High-Efficiency Bragg Gratings in Photothermorefractive Glass
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High-efficiency Bragg gratings in photothermorefractive glass Oleg M. Efimov, Leonid B. Glebov, Larissa N. Glebova, Kathleen C. Richardson, and Vadim I. Smirnov Photosensitive silicate glasses doped with silver, cerium, fluorine, and bromine were fabricated at the Center for Research and Education in Optics and Lasers. Bragg diffractive gratings were recorded in the volume of these glasses with a photothermorefractive process ~exposure to UV radiation of a He–Cd laser at 325 nm is followed by thermal development at 520 °C!. Absolute diffraction efficiency of as much as 93% was observed for 1-mm-thick gratings with spatial frequencies up to 2500 mm21. No decreasing of diffraction efficiency was detected at low spatial frequencies. Original glasses were transparent ~absorption coefficient less than 1 cm21! from 350 to 4100 nm. Induced losses in exposed and developed glass decreased from 0.3 to 0.03 cm21 between 400 and 700 nm, respectively, and did not exceed 0.01–0.02 cm21 in the IR region from 700 to 2500 nm. Additional losses caused by parasitic structures recorded in the photosensitive medium were studied. © 1999 Optical Society of America OCIS codes: 050.7330, 090.2900, 160.2750. 1. Introduction transfer characteristic, high resolution, and low Increasing applications for holographic optical ele- noise, be indefinitely recyclable or relatively inexpen- ments have resulted in continued development of sive. While several materials have been studied, new effective and reliable photosensitive media. none has been found so far that meets all these re- The recent Hariharan book1 notes that the main pho- quirements.”1 The lack of available materials for tosensitive materials available for high-efficiency ho- phase holography stimulates the search for new ap- logram recording are silver halide photographic proaches. In this paper we describe properties and emulsions, dichromated gelatin, photoresists, pho- performance of a new inorganic glass as a medium for topolymers, photothermoplastics, polymers with hologram recording, which meets practically all the spectral hole burning, and photorefractive crystals. requirements indicated above. Each of these materials has their merits, but all have Use of inorganic photosensitive glasses for phase drawbacks. Organic materials ~photographic emul- hologram recording was described several years ago sions, dichromated gelatin, and photopolymers! are in Refs. 2–4. Bragg gratings were recorded in lith- sensitive to humidity. Moreover, they significantly ium aluminum silicate2 and sodium zinc aluminum shrink in the development process. Inorganic mate- silicate3,4 glasses doped with silver and cerium by rials ~photorefractive crystals! have low resistance to exposure to UV radiation followed by thermal treat- elevated temperatures and produce additional pat- ment. This phenomenon was named the photother- terns because of exposure to the beam diffracted from morefractive ~PTR! process. Glasses, which possess the recorded grating. “The ideal recording material such properties, were called photothermorefractive for holography should have a spectral sensitivity well glasses. matched to available laser wavelengths, a linear The photothermal process based on precipitation of dielectric microcrystals in the bulk of glass exposed to UV radiation was discovered by Stookey in 1949.5 This two-step process ~exposure and thermal devel- The authors are with the Center for Research and Education in opment! was used to record a translucent image in Optics and Lasers, University of Central Florida, Orlando, Florida 32816-2700. glass because of light scattering caused by a differ- Received 31 August 1998; revised manuscript received 9 Novem- ence between the refractive indices of a precipitated ber 1998. crystalline phase and the glass matrix. Later, col- 0003-6935y99y040619-09$15.00y0 ored images were recorded in similar glasses by pho- © 1999 Optical Society of America tothermal precipitation of a number of complex 1 February 1999 y Vol. 38, No. 4 y APPLIED OPTICS 619 crystals of different compositions, sizes, and shapes.6–8 toionization, has a maximum near 300 nm and a The link of processes, which occur in these glasses long wavelength tail up to 400 nm. This means and produce coloration, was described in Refs. 5 and that several commercial lasers such as N2, Ar, He– 6. According to these studies, the first step is the Cd, etc. can be used for recording. Once developed, exposure of the glass sample to UV radiation, which holograms in PTR glass were not destroyed by fur- produces ionization of a cerium ion. The electrons ther exposure to visible light. The price of this released from cerium are then trapped by a silver ion. photosensitive silicate glass is rather small in com- As a result, silver is converted from a positive ion to parison with other photosensitive materials. Un- a neutral atom. This stage corresponds to a latent fortunately, this new material