Mechanisms of calcium imbalance in tetrabromobisphenol A-induced neurotoxicity

Optical Materials 47 (2015) 462–464

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Significant blue-shift in photoluminescence excitation spectra of Nd3+:LaF3 potential laser medium at low-temperature Ren Arita a,⇑, Yuki Minami a, Marilou Cadatal-Raduban a,b,c, Minh Hong Pham a,c,d, Melvin John Fernandez Empizo a, Mui Viet Luong a, Tatsuhiro Hori a, Masahiro Takabatake a, Kazuhito Fukuda a, Kazuyuki Mori a, Kohei Yamanoi a, Toshihiko Shimizu a, Nobuhiko Sarukura a, Kentaro Fukuda e, Noriaki Kawaguchi e, Yuui Yokota f, Akira Yoshikawa e a

Institute of Laser Engineering, Osaka University, 2-6 Yamadaoka, Suita, Osaka 565-0871, Japan Centre for Theoretical Chemistry and Physics, Institute of Natural and Mathematical Sciences, Massey University, Albany, Auckland 0632, New Zealand c Institute for Academic Initiatives, Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan d Institute of Physics, Viet Nam Academy of Science and Technology, 10 Dao Tan, Ba Dinh, Hanoi, Viet Nam e Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan f New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan b

a r t i c l e

i n f o

Article history: Received 27 March 2015 Received in revised form 28 May 2015 Accepted 8 June 2015 Available online 12 June 2015 Keywords: Nd3+:LaF3 Fluoride crystal Vacuum ultraviolet Temperature dependent Laser material

a b s t r a c t Temperature-dependent optical properties of bulk Nd3+:LaF3 crystals are reported. A blue-shift in the photoluminescence excitation (PLE) spectrum is observed at 30 K. The 173.2-nm emission peak wavelength at 300 K shifted to 172.8 nm at 30 K, consistent with the 6-nm blue-shift in transmission edge and 2437-cm 1 increase in the lowest energy level of the 4f25d configuration. Thermal broadening of the 5d–4f emission bands with increasing temperature is also observed as the dip at around 178.5 nm present at 30 K disappears at 300 K. A smaller spectral overlap between the PLE and emission spectra is observed as temperature is decreased. Our results suggest that absorption cross-section at the peak fluorescence wavelength is expected to decrease at 30 K. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Vacuum ultraviolet (VUV) radiation is important not only for fundamental research such as spectroscopy [1] but also for industrial applications such as nanofabrication [2] and optical lithography. Synchrotron radiation and free electron lasers are ideal short-wavelength light sources because of their wavelength tunability. These facilities, however, require large-scale equipment and precise management. A compact short-wavelength solid-state laser system is therefore desirable. In the ultraviolet (UV) region, the 5d–4f radiative transitions from Ce3+-doped LiCaAlF6 crystals were extensively studied. As a result, wide-tunable (280–311 nm) [3] laser and ultra-fast chirped pulse amplification systems [4] have been achieved. On the other hand, few reports discuss the lasing properties of solid-state laser media in the VUV region. The presently available VUV laser systems are using non-tunable gas media such as ArF at 193 nm, Xe2 at 172 nm, F2 at 157 nm, Kr2 at 147 nm, ⇑ Corresponding author. E-mail address: [email protected] (R. Arita). http://dx.doi.org/10.1016/j.optmat.2015.06.020 0925-3467/Ó 2015 Elsevier B.V. All rights reserved.

