UV and blue upconversion in Tm3+-doped fluoroaluminate glass by 0.655 μm excitation

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Journal of Non-CrystalhneSohds 135 (1991)90-93 North-Holland

Letter to the Editor

UV and blue upconversion in Tm3+-doped fluoroaluminate glass by 0.655 Ixm excitation K. H i r a o a, S. T a n a b e b, S. K i s h i m o t o a, K. T a m a i a a n d N. Soga a a Department of Industrial Chemtstry, Faculty of Engineering, and b Department of Chemtstry, College of Ltberal Arts and Sctences, Kyoto Unwerstty, Sakyo-ku, Kyoto 606-01, Japan

Recewed 24 June 1991

Intense upconverslon fluorescence could be observed m the UV and blue region m Tm3+-doped fluoroalumlnate glass m the system AtF~-CaF2-BaF 2-YF3-TmF3 by single wavelength pumping with a DCM-dye laser at pump powers less than [ I found that the dependences of fluorescence intensity on the incident excitation power were quadratic both tor 0 30 ancl 0 45 I~m emission, which originates from ID2 and for 0 48 ~m emission from IG4, respectively, indicating the presence of two step excltatton mechanisms

Frequency upconversion of rare-earth ions doped in glasses are attracting a great interest because of the possibility of infrared-pumped visible lasers. A number of studies have been carried out on Er 3+- or Ho3+-doped glasses [1-3]. These ions can efficiently convert infrared radiation from I I I - V group diode lasers into green emission around 0.55 ~m. On the other hand, one characteristic of the Tm 3+ ion is the presence of stable excited levels, emitting blue fluorescence. A blue laser is an especially attractive material for higher density optical data storage. Fluoride glasses may be a good candidate for the Tm 3+ blue upconversion laser, since the nonradiative loss due to multiphonon relaxation is decreased due to their lower phonon energy [4]. Recently, it was reported that a blue upconversion phenomenon was found for a fluorozirconate glass by co-pumping both at 676.4 and 647.1 nm using a krypton ion laser [5]. These two wavelengths correspond to the ground state absorption (GSA) and excited state absorption (ESA) of Tm 3+ and thus upconversion could be attained. In this study, fluoroaluminate glass was chosen as a host of Tm 3+ ion and the upconversion charac-

teristics were investigated by pumping with a single wavelength. Glass in the composition of 40.5AIF 3 • 21.8CaF 2 • 21.8BaF z • 14.9YF 3 • 1TmF 3 was prepared by using reagent grade A1F3, CaF 2, BaF z, YF3, and TmF 3. A small amount of N H 4 F " H F was also added to complete the fluorination. The powders of batch composition were well mixed in an alumina mortar and melted in a platinum crucible at 1000°C for 15 min. The melt was poured on a stainless steel and pressed with a stainless plate. The glass obtained was annealed at 4 3 5 ° C for 15 min and cut into 4 × 4 × 1 mm shape and polished with diamond paste. Absorption spectrum was measured with a Hitachi-330 self-recording spectrophotometer in the wavelength range 200-2000 nm. The spectrum is shown in fig. 1. The energy level of Tm 3+ is shown in fig. 2. Each assignment in fig. 1 corresponds to the upper level of the transition from the ground 3H 6 multiplet. Fluorescence spectra were recorded with a Hitachi-850 Fluorescence Spectrophotometer. As an excitation source, a dye laser apparatus (Spectra Physics Model 375 Dye Laser) excited by an Ar +