has not met all image formation, and no significant coloration occurs. requirements formulated in Ref. 1 because of strong The next step is a thermal development, which is scattering, which arises in the process of develop- a process that includes two stages. The first stage ment. This scattering resulted in low absolute dif- involves the high diffusion rate that silver atoms fraction efficiency of gratings in PTR glasses, which did possess in silicate glasses. This diffusion leads to not exceed 45%. Thus a new and promising material the creation of tiny silver crystals at relatively low for holographic optical elements, which was an- temperatures ~450–500 °C!. A number of silver nounced in Refs. 3 and 4, was not free of all drawbacks. clusters arise in exposed regions of glass after aging The goal of this research was to find if high losses at elevated temperatures. These silver particles are an intrinsic feature of PTR glass or if, with the serve as the nucleation centers for sodium and flu- proper choice of glass technology and conditions of orine ion precipitation, and cubic sodium fluoride exposure and thermal development, this phenome- crystal growth occurs at temperatures between 500 non can be eliminated. To do this we developed a and 550 °C. Further heat treatment leads to the laboratory technology of PTR glass melting and sam- growth of elongated pyramidal complex Na,Ag-F,Br ples preparation; studied absorption spectra of orig- crystals on the surface of cubic NaF crystals. This inal, exposed, and developed PTR glasses in the UV, mixture of crystals can produce opal coloration in visible, and IR regions; and studied Bragg grating the case of large crystal sizes or yellow coloration optical properties versus conditions of UV exposure caused by colloidal silver precipitated on interfaces and thermal treatment. Different types of losses, of dielectric crystals. A second exposure to UV fol- which appeared in the process of hologram recording, lowed by a second heat treatment produces differ- were studied, and requirements for sample prepara- ent coloration because of metallic silver reduction tion and recording procedure were formulated. on the surfaces of the dielectric pyramids. The fi- nal resulting coloration depends on the size and 2. Experiment aspect ratio of these silver particles. This multi- The same photosensitive glass of approximate com- ~ ! stage photothermal process in photosensitive glass position mol.% 15Na2O-5ZnO-4Al2O3-70SiO2- was proposed for decoration, color photography, 5NaF-1KBr-0.01Ag2O-0.01CeO2 was studied in this sculpture, and even for holography. However, no research as in previous research.3,4 The glass was difference of refractive indices of exposed and un- melted in an electrical furnace ~DelTech Model DT- exposed sites was found at that time, and the first 31-RS-OS! in 400-ml fused-silica crucibles at phase holograms were recorded in photosensitive 1460 °C for 5 h. Stirring was used to homogenize glasses many years later.2,3 the melt. After the melting, the glass was poured It was found in Refs. 3 and 4, which describe onto a thick metal slab to avoid spontaneous crys- dielectric crystal precipitation in glasses exposed to tallization. The glass casting underwent anneal- radiation of the nitrogen laser at 337 nm, that a ing with a cooling rate in the region of structural refractive-index decrease of approximately 5 3 1024 and stress relaxation of 0.2 °Cymin in a Lindbergy occurs. The refractive index of NaF in the red BlueM box furnace. Thermal development was 5 spectral region is nNaF 1.32 in comparison with produced by keeping the exposed specimen in the 5 ~ the refractive index of PTR glass nPTR 1.49 Ref. same furnace at a temperature of 520 °C from pe- 4!. The small value of refractive-index change is riods of a few of minutes to several hours. Polished due to the small volume fraction of the precipitated glass samples from 0.5- to 10-mm thickness of 12 crystalline phase. However, it is sufficient enough mm 3 25 mm in size were prepared with a Buehler to result in a high-efficiency Bragg grating record- Ecomet-3 grinderypolisher with an Automet-2 ing in samples with thicknesses more than several power head. Optical homogeneity of samples was hundreds of micrometers. Conditions of glass ex- tested by the shadow method in the divergent beam posure and development were found in that re- of a He–Ne laser. search to create Bragg gratings with relative Absorption spectra from 200 to 5000 nm were mea- diffraction efficiency up to 90% and angular selec- sured with a double-beam Perkin-Elmer 330 and a tivity up to 2 mrad. The maximum recorded spa- single-beam Perkin-Elmer 1600 Fourier-transform tial frequency was 2500 mm21. These gratings infrared spectrophotometers. To avoid uncertainty were stable up to 400 °C. The photosensitivity was in the absorption coefficient caused by dispersion of found in the range of several joules per square cen- surface losses in such a wide spectral area, optical ~ ! ~ ! timeter at a nitrogen laser wavelength 337 nm .