Rigorous broadband investigation of liquid-crystal plasmonic structures using finite-difference time-domain dispersive-anisotropic models (original) (raw)

Modeling of metallic nanostructures embedded in liquid crystals: application to the tuning of their plasmon resonance

Optics Letters, 2009

We numerically investigate arrays of metallic nanoparticles deposited on a glass substrate and covered by a liquid-crystal material. Extinction spectra at normal incidence are computed using the finite-difference time-domain method, and we show that by rotating the optical axis around an axis orthogonal to the main direction of illumination, it is possible to tune the resonance of the system according to a simple law. The spectral width of the tunability is studied as a function of different parameters.

FDTD Modeling of Gold Nanoparticles in a Nematic Liquid Crystal: Quantitative and Qualitative Analysis of the Spectral Tunability

The Journal of Physical Chemistry C, 2010

In this paper we numerically investigate an active plasmonic device that is based on an ordered array of gold nanocylinders with an elliptic basis embedded in a nematic liquid crystal (LC) layer. The localized surface plasmon resonance (LSPR) of the gold cylinders depends on the refractive index of the surrounding media, which depends on the orientation of the optical axis. Rotating the optical axis causes the LSPR to shift, and we will show that by modifying geometrical parameters we can increase the sensitivity of the LSPR to the orientation of the optical axis and thereby enhance the switching effects.

Theoretical Study of the Anchoring Influence on Plasmonic Resonance Tunability of Metallic Nanoparticles Embedded in a Liquid Crystal Cell

Plasmonics, 2013

In this paper, the localized surface plasmon resonance (LSPR) peak position of an ordered gold nanoparticles array embedded in a nematic liquid crystal (LC) media is investigated using finite-difference time-domain method. The influence of the anchoring effects between nematic LC molecules and glass substrate on the shift of LSPR wavelength is taken into account, and results are compared with the case of a perfect alignment of the LC molecules. Keywords FDTD • Liquid crystal • Plasmon resonance Liquid crystals (LCs) are an outstanding material as a surrounding environment for metallic nanoparticles, owing to their anisotropy. Indeed, by applying an external electric field, the environment of the particle can be modified, leading to a localized surface plasmon resonance (LSPR) shift [1]. Using LCs in conjunction with gold gratings [2] or nanoparticles, for example, gold nanorods [3, 4], gold nanodots, and nanodisks [5-7], gold dimer [8], and nanohole arrays [9, 10] have been reported experimentally. Furthermore, some papers have reported numerical investigation of metallic arrays or particles covered by nematic LCs [11-15]. But all of these works neglect the anchoring phenomenon at the interface of LC molecules and glass substrates or consider that a weak anchoring condition is met.

Anisotropic shift of surface plasmon resonance of gold nanoparticles doped in nematic liquid crystal

Optics express, 2014

Study of the liquid crystal (LC) director around nanoparticles has been an important topic of research very recently, since it allows design and fabrication of next-generation LC devices that are impossible in the past. In our experiment, alkanethiol-capped gold nanoparticles (GNPs) were dispersed in nematic LC. Analysis of the LC director around GNPs was performed by investigating the behavior of surface plasmon polariton (SPP) absorption peaks of the GNPs using spectrophotometry technique. It is found that the incident linearly polarized light orientated at 0°, 45°, and 90° angles with respect to the rubbing direction experiences varying interaction with the LC medium. The corresponding transmission of light reveals the anisotropic shift in wavelength of SPP peak. The anisotropic behavior of SPPs of the GNPs is in agreement with theoretical calculations.

Numerical modelling of light propagation in surface plasmon resonance sensor with liquid crystal

Condensed Matter Physics, 2018

The five-layer nanorod-mediated surface plasmon sensor with inhomogeneous liquid crystal layer was theoretically investigated. The reflectance as the function of the incident angle was calculated at different voltages applied to the liquid crystal (LC) for different analyte refractive indices. By changing the LC director orientation one can control the position of the reflective dips and choose the one that is the most sensitive to the analyte refractive index. At the chosen angle of incidence, the analyte refractive index can be found from the reflectance value. The director reorientation effect is stronger when the prism refractive index is between ordinary and extraordinary refractive indices of the LC. In this case, the voltage increase and the prism refractive index decrease have a similar effect on the reflectance features.