and Ar2 at 126 nm. Compared to gas media, solid-state laser media is more advantageous because of its wide spectral width. The 5d–4f radiative transitions in trivalent rare earth ions such as Nd3+, Tm3+ and Er3+, are excellent for VUV emission [5]. Fluoride crystals doped with these rare earth ions are prominent candidates for VUV laser media because of their wide band gaps. Lasing from Nd3+-doped lanthanum fluoride (Nd3+:LaF3) has been reported by Waynant and Dubinskii [6–8]. Possibility of an all-solid-state VUV laser via step-wise excitation and two-photon excitation in Nd3+:LaF3 [9] and Nd3+-doped lutetium lithium fluoride (Nd3+:LuLiF4) [10], respectively has also been reported. Our previous research revealed that Nd3+-doped lanthanum barium fluoride (Nd3+:La0.9Ba0.1F) has a broader fluorescence bandwidth compared with Nd3+:LaF3 and suggest the potential of Nd3+:La0.9Ba0.1F as a tunable laser material or as gain medium for short pulse amplification [11,12]. However, nobody has reproduced VUV lasing from rare-earth-doped fluorides including Nd3+:LaF3. A better understanding of the optical properties of rare earth-doped fluorides in the VUV region is needed. In particular, studying the temperature-dependent excitation spectrum of solid-state media

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2. Experiment

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3. Results and discussion Fig. 1 shows the transmission spectra of Nd3+:LaF3 from 100 nm to 180 nm at 300 K, 200 K, 77 K and 30 K. The transmission edge at room temperature is 169.2 nm. The transmission spectra are not corrected for surface reflections, but it is apparent that the transmission edge is blue-shifted to 162.5 nm at 30 K. The transmission edge is defined for both temperatures as the wavelength at which the transmittance has decreased to 10%, in accordance with previous reports [16]. The blue shift is due to the decrease in the amplitude of crystal lattice vibrations at low temperature resulting from the thermal contraction of the lattice [16,17]. Operating at low temperatures would improve the transparency of Nd3+:LaF3 to VUV radiation. Among the reported fluoride crystals, Nd3+:LaF3 still seems to be the most suitable material for VUV laser applications. The high absorption coefficient of Nd3+:LaF3 at 157 nm makes the F2 laser an ideal pumping source to achieve VUV lasing. Photoluminescence excitation (PLE) spectra were obtained at the same temperature settings: 300 K, 200 K, 77 K and 30 K, as shown in Fig. 1. The emission wavelength was fixed at the 172-nm fluorescence peak of Nd3+:LaF3 [6–8,11,12]. The small peak

Transmittance (%)

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The Nd3+:LaF3 sample was prepared by the Czochralski growth method with 1% Nd3+ doping concentration. The bulk sample was cut into 10  10  10 mm3 cuboid crystal with all of its sides polished for an optical finish. Nd3+:LaF3 was placed inside the vacuum chamber maintained at a pressure less than 5  10 3 Pa for temperature-dependent PL measurements. A liquid helium cryostat and a heater were used to vary the temperature from 30 K to 300 K. The crystal was irradiated with a F2 laser operating at 157-nm and 1-mJ pulse energy. The optical path from the laser to the vacuum chamber was purged with nitrogen gas to decrease VUV absorption by oxygen. Fluorescence from Nd3+:LaF3 was collected and focused onto the entrance slit of a Seya-Namioka spectrometer by a couple of MgF2 lenses. The luminescence intensity from the Nd3+:LaF3 was measured using a CCD camera (ANDOR D0434-FI-Z). For temperature-dependent PLE measurements, the vacuum chamber was kept at a pressure of less than 10 5 Pa. The temperature was also maintained using a liquid helium cryostat and a heater. The crystal was excited using the 100 nm to 180 nm synchrotron radiation from the BL7B line at the UVSOR facility of the Institute for Molecular Science, Japan [15]. The 5d–4f transition fluorescence around 172 nm from Nd3+:LaF3 was selected using a band-pass filter (Peak 172 nm, FWHM 26.4 nm), and the fluorescence intensity was measured using a photomultiplier tube (Hamamatsu Photonics R6836). The sample and detector were positioned to minimize surface-reflected and scattered excitation beams from reaching the detector. The optical path from the MgF2 window of the vacuum chamber to the photomultiplier tube was also purged with nitrogen gas.

