Pawel Karpinski

Biopolymer based system doped with nonlinear optical dye as a medium for amplified spontaneous emission and lasing

In this paper, we present results of detailed studies on amplified spontaneous emission (ASE) and lasing achieved in a double-layer system consisted of a biopolymer based matrix loaded with 3-(1,1-dicyanoethenyl1)-1phenyl-4,5dihydro-1H-pyrazole organic nonlinear optical dye and photochromic polymer. The laser action was achieved via distributed feedback configuration with third order of Bragg scattering on surface relief grating generated in photochromic polymer. To excite the luminescence, we have used 6 ns pulses of Nd:YAG laser at 532 nm. The ASE and lasing thresholds were estimated to be 17 mJ/cm2 and 11 mJ/cm2, respectively.

Photochromic and nonlinear optical properties of azo-functionalized POSS nanoparticles dispersed in nematic liquid crystals

It is well known that addition of a small amount of nanoparticles to liquid crystals may enhance their nonlinear optical properties. To date, in most reports, semiconductor- or metallic-core nanoparticles in LCs have been described. Here, we report on the synthesis of two types of novel nanoparticles based on a cubic siloxane cage (RSiO3/2)8, known as polyhedral oligosilsesquioxane (POSS) with the Si–O skeleton surrounded by covalently attached eight groups (R) of azo-benzene mesogenes differing in the length of their aliphatic chains. These dielectric-core nanoparticles substituted with mesogenic groups are well adapted to the calamitic liquid crystal matrix. Azo-benzene units add an important light-driven functionality to the system via their conformational photoisomerisation. We report on the photochromic properties of these new compounds in chloroform. By embedding the nanoparticles in a nematic liquid crystalline matrix, a functional hybrid nanocomposite material has been obtained. Our research was focused on the characterization of the third order nonlinear optical properties in this system linked to photochromism. The experimental results of all-optical switching in an optical Kerr effect configuration are discussed in the framework of light-induced modification of the aliphatic chains containing azo-benzene photoisomerizing moieties and following local director reorientation in nematic liquid crystals. The described properties are useful in the field of real-time holography.

Investigation of enhancement of photoinduced reorientation of liquid-crystal molecules in the presence of azo-dye and gold nanoparticles

The optical-Kerr-effect configuration is used to study the enhancement of photo-induced molecular reorientation in a twisted nematic liquid crystal due to the presence of metal nanoparticles. The system consisting of a liquid crystal with photoisomerizable dye Disperse Orange 3 shows an enhancement of the Kerr-like response at 532 nm excitation light when doped with 15 nm in size gold nanoparticles. A simple theoretical model predicting the behavior and magnitude of the expected response has been formulated. The contribution of localized surface plasmon of gold nanoparticles to light-induced torque is discussed.

Optical phase conjugation in azo-dye doped chiral liquid crystal

We report on optical phase conjugation phenomenon observed in chiral nematic liquid crystal showing band gap type Bragg reflection. The phase conjugate to the signal beam is observable only in the small temperature interval when the Bragg condition is fulfilled and only for circularly polarized light. The optical phase conjugation signals were observed at low cw laser light intensities (<100 mW/cm2, λ = 532 nm). Estimated value of third order optical susceptibility χ(3) = 2.8 × 10−17 m2/V2 is attributed to enhancement due to photoisomerisation of azo-dye (disperse red 1) inducing molecular reorientation process of liquid crystal molecules.

Refractive index and surface relief grating formation in DNA based dye-doped films

AbstractThe refractive index and surface relief grating formation in deoxyribonucleic (DNA) based thin films, functionalized with the well known fluorophore LDS698, was studied in holographic two beam coupling geometry and as function of the polarization of incident laser beams. The DNA biopolymer used was functionalized with cetyltrimethylammonium chloride (CTMA) surfactant. If both beams polarization are parallel (p-p) a refractive index grating is formed, as is usually observed, this is also the case when both beams polarizations are perpendicular (s-p). This fact is tentatively interpreted as due to the DNA matrix chirality. Also formation of a surface relief refractive index grating is observed in these materials for the first time, in contrary to earlier observations reported in literature. This effect is tentatively explained as due to the creation of an electrostatic bond between DNA and the guest molecules, which are electrically charged.

