Photoassisted poling of azo dye doped polymeric films at room temperature

The reversible cis-trans photoisomerization of disperse red 1 (DR1) in PMMA thin films has been demonstrated to be strongly polarization sensitive [1]. In this communication two mechanisms are identified: the angular hole burning and the angular redistribution of molecules. It is shown that, in the presence of a DC electric field, the redistribution is not centrosymmetric and produces a poling of the film. The evolution of the second-order nonlinear susceptibility, ⁽²⁾, is monitored by measuring the electro-optic effect by attenuated total reflection and by second-harmonic generation.     

Compositional stability in GeO x and SbO x thin films for optical-storage applications

Laser-induced transformations (optical and compositional) of amorphousMO _x thin films are studied as a function of the oxygen content and the nature of theM element,M being either a semiconductor (Ge) or a semi-metal (Sb). A high optical contrast in reflectivity is always found associated to the first laser pulses, this process being unaffected by the presence of oxygen. Next pulses lead to a process which may involve optical changes depending on the environmental conditions. Whereas the former optical changes are most likely related to a structural process which is completed before oxidation starts, the latter ones occur when irradiating at high oxygen pressures (1.2 bar) and thus they are easily related to an oxidation process. At low oxygen pressures (close to the oxygen partial pressure in air), the films show a good compositional stability upon irradiation together with a high optical contrast.     

A study of some optical properties of hafnium dioxide (HfO2) thin films and their applications

2 ), a series of films ranging in thickness from 50 to 10000 nm was prepared by using an electron beam gun inside an evacuated coating chamber of pressure 1×10^-5 mbar. The films were obtained on optical glass substrate by using oxygen with a backfill pressure of 2.4×10^-4 mbar during the deposition processes. The optical constants of the films were computed in the spectral wavelength region (350–2000 nm) from the transmission, reflection and thickness measurements. A computer program was created to determine two optical parameters n and k of the films, and this was achieved by entering the practical results into the computer program, which solved a series of equations for each wavelength. The effects created by changing various evaporation conditions (thickness, substrate temperature and evaporation rate) were studied in the spectral wavelength range, and the optimum values of the various conditions were obtained while achieving the best optical performance. According to the investigations of the HfO₂ material, two applications of the anti-reflection (AR) multi-layer coatings were achieved in two different spectral wavelength ranges. The first application was measured in the visible and near infra-red (VIS/NIR) range from 500 nm to 850 nm deposited on the glass substrate. The second application was measured in the infrared (IR) range from 7500 nm to 11500 nm deposited on germanium substrate. Computer modelling for designing the optical multi-layer system has been presented. The theoretical formulation and experimental results with the same specification were achieved. The correlation between the theoretical and the experimental results reveals a close agreement that offers a convenient method for predicting and controlling the multi-layer coating. By continuous measurement of the optical and mechanical (durability) performances of the coating process, high-quality films were produced in the manufacture of various optical devices. Received: 16 April 1997/Accepted: 5 August 1997     

Laser-induced self-phase modulation in polymeric films

Self phase modulation is observed in a polymeric thin film for the first time. It is found that this effect can occur at relatively low laser intensity. Experimental results obtained are compared with theoretical predictions.     

Thin films by regular patterns of metal nanoparticles: Tailoring the optical properties by nanodesign

Ultrathin metal films consisting of regular two-dimensional arrays of 44–95 nm sized silver nanoparticles were fabricated by electron-beam lithography. By independent variation of particle shape and interparticle distance, respectively, the film's optical extinction maximum wavelength can be tuned in a range from 460 nm to 520 nm and the bandwidth between 50 and 100 nm. The results suggests the potential of lithographically designed nanoparticle films as thin layer material with tailorable optical properties for applications in microoptics and optoelectronics.     

A study of X-ray luminescence and spectral compatibility of europium-activated yttrium-vanadate (YVO4: Eu) screens for medical imaging applications

The X-ray luminescence efficiency of laboratory-prepared YVO₄: Eu screens and their spectral compatibility to common optical detectors were studied under medical fluoroscopy conditions. YVO₄: Eu screens were prepared by sedimentation and with different coating thickness. Luminescence efficiency of the YVO₄: Eu screens was measured at various X-ray tube voltages (50–250 kVp) and for screens of different coating thicknesses (20–180 mg/cm²). Spectral response was also measured and spectral matching factors between the YVO₄: Eu screens and some common optical detectors (photocathodes, photodiodes, photographic emulsion) were calculated. Experimental results on efficiency were fitted by formulas of the theoretical model developed by Hamaker and Ludwig in order to determine phosphor intrinsic X-ray-to-light conversion efficiency and intrinsic optical characteristics, such as coefficients related to light scattering and absorption. Although the luminescence efficiency of YVO₄: Eu screens was found to be relatively low (3–11 M s/mR m²), the matching factor of YVO₄ : Eu screens with some red sensitive optical detectors was excellent, of the order of 0.96. High spectral compatibility may indicate that YVO₄: Eu scintillators could be used in medical image detectors.     

