Photoinduced Gratings Research Articles

CdSe/CdS core/shell quantum dots as sensitizer of a photorefractive polymer composite

CdSe/CdS core/shell, tri-n-octylphosphine oxide passivated, quantum dots are used to sensitize a photorefractive polymer composite. The composite also consists of poly(N-vinylcarbazole) as the nominally charge transporting matrix and an electro-optic chromophore. The efficacy of sensitization and consequent photorefractive performance is investigated using transmission spectroscopy and ellipsometry, two-beam coupling and degenerate four-wave mixing experiments. The photorefractive nature of the photo-induced grating is confirmed by the observation of asymmetric two-beam coupling. Four-wave mixing reveals record diffraction efficiencies for a nano-particle-sensitized photorefractive polymer at the field levels applied (1.3% at 70 V.μm−1). A recently developed analytical technique is used to extract space-charge field rise time values from degenerate four-wave mixing transients. In turn, analysis of the dependence of the rise time on applied field is used to determine the zero-field charge dissociation efficiency to be 3.6 × 10−5 ± 0.5 × 10−5. It is further shown that the magnitude of this parameter accounts for most of the difference in photorefractive response rate between the present material and a similar C60 sensitized composite.

Acousto-optic bandpass filter

The acousto-optic bandpass filter is a device whereby one or more bands of optical wavelengths may be selected for further transmission. In this device, all light within the optical bandwidth of operation is first coupled from the core mode of a optical fiber to a specific cladding mode by a long-period photo-induced grating. These cladding mode lightwaves then enter an acousto-optic interaction region where selected RF frequencies of flexure waves, induced by an acoustic wave amplifier, re-couple selected bands of wavelengths back into the core mode. The interaction region is isolated from the broadband coupler by an acoustic absorber to limit acoustic interaction to the interaction region.

Photofabrication of periodic microstructures in azodye-doped polymers by interference of laser beams

Volume holographic gratings and two-dimensional periodic microstructures in azodye-doped polymethylmethacrylate were fabricated, respectively, by interference of two coherent beams of a femtosecond laser and by interference of three coherent beams of a nanosecond laser. The dependence of the first-order Bragg diffraction efficiency and the photoinduced refractive-index modulation of the gratings on the intensity of the writing light was investigated. The measurements of the absorption spectra before and after irradiation with the writing light suggest that the photoinduced gratings were refractive-index-modulated gratings, which arose from a photoinduced decomposition reaction of the azodye molecules through multiphoton absorption. In the experiments involving the interference of three beams, the period of the two-dimensional periodic microstructures was changed by adjusting the angle between the three writing beams.

Thermally stabilized photoinduced Bragg gratings

Bragg gratings were printed in Ge–SiO2 and Ge–B–SiO2 thin glass films by KrF excimer laser irradiation through a phase mask. The diffraction efficiency of the grating printed in the Ge–B–SiO2 film was approximately one order of magnitude greater than that of the Ge–SiO2 film. Although the gratings were almost erased upon annealing at temperatures lower than 500 °C, a grating with much higher diffraction efficiency than before annealing was formed exclusively in the Ge–B–SiO2 film after annealing at 600 °C. The diffraction efficiency of the grating was unchanged upon repeated heating between room temperature and 600 °C.

Photoinduced reversible optical gratings in photochromic diarylethene-doped polymeric thin films

We report on our investigation of the potential for all-optical reversible photostructuring of the photochromic molecule 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BFCP). The light-induced isomerization of BFCP was studied in solution as well as in polymeric thin films. The absorption change that is due to the photochromic reaction of BFCP was determined by conventional spectroscopic measurements. The absolute values of the refractive indices of doped polymer films were measured by the waveguide grating coupling technique for the two states of the photochromic molecule. The dynamics of the reactions and the light-induced refractive-index changes Δn of doped polymer samples were investigated in holographic grating experiments. Changes of as much as Δn=(3.5±0.5)×10-4 were obtained at 806 nm for 41.16-wt. % doping. Finally, a modified two-beam coupling experiment permitted the separation of absorption and refractive-index contributions to the grating buildup and the determination of the phase shifts of these gratings with respect to the intensity grating.

Experimental studies of diffraction by photoinduced permanent gratings in nanotube-doped liquid crystals

We report on our observation of the formation of permanent holographic gratings induced in nematic liquid crystals doped with carbon nanotubes by the simultaneous action of both a spatially modulated light and dc electric field. Properties of these irreversible gratings were investigated with a normally incident laser beam of linear polarization. The polarization dependence of the diffraction intensity of the probe reveals the anisotropic nature of the permanent index gratings.

