Smart Optical Materials by Sol-Gel Method
by R. Reisfeld
Dept. of Inorganic and Analytical Chemistry , The Hebrew University, Jerusalem 91904, Israel

Tunable Laser samples prepared by the sol-gel method

Tunable lasers in the visible based on sol-gel technology   Incorporation of perylimide and pyrromethene dyes into glasses prepared by the sol-gel method enables the design of new types of stable solid lasers tunable in the visible range. These can be prepared either in the form of slabs or rods or as waveguiding active media deposited on glass or polymer supports. The difference between the refractive indices of the film and its support determines whether waveguiding occurs in the film or a leaking waveguide laser is formed.

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The efficiency of the lasers obtained recently is up to 70% when appropriate optical pumping is designed.

The recently developed tunable dye lasers in the visible are obtained by incorporation of stable laser dyes into glasses prepared by the sol-gel method. In order that the lasers be photostable and more efficient it is essential that the dye molecules penetrate into the glass matrices in a unimolecular form and be protected from the surroundings.

Until recently, liquid dye lasers were the main systems used to achieve tunability in the visible, and the only commercial choice for tunable lasers between 400 and 660 nm. However, in the last few years an intensive effort was devoted to produce embedded organic dyes in various solid matrices, with the goal of achieving solid-state dye laser devices that may replace the liquid dye lasers; e.g. laser dyes were incorporated into silica-gels, xerogels, alumina gels, ORMOSILS, and composite glasses. A solid-state dye laser has advantages over a liquid dye laser by not being a volatile solvent, non-flammable, toxic, and by its compact size and mechanical stability. Still, for applications that require high powers, at either cw or pulsed high-repetition-rate operation, the problem of heat dissipation is a serious impediment for their utilization. In liquid dyes on the other hand, a jet or a flowing solution are handy practical ways of solving the heat problem. In both cases, photostability is a feature of prime importance in selecting a laser dye. Lasers obtained by impregnation of the perylimide dyes into sol gel glasses where the dyes are enclosed in the pores of the glass seem to be so far the most photostable system.

In 1989 we succeeded for the first time to prepare a photostable tunable laser by impregnating the orange perylene derivative (perylimide) dye "BASF-241" dissolved in MMA, into a silica-gel. The method of Pope and Mackenzie, which allows polymerization of MMA in the pores of the glass, was applied here. The dye which is orders of magnitude more stable than the conventional laser dyes impregnated in the glass provided an efficient solid-state laser material. This laser was tunable in the range 568-583 nm.

The extension of the lasing range picking around 613 nm was performed by impregnating the red perylimide dye into composing silica gel PMMA glasses. This wavelength is important for medical PDT and diagnostics: Human blood and tissue absorption is small in the red, allowing the preferential absorption of light by a photo-active cancer therapeutic agent, such as hematoporphyrin derivative, which concentrates in tumors.

The perylimide dyes were dissolved in a methylmethacrylate (MMA) monomer to form solutions of different concentrations in the range 10^-6-10^-3 mol/liter. Highly porous silica-gel bulk glasses (density about 0.7 g/cm³) were prepared, by the sol-gel method and were dried by slow heating (100°C/day) from room temperature to 500°C. Then, the bulks were immersed in the dye-doped solution of the MMA monomer, which was simultaneously catalyzed by the addition of 2% benzoyl peroxide. The MMA-dye solution thus diffused into the silica-gel glass pores, and polymerized therein. After this process of dye impregnation, the bulks were re-immersed in an MMA-dye solution, which at this stage was catalyzed for full polymerization by 0.5% benzoyl peroxide, and kept in a sealed container at 40°C for about a week. The samples were then withdrawn, cleaned, and polished, to obtain parallel-piped slabs of approximate dimensions 10x10x3 mm³, with clear smooth surfaces.  The density of the composite glass was d = 1.447±0.005 g/cm³ and the refractive index n = 1.472±0.003.

Later solid state dye lasers were made by, incorporating photostable laser dyes in organically modified silicate (ORMOSIL) and composite glass matrices. Three different types of pyrromethene (PM) were used: PM 567, PM 580 and PM 597. Their formulae are presented in figure 2. The ORMOSIL glass samples were prepared by one step process at room temperature by sol gel technology, which led to the formation of hybrid organic/inorganic materials. The composite glass samples are made by biphase process: (a) preparation of porous silica gel, (b) impregnation of the dye dissolved in methyl methacrylate (MMA) into the silica gel and polymerization of the MMA.

A number of various glass compositions were tested and their efficiencies and half output energy lifetimes were measured. The optimal slope efficiency for the lasers pumped transversally, by frequency doubled Nd-YAG laser, approaches 42%. in PM 597 in composite glass.

Organically modified silicates (ORMOSILs) usually exhibit lower porosity and enhanced mechanical properties which allow cutting, grinding and polishing prior to heat treatment. A typical ORMOSIL gel network contains a significant amount of organic functionalities, which offers great flexibility with respect to the chemical compatibility of the gels with the dye to be incorporated.

Various organic laser dyes have been incorporated into ORMOSILs, derived from polydimethylsiloxane and tetraethoxysilane. These sol-gel derived PT-ORMOSILs proved to be good hosts for the laser dyes in terms of stability and optical gain.

The mechanical and optical properties of glasses prepared by the sol-gel are being improved constantly by modifying the sol process and using a variety of organofunctional silicon alkoxides. 

Modified alkoxide precursors, RSi(OEt)3 or RSi(OMet)3 (where R is a non-hydrophobic group)

methyltriethoxysilane (MTEOS),  vinyltriethoxysilane (VTEOS),  amyltriethoxysilane (ATEOS), 3-(trimethoxysilyl)propylmethacrylate (TMSPMA), methyltriethoxysilane (MTEOS), 3-glycidoxypropyltrimethoxysilane (GLYMO)  methyltrimethoxysilane (MTMOS)

The permanent (R) organic group decreases the mechanical tensions during the drying process. Functionalized alkoxides F-R’-Si(OEt)₃, where F is a chemical function such as an amino or isocyanate group and R’ is an alkyl spacer, allow to covalently graft onto the xerogel matrix to avoid phase separation and consequently to increase the concentration of the guest molecules. After drying, optically clear and dense inorganic-organic hybrid xerogels (30 mm diameter and 15 mm thick) were obtained and described recently.

Fig 1. BASF LFR 300 – molecular structure.

Fig. 2. Molecular structure of pyrromethene dyes

Planar active wave guides

The forming of active planar waveguides by the sol-gel method is feasible by incorporation of variety of laser dyes at room temperature. Light amplification in a dye-doped in these devices is achieved.

The combination of the tunability and high efficiency of laser dyes with the high power density that can easily be achieved in planar waveguide makes devices based on dye-doped waveguides very promising.

Glass waveguiding films were prepared from titania and modified silica using the sol-gel method and doped with the laser dye rhodamine B. The guided and amplified fluorescence (pumped by a double frequency Nd-YAG laser) was coupled out either by static grating written on the film or by a prism. The gain of the emitted superradiance was determined from the amplified spontaneous emission intensity dependence on the pumped strip length. A maximum net gain of 54 dB/cm was measured.