100 50

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PLE Intensity (arb. units)

in the VUV region could provide vital information for optimum lasing conditions. Room temperature excitation spectrum of some rare-earth-doped fluorides, including Nd3+:LaF3, has been reported by Yang et al. [13,14]. However, data on temperature-dependent PLE spectrum of Nd3+:LaF3 in VUV region are lacking. In this paper, we measured the temperature-dependent transmission, photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Nd3+:LaF3. Thermal broadening of the 4f25d energy levels at room temperature was observed in the emission spectra. The PLE spectrum was significantly blue-shifted at 30 K. The spectral overlap between the absorption and emission spectra should decrease at low temperature.

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Wavelength (nm) Fig. 1. Transmission and photoluminescence excitation (PLE) spectra of Nd3+:LaF3 at 300 K, 200 K, 77 K and 30 K. Transmission and PLE plots are not corrected for surface reflections. The transmission edge is blue shifted from 169.2 nm (300 K) to 162.5 nm (30 K). This 6-nm blue shift is comparable to the PLE shift from 171 nm (300 K) to 165 nm (30 K). The small PLE peak around 117 nm is characteristic of the LaF3 host. A smaller spectral overlap between the PLE and transmission spectra is observed at 30 K compared to 300 K.

around 117 nm is characteristic of the LaF3 host with a band gap of about 10.3 eV at room temperature [18]. This peak results from the energy transfer from the conduction band of the host material to the 5d-level of the Nd3+ ion. The peak-shift from 114 nm at 30 K to 117 nm at 300 K is attributed to the increase of the band gap energy of the LaF3 host. Broad PLE bands from 110 nm to 165 nm and from 114 nm to 171 nm for 30 K and 300 K, respectively are observed. The PLE spectra confirms that a wide range of VUV sources, including the F2 laser, can be used to excite Nd3+:LaF3 to achieve lasing. The long-wavelength edge is blue shifted by about 6 nm (from 171 nm to 165 nm) when the temperature is decreased from 300 K to 30 K. In general, the PLE spectrum mimics the absorption spectrum. Fig. 1 compares the PLE and transmission spectra. The 6.3-nm blue shift in the transmission edge (169.2 nm at 300 K to 162.6 nm at 30 K) is comparable to the 6-nm blue shift in the PLE spectra (171 nm at 300 K to 165 nm at 30 K). The photoluminescence (PL) spectra of Nd3+:LaF3 at 300 K, 200 K, 100 K, and 30 K when excited by the 157-nm emission of a F2 laser are shown in Fig. 2. The room temperature PL peak at 173.2 nm originates from the allowed dipole transition from the lowest energy level of the 4f25d configuration at about 59,101 cm 1 to the 4I11/2 multiplet of the 4f3 configuration at about 2000 cm 1. The position of the 4f25d lowest energy level was determined from the room-temperature transmission edge. The PL wavelength corresponds well with previous reports [11,19]. On the other hand, a 172.8-nm PL peak with a dip at around 178.5 nm is observed at 30 K. The dip present at 30 K and disappearing at 300 K is also observed in the PLE spectrum. At room temperature, the lowest energy level of the excited-state configuration where the 173-nm emission originates from is broadened due to thermal broadening. This results to the smoothening of the PLE and PL spectra. The dip in the PL spectrum was also observed by Aleksanyan et al. at 18 K [20]. Similar to the room temperature PL peak, the 172.8-nm emission peak also originates from spin allowed 4f–5d transitions with the lowest energy level of the 4f25d configuration shifted to about 61,538 cm 1. The lowest

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PL Intensity (arb. units)

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λex=157nm(F2 laser) 30K 100K 200K 300K

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shift of the PLE spectrum compared with the PL spectrum, as seen in Fig. 3. The temperature of the laser crystal gain medium is expected to increase during pumping and after lasing is achieved. The heat produced within the crystal during application as laser gain media will lead to the increase in overlap between the absorption and emission spectra and ultimately, the quenching of laser emission. Fig. 3 suggests that the absorption cross-section at the peak fluorescence wavelength is expected to decrease at 30 K, leading to lower self-absorption of emission. Further investigation is needed to quantify dn/dT for Nd3+:LaF3 and its effects on the absorption coefficient at the emission wavelengths as well as the absolute intensities of the emission peaks.

