Permanent Gratings Research Articles

The fabrication of permanent holographic gratings in bulk polymer medium by a femtosecond laser

Refractive-index-modulated holographic gratings have been recorded for the first time in the interior of polymer bulk, by a femtosecond laser at 800 nm. The polymer bulk was prepared by doping an azo dye, HPNA in polymethylmethacrylate. This sample preparation process is quite simple. Multi-photon absorption and, as a result, photoinduced changes in isomer population were postulated as a brief discussion on the mechanism of the grating formation.

Switchable Gratings from Polymerized Nematic Emulsions

Permanent diffraction gratings can be built in polymer dispersed liquid crystals by using either photomasks or crossed laser beams, in order to modulate the light intensity across the film during the curing process. The intensity of light diffracted from these gratings can be controlled with an applied electric field. In this paper, we report our results on the formation of permanent gratings in nematic emulsions, photopolymerized through a photomask. The gratings are permanently stored and the diffraction efficiency can be modulated with an electric field. The grating structure has been investigated by optical and scanning electron microscopy. The advantages of using nematic emulsions in photopatterning are discussed.

Observation of self-diffraction effect in Acid blue 7 dye soaked gelatin films

Abstract Permanent self-diffraction gratings are formed in red sensitive Acid blue 7 dye soaked gelatin films under illumination of an interference pattern by two linearly polarized beams from a He-Ne laser at 632.8 nm. Growth of the self-diffraction grating is monitored by measuring intensities of various diffraction orders. Systematic studies are carried out to investigate the influence of various parameters involved in diffraction efficiency of the grating such as time of exposure, concentration of dye in the gelatin-coated plate, intensity of interfering beams and intensity ratio of interfering beams. Efficient gratings with ten diffracted orders are formed. Several interesting observations are made by blocking one of the writing beams and an attempt is made to analyse these results.

Observation of self-diffraction effect in acid blue 7 dye soaked gelatin films

Permanent self-diffraction gratings are formed in red sensitive Acid blue 7 dye soaked gelatin films under illumination of an interference pattern by two linearly polarized beams from a He-Ne laser at 632.8 nm. Growth of the self-diffraction grating is monitored by measuring intensities of various diffraction orders. Systematic studies are carried out to investigate the influence of various parameters involved in diffraction efficiency of the grating such as time of exposure, concentration of dye in the gelatin-coated plate, intensity of interfering beams and intensity ratio of interfering beams. Efficient gratings with ten diffracted orders are formed. Several interesting observations are made by blocking one of the writing beams and an attempt is made to analyse these results.

Permanent narrow-band reflection holograms for infrared light recorded in LiNbO3:Ti:Cu channel waveguides

Permanent refractive-index gratings are generated by thermal fixing of holograms in photorefractive lithium niobate channel waveguides. The guides are fabricated by successive indiffusion of titanium stripes and thin layers of copper. After high-temperature recording with green light, refractive-index modulations exceeding Δn=8×10-5 for light of the telecommunication wavelength 1550 nm appear without the need of any development process of the written holograms. The gratings are stable in the dark and no compensation mechanism via dark conductivity is observed. Thus this method may be well suited for long-time applications in holography and integrated optics.

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.

Spatial periodicity of photorefractive orientational gratings in dye-doped polymer–liquid crystal composite

Permanent photorefractive orientational gratings induced in dye-doped polymer dispersed liquid crystal show unusually the same refractive index spatial periodicity of the interference light pattern. The quadratic response of the material to the local electric field predicts a grating wave vector twice of the intensity pattern one. In this paper we propose an explanation of this periodicity and experimental results that support our idea.

Permanent polarization gratings in photosensitive Langmuir–Blodgett films

An investigation was carried out on thin permanent phase polarization gratings recorded in Langmuir–Blodgett films using two orthogonal circularly polarized Ar-ion laser beams. The films are composed of amphiphilic azo-dye molecules and manifest an extremely large value of photoinduced optical anisotropy, Δn≈0.36. The experimental results are in excellent accord with the theoretical model based on the Jones matrix representation of the polarization pattern. The gratings are stable at least, for half a year.

Réseaux de diffraction permanents modulables engendrés par des transitions liquide/solide locales induites par laser

we report on a new procedure to obtain permanent diffraction gratings on solid substrates. Gratings are generated using laser-induced liquid/solid phase transitions in photosensitive binary aqueous solutions in the interference pattern of two interfering pump beams. The important observed diffraction efficiencies open a promising potential for practical developments.

