Inscription of holographic gratings using circularly polarized light: influence of the optical set-up on the birefringence and surface relief grating properties

The polarization properties of the optical set-up used for holographic recording of diffraction gratings on azopolymer thin films are analyzed. The state of polarization of circularly polarized light is fully analyzed after reflection on a mirror at various incidences (Lloyd-mirror set-up). The Stokes analysis is performed using a photopolarimeter and the phase shift, the ellipticity and the azimuth orientation are compared with those calculated from Fresnel formulae. At large angles of incidence, an initially right circularly polarized (RCP) beam becomes elliptically polarized with an azimuth of nearly +45°. From these results, holographic diffraction gratings are recorded on an azobenzene-containing polymer thin film using (i) co- and contra-circularly polarized beams and (ii) a right circularly polarized beam interfering with a +45° linearly polarized light beam. Using Jones-matrix formalism, the polarization states of the diffracted orders from the birefringence (Δn) and the surface-relief (2Δd) gratings are derived and compared with experimental measurements. Finally, the induced local birefringences and surface-relief amplitudes are discussed in connection with atomic force microscopy measurements. The diffraction efficiencies obtained under the (+45°+RCP) and (LCP + RCP) (where LCP = left circularly polarized) configurations are thus compared and discussed. Received: 5 October 2001 / Revised version: 26 November 2001 / Published online: 17 January 2002     

Photoinduced macroscopic chiral structures in a series of azobenzene copolyesters

A study of the propagation of elliptically polarized light and the resulting formation of macroscopic chiral structures in a series of azobenzene side-chain copolyesters, in which the morphology is varied from liquid crystalline to amorphous, is reported. Real-time measurements are presented, showing the dynamic behavior of the photoinduced rotation of the polarization ellipse in the different samples. The relationship between the ellipticity of the recording light and the linear birefringence induced is studied. A numerical solution that takes into account the influence of the photoinduced linear dichroism on the light propagation through the samples is presented. Received: 10 June 2002 / Revised version: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +45-4677/4588, E-mail: lian.nedelchev@risoe.dk RID="**" ID="**"Permanent address: Department of Chemistry and Physics, The Nottingham Trent Unversity, Clifton Lane, Nottingham NG11 8NS, England     

Femtosecond-laser-induced nanostructure formed on hard thin films of TiN and DLC

We report observation of nanostructures formed on thin TiN and DLC films that were irradiated by 800- and 267-nm, femtosecond (fs) Ti:sapphire laser pulses at an energy fluence slightly above the ablation threshold. On the ablated thin-film surfaces, the linearly polarized fs pulses produce arrays of fine periodic structures that are almost oriented to the direction perpendicular to the laser polarization, while the circularly polarized light forms fine-dot structures. The size of these surface structures is 1/10–1/5 of the laser wavelength and decreases with a decrease in the laser wavelength. Received: 3 September 2002 / Accepted: 4 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +81-778/62-3306, E-mail: yasuma@fukui-nct.ac.jp     

Theory of optical bistability in a non-linear quasi-waveguide

Optical bistable behavior in a quasi-waveguide containing non-linear film has been experimentally investigated in several publications in the past years; however, the physical mechanism for optical bistability has not been theoretically explained. In the present letter, we propose a theoretical model and successfully explain the observed optical bistability of both the reflected light and the scattered light (m-lines) in a non-linear quasi-waveguide. The optical bistability in the non-linear quasi-waveguide is due to the scattering-type wavevector mismatch mechanism. Received: 10 July 2002 / Revised version: 10 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: htwang@nju.edu.cn     

Polarisation-sensitive optical coherence tomography for material characterisation and strain-field mapping

Optical coherence tomography (OCT) is a relatively new imaging technique capable of recording cross-sectional images of transparent and turbid structures with micrometer-scale resolution. Originally developed for biomedical imaging applications, this technique also has a great potential for non-destructive material characterisation and testing. Polarisation-sensitive (PS) OCT is a recent extension of classical OCT that measures and images birefringence properties of a sample, which, however, has not yet been applied to materials science. We present imaging of glass-fibre-enforced epoxy resin compounds and the detection of dry spots, where the epoxy did not properly penetrate the glass-fibre structure. Furthermore, we demonstrate PS-OCT imaging of the birefringence properties of different materials. The mapping of strain fields of samples under uniaxial and non-uniform external stress and the detection of flow patterns in injection-moulded plastic parts could be demonstrated with this technique for the first time. Received: 21 October 2002 / Accepted: 22 November 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. Fax: +43-732/9015-5618, E-mail: david.stifter@uar.at     

High-peak-power diode-pumped passively Q-switched Nd:YVO<Subscript>4</Subscript> laser

