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Optical properties of zirconia–titania–ORMOSIL films for temperature ...

Optical properties of zirconia–titania–ORMOSIL films for temperature ...

X.L. Zhu et al. / Optics

X.L. Zhu et al. / Optics Communications 251 (2005) 322–327 323 fier had the capability of absorbing the stress induced during the evaporation process of film synthesis from solutions [1–3]. The sol–gel polymerization of zirconia–titania– ORMOSIL (ZrO 2 –TiO 2 –ORMOSIL) films is interesting for its potential applications due to their remarkable optical properties. During the synthesis process, by varying the ratio of TiO 2 contents in the film, the desired values of the refractive index of the dried films can be achieved, and by varying the ratio of ORMOSIL, the thermal expansion coefficient of the films may be adjusted to low value or even around zero [4,5], almost no stress remained after drying and ageing process. More important, the ZrO 2 contents can lead to an improved chemical durability of the sol–gel method derived glass structure of the films [6,7]. This kind of inorganic–organic hybrid glass films with lower optical loss, better thermal stability and lower thermal expansion coefficient has potential application of thermo-optics (TO) switch in integrated optics. In our experiments, up to 10 lm ZrO 2 –TiO 2 –ORMOSIL waveguide films were successfully fabricated by dip-coating on glass or quartz substrates. The determination of the refractive index (n), the extinction coefficient (k), and the thermal-optic coefficient (dn/dT) of the thin films is very important for thermo-optic devices applications, such as optical switch and optical cross-connect. A scanning spectrroscopic ellipsometer is an effective method to measure the critical parameters (n and k) of the thin films [8–10]. Both n and k define the propagation and loss of an optical wave in the thin film [11–13]. In our previous work, we presented a novel thermo-optic coefficient measurement method for active materials using tunable distributed feedback (DFB) configuration by varying temperature [14]. In a DFB waveguide arrangement, the emission wavelength follows the Bragg backwards scattering condition of k L =2n eff K/M, where n eff is the effective refractive index of the gain medium at wavelength k L , K is the period of the modulation, and M is the Bragg scattering order. When the periodic modulation is optically induced by another pump laser at wavelength kp, the period K is given as K = kp/2sin(h), where h is half of the intersection angle of pump beams. The active film acts as a planar waveguide, and DFB laser beams can propagate within the film according to the supported modes characterized by n eff . Wavelength tuning can be performed by varying the period of the modulation, or changing the refractive index (n eff ) based on the Bragg backwards scattering condition. In experiments, wavelength tunable output were demonstrated in DFB waveguide lasers by varying the intersection angle of the pump beams [7,15]. Since the change of refractive index of the films depends on temperature, wavelength tunable can also be realized by varying temperature, and the thermal refractive index coefficient of the active thin films can be deduced finally with the temperature tuning data [16]. In this paper, the optical properties of Rhodamine 6G (R6G) dye doped ZrO 2 –TiO 2 –ORMOSIL films were investigated, and the wavelength tunable output of DFB waveguide lasers was demonstrated by varying the temperature of the active films. 2. Experiments R6G dye doped ZrO 2 –TiO 2 –ORMOSIL thin films used in our experiments were fabricated by sol–gel method at room temperature. The synthesis procedure [4,5] was sketched in block diagram in Fig. 1. The initial acid catalysed silica–titania sols were prepared from zirconium n-propoxide, titanium butoxide and c-glycidyloxypropy-ltimethoxysilane (GLYMO), the typical molar ratios of the major compositions of the sols were zirconium n-propoxide:Ti(OBu t ) 4 :GLYMO:water = 3: 3:4:20. For thick film fabrication, it was essentially to adopt modified solutions, such as acetic acid, acetyl acetone and ethyl acetoacetate into the mixture. R6G dye was at last added into the sols after the hydrolysis stage of the mixture until the desired initial dye concentration reached. For DFB waveguide laser application, the initial dye concentration was typically of 2 · 10 3 M/L. R6G dye doped ZrO 2 –TiO 2 –ORMOSIL films were obtained by dip-coating using the prepared precursors on the glass or quartz substrates. After aging and drying at room temperature for weeks (typically for two weeks in our fabrication), and heated in thermostat at 50 °C for days (typically

324 X.L. Zhu et al. / Optics Communications 251 (2005) 322–327 Fig. 1. Schematic block diagram of sol–gel procedures for films preparation. for five days), the dried films were ready for optical measurement and DFB waveguide experiments. The surface of the ZrO 2 –TiO 2 –ORMOSIL films was examined by an atomic force microscope (AFM). A rather lower line roughness of 2 nm (max.) was achieved, and the rms surface roughness was about 0.26 nm, see the AFM image in Fig. 2. Such small roughness enabled them to be used as Fig. 2. The AFM image of ZrO 2 –TiO 2 –ORMOSIL film.

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