Photorefractive index gratings in SnO2:SiO2 optical fibers

Permanent photorefractive gratings with considerable refractive index modulations (∼3×10−4) were written in silica optical fibers doped with very low levels of SnO2(∼0.15 mol %) using ultraviolet (UV) laser radiation at 248 nm. The photoinduced refractive index change does not show any sign of saturation even for long exposure times (i.e., total fluence &amp;gt;20 kJ/cm2). A comparison with GeO2:SiO2 fibers shows that under similar UV intensities saturation takes place for shorter exposure time and comparable refractive index changes are obtainable for GeO2 concentrations ∼10 mol %.

UV-induced permanent gratings in sol–gel germanosilicate thin films

Germanosilicate thin films have been elaborated by the sol–gel process and the dip-coating technique. Pulsed or continuous wave UV laser (244 nm) was used to write permanent gratings in these films. In the case of exposure to cw laser, the grating diffraction efficiencies were measured using a focused beam from a He–Ne laser at 633 nm and photo-induced changes in refractive index as high as 4×10 −3 have been obtained. The thermal behaviour of these gratings has been investigated showing a good stability up to 400°C. Exposure to pulsed fringe pattern led to a glass photo-expansion modulated by a strong corrugation which can be due mainly to photo-ablation at the places of the bright fringes. The waveguide surface at the grating places was investigated through Atomic Force Microscopy (AFM) and microscopic profilometry techniques. Preliminary results on the kinetics of the grating growths are also reported.

Photoinduced orientations of azobenzene chromophores in two distinct holographic diffraction gratings as studied by polarized Raman confocal microspectrometry

Optically isotropic thin films (0.5 μm) of an amorphous side-chain copolymer containing 10% azobenzene moieties and 90% meso-azobenzene groups (K1-10 sample) were irradiated by two interfering orthogonal (±45°) or parallel (p + p) linearly polarized laser beams; permanent holographic diffraction gratings were thus inscribed. From atomic force microscopy (AFM) measurements, relatively weak amplitudes (about 50–60 nm) in the surface relief modulation (SRM) are detected but strong perturbations in the profiles and new substructures are evidenced at half-period positions. Using a confocal microspectrometric technique we have recorded various pre-resonance enhanced polarized Raman spectra from a large grating area and obtained spatially resolved Raman images of the intensity variations. This yields new insight into the photoinduced orientation effects, the angular distributions of the chromophore species, and finally the grating formation mechanisms. Different theoretical equations of the polarized Raman scattering intensities, taking account of the pump polarization directions and the high numerical aperture objective used, are derived. Then, an analysis of the experimental results allows us to extract values of the second 〈P2〉 and fourth 〈P4〉 coefficients in the chromophore orientation functions at several positions of the SRM, in particular at the top and bottom regions, and to calculate the corresponding normalized distribution functions ia information entropy theory. In both gratings, somewhat broader distributions are found in the bottom regions, regions of greater polymer removal. Thus, mass-transport phenomena have perturbed, to a weak extent in the (±45°) first case and to a larger extent in the (p + p) second case, the primary photoinduced orientations. So, two simultaneous and superimposed mechanisms appear to be responsible for the observed complex SRM in the highly birefringent K1-10 copolymer under study and significant second-order contributions are suggested. The grating surface profiles are thus reproduced by fitting the SRM amplitudes at the fundamental and doubled frequency and the corresponding phase constant parameters: a good agreement between the observed (AFM) and simulated profiles is obtained.

Fast optical recording in dye-doped liquid crystals

Light-induced director reorientation in dye-doped nematic liquid crystals was recently reported to be an efficient method of writing permanent holographic gratings with high sensitivity [Phys. Rev. Lett. 82, 1855 (1999)]. We report the achievement of stable director reorientation in the same materials by means of a single 4-ns pulse of the second harmonic of a Nd:YAG laser. Fast recording of high-resolution holographic gratings (more than 500 lines/mm) can be obtained with an energy density as low as 7x10(-3)J /cm(2) .

Polymer dispersed liquid crystals: effects of photorefractivity and local heating on holographic recording

We report a detailed investigation of the photorefractive origin of permanent orientational gratings recorded by holographic technique in dye-doped polymer-dispersed liquid crystal (PDLC) films. This investigation was performed using wave-mixing characterization by two beam coupling (TBC) experiments. We determined the TBC gain by means of asymmetric energy transfer measurements and the phase shift by TBC translation technique measurements. The photorefractive origin of the effect were proved by the experimental results. Nevertheless, some peculiarities showed the presence of other mechanisms that combined with the photorefractivity to give the observed storage effect. The long time stability and some characteristic of the recorded structure have been explained as a thermal fixing of the grating. During the writing process, due to the strong light absorption, the sample was locally heated; under these conditions, the space charge field effect can modify the droplets interfaces and consequently the orientation of liquid crystal inside the droplets. After removing the two writing beams, the new configuration was frozen.

