Andres Diaz

Tunable Frequency Selective Surfaces and Negative-Zero-Positive Index Metamaterials Based on Liquid Crystals

We utilize the properties of aligned nematic liquid crystal (LC) cells in the design of: (i) a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values and (ii) micron-scale metallodielectric and all-dielectric tunable frequency selective surfaces (FSSs). The metamaterial is constructed by randomly doping a liquid crystal substrate with coated dielectric (non-magnetic) spheres and can be utilized over a large spectral range. FSSs with a liquid crystal superstrate are synthesized using conventional and genetic algorithm methods to exhibit broadband tunable filter characteristics at mid-infrared (mid-IR) wavelengths. These LC tunable FSS structures can be used to develop a new class of infrared/optical switches for terahertz applications.

Nonlinear-absorbing fiber array for large-dynamic-range optical limiting application against intense short laser pulses

A complete quantitative description of a nonlinear fiber array for optical limiting application against laser pulses in the picosecond–nanosecond regime is presented. We discuss the dynamics of the molecular photonic processes accompanying the propagation of a laser pulse through the fiber core made of materials that possess reverse saturable absorption, two-photon absorption (TPA), and excited-state absorption (ESA), and we detail the optical limiting effectiveness and limitations of these nonlinear absorption processes individually and in concert. In particular, we demonstrate the importance of excited-state population recycling in extending the dynamic range of the limiting action. Experimental results obtained from a particular fiber core material that possesses TPA and ESA show good agreement with theoretical expectations and demonstrate the optical limiting capability of such a nonlinear fiber array.

Liquid-Crystals for Tunable Photonic Crystals, Frequency Selective Surfaces and Negative Index Material Development

We present the results of experimental and theoretical studies that demonstrate the feasibility of realizing electro-optical switching, frequency filtering and negative index of refraction. In liquid crystal infiltrated TiO2 inverse opal 3-D photonic crystals, we observed a sizeable electrical tuning of the Bragg reflection peak. In frequency selective structures with a liquid crystal over-layer, the tuning range can be as wide as 380 nm, operational throughout the entire visible – far infrared spectra range. Furthermore, some of the structures can be designed to exhibit a negative index of refraction property over a predetermined frequency interval.

Patterns of nocturnal rehydration in root tissues of Vaccinium corymbosum L. under severe drought conditions

Although roots in dry soil layers are commonly rehydrated by internal hydraulic redistribution during the nocturnal period, patterns of tissue rehydration are poorly understood. Rates of nocturnal rehydration were examined in roots of different orders in Vaccinium corymbosum L. ‘Bluecrop’ (Northern highbush blueberry) grown in a split-pot system with one set of roots in relatively moist soil and the other set of roots in dry soil. Vaccinium is noted for a highly branched and extremely fine root system. It is hypothesized that nocturnal root tissue rehydration would be slow, especially in the distal root orders because of their greater hydraulic constraints (smaller vessel diameters and fewer number of vessels). Vaccinium root hydraulic properties delayed internal water movement. Even when water was readily available to roots in the wet soil and transpiration was minimal, it took a whole night-time period of 12 h for the distal finest roots (1st to 4th order) under dry soil conditions to reach the same water potentials as fine roots in moist soil (1st to 4th order). Even though roots under dry soil equilibrated with roots in moist soil, the equilibrium point reached before sunrise was about –1.2 MPa, indicating that tissues were not fully rehydrated. Using a single-branch root model, it was estimated that individual roots exhibiting the lowest water potentials in dry soil were 1st order roots (distal finest roots of the root system). However, considered at the branch level, root orders with the highest hydraulic resistances corresponded to the lowest orders of the permanent root system (3rd-, 4th-, and 5th-order roots), thus indicating possible locations of hydraulic safety control in the root system of this species.

Dynamics of cross-polarization-stimulated orientation scattering in a nematic liquid-crystal film

We have studied the dynamic evolution of stimulated cross-polarization scattering in a nematic liquid-crystal film. The effect is initiated by two-beam coupling between a cw incident polarized laser and its orthogonally polarized multifrequency noise components scattered by fluctuations in the director axis. Complex time- and intensity-dependent oscillations and energy exchanges among the pump and stimulated waves were observed. Imparting an ac modulation to the film further amplified the characteristic oscillations while it suppressed the background noise. The conditions for stimulated laser generation and the observed frequency spectra of the intensity oscillations are in good agreement with a quantitative theory that accounts for the temporal-spatial dependence of the wave mixing process.

Nonlinear molecular photonics and controlled laser transmission through a nonlinear fiber array

We have performed a critical analysis of optical limiting materials exhibiting the two typical absorption mechanisms of reverse saturable absorption (RSA) and two-photon absorption (TPA). The limiting characteristics of beam propagation in nonlinear liquids that exhibit RSA and TPA are discussed, and guidelines for materials and parameter optimization that will result in practical large dynamic range devices are given.

Refractive index engineering of liquid crystals with nano-particulates

We study the enhancement and control of the refractive index of liquid crystals with dispersed gold and silver nanoparticles. The maximum obtainable variation in the real and imaginary parts of the effective refractive index of the solution by reorientation of the liquid crystal molecules is calculated, and the results obtained with gold and silver nanospheres are compared. The effect of size, concentration, and composition of the nano-particulates (solid spheres vs. silver- or gold-coated silica nanoshells) on the refractive index and its wavelength dependence is also considered.

Nano-spheres dispersed nematic liquid crystals for broadband tunable negative-zero-positive index materials

It is demonstrated that by introducing gain and nano-spheres in aligned nematic liquid crystals, one can design metamaterials which possess broadband tunable negative-zero-positive index with relatively low loss compared to other material systems.

Nanosecond-cw visible-IR all-optical switching and nonlinear transmission with nonlinear organic optical liquids and liquid crystals

We present the results of nonlinear transmission in various ordered and disordered mesophases of liquid crystals, and demonstrate that in bulk or guided wave geometry, they are capable of clamping the transmission of pulsed or cw lasers to below the Maximum Permissible Exposure level of eyes and optical sensors in the entire visible - infrared region.

Dynamic polarization switching and stimulated orientational scattering in nematic liquid crystals

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.