Redalyc.The Role of Nd3+ in the Population Inversion Between 1G4 and 3H6 States of Tm3+ in Yb:Nd:Tm:YLF Crystal

Exacta ISSN: 1678-5428 [email protected] Universidade Nove de Julho Brasil

Henriques Librantz, André Felipe; Coronato Courrol, Lilia; Gomes, Laércio; Ranieri, Izilda Marcia; Baldochi, Sonia Lícia The role of Nd3+ in the population inversion between 1G4 and 3H6 states of Tm3+ in Yb:Nd:Tm:YLF crystal Exacta, vol. 5, núm. 1, janeiro-junho, 2007, pp. 113-118 Universidade Nove de Julho São Paulo, Brasil

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The role of Nd3+ in the population 1 3 3+ inversion between G4 and H6 states of Tm in Yb:Nd:Tm:YLF crystal 1

André Felipe Henriques Librantz Professor pesquisador – Uninove; Pesquisador colaborador – Ipen-USP. São Pauilo – SP [Brasil] [email protected]

Lilia Coronato Courrol Professora pesquisadora – Unifesp. São Pauilo – SP [Brasil] [email protected]

Laércio Gomes Pesquisador – Ipen-USP. São Pauilo – SP [Brasil] [email protected]

Izilda Marcia Ranieri Pesquisador – Ipen-USP. São Pauilo – SP [Brasil] [email protected]

Sonia Lícia Baldochi Pesquisador – Ipen-USP. São Pauilo – SP [Brasil] [email protected]

In this paper, we present the energy transfer rates of YLF:Yb: Tm:Nd system, identifying the most important processes that lead to the thulium blue upconversion emission under excitation around 792 nm. The calculation of the rate equations system based on numerical method using Runge-Kutta method was 1 employed for analyzing the population inversion between G4 3 3+ and H4 states of Tm . Population inversion was achieved only in Yb:Nd:Tm doped YLF crystal, which exhibits a pumping rate threshold of 620 s-1. The Nd3+ absorption near 792 nm is very important because the excitation is rapidly transferred to Yb3+, that also interacts with Tm3+. Key words: Tm3+ blue emission upconversion. Numerical simulation.

Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007. 113 1 Introduction composition of 1.02 LiF: 1 LnF3. LiF powder (Al- pha-Johnson Matthey, 99.9%) was zone-refined Thulium-doped materials generate blue laser before it was added to the mixture. emission through the nonlinear conversion of ra- The studied crystal was grown by the Czo- diation from the infrared into visible range (GOS- chralski method using diameter automatic control, NELL et al., 2003), after various cross-relaxation with growing rate of 1.30 mm/h and rotation rate (CR) and excited state absorption (ESA) processes of 15 rpm for the <100>-oriented boule. During (CHADEYRON et al., 2002). Particularly, blue la- the process, the atmosphere inside the Czochral-

ser is important in the compact disc industry, opti- ski furnace was composed by Ar (1.4 bar) and CF4 cal storage systems, color displays and in new med- (0.2 bar). ical and dentistry applications and in atmospheric The YLF:Yb:Tm:Nd crystal with 60mm in and physics research (MEIJERINK et al., 2002). length and 20mm in diameter was used. Samples

LiYF4 (YLF) crystals doped with thulium were cut from this crystal (middle). Nd concen- (Tm3+) and also co-doped with ytterbium (Yb3+) tration was measured to be 1 mol%. The samples (BASS et al., 1995; CASSANJES et al., 2002) are were cut and polished with 2 mm thickness. well-known as active media that generate stimu- The absorption spectra of the glasses were lated radiation on a number of lines over a wide measured by using a spectrophotometer (Cary/ spectral range from 450 nm to 2350 nm, upon OLIS17D) operating in the range of 300-2000 selective laser, laser diode, and flash lamp pump- nm. For emission and lifetime measurements of ing (JOUBERT, 1999; BASS et al., 2004; GOMES the excited states of the Yb3+,Tm3+ and Nd3+ ions. et al., 2004). It was demonstrated that the addi- The sample was excited by laser radiation generat- tion of Nd3+ as a second sensitizer for YLF:Yb:Tm ed by tunable optical parametric oscillator (OPO), crystals improves the upconversion mechanism which was pumped by the second harmonic of a that gives rise to the Tm3+ blue emission in 475 nm Q-switched Nd-YAG laser (Brilliant B from Qu- (BALDOCHI et al., 2005). antel) of 4 ns and 10 HZ; and observed by a 0.25 In this work, we compare the blue thu- m Kratos monochromator, EGG Bos car Proces- lium emission obtained after pumping the YLF: sor, EMI S-20 PMT and InSb 77K infrared detec- Yb(20mol%):Tm(0.5mol%):Nd(1%) crystal at two tor (Judson). different wavelengths: 792 nm or 960 nm. All the fluorescence decay times were measured at 300 K. In order to isolate these lumines- cence signals, band pass filters with ~80 % trans- 2 Experimental setup mission at 1200 and 2000 nm (each with a half width of 15 nm and an extinction coefficient of The rare earth fluorides were prepared from ~10-5) was used. pure oxide powders (Alpha-Johnson Matthey, 99.99%) by hydrofluorination at high temperature in HF atmosphere. The powder was contained in 3 Results and discussions a cylindrical platinum boat, which was inserted in

a sealed platinum tube. The LiF-LnF3 (Ln=Y, Yb, YLF:Yb:Tm:Nd crystal has two main absorp- Nd, and Tm) mixture was melted using an open tions in the near infrared at ~960 nm (Yb3+) and platinum boat in the same atmosphere, with a near 792 nm due to Nd3+ and Tm3+ ions, which

