High temperature persistent spectral hole burning of Sm2+ in fluorohafnate glasses

]OURNA L OF

Journal of Non-Crystalline Solids 152 (1993)267-269 North-Holland

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Letter to the Editor

High temperature persistent spectral hole burning of in fluorohafnate glasses K. H i r a o a, S. T o d o r o k i a, K. T a n a k a a n d T. K u s h i d a c

a,

S m 2+

N. Soga a, T. I z u m i t a n i ~, A. K u r i t a c

a Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606-01, Japan b Hoya Corporation, 3-3-1 Musashino, Akishima-shi, Tokyo 196, Japan c Department of Physics, Faculty of Science, Osaka University, Toyonaka 560, Japan Received 29 July 1992 Revised manuscript received 20 October 1992

Persistent spectral hole burning of Sm 2÷ in a fluorohafnate glass was observed. The hole was burned even at 180 K. In this system, the photochemical process is likely to be dominant because of the absence of an anti-hole adjacent to the hole.

Persistent spectral hole burning materials are of interest to the development of optical memories [1]. Many attempts have been made to prepare hole burning materials using organic compounds doped with dye [2-4] and inorganic crystals containing transition metal or rare-earth ions [5-8], although operating temperatures of such systems are restricted to low temperature. Room temperature hole burning was recently observed for the first time for Sm 2÷ in SrFCll/2Brl/2 single crystal [9]. There has been no known report of hole burning in inorganic glasses above liquid helium temperature although glasses are expected to be advantageous with larger inhomogeneous linewidth due to various sites of Sm 2÷ unlike crystal system and easier preparation of large samples. We report successful observation of sharp hole burning in Sm2+-doped fluorohafnate glasses at 180 K. This is important not only because it is the first observation of hole burning

Correspondence to: Dr K. Hirao, Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan. Tel: + 81-75 753 5531. Telefax: + 81-75 761 8846.

of Sm 2+ in a glass system but also because the temperature is far above liquid nitrogen temperature. The glass was prepared using HfF4, A1F3, YF3, CaF2, SrF 2, BaF 2, NaF and SmF 3 as starting materials, which were mixed thoroughly and melted in a glassy carbon crucible in strong reducing atmosphere in order to reduce Sm 3+ to Sm 2÷. Transparent brown-colored glass was obtained. This coloring is due to strong absorption of f ~ d (4f 6 7F 0 --~ 4f 5 5d) transition of Sm 2+. The sample employed for optical measurement was cut and polished to 5 × 25 × 25 mm. The fluorescence spectrum of the sample under excitation by Ar ÷ ion laser is shown in fig. 1, where both emission of Sm 3+ and Sm 2÷ were observed. The linewidth of 5D 0 ~ 7F 0 line for Sm 2÷ ion was 71 cm-~ which is about three times that of BaFClxBrx_ x : Sm 2÷ systems [6,7] because of its amorphous random structure. It is noted that large inhomogeneous linewidth is desirable for data storage. The measurement of the hole burning effect was carried out as follows. The sample was kept at a fixed temperature between 4.5 and 180 K using a gas-flow-type cryostat and was irradiated

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K. Hirao et al. / Spectral hole burning o f Sin 2 + in fluorohafnate glasses

268

with a DCM dye laser pumped by an Ar ÷ laser. The dye laser wavelength was tuned to resonate with the 5D 0 *--7Fo line. As soon as the irradiation was discontinued, the excitation spectrum for the 5D o --* 7F2 fluorescence around 723 nm was measured by varying the wavelength of the exciting DCM laser. Figure 2 shows the excitation spectra obtained at 4.5 and 140 K after irradiation with the DCM laser for 460 and 1190 s, respectively. It should be noted that the hole is clearly observed at 140 K. Moreover, we observed a broad hole at room temperature. The hole width is about 1.5 cm-1 at 4.5 K and about 10 cm-1 at 140 K. The saturation effect is considered to make a considerable contribution to the hole spectrum in these cases. For the spectrum at 4.5 K, the linewidth of the laser light is non-negligible. Therefore, it is probable that the homogeneous width of the 7F0 ---* 5D 0 line in this material is much narrower than the hole-widths shown in fig. 2. No 'anti-hole', or increased emission peak, was observed around the burnt hole as shown in fig. 2. Any anti-holes are caused by photo-induced redistribution of Sm 2÷ ions within the inhomogeneous linewidth. Therefore, it is clearly suggested that the dominant burning mechanism in this system is not photo-physical such as Sm2A+

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Fig. 1. Fluorescence spectrum at 4.5 K excited by As + ion laser (all line) and energy diagram of Sm 2+.

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Wavelength / nm Fig. 2. Bottom: Excitation spectrum at 4.5 K and 140 K of Sm2+-doped fluorohafnate glass obtained by monitoring the 5D 0 --* 7F2 emission before and after irradiation with a DCM dye laser of 681.8 nm. Burning time is 460 and 1190 s at 4.5 and 140 K, respectively. The persistent hole burning is observed. Top: The difference signal at 4.5 K, magnified by a factor of 2.

where the photon energy of hto is resonant with Sm~,÷ but not with Sm~+. In other words, the photochemical hole burning is likely to be dominant, where this photo-ionization reaction is described as Sm2++(trap )

h a , Sm3++(trap)-"

(2)

Figure 3 shows the burning time dependence of the hole depth. A decrease of hole depth with increasing temperature is due to both an increase of hole width (homogeneous linewidth of Sm 2÷) and thermal assisted reverse reaction of eq. (2) (releasing electrons from traps). The dominant electron trap in the present material can be Hf 4+ or Sm 3+. Or, if F 2 molecular ions and F ° defects are present in the present strongly reduced glass, these are likely to be the dominant traps. These defects are reported to be stable at 180 K in X-irradiated fluoride glasses [10]. Further studies are necessary to clarify the actual hole burning mechanism.

K. Hirao et al. / Spectral hole burning of Sm 2 + in fluorohafnate glasses

269

terns is superior to that of other materials such as organic or crystal, because of their high chemical and thermal stability and ease of preparation.

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References

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i

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200

i

400

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600

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i

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Fig. 3. The burning time dependence of the persistent holedepth obtained by monitoring the 5D 0 ~ 7F2 emission in the presence o f burning irradiation of a DCM dye laser of 681.8 am,

In conclusion, persistent spectral hole burning of Sm 2÷ in glass matrix was observed for the first time to our knowledge. The hole was burnt at 180 K, which is the highest temperature of an inorganic glass material. From the point of view of practical applications, hole burning in glass sys-

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