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Cadatal et al. Vol. 25, No. 7 /July 2008 /J. Opt. Soc. Am. B B27

Vacuum ultraviolet optical properties of a micro-pulling-down-method grown Nd3+: „La0.9,Ba0.1…F2.9

Marilou Cadatal,1,2,3,* Yusuke Furukawa,3 Young-Seok Seo,3 Shingo Ono,4 Elmer Estacio,3 Hidetoshi Murakami,3

asushi Fujimoto,3 Nobuhiko Sarukura,3 Masahiro Nakatsuka,3 Kentaro Fukuda,5,6 Rayko Simura,6 Toshihisa Suyama,5

Akira Yoshikawa,6 and Fumio Saito6

1Institute for Molecular Science (IMS), Myodaiji, Okazaki, Aichi 444-8585, Japan2The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan

3Institute of Laser Engineering Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan4Nagoya Institute of Technology, Gokiso, Showa, Nagoya, Aichi 466-8555, Japan

5Tokuyama Corporation, Shibuya-ku, Tokyo 150-8383, Japan6Institute of Multidisciplinary Research for Advanced Materials Tohoku University, 2-1-1 Katahira, Aoboa-ku

Sendai, 980-8577, Japan*Corresponding author: cadatal-m@ile.osaka-u.ac.jp

Received December 3, 2007; accepted January 31, 2008;posted February 28, 2008 (Doc. ID 90369); published April 7, 2008

Nd3+: �La1−x ,Bax�F3−x �x=0.1� is efficiently grown by the micro-pulling-down method. Characterization of itsoptical properties reveals that it is transparent in the vacuum ultraviolet region with an absorption edge ataround 180 nm. It has fluorescence centered at 175 nm with a bandwidth of 12 nm and a lifetime of 6.1 ns asmeasured by a vacuum ultraviolet streak camera and spectrometer combination. It is proposed to be suitableas a vacuum ultraviolet scintillator and a potential laser material. © 2008 Optical Society of America

OCIS codes: 300.2530, 260.7210, 160.5690.

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. INTRODUCTIONesearch efforts have continuously been geared toward

he generation of vacuum ultraviolet (VUV) radiationrom laser materials for scientific and industrial applica-ions such as photolithography [1,2] and spectroscopy3,4] and from scintillators for gamma ray detection [5].

hile there are available VUV laser sources such as ex-imer lasers based on rare gas dimers like Kr2 and Xe26,7] and frequency conversion in nonlinear crystals [8],hese have limited tunability and low conversion effi-iency, respectively. In 1976 Yang and DeLuca proposedhe generation of VUV radiation from dipole allowed in-erconfigurational d– f transitions in rare-earth-dopedide-bandgap fluorides [9]. This is an attractive schemeecause of its simplicity, reliability, and efficiency. It islso possible to obtain good-quality laser beams, con-rolled adjustment of the spectral width, and multiwave-ength operation from only one laser oscillator [10]. Sincehe first experimental verification of their proposal [11],everal studies have reported luminescence from severalrivalent rare-earth-doped fluorides [12–14]. Amonghese, Ce3+-activated fluorides, particularlye3+:LiCaAlF6 [15–17] and Ce3+:LiSrAlF6 [18,19] haveeen proved to be efficient ultraviolet (UV) active media.n spite of the encouraging results delivered by these ma-erials, transition energies are not large enough for VUVmission. Only Nd3+:LaF3 has been reported to lase inhe VUV region [20,21]. Work on this material, however,id not progress owing to rapid saturation of the laserutput brought about by loss mechanisms such as colorenter formation resulting from excited state absorption.oreover, it had limited tunability and required electron

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eam pumping. Because of this, there is a continuingearch for appropriate activator–matrix complexes andctive medium–pump source combinations suitable forenerating efficient VUV tunable lasers.

On the other hand, even though the number of knowncintillators amounts to several hundred, new and betteraterials are still needed to satisfy many requirements

ependent on particular applications. For example, large-ized ZnO crystal grown by the hydrothermal method hasecently been shown to be a fast scintillator for extremeV photolithography [22]. High-performance scintillators

hould both be fast and efficient [23]. Nd3+-doped fluo-ides are scintillator candidates because of their charac-eristic decay times of the order of 10 ns arising from thearity-allowed d– f transition [24]. Crystals having highight yields can also be engineered by choosing the rightctivator–host material combination. Their ability to emitn the VUV region is also advantageous for specific appli-ations. For instance, gamma ray scintillators that emitUV fluorescence are strongly required for a gas scintil-

ation microwire stripped detector for high-resolution pos-tron emission tomography application [25]. Material re-earch for VUV scintillators is also ongoing.

