Enhanced Multiphoton Absorption Induced Luminescence in

Fresnoite, Ba2TiSi2O8, is a ferroelectric crystal with a very large second-order optical nonlinearity. X-ray diffraction analysis and Raman spectra in...
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J. Phys. Chem. C 2007, 111, 17118-17121

Enhanced Multiphoton Absorption Induced Luminescence in Transparent Sm3+-doped Ba2TiSi2O8 Glass-ceramics Bin Zhu, Songmin Zhang, Geng Lin, Shifeng Zhou, and Jianrong Qiu* State Key Laboratory of Silicon Materials, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: May 21, 2007; In Final Form: August 9, 2007

Fresnoite, Ba2TiSi2O8, is a ferroelectric crystal with a very large second-order optical nonlinearity. X-ray diffraction analysis and Raman spectra indicate that Ba2TiSi2O8 crystals are precipitated in the glass-ceramics after heat treatment at 750 °C for 2 h. Near-infrared to visible upconversion luminescence is observed in Sm3+-doped glass and glass-ceramic samples under infrared femtosecond laser irradiation, and the Sm3+doped glass-ceramic shows strongly enhanced mutiphoton absorption induced luminescence and improved saturate threshold compared with the as-prepared glass. The mechanisms of the observed phenomena are discussed.

1. Introduction Conversion of near-infrared to visible and ultraviolet light, so-called upconversion,1,2 is one approach to the development of compact and efficient visible lasers,3 which has promising applications in optical storage,4 and three-dimensional displays.5 Therefore, the investigations of upconversion processes in transparent solid materials containing rare earth ions have never been discontinued. Furthermore, enhancement upconversion luminescence based on energy transfer in the multi-ion doped host materials has attracted considerable attention.6-9 The process is a so-called sensitization process in which one species is excited by incident radiation and transfers its excitation energy to the other species in its ground or excited state. This resulting ion-pair interaction energy transfer has been extensively studied in many codoped glasses/crystals and has been explained on the basis of several models.7 However, there have been few reports on the energy transfer upconversion luminescence on the basis of absorption of second-harmonic wave of the excitation laser due to the nonlinear optical nano- and microcrystallites in transparent rare-earth-doped glass-ceramics. Recently, we observed Eu3+-doped transparent glass-ceramics containing nonlinear optical microcrystallites showing strongly enhanced upconversion luminescence as compared with the asprepared glass.10 Fresnoite (Ba2TiSi2O8) is a ferroelectric crystal with a very large second-order optical nonlinearity.11 In this study, we prepared Sm3+-doped glass-ceramics containing Ba2TiSi2O8 microcrystals and studied the upconversion luminescence properties of Sm3+-doped BaO-TiO2-SiO2 glass and glass-ceramics. 2. Experimental Methods The glass with a composition of 40BaO-20TiO2-40SiO20.5Sm2O3 (mol %) was prepared using 4N-purity grade BaCO3, TiO2, SiO2, and Sm2O3. The batch was mixed thoroughly and was melted in a Pt crucible at 1550 °C for 2 h in air. The melt was poured onto a steel plate and was cooled to room temperature. The cooling rate was estimated to be about 300 * To whom correspondence should be addressed. Telephone: 008657188925079; fax: 008657188925079; e-mail: [email protected].

