Artigos Study of UV fluorescences induced from 4f3 t 4f25d multistep absorptions of Nd3+ions in YLiF4 and LuLiF4 crystals 1 André Felipe Henriques Librantz Doutor em Tecnologia Nuclear [Materiais] – Ipen; Pesquisador-colaborador – Cnen/Ipen; Professor na graduação [Ciência da Computação] – Uninove. librantz@ipen.br, São Paulo – SP [Brasil] Luiz Vicente Gomes Tarelho Doutor em Tecnologia Nuclear [Materiais] – Ipen; Pesquisador-colaborador – Cnen/Ipen. ltarelho@ipen.br, São Paulo – SP [Brasil] Laércio Gomes Doutor em Tecnologia Nuclear [Materiais] – Ipen; Pesquisador-colaborador – Cnen/Ipen. lgomes@ipen.br, São Paulo – SP [Brasil] Izilda Márcia Ranieri Doutora em Tecnologia Nuclear [Materiais] – Ipen; Pesquisadora-colaboradora – Cnen/Ipen. iranieri@ipen.br, São Paulo – SP [Brasil] Nd3+ ultraviolet (UV) fluorescence induced by multiphotonic laser excitations was studied in doped Nd:YLiF4 (YLF) and Nd:LuLiF4 (LLF) crystals by using the time resolved spectroscopy technique. The UV luminescences are due to transitions between the 4f25d and the 4f3 electronic configurations of Nd3+ ions. The 4f25d configuration can be reached by direct pumping or by multiphotonic excitation, both processes give rise to the UV band emission with structure due to the strong phonon coupling expected for 5d orbital involvement in the transition. The multiphotonic excitation process is due to three photons (532 nanometers [nm]) sequential absorptions by metastable levels of the 4f3 configuration split by crystalline local field. The sequential excitation of Nd by the laser excitation is attributed to the 4I 9/2 + 532 nm T 4G7/2 ground state absorption followed by the 4G7/2 + 532 nm T 2F5/2 and 2F5/2 + 532 nm T 4f25d excited state absorptions. The UV emissions due to 4f25d configuration are parity allowed, having lifetime of 35 nanoseconds (ns) in contrast to UV emissions from 4f3 configuration which are induced by two absorption steps and are parity forbidden showing longer lifetime of 8 microseconds (ms) and narrow lines. The polarization effects of the UV emissions were studied and their behaviors are dependent on the excited state configuration involving or not involving the 5d orbital. The allowed UV emission positions were affected by the host variation more than the ones originated from the 4f3 configuration as expected. The electronic energy of the 4f25d configuration shifts to lower energy for increasing the crystal field. Key words: Multiphotonic excitation process. Ultraviolet radiation. UV transition. Exacta, São Paulo, v. 4, n. 1, p. 179-183, jan./jun. 2006 179 1 Introduction 70 Rare earth ions-doped crystals are very useful laser media for generating laser radiation in 60 4f 25d the visible and infrared region. Solid state materials doped with Nd are very promising for using as 50 laser medium for ultraviolet (UV) region (THOGERSEN; GILL; HAUGEN, 1996). The excitation can be performed directly to the level of inter- 40 2 F5/2 est or sequentially pumped. The most interesting pumping mechanism is the three steps excitations 30 at 532 nanometers (nm) which has the advantage of matching the second harmonic of Nd:YAG laser which is one of the most disseminated laser for 2 20 G7/2 4 F 9/2 4 I15/2 optical pumping systems (THOGERSEN; GILL; HAUGEN, 1996; VENIKOUAS et al., 1984). The 4fn T 4fn-15d transitions are characterized by 10 a strong environmental interaction and they are responsible for high oscillator strength and broad band absorption and emission spectra in the UV 0 4 I 9/2 4fn transitions are parity forbidden and they are Graphic 1: Energy level diagram of Nd:YLF showing the three-photon excitation scheme and the observed transitions from decay of the 4f 25d configuration2 (1 nm= 250 cm-1) sharp and weak because they take place only due Source: Toghersen and collaborators (1996). range. Otherwise, the intraconfigurational 4fn T to the crystal field configuration mixing (KOLLIA et al., 1998; POWELL, 19981). spectra were performed using a Cary-Olis 17 D 3 The spectrum characterization of the 4f T 2 3 3 double-beam spectrophotometer interfaced to a 4f 5d and 4f T 4f transitions provides informa- computer. Using a time resolved spectroscopy tion about the local level structure and electron- system of 10 nanoseconds (ns) of resolution phonon coupling differences between 4f3 and the the emission spectra and decay time determi- 