Showing papers by "Iam-Choon Khoo published in 2007"
TL;DR: Numerical results show that the index of refraction for the proposed structure can be changed over the range from ?
Abstract: Near-infrared metamaterials that possess a reconfigurable index of refraction from negative through zero to positive values are presented. Reconfigurability is achieved by cladding thin layers of liquid crystal both as a superstrate and a substrate on an established negative-index metamaterial, and adjusting the permittivity of the liquid crystal. Numerical results show that the index of refraction for the proposed structure can be changed over the range from -1 to +1.8 by tuning the liquid crystal permittivity from 2 to 6 at a wavelength of 1.4 μm.
187 citations
TL;DR: Alkanethiol-capped gold nanoparticles dispersed in n-dodecane studied by spectroscopic ellipsometry and were modeled using Mie scattering theory in good agreement with the theoretical expectation that such dispersed plasmonic nanoparticles can act as low or tunable refractive index materials at specific optical wavelengths.
Abstract: Alkanethiol-capped gold nanoparticles dispersed in n-dodecane were studied by spectroscopic ellipsometry and were modeled using Mie scattering theory The refractive index in the visible and near-infrared depended on the volume fraction of gold nanoparticles, in good agreement with the theoretical expectation that such dispersed plasmonic nanoparticles can act as low or tunable refractive index materials at specific optical wavelengths
157 citations
TL;DR: In this paper, a full-wave analysis technique based on the finite element-boundary integral method is developed and used to rigorously treat the scattering from periodically structured metamaterials incorporating anisotropic liquid crystals (LCs) and dispersive materials.
Abstract: A full-wave analysis technique based on the finite element-boundary integral method is developed and used to rigorously treat the scattering from periodically structured metamaterials incorporating anisotropic liquid crystals (LCs) and dispersive materials. Reconfiguration of the negative-index metamaterials is achieved by controlling the magnetic resonance via tuning permittivity of the embedded anisotropic LCs. Numerical results show that the refractive index of the metamaterials can be reconfigured by tuning the director orientation of anisotropic LCs or by using temperature-dependent LCs. The design configurations and their characteristics in the near- and the mid-infrared ranges are presented.
137 citations
TL;DR: In this article, the authors present a theoretical analysis and experimental demonstration of optically self-activated polarization switching process in a 90° twist alignment dye-doped nematic liquid crystal, and demonstrate the feasibility of designing efficient low threshold polarization and fast switching and optical limiting devices for cw and microsecond lasers.
Abstract: The authors present a theoretical analysis and experimental demonstration of optically self-activated polarization switching process in a 90° twist alignment dye-doped nematic liquid crystal. The underlying mechanisms are laser induced dye-assisted director axis reorientation and order parameter modifications. Experimental observations are in good agreement with the theoretical expectations derived from a modified Jones matrix calculation, and demonstrate the feasibility of designing efficient low threshold polarization and fast switching and optical limiting devices for cw and microsecond lasers.
26 citations
TL;DR: In this article, the authors present a review of recent progress in the studies of the nonlinear and electro-optics of liquid crystals, particularly in their meta-material forms, and obtain an analytical expression for the "ultimate" optical nonlinearity of nematic liquid crystals.
Abstract: We present a review of recent progress in the studies of the nonlinear- and electro-optics of liquid crystals, particularly in their meta-material forms. An analytical expression for the "ultimate" optical nonlinearity of nematic liquid crystals is obtained, and several routes to realizing such optical nonlinearities are discussed. We also describe two approaches for realizing tunable or reconfigurable negative-zero-positive index materials: (1) planar nano-structured frequency selective surfaces [FSS] containing nematic liquid crystals; (2) core-shell nano-spheres randomly distributed in bulk nematic liquid crystal matrix. Such metamaterials can be designed for applications in the visible-infrared, as well as Terahertz and microwave regimes. These liquid crystalline meta-materials are capable of supra-nonlinearities characterized by refractive index changing coefficients of over 1 cm2/watt and microseconds response times.