114 Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007. Artigos

20 Detector InSb ou Yb:Nd:Tm:YLF fotomultiplicadora Emission: 1 µm Nd 3+ (4 F ) 15 3/2 Excitation at 792 nm Monocromador ou filtro 10

Signal (arbit. units) 3+ 2 Yb ( F 5/2 ) 5 Lentes OPO GSH Nd:YAG Signal (arbit. units)

Amostra 0

0 1 2 3 4 TimeTime (ms) Pré-amplificador Computador Box-car ou Figure 2: Emission decay curve (1.06mm) excited osciloscópio by pulsed laser at 792 nm in Yb:Tm:Nd:YLF crystal Source: The authors.

Figure 1: Schematic diagram of the experimental Yb:Tm:YLF setup 8 Emission at 1 µm Source: The authors. Excitation: 792 nm can be used for solid state laser system pumped by diode laser. The most intense absorption is near 4 960 nm due to high concentration of Ytterbium Signal (arbit. units) in the sample. Besides that, Neodymium absorp- Signal (arbit. units) tion at ~792 nm is approximately 8 times more 0 intense than Thulium absorption (at) due to the 0 1 2 3 4 5 6 7 8 highest absorption cross section of Nd3+ in the π TimeTime (ms) polarization. Figure 3: Emission decay curve (1.06mm) excited When YLF sample containing Tm3+ co- by pulsed laser at 792 nm in Yb:Tm:YLF crystal Source: The authors. doped with Yb3+ and Nd3+, excited at 792 nm, a strong blue emission is observed with maximum dicates that Nd ions significantly contribute to the at 475 nm, due to the transition 1G T 3H . It is 4 6 population of 1G excited level. more intense than the red emission at ~ 650 nm. 4 Figure 2 shows the 1.06 m emission, in which the µ 3.1 Rate equations system fast component is due to Nd (4F ) T Yb transfer 3/2 Figure 4 shows a simplified energy level (process b). Figure 3 shows the 1.06µm emission scheme of the (Yb3+, Tm3+, Nd3+) system consid- 3+ from Yb indirectly excited by Tm T Yb transfer ered for continuous laser pumping at 797nm. (process d). According to the energy levels diagram of We observe also that the Nd(1 mol%) effect Figure 4, the following rate equations were de- in the Yb:Nd:Tm:YLF crystal causes an enhance- rived for Yb:Nd:Tm:YLF system under continu- ment of the Tm3+ blue emission due to the Nd(4F ) 3/2 ous laser pumping at 792 nm. The n1 and n2 are 3 3+ T Tm( H4) (process h, seen in Figure 3), which in- the Yb population, n3, n4, n5 and n6 refers to the

Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007. 115 Table 1: Rate equation of the system and spectroscopic parameters

dn1 n2 = + gn2n5 + en3n3 - dn1n5 + fn2n4 - bn1n8 + cn2n7 dt t2

dn2 n2 = + dn1n5 - gn2n5 - en2n3 - fn2n4 + bn1n8 - cn2n7 dt t2 dn n b b 3 = - R n + 4 + 53 n + 63 n + dn n - en n p 3 t 5 6 1 5 2 3 dt 4 t5 t6 dn n b b 4 = - 4 + 54 n + 64 n - fn n + en n dt t t 5 t 6 2 4 2 3 Figure 4: Energy levels scheme and energy 4 5 6 transfer mechanisms of Yb/Tm/Nd system. dn n b 5 = - R n + 5 + 65 n - dn n - gn n - hn n + fn n Continuous line (up) 797 nm excitation, dashed dt p 3 t t 6 1 5 2 5 5 8 2 4 line (up) 965 nm excitation. Dash dot line (down) 5 6 blue thulium emissions. Dot lines (up and down) dn n 6 = - 6 + hn n + gn n Yb emission and cross relaxation processes 5 8 2 5 dt t6 Source: The authors. dn7 n8 = - 7.8 Rpn7 + + bn1n8 + hn5n8 dt t8 3+ 3+ Tm and n7, n8, are the Nd populations, in which dn n 8 = 7.8 R n - 8 - bn n - hn n dt p 7 t 1 8 5 8 n1+n2= 20 mol%, n3+n4+n5+ n6=0.5 mol% and 8