0022-3093/91/$03 50 © 1991 -Elsevter Science Pubhshers B V All rights reserved

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laser (Coherent innova 70) was used with D C M dye, having the emission wavelength range of 630-700 nm, which corresponds to Tm3+ :3F2,3 <---3H 6 ground state absorption. The excitation wavelength was tuned with a tuning wedge and the power was controlled by the power of Ar + laser and determined by a power m e t e r (Coherent 210 Power Meter). The maximum power of 0.65 txm radiation obtained was 360 mW. Efficient upconversion was obtained with 0.655 Ixm exotat~on. The fluorescence spectrum with 0.655 txm excitation is shown in fig. 3 for various pumping powers. The largest p e a k is due to the Rayleigh scattering of the incident light. Also shown ~s the upconversion fluorescences at 363, 451,476 nm and one-step fluorescence at 793 nm. The blue emissions at 451 nm and 476 nm could be clearly observed by human eye even with pumping powers less than 100 mW. The wavelength of 655 nm is intermedmte between the ESA and G S A bands. In this glass, the upconverslon is attained by a single p u m p because of the overlap of these two absorption bands at 655 nm. In the case of Tm3+-doped silica fiber reported by H a n n a et al. [6], the emission around 460-470 nm was observed in the fluorescence spectra, but the emission from the 1D 2 level and that from t h e 1G 4 level was not separated well, presumably due to the large Stark splitting and inhomogeneous broadening in the oxide host. On the other hand, for the fluoride host m this study, the

emissions at 451 nm and 476 nm are clearly separated. From the optical absorption spectrum, it is suggested that the former is due to the 1D 2 --* 3H 4 transition and the latter t o 1G 4 ---> 3H 6. In addition to the blue emissions, the 360 nm emission in the U V range is clearly observed, which is due to 1D 2 --* 3H 6. The relative intensity of this U V emission is larger c o m p a r e d with the spectrum of Tm3+-doped silica glass [6], where the peak around 370 nm is only weakly detected. This may be due to the different transition probability of 1D 2 - - * 3 H 6 a n d not due to the larger population of the emitting level, since the emitting levels of 360 nm and 450 nm are the same

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Fig 3 Emission spectra of Tm3+-doped glass p u m p e d by 0 655 l~m with the power of (a) 20 mW, (b) 60 roW, (c) 100 mW, (d) 150 mW, (e) 200 mW, (f) 250 mW, (g) 300 m W and (h) 360 mW.

(1D2). As seen in fig. 3, the intensity of upconversion emission increased drastically with increasing excitation power, while the intensity at 793 nm increased linearly. At higher pumping power, the intensity of upconversion fluorescence is comparable to that of one-step fluorescence. The power dependence of emission intensity on the incident pumping power is plotted in fig. 4. Apparently, the 793 nm emission shows linear dependence against pumping power while the other

emissions have non-linear dependence. The loglog plot is shown in fig. 5. Each slope corresponds to the number of steps needed for the excitation,

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Fig 4 D e p e n d e n c e of the emission intensity on the 0 655 ~ m pumping power The lines are drawn as a guide for the eye

Fig 5 The log-log plot of the emission intensity and pumping power v , 793 nm; zx, 451 nm, ©, 363 nm; and [:3, 476 nm Each hne indicates the slope of expected n u m b e r of steps, 2 or 1 Both slopes of 2 and 1 are suggested by the lower and upper mangles, respectwely.

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although there seems to be the effect of saturation at higher pumping power. Therefore, both UV and blue emissions having a slope of 2 should occur by a two step process with 0.65 txm excitation in this fluoroaluminate glass. The saturation effect is probably due to the depopulation of the ground level by higher power excitation. Since the concentration of Tm 3+ is low, the main mechanism leading to the present upconversion is likely to be ESA. Thus the intermediate level of the upconversion, from which ESA occur, should be clarified. According to the energy matching, one possibility is the 3F4 level, which is populated after the multiphonon decay from 3F2,3 and pumped with the incident photon of the same energy to 1D 2 level. As far as the emission from 1D 2 level is concerned, this process seems to be reasonable, owing to the absence of other higher

93

levels at a suitable energy and rather large energy gap of 3F4 to the next lower level, 3H 5. Further investigation is now being carried out on the detailed upconversion process of this glass.

References [1] D C Yeh, W A Slbley, M Suscavage and M G Drexhage, J Appl Phys 62 (1987)266 [2] J Y. Allaln, M Monene and H Poignant, Electron Lett. 26 (1990) 261 [3] S Tanabe, K Hlrao and N Soga, J Non-Cryst Sohds 122 (1990) 79 [4] C B Layne and M.J. Weber, Phys Rev B16 (1977) 3259 [5] J Y Allam, M Monerle and H Potgnant, Electron Lett 26 (1990) 166 [6] D C Hanna, R M Percival, I R Perry, R G Smart, J E Townsend and A C Tropper, Opt Commun 78 (1990) 187