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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) 322–327 325 low attenuation waveguides [17–19]. The thickness of the ZrO 2 –TiO 2 –ORMOSIL films was detected by a surface profilometer (a-step 500). Using the same precursors, films with different thickness from 0.97 to 10.04 lm were fabricated by varying the withdrawing speed from 0.25 to 2.5 mm/s. The refractive index n and the extinction coefficient k of the undoped ZrO 2 –TiO 2 –ORMOSIL films at room temperature were measured by a home-made scanning spectroscopic ellipsometer [20]. At the emission wavelength of 600 nm, the refractive index and the extinction coefficient of the dried films were n 1.548 and k 2.50 · 10 5 respectively, Fig. 3 was the results of n and k vs. wavelength. Fig. 4(a) shows the transmittance spectral of the undoped ZrO 2 –TiO 2 –ORMOSIL films with typical thickness of about 5 lm. The absorption curve cut-off near the wavelength of 300 nm, and it was transparent within the visible and infrared spectral range, this result could also be induced from the spectra of the extinction coefficient k [17,18]. So n (a) k (b) 1.70 1.65 1.60 1.55 0.010 0.005 0.000 300 400 500 600 700 800 Wavelength (nm) 300 400 500 600 700 800 Wavelength (nm) Fig. 3. Spectra of (a) the refractive index n and (b) the extinction coefficient k of ZrO 2 –TiO 2 –ORMOSIL film. Transmitance (%) (a) Absorption (%) (b) 1.0 0.5 0.0 300 400 500 600 700 800 1.0 0.5 0.0 300 400 500 600 700 800 Wavelength (nm) Fig. 4. Transmittance spectral of the undoped ZrO 2 –TiO 2 – ORMOSIL films (a) and absorption spectra of R6G dye doped (b). this kind of inorganic–organic hybrid films could be used as laser dye host [19,21–24] from near UV to infrared spectral for DFB waveguide lasing. In this experiment, when R6G dye was doped into the films, the absorption peak occurred near 532 nm (Fig. 4(b)). The experimental arrangement of temperature tunable DFB waveguide laser was similar to our previous work [14]. A holographic grating of 1800 lines/mm was used as a beam splitter for the 532 nm pump beam from a nanosecond Q-switched Nd:YAG laser. A cylindrical lens with 500 mm focal length was used to focus the pump beams. The R6G dye doped ZrO 2 –TiO 2 –ORMOSIL film was housed in a specially designed home-made oven. A thermocouple with 0.1 °C accuracy was placed in contact with the film for temperature measurement. The film was heated at a rate of 0.5 °C/min. The DFB waveguide laser was coupled by a fiberbundle into the entrance port of a 0.3 m focal length spectrograph/intensified charge coupled device (ICCD) detection system, the wavelength resolving capability of the detection system (CCD) was 0.04 nm. The DFB waveguide laser

326 X.L. Zhu et al. / Optics Communications 251 (2005) 322–327 Wavelength (nm) 604 603 602 601 600 599 598 597 596 Heating process Cooling process Linear fit 20 3 0 40 5 0 60 7 0 80 9 0 1 0 0 110 1 2 0 130 Temperature (˚C) Fig. 5. DFB waveguide emission wavelength k L vs. temperature T. operated at the second order (M = 2) of Bragg scattering, and the emission wavelength at room temperature should be pre-set, typically near the central of the gain spectral curve. Fig. 5 was the experimental results of emission wavelength k L vs. temperature T for ZrO 2 –TiO 2 – ORMOSIL DFB waveguide laser, the thickness of the film was around 1.0 lm, and the pump energy deposited in the film was about 10 lJ. Approximate 5.5 nm wavelength tuning range was achieved when temperature was increased from 20 to 120 °C, the linewidth was less than 0.5 nm. The tuning curves showed that the variation of k L appeared perfectly linear except for the discontinuity near 70 °C. The slope of the linear fitted line was 0.065 and 0.049 nm/°C, respectively. The discontinuity of the temperature tuning curve indicated a phase transition occurrence within the film structure. When the film was cooled down from 120 to 70 °C subsequently, it was found that the emission wavelength followed the same k L vs. T trace detected in the heating process, but from 70 to 20 °C, the slope of cooling curve was appreciably larger than that of heating curve, which revealed that the refractive index of the film became larger after heating process. This slight hysteresis structure in the k L vs. T trace may be due to the unrelaxed strain in the hybrid film, the change of refractive index n in this temperature range is largely a result of the change in volume, and the volume is in turn affected by the deformation during heating or cooling process. The sol–gel materials may not completely recover its elasticity in the heating/cooling cycle and thus results in the hysteresis structure [14]. The ellipsometry data also indicated the variation of refractive index n of the films treated at different temperature. The effective refractive index n eff of the ZrO 2 –TiO 2 –ORMOSIL film as a function of temperature T was deduced followed the Bragg scattering equation using the wavelength tuning data. For the active film, the change of refractive index depending on temperature was linear in the various temperature ranges where no phase transition occurred, and whose slope could give dn eff /dT respectively. The thermal-optic coefficient (dn eff / dT) of R6G doped ZrO 2 –TiO 2 –ORMOSIL films was of the order of 1.6 · 10 4 and 1.3 · 10 4 °C 1 in its two different phase. The negative value of dn eff /dT revealed that the thermal expansion coefficient of ZrO 2 –TiO 2 –ORMOSIL film was larger than the thermal electronic polarizability coefficient [25]. For comparison, we also performed temperature tuning of DFB waveguide lasers for R6G dye-doped ZrO 2 films and ZrO 2 –TiO 2 films in the same configuration. The thermal-optic coefficient of ZrO 2 film and ZrO 2 –TiO 2 film was 2.7 · 10 4 and 2.1 · 10 4 °C 1 , both were larger than that of ZrO 2 –TiO 2 –ORMOSIL film. The results indicated that both ZrO 2 films and ZrO 2 –TiO 2 films had larger thermal expansion coefficient than that of ZrO 2 –TiO 2 –ORMOSIL films. Multi-mode emission of temperature tunable R6G doped ZrO 2 –TiO 2 –ORMOSIL films DFB waveguide laser was demonstrated in experiments. When R6G doped film with the thickness of 3.7 lm was used as the gain material, four waveguide modes lasing was recorded, the trace Emission Intensity (a.u.) 70000 60000 50000 40000 30000 20000 10000 0 570 580 590 600 610 620 wavelength (nm) Fig. 6. Multi-mode emission of DFB waveguide laser.

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