Liquid Crystals as an Active Medium: Novel Possibilities in Plasmonics

Nanospectroscopy, 2015

LCs show advantages compared to pure LCs because of their improved electro-optical characteristics due to enhanced photoluminescence [1], reduced driving voltage , improved order parameter and dielectric anisotropy , fast response time and spontaneous vertical LC alignment . On the other hand, if NPs are considered for their plasmonic properties, they can find in LCs a convenient host that can promote their nanoscale self-organization. Depending on the considered phase (nematic, cholesteric or smectic), LCs show short or longrange order. By accurately matching the capping material of the NPs, their size and shape with the right LC host it is possible to induce NPs to follow this order and hence achieve self-organization . Even if noticeable, this is not the only advantage of using LCs as a host for NPs. Being birefringent, LCs represent a possible way for realizing optical metamaterials with tunable functionalities: indeed, according to Mie's theory, the localized surface plasmon resonance (LSPR), typical of NPs, depends on the refractive index of the medium surrounding them . For LCs, this value can be finely controlled in a broad interval (typically from 1.5 to 1.7) by means of external stimuli (electric fields or by temperature changes). When combining plasmonic NPs with LCs, depending on the chosen LC mesophase, many configurations emerge. In the following, we will consider several of them in detail, providing evidence for advantages and potential drawbacks. Considering these configurations, the main distinction derives from the way plasmonic subunits are put in contact with the LC phase. So that, we identify systems where metal nanoparticles are immobilized on a substrate and layered with LCs or the case where NPs are homogeneously dispersed in the bulk of the mesophase. Even if extensive, this review is not conceived to be exhaustive: many configurations have already been exploited but many more are still to come, which explains the wide interest demonstrated by the scientific community to this topic.

High contrast modulation of plasmonic signals using nanoscale dual-frequency liquid crystals

Optics …, 2011

We have designed and simulated a dual-frequency liquid crystal (DFLC) based plasmonic signal modulator capable of achieving over 15 dB modulation depth. The voltage-controlled DFLC is combined with a groove and slit configuration and its operation is discussed. Using the finitedifference time domain (FDTD) method, simulations were conducted to discover the groove-slit separation distance that enabled a practically useful modulation depth for the two states of the DFLC. Moreover, we have shown that significant improvement in modulation depth can be achieved by addition of a second groove to the design structure. Additionally, a performance analysis indicates a switching energy on the order of femtojoules and a switching speed on the order of 100 microseconds. Results of this investigation can be useful for the future design, simulation, and fabrication of DFLC-based plasmonic signal modulating devices, which have application in electro-optical and all-optical information systems.

Tunable Plasmonic Sensor with Metal-Liquid Crystal-Metal Structure

IEEE Photonics Journal, 2015

A new tunable plasmonic biosensor is proposed by embedding liquid crystals in the slit region of a corrugated plasmonic nanoslit array. The absorption spectrum of the structure and, thus, the sensing wavelength can be controlled by applying voltage to the electrode parts of the device. The interaction between the waveguide mode inside the slit and the surface plasmon polaritons on the corrugation surface can be modified using the electrooptical effect of the liquid crystals. Our numerical simulations with the finite-difference time-domain (FDTD) method reveal a large tuning of the absorption spectrum up to 100 nm. In addition to the tunability feature, the structure has a high sensitivity of 570 nm/RIU. The sensing performance of the device is evaluated by performing homogeneous biochemical sensing simulations. The special feature of the proposed structure gives it an opportunity to be used as an efficient element in integrated photonics circuits for miniaturization and tuning purposes.

Plasmonic Resonance Tunability and Surface-Enhanced Raman Scattering Gain of Metallic Nanoparticles Embedded in a Liquid Crystal Cell

The Journal of Physical Chemistry C, 2013

The localized surface plasmon resonance positions and the surface-enhanced Raman scattering (SERS) gain (G SERS) of an ordered array of gold nanocylinders with elliptic basis, embedded in a nematic liquid crystal (LC) media, are investigated using the finite-difference time-domain method. Structures are designed so that two resonances occur. The influence of the anchoring effects between nematic LC molecules and glass substrate on the shift of the double-resonance wavelengths is taken into account, and results are compared with the case of a perfect alignment of the LC molecules and an isotropic material.

Controlling Plasmon Resonance of Gold and Silver Nanoparticle Arrays with Help of Liquid Crystal

Photonics

The tunability of plasmonic resonances in gold and silver nanosphere arrays on a glass substrate, embedded in a liquid crystal matrix, was explored. The calculations involving the finite element method revealed that the optical properties of these arrays can be modulated by reorienting the liquid crystal. When the liquid crystal director was reoriented between planar and homeotropic configurations in the plane containing the incident wave polarization vector, the plasmonic resonance wavelength shifted within an approximately 100 nm range. A reduced shift of about 40 nm was observed when the reorientation occurred in the plane perpendicular to the polarization. Both metal nanosphere arrays showed notable near-field amplification. Gold achieved up to 18 times the amplification of the incident wave electric field, while silver reached 16 times but showed a remarkable 40 times amplification at the inter-band transition resonance wavelength. This research underscores the potential of usi...