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We report on the experimental observation of a Tamm plasmon state in the near-IR region characterized by an anomalously high energy level located in the upper half of the photonic band gap. Such a "blue"Tamm plasmon was demonstrated at the interface between a conventional, completely periodic Bragg reflector and a nanostructured nonresonant thin gold grating. We study the effect of the grating period on the characteristics of the anomalous state and show that the anomaly results from a nontrivial topology of the nanograting's optical near field, which cannot be captured by the effective medium approach and transfer matrix method commonly employed in the analysis of Tamm plasmons.

We demonstrate experimentally how to turn a conventional distributed Bragg reflector into a polarization selecting mirror operating in the near-IR at normal incidence without diffraction and with high extinction ratio. Our approach involves combining a dielectric multilayer composite with a sub-wavelength metal wire-grid nanograting, which can be routinely fabricated using well-established planar fabrication techniques. Moreover, the design and working principle of our nanostructured hybrid mirror enable it to operate as a surface sensor and allow straightforward integration of the mirror with functional materials for tuning its wavelength/polarization extinction ratio.

We demonstrate experimentally that Tamm plasmons in the near-IR can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. More crucially, not only do Tamm plasmons survive the nano-patterning of the metal film, but they also become sensitive to external perturbations, as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface we were able to red-shift the spectral position of Tamm plasmon by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of this notoriously inert excitation within a 10 nm range., Comment: 14 pages, 3 figures

Focused ion beam (FIB) milling is an important rapid prototyping tool for micro- and nanofabrication and device and materials characterization. It allows for the manufacturing of arbitrary structures in a wide variety of materials, but establishing the process parameters for a given task is a multidimensional optimization challenge, usually addressed through time-consuming, iterative trial-and-error. Here, we show that deep learning from prior experience of manufacturing can predict the postfabrication appearance of structures manufactured by focused ion beam (FIB) milling with >96% accuracy over a range of ion beam parameters, taking account of instrument- and target-specific artifacts. With predictions taking only a few milliseconds, the methodology may be deployed in near real time to expedite optimization and improve reproducibility in FIB processing.

A concept of an efficient ultrathin spatial phase modulator for correcting terahertz wavefronts in transmission with subwavelength spatial resolution is experimentally demonstrated. It exploits a combination of spatially addressable resonant planar metamaterials (so-called metasurfaces) and an optically thin (

Photovoltaic light modulators integrate liquid crystals and solar cells and offer an exciting prospect for autonomous, smart displays and visors. Illumination produces a photovoltage that modifies the liquid crystal alignment and light transmission. However, determining their properties, for example, the voltage dropped across the liquid crystal, different pretilts or anchoring energies, inherent to asymmetric cell designs, poses significant challenges. We have successfully applied to such photovoltaic modulators a new measurement methodology based on wide-area cross-polarized intensity measurements, coupled to an Ericksen-Leslie model. We have implemented it in a versatile optical analyzer, driven by a Matlab graphical user interface.

The current development of new liquid crystal devices often requires the use of thin cells and new experimental materials. Characterizing these devices and materials with optical methods can be challenging if (1) the total phase lag is small (“thin cells”) or (2) the liquid crystal optical and dielectric properties are only partially known. We explore the limitations of these two challenges for efficient characterization and assessment of new, to the best of our knowledge, liquid crystal devices. We show that it is possible to extract a wealth of liquid crystal parameters even for cells with a phase lag of Δ Φ ≈ π , such as E7 liquid crystal in a 1.5 µm cell, using cross-polarized intensity measurements. The reliability of the optical method is also demonstrated for liquid crystals without precise values of dielectric or refractive index coefficients.

We demonstrate experimentally that Tamm plasmons can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. Not only do Tamm plasmons survive the nano-patterning of the metal film, but they also become sensitive to external perturbations, as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface we were able to red-shift a Tamm plasmon by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of this notoriously inert excitation within a 10 nm range.

We report the generation of space-time non-separable toroidal pulses of complex topological structure. Such pulses, a formation of singularities propagating in free space, are ideal probes for toroidal and anapole modes in matter.

