HOLOGRAPHY The effect of decoloration on the properties of volume phase holograms based on silicate photothermorefractive glass A. S. Zlatov, a I. O. Chëporov, Yu. L. Korzinin, and N. V. Nikonorov St. Petersburg State University of Information Technologies, Mechanics, and Optics, St. Petersburg Submitted April 13, 2010 Opticheski Zhurnal 77, 22–24 December 2010 This paper discusses the decoloration process of volume phase holograms based on photothermorefractive PTR glass under the action of the second harmonic of the pulsed radiation of a neodymium laser. It is shown that the photodecoloration process increases the refractive-index modulation of PTR glass and reduces its absorption in the visible region. © 2010 Optical Society of America. INTRODUCTION Photothermorefractive PTR glasses are promising ma- terials for creating efficient holographic volume elements. 1 Holograms based on these glasses possess high diffraction efficiency and spectral-angular selectivity. The high thermal and optical strength of PTR glasses makes it possible to use such holographic optical elements in powerful laser systems. Moreover, holograms in PTR glass possess high chemical stability and mechanical strength, and are virtually no differ- ent in this respect from K8 commercial optical glass. The recording of holograms on PTR glasses is based on the process of photothermoinduced crystallization of glass, 2 the essence of which is as follows: UV radiation and heat processing cause crystallization centers in the form of colloi- dal silver particles to be created in the glass matrix. Subse- quent heat treatment causes complex NaF–AgBr nanocrys- tals to grow at these centers. An amplitude-phase hologram is recorded as a result of the induced absorption of the col- loidal silver and the difference in refractive index of the glass phase and the crystalline phase. Because this process is irre- versible, the image is not erased in the process of reading, and there are no limitations on the lifetime of the volume amplitude-phase hologram. However, the presence of a broad absorption band of colloidal silver with a maximum in the 440– 460-nm region, along with scattering at the microcrys- talline phase, restrict the use of this material in the visible region. It is shown in Ref. 3 that the absorption band of colloidal silver can be reduced by irradiating it with the sec- ond harmonic of a pulsed nanosecond neodymium laser. This paper discusses the process of using the second harmonic of a pulsed Nd 3+ YAG laser to photodecolor vol- ume phase holograms recorded in the volume of PTR glass. The properties of the volume phase holograms were investi- gated before and after they were photodecolored, and the changes of their main characteristics were established. THE OBJECT OF STUDY AND THE EXPERIMENT Aluminum-zinc silicate PTR glasses with a high fluorine concentration, doped with cerium, silver, and antimony ions were investigated. Holograms with a spatial frequency of 1000 mm -1 were recorded, using a He–Cd laser at wavelength 325 nm and a symmetric two-beam layout. The samples were heat-treated at T =520 °C. The samples were decolored with a pulsed Nd 3+ YAG laser 10 ns operating at the third harmonic 532 nm. The spectrum of the photoinduced absorption in the vis- ible region was measured, along with the angular selectivity contour and the diffraction efficiency of the hologram at the wavelength of a He–Ne laser at 633 nm. 4 As is well known, 5 the dependence of the diffraction efficiency of transmissive three-dimensional phase holo- grams on the refractive-index modulation amplitude has an oscillatory character:  = sin 2  1 , where  1 = n 1 T /  cos  0 , n 1 is the refractive-index modulation am- plitude, T is the thickness of the medium,  is the wave- length of the reconstructing radiation in air, and  0 is the angle of incidence of the reconstructing beam on the holo- gram in the medium. It should be pointed out that, under Bragg conditions, identical diffraction efficiencies are achieved when  1 = k  arcsin   where k =1,2,3,.... To choose k, i.e., to unambiguously determine  1 , the shape of 300 400 500 600 0.5 0.4 0.3 0.2 0.1 0 Wavelength, nm Absorption coefficient, cm 1 1 2 3 FIG. 1. Attenuation spectra of PTR glass before 1 and after 2 UV irra- diation and heat treatment, 3—after irradiation with the second harmonic of a Nd 3+ YAG laser. 753 753 J. Opt. Technol. 77 12, December 2010 1070-9762/2010/120753-02$15.00 © 2010 Optical Society of America