Granqvist, âThermal lens and photoacoustic spectroscopy to determine the .... sample is mounted on a manual rotation stage (RSP1, THORLABS, accuracy: 1°). A. 500 KHz cylindrical PZT piezoelectric transducer (10.0 mm in diameter) is ...
Optical absorption photoacoustic measurements for determination of molecular symmetries in a dichroic organic-film Vicente Torres-Zúñiga, Rosalba Castañeda-Guzmán, S. Jesús Pérez-Ruiz, Omar G. Morales-Saavedra and Manuel Zepahua-Camacho. Centro de Ciencias Aplicadas y desarrollo Tecnológico, Universidad Nacional Autónoma de México, CCADET-UNAM. Cd. Universitaria A.P. 70-186 C.P. 04510 México D.F.
Abstract: A novel approach to specify symmetries and main optical axes in anisotropic polymeric films is proposed. This method is based on the analysis of the optical absorption via the pulsed laser photoacoustic (PLPA)technique in a common polarizer film, while rotating the polarizer axis at normal incidence. Since the PLPA-signals are directly proportional to the optical absorption, it is shown that a symmetric and complementary Malus’s law can be obtained over full root mean square (RMS)- and correlation (CA)-analysis of the PLPA-signals. Such data processing reveals the main material directions of the constituting film molecules defining the symmetry structure of the sample. PLPA-results were compared to the pure optical transmission experiments and show unambiguous information, allowing this technique to be used in nonstandard and opaque polymeric films, where the analysis of the optical measurements represents a difficult task, and in general, in anisotropic media. © 2008 Optical Society of America OCIS codes: (260.0260) Physical optics; (260.5430) Polarization; (160.1190) Anisotropic optical materials; (160.5470) Polymers; (310.0310) Thin films; (310.5448) Polarization; (110.5125) Photoacoustics.
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(C) 2008 OSA
Received 23 Sep 2008; revised 10 Nov 2008; accepted 19 Nov 2008; published 1 Dec 2008
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1. Introduction A dichroic polarizer film is a highly oriented, linear anisotropic material with a periodic structure; these films are employed to adjust the light intensity of several light sources, transforming the polarization direction vector as function of the film polarizing axis angle. In this context, polymeric dichroic polarizers are of great importance in standard optical and optoelectronic applications. For example, such polarizers can act as light-modulating passive optical elements in Liquid Crystal Displays (LCD) and low-power laser devices, among others [1]. The polarizer’s quality can be determined by the well-known optical Malus' law. This expression describes the light intensity output as function of the angle between two consecutive linear polarizers. This optical test can be directly performed with a polarized light source and a standard optical power-meter, obtaining the well-known and representative quadratic sinusoidal transmission curve. On the other hand, the PLPA-technique, takes advantage of the excitation of a material with a pulsed laser source which produces among other physical effects, acoustic signals on the lattice of the material. The acoustic waves (in this particular case, the ultrasonic waves), are produced within the illuminated material volume due to thermal processes such as a nonradiative response of the absorbed light; thus the PLPA-signal’s amplitude can be considered to be a directly proportional measurement of the energy absorption of the electromagnetic wave. In a typical PLPA-experiment the acoustic waves travel at the characteristic sound speed of the material under study reaching a PZT (PbZrTiO3 ceramic) detector/microphone which allows the analysis of the PLPA-data via numerical correlation and RMS-processes. The PLPA-technique has emerged as an effective and accurate tool for the evaluation of thermal- (phase transitions) [2, 3] and optical- (optical absorption coefficients in solids) [4, 5, 6] processes occurring in different materials such as semiconductors, polymers, organic and inorganic crystals, etc [7, 8, 9, 10]. In short, several structural material characteristics can be accurately studied by PLPA-technique and the adequate analysis of the obtained data. Most common experimental techniques applied in order to determine the optical absorption properties in materials are indirect procedures. Most of these techniques use direct optical transmission spectra and mathematical processing to obtain the complementary optical absorption data. In this work, we first show the reconstruction of the optical Malus’s law from the absolute absorption point of view via the photoacoustic signal analysis. Concretely, an organic linear polarizer film is excited at normal incidence by a pulsed laser beam and the generated photoacoustic signals are monitored as function of the polarizer axis angle while rotating the sample. It was observed that the obtained experimental curve is complementary and exhibits reciprocal symmetry to the sinusoidal optical Malus’s law. Moreover, we also demonstrate that a complete RMS- and CA-analysis of the photoacoustic signals as function of the polarizer axis angle permits monitoring of the morphological structure of the dichroic polymer film within the linear optical regime. This brings the possibility to derive, at first instance, the macroscopic structural properties of materials, such as the molecular order and anisotropy, which represent an important advantage particularly for low transparent materials where the analyses of purely optical measurements represent a difficult task.
#101819 - $15.00 USD
(C) 2008 OSA
Received 23 Sep 2008; revised 10 Nov 2008; accepted 19 Nov 2008; published 1 Dec 2008
8 December 2008 / Vol. 16, No. 25 / OPTICS EXPRESS 20725
2. Experimental section 2.1 Materials: iodine-impregnated polyvinyl alcohol In this work, we have implemented a ~600 μm thick commercial linear polarizer film (Kenko, IN2170) which is commonly used in photographic studios, optical laboratories and as substrate support in the liquid crystal display industry, among other low optical power applications. This polarizer is built-up from a micro-structured dichroic sheet sandwiched between two strain-free glass plates. This specific kind of polarizer is commonly designed for visible (380–780 nm) applications. The polymer dichroic film is also known as a Polaroid-H sheet, which is an iodine-impregnated polyvinyl alcohol (PVA) sheet. PVA is a well oriented polymer containing hydroxyl groups that give rise to intermolecular and intramolecular hydrogen bondings. PVA is classified into three different types: isotactic, atactic, and syndiotactic, according to the stereoregularity of its hydroxyl groups. Physical properties of PVA are highly dependent on the degree of syndiotacticity, which is primarily determined by the choice of the vinyl ester monomers [11, 12, 13]. To manufacture Polaroid-H films, a sheet of PVA is heated and stretched in one direction while softened, this procedure induces the alignment of the long polymeric molecules along the stretch direction. When dipping the stretched PVA-film in an iodine solution, the iodine atoms attach themselves to the aligned polymeric chains, providing electrons to the film structure; as a consequence high electronic mobility along the aligned chains is possible, but not in the perpendicular direction. Therefore, optical waves traveling across the organic film with electric fields parallel to the polymeric chains (low transmission optical axes) are strongly absorbed due to dissipative effects produced by the allowed electron mobility. Thus, the respective perpendicular film direction of the PVA-chains can be considered as the "transparent" optical axis, since electrons cannot freely move to absorb energy. This kind of polarizer looks neutral in color when viewed under unpolarized light and is remarkably free of light scattering due to the tiny dichromophore molecular dimensions [11]. 2.2 Detection of the photoacoustic signals The experimental setup for the PLPA-signal detection is schematically shown in Fig. 1. The sample is illuminated by a p-polarized Q-switched frequency-doubled Nd:YAG laser system (Minilite II from Continuum, USA) along the z-direction, the laser system generates light pulses of ∼7 ns and operates at a repetition rate of 10 Hz with maximum output energy of ∼5.0 mJ per pulse at a wavelength of 532 nm. The beam is not focused in order to avoid lateral diffusion of heat or sample damage. A fast photodiode detector (Thorlabs Inc. model 201/579-7227) with a rise time
