Stanislaw Bartkiewicz

High gain of light in photoconducting polymer–nematic liquid crystal hybrid structures

Abstract A liquid crystal panel useful for a realtime holography exhibiting also very high exponential gain coefficient is presented. We prepared nematic liquid crystal layer sandwiched between photoconducting polymeric layers (poly( N -vinylcarbazole)/trinitrofluorene). The first-order diffraction efficiency measured in degenerate two-wave mixing experiment amounts to 44%. The most important feature of the presented panel is its ability to switch energy from beam to beam. In two-wave coupling experiment multiple orders of diffraction are present and a very high two-beam coupling gain ratio (12) and a net exponential gain coefficient Γ =3700 cm −1 have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam incidence bisector at 15°. The time constants of grating formation and erasing in the studied system are functions of applied voltage and can be made as short as few milliseconds at favorable conditions.

Optical phase conjugation in dye-doped nematic liquid crystal

Optical phase conjugation in dye-doped nematic liquid crystals has been obtained with 532 nm radiation from a cw doubled YAG laser of total power of 50 mW. A 40° twisted nematic cell configuration has been used. The degenerate four-wave mixing (DFWM) experiment allowed for measurements of build-up time of phase conjugate signal and phase conjugate reflectivity as a function of input beam intensities and grating spacing. The PC reflectivities reached 2.6% and build-up times were of the order of 200 ms. The origin of the phase conjugate signal is linked with photoconductivity enhanced dynamic holographic refractive index grating formation observed in this and other similar systems.

Optical amplification with high gain in hybrid-polymer–liquid-crystal structures

Efficient coherent light amplification with a very high net exponential gain Γ≈2600 cm−1 has been observed in sandwich-type hybrid-polymer–liquid-crystal structures. The amplification is a result of an energy exchange between two beams in a two-beam coupling experiment. The reported gain coefficient has been measured in a tilted sample configuration within a Raman–Nath diffraction regime at electric fields of the order of 1 V/μm.

Amplified spontaneous emission in the spiropyran-biopolymer based system

Amplified spontaneous emission (ASE) phenomenon in the 6-nitro-1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indolin] organic dye dispersed in a solid matrix has been observed. The biopolymer system deoxyribonucleic acid blended with cationic surfactant molecule cetyltrimethyl-ammonium chloride served as a matrix. ASE appeared under sample excitation by UV light pulses (λ=355 nm) coming from nanosecond or picosecond neodymium doped yttrium aluminum garnet lasers and has been reinforced with green (λ=532 nm) light excitation followed UV light pulse. The ASE characteristics in function of different excitation pulse energies as well as signal gain were measured.

Polarization Dependence of Holographic Grating Recording in Azobenzene-Functionalized Polymers Monitored by Visible and Infrared Light

The holographic grating recording process in thin films of amorphous azobenzene-functionalized polymers has been widely reported in the literature. In spite of the many reports, little is known about the mechanisms responsible for different temporal behaviors of the diffraction efficiency during long recording times. Here, we report on experimental studies of the diffraction efficiency changes during the holographic diffraction grating recording process in photochromic polymer. The gratings were inscribed for four different polarization combinations of the recording beams: s-s, p-p, s-p and right to left circular polarization (RCP-LCP) employing the degenerate two-wave mixing technique. The grating recording process was simultaneously monitored by three different wavelengths: 514.5 nm (writing) and 632.8 and 904 nm (reading). The temporal evolution of the diffraction efficiency (for all polarization configurations and for each wavelengths) was precisely fitted within the model, which assumes simultaneous formation of the absorption grating and three coupling phase gratings shifted by 0 or pi with respect to each other. Two of the phase gratings originate from the refractive index grating changes in the bulk (volume) of a material and the third one from the surface relief modulation. The model enabled us to extract relevant parameters for each grating such as the build-up time constant, its final amplitude, and the phase shifts between phase gratings. Performed studies and the discussion of results revealed the main differences in the diffraction grating formation process for s-s, p-p, s-p, and RCP-LCP polarization configurations.

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.

Mechanism of optical recording in doped liquid crystals

A new principal mechanism underlying real-time holographic grating recording in a thin cell filled with a nematic liquid crystal doped with a dye is proposed. In the experiment a laser light interference pattern is mapped in the form of a spatial modulation of index of refraction. This is achieved by respective modulation of an electric field in the vicinity of the nematic liquid crystal due to its bulk photoconductivity and subsequent space charge field formation. In the spatially modulated electric field the nematic director is locally reoriented to some degree, leading to the appearance of a phase Δπ grating due to the birefringent properties of a nematic. The externally applied electric field plays an important role in the whole process: it changes the initial planar alignment of liquid crystal molecules and enhances the photogeneration quantum yield. The role of electrical edge conditions in the cell is pointed out as they determine the field distribution in the liquid crystal, which is generally different from the space charge field distribution described for photorefractive materials.

Observation of high gain in a liquid-crystal panel with photoconducting polymeric layers

A novel, to our knowledge, liquid-crystal panel suitable for real-time holographic purposes has been prepared. A nematic liquid-crystal layer sandwiched between photoconducting polymeric layers, when exposed to a sinusoidal light-intensity pattern, shows efficient formation of refractive-index gratings. The unique feature of the presented panel is its ability to switch energy from beam to beam in a manner similar to the charge-diffusion-controlled photorefractive effect. In a two-wave-mixing experiment multiple orders of diffraction are present, and a very high two-beam coupling-gain ratio (2.5) and a net exponential gain coefficient of ? = 931 cm(-1) have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam-incidence bisector at 45 degrees . The time constants for grating formation and erasure in the studied system are functions of the applied voltage and can be made as short as a few milliseconds under favorable conditions. The mechanism of beam coupling is linked with an electric-field-driven reorientation of the nematic director as a result of a spatially modulated space-charge field created by light in a photoconducting poly(3-octyl)thiophene polymeric layer.

Three gratings coupling during the holographic grating recording process in azobenzene-functionalized polymer

The authors present a mechanism explaining complex diffraction efficiency dynamics of the holographic grating recording process in azobenzene-functionalized polymer. The mechanism assumes simultaneous formation of three coupling phase gratings with different recording time constants and constant, equal to 0 or π, phase shifts between them. Two of gratings originate from the refractive index changes in the bulk of material and the third one from the surface relief modulation. Diffraction efficiency dynamics curves calculated using the proposed model are in a very good agreement with the experimental results.

Dynamic charge-carrier-mobility-mediated holography in thin layers of photoconducting polymers

The dynamic holography technique is proposed for the measurement of the charge-carrier-mobility in thin layers of a photoconducting polymer, used in optically addressed liquid-crystal spatial light modulators. The photorefractive properties of these modulators are studied under short-pulse (20 ps, 532 nm) laser illumination conditions and the charge mobility in the photoconducting polymer (μh=10−7 cm2/V s) is obtained from the temporal evolution of intensity of the first-order diffracted beam. A mechanism responsible for the grating formation is proposed and discussed.