Surface Plasmon Polariton Excitation in Metallic Layer Via Surface Relief Gratings in Photoactive Polymer Studied by the Finite-Difference Time-Domain Method

We performed numerical investigations of surface plasmon excitation and propagation in structures made of a photochromic polymer layer deposited over a metal surface using the finite-difference time-domain method. We investigated the process of light coupling into surface plasmon polariton excitation using surface relief gratings formed on the top of a polymer layer and compared it with the coupling via rectangular ridges grating made directly in the metal layer. We also performed preliminary studies on the influence of refractive index change of photochromic polymer on surface plasmon polariton propagation conditions.

Fluorescence and SHG in organic nanocrystals of DCNP

Organic nanocrystals (ONCs) similarly to inorganic ones show interesting size effects. It has been already observed that their luminescence properties can be changed in terms of enhanced luminescence and spectral shift of fluorescence bands as compared to bulk materials. Usually these effects are observed for much larger size of nanoparticles from 50 nm up to 1000 nm. Noncentrosymmetric and fluorescent organic crystals are rare but particularly interesting as they exhibit both nonlinear optical properties like Second Harmonic Generation (SHG) and fluorescence. Such ONCs could serve as fluorescent and SHG probes for bio-imaging purposes simultaneously. This could be beneficial for situation in which the dynamics of ordering of biological systems has to be studied. Here, we report on 3-(1,1-dicyanoethenyl)-1-phenyl-4,5- dihydro-1H-pyrazole (DCNP) compound which forms nanocrystals, exhibits large shifts of fluorescence maximum with size and strong SHG signals. When embedded in polymeric or biopolymeric matrix DNA-CTMA shows efficient amplified spontaneous emission. The unique properties of this compound in its various forms from molecules, ONCs to macroscopic single crystals are studied and discussed.

Bidirectional molecular reorientation induced by localized surface plasmon

We report about the opposite direction of molecular reorientation induced by the light of wavelength at two sides of the plasmonic resonance spectrum. The molecular reorientation is highly enhanced due to the electric field concentration for wavelength close to the resonance of the metal nanoparticle. We theoretically investigate and confirm by discrete–dipole approximation method calculations the mechanism of reorientation of nematic liquid crystal molecules upon interaction with localized surface plasmon.

Laser inscription of surface structures and induction of optical anisotropy in azo-benzene substituted photochromic polymers and other systems

We present trial calculations of surface light-induced patterns in photochromic azo-substituted polymers. Using microscope with nanopositioning stage various birefringence and surface structures have been recorded in photochromic azo-functionalized polymers. By systematic approach to the inscription experiment and controlling cw or pulsed laser light intensity, its polarization and beam scan speed we observed the dynamics of molecular photoorientation and its relation to mass transport. We discuss properties of holographically inscribed polarization gratings and analyze them spatially by monitoring of microscopic local diffraction efficiency. We report how azo-benzene molecules can work in other systems, i.e. azobenzene functionalized POSS molecules embedded in nematic liquid crystal.

Second harmonic generation and two-photon excitation fluorescence from individual nanocrystals of pyrazoline derivatives

We measure the quadratic nonlinear optical properties, namely second harmonic generation (SHG) and two-photon excitation fluorescence (TPEF), of nanocrystals of different derivatives of pyrazoline. The derivatives of pirazoline, like for example DCNP are supposed to have high second order nonlinear susceptibility. We use a nonlinear optical microscopy setup, samples were excited with a femtosecond laser operating at 800 nm wavelength, both signals (SHG and TPEF) were measured simultaneously using a dichroic mirror and for SHG a narrow spectral bandwidth filter centered at 400 nm, respectively. The details of the used nonlinear polarimetric microscopy technique can be found in reference. We have produced the nano-size crystals using the precipitation method and estimated the average size of these nanocrystals to be 300 × 600 nm using atomic force microscopy (AFM).