Polarization dependence in the UV laser desorption of NO from NiO(100)

- and -polarized radiation. When the yield is related to the incident photon flux, desorption cross sections of (1.9±0.3)×10^-17 cm² and (3.3±0.5)×10^-17 cm² for - and -polarized light, respectively, are deduced. In order to assess the importance of substrate excitation the optical constants of NiO have been determined for the photon energy employed. When the desorption yield is then related to the photon flux absorbed in the NiO film, much larger cross sections are obtained, and an even larger effectiveness of -polarized light. For a direct optical excitation of charge transfer states the implications for the symmetry character of the states involved are discussed. It is concluded that excitations to A^′′ states are preferred. Received: 21 October 1998     

Propagation and deflection of guided modes in planar waveguides via grating rotation

Propagation and deflection of guided modes in a polymer slab waveguide is studied. Rotation of the grating structure with respect to the plane of incidence deflects the waveguide modes in the plane of the sample so that the propagation direction is no longer perpendicular to the grating grooves. The deflection angles are measured as a function of the grating rotation angle and the experimental results are compared with theoretically expected values. The difference in polarization behaviour between waveguide and plasmon surface polariton grating rotation coupling is shown.     

Effect of intrinsic-gain fluctuations on quantum noise of phosphor materials used in medical X-ray imaging

The quality of a medical image depends, among other parameters, on quantum noise. Quantum noise is affected by the fluctuations in the number of optical quanta produced within the phosphor, per absorbed X-ray (i.e. phosphor intrinsic-gain fluctuations). This effect is considered by means of a factor, called in this study intrinsic-gain noise factor, IGNF(E). In existing theoretical models of quantum noise, the corresponding factor is taken to be equal to one. In this paper, an expression that accounts for the coefficient of variation of the phosphor intrinsic gain is introduced. This expression takes into account the process of electron–hole pair conversion to optical photons and the frequency distribution function of the emitted optical photon energy. Subsequently IGNF(E) is expressed in terms of this coefficient of variation. IGNF(E) has been calculated for several phosphors and for various energies. For all medical X-ray energies studied, phosphors that exhibit a high relative fluctuation of emitted optical photon energy, IGNF(E) exceeds by 2% to over 17% the corresponding factor of the existing theoretical models of quantum noise. Received: 25 March 1999 / Accepted: 29 March 1999 / Published online: 14 July 1999     

Silicone polymer deposition by CO2 laser induced decomposition of silane in the presence of methyl methacrylate

Infrared multiphoton decomposition of monosilane in the presence of methyl methacrylate results in the formation of gaseous methane, acetylene, butenes and carbon monoxide along with a solid polymer whose chemical mechanism of formation is discussed in line of the poly(dimethylsiloxane) structure inferred from ESCA and IR spectral analysis.     

Evaluating Zn2SiO4:Mn phosphor for use in medical imaging radiation detectors

2 SiO₄:Mn phosphor was evaluated for use in radiation detectors of medical imaging systems. Zn₂SiO₄:Mn was used in the form of laboratory-prepared fluorescent layers (screens) with coating weights from 18 to 150 mg/cm². The phosphor was excited to luminescence by low-energy X-raysusing X-raytube voltages ranging from 15 to 50 kVp. The number of emitted optical photons per incident X-rayquantum was thus determined for various X-rayenergies and phosphor coating weights. The optical emission spectrum was also measured and it was used to evaluate the spectral compatibility of Zn₂SiO₄:Mn with radiographic films, photocathodes and the Si photodiode. Finally, phosphor optical properties were estimated by fitting a theoretical model to experimental data. Results showed that Zn₂SiO₄:Mn is more efficient for low-energy X-rays. Its intrinsic conversion efficiency was found equal to 0.08, which is comparable to that of actually used phosphors. Zn₂SiO₄:Mn was also adequately compatible with orthochromatic films and the ES-20 photocathode, thus being appropriate for low-voltage radiography and fluoroscopy. Received: 31 July 1998/Accepted: 3 August 1998     

Investigation of localized surface plasmons with the photon scanning tunneling microscope

A Photon Scanning Tunneling Microscope (PSTM) with probe-sample distance control by electron tunneling is used to probe the localized surface-plasmon fields of individual nanometric silver particles. As samples, conventional island films produced by thermal evaporation and regular particle arrays produced by an electron-beam-lithography-based technique, respectively, are used. In either case the strength and spatial localization of the surface-plasmon fields strongly depend on the excitation wavelength. The results are interpreted as different resonance frequencies of individual particles or of different sample areas. On regular arrays consisting of particles with a smallest diameter of 40 nm, the PSTM maps represent the plasmon field strength spatially resolved for individual particles.     

Modified complex method for constrained design and optimization of optical multilayer thin-film devices

A new computer design program based on the modified-complex method has been developed for constrained optimization and refinement of optical thin-film multilayer devices. This program is having a provision to include both limiting constraints as well as constraint equations. Constraints are suitably accommodated to generate designs which are practicable and withstand high laser power. Various rapid convergent processes like dynamic contraction and expansion of feasible vertices are incorporated for efficient scanning of the constraint parametric space. A broad band IR antireflection coating has been designed to test its relative efficiency with respect to other available methods. A wide varieties of highly useful multilayer devices have been successfully developed using this method.     

