Journal of Crystal Growth 362 (2013) 167–169
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Micro-pulling down method-grown Er3 þ :LiCaAlF6 as prospective vacuum ultraviolet laser material Marilou Cadatal-Raduban a,n, Toshihiko Shimizu a, Kohei Yamanoi a, Kohei Takeda a, Minh Hong Pham a, Tomoharu Nakazato a, Nobuhiko Sarukura a, Noriaki Kawaguchi b,c, Kentaro Fukuda b,c, Toshihisa Suyama b, Takayuki Yanagida c, Yuui Yokota c, Akira Yoshikawa c a
Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan Tokuyama Corporation, 3-chome, Shibuya-ku, Tokyo 150-8383, Japan c Institute for Materials Research (IMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan b
a r t i c l e i n f o
abstract
Available online 31 October 2011
We report the successful growth of trivalent erbium-doped lithium calcium aluminum fluoride (Er3 þ :LiCaAlF6) using the micro-pulling down method. Several absorption bands were observed at 139 nm (71,942 cm 1), 149 nm (67,114 cm 1), and 162 nm (61,728 cm 1), which can be ascribed to 4f-5d transitions in Er3 þ . Evaluation of its optical properties using the 157-nm emission of a F2 laser reveal that it has a 163-nm vacuum ultraviolet fluorescence with 1.3-ms decay time, involving a transition originating from the high-spin state of the 4f 105d excited state configuration. This is one of the shortest emission wavelengths from solid-state materials reported at room temperature. & 2011 Elsevier B.V. All rights reserved.
Keywords: A2. Growth from melt A2. Single crystal growth B1. Fluorides B3. Solid state lasers
1. Introduction Efficient and economical crystal growth methods are indispensable for developing new materials. It is for this purpose that crystal growth techniques are continuously being improved in order to grow high-quality rare-earth doped fluorides. The micropulling down (m-PD) method is an example of a recently developed crystal growth technique. Owing to a fast growth speed, a high quality crystal can be grown using less than 1 g of raw material in 5–12 h, thereby allowing growth of crystals at a shorter time and at a lower cost compared with conventional growth methods [1]. Recent reports showed the feasibility of growing VUV luminescent materials economically and efficiently using the m-PD method [2–4]. Lately, laser-quality crystals have been grown, thereby enabling the demonstration of lasing in the ultraviolet region from a m-PD method-grown cerium-doped lithium calcium aluminum fluoride (Ce3 þ :LiCaAlF6 or Ce:LiCAF) [5]. Meanwhile, vacuum ultraviolet (VUV) light sources have extensive applications in various fields such as photolithography [6] and spectroscopy [7]. Sources of VUV radiation are currently limited to excimer lasers [8] and nonlinear methods using frequency mixing in gases [9,10] metal vapors [11], and nonlinear crystals [12]. Unfortunately, these sources are either cumbersome or difficult to maintain. Hence, there has been ongoing research in developing more efficient laser materials in the VUV region.
n
Corresponding author. E-mail address:
[email protected] (M. Cadatal-Raduban).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.035
Rare-earth doped fluorides have been shown to be prominent VUV laser material candidates because of their wide bandgap. With the proper pump source, direct emission from these rareearth doped fluorides would provide a much simpler VUV light source. VUV fluorescence from rare-earth doped fluorides was first reported from neodymium (Nd3 þ )-, thulium (Tm3 þ )-, and erbium (Er3 þ )-doped trifluorides [13]. Among the three, fluorescence from Er3 þ -doped yttrium lithium fluoride (Er3 þ :YLiF4) had the shortest emission wavelength, with potential tunability from 165 nm to 172 nm [13]. Meanwhile, the choice of host is also an important consideration in that the host should have a transmission window at the emission wavelength. LiCAF was shown to be an excellent host in the ultraviolet (UV) region [5]. Owing to its short transmission edge at 112 nm, LiCAF would also be a suitable host in the VUV region. In this paper, we report the successful growth of Er3 þ :LiCaAlF6 (Er:LiCAF) crystal using the m-PD method. We also discuss its fluorescence characteristics with 157-nm F2 laser excitation. Its prospect as a VUV laser material is also evaluated. If lasing were successfully achieved from this crystal, it would have one of the shortest emissions among the rare-earth doped fluorides reported.
2. Experiment The m-PD apparatus modified for fluoride crystal growth is described in several papers [1–3,5]. High purity raw materials (Stella Chemifa Corp.) in stoichiometric ratios were thoroughly mixed and put into a graphite crucible. The growth chamber was
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then evacuated to 10 4 Torr by rotary and diffusion pumps. The crucible was baked at 600 1C for 1 h, using radio frequency heating in order to remove oxygen traces from the moisture of raw materials and adsorbates on the chamber surface. Simultaneously, the chamber was evacuated to 10 5 Torr. After baking, the recipient was filled with a mixture of Ar and CF4 until ambient pressure. The crucible was then heated to the melting temperature of about 1450 1C. The single crystal was grown at a pulling rate of 0.1 mm/min and with complete solidification of the melt charged in the crucible. The Er:LiCAF crystal was then excited by the 157 nm emission of a F2 laser operating at 100 Hz repetition rate, 1 mJ pulse energy, and 5 ns pulse duration inside a vacuum chamber, which was evacuated to 10 6 Torr. Fluorescence was collected by an MgF2 lens and focused onto the entrance slit of a VUV spectrograph, which recorded the spectral profile. Temporal profile measurements were carried out using a VUV Seya-Namioka spectrometer and streak camera system connected to the sample chamber. A more detailed description of the VUV streak camera design and experimental set-up can be found elsewhere [14,15]. Measurements were done at room temperature.