We report on a passively Q-switched diode-pumped Nd:YVO₄ laser polarized along the a axis (corresponding to the smallest value of emission cross section at 1064 nm), generating 157-μJ pulses with 6.0-ns time duration (>20 kW peak power) and 3.6 W of average power at 1064 nm with good beam quality (M²<1.4). The selection of the polarization was performed by a novel technique relying on the birefringence of the laser crystal and on the misalignment sensitivity of the resonator. Received: 30 September 2002 / Revised version: 22 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +39-382/422583, E-mail: agnesi@ele.unipv.it     

Inverse Relaxation of Photoinduced Birefringence in a Liquid-Crystalline Azobenzene Side-Chain Polymer

Photoinduced birefringence in a liquid-crystalline azobenzene side-chain polymer is investigated. It is observed that the birefringence does not show any decay but increases after switching off the pump light at room temperature. The magnitude of the birefringence relaxation is found to depend on the exposure dose of the pump light. A discussion about the mechanism of the inverse relaxation of birefringence is presented.     

Pulsed-laser-deposited epitaxial aluminum nitride films on (111) Si for surface acoustic-wave applications

Epitaxial (001) aluminum nitride (AlN) thin films on (111) Si substrates are prepared using pulsed-laser deposition. The epitaxial structure of the as-prepared thin films is characterized by checking the X-ray-diffraction θ-2 θ scan and pole-figure, using scanning electron microscopy, infrared radiation (IR) spectroscopy and Raman spectroscopy. The surface acoustic-wave resonance at 345 MHz for a 1.5 μm thick AlN film on a (111) Si substrate is observed using an inter-digital electrode. Received: 18 September 2001 / Accepted: 29 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: liujm@nju.edu.cn     

Calculation of the temperature and thermal expansion of a STM tip heated by a short laser pulse

A mathematical model for the calculation of the temperature field in a scanning tunneling microscope (STM) tip under laser illumination is developed. The duration of the laser pulse is a few nanoseconds or shorter. A Gaussian distribution of the laser light intensity in time and space is assumed. Two different mechanisms of tip heating are taken into account: 1. due to an enhanced electric field on the tip; 2. due to heating of the side surface of the tip by the focused spot of laser light. An average tip temperature is calculated using the heat conductivity equation. The enhanced electric field on the tip is calculated by the method of boundary integral equations. Received: 20 August 2002 / Revised version: 4 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-2551/962-490, E-mail: sklein@fh-muenster.de     

Nanoscale domain switching mechanism in Pb(Zr,Ti)O<Subscript>3</Subscript> thin film

Pb(Zr,Ti)O₃ (PZT) ferroelectric thin film was prepared by the sol-gel technique and crystallized with a (111) preferred orientation. The domain structure and polarization reversal behavior were investigated by using scanning force microscopy (SFM) piezoresponse mode at the nanometer scale. A step structure of approximately 30 nm in width was directly observed, which was formed during the polarization reversal process. The presence of the step structure reveals that the forward domain-growth mechanism is the dominant domain-switching process in our PZT thin films. Received: 6 August 2002 / Accepted: 9 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/5241-3122, E-mail: huarongzeng@163.net     

Mechanisms of optical anisotropy induction in azopolymer films

The process of optical anisotropy indiction in azopolymer films upon irradiation by polarized UV light is studied by means of UV and IR birefringence spectroscopy. A substantial dependence of the character of the relaxation of the induced anisotropy on the exposure time to the exciting light is revealed: whereas the process of anisotropy induction is reversible at small exposure times, at large exposure times the induced anisotropy virtually does not relax and even increases after the irradiation is terminated. A difference between kinetic curves of dichroism and birefringence induction is shown: at exposure times corresponding to the saturation of the induced dichroism a slow increase in the birefringence curve is observed. Results obtained are explained in terms of the assumption of partial ordering of macrochain fragments in irradiated films. The assumption is substantiated by measurements of polarized IR spectra. Institute of Physics of the National Academy of Sciences of Ukraine, 46, Nauka Ave., Kiev, 252650, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 121–125, January–February, 1998.     