Wavelength (nm) Fig. 2. Photoluminescence (PL) spectra of Nd3+:LaF3 at 300 K (solid-black), 200 K (dashed-black), 100 K (dashed-gray) and 30 K (solid-gray) when excited by the 157-nm emission of a F2 laser. Peak wavelengths are at 173.2 nm and 172.8 nm for 300 K and 30 K, respectively. The dip in the 30 K fluorescence is due to thermal contraction at low temperature. Difference in intensities could be due to dn/dT effects.

energy level of the excited state configuration is estimated from the transmission edge at 30 K. The emission peak appears to be red-shifted by about 0.4 nm when temperature is increased from 30 K to 300 K. The lowest energy level of the excited state configuration is determined from the transmission edge, which is at 59,101 cm 1 at 300 K and 61,538 cm 1 at 30 K. The increase in temperature thus resulted to a downward shift of 2437 cm 1 in the lowest energy level of the excited state configuration. The emission peak originates from the lowest energy level of the 4f25d configuration to the 4I11/2 multiplet of the 4f configuration. The downward shifting in the 4f25d lowest energy level caused the red-shift (2500 cm 1) in the emission peak at 300 K. Fig. 3 compares the PLE and PL spectra. The contribution of the change in refractive index (dn/dT) and surface reflections were not considered here, and thus caution is taken in interpreting the intensities. Nevertheless, there appears to be a greater overlap in the two spectra at 300 K compared to 30 K. This suggests that the spectral overlap between the absorption and PL spectra should decrease at low temperature. As discussed earlier, the disappearance of the 178.5-nm dip in the PL spectrum when temperature is increased can be attributed to the thermal broadening of the 5d–4f emission bands. Similar broadening is expected for the 4f–5d absorption spectrum when temperature is increased. This broadening will lead to a greater overlap between the emission and absorption spectra when temperature is increased. The decrease in spectral overlap between the absorption and PL bands at low temperature will also be a result of the larger blue

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Wavelength (nm) Fig. 3. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra at 300 K (black) and 30 K (gray). A greater overlap in the two spectra is observed at 300 K compared to 30 K.