3. Results and discussion Fig. 1 shows the transmission spectra of a 1 mol% and 2 mol% Er3 þ -doped LiCAF. Several absorption bands were observed at 139 nm (71,942 cm 1), 149 nm (67,114 cm 1), and 162 nm (61,728 cm 1). These can be ascribed to 4f-5d transitions in Er3 þ .
Fig. 1. Transmission spectra of a 1 mol% and 2 mol% Er3 þ -doped LiCAF. Absorption bands were observed at 139 nm, 149 nm, and 162 nm. The inset shows a photograph of the m-PD method-grown Er(1%):LiCAF.
The optical characteristics of the 1 mol%-doped crystal were evaluated in this work because it has higher transmission compared with the 2 mol%-doped crystal. The inset in Fig. 1 shows the photograph of the m-PD method-grown Er(1%):LiCAF. It is clear and void of cracks or inclusions. The spectral profile obtained from the VUV spectrograph is shown in Fig. 2a. The 157-nm excitation that was scattered after hitting the sample is also shown for reference. The spectral image was calibrated using this known wavelength. The dominant luminescence peak is centered at around 163 nm (61,350 cm 1), while a weaker peak is observed at around 181 nm (55,249 cm 1). Taking into consideration the energy levels of Er3 þ [16], the dominant peak at 163 nm can be attributed to interconfigurational transition from the 4f105d excited state configuration to the 4I15/2 level of the 4f 11 ground state configuration. This 163-nm emission is shorter than the previously reported 172-nm emission from Nd3 þ :LaF3 at room temperature [17,18]. The temporal profile of the 163-nm peak obtained from the streak camera image is shown in Fig. 2b. The luminescence decay time is estimated to be 1.27 ms. The temporal profile of the excitation laser pulse is also shown for reference. Spin configurations for lanthanides with a more than halffilled 4f shell will have two possibilities for the 4f n 15d excited state configuration. When an electron gets promoted from the 4f n ground state configuration to the 4f n 15d excited state configuration, its spin in the d orbital can either be parallel or antiparallel to the unpaired spins in the 4f n 1 core. The first case will give rise to a high-spin (HS) state while the latter will result in a low-spin (LS) state. Although the HS state will be lower in energy, transitions between the ground state and this excited state will be spin-forbidden. As a consequence, spin-forbidden 4f n-4fn 15d transitions can be expected for all lanthanide ions with n 47. A previous work has reported the presence of weak 4f-5d excitation bands corresponding to spin-forbidden transitions in trivalent Tb (n¼ 8), Dy (n ¼9), Ho (n¼10), Er (n¼11), and Tm (n ¼12) doped to YLiF4 [19]. It then follows that, given sufficient excitation energy to access both states, VUV emission resulting from interconfigurational 5d-4f transitions in Er3 þ (4f 11) doped to LiCAF may consist of a fast emission with decay time ranging from a few to tens of nanoseconds or a slow emission with decay time in the microsecond range or both fast and slow emissions, depending on the interplay between the LS and HS states. Fast emission has been established to arise from spinallowed transition, originating from the LS (2Sþ1¼4) state of the 4f 105d configuration. On the other hand, slow emission arises from spin-forbidden transition. Accordingly, this is transition from the HS (2Sþ 1¼6) state of the 4f 105d configuration. A more detailed
Fig. 2. (a) Spectral profile of Er(1%):LiCAF showing VUV fluorescence peaks at 163 nm and 182 nm. (b) Temporal profile of the 163-nm peak. The dashed line is the exponential fitting to the fluorescence decay.
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of the shortest emission wavelengths from solid-state materials reported. With the proper pump source, direct emission from this crystal would provide a simple, short wavelength light source that could open up potential applications in the VUV region. References
Fig. 3. Energy level diagram of Er3 þ showing the transitions resulting to the 163-nm and 182-nm fluorescence peaks.
treatment of the ground state and excited state spin configurations of Er3 þ in various fluoride hosts can be found in several publications [19–24]. The slow decay time observed in our m-PD-grown Er:LiCAF sample indicates that the transition involves the HS state of the 4f105d excited state configuration and is thus, spin-forbidden. This is further supported by the weak absorption band with peak at 162 nm (61,728 cm 1) that was previously observed from Er:LiCAF [15]. The 157 nm (63,694 cm 1) excitation populates this level at 61,728 cm 1 and subsequent interconfigurational transition to the 4f11 (4I15/2 ) ground state configuration resulted in the slow 163-nm emission. A detailed energy level diagram of the transitions in Er:LiCAF is shown in Fig. 3. The slow decay time, in the ms regime, would facilitate a lower lasing threshold. If lasing were successfully achieved from this crystal, it would have one of the shortest emissions among the rare-earth doped fluorides reported.
4. Conclusion In conclusion, we have reported the successful growth of Er:LiCAF by the m-PD method. Excitation with a F2 laser emitting at 157 nm resulted in VUV luminescence at 163 nm with a decay time of 1.27 ms. The 163-nm emission is ascertained to be a spinforbidden transition originating from a high-spin excited state, further explaining the rather long decay time. The 163-nm emission wavelength of Er:LiCAF is shorter than the previously reported 172-nm emission from Nd:LaF, therefore making it one
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