G. B. Hadjichristov

Gradient polymer-disposed liquid crystal single layer of large nematic droplets for modulation of laser light

The light modulating ability of gradient polymer-disposed liquid crystal (PDLC) single layer of large droplets formed by nematic E7 in UV-cured polymer NOA65 is studied. Operating at relatively low voltages, such PDLC film with a of thickness 10-25 μm and droplet size up to 50 μm exhibits a good contrast ratio and is capable of producing a large phase shift for the propagating coherent light. For a linearly polarized He-Ne laser (λ=633 nm), an electrically commanded phase shift as large as π/2 can be obtained by the large-droplet region of the film. The electrically produced phase shift and its spatial profile controlled by the thickness of the gradient PDLC single layers of large nematic droplets can be useful for tunable spatial light modulators and other devices for active control of laser light.

Characterization of Chemical Bonding in Ion-Implanted Polymers by Means of Mid-Infrared Reflectivity:

An optical approach for structural characterization of the modified surface layer in ion-implanted polymers is proposed. The mid-infrared reflectivity from the implanted surface is analyzed in terms of an oscillator dispersion model combined with the theory of differential reflection spectroscopy. The degree of destruction of a specific chemical bond is determined by the relative drop of the oscillator strengths associated with the corresponding vibrational modes. As an example, this methodology is applied to poly(methylmethacrylate) (PMMA) implanted with 50 keV silicon ions at fluences in the range 3 × 1014 to 1 × 1017 ions/cm2. The scission rates for the C=O, C–O–C, and C–H bonds, as well as the static dielectric constant of the ion-modified material, are calculated as a function of the ion fluence. Further, a lower-limit estimate of 120 nm for the thickness of the ion-modified layer is obtained.

Electro-optical response of polymer-dispersed liquid crystal single layers of large nematic droplets oriented by rubbed teflon nanolayers

The surface orienting effect of rubbed teflon nanolayers on the morphology and electro-optical (EO) response of polymer-dispersed liquid crystal (PDLC) single layers of large nematic droplets was studied experimentally. In PDLC composites of the nematic liquid crystal (LC) E7 and NOA65 polymer, single droplets of LC with diameters as larger as 10 μm were confined in layers with a thickness of 10 μm, and the nematic director field was efficiently modified by nanostructuring teflon rubbing of the glass plates of the PDLC cell. For layered PDLCs arranged and oriented in this way, the modulated EO response by the dielectric oscillations of the nematic director exhibits a selective amplitude-frequency modulation controllable by both temperature and voltage applied, and is simply related to the LC droplet size. That may be of practical interest for PDLC-based modulators operating in the infrasound frequency range.

Two-photon resonance four-wave mixing spectroscopy in polar media

The effect of the permanent electric-dipole moments of polar media on the two-photon resonance four-wave mixing (TPR FWM) as described by chi (3)( omega 4= omega 1- omega 2+ omega 3) is studied. The spectra corresponding to different processes of TPR FWM are modelled for a three-level system. It is demonstrated that the difference- and sum-frequency TPR FWM spectroscopy is sensitive to the difference between permanent dipole moments of the two levels coupled by the TPR.

Kramers-Kronig relations in FWM spectroscopy

The Kramers-Kronig (K-K) relations are verified for the third-order non-linear optical susceptibilities which are described by known expressions, obtained by the density matrix formalism. Possible applications of the K-K relations in non-linear Raman spectroscopy techniques are also discussed. Coupling of the real and imaginary parts of CARS excitation and probe profiles, as well as a relation between the spectra of two variants of OHD-RIKE are demonstrated. Some cases are shown when the susceptibility does not obey the K-K relations. This result is not a consequence of a break of the causality principle.

Electro-optically responsive composites of gold nanospheres in 5CB liquid crystal under direct current and alternating current joint action

Direct current (DC) electro-optical (EO) control of transmitted laser beam intensity based on EO controlled coherent light scattering and diffraction by stationary longitudinal texture pattern (LTP) is achieved in planar-oriented cells with a composite mixture of polymer-coated gold spherical nanoparticles (Au-NPs) with a mean diameter of about 12 nm and the room-temperature nematic pentylcyanobiphenyl (5CB). At relatively low DC voltage of about 5 V, the effective scattering/diffraction by Au-NPs/5CB composites leads to a spatial spreading of transmitted coherent light from a low-power continuous wave laser beam, resulting in a drastic reduction of its local intensity. The effect is polarization dependent and is strongest when the polarization of the input laser beam is along the LTP. The EO response of Au-NPs/5CB mixtures is studied under DC and alternating current (AC) joint action with the aim of the potential use of these composite materials as EO controlled diffusers. The specific V-shaped sharp dip...

Single-Layered PDLC for Diffractive Optics

The electrically and spatially controllable coherent light diffraction by single-layered polymer-dispersed liquid crystal (PDLC) films is studied. Particularly, a wedge-formed thin film of E7/NOA65 microscale PDLC is examined. The PDLC single layer of thickness varying from a few micrometers to 25 μm contains liquid crystal (LC) droplets with double-truncated spherical shapes and a linear-gradient size distribution along the film length. The single-layer arrangement and compact packing of the micrometer-sized PDLC structure render the ellectrically-commanded coherent light diffraction controlled by LC/polymer interface.

Ion-implanted polymethyl methacrylate beam splitter/coupler for 1.55 μm applications

The applicability of layers of ion-implanted polymethyl methacrylate (PMMA) for beam splitting of laser light at the telecommunications wavelength of 1.55 mum is examined. Bulk PMMA is studied, subjected to low-energy (50 keV) silicon ion implantation at various ion fluences in the range from 10(14) to 10(17) cm(-2). The formed ultrathin near-surface ion-implanted layer of a thickness of about 100 nm, buried in a depth of approximately 100 nm, can be used to split (or combine) laser beams at 1.55 microm with a low absorption loss.

Interference effects between raman and parametric stimulated emission

The interaction between stimulated electronic Raman scattering and four wave parametric emission in potassium has been studied on the basis of common real states and emission channels. The modified intensity Stokes dependence and higher saturation pump intensity have been accounted for by the model developed. Two tunable pump waves are used to initiate the two steps, involving a dipole forbidden 4S − 3D transition.

Optical four-wave mixing in bulk polymers

A multiplex CARS scheme is used to study the bulk polymers poly-(methyl methacrylate) (PMMA) and poly-(vinyl toluene) (PVT). A coherent signal resulting from optical four-wave mixing is obtained with electronic and Raman-resonant parts. The magnitude of the third-order optical susceptibility tensor of these polymers is experimentally estimated. Significant enhancement of the total susceptibility by the Raman resonances is observed for PVT.