4. Conclusion In conclusion, temperature-dependent measurements of the VUV optical properties of bulk Nd3+:LaF3 crystal were reported. A blue shift in the transmission edge of 6 nm is observed when temperature was decreased to 30 K. This shift is mirrored in the PLE spectrum showing a blue shift of 6.3 nm at the same temperature. At 30 K, the emission spectra exhibited a dip around 178.5 nm that disappears as temperature is increased to 300 K as a result of thermal broadening of the 5d–4f emission bands. The spectral overlap between the PLE and emission spectra increases with temperature, indicating that emission self-absorption could decrease at low temperature. Further investigation is needed to quantify dn/dT for Nd3+:LaF3. References [1] D.J. Jones, R.H. French, H. Mullejans, S. Loughin, A.D. Dorneich, P.F. Carcia, J. Mater. Res. 14 (1999) 4337–4344. [2] H. Sugimura, L. Hong, K.H. Lee, Jpn. J. Appl. Phys. Part 1 – Regular Pap. Brief Commun. Rev. Pap. 44 (2005) 5185–5187. [3] S.V. Govorkov, A.O. Wiessner, T. Schroder, U. Stamm, W. Zschocke, D. Basting, Efficient high average power and narrow spectral linewidth operation of Ce:LiCAF laser at 1 kHz repetition rate, in: W. Bosenberg, M. Fejer (Eds.), Advanced Solid State Lasers, Optical Society of America, Coeur D’Alene, Idaho, 1998. p. UL1. [4] Z.L. Liu, T. Kozeki, Y. Suzuki, N. Sarukura, K. Shimamura, T. Fukuda, M. Hirano, H. Hosono, IEEE J. Sel. Top. Quantum Electron. 7 (2001) 542–550. [5] T. Yanagida, N. Kawaguchi, K. Fukuda, S. Kurosawa, Y. Fujimoto, Y. Futami, Y. Yokota, K. Taniue, H. Sekiya, H. Kubo, A. Yoshikawa, T. Tanimori, Nucl. Instrum. Meth. Phys. Res. Sect. A – Accel. Spectrometers Detectors Assoc. Equip. 659 (2011) 258–261. [6] M.A. Dubinskii, A.C. Cefalas, C.A. Nicolaides, Opt. Commun. 88 (1992) 122–124. [7] M.A. Dubinskii, A.C. Cefalas, E. Sarantopoulou, S.M. Spyrou, C.A. Nicolaides, R.Y. Abdulsabirov, S.L. Korableva, V.V. Semashko, J. Opt. Soc. Am. B – Opt. Phys. 9 (1992) 1148–1150. [8] R.W. Waynant, Appl. Phys. B – Photophys. Laser Chem. 28 (1982). pp. 205–205. [9] T. Nakazato, M. Cadatal-Raduban, K. Yamanoi, M. Tsuboi, Y. Furukawa, M. Pham, E. Estacio, T. Shimizu, N. Sarukura, K. Fukuda, T. Suyama, T. Yanagida, Y. Yokota, A. Yoshikawa, F. Saito, IEEE Trans. Nucl. Sci. 57 (2010) 1208–1210. [10] K. Yamanoi, Y. Minami, R. Nishi, Y. Shinzato, M. Tsuboi, M.V. Luong, T. Nakazato, T. Shimizu, N. Sarukura, M. Cadatal-Raduban, M.H. Pham, H.D. Nguyen, Y. Yokota, A. Yoshikawa, M. Nagasono, T. Ishikawa, Opt. Mater. 35 (2013) 2030–2033. [11] M. Cadatal, Y. Furukawa, S. Ono, M. Pham, E. Estacio, T. Nakazato, T. Shimizu, N. Sarukura, K. Fukuda, T. Suyama, A. Yoshikawa, F. Saito, J. Lumin. 129 (2009) 1629–1631. [12] M. Cadatal, Y.-S. Seo, S. Ono, Y. Furukawa, E. Estacio, H. Murakami, Y. Fujimoto, N. Sarukura, M. Nakatsuka, T. Suyama, K. Fukuda, R. Simura, A. Yoshikawa, Jpn. J. Appl. Phys. 46 (2007) L985. [13] K.H. Yang, J.A. Deluca, Appl. Phys. Lett. 29 (1976) 499–501. [14] K.H. Yang, J.A. Deluca, Phys. Rev. B 17 (1978) 4246–4255. [15] K. Fukui, H. Nakagawa, I. Shimoyama, K. Nakagawa, H. Okamura, T. Nanba, M. Hasumoto, T. Kinoshita, J. Synchrotron Radiat. 5 (1998) 836–838. [16] A.H. Laufer, J.A. Pirog, J.R. McNesby, J. Opt. Soc. Am. 55 (1965) 64–66. [17] S.A. Johnson, H.G. Freie, A.L. Schawlow, W.M. Yen, J. Opt. Soc. Am. 57 (6) (1967) 734–737. [18] P. Dorenbos, J.T.M. deHaas, C.W.E. vanEijk, J. Lumin. 69 (1996) 229–233. [19] Y. Shinzato, K. Yamanoi, R. Nishi, K. Takeda, T. Nakazato, T. Shimizu, N. Sarukura, M. Cadatal-Raduban, K. Fukuda, S. Kurosawa, Y. Yokota, A. Yoshikawa, T. Togashi, M. Nagasono, T. Ishikawa, Appl. Phys. Exp. 6 (2013). [20] E. Aleksanyan, V. Harutunyan, M. Kink, R. Kink, M. Kirm, Y. Maksimov, V.N. Makhov, T.V. Ouvarova, Opt. Commun. 283 (2010) 49–53.