6 citations
TL;DR: In this paper, the optical properties of gold and silver nanoparticles dispersed in different organic liquids in the visible to near-IR range were investigated using spectroscopic ellipsometry and a reasonable agreement with theoretical results based on Mie scattering was obtained.
Abstract: Metamaterials are of substantial current interest because they may exhibit unusual and/or configurable optical responses We studied the optical properties of gold and silver nanoparticles dispersed in different organic liquids in the visible to near-IR Calculation of the refractive indices of metallic nanospheres or metallic-coated silica spheres in liquid crystals show the possibility of tuning and varying the refractive index by reorientation of the liquid crystal molecules Measurements of the refractive indices of gold nanoparticles in dodecane were experimentally studied by using spectroscopic ellipsometry and a reasonable agreement with the theoretical results based on Mie scattering was obtained Finally, the effect of gold and silver nanospheres on the nonlinear absorption properties of an organic liquid (L34, a 4,4'-dialkyl phenyleneethynylene) was studied The results suggest that metallic nanoparticles dispersed in a host organic fluids can be good materials for fabrication of low and tunable index materials in the visible to near-IR wavelength range, and for the enhancement of the nonlinear absorption of liquids used in switching applications
5 citations
TL;DR: In this article, the dispersion of nanoparticles in a nematic liquid crystal host medium provides an efficient way of designing tunable optical metamaterials that respond from the visible to the Terahertz and microwave regime.
Abstract: The dispersion of nanoparticles in a nematic liquid crystal host medium provides an efficient way of designing tunable optical metamaterials that respond from the visible to the Terahertz and microwave regime. As studied in this article, these systems exhibit much larger birefringence than the nematic host and possess sub-unity, zero, and even negative refractive indices with unique reflection and transmission properties. These tunable optical metamaterials will enable a wide range of application possibilities in next-generation reflective, transmissive, modulation and switching elements and devices.
4 citations
TL;DR: In this article, a class of liquid crystalline photonic metamaterials that exhibit tunable negative-zero-positive refractive indices was described and incorporated into a gain medium such as laser dye.
3 citations
08 Feb 2007
TL;DR: In this paper, a nonlinear optical metamaterial formed by randomly dispersing coated core-shell nano-spheres in aligned nematic liquid crystals was described, which exhibited effective refractive indices ranging from negative, through zero to positive values.
Abstract: We described a nonlinear optical metamaterial formed by randomly dispersing coated core-shell nano-spheres in
aligned nematic liquid crystals. Such material will exhibit effective refractive indices ranging from negative, through
zero to positive values. The nano-spheres dispersed liquid crystal possesses an effective birefringence that is larger than
the natural birefringence of the host liquid crystal, which effectively enhances the nonlinear optical response associated
with optical field induced director axis reorientation.
2 citations
TL;DR: Stimulated Orientational Scattering (SOS) as mentioned in this paper uses the angular reorientation of the director axis in liquid crystals to produce cross-polarized light amplification, similar to photorefractivity.
Abstract: Stimulated Orientational Scattering (SOS) uses the angular reorientation of the director axis in liquid crystals to produce
cross-polarized light amplification. Akin to photorefractivity, SOS uses grating formation and the resulting phase-matching
to scatter incident radiation into a coherent, cross-polarized signal beam. This paper provides a brief review of
the theory underlying SOS, a discussion of the simulation of SOS dynamics, and empirical results of the SOS effect
acting in a thin film (300 μm) planar sample of the liquid crystal E7 induced by an Argon ion laser at a wavelength of
488 nm.
1 citations
TL;DR: In this article, the authors present a theoretical model and some experiment demonstrations of all-optical passive switching processes with 90° twist-aligned nano-doped nematic liquid crystal cells sandwiched between two crossed polarizers.