Surface-sustained permanent gratings in nematic liquid crystals doped with carbon nanotubes

Response properties of photoinduced gratings were studied in planar cells of liquid crystal doped with multiwalled carbon nanotubes. The grating formation time deduced from beam-coupling measurements was obtained to be ~30 ms. Absorption spectroscopy implies that the permanent gratings are associated with periodically distributed carbonaceous material adsorbed on the inner surfaces of the cell windows under prolonged illumination to the 514.5-nm beams, giving rise to the persistent lightinduced modulation of the easy axis.

Optically written solid-state lasers with broadly tunable mode emission based on improved poly (2,5-dialkoxy-phenylene-vinylene)

We report an optical written distributed feedback (DFB) structure with broadly and continuously tunable mode emission (Δλ=30 nm) based on polycondensation-type poly[2methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH–PPV). In a Lloyd-mirror configuration we realize first-order feedback by dynamic photoinduced in-plane gratings, which can also be imprinted permanently. This technique is a versatile fabrication tool for polymeric DFB laser structures while also providing an efficient method to probe the lasing performance of organic solid-state laser materials. Furthermore, features of the photoinduced absorption spectra of improved MEH–PPV indicate favorable material properties towards further homopolymer solid-state laser applications.

Photoinduced permanent gratings inside bulk azodye-doped polymers by the coherent field of a femtosecond laser

Permanent holographic gratings were fabricated in bulk azodye-doped polymethylmethacrylate by the coherent field of a femtosecond laser. Optical microscopy and atomic force microscopy observations revealed that the gratings consist of two parts, surface relief gratings on both surfaces and refractive index modulated volume gratings in the interior of the polymers. The diffraction efficiency of the first-order Bragg for the gratings was estimated to be 90%.

Surface and volume contributions to total diffractional efficiency in As 2S 3 thin film glasses

Chalcogenide glasses such as As 2S 3 illuminated by near bandgap light ( E g∼2.4 eV) exhibit photodarkening and photoexpansion. An exposition by a sinusoidal intensity holographic pattern thus will create a combination of two superposed volume ( δn) and surface ( δL) gratings [J. Lightwave Technol. 15 (8) (1997) 1343; IEEE Photon Technol. Lett. 8 (8) (1996) 1041]. We present studies of contribution of each grating to the total diffractional efficiency (DE) of these photoinduced gratings, recorded on vapor-deposited As 2S 3 thin films. An index matching technique is used to achieve a situation where mainly δn contributes to the total DE. Also, measuring the dynamic evolution of both δL and total DE allows to calculate δn (up to 5×10 −2) contribution via the theoretical formula. Both techniques show that the main contribution to the total DE for our films (of various thickness, i.e. 1.4, 3.0 and 5.0 μm) under weak illumination (under 2.2 mW/mm 2), is due to the grating of the refractive index modulation δn.

Analysis of gratings induced in azo-dye doped liquid crystals

We investigate the formation of photoinduced gratings in a homeotropic film of nematic liquid crystal doped with methyl red. Using a grating translation technique, we determine the time evolution of the amplitudes and phase shifts of both index and absorption modulations. With low writing intensities, a fast grating caused by photoisomerization of the dye competes with a slower grating, originating from a reorientation of the director. This slower grating only appears if the light polarization has a nonzero projection on the director, i.e. at oblique incidence. The grating decays in the dark. We suggest that its formation may be explained by a light-induced dielectric torque, rather than a photorefractive effect. With high intensities, an even slower grating is formed at normal incidence. We stipulate that it originates from a change in the anchoring parameters caused by the adsorption of photoisomerized dyes on the surfaces. The resulting reorientation of the director is permanent.

Photoinduced Birefringence and Surface Relief Gratings in Novel Polyurethanes with Azobenzene Groups in the Main Chain

Three novel polyurethanes with V-shaped bisazo groups (PU1 and PU2) or rodlike monoazo groups (PU3) in the main chain were synthesized. On irradiation with a linearly polarized laser beam, birefringence was induced in the three films to the level of 0.035, 0.037, and 0.044 for PU1, PU2, and PU3, respectively. The stable birefringence obtained at Tg − T = 33 °C was 71%, 46%, and 19% of that achieved at room temperature for PU1, PU2, and PU3, respectively. The relative stability of the photoinduced birefringence of PU3 decreased linearly with the increase of temperature, while that of PU1 and PU2 was constant even close to Tg. Surface relief gratings with high diffraction efficiency were inscribed on these main-chain polymers. The photoinduced gratings are only partially erasable by heating above the Tg of the polymers.

