Photoinduced Refractive Index Change Research Articles

Preparation of diarylethene copolymers and their photoinduced refractive index change

Diarylethene copolymers were synthesized from 1-(6 0 -vinyl-2 0 -methylbenzo[b]thiophene-3 0 -yl)-2-(2 00 -methylbenzo[b]thiophene-3 00 -yl)hexafluorocyclopentene (VMBTF6) and 1-[6 0 -(4 000 -vinylbenzoyl)-2 0 -methylbenzo[b]thiophene-3 0 -yl]-2(2 00 -methylbenzo[b]thiophene-3 00 -yl)hexafluorocyclopentene (VBMBTF6) via living free radical techniques using stable TEMPO derivatives. The diarylethene content was controlled by the feed ratio of diarylethene derivatives and styrene. Transparent photochromic polymer films were prepared from the diarylethene copolymers with narrow molecular weight dispersion (PD � 1:3) by spin coating method. Photoinduced refractive index changes (DnTE) of the polymer films, with 25 mol% of diaryethene content, accompanied by the photochromic change were determined as 0.0009 and 0.0030 for poly(styrene-ran-VMBTF6) and poly(styrene-ran-VBMBTF6), respectively. 2002 Published by Elsevier Science B.V.

Growth Modeling of Fiber Gratings: A Numerical Investigation

We have developed a simple method to analyze the growth process of fiber gratings (FGs) in the core of photorefractive optical fibers. It is based on the exponential saturation model and it takes into account the type of photosensitive fiber used, the exposure times, and the incident optical power. The photorefractive fiber has been characterized by two parameters: the saturation photoinduced refractive index change and the saturation energy, which can be obtained experimentally. We carry out a numerical investigation for the growth process of a uniform FG. The experimentally observed phenomena of shifting the Bragg optical frequency with exposure times and the appearance of higher-order Bragg resonances are predicted and discussed.

Photo-induced refractive index change in hydrogenated amorphous silicon oxynitride

Refractive index change is shown to be induced by the irradiation of ultraviolet photons in hydrogenated amorphous silicon oxynitride films prepared by plasma-enhanced chemical vapor deposition. The mechanism of the index change and its dependence on the nitrogen content were investigated by electron spin resonance and scanning electron microscopy. It is concluded that the index change is due mainly to densification, and that the contribution of the formation of paramagnetic defects is only slight. To demonstrate the versatility of this refractive index change, a planar diffraction grating was fabricated.

Ultrafast all-optical spatial light modulation based on complex refractive index changes of copper phthalocyanine-doped polymer film upon photoexcitation in guided mode geometry

Ultrafast all-optical reflectance control was achieved by photoinduced complex refractive index changes in thin poly(vinyl alcohol) (PVA) films containing water-soluble phthalocyanine at the incident angle corresponding to a sharp guided mode resonance upon excitation with a ns pulsed laser as a writing beam. The rise time of reflectance was less than 20 ns and the fall time was controlled by the lifetime of the excited triplet phthalocyanine, 31 ns for copper phthalocyanine (CuPcS). The spatial resolution was determined to be better than about 3 μm by using a photochromic dye. The present system based on photoexcitation of CuPcS will allow all-optical spatial light modulation upto about 30 MHz.

Large Photoinduced Refractive Index Change of Polymer Films Containing and Bearing Norbornadiene Groups and Its Application to Submicron-Scale Refractive-Index Patterning

We measured photoinduced refractive index changes of polymer films containing various norbornadiene compounds. Poly(methyl methacrylate) (PMMA) films containing 51.6 wt% of 5,6-(2-benzofuryl)-2,3-di(trifluoromethyl)-1,4,7,7-tetramethyl-2,5-norbornadiene (NBD4) showed refractive index changes of 0.037 during photoisomerization. A polymer film bearing thiophenyl- and benzofuryl- substituted norbornadiene groups (PolyTBFNBD) showed a refractive index change of 0.05–0.06 for 594–632.8 nm during photoisomerization. This value is the largest value of refractive index change so far in a nonresonant region induced by photochemical reactions. Submicron-scale patterning of pure refractive-index contrast on the PolyTBFNBD film was also demonstrated by using near-field scanning optical microscopy (NSOM).

Observation of a fast-formed absorption grating and a slowly formed phase grating in undeveloped dichromated gelatin

Real-time diffraction efficiency is measured during hologram formation in undeveloped dichromated gelatin (DCG) film. The main features of real-time diffraction efficiency of undeveloped DCG reveal double peaks with increasing exposure time. Adopting the grating translation technique, we show that the first peak originates from the absorption grating and that the latter is related to the phase grating. To explain such complicated hologram-formation dynamics, we present a phenomenological model for mixed gratings associated with photoinduced refractive-index changes and absorption changes. We have compared the model results with the experimental results, which show good agreement.