We demonstrate experimentally that optical Tamm states corresponding to Tamm plasmons can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. Not only do such states survive the nano-patterning of the metal film, but they also become sensitive to external perturbations, as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface we were able to red-shift Tamm plasmons by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of these notoriously inert excitations within a 10 nm range.

We report on a discovery that homogeneous plasmonic non-diffracting metasurfaces of a certain type respond differently to spatially coherent and incoherent light, enabling robust speckle-free discrimination between different degrees of coherence. The effect has no direct analogue in natural optical materials and may find applications in compact metadevices enhancing imaging, vision, detection, communication and metrology.

Planar metamaterials represent a powerful paradigm of optical engineering, which enables one to control the flow of light across structured material interfaces in the absence of high-order diffraction modes. We report on a discovery that planar metamaterials of a certain type, formed by nanopatterned metal films, respond differently to spatially coherent and incoherent light, enabling robust speckle-free discrimination between different degrees of light coherence. The effect has no direct analogue in natural optical materials and may find applications in nanoscale metadevices enhancing imaging, vision, detection, communication, and metrology.

We report on a discovery that homogeneous metallic non-diffracting metamaterials of a certain type respond differently to spatially coherent and incoherent light, enabling robust speckle-free discrimination between different degrees of coherence. The effect has no direct analogue in natural optical materials and may find applications in compact metadevices enhancing imaging, vision, detection, communication and metrology.

We report on a discovery that homogeneous metallic non-diffracting metasurfaces of a certain type respond differently to spatially coherent and incoherent light, enabling robust speckle-free discrimination between different degrees of coherence. The effect has no direct analogue in natural optical materials and may find applications in compact metadevices enhancing imaging, vision, detection, communication and metrology.

Valley polarized topological states of light allow for robust waveguiding which has been demonstrated for transverse-electric modes in THz and near-infrared parts of the spectrum. As the topological protection relies on guiding the light via a highly structured surface, direct imaging of the photonic modes at sub-unit cell resolution is of high interest but challenging in particular for transverse-magnetic modes. Here, we report mapping the transverse-magnetic modes in a valley photonic crystal waveguide using scattering-type scanning near-field optical microscopy at the optical telecom C-band wavelength. The waveguide based on a triangular air-hole motif with broken inversion symmetry is fabricated from suspended Germanium layer. We observed the launching and guiding of the transverse-magnetic edge mode along the boundary between topologically distinct domains with opposite valley Chern indices. These results are supported by theoretical simulations, and provide insight into the design and use of topological protected states for applications in densely integrated optical telecommunication devices.

Breaking the diffraction limit in imaging microscopes with far-field imaging options has always been the thrust challenge for optical engineers and biologists over the years. Although structured illumination microscopy and Bessel beam assisted imaging has shown the capability of imaging with sub-diffraction resolutions, they rely on the use of objective lenses with large numerical apertures (NA). Hence, they fail to sustain resolutions at larger working distances. In this context, we demonstrate a method for nanoscale resolution imaging at longer working distances, named as Structured Illumination Bessel Microscopy (SIBM). The proposed method is envisaged for both biological and engineering applications that necessitate high imaging resolutions at large working distances. EDB (Economic Devt. Board, S’pore) MOE (Min. of Education, S’pore) Published version

We demonstrate experimentally for the first time the generation of electromagnetic "Flying Donuts", few-cycle pulses of toroidal topology with non-separable spatial and temporal structure, and discuss applications in the study of anapole excitations in matter.

We fabricated and characterized a meta-pixel – metamaterial-based active optical device incorporating a twisted nematic liquid crystal cell. The meta-pixel exhibits directionally asymmetric transmittance and reflectance, which can be controlled electrically by realigning the liquid crystal. Apart from defining the optical response of the meta-pixel, the metamaterial also plays a vital role in implementing electrical control and alignment of the liquid crystal. Such a meta-pixel can find applications in a new generation of miniature displays, compact tunable optical filters and modulators.

