Hendry Izaac Elim

Transparent nanohybrids of nanocrystalline TiO2 in PMMA with unique nonlinear optical behavior

PMMA is one of the most versatile polymeric materials for applications in various technological areas including optics and electro-optics. While the current applications of PMMA in optics and electro-optics are limited by their linear optical behavior, we report here in this paper the unique nonlinear optical behavior of nanohybrids consisting of nanocrystalline TiO2 in PMMA. Transparent thin films of TiO2–PMMA nanohybrid on substrates were synthesized by in-situ sol–gel and polymerisation, assisted by spin coating. Using titanium isoproproxide (Ti-iP) as the starting material for nanocrystalline titania, together with methyl methacrylate and 3-(trimethoxysilyl)propyl methacrylate, nanohybrids containing up to 80% Ti-iP in PMMA were successfully realized. The resulting nanohybrid thin films coated on quartz substrates are optically transparent and demonstrate large nonlinear optical behavior, with an ultrafast response of <1.5 ps. The highest two-photon absorption coefficient (β) and nonlinear refractive index (n2) are observed with the nanohybrid thin film of 60 wt% Ti-iP in PMMA, as confirmed by the Z-scan technique.

Ultrafast optical nonlinearity in poly(methylmethacrylate)-TiO2 nanocomposites

With 780-nm, 250-fs laser pulses, ultrafast optical nonlinearity has been observed in a series of thin films containing poly(methyl methacrylate) (PMMA)-TiO2 nanocomposites, which are synthesized by a simple technique of in-situ sol-gel/polymerization. The best figures of merit are found in one of the films prepared with a 60 wt % of titanium isopropoxide. Transmission electron microscopy shows the presence of 5-nm-diameter particles in the film. The observed optical nonlinearity has a recovery time of ∼1.5 ps. These findings suggest the strong potential of PMMA-TiO2 nanocomposites for all-optical switching.

Controlling the crystallinity and nonlinear optical properties of transparent TiO2–PMMA nanohybrids

Titania–polymer nanohybrid thin films represent a new class of potential materials for optoelectronic applications. While most such nanohybrid thin films lack control in crystallinity, we report in this paper transparent nanohybrids of titania-polymethyl methacrylate (TiO2–PMMA) thin films having a remarkably enhanced nanocrystallinity. Post-treatments with water vapor at relatively low temperatures were applied on these thin films, following in situ sol–gel polymerization. They promoted rearrangement of flexible Ti–O–Ti bonds leading to enhanced crystallization of the TiO2 phase. The degree of TiO2 crystallinity in the resulting nanohybrid films was studied by using XRD, FTIR, UV–Vis spectroscopies and HRTEM. Both linear and nonlinear optical responses increase with the enhancement of TiO2 crystallinity in the nanohybrids. The highest two-photon absorption coefficient (β) and nonlinear refractive index (n2) were observed for the nanohybrid thin films with highest TiO2 crystallinity, as confirmed by open and closed aperture Z-scan techniques using 250 fs laser pulses at 800 nm, having a value of 2260 cm GW−1 and 6.2 × 10−2 cm2 GW−1, respectively.

Preparation of carbon nanoparticles with strong optical limiting properties by laser ablation in water

Carbon nanoparticle colloids were prepared by Nd:YAG laser ablation of a carbon target immersed in water. The nanoparticles were collected on substrates and analyzed with micro-Raman spectroscopy and electron microscopy. Optical limiting properties of the carbon nanoparticle colloids towards 532 nm wavelength were characterized using a nanosecond Nd:YAG laser. A strong optical limiting response was detected for the carbon nanoparticle colloids with C60 as reference at input laser fluence above 0.6 J/cm2. The effects of carbon nanoparticle concentrations upon the optical limiting behavior were also investigated. A convenient method for preparing carbon nanoparticle based optical limiters is proposed in this investigation.

Excitonic nonlinear absorption in CdS nanocrystals studied using Z-scan technique

Irradiance dependence of excitonic nonlinear absorption in cadmium sulfide (CdS) nanocrystals has been studied by using Z-scan method with nanosecond laser pulses. The wavelength dependence of nonlinear absorption has also been measured near the excitonic transition of 1S(e)–1S3/2(h). We observe the saturable absorption, which can be described by a third-order and a fifth-order nonlinear process for both 3.0-nm-sized and 2.3-nm-sized CdS nanocrystals. The experimental results show that the excitonic nonlinear absorption of CdS nanocrystals is greatly enhanced with decreasing particle size. A two-level model is utilized to explain both irradiance and wavelength dependence of the excitonic nonlinearity.