Large Photoinduced Refractive Index Change in a Polyimide Film by Charge-Transfer Complex Formation with a Polymer-Bound Phenylazide Fragment

A large photo-induced refractive index change as large as 0.014 at 632.8 nm in thermostable transparent polymer films has been realized for the first time by using photoirradiation of polyimide precursor films containing a phenylazide and successive imidization. The phenylazide fragment forms a charge-transfer complex with imide moiety. The films maintained high transmittance in a wide visible region in contrast to usual polyimide films. This value of refractive index change is sufficient to make thermostable channel waveguides and other optical devices.

Low-cross-talk polarization-insensitive 10-GHz-spaced 128-channel arrayed-waveguide grating multiplexer–demultiplexer achieved with photosensitive phase adjustment

We successfully fabricated a polarization-insensitive 10-GHz-spaced 128-channel arrayed-waveguide grating whose cross talk ranged from -39 to -36 dB . The optical phases of all arrayed waveguides were adjusted simultaneously by means of a photoinduced refractive-index change under ArF excimer laser irradiation through metal masks.

In situ observation of photoinduced refractive-index changes in filaments formed in glasses by femtosecond laser pulses

We investigated the relationship between the formation of filaments and local refractive-index changes induced by femtosecond laser pulses in silica glass. In situ observation revealed that the location of a filament coincided with that of the refractive-index change. Observation also showed that the region of refractive-index change was elongated toward the upstream direction of the laser pulses with increasing exposure time. The region of refractive-index change was several hundred micrometers long, and its diameter was smaller than 2 mum. The refractive-index change was confirmed by two of three different methods to be as large as 0.8 x 10(-2).

Preparation of Diarylethene Substituted Polymer Films for Optical Recording

ABSTRACTEffect of photo-irradiation on the color change of diarylethene polymer films was investigated. Diarylethene polymer films were prepared from photopolymerization of a radical curable mixture of diarylethene compound and a fluoroalkylacrylate containing a photo initiator. 1-[6'-(Methacryloxyethyloxycarbonyl)-2'-methylbenzo[b]thiophene-3'-yl]-2- (2"-methylbenzo[b]thiophene-3"-yl)hexafluorocyclopentene (MMBTF6) was synthesized from 2,3-bis(2-methylbenzo[b]thiophene-3-yl)hexafluorocyclopentene (BTF6) in three steps. 2,2,3,3-Tetrafluoro-1,4-butyldiacrylate (TFBDA) was synthesized from the corresponding diol and acryloyl chloride in the presence of a base. The photocurable mixture was coated on a substrate and subjected to antinic irradiation, to afforded homogeneous transparent film. A mask image was recovered on the film by a light of 365 nm and read by a visible light (λ > 700 nm) without destruction of the image. To erase the recorded image, a white light or a visible light was irradiated. Photochromic quantum yield and photo-induced refractive index change of the diarylethene bound polymer film were determined as 0.12 and 0.0006 respectively.

Reflection-Mode All-Optical Parallel Switching in Guided Wave Geometry Including Photochromic Compounds

A novel all-optical parallel switching device was demonstrated by using photoinduced changes of a complex refractive index in a guided mode thin film composed of silver and polymer containing photochromic dyes. An incident angle of a probe beam was set at a value corresponding to the minimum reflection due to the guided mode. Pulsed laser excitation caused changes of absorption of the polymer films due to photochromism, which resulted in the increase of reflection because the guided mode condition was changed. According to the Kramers-Kronig relationship the real part also changes, which results in the shift of the minimum. Appropriate wavelength can be selected for all-optical switching in this geometry. We used photochromic spiropyran dispersed in polystyrene. Self-held parallel switching was achieved upon UV or visible laser excitation with response times less than 20 ns.

Heat Conduction in Nano-Environment Observed in Cooling Processes of Colloidal Silver Nanoparticles in Water

Ultrafast dynamics of colloidal silver nanoparticles in an aqueous solution was investigated by observing the ultrafast lensing effects caused by a photo-induced refractive index change. The ultrafast response consisted of an instantaneous peak by the optical Kerr effect and a subsequent bi-exponential decay component. The decay component was independent of the relative polarization planes of pump and probe beams and had temperature-dependent time constants ranging from 0.4 to 2.2 ps and from 10 to 100 ps. A minimum at around 7°C was found for the time constant of the fast component. The results suggest that the structure of water molecules adjacent to the silver surface is affected by hydrogen bonds in a temperature-dependent manner similar to that of the molecular structure of bulk water.

