In this study, the optical nonlinearity of LCs with cell substrates coated with gold-nanoparticle (AuNP) -doped PVA alignment layers were examined using the Z-scan technique. The results show that the nonlinear refractive index n2 of the sample is enhanced by the gold nanoparticles doped in the alignment layers, because of the thermal effect of the absorption by the surface of the sample through the localized surface plasmon resonance (LSPR) of the gold nanoparticles. As the concentration of AuNPs in the alignment layers of the LC sample increases, the thermal effect of the LSPR increases, and |n2| observably increases. Furthermore, the self-defocusing effect (n2<0) of the sample can be modulated by the application of an external voltage, and a self-focusing effect (n2<0) can be observed when samples are illuminated by a high-intensity laser with the application of a high voltage. Therefore, the magnitude and the sign of n2 of the sample can be modulated by combining the applied electric field and the optical field.
Two-dimensional (2-D) square lattice (SL) photonic crystals (PCs) are fabricated and their optical/electro-optical
properties are studied. The PCs are based on polymer-dispersed liquid crystals (PDLC) that are formed using twobeam
interference with double-exposures. The PC structure that is observed using a scanning electron microscope
(SEM) matches with the calculated interference pattern. The results of optical/electro-optical studies demonstrate
that superprism and negative refraction effects occur at certain incident angles over a range of frequencies, and are
consistent with the simulated ones. Moreover, the negative refraction efficiency is electrically controllable.
This work studies photo-induced reorientation in azo-dye-doped liquid crystal (ADDLC) films by observing the
diffraction patterns produced by self-phase modulation (SPM). The experimental results show that not only the
magnitude but also the sign of the refractive index change (Δn) of an ADDLC sample pumped by a green laser can be
modulated by the beam intensity (IG). The reason is that different IG give rise to different trans/cis concentration ratios,
and then induce different overall torques on LC molecules. The trans and cis isomers contribute negative and positive
torques, respectively, that reorient LC molecules perpendicular and parallel to the polarization of the pump laser. The
reorientation effect in the ADDLC sample illuminated by the biphotonic lasers (green and red lasers) is also investigated
by observing the self-phase modulation diffraction pattern. The results indicate that red light can induce |Δn| to decrease
and increase at low and high IG, respectively. Besides, the
dye-induced torque can be changed from negative to positive
by adding red light. This is due to the fact that cis isomers produce an extra positive torque to the LCs after absorbing red
light.
This paper proposes an optically tunable focal intensity microlens array (MLA) by using a focusing unit with
birefringent liquid crystalline polymer (LCP) and a tuning unit with photoalignment for controlling the polarization state
of the incident light. Due to the different refractive indices of LCP, it acts as a positive or negative microlens with
respect to the polarization state. The resultant tunable focal intensity MLA shows the fast optical switching time without
voltage and the multi-stable characteristics.
This study describes an electrically tunable two-dimensional (2D) liquid crystal (LC) holographic polarization
grating (HPG). It is a twisted nematic (TN) grating array, which is obtained by orthogonally overlaying two
crossed 1D polarization holograms. A 1D polarization hologram has a rotating linear polarization pattern,
generated by the interference of two orthogonal circularly (OC) polarized beams on a substrate that is coated
with an azo-dye doped PVA layer. The images under a polarized optical microscope (POM) and the diffraction
patterns from the 2D grating are simulated using the Jones matrix and a Fourier transformation. The
experimental results agree with the simulated results. This work represents a substantial advance toward the
realization of highly functionalized passive optical devices in which both the beam propagation direction and the
polarization state can be controlled in two dimensions.
The amplitudes of terahertz radiation are measured for a series of GaAs surface intrinsic-n+ (SIN+) structures with
various built-in surface electric fields as the bias. As the surface field is lower than the so-called "critical electric field"
related with the energy difference between the &Ggr; to L valley of the semiconductor, the amplitude is proportional to the
product of the surface field and the number of photo-excited carriers. As the intensity of surface field exceeds the critical
field, the THz amplitude is independent of the surface field but proportional the number of the photo-excited carriers.
Our study proposed two optimal conditions for an SIN+ structure to serve as a THz emitter: the width of its intrinsic layer
is nearly equal to the penetration depth of the pump beam, and the intensity of built-in electric field is nearly equal to the
critical electric field. Notably, the critical field determined from the THz amplitude under various electric fields provides
one way to estimate the &Ggr; to L valley splitting in semiconductors.
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