The pioneering realization of suspensions of ferroelectric nanoparticles in a nematic host was one of excellent contributions of Yuri Reznikov to the science of liquid crystals [Y. Reznikov et al., Appl. Phys. Lett. 82 (2003) 191]. This achievement created great excitement as entailed the enhancement of materials dielectric properties and increase of the phase transition temperature between nematic and isotropic phase. In this contribution, we examine the spectrum of fundamental fluctuations in one of his ferroelectric suspensions by dynamic light scattering measurements, which gives insight into the coupling between the particles and the orientation of the liquid crystalline matrix. We set side by side these results with the equivalent ones obtained for the case of stable suspensions of ferromagnetic nanoplatelets in a nematic matrix showing macroscopic ferromagnetic ordering. The underlying origin of the difference between the particle-matrix coupling observed in both cases is discussed and its effect on the orientational fluctuations spectrum is compared., 13 pages, 3 figures

The search for new paradigms for all-optical processing plays a pivotal role in current photonics research. In this perspective, light-defined waveguides (LDW) are of central importance. LDW can be roughly divided into two groups: permanent [1] and real-time [2]. In the first case, LDW persist even after the writing beam is shut off; in the second case, the effect requires the presence of an input (writing) beam. The latter corresponds to the formation of a spatial soliton, a shape-preserving wavepacket arising from the balance between diffraction and self-focusing [2]. Here, we show how spatial soliton waveguides in nematic liquid crystals (NLC), namely nematicons [2], can be rendered permanent via UV photo-polymerization.

We demonstrate the electro-optic control of self-confined beams via frequency modulation in dual-frequency nematic liquid crystals. Proper design of comb electrodes in a planar cell allows the maximization of the overall steering angle when only walk-off variations are used. A span angle as large as two times the maximum walk-off angle is achieved.

Spatial solitons can affect and enhance random lasing in optically-pumped dye-doped nematic liquid crystals. Upon launching two collinear beams in the sample, the first to pump the fluorescent guest molecules and the second to induce a reorientational soliton, strikingly the second beam not only guides the emitted photons in the soliton waveguide, but also enhances the lasing efficiency and modulates its spectral width. By altering the scattering paths of the emitted photons, the soliton also contributes to the selection of the lasing modes, as further confirmed by the observed kinks in the input/output characteristics. These experimental results demonstrate that random lasing can be efficiently controlled by a light beam which does not interact with the gain molecules, opening a route towards light-controlled random lasers. publishedVersion

Light-controlling-light is one of the most advanced paradigms in optical signal processing, including light-induced waveguides as well as all-optical switching and routing. Other fundamental aspects of all-optical processing are optical memories and sequential elements, which require responses depending on the evolution history of the system, such as the hysteresis stemming from optical multistability. Hereby, we report on optical bistability and hysteresis in cavityless geometries and in the presence of self-localized beams, i.e., spatial optical solitons, exploiting the nonlocal reorientational nonlinearity of nematic liquid crystals (NLC). When the optic axis of NLC is initially orthogonal to the applied electric field, the molecular dipoles start to rotate above a threshold named after the Fréedericksz transition, the latter being usually of the second order. Here, we show that such transition become of the first order via the intrinsic feedback provided by self-focusing, in turn leading to the appearance of a hysteresis loop between diffracting and self-confined beams. We report on hysteresis of the beam size versus input power, as well as hysteresis versus applied voltage at a fixed beam power. Our findings introduce a novel kind of cavity-less optical bistability with propagating light beams and disclose a novel approach to information storage based on light self-localization.

Stiction is one of the major reliability issues limiting practical application of nano-electro-mechanical systems (NEMS), an emerging device technology that exploits mechanical movements on the scale of an integrated electronic circuit. We report on a discovery that stiction can be eliminated by infiltrating NEMS with nematic liquid crystals. We demonstrate this experimentally using a NEMS-based tunable photonic metamaterial, where reliable switching of optical response was achieved for the entire range of nanoscopic structural displacements admitted by the metamaterial design. Being a more straightforward and easy-to-implement alternative to the existing antistiction solutions, our approach also introduces an active mechanism of stiction control, which enables toggling between stiction-free and the usual (stiction-limited) regimes of NEMS operation. It is expected to greatly expand the functionality of electro-mechanical devices and enable the development of adaptive and smart nanosystems.