2000 Macromolecular Science and Engineering Award LectureThe versatility of azobenzene polymers

The well-known trans–cis–trans photoisomerization of azobenzenes produces at least three different kinds of motion in the polymer materials to which the azobenzenes are bound. The first is a photoinduced motion of the azobenzene groups only, and they can align in a selected position with respect to the light polarization. The second is a macroscopic motion of huge amounts of polymeric material, producing surface deformation, and the third is a reorganization of smectic domains in liquid crystalline polymers. These motions and their consequences are briefly discussed in relation to the polymer structure and some possible photonic applications are mentioned.Key words: photoinduced orientation, azobenzene polymers, surface gratings, photonics, thermochromism, photochromism, photorefractivity, photoinduced chirality and switching.

Properties of CdF2:Ga as a medium for real-time holography

The metastable nature of the excited state of bistable Ga centers in semiconducting CdF2 crystals allows for the use of CdF2:Ga and CdF2:Ga,Y crystals as materials for real-time holography over a wide range of response times (1–1000 ms). The characteristics of these materials and optimal conditions for their use are discussed in the framework of a model that describes the decay of photo-induced gratings written in them.

Progress in erasable and permanent polymer based DFB-structures with multicolour tunable laser emission

Highly luminescent organic solid state media including molecular dye doped guest-host systems and novel conjugated polymers currently attract a lot of interest for application in photonic technology. We demonstrate organic solid-state lasers with broadly and continuously tuneable mode emission (Δλ = 40nm, FWHM < 1nm). Feed-back onto polymeric DFB-structures results from dynamic and permanent photoinduced in-plane gratings. A write and read - technique for easy and low cost fabrication of single and multiple grating devices is presented. The combination and optimization of tuneable gratings with specific confinement features as from different waveguiding formats open-up new approaches in integrated optics.

Diffraction efficiency increase by corona discharge in photoinduced surface-relief gratings on an azo polymer film

The diffraction efficiency of a surface-relief grating (SRG) was dramatically increased by corona discharge. We fabricated this SRG by applying surface modulation to azo polymer films, using laser interference fringes. The electric charge was deposited upon the SRG by a corona discharge in an oven. The first-order diffraction efficiency measured before and after corona discharge above the glass-transition temperature increased from ~2% to ~40% in a SRG written with a circularly polarized beam. The relief depth of the SRG as measured with an atomic-force microscope also increased, from ~130 nm to ~450 nm. The increase in diffraction efficiency was independent of the writing polarization.

Evidence for Photoinduced Molecular Migration Mediated Surface-Relief Grating Formation in Azo-Dye Polymers

Direct and reversible holographic recording of surface-relief gratings in azo-dye polymers was recently evidenced using atomic force microscopy. Irradiation with an interference pattern between polarized laser beams was observed to lead to quantitative mass-transport. The surface gratings have a negative amplitude that can be twice as large as the polymer film thickness. The origin of such photo-driven mass transport is still controversial. We provide here experimental evidence that the chromophores migrate from the high to low the intensity regions. Chain migration in such doped or grafted guest-host polymer systems results from the photoinduced migration of the guest chromophores.

Highly Stable Optically Induced Birefringence and Holographic Surface Gratings on a New Azocarbazole-Based Polyimide

An azocarbazole-based polyimide was synthesized from an azocarbazole diamine monomer and 4,4‘-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) by a two-step polycondensation reaction. Birefringence experiments have been performed on the pure polyimide and on the polyimide mixed with small azo chromophores. Because of its high Tg, the polyimide has a very stable induced birefringence (only 14% loss during the relaxation process). The stability of the mixtures is also very high. Surface relief diffraction gratings were inscribed on these systems. The measured diffraction efficiency was about 0.25% for a diamine-doped polyimide exposed for 1 h to an irradiance of 200 mW/cm2. The surface profile studied by atomic force microscopy showed an amplitude of 30 nm and a regular spacing of 700 nm on a 255 nm thick film. The photoinduced gratings had a high stability without further surface deformation after baking at 240 °C for 1 h in air.

Polarization analysis of diffracted orders from a birefringence grating recorded on azobenzene containing polymer

Polarization analyses of transmitted and first-diffracted orders have been performed on photoinduced gratings in azopolymers. The polarization states of diffracted orders depend strongly on irradiation time because of the contribution from both a phase grating at short time and a surface relief grating at longer time. These results are nicely explained using a Jones’ matrix approach which predicts the polarization states of transmitted and diffracted orders of polarization holographic gratings.

Photoinduced surface-relief grating in a cubic crystal in a constant electric field

A theoretical model gibing a time-independent description of a photorefractive surface-relief grating in a cubic crystal placed in an external constant electric field is proposed. The model is used to calculate the height of the surface barrier of a Bi12TiO20 crystal for the (001) cut, where there is no interaction of light beams in the interior volume of the medium.