Gratings In Thin Films Research Articles

Direct fabrication of surface relief gratings in chalcogenide glasses by excimer laser interference lithography

In this paper we study the possibility to realise surface relief gratings in thin chalcogenide glass films by holographic exposure using a pulsed KrF excimer laser. Gratings with a period of 540 nm and depths of 100–300 nm were patterned at the surface of 1 μm thick films. Due to coupling of an incident near-infrared laser beam to waveguide modes a resonance-like polarisation dependent decline of transmission was observed at specific incidence angles. Just one laser pulse with a fluence of 12 mJ/cm2 per beam was sufficient to achieve the required grating parameters in sulphide glasses with low Tg.

Electro-optically tunable second-harmonic-generation gratings in ion-exfoliated thin films of periodically poled lithium niobate

The authors demonstrate that thin, single-crystal films fabricated by ion exfoliation of a bulk periodically poled LiNbO3 (PPLN) crystal reduced the tuning voltages for electro-optically tunable harmonic generation in PPLN devices. The tuning voltage of ±150V resulted in 0.5nm total tuning of the second harmonic signal in the authors’ 10-μm-thick device, compared to an order of magnitude higher tuning voltage value needed in a comparable bulk device.

Influence of two-dimensional bragg diffraction on the competition of the waveguide TE0 and TM0 modes during formation of spontaneous gratings in AgCl-Ag thin films

An intricate space-time instability of patterns of small-angle scattering and diffraction of a laser beam on the structures appearing in AgCl-Ag films under exposure to the same beam is investigated. The instability is related to the formation of spontaneous gratings in the film resulting from the interference of the incident beam with the waveguide modes scattered in the film. The existence of a two-dimensional Bragg diffraction on the TE0 and TM0 modes with subsequent formation of secondary spontaneous gratings is revealed and is theoretically substantiated. It is established that the difference in the indicatrices of the radiation scattered into the TE0 and TM0 modes for the s-and p-polarizations manifests itself in different kinds of diffraction and small-angle scattering patterns at the output of illuminated samples.

Strong Bragg gratings photoinduced by 633-nm illumination in evaporated As2Se3 thin films.

Bragg gratings are used in several photonic devices to reflect, and thus to isolate, specific wavelengths of light. Gratings can be photoinduced in chalcogenide glasses by illumination of bandgap light in an interference pattern. We used holographic interferometry to create Bragg gratings in amorphous As2Se3 thin films with a period of 0.56 microm by illumination with 633-nm light. The quality of the gratings was tested in real time, and refractive-index modulations as high as 0.037 were measured. These gratings were found to be stable over a period of several months if they were kept in the dark.

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.

Specific features of holographic recording of diffraction gratings in thin films of fullerene-containing organic systems

Thin films based on fullerene-containing polyimides COANP and PNP were studied using methods of dynamic holography under conditions of Raman-Nath diffraction. A significant increase in the diffraction efficiency of the fullerene-containing samples was detected, and the threshold of the transition from phase grating to amplitude grating was determined. The qualitative explanation of the results is based on the spectral features and the thermal properties of the systems under study.

Analysis of temporal behavior of beams diffracted by volume gratings formed in photopolymers

The temporal behavior of beams diffracted by volume gratings in photopolymer thin films are measured and analyzed by solution of the diffusion equation for the monomer concentration inside the thin films. Two contributors to the refractive-index change that forms the volume gratings are assumed: One is the phase grating formed by modulation of the monomer concentration, and the other is the phase grating formed by modulation of the density of the polymeric materials. The phase grating that is due to monomer modulation is responsible for the initial fast rise and decay of the diffracted signal, and the phase grating that is due to modulation of density of the polymeric materials is responsible for the slowly rising and then steady signal. The temporal behavior of the diffracted beams is determined by the ratio of magnitudes of the incident beam intensity and the diffusion coefficient.

Linear and non-linear gratings in DR1 side chain polymers

Linear (refractive index) and non-linear (second order non-linear optical susceptibility) gratings are of primary interest in optoelectronics. While linear gratings may be used for filtering or wave length selective coupling, non-linear gratings may find applications in active devices such as second harmonic generators, parametric generators or all optical switches through cascading of second order susceptibilities. We report on a simple method for the inscription of linear or non-linear gratings in DR1 side chain polymer thin films or waveguides. The technique is based on the photoisomerisation induced reorientation properties of such a material under irradiation with an argon ion laser. It avoids the use of interdigited electrodes for the selective orientation of the dyes, can be easily applied to a waveguide geometry and results into a good orientational stability. Sample characterization involves polarized microscopy and second harmonic generation.