Ultrafast absorptive and refractive nonlinearities in multiwalled carbon nanotube films

By using femtosecond laser pulses at a wavelength range from 720 to 780nm, we have observed absorptive and refractive nonlinearities in a film of multiwalled carbon nanotubes grown mainly along the direction perpendicular to the surface of quartz substrate. The z-scans show that both absorptive and refractive nonlinearities are of negative and cubic nature in the laser irradiance range from a few to 300GW∕cm2. The magnitude of the third-order nonlinear susceptibility, χ(3), is of an order of 10−11esu. The degenerate pump–probe measurement reveals a relaxation time of ∼2ps.

Gain narrowing and random lasing from dye-doped polymer-dispersed liquid crystals with nanoscale liquid crystal droplets

Dye-doped polymer-dispersed liquid crystals have been studied for random lasing. The dye-doped polymer-dispersed liquid crystal film was fabricated by photoinitiated polymerization with a collimated 514.5nm Ar+ laser beam. Scanning electron microscopy analysis showed that most liquid crystal droplets in polymer matrix ranged from 20to80nm. Gain narrowing and random lasing from dye-doped polymer dispersed liquid crystals were observed under the excitation of a frequency-doubled Nd:YAG (yttrium aluminum garnet) laser operating at a wavelength of 532nm. The possible mechanism was proposed to explain the random lasing. The threshold of the random lasing was about 25μJ/pulse. The linewidth of the lasing peaks was about 1nm. With the film thickness of 6.5μm, the lasing mode was nearly transverse electric polarized.Dye-doped polymer-dispersed liquid crystals have been studied for random lasing. The dye-doped polymer-dispersed liquid crystal film was fabricated by photoinitiated polymerization with a collimated 514.5nm Ar+ laser beam. Scanning electron microscopy analysis showed that most liquid crystal droplets in polymer matrix ranged from 20to80nm. Gain narrowing and random lasing from dye-doped polymer dispersed liquid crystals were observed under the excitation of a frequency-doubled Nd:YAG (yttrium aluminum garnet) laser operating at a wavelength of 532nm. The possible mechanism was proposed to explain the random lasing. The threshold of the random lasing was about 25μJ/pulse. The linewidth of the lasing peaks was about 1nm. With the film thickness of 6.5μm, the lasing mode was nearly transverse electric polarized.

Refractive index control and Rayleigh scattering properties of transparent TiO2 nanohybrid polymer.

The surface of TiO2 nanoparticles (NPs) was modified by a coupling agent of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane to render them highly compatible with organic monomer mixtures avoiding aggregation. Such TiO2 NPs were then chemically attached with a prepolymer. The refractive index of hybrid TiO2 NPs-polymer was increased dramatically in comparison with that of pure polymer, and it can be controlled by adjusting the content of TiO2 NPs. The Rayleigh scattering of hybrid TiO2 NPs-polymer was size-dependent and increased with the increase of the TiO2 NPs content. When Rayleigh scattering of the hybrid material prepared with a milling and centrifuge process was significantly lowered, one can obtain a transparent sample, which is good for optical devices.

Effect of liquid crystal concentration on the lasing properties of dye-doped holographic polymer-dispersed liquid crystal transmission gratings

Optically pumped single-mode lasing was achieved from a 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran dye-doped holographic polymer-dispersed liquid crystal transmission grating with various liquid crystal concentrations, which played an important role in the lasing generation and wavelength selection. With the decrease of the liquid crystal concentration, under the excitation of a frequency-doubled Nd:yttrium aluminum garnet laser operating at 532nm, the lasing wavelength was blueshifted, and the full width at half maximum of the lasing peak became narrower. The lowest threshold pumping energy was found to be about 5μJ∕pulse at a liquid crystal concentration of 19.7wt%. The lasing emission was thermally switchable due to the change of the refractive index modulation.

A Novel Preparation of High-Refractive-Index and Highly Transparent Polymer Nanohybrid Composites

In an attempt to fabricate a high-refractive-index (RI) as well as a highly transparent polymer nanohybrid composite, a combination of a polymer matrix, nanoparticles, and nanoparticle-surface coupling agent was tailored systematically. We developed an effective and versatile hybridizing process for further increasing the RI of the nanohybrid composites. By using various indispensable steps, we achieved the highest RI of n (594 nm) ≈2 and a highly transparent (transmittance ≈93% at 460 nm) polymer nanohybrid composite.