We have developed a facile method for preparing magnetic nanoparticles which couple strongly with a liquid crystal (LC) matrix, with the aim of preparing ferronematic liquid crystal colloids for use in magneto-optical devices. Magnetite nanoparticles were prepared by oxidising colloidal Fe(OH)(2) with air in aqueous media, and were then subject to alkaline hydrothermal treatment with 10 mol dm(-3) NaOH at 100°C, transforming them into a polydisperse set of domain magnetite nanorods with maximal length ~500 nm and typical diameter ~20 nm. The nanorods were coated with 4-n-octyloxybiphenyl-4-carboxylic acid (OBPh) and suspended in nematic liquid crystal E7. As compared to the conventional oleic acid coating, this coating stabilizes LC-magnetic nanorod suspensions. The suspension acts as a ferronematic system, using the colloidal particles as intermediaries to amplify magnetic field-LC director interactions. The effective Frederiks magnetic threshold field of the magnetite nanorod-liquid crystal composite is reduced by 20% as compared to the undoped liquid crystal. In contrast with some previous work in this field, the magneto-optical effects are reproducible on time scales of months. Prospects for magnetically switched liquid crystal devices using these materials are good, but a method is required to synthesize single magnetic domain nanorods.

Tetraoctylammonium bromide (TOAB)-coated gold nanoparticles dispersed in nematic liquid crystals lead to an enhancement of the nonlinear index coefficient, n2. A small amount of particles is sufficient to increase n2 and two-beam coupling gain by more than an order of magnitude while maintaining high optical quality and the ability of the suspensions to withstand strong applied electric fields. However, TOAB plays a crucial role in increasing the pretilt angle and reducing the anchoring strength, which leads to a stronger modulation of both the reorientation and the refractive index of liquid crystal suspensions.

Dye-doped nematic liquid crystals support random lasing under optical pumping, as well as reorientational optical spatial solitons acting as all-optical waveguides. By synergistically combining these two responses in a collinear pump-soliton geometry, the resulting soliton-enhanced random laser exhibits higher conversion efficiency and better directional properties. After a short account on random lasers and solitons in nematic liquid crystals–nematicons–we describe our experimental results on nematicon-molded random lasers.

We demonstrate a guided-wave random laser configuration by exploiting the coexistence of optical gain and light self-localization in a reorientational nonlinear medium. A spatial soliton launched by a near-infrared beam in dye-doped nematic liquid crystals enhances and confines stimulated emission of visible light in the optically-pumped gain-medium, yielding random lasing with enhanced features.See also erratum at:Appl. Phys. Lett. 110, 019902 (2017); https://doi.org/10.1063/1.4973864

paper, we report on the influence of sub-micron ferroelectric particles on optical, dielectric and electro-optical properties of cholesteric host. An ultra-small fraction of ferroelectric nanoparticles in a cholesteric liquid crystal greatly increases the birefringence of the latter and its dielectric anisotropy that results in a broadening of the reflection band, decreasing of the driving voltage of the switching between bistable textures and in an increasing of its steepness. 1. Introduction helical structure of chiral nematic liquid crystals (cholesterics) brings a lot of fascinating properties. If the helical pitch is comparable to the wavelength of light a distributed optical feedback results in selective light reflection and giant optical activity. Effective control of helical pitch value and chiral structures of cholesterics by external field and temperature makes cholesterics extremely promising for uses in photonic and electro- optic devices, such as photonic crystals, light shutters and switches, adaptive laser-optics and lasing. Effect of electric-field-driven texture transition between planar and focal conic textures makes a base of operation of bistable cholesteric displays (1), future of which looks incredible due to fascinating possibility to make flexible polymer bistable LCD's by roll-to-roll technology (2). Despite substantial advantages, a wide use of cholesteric LCD's is still restrained by high driving voltages and rather low contrast of the devices. Therefore, development of new cholesteric materials is a crucial task for a widespread application of cholesteric LCD's. Recently a new class of LC systems has been proposed - diluted colloids of ferroelectric nanoparticles in nematic matrixes (3-4). These systems macroscopically appear identical to a pure nematics providing, however, strongly enhanced optical and electro-optical properties. In particular, ferroelectric nanoparticles strongly increase dielectric anisotropy and birefringence of the nematic host. Our preliminary studies also showed advanced characteristics of cholesterics doped with ferroelectric nanoparticles. Introducing the particles leads to the decrease of driving voltages, increase of the both reflection contrast and the steepness of the transition (5).