K/s. The obtained as-prepared glass samples are transparent. Some of the as-prepared glass samples were then heat-treated at 750 °C for 2 h. After annealing, microcrystallites with the same stoichiometric composition of the as-prepared glass were precipitated inside samples. Hereafter, the as-prepared glass samples are called the glasses and the heat-treated samples are glass-ceramics. The samples were cut and polished to the size of 5 × 5 × 10 mm3 for the optical measurements. A regeneratively amplified Ti:sapphire laser, which emits 120 femtosecond, 1 kHz, mode-locked pulses, was used as the pump source. The laser beam was focused into the center of the samples by an optical lens with a focal length of 100 mm along the longitudinal direction of the samples, and fluorescence spectra excited by the femtosecond laser were recorded from the sample side by a ZOLIX SBP300 spectrophotometer. The emission intensity was corrected for absorbed power, and photoluminescence (PL) spectra were corrected using a standard luminescent material (Y2O3:Eu3+). The experimental error of luminescence intensity was within (10%. Absorption spectra of the samples were measured using a Hitachi U-4100 spectrophotometer. The PL spectra were measured using a Hitachi 850-type fluorescence spectrophotometer with a Xe lamp as the excitation light source. X-ray diffraction (XRD) measurements were carried out using a Rigaku D/MAX-RA diffractometer with Cu KR as the incident radiation at room temperature. Raman spectra were measured by a Raman spectrometer (Renishaw inVia) with 514 nm Ar+ laser excitation. Refractive indices were measured on a Metricon-2010 prism coupler at 632.8 nm. 3. Results and Discussion Figure 1 shows the absorption spectra of the glass and glassceramic samples. No apparent change is observed in the absorbance from 350 to 850 nm. As shown in Figure 1, both show two absorption peaks at about 403 and 471 nm. These peaks can be assigned to the transitions of Sm3+ from the ground state 6H5/2 to the excited stated.12 It should be pointed out that the glass sample still remains transparent after heat-treatment at 750 °C for 2 h. The refractive indices of the glass and the glass-ceramic are 1.734 and 1.741, respectively. Figure 2 shows the X-ray diffraction patterns of glass and glass-ceramic samples, together with the reference of the

10.1021/jp0739061 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/12/2007

Enhanced Multiphoton Absorption Induced Luminescence

Figure 1. Absorption spectra of Sm3+-doped BaO-TiO2-SiO2 glass (a) and glass-ceramic (b) in the wavelength region of 350-800 nm. The thickness of the samples is 2 mm.

Figure 2. XRD patterns of the Sm3+-doped BaO-TiO2-SiO2 glass (a) and glass-ceramic heat-treated at 750 °C for 2 h (b). The diffraction data of Ba2TiSi2O8 is also included.

Ba2TiSi2O8 crystal. There is only one broad peak shown in the as-prepared glass, whereas there are several sharp peaks superimposed on the broad peak in the glass-ceramic sample. These sharp peaks are consistent with the diffraction peaks of Ba2TiSi2O8 crystals. We found that there was little difference in the XRD diffraction patterns of the surface part and inside of the glass-ceramic sample, indicating that the Ba2TiSi2O8 microcrystallites homogeneously existed in the annealed sample. Therefore, microcrystalline particles of Ba2TiSi2O8, which possess second-order optical nonlinearities11 and have the same stoichiometric composition as the glass matrix, were precipitated in the glass after heat-treatment. We also used Raman spectroscopy to confirm the generation of crystals. From Figure 3, apparent spectroscopic changes can be found after the sample was heat-treated at 750 °C for 2 h. First, the broad band at 858 cm-1 in the original glass can be assigned to the stretching mode of the short Ti-O* bond (O* denotes an apical oxygen), the Ti-O- bonds (O- denotes a nonbridging oxygen), and terminal SiO3 groups, and the band at 320 cm-1 is most likely due to the ν(Ba-O) mode.13 The band at 858 cm-1 becomes narrow, and the strongest peak at 860 cm-1, assigned to the vibration of the short Ti-O bond, becomes more and more distinct. Additionally, the band at 320 cm-1 splits into several peaks at 224, 271, 340, and 375 cm-1 because of the translational and bending modes of the Si2O7 and TiO5 groups.14 Two peaks at 589 and 665 cm-1 are assigned to the ν(TiO4) and νs(Si-O-Si) modes, respectively, and appeared after heat-treatment, which indicated that the crystal structure consisting of corner-linked TiO5 pentahedra and Si2O7 pyro-

J. Phys. Chem. C, Vol. 111, No. 45, 2007 17119

Figure 3. Raman spectra of the Sm3+-doped BaO-TiO2-SiO2 glass (a) and glass-ceramics heat-treated at 750°C for 2 h (b).