Interfaces with vapor-evaporated polyaniline thin films

Interfaces with vapor-deposited polyaniline thin films are investigated using angle-resolved X-ray photoemission spectroscopy. We demonstrate that a diffuse interface is formed with sodium deposited on top of polyaniline, and the distribution of sodium atoms, in the near surface region of the polyaniline thin film, is quite uniform. In comparison, the interface between polyaniline and another polymer, PVDF-TrFE, is rather abrupt and a flat-band model can be applied to the interface band structure. Received: 29 November 2002 / Accepted: 16 December 2002 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-402/472-2879, Email: pdowben@unl.edu     

Optical properties of SrTiO<Subscript>3</Subscript> thin films by pulsed laser deposition

SrTiO₃ thin films were prepared on a fused-quartz substrate by pulsed laser deposition (PLD). Dense and homogeneous films with a thickness of 260 nm were prepared. Optical constants (refractive index n and extinction coefficient k) were determined from the transmittance spectra using the envelope method. The optical band gap energy of the films was found to be 3.58 eV, higher than the 3.22 eV for bulk SrTiO₃, attributable to the film stress exerted by the substrate. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The refractive index and the packing density for the PLD-prepared SrTiO₃ thin films are higher than those for the SrTiO₃ films prepared by physical vapor deposition, sol–gel and RF sputtering. Received: 18 March 2002 / Accepted: 7 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

Pulsed laser deposition and in situ diagnostics of the process applied to shape-memory alloys

NiTi films deposited by pulsed laser ablation on Si/SiO₂ are shown to exhibit structural and functional properties related to the shape-memory effect. Film characterization suggests that relevant temperatures for the solid-to-solid transformation responsible for the shape memory are in substantial agreement with those of the bulk target material, demonstrating a good congruency of the deposition process. Besides the technological interest for this class of thin films, our findings point out the suitability of laser ablation for metal alloy deposition. An investigation based on in situ ion-mass spectroscopy and covariance mapping analysis allows us to determine the main vapor-phase processes leading to the formation of stoichiometric clusters expected to play a relevant role in assisting the growth of NiTi thin films. Received: 6 August 2001 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-50/2214-333, E-mail: fuso@df.unipi.it     

The photo-neutronrefractive effect

Since its discovery in 1966, the photorefractive effect, i.e. the change of the refractive index upon illumination with light, has been studied extensively in various materials and has turned out to play a key role in modern optical technologies like photonics. This article focuses on substances that change their refractive index for neutrons when irradiated with light. In analogy to light optics, we call them photo-neutronrefractive. After a short introduction to the relevant concepts of neutron optics, two materials exhibiting this effect, a photopolymer and an electrooptic crystal, are presented. Further, we discuss the progress made concerning the development of creating light-induced gratings for neutron diffraction, which culminated in the setup of an interferometer for cold neutrons. Experiments performed on photo-neutronrefractive materials are surveyed and the variety of corresponding results obtained is presented, including a discussion of their impact on material science, neutron optics, and the foundations of physics. Received: 23 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +43-1/4277-9511, E-mail: martin.fally@exp.univie.ac.at     

偶氮类有机材料光致双折射的测量

研究了2种偶氮聚合物薄膜的光致双折射效应,并探讨了泵浦光偏振态和光强对光致双折射的影响.采用远离共振区的He-Ne光(633nm)作为探测光,用Ar+激光(488nm)作为泵浦光,通过测量相关参量得到了样品的光致双折射值.实验结果表明:改变泵浦光的偏振态可以控制光致双折射值和探测光的透过信号强度,在0°～45°范围内,泵浦光光强存在最佳值,此时偶氮薄膜实验样品具有最大的双折射值.     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

Electromagnetically induced transparency mediated by phonons in strongly coupled exciton–phonon systems

It is shown theoretically that electromagnetically induced transparency (EIT) due to strong exciton–phonon coupling can occur in strongly coupled exciton–phonon systems such as polymers and organic semiconductors and lead to ultra-slow light effects. The results indicate that the strong coupling of excitons and phonons is important, but the exciton– exciton interaction plays a small role in the generation of the EIT. Numerical results for polydiacetylene–toluene sulfonate are also presented. This EIT in a solid-state medium might be utilized for efficient multiwave mixing and quantum nondemolition measurements, as well as for novel acousto-optical devices. Received: 21 August 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. E-mail: zhukadi@yahoo.com     

Generation of 99-mW continuous-wave 285-nm radiation for magneto-optical trapping of Mg atoms

We have developed a tunable intense narrow-band 285 nm light source based on frequency doubling of 570 nm light in BBO. At input powers of 840 mW (including 130 mW used for locking purposes) we generate 99 mW UV radiation with an intensity profile suitable for laser-cooling experiments. The light is used for laser cooling of neutral magnesium atoms in a magneto-optical trap (MOT). We capture about 5×10⁶ atoms directly from a thermal beam and find that the major loss mechanism of the magnesium MOT is a near-resonant two-photon ionization process. Received: 15 February 2002 / Revised version: 13 August 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +45-4588/7762, E-mail: dnm@mic.dtu.dk Present address: Mikroelektronik Centret, Technical University of Denmark, Orsteds Plads, Bldg. 345 East, 2800 Kgs. Lyngby, Denmark