Innovation in crystal growth methods plays an impor-ant role in developing VUV laser materials and scintilla-ors. Classical methods for mass production of single crys-als for industrial applications, for example Czochralskind Bridgman, have already been well established [26].owever, these techniques are time and raw-material

onsuming, especially for single-crystal material screen-ng. It is also difficult to combine screening with the es-ablishment of appropriate growth conditions required for

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igh-quality crystal production [26]. The micro-pulling-own (�-PD) method is a relatively new crystal growthechnique developed mainly at Tohoku University, Japan,o meet the challenges in materials research. Recent im-rovements of this method made the quality of �-PDrown crystals comparable with those prepared by Czo-hralski, Bridgman–Stockbarger, or other classicalrowth techniques [27]. Owing to a fast growth speed, aigh quality crystal can be grown by using less than 1 g ofaw material in 5–12 h, thereby allowing the growth ofarge crystals in a shorter time and at a lower cost com-ared with the other melt growth methods mentionedbove [27]. Moreover, this method has the capability ofontrolling the shape of the grown crystal to produce fi-ers, rods, and tubes [27]. This method was initially de-igned for the growth of oxide crystals, and several stud-es have demonstrated the advantages of this method28–30]. However, because fluoride materials have re-eived considerable attention, innovation of the �-PD ap-aratus to accommodate fluoride crystal growth was car-ied out. Moreover, extensive research is devoted to themprovement of this method to facilitate the screening ofuoride host lattices, dopant concentration, and optimiza-ion of the growth parameters. The demonstration of ra-io luminescence from �-PD-grown Ce:PrF3 shows thathis method is capable of efficiently growing fluoride crys-als [31]. In this paper, we report the successful growth ofd3+: �La0.9,Ba0.1�F2.9 crystal by using the �-PD methododified for fluoride crystal growth. For comparison, we

ave also grown Nd3+:LaF3, using the same growth pa-ameters. The optical characteristics of these materials inhe VUV region are investigated and compared. Our re-ults not only reinforce �-PD’s merits as a useful tool foruoride crystal growth but also advance the developmentf a VUV scintillator and laser material.

. EXPERIMENTAL SETUPd3+:La�1−x�BaxF3−x �x=0.1� or Nd3+: �La0.9,Ba0.1�F2.9 is

rown by using the �-PD apparatus for fluoride crystalrowth shown in Fig. 1. High-purity LaF3, BaF2, anddF3 are prepared as starting materials at a molar ratio

f 90:10:1. They are thoroughly mixed and placed inside a

ig. 1. (Color online) The �-PD apparatus modified for fluoriderystal growth, used to grow Nd3+: �La ,Ba �F .

0.9 0.1 2.9

raphite crucible. The growth chamber is then evacuatedo 10−4 Torr by rotary and diffusion pumps. Using radiorequency heating, the crucible is baked at 600°C for 1 ho remove oxygen traces from the moisture of raw mate-ials and adsorbates on the chamber surface. Simulta-eously, the chamber is further evacuated to 10−5 Torr.fter baking, the recipient is filled with a mixture of ar-on and CF4, which will serve as the growth atmosphere,ntil ambient pressure is reached. The crucible is theneated to the melting temperature of about 1450°C. Theingle crystal is grown with a pulling rate of 0.1 mm/minnd with complete solidification of the melt charged in therucible. A detailed description of the �-PD apparatus cane found in [32]. The phase diagram for the LaF3–BaF2ystem shows that La�1−x�BaxF3−x with 0.2�x�0.5 woulde eutectic and opaque [33]. In our experiment, we ob-erved that the crystal is already cloudy when x=0.15;hus we chose x=0.1. For reference, Nd3+:LaF3 crystal islso grown from the stoichiometric mixture of high-purityaF3 and NdF3 by using the same growth parameters.or 1 mol.% Nd doping concentration, the number densityf Nd3+ ions in �La0.9,Ba0.1�F2.9 and LaF3 host is 1.651020 cm−3 and 1.81�1020 cm−3, respectively. The as-

rown crystals are 20-mm in length and 2-mm in diam-ter. Both crystals have no visible cracks or inclusions. Ahotograph of the actual samples used in fluorescenceeasurements is shown in Fig. 2.The absorption coefficient is measured by using a

