed with CNCs, and then free-standing films were formed by evaporation-induced self-assembly. The chiral nematic dry films that formed were then exposed to hydrochloric acid vapor for subsequent GA cross-linking with CNCs. The GA cross-linked CNC films had a highly ordered chiral nematic organization. The enhanced water stability of the films was demonstrated by using GA cross-linked CNC films as freestanding template substrates for conducting polymers (polypyrrole) and metal oxides (iron oxide) to form flexible chiral nematic photonic hybrids.

Localized modes in chiral photonic structures

We discuss chiral structures in self-organizing, artificial, and biological materials. A review of experimental studies and recent advances in the localization of light in chiral structures is given. The behavior of polarized resonant modes in such structures is examined using the example of a one-dimensional photonic crystal containing liquid crystal materials. The anomalous spectral shifts of transmission peaks are interpreted as the contribution of the geometric phase caused by the twisting of the layers of the liquid crystal. The optical Tamm state localized at the boundary between chiral and nonchiral mirrors in the form of a cholesteric layer and a polarization-preserving anisotropic mirror is analytically and numerically described. Considerable attention is paid to the presentation of the properties of localized optical modes in the cholesteric with a resonant metal-dielectric nanocomposite. New possibilities for controlling the properties of the photonic structure are noted, due to the combination of the dispersion of the resonant medium and the intrinsic dispersion of the cholesteric. Attention is focused on controlled hybrid modes in the cholesteric structure formed by the coupling of localized modes. Possible applications and further ways of developing the concept of chiral photonic structures are deliberated.

Integration of Optical Surface Structures with Chiral Nanocellulose for Enhanced Chiroptical Properties.

The integration of chiral organization with photonic structures found in many living creatures enables unique chiral photonic structures with a combination of selective light reflection, light propagation, and circular dichroism. Inspired by these natural integrated nanostructures, hierarchical chiroptical systems that combine imprinted surface optical structures with the natural chiral organization of cellulose nanocrystals are fabricated. Different periodic photonic surface structures with rich diffraction phenomena, including various optical gratings and microlenses, are replicated into nanocellulose film surfaces over large areas. The resulting films with embedded optical elements exhibit vivid, controllable structural coloration combined with highly asymmetric broadband circular dichroism and a microfocusing capability not typically found in traditional photonic bioderived materials without compromising their mechanical strength. The strategy of imprinting surface optical structures onto chiral biomaterials facilitates a range of prospective photonic applications, including stereoscopic displays, polarization encoding, chiral polarizers, and colorimetric chiral biosensing.

Study of chiroptical effects on the novel chiral photonic structure

Purpose The purpose of this paper is to demonstrate a novel chiral photonic crystal with thin thickness and small unit cells via numerical calculations. The multi-band circular dichroism is found in a wide frequency range from 400 to 600 THz by studying the transmission properties. Design/methodology/approach To investigate this chiral photonic structure, refection coefficients are analytically computed using finite element method. Numerical results are given, and physical properties are discussed, including the optical rotation, the circular dichroism and the absorption. Findings The results of this modeling and simulation under COMSOL multiphysics environment have led the authors to study the scattered parameters such as the coefficient of transmission (S21) and the coefficient of reflection (S11) for a 2D CPC nanostructure. The authors have also developed script under the Matlab environment which studies absorption and circular dichroism and ensure the existence of optical activity. According to the obtained results, the coefficient of transmission is proportional to the parameter of chirality. Originality/value The authors have designed a novel chiral photonic structure that exhibits larger circular dichroism. The CD spectrum has typically both positive and negative bands. The design principles defined in this work, which combine the concepts of the photonic crystal with the chiral structure (optical activity, circular dichroism and absorption), represent a model for simulation of the properties of a more complex chiral photonic structure. These results led to realization of novel circularly polarized devices in nanotechnologies.

Локализованные моды в хиральных фотонных структурах

Локализованные моды в хиральных фотонных структурах

Polarization Conversion Effect in Biological and Synthetic Photonic Diamond Structures

AbstractPolarization of light is essential for some living organisms and many optical applications. Here, an orientation dependent polarization conversion effect is reported for light reflected from diamond‐structure‐based photonic crystals (D‐structure) inside the scales of a beetle, the weevil Entimus imperialis. When linearly polarized light propagates along its 〈100〉 directions, the D‐structure behaves analogous to a half‐wave plate in reflection but based on a different mechanism. The D‐structure rotates the polarization direction of linearly polarized light, and reflects circularly polarized light of both handednesses without changing it. This polarization effect is different from circular dichroism occurring in chiral biological photonic structures discovered before. The structural origin of this effect is symmetry breaking inside D‐structure's unit cell. This finding demonstrates that natural photonic structures can exploit multiple functionalities inherent to the design principles of their structural organization. Aiming at transferring the inherent polarization effect of the biological D‐structure to technically realizable materials, three simplified biomimetic structural models are derived and it is theoretically demonstrated that they retain the effect. Out of these structures, functioning woodpile structure prototypes are fabricated.

