Nd: (La1-x,Bax)F3-x as Vacuum Ultraviolet Scintillator and New Potential Laser Material

At present, few laser sources are available in the vacuum ultraviolet (VUV) region and most of them are either complicated or expensive. Among these are excimer lasers based on rare gas dimers [1]. However, limited tunability and the use of e-beam excitation impose serious experimental restrictions. Tunable radiation can be generated from nonlinear methods using frequency mixing in gases [2,3], metal vapors [4] and nonlinear crystals [5] but this scheme is characterized by low conversion efficiency. Current injection-type semiconductor laser diodes are more compact; however their operating wavelength cannot extend below 200 nm because of their narrow band gap. An attractive solution is to take advantage of the allowed radiative interconfigurational d-f transitions of rare – earth (RE) activated ions in wide band gap dielectric crystals due to its simplicity, reliability, and efficiency. With this scheme, it is possible to obtain good quality laser beam, controlled adjustment of the spectral width, and multi-wavelength operation from only one laser oscillator [6]. Several works have studied the potential of Nd-doped fluorides such as LaF3 as laser materials in the VUV. Dubinskii et al and Waynant et al demonstrated the ability of Nd:LaF3 to lase at 172 nm by optical [7] and electron beam pumping [8], respectively. After these efforts, no successful material research for VUV laser media is reported. On the other hand, gamma ray scintillator that emits vacuum ultraviolet fluorescence is strongly needed for gas scintillation micro-wire stripped detector for high resolution PET application [9]. Material research for this application is still ongoing. To efficiently address the need for suitable materials, we use the micro-pulling down (micro-PD) method for crystal growth that enable shorter growth time at a lower cost compared with other melt growth methods like Czochralski or Bridgeman – Stockbarger [10]. In this paper, Nd:(La1-x,Bax)F3-x as new scintillator and laser material is explored. Single crystal was efficiently grown using the micro-PD method modified for fluoride crystal growth [11]. Compared to Nd:LaF3, it has higher VUV transmission, shorter transmission edge; and its fluorescence peak located at 175 nm is broader and more intense. The sample was grown using an RF heated micro-PD apparatus with graphite crucible [11]. High purity LaF3, BaF2, and NdF3 with 90 mol%:10 mol%:1 mol% molar ratio were used as starting materials to obtain Nd:(La1-x,Bax)F3-x where x = 0.1. The 20-mm long, 2-mm wide single crystal was grown with a pulling rate of 0.1 mm/min. The growth atmosphere is mixture of Ar and CF4. The crystal was grown with complete solidification of the melt charged in the crucible. Transmission characteristics are highly influenced by the host material. Figure 1 shows that the transmission edge of undoped (La1-x,Bax)F3-x located at 160 nm is at a shorter wavelength compared to LaF3 at 180 nm. This implies that Nd:(La1-x,Bax)F3-x would have better VUV transmission characteristics compared to Nd:LaF3.