Peculiarities of spontaneous-grating formation in light-sensitive films under elliptically polarized laser radiation

The formation of spontaneous gratings in thin waveguide AgCl–Ag photosensitive films was studied under elliptically polarized laser radiation. New peculiarities of the gratings growth were found (i) a deviation in angular position of the wave vectors of the most probable microgratings from π / 2 about the major axis of the polarization ellipse; and (ii) an asymmetry, which depends on the ellipticity sign, in the diffraction and small-angle scattering patterns. The experimental results are explained within the framework of a model that shows the scattering of incident light by small prolate absorptive Ag spheroids oriented across the major axis of the ellipse and arising from the stage of photoinduced dichroism that precedes grating formation. We show that the dependence of the diffraction patterns' asymmetry on the ellipticity sign is related to an instability due to the angular position of the wave vectors of the most probable microgratings. PACS No. 78.65

Pulsed laser recording of gratings in SiO-Cu quantum dot thin films

Interference patterns are written by two intersecting pulsed laser beams in thin films composed of Cu quantum dots in SiO glass. The rapid local optical heating of the metal clusters forming the quantum dots in the glass film results in the writing of patterns in the thin film. The high cluster temperatures attained drive the selective growth of the larger quantum dots and also result in a stress induced peeling of the film from the substrate. Estimates of the kinetics of the selective quantum dot growth agrees well with experimental observations.

The influence of substituents on the orientational behaviour of novel azobenzene side‐chain polyesters

AbstractA series of new mesogenic azobenzene propanediols with nitro, methoxy, cyano and no substituent have been synthesized and their mesomorphic behaviour mapped. The diols are employed in a melt transesterification with diphenyl tetradecanedioate to produce side‐chain polyesters with high molar masses. Diols and polyesters have similar UV‐visible spectra reflecting the acceptor character of the substituent. Polarization FTIR spectroscopy has been used to determine the mesogenic order parameter for the polyesters with different substituents at four laser irradiation wavelengths (413, 458, 488 and 515 nm). Diffraction gratings in thin polyester films have been recorded through polarization holography. Cyano and unsubstituted azobenzene polyesters are shown to exhibit the highest diffraction efficiencies on the order of 13%.

Diffraction from a short-cavity Fabry-Pérot: application to photorefractive quantum wells

Diffraction efficiencies from absorption and index gratings in short-cavity Fabry-Pérot multiple quantum well structures are strong functions of film thickness. Diffraction efficiencies of transmitted and reflected waves vary by an order of magnitude by adjusting the device thickness to bring the Fabry-Pérot resonance condition close to the excitonic absorption. Asymmetric Fabry-Pérot quantum-well reflection modulators produce the largest diffraction efficiencies, with input diffraction efficiencies approaching 6%. Analytic expressions for the diffraction efficiencies are derived in the Raman-Nath regime for a single thin film and for multilayer structures. These results apply in general for dynamic holography and transient dielectric gratings in thin films, but is illustrated specifically for photorefractive semiconductor quantum wells.

Laser-induced acoustic phonon gratings in semiconductor thin films

Laser induced ultrasonic standing waves are observed in molecular beam deposited ZnSe thin films using the transient grating technique. This observation is attributed to a very short trapping time for single photon absorption in the band tail. The period of the standing wave is used to determine the acoustic phonon velocity in the ZnSe thin film which indicates that it is longitudinal acoustic phonons which are excited.

Polarization analysis of induced gratings in nematic liquid crystal films

A detailed analysis of classical diffraction on the optically induced phase gratings in thin films of the nematic liquid crystals 5CB, 6CB, and MBBA is presented. It enables a quantitative separation of thermal contributions from reorientational contributions to the gratings to be made by a simple measurement of the gratings' diffraction efficiency with a polarized probe laser.

Time dependence and diffraction efficiency of optically-induced phase gratings in nematic liquid crystal films

The time dependence and diffraction efficiency of optically-induced phase gratings in thin films of the nematic liquid crystals 5CB and 6CB are reported. The experimental results are in quantitative agreement with a three-dimensional model for the induced grating. This analysis provides an independent determination of the elastic constants K2 and K3 as well as a vicosity coefficient ν̄.

Scanning-electron-microscope-written gratings in chalcogenide films for optical integrated circuits

Scanning-electron-microscope-written gratings in amorphous chalcogenide thin films have interesting and promising characteristics as guided-wave optical components. As a first trial to demonstrate their use in integrated optics, we have designed and fabricated the gratings for deflecting and splitting the guided waves and experimentally confirmed their excellent optical performance of high efficiency and wide acceptanceangle.

Diffraction efficiency of continuously etched gratings in As 2S 3 films

The diffraction efficiency of gratings in chalcogenide glass thin films during the chemical etching process is described. The experiment is performed by a simultaneous etching and detection technique, and two absolute maxima with intermittent interference maxima and minima are observed. Theoretical analysis including interference effect is presented, and excellent agreement with experimental curve is obtained. Application of this technique to lithographic process for etching control is briefly discussed.