Laser-induced transient and permanent gratings in Eu 3+-doped dual alkaline earth silicate glasses

We have studied the formation of laser induced permanent and transient refractive index gratings in Eu 3+-doped dual alkaline earth silicate glasses. The growth, decay, and erasure of the grating are reported as functions of the write-beam power and crossing angle. The concentrations of the alkaline earth modifiers affected the diffraction efficiency of the induced grating. The diffraction efficiency of the permanent grating decreased as the concentrations of the larger and more massive alkaline earth modifiers (Ca, Sr, Ba) increased. The strength of the transient signal increased with the strength of the permanent grating. The permanent change in the index of refraction was attributed to the migration of small modifiers such as Na and Mg through the glass network. The energy to activate these modifiers was provided by the non-radiative relaxation of excited Eu 3+. The decay of the transient signal was the same as that of the fluorescence of the 5D 0 → 7F 0 states and was independent of the write-beam power. We suggest that the results of this research indicate that the use of large alkaline earth modifiers such as Ca, Sr, and Ba with concentrations less than 2 mol% produce a modest improvement in the diffraction efficiency in the rare earth-doped silicate glasses.

Optical Storage Effect in Dye Doped Polymer Dispersed Liquid Crystals

We report an accurate analysis of an optical storage effect observed in dyedoped PDLC films. Permanent diffraction gratings are produced using standard holographic techniques with different kind of PDLC samples. For all these samples the polymerisation process is completed before the writing procedure. The analysis of the experimental results brought us to distinguish the role of the PDLC components in the observed phenomenon and propose some hypothesis about the possible physical mechanisms at the basis of the storage effect.

Laser Written Permanent Gratings in a New Liquid Crystalline Organometallic Polymer

Interest in materials for optical storage effect has been intense in the last years. In this paper we report the observation of permanent gratings in a new liquid crystalline side-chain polymer containing a mononuclear cyclopalladated azobenzene complex. Two-beam coupling experiments indicate photorefractive properties. A gain of 900 cm−1, without applied electric field, was determined by two beam coupling measurements. A significant feature of this high gain material is that this effect occurs in an easily processable pure compound.

Writing and Erasure of Holographic Gratings in Dye-Doped Polymer Dispersed Liquid Crystals

We Demonstrate that is possible to write and erase holographic gratings in dye-doped polymer dispersed liquid crystals (PDLC). PDLC samples were irradiated after complete curing using two beams in an interferometric geometry. In this way permanent holographic gratings were written with a resolution up to 100 lines/mm. The erasure was obtained by a single beam, while successive writing were possible again by two beams interference, without any degradation after many cycles. The origin of the effect can be the onset of periodic distribution of droplets of different shape and spatial diffusion of the dye molecules.

Dye-doped liquid crystals as high-resolution recording media

Light-induced anchoring of the molecular director is reported to be an efficient method for writing permanent holographic gratings in dye-doped liquid crystals. We have achieved higher sensitivity and spatial resolution in these materials with other methods. An energy density as low as 10(-1) J/cm(2) was sufficient to write gratings with a resolution higher than 100 lines/mm.

UV induced permanent gratings in Ce 3+ or Eu 2+ doped PZG glass thin-film waveguides deposited on CaF 2 substrates

Ce 3+ or Eu 2+ doped PZG (PbF 2, ZnF 2, GaF 3) glass thin films fabricated by vapor phase deposition on CaF 2 substrates have been shown to possess high photosensitivity. As an illustration, strong permanent gratings have been written in the PZG glass by using a continuous wave UV laser ( λ = 244 nm). The kinetics of the grating growths were monitored by probing the efficiencies of the holograms at a wavelength of 633 nm. The mechanisms that could form the gratings have been analyzed. The UV induced changes in absorption coefficient at 633 nm or the UV induced surface modulation turned out to be too low to explain the order of magnitude of the grating efficiency. The gratings are assumed to come from changes in refractive index as high as 1.3 × 10 −2 and 3.9 × 10 −3 in the Eu 2+ and Ce 3+ doped samples respectively.