Figure 4. Emission spectra of the Sm3+-doped glass (a) and glassceramic (b) under femtosecond laser irradiation (the excitation power is 32 mW) and 400 nm light excitation. The insets are the photographs of the Sm3+-doped glass and glass-ceramic samples irradiated by focused femtosecond laser with the same pump power.

silicate groups has been organized.13 All of the sharp peaks were in good agreement with the Raman spectrum of a Ba2TiSi2O8 crystal.13,14 When the Sm3+-doped glass and glass-ceramic were irradiated by the femtosecond laser, red emissions could easily be seen at the focused spots by the naked eye. Furthermore, strongly enhanced upconversion luminescence was observed in the heattreated glass-ceramic as compared with that of as-prepared glass. Figure 4 shows the emission spectra of the Sm3+-doped glass and glass-ceramic irradiated by the 800 nm femtosecond laser. For comparison, the emission spectra of the as-prepared glass and glass-ceramic under excitation of the 400 nm monochromatic light from a Xe lamp are also given in Figure 4. The insets in Figure 4 are photographs of the glass and glass-ceramic samples irradiated by focused femtosecond laser with the same pump power. The brightest spot in the middle of the glass corresponds to the focus point of the femtosecond laser. The spectra of the Sm3+-doped glass and glass-ceramic excited by the 800 nm femtosecond laser exhibit four emission peaks at about 565, 602, and 648 nm, which are similar to the spectra of the Sm3+-doped glass and glass-ceramic excited by 400 nm monochromatic light from a Xe lamp. These three emission peaks can be attributed to the 4G5/2 f /6HJ (J ) 5/2, 7/ , 9/ ) transitions of Sm3+.12 Most importantly, an emission 2 2 peak is also observed at about 400 nm in the 800 nm femtosecond laser irradiated glass-ceramic sample, whereas it is absent in the as-prepared glass. It indicates that the emission at about 400 nm is not because of the diffraction of the

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Figure 5. Luminescent intensity of the 4G5/2 f /6H7/2 transition of Sm3+-doped glass (a) and glass-ceramic (b) as a function of the femtosecond laser pump power.

spectrophotometer. This emission peak is due to the second harmonic generation of the femtosecond laser when excited by the femtosecond laser. We did not observe any detectable structural change in the glass and glass-ceramic after the excitation of the femtosecond laser by optical microscope. Therefore, we think the observed upconversion luminescence is not originated from multiphoton ionization, which may result in destructive breakdown and is a typical phenomenon of multiphoton absorption induced luminescence. Generally, it is impossible to obtain visible emission from an infrared pumping via a single-photon process. Therefore, it is expected that the emission of the Sm3+ ions excited at 800 nm requires at least a two-photon excitation. The multiphoton process strongly depends on the laser intensity. A relationship between the pumping power and the fluorescence intensity can be used to describe the multiphoton process (eq 1),15

I ∝ Pn

(1)

where I is the integrated intensity of the upconversion fluorescence, P is the pump power of the femtosecond laser, and n is the photon number. The number of photons must satisfy the condition that the total energy of n photons exceeds or equals the excitation energy required by excited states. The value of n can be experimentally determined. By varying the pumping power of femtosecond laser at a fixed focused point, we can obtain a series of fluorescence spectra. Therefore, the number of photons (n) can be determined from the slope coefficient of the linear fitted line by plotting the logarithmic transformation of the pumping power and fluorescence intensity. The log-log relationships of the pumping power of femtosecond laser and the fluorescence intensity of the Sm3+-doped glass and glassceramic are shown in Figure 5. As can be seen in Figure 5, a linear correlation between the logarithmic transformation of the excitation power and the upconversion luminescence intensity is observed in the glassceramic, up to the excitation power of 105 mW (Log P ) 2.02). On the contrary, in the as-prepared glass, the upconversion luminescence intensity becomes saturated when the excitation power is above 55 mW (Log P ) 1.74). The saturated threshold is greatly improved in the glass-ceramic because of the precipitation of Ba2TiSi2O8 crystallites. In the case of glassceramic, the laser energy injected into the glass-ceramic sample may be effectively transformed to second harmonic generation because Ba2TiSi2O8 is one of the most efficient nonlinear optical crystals,11 thus resulting in the increase of the saturate threshold. When the pump power is lower than the threshold value, the