4f25d configurations (KOECHNER, 1986). The pumping scheme used in this work is illustrated in Graphic 1. The main UV emissions originated form the 4f25d configuration are indicated in this illustration. nation were provided. The laser pumping system consists of a frequency doubled Nd:YAG pulsed laser whose beam intensity is reduced and focused on the samples. The emission of the samples is focused into the monochromator that disperses and directs the light to the EMI 2 Experimental setup S-20 photomultiplier tube. The detection system is connected to the 200 megahertz (MHz) 180 YLF and LLF samples were cut and polished Tektronix oscilloscope and a Box-Car gated in- properly with the c-axis parallel to the longest tegrator coupled to a microcomputer as showed side of the rectangular samples. The absorption in the Illustration 1. Exacta, São Paulo, v. 4, n. 1, p. 179-183, jan./jun. 2006 Artigos energy limit of the 4f 2 5d band of Nd 3+ in YLF Detector Sample Q-switched Nd:YAG laser (frequency doubled) lies at ~ 55,000 cm-1 so, the three-photon sequential absorptions of 532 nm, can excite Nd ion to this configuration. The UV fluorescence spectra of Nd in YLF crystal are exhibited in Illustrations 3 and 4. The emission spectrum of Injection telescopy nm are exhibited by Illustration 3. They were Monochromator Signal analyser system (box-car averager and scope) 4f 2 5d T 2G7/2 at 230 nm and 4f 2 5d T 4F 9/2 at 260 discriminated by using a narrow static gate (2 ns) and a delay time of 20 ns, in order to detect Photomultiplier only the allowed UV emissions. The two-photon that induced emissions from 4f3 configuration Illustration 1: Experimental setup for luminescence measurements are also present in the experiment and were op- Source: The authors. tically discriminated by using a narrow static The reduction of laser energies to hundred of microjoules (mJ) is necessary to avoid the self focusing and thermal lens problems in the millijoule (mJ) energy range which can destroy the samples (KOLLIA et al., 1998). The thermal lens effects produce a strong signal decrease and a bad signal-noise ratio, which can disguise the UV emission signal. A non-divergent beam and energies ranging from 4 to 100 mJ can be used to improve UV fluorescence performance. gate of 2 ns using a longer delay time of 2 microseconds (ms) in the box-car averager. By the comparison of both integrated emission signals from distinct electronic configurations, we must say that UV emission from 4f 2 5d is about ten times stronger than the one from 4f3 configuration, besides having an absorption process of order two. The emission spectrum of Illustration 4 exhibits the following transitions: 2F5/2 T 4I15/2 at 310 nm, 2F5/2 T 4I13/2 at 290 nm, 2F5/2 T 4I11/2 at 275 nm and 2F5/2 T 4I 9/2 at 260 nm. The measured lifetime of 2F5/2 state is 8 ms in contrast with the measured lifetime of 35 ns found for 3 Results and discussion the 4f 2 5d configuration. By comparing these re- In order to obtain a time resolved spectrosco- sults one concludes that the 4f 2 5d configuration py of the emissions and to investigate polarization has an emission spectra larger than the emis- effects they were measured both UV emissions sion from 4f3(2F5/2) state. This comes from the from 4f3 and 4f25d excited states, which follow in fact that 4f3 configuration produces inner states the sub-sections. shielded by the closed 5p 6 orbital thus producing optical transitions with very small phonon 3.1 UV Emissions and the Polarization Effects coupling. As a consequence, the polarization effects in the UV emissions from each configura- Nd:YAG-Q- tion must have different effects. The strongest switched laser operating at 532 nm with a rep- polarization effects are observed for the internal etition rate of 10 hertz (Hz) was used in all the 4f3 transitions, while the 4f 2 5d-mixed configu- investigations of UV fluorescences. The lower ration is slightly polarized. The frequency doubled Exacta, São Paulo, v. 4, n. 1, p. 179-183, jan./jun. 2006 181 16 LLF crystal is diminished by approximately 180260 p 14 12 Signal (a.u.) 200 cm-1 in comparison with the position of this ND:YLF 300 K delay: 20 ns 10 8 level in YLF, due to the strongest crystal field felt by Nd ions in LLF crystal when Y3+ neighbors are substituted by Lu3+ ions which have smallest s 6 230 ionic radius. On the other hand, the 4f3 configu- 275 ration has UV emissions showing a very small 4 shifting caused by the local field increasing. As 2 0 200 observed, the 2F5/2 T 4I 9/2 emission showed a 20 220 240 260 280 300 Wavelenght (nm) 320 340 Graphic 2: Polarized emission spectrum of the 4f 25d configuration of Nd:YLF crystals, after three photons absorptions of 532 nm 12 s polarization 300 K 10 Signal (a.u.) Source: The authors. 