2-purged VUV spectrometer (Bunkoh-keiki, type KV-01) and a semi-double-beam system that uses a deute-ium lamp. The samples used are 1 mm thick. For fluo-escence measurements, an F2 laser (MPB Tech., PSX-00) operating at 157 nm, 100 Hz repetition rate, 1 mJulse energy, and 5 ns pulse duration excites the samplelaced inside a vacuum chamber. The optical path fromhe laser to the vacuum chamber is also purged with N2as to prevent absorption of the 157 nm excitation by oxy-en. A MgF2 lens is used to collect and focus the fluores-ence onto the entrance slit of a spectrograph. The spec-rograph has a 1 nm resolution. A VUV CCD array detectshe fluorescence spectrum. In all measurements, the spec-rum is corrected for the CCD detector spectral response.

Temporal measurements to determine the fluorescenceifetimes of Nd3+: �La0.9,Ba0.1�F2.9 and Nd3+:LaF3 are per-

ig. 2. (Color online) As-grown Nd3+: �La0.9,Ba0.1�F2.9 andd3+:LaF crystals.

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Cadatal et al. Vol. 25, No. 7 /July 2008 /J. Opt. Soc. Am. B B29

ormed with a streak camera and a Seya-Namioka VUVpectrometer combination. The spectrometer is connectedo the vacuum chamber where the fluoride sample islaced. The schematic diagram of the system is shown inig. 3. The entire system is maintained at a vacuum levelf 10−6 Torr. Moreover, all measurements are spectrallynd temporally calibrated.

. RESULTSransmission characteristics are highly influenced by theost material. Fluorides are promising because they areapable of emitting in the VUV region because of theirroad bandgaps. They are also highly transparent in theUV region. This makes them suitable not only as laseraterials but also as VUV optical materials. It is also pos-

ible to achieve high doping efficiency, therefore providingore flexibility in engineering their optical properties

uch as light output and lifetime [34,35]. Moreover, therere nontoxic and robust hosts that can be chosen from aide range of complex fluorides. Figure 4 shows the ab-

orption coefficient, �, of 1 mm thick �La0.9,Ba0.1�F2.9,aF3, and BaF2 hosts in the VUV region. Their absorptiondges are estimated to be about 180, about 210, and about38 nm for �La0.9,Ba0.1�F2.9, LaF3, and BaF2, respectively.learly, �La0.9,Ba0.1�F2.9 has a shorter transmission edgend is more transparent in the VUV region comparedith LaF3. This indicates that it will have better VUV

ransmission characteristics, given the same doping ion.oreover, its short-wavelength transmission edge makes

t more suitable as a short-wavelength solid-state opticalaterial.Figure 5 shows the VUV fluorescence from

-PD-method-grown Nd3+: �La0.9,Ba0.1�F2.9. The plot alsohows the 157 nm excitation for reference. This is the ex-itation that is detected after being scattered from theurface of the sample. After considering the energy levelsor Nd3+ activator ions, the dominant peak located at75 nm may be attributed to the allowed dipole transitionrom the 4f25d configuration to the 4I15/2 multiplet of the

ig. 3. (Color online) Schematic diagram of the VUV spectrom-ter and streak camera system used to measure the temporalharacteristics of micro-PD-method-grown Nd3+: �La0.9,Ba0.1�F2.9nd Nd3+:LaF3. Measurements are carried out under a vacuumevel of 10−6 Torr.