slope coefficient of the fitted line is 1.7 for the as-prepared glass and 1.7 for the glass-ceramic. This indicates that the upconversion luminescence originates from a two-photon process. Therefore, two-photon simultaneous excitation is dominant for the upconversion luminescence in the as-prepared glass because there is no resonant absorption at 800 nm (Figure 1). As shown in Figure 5, the upconversion luminescence intensity of the glass-ceramic is about 5 times more than that of the glass under the same condition. We suggest that the enhanced emissions can be attributed to the absorption of the second harmonic generation due to the precipitated Ba2TiSi2O8 microcrystalline particles with second-order optical nonlinearities. The 800 nm femtosecond laser was converted to a 400 nm laser in the glass-ceramic at first, and then the later was reabsorbed by Sm3+. It should be pointed out that the half-width at middle maximum of the femtosecond laser is about 15 nm, and the wavelength of the second harmonic generation covers the wavelength region from 393 to 408 nm. The second harmonic generation is reabsorbed effectively by Sm3+ via 6H5/2 f 4G11/2, 4L 4 6 4 4 4 6 15/2, K11/2, P3/2, F7/2, L13/2, P5/2, and P5/2 transitions at 12 403 nm. When we turn the laser wavelength to 1100 nm, only second harmonic generation at 550 nm was observed, and no luminescence from Sm3+ was detected. In addition, we did not observe such enhanced luminescence in Tb3+-doped glassceramics, because there is no absorption from 380 to 420 nm due to transitions of Tb3+. Furthermore, the efficiency of luminescence because of multiphoton excitation is far less than that of the second harmonic generation.16 Therefore, the upconversion luminescence intensity is strongly enhanced in the glass-ceramic because of the energy-transfer from the second harmonic generation of the microcrystallites to the Sm3+. Our results demonstrated a basic idea for the enhancement of multiphoton absorption induced luminescence. If rare earth ions with absorption at wavelength λ are doped in or codoped with microcrystallites with second-order nonlinear optical property, then the effective second harmonic generation (λ) can be absorbed by the rare-earth ions when excited by light at 2λ and results in enhanced multiphoton absorption induced luminescence. This idea will be useful in designing materials for three-color, solid-state three-dimensional display and medical imaging devices. 4. Conclusions In conclusion, the visible upconversion luminescence in Sm3+-doped glass and glass-ceramic have been experimentally demonstrated by focused infrared femtosecond laser pumping, and strongly enhanced upconversion luminescence was observed in the heat-treated glass-ceramic as compared with that of asprepared glass when excited by the femtosecond laser. The intensity of the upconversion luminescence is proportional to the square of the excitation power. The saturate threshold of the glass-ceramics also increases greatly as compared with the as-prepared glass. Because bright and localized emission can be induced in the rare-earth doped glass-ceramics by the femtosecond laser, the present technique will be promising for three-dimensional display devices. Acknowledgment. This work was financially supported by the National High Technology Research and Development Program of China (G20060914). This work was also supported by the Program for Changjiang Scholars and Innovative Research Team in University. References and Notes (1) Yang, L.; Wang, C.; Dong, Y.; Da, N.; Hu, X.; Chen, D.; Qiu, J. Opt. Express. 2005, 13, 10157.

Enhanced Multiphoton Absorption Induced Luminescence (2) Yang, L.; Dong, Y.; Chen, D.; Wang, C.; Hu, X.; Da, N.; Zhao, G.; Xu, J.; Jiang, X.; Zhu, C.; Qiu, J. Opt. Express. 2006, 14, 243. (3) Johnson, L.; Guggenheim, G. Appl. Phys. Lett. 1971, 19, 44. (4) Parthenopoulos, D.; Rentzepis, P. Science 1989, 245, 843. (5) Downing, E.; Hesselink, L.; Ralston, J.; Macfarlane, R. Science 1996, 273, 1185. (6) Wang, Y.; Ohwaki, J. Appl. Phys. Lett. 1993, 63, 3268. (7) Kumar, K.; Rai, S. Solid State Commun. 2007, 142, 58. (8) Biswas, A.; Maciel, G.; Friend, C.; Prasad, P. J. Non-Cryst. Solids. 2003, 316, 393. (9) Hu, Z.; Wang, Y.; Ma, E.; Chen, D.; Bao, F. Mater. Chem. Phys. 2007, 101, 234.

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