10 260 Signal (a.u.) cm-1 peak shift. ND:YLF 300 K delay: 20 ms s 8 6 4 2 5 0 200 220 240 260 280 300 Wavelenght (nm) 275 290 p Nd:YLF 310 Nd:LLF 0 200 220 240 260 280 300 Wavelenght (nm) 320 340 Graphic 3: Polarized emission spectrum of the 4f3 configuration after two photons absorption of 532 nm in Nd:YLF crystal Graphic 4: Emission spectrum of the 4f 25d configuration showing the differences between the host lattices Source: The authors. 1.0 Source: The authors. 0.9 0.8 3.2 Host lattice effects doped crystals of YLF and LLF with laser pulsed excitation at 532 nm. Graphic 4 shows the measured 4f 25d-emission spectrum of both crystals for comparison. The spectrum shape of both UV emissions is similar. However, a strong shift of main peaks by 180 and 200 cm-1, towards the lowest energy, was observed for LLF crystal. 182 Signal (a.u.) UV emissions were induced in both Nd- 0.7 4f 3 -4f 3 transition 0.6 0.5 0.4 b = 1.8 0.3 0.2 0.1 0.0 20 40 60 80 100 120 140 160 180 200 220 Energy (mJ) This suggests that the total energy of the bottom Graphic 5: Process order of the excitation mechanism of the 4f3 configuration. The fit is S = cE1.8 of the 4f 25d mixed configuration of Nd ions in Source: The authors. Exacta, São Paulo, v. 4, n. 1, p. 179-183, jan./jun. 2006 Artigos 1.0 observation validates the use of LLF crystal as a 0.9 promising system for UV laser operation near 260 0.8 nm. The spectral discrimination of a long lived Signal (a.u.) 0.7 4f 25d-4f 3 transition 0.6 UV emission of 4f3 configuration from the UV 0.5 fast emission component allowed us to clarify the 0.4 multistep excitation mechanism involved in neo- b=3 0.3 dymium UV fluorescence for laser applications. 0.2 0.1 0.0 20 40 60 80 100 120 140 Energy (mJ) 160 180 200 Graphic 6: Process order of the excitation mechanism of the 4f25d configuration. The fit is S = cE3 Notes 1 The authors thank the financial support of Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). Graphics 5 and 6 show the behavior of the 2 See chapter 10. 260 nm UV emissions intensities versus the la- 3 This transformation is related to the equation c = l x f, where c is the light speed, l is the wavelength and f is the frequency that is proportional to the photon energy. Source: The authors. ser pumping energy at 532 nm. Illustration 6 a shows the behavior of 2F5/2 state emission at 260 nm from the 4f3 configuration, giving an energy power dependence of second order (c = 1,8) for a pumping energy varying from 30 to 200 mJ. The UV emission at 260 nm from the 4f25d configuration exhibits power law dependence according to the three order process (Graphic 7). In this case, it is seen a luminescence decrease for pumping energy higher than 130 mJ. 4 Final considerations The Nd:LLF crystal shows a three photon multistep absorption at 532 nm allowing the population of a 4f25d configuration, similar as it was previously observed in Nd:YLF crystal. This References KOECHNER, W. Solid-state laser engineering. 4. ed. New York: Springer, 1986. KOLLIA, Z. et al. On the 4f25d T 4f3 interconfigurational transitions of Nd3+ ions in K 2YF5 and LiYF4 crystal hosts. Optics Communications, Amsterdam, vol. 149, no. 4-6, pp. 386-392, 1998. POWELL, C. R. (Ed.). Physics of solid-state laser materials. 1. ed. New York: AIP Press, 1998. THOGERSEN, J.; GILL, J. D.; HAUGEN, H. K. Stepwise multiphoton excitation of the 4f25d configuration in Nd3+:YLF. Optics Communications, Amsterdam, vol. 132, pp. 83-88, 1996. VENIKOUAS, G. E. et al. Spectroscopy of Y3A l5O12:Nd3+ under high-power, picosecond-pulse excitation. Physical Review B, Ridge, vol. 30, no. 5, pp. 2,401-2,409, 1984. Recebido em: 23 fev. 2006 / aprovado em: 28 abr. 2006 Para referenciar este texto LIBRANTZ, A. F. H. et al. Study of UV fluorescences induced from 4f3 T 4f25d multistep absorptions of Nd3+ ions in YLiF4 and LuLiF4 crystals. Exacta, São Paulo, v. 4, n. 1, p. 179-183, jan./jun. 2006. Exacta, São Paulo, v. 4, n. 1, p. 179-183, jan./jun. 2006 183