f3 configuration [36]. Using the same setup, the fluores-ence spectrum from Nd3+:LaF3 is obtained. This is alsohown in the same figure. The VUV fluorescence peak isocated at around 172 nm, which is in good agreementith previous reports [20,21]. Both samples have the

ame dimensions and are grown by the same technique.he shift of the Nd3+: �La0.9,Ba0.1�F2.9 fluorescence peako a longer wavelength compared with Nd3+:LaF3 may beue to the effect of BaF2. The transmission of a BaF2 hosts shown in Fig. 4 for reference. Nd3+: �La0.9,Ba0.1�F2.9 isbserved to have a broader fluorescence compared withd3+:LaF3. It has a fluorescence bandwidth (full width atalf-maximum) of 12 nm, while Nd3+:LaF3 has a band-idth of 8 nm. In this regard, Nd3+: �La0.9,Ba0.1�F2.9 willave better laser characteristics in terms of better tun-bility and the capability for amplification of shorterulses. For instance, with a bandwidth of 12 nm, about.25 fs pulses can be amplified, while an 8 nm bandwidthould amplify about 3.7 fs pulses. Moreover, laser emis-ion from Nd3+: �La0.9,Ba0.1�F2.9 has better prospects,ince this material is transparent at the175 nm emission.

detailed discussion of the advantages of this materialver Nd3+:LaF3 can be found in [32].

ig. 4. Absorption coefficient, �, of undoped �La0.9,Ba0.1�F2.9,aF3, and BaF2. �La0.9,Ba0.1�F2.9 is more transparent in the VUVegion. It has an absorption edge at 180 nm compared, with LaF3ith an absorption edge at 210 nm.

ig. 5. F2-laser-induced fluorescence from Nd3+: �La0.9,Ba0.1�F2.9solid curve) and Nd3+:LaF3 (dashed curve) with peaks at 175nd 172 nm, respectively. The spectra are corrected for the spec-rograph and CCD detector spectral responses.

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B30 J. Opt. Soc. Am. B/Vol. 25, No. 7 /July 2008 Cadatal et al.

The streak camera image of the (157 nm emission)2-laser-induced Nd3+:LaF3 fluorescence is shown in Fig.. Also shown in the same figure is the 157 nm excitationulse for reference. The location of the fluorescence peaks in good agreement with the spectrum that was mea-ured by a VUV spectrograph and CCD array detector ashown in Fig. 5. The temporal profiles of the VUV fluores-ence from Nd3+: �La0.9,Ba0.1�F2.9 and Nd3+:LaF3 arehown in Fig. 7. The fluorescence peak at 175 nm fromd3+: �La0.9,Ba0.1�F2.9 has a lifetime of 6.1 ns. On the

ther hand, the 172 nm fluorescence from Nd3+:LaF3 haslifetime of 8.9 ns. Fluorescence decay from both samplesre single exponential. The leveling off beyond 30 ns ob-erved from Nd3+: �La0.9,Ba0.1�F2.9 is due to backgroundoise. Also shown in the same figure is the temporal pro-le of the 157 nm F2 laser excitation pulse. It has a pulse-idth of 5 ns. The time scale of the excitation pulse is

hifted to coincide with the fluorescence for clarity. Inerms of scintillation properties, Figs. 4 and 7 show thatd3+: �La0.9,Ba0.1�F2.9 is more suitable as a VUV scintil-

ator. It is transparent at the emission wavelength,hereby resulting in better light yield. It also has a fasteruorescence decay time.

. SUMMARYn summary, we have successfully grown ad3+: �La0.9,Ba0.1�F2.9 crystal by using the �-PD method.

nvestigation of its VUV optical properties in terms ofransmittance, F2-laser-induced fluorescence, and life-ime reveal that it offers several advantages overd3+:LaF3. For instance, it is more transparent in theUV region with a short-wavelength absorption edge atround 180 nm. Moreover, it has a fluorescence peak at75 nm with a bandwidth of 12 nm. These make it a po-ential laser material with better tunability and with the

ig. 6. (Color online) VUV streak camera image of the 172 nmuorescence from a Nd3+:LaF3 crystal excited by a F2 laser. Alsohown is the streak camera image of the 157 nm excitation pulse.he image is temporally and spectrally calibrated.

apability for amplification of shorter pulses. Moreover, itas good prospects as a VUV scintillator because of a fastuorescence decay time and transparency in the VUV re-ion.

CKNOWLEDGMENTShis work was partially supported by the Ministry ofducation, Culture, Sports, Science, and Technology ofapan, Grant-in-Aid for Young Scientists (A), 19686001,007. We thank Tsuguo Fukuda from the Institute of Mul-idisciplinary Research for Advanced Materials (IMRAM),ohoku University, for his useful discussions and sugges-ions.

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