Spontaneous symmetry breaking (SSB) occurs when noise triggers an initially symmetric system to evolve toward one of its nonsymmetric states. Topological and optical SSB involve material reconfiguration/transition and light propagation/distribution in time or space, respectively. In anisotropic optical media, light beam propagation and distribution of the optic axis can be linked, thereby connecting topological and optical SSB. Using nonlinear soft matter, namely uniaxial liquid crystals, we report on simultaneous topological and optical SSB, showing that spatial solitons enhance the noise-driven transition of the medium from a symmetric to an asymmetric configuration, while acquiring a power-dependent transverse velocity in either of two specular directions with respect to the initial wavevector. Solitons enhance SSB by further distorting the optic axis distribution through nonlinear reorientation, resulting in power-tunable walk-off as well as hysteresis in beam refraction versus angle of incidence.

We demonstrate that light refraction at a straight interface between an isotropic dielectric and a nematic liquid crystal can change from positive to negative depending on power. The phenomenon relies on the reorientational response and the all-optical rotation of the optic axis, causing in turn variations in walk-off and beam self-steering.

Optical waveguides are usually fabricated by top-down methods. Hereby, this study demonstrates a bottom-up approach based on nonlinear optics in reorientational nematic liquid crystals with polymerizable materials. Near-infrared optical spatial solitons—nematicons—in conjunction with UV exposure are employed in order to achieve (i) real-time signal waveguiding for point-to-point beam-induced interconnects by effectively quenching the fluctuations of nematicon trajectories through polymer stabilization; (ii) the molding of permanent channel waveguides in a crosslinked polymer network, so that they persist after turning off the soliton beam and confine signals of various wavelengths and intensities. These findings introduce a novel, highly versatile platform and a wealth of possibilities for guided-wave photonics, signal addressing, and processing in liquid crystalline soft matter.

Refraction at a dielectric interface can take an anomalous character in anisotropic crystals, when light is negatively refracted with incident and refracted beams emerging on the same side of the interface normal. In soft matter subject to reorientation, such as nematic liquid crystals, the nonlinear interaction with light allows tuning of the optical properties. We demonstrate that in such material a beam of light can experience either positive or negative refraction depending on input power, as it can alter the spatial distribution of the optic axis and, in turn, the direction of the energy flow when traveling across an interface. Moreover, the nonlinear optical response yields beam self-focusing and spatial localization into a self-confined solitary wave through the formation of a graded-index waveguide, linking the refractive transition to power-driven readdressing of copolarized guided-wave signals, with a number of output ports not limited by diffraction.

The voltage transfer function is a rapid and visually effective method to determine the electrical response of liquid crystal (LC) systems using optical measurements. This method relies on cross-polarized intensity measurements as a function of the frequency and amplitude of the voltage applied to the device. Coupled with a mathematical model of the device it can be used to determine the device time constants and electrical properties. We validate the method using photorefractive LC cells and determine the main time constants and the voltage dropped across the layers using a simple nonlinear filter model.

We experimentally demonstrate a family of compact and efficient IR and THz electro-optical modulators based on active planar metamaterials (metasurfaces) hybridised with liquid crystals.

We investigate optical bistability with light beams propagating in reorientational nematic liquid crystals. In a given interval of input powers, light beams can propagate in either diffracting or soliton states, depending on the “history” of the system. We use biased planar samples exhibiting an abrupt self-focusing response with threshold.

Achieving an efficient spectral tuning in liquid-crystal (LC)-loaded active photonic metamaterials has so far remained a challenge due to strong surface anchoring of LC molecules. This paper reports on a novel approach in the development of hybrid metamaterials that enables to overcome this problem and engage for the first time in-plane switching of liquid-crystal molecules on the nanoscale. Combined with the usual volume switching, it unlocks the full potential of the liquid crystals as a functional component of active metamaterial hybrids operating at optical frequencies. As a result, the resonant response of an active metasurface can now be controlled both in terms of its magnitude and wavelength with the spectral tunability approaching the theoretical limit of 9%. This mechanism of two-way active switching of the hybrid metamaterial is also confirmed theoretically by simulating the distribution of the LC director around the metamaterial fabric.