(Invited) Light Management for Engineering Luminescence in Nanoscale Environments By Numerical Optimization

The spontaneous emission rate of fluorescencent species in general is affected by its environment. Geometrical structure and material composition of the environment can yield strongly increased rates. This effect is known as Purcell effect, and the magnitude of enhancement of the emission rate in a cavity is known as Purcell factor. The Purcell factor is proportional to the cavity mode's quality factor (Q factor) and inversely proportional to the modal volume. This implies that strongly resonant structures with small length scales (of the order of the wavelength of light) can greatly influence fluorescence. However, not only the emission rate can be controlled by a suitably chosen environment, but also the directional properties of the emitted light field can be engineered by placing the emitter in custom-made environments. In the regime of optical wave phenomena on nano-meter length scales (nano optics), Maxwell's equations for the 3D, vectorial electromagnetic field distribution can to be solved numerically in order to quantify the expected emission rate and far field radiation patterns. In order to obtain specific functionalities of optical devices, numerical optimization is performed. I.e., in numerical computations, structural parameters (layer thicknesses, structure dimensions, material composition, etc) are varied until a parameter set is reached which yields best performance for the specific functionality. This numerically optimized structure can then serve as design pattern for experimental realizations. We review our recent developments of finite-element method based and machine-learning inspired optimization algorithms [1]. We have applied these methods in order to optimize light extraction efficiency of quantum dots embedded in deterministically fabricated cavities [2] and in periodic nano-structures [3], as well as extraction efficiency from structured light emitting diodes (LED, OLED) [4]. We have also investigated possibilities to manipulate emission properties in chiral photonic structures [5], and to design vertical cavity lasers [6]. Further, in applications like light trapping in solar cells, for enhanced photovoltaic efficiency, the related problem of maximizing coupling of light of specific spectral properties into nanostructured solar cells can be treated [7]. In this contribution we will further present recent results on optimization of photon extraction efficiency from single photon sources for applications in quantum technology. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 675745 (MSCA-ITN-EID NOLOSS) and from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 14IND13 (PhotInd). We further acknowledge funding by Einstein Foundation Berlin (ECMath-SE6) and by Freie Universität Berlin (Dahlem Research School). [1] P.-I. Schneider, et al. Global optimization of complex optical structures using Bayesian optimization based on Gaussian processes. Proc. SPIE 10335, 103350O (2017).[2] M. Gschrey, et al. Highly indistinguishable photons from deterministic quantum-dot microlenses utilizing three-dimensional in situ electron-beam lithography. Nat. Commun. 6, 7662 (2015).[3] C. Barth, et al. Increased fluorescence of PbS quantum dots in photonic crystals by excitation enhancement. Appl. Phys. Lett. 111, 031111 (2017).[4] L. Zschiedrich, et al. Numerical analysis of nanostructures for enhanced light extraction from OLEDs. Proc. SPIE 8641, 86410B (2013).[5] P. Gutsche, et al. Locally enhanced and tunable optical chirality in helical metamaterials. Photonics 3, 60 (2016).[6] N.N. Ledentsov, et al. Direct Evidence of the Leaky Emission in Oxide-Confined Vertical Cavity Lasers. IEEE J. Quant. Electron. 52, 2400207 (2016).[7] K. Jäger, et al. Simulations of Sinusoidal Nanotextures for Coupling Light into c-Si Thin-Film Solar Cells. Opt. Express 24, A569 (2016).

Fabrication of helical photonic structures with submicrometer axial and spatial periodicities following "inverted umbrella" geometry through phase-controlled interference lithography.

In this Letter we report for the first time, to the best of our knowledge, a phase spatial light modulator (SLM)-based interference lithography (IL) approach for the realization of hexagonally packed helical photonic structures with a submicrometer scale spatial, as well as axial, periodicity over a large area. A phase-only SLM is used to electronically generate six phase-controlled plane beams. These six beams from the front side and a direct central backside beam are used together in an "inverted umbrella" geometry setup to realize the desired submicrometer axial periodic chiral photonic structures through IL. The realized structures with 650nm spatial and 353nm axial periodicities on negative photoresist can be used as an optical filter and refractive index sensor, as evidenced from the FDTD-based simulation study on its optical properties. Further, the fabricated templates can be transferred to metals such as silver or aluminum for the realization of a metamaterial-based broadband circular polarizer ranging from 1 to 3.5μm of near-infrared spectra.

Controlling circular polarization of light emitted by quantum dots using chiral photonic crystal slabs

We study the polarization properties of light emitted by quantum dots that are embedded in chiral photonic crystal structures made of achiral planar GaAs waveguides. A modification of the electromagnetic mode structure due to the chiral grating fabricated by partial etching of the waveguide layer has been shown to result in a high circular polarization degree ${\ensuremath{\rho}}_{c}$ of the quantum dot emission in the absence of external magnetic field. The physical nature of the phenomenon can be understood in terms of the reciprocity principle taking into account the structural symmetry. At the resonance wavelength, the magnitude of $|{\ensuremath{\rho}}_{c}|$ is predicted to exceed 98%. The experimentally achieved value of $|{\ensuremath{\rho}}_{c}|=81%$ is smaller, which is due to the contribution of unpolarized light scattered by grating defects, thus breaking its periodicity. The achieved polarization degree estimated removing the unpolarized nonresonant background from the emission spectra can be estimated to be as high as 96%, close to the theoretical prediction.

Polarization control of quantum dot emission by chiral photonic crystal slabs.

We investigate theoretically the polarization properties of the quantum dot's (QDs) optical emission from chiral photonic crystal structures made of achiral materials in the absence of external magnetic field at room temperature. The mirror symmetry of the local electromagnetic field is broken in this system due to the decreased symmetry of the chiral modulated layer. As a result, the radiation of randomly polarized QDs normal to the structure becomes partially circularly polarized. The sign and degree of circular polarization are determined by the geometry of the chiral modulated structure and depend on the radiation frequency. A degree of circular polarization up to 99% can be achieved for randomly distributed QDs, and can be close to 100% for some single QDs.