Significant photoinduced refractive index change observed in porous silicon Fabry–Pérot resonators

It is shown that a porous silicon Fabry–Pérot resonator device exhibits a significant nonlinear optical transmission behavior at wavelengths in the near-infrared region. The input–output (transmission–incident power) response curve shows superlinear or sublinear behavior dependent on excitation wavelengths around the resonant position. These characteristics are interpreted as a result of nonlinear refractive index change, including by enhanced optical power in the Fabry–Pérot cavity configuration and carrier accumulation in silicon nanocrystallites. The result leads to further progress of nanocrystalline silicon into functional photonic device applications.

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 >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 %.

Structures and Optical Properties of Defects Correlated with Photo-Induced Refractive Index Changes in Ge-Doped SiO<sub>2</sub> Glass

Structures and Optical Properties of Defects Correlated with Photo-Induced Refractive Index Changes in Ge-Doped SiO<sub>2</sub> Glass

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 changes in the optical constants of Ge–Se–AgI thin films

The photoinduced changes in the complex refractive index n = n − ik of thin films from the Ge–Se–AgI system with constant ratio Ge/Se=1/4 and concentrations of AgI of 0, 5 and 10 mol% are studied by real time measurements of reflectance ( R) and transmittance ( T) of the films. The phase delay ( δ) between the components of the transmitted wave, which is proportional to the birefringence of the film, is measured in real time as well. The changes in the average value of the refractive index (Δ n), and in the average value of the absorption index (Δ k) in the imaginary part of n as well as the induced optical anisotropy are estimated by solving the inverse optical problem. It is found that involving small quantities of AgI into a Ge–Se matrix increases the sensitivity of the films, but the anisotropic effects are comparatively weakly in them. The maximum values of changes in the average refractive index (Δ n=0.025) and in the average absorption index (Δ k=−0.03) are obtained in the films, containing 5 and 10 mol% AgI, respectively. Considerable values of the photoinduced anisotropy – birefringence ( Δn a≈−0.002) and dichroism ( ΔD≈0.05, which corresponds to Δk a≈0.035) are observed in films without addition of AgI (“pure” Ge–Se film).

Large photoinduced refractive index changes in pulsed-laser-deposited lead germanate glass waveguides with controllable refractive index sign change

A study of the photosensitivity of lead germanate glasses grown by pulsed laser deposition is presented. Lead germanate films were grown from stoichiometric bulk targets, on borosilicate glass substrates in an oxygen atmosphere, to form optical waveguides. The films grown exhibited high absorption in the ultra violet spectral area (around 240 nm) which indicated the possibility of high photosensitivity. The photosensitivity of the materials was examined by phase mask and interferometric grating recording over a wide ultra violet spectral region (325nm-193nm) using various continuous and pulsed laser sources. Very high refractive index changes (up to 1%) have been observed along with variable sign of the refractive index change depending on the oxygen background pressure used during growth and recording intensity

Raman spectra and photostructural changes in the short-range order of amorphous As–S chalcogenides

Photoinduced changes of optical transmission, diffuse reflectance, refractive index and Raman spectra of As x S 100− x stoichiometric ( x=40) and non-stoichiometric ( x=38, 42) films, bulk glasses and glassy powders were measured. The photoinduced changes of the absorption edge, diffuse reflectance and refractive index were accompanied by changes in Raman band intensities and positions and, therefore, by changes of structure. The Raman spectra of freshly evaporated films, and spectra of As-rich, contain some bands of As 4S 4 molecules whose content (e.g. ⩾20 mol% in As 42S 58 freshly evaporated films, ∼10 mol% for As 42S 58 glasses) depends on the stoichiometry and state of the films (freshly evaporated, exposed or annealed films) or upon the state of samples (bulk, powdered glass, exposed or annealed powder). The As 4S 4 content decreased by light exposure and also by annealing, while irradiation increases the content of As–As bonds. A model is proposed in which the photoinduced changes of the optical properties in As–S glasses are connected with photostructural changes, which change the short-range order, namely the densities of individual chemical bonds.

Composition dependence of the photoinduced refractive-index change in lead silicate glasses

A large photoinduced refractive-index change (as great as Dn=0.21+/-0.04) is obtained in lead silicate glasses by irradiation with the frequency-quadrupled output of a Q -switched YAG laser (266 nm). An approximately exponential relationship is found between the photoinduced refractive-index change and the lead cation mole fraction over the composition range from 18.7% to 57%. The induced refractive-index change is permanent and shows no decay after heating to 360 degrees C during 1 h. Dispersion of the refractive-index change suggests that the photosensitivity is associated with changes in the intrinsic glass absorption edge.