We investigated bistability with light beams in reorientational nematic liquid crystals. For a range of input powers, beams can propagate as either diffracting or self-trapped, the latter corresponding to spatial solitons. The first-order transition in samples exhibiting abrupt self-focusing with a threshold is in agreement with a simple model.

We experimentally demonstrate intensity and phase modulation of terahertz radiation using actively controlled large-area planar metamaterial (metafilm) hybridized with a 12 um thick layer of a liquid crystal. Active control was introduced through in-plane electrical switching of the liquid crystal, which enabled us to achieve a reversible single-pass absolute transmission change of 20 % and a phase change of 40 deg at only 20 V., 6 pages, 3 figures

We experimentally demonstrate efficient electrical modulation and control of the near-IR response of plasmonic metamaterials loaded with liquid crystals (LC) by exploiting micro-scale volume and (for the first time) nano-scale in-plane LC-switching modes. We also show that in the resulting hybrid optical cell the metamaterial nano-structure can replace all three essential components of an LC device: (i) LC-alignment layer; (ii) transparent electrode and (iii) polarizer; making the hybrid cell more compact than the conventional LC devices and easy to integrate into plasmonic and nano-photonic circuits. The relative ease of on-demand engineering resonant bands (i.e. colours) and tailoring reflection/refraction phenomena in metamaterials is particularly appealing for application in high-resolution and emerging display technologies including near-to-eye and virtual retina displays, holographic and 3D imaging.

We investigate, both theoretically and experimentally, spatial soliton interaction with dielectric interfaces in a strongly anisotropic medium with non-locality, such as nematic liquid crystals. We throw light on the role of refractive index gradients as well as optic axis variations in both voltage- and self-driven angular steering of non-local solitons. We specifically address and then employ in experiments a suitably designed electrode geometry in a liquid crystalline cell in order to define and tune a graded dielectric interface by exploiting the electro-optic response of the material through the in-plane reorientation of the optic axis in two distinct regions. We study both refraction and total internal reflection as well as voltage controlled steering of spatial solitons.

We investigate light self-trapping and nematicon generation in reorientional nematic liquid crystals, addressing the role of phase-front curvature and size of the TEM00 input beam excitation. The experimental results in planar cells are in excellent agreement with a semi-analytical model and numerical simulations. These findings can substantially aid characterization and generation of spatial solitons in liquid crystals and be readily generalized to other nonlocal nonlinear media and soft matter systems.

Spectroscopic ellipsometry is a technique especially well suited to measure the effective optical properties of a composite material. However, as the sample is optically thick and anisotropic, this technique loses its accuracy for two reasons: anisotropy means that two parameters have to be determined (ordinary and extraordinary indices) and optically thick means a large order of interference. In that case, several dielectric functions can emerge out of the fitting procedure with a similar mean square error and no criterion to discriminate the right solution. In this paper, we develop a methodology to overcome that drawback. It combines ellipsometry with refractometry. The same sample is used in a total internal reflection (TIR) setup and in a spectroscopic ellipsometer. The number of parameters to be determined by the fitting procedure is reduced in analysing two spectra, the correct final solution is found by using the TIR results both as initial values for the parameters and as check for the final dielectric function. A prefitting routine is developed to enter the right initial values in the fitting procedure and so to approach the right solution. As an example, this methodology is used to analyse the optical properties of BaTiO(3) nanoparticles embedded in a nematic liquid crystal. Such a methodology can also be used to analyse experimentally the validity of the mixing laws, since ellipsometry gives the effective dielectric function and thus, can be compared to the dielectric function of the components of the mixture, as it is shown on the example of BaTiO(3)/nematic composite.

We study the interaction of a spatial soliton waveguide with a voltage defined and electrically tuned interface in nematic liquid crystals, whereby the optic axis is reoriented through the use of patterned electrodes. We investigate refraction and total internal reflection of nematicon wavepackets, disclosing the role of anisotropy and obtaining a remarkable in-plane steering as large as 55°.