Entropy as a measure of the performance of phosphor materials used in medical imaging radiation detectors

In information theory, entropy expresses the information gain obtained after detection of a signal concerning the state of a parameter of interest. In this study, entropy has been expressed in terms of physical quantities (emitted optical fluence and MTF) related to the imaging performance of phosphor materials, which are employed in medical imaging radiation detectors. Four phosphor materials, used in the form of laboratory-prepared fluorescent layers (screens), were compared on the basis of their entropy performance. Measurements were performed using 30- and 80-kVp X-ray beams often employed in X-ray imaging. Results showed that phosphor materials with high density and effective atomic number exhibit high entropy performance, especially at the higher X-ray tube voltage of 80 kVp. Entropy values are also affected by the type of activator, which determines the intrinsic X-ray-to-light conversion efficiency, and the spectrum of emitted light. The proximity of the incident X-ray quanta energy to the energy of the K-shell threshold for photoelectric absorption is an additional important factor which increases entropy. This effect was more apparent in the performance of yttrium-based phosphors at the lower voltage of 30 kVp. Received: 7 January 2000 / Accepted: 28 March 2000 / Published online: 23 August 2000     

Characterization of polymer strip waveguides by leaky-mode spectroscopy

Strip waveguides, recorded by UV lithography into photopolymer films were characterized as leaky guides. The effective modes indices of the guides have been measured by the prism coupling method. It is demonstrated that application of the effective index method on leaky strip guides allows the determination of the optical pattern parameters throughout an exposure or annealing procedure.     

Propagation characteristics of Ag+ ion-exchanged stripe waveguides

A detailed experimental study of stripe waveguides formed by Ag⁺/Na ion-exchange has been carried out. Propagation constants were precisely measured by excitation of single output pq-lines. The resulting data allowed us to approximate the transverse silver distribution by a rectangular profile. The effective index method is used for calculating theoretical propagation constants, assuming no sideways diffusion. A comparison between experiment and theory is made over the range of mask widths studied.     

Photorefractive properties of ion-implanted waveguides in strontium barium niobate crystals

0.61 Ba_0.39Nb₂O₆, SBN61), either by proton or helium ion implantation. Proton-implanted samples show a large increase of dark conductivity that reduces or even prevents the recording of refractive index gratings. For waveguides formed by helium implantation this effect is absent, and they can be used for efficient holographic recording. Photorefractive properties of the waveguides are investigated by two-beam coupling. After implantation with 2.0 MeV He⁺ and doses of (0.5-5)×10¹⁵ cm^-2, the samples have to be polarized again, because heating or charge effects at the crystals surface during the implantation process decreases or even reverses the effective electrooptic coefficients in the waveguiding layer. For repoled samples, we find logarithmic gain coefficients of up to 45 cm^-1 with time constants for the build-up of the purely π/2-shifted refractive index grating of the order of 1 ms for the blue lines of an Ar⁺ laser. Photoconductivity depends nonlinearly on light intensity with an exponent x≈0.55. With increasing implanted helium dose, both electronic and nuclear damage of the waveguiding layer grows, and the photorefractive properties of the waveguides are considerably degraded. Received: 20 February 1997/Revised version: 1 May 1997     

N2 production in e-beam pumped XeF lasers containing NF3

The rate of build-up of N₂ was measured in electron-beam-irradiated Ne/Xe/NF₃ mixtures using mass spectroscopy. the amount of N₂ produced indicated that N₂ is the primary nitrogen bearing stable species created in these mixtures. The rate constant for dissociative electron attachment to the NF₂ fragments produced in electron attachment to NF₃ is estimated to be 5×10^–8 cm³/s in order to explain the amount of N₂ produced.This work was supported by DARPA under Contract No. DAA01-82-C-A125 and monitored by MICOM     

A planar electrooptic beam splitter/modulator with a pair of parallel electrodes

A new planar electrooptic beam splitter with a pair of parallel electrodes is proposed and its characteristics are demonstrated on LiNbO₃. At a voltage up to 80 V and a 9.5 m electrode separation, the two output beams can be scanned over a total angle of 3° in the guide plane. The structure can also be applied to amplitude modulation, requiring 15 V for complete on-off modulation. The device permits convenient fabrication and has a potential to operate at high speeds.     

Laser-induced etching of silicon

Conventional fabrication method of porous silicon is anodisation of single crystal silicon in hydrofluoric acid. In this report, we show that it is possible to fabricate porous silicon by laser-induced etching. An earlier report by us has demonstrated the dependence of porous silicon photoluminescence characteristic on the etching laser wavelength [1]. Here we used 780 nm line from a diode laser as the etching source, and the optimum etching conditions were obtained. A simple model was proposed to explain the etching process. Scanning Electron Microscope (SEM) images of the samples support the proposed process.