Abstract: We present a theoretical model and some experiment demonstrations of all-optical passive switching processes with 90° twist-aligned nano-doped nematic liquid crystal cells sandwiched between two crossed polarizers. The photosensitive dopants give rise to laser induced dye-assisted director axis reorientation and order parameter modifications, which in turm produce an intensity dependent polarization switching and hence a transmission modulation capability. Experimental observations are in good agreement with our expectation derived from modified Jones matrix analysis and also demonstrate the feasibility of an efficient [microwatt power] low threshold polarization and fast switching [microseconds] all optical limiting device for visible as well as infrared lasers or bright light sources.
09 Jun 2007
TL;DR: In this paper, a near-IR metamaterial with a tunable index of refraction from negative, through zero, to positive values has been proposed, where liquid crystal layers are incorporated both as a superstrate and a substrate onto a conventional NIM.
Abstract: A near-IR metamaterial having a tunable index of refraction from negative, through zero, to positive values has been proposed. Liquid crystal layers are incorporated both as a superstrate and a substrate onto a conventional NIM. Tuning the permittivity of the liquid crystal layers controls the value of the effective permittivity in an averaging manner. Combined with proper magnetic resonances provided by a pair of silver strips separated by a thin layer of alumina, the effective index of refraction can be reconfigured between negative, zero, and positive values. The ability to vary the permittivity of the liquid crystal layers also provides a method to control the bandwidth of the negative index behavior.
15 Oct 2007
TL;DR: In this paper, the reflection and transmission coefficients in the negative index materials were derived based on the finite differential time domain approach, where the reflection coefficients were obtained depending on permittivity and permeability at a fixed frequency.
Abstract: We will discuss the reflection and transmission coefficients in the negative index materials. Based on the finite differential time domain approach, we obtain the reflection and transmission coefficients depending on permittivity and permeability at a fixed frequency. The transmissivity and reflectivity will be also examined. Simulation results are in good agreement with the theoretical ones. This technique can be easily extended to the multi-layered structure with negative index materials to design the optical system.
30 Jul 2007
TL;DR: In this paper, the feasibility of realizing liquid crystalline supra-nonlinear optical metamaterials with tunable negative-zero-positive refractive indices and nonlinear coefficient far exceeding 1 cm2/W in the visible - infrared regime was demonstrated.
Abstract: We demonstrate by theory and experiments the feasibility of realizing liquid crystalline supra-nonlinear optical metamaterials with tunable negative-zero-positive refractive indices and nonlinear coefficient far exceeding 1 cm2/W in the visible - infrared regime.
23 Jul 2007
TL;DR: In this paper, a nonlinear neat organic liquid (L34) in bulk or wave-guided [fiber core] structure was proposed for all-optical switching in an extremely wide time scales spanning sub-nanoseconds to cw regime.
Abstract: In this presentation, we describe another promising organic material that enables all-optical switching in an extremely wide time scales spanning sub-nanoseconds to cw regime - a nonlinear neat organic liquid (L34) in bulk or wave-guided [fiber core] structure. In the visible spectrum [400-700 nm], quantitative z-scan and pump-probe techniques have shown that in the sub-nanosecond time scales, the dominant nonlinear absorption processes are two-photon coupled to excited state absorption processes characterized by an intrinsic two photon absorption coefficient of ~5 cm/GW, and an intensity dependent effective two-photon absorption coefficient that can be over 200 cm/GW. For longer time scales, the transparency of the liquid allows doping with appropriate absorbers to generate efficient thermal/density and nonlinear scattering effects [as well as guided mode extinction in fiber geometry] for all-optical switching operations. Accordingly, one may envision single material constituent device capable of all-time scale optical switching applications.
09 Jun 2007
TL;DR: In this article, the authors present a method to realize the index of refraction tuning of a metamaterial consisting of LC-dispersed coated spheres, which can be used to achieve tunability in a variety of different terahertz and optical devices.
Abstract: Nematic LC's offer an excellent path toward achieving tunability in a variety of different terahertz and optical devices. We have presented technology that can take advantage of the properties of LC to realize the index of refraction tuning. Metamaterials consisting of LC-dispersed coated spheres were designed to have an index of refraction that tuned from negative to zero to positive at the frequency of operation.