n7+n8= 1mol%. Energy transfer rates (from best fitting) (given in s-1): b = 98246 I R = ó P c = 241 P 35 d = 238 hνP e = 2000 f = 14510 -1 is the pumping rate (s ), IP is the intensity of g = 960 2 h = 14912 pumping given in W/cm and hνP is the photon energy of pumping radiation. Cross-section absorp- Lifetimes values used in the calculation: τ2 = 2 ms -21 2 τ4 = 15 ms tion σ35 = 7.3 x 10 cm for Tm, and σ78 = 5.7 x τ5 = 1 ms -20 2 10 cm for Nd, respectively. τ6 = 0.75 ms τ8 = 0.57 ms Rate equation of the system, energy transfer Source: The authors. rates and lifetimes values used in the calculation are listed in the Table 1. Yb:Nd:Tm:YLF 2.0 3 Figures 5a and 5b exhibit the Tm (n5 and n6) (a) 3 (b) n (1G ) n5 ( H4) 6 4 population dependence on the pumping rate (in- best fitting best fitting (p=1.1) (p=1.7) tensity) obtained from the numerical simulation 1.5 2

of the described rate equations. These results are mol %) -3 in agreement with the experimental dependence 1.0

3+ observed for these Tm levels in Yb:Nd:Tm:YLF 1 0.5

crystals excited with diode laser at 792 nm. Population (10 Figure 6 shows the population difference 0.0 3+ 0 (n6-n3) of Tm obtained by means of numeri- 0 10 20 30 40 0 100 200 300 cal simulation of rate equations system for the -1 -1 RP (s ) RP (s ) 792 nm pumping of Tm and Nd ions. Populations Figure 5: Energy excitation dependence of (a) of each excited level involved were taken after the 3 1 H4 and (b) G4 levels system getting the equilibrium. We observed that Source: The authors.

116 Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007. Artigos

0.2 laser excitation to discriminate the energy transfer Yb:Nd:Tm:YLF [Nd]= 1 mol% parameter or rates for each process involving Nd: 0.1 [Nd]= 0 Yb), Yb:Tm) and Nd:Tm). Rate equations of the

0.0 system were numerically solved and the populations of the involved levels were obtained, which -0.1 allowed the obtaining of the population inversion

-0.2 1 3 3+ for the G4 T H6 emission of Tm in Yb:Nd:Tm:

Population Inversion (mol %) YLF crystal. A comparison with the results also -0.3 obtained for Yb:Tm:YLF showed that Nd (1 mol%) -0.4 100 200 300 400 500 600 700 800 900 1000 played an important role in the population inver- -1 RP(s ) sion, which shows that this triply-doped system can be used for the development of laser emitting Figure 6: Variation of Tm3+ blue emission intensity as a function of the pumping rate in the samples in the blue region operating in continuous pump- of Yb:Tm:Nd:YLF: and Yb:Tm:YLF (dashed line) ing regime at 792 nm. Source: The authors. the presence of Nd (1 mol%) promotes popula- Note tion inversion for a pumping rate since 620 s-1. On the other hand, the Yb:Tm:YLF system does not 1 Article originally presented at the XXX ENFMC – Annals show population inversion as it can be seen by the of Optics 2007 dashed curve exhibited in the Figure 6. From this investigation using time resolved Acknowledgements spectroscopy, we can infer that the most effective way of populating 1G excited state of Tm3+ after 4 The authors would like to thank FAPESP, 792 nm excitation occurs according to the follow- CNPq and Uninove for financial support. ing sequence:

i) (a) Ground state absorption of Nd and Tm (a’) at 792 nm; References 4 BALDOCHI, S. L.; COURROL, L. C.; GOMES, ii) Neodymium deactivation (10 µs): Nd( F3/2) T L.; RANIERI, I. M.; TARELHO, L. V. G.; VIEIRA Yb(2F ) (process b); 5/2 JUNIOR, N. D. Enhancement of blue upconversion

iii) Three sequential cross relaxation processes: mechanism in YLiF4:Yb:Tm:Nd crystals. Journal of Applied Physics, Argonne, vol. 98, no.11, p. 113504, 1) Yb(2F ) T Tm (3F ) (process e); 2) Yb(2F ) 5/2 4 5/2 2005. T Tm (3H ) (process f) and 3) Yb(2F ) T Tm 4 5/2 BASS, M.; CHAI, B. H. T.; HONG, P.; ZHANG, X. X. 1 ( G4) (process g). Blue upconversion with excitation into Tm ions at 780 nm in Yb- and Tm-codoped fluoride crystals. Physical Review B, Orlando, vol. 51, pp. 9298-9301, 1995. BASS, M.; CASSANHO, A.; JENSSEN, H.; MILLIEZ, Conclusions J.; RAPAPORT, A. Role of pump duration on temperature and efficiency of up-conversion in fluoride In this work, we used a time resolved spectro- crystals co-doped with ytterbium and thulium. Optics Express, Washington, vol. 12, no. 21, pp. 5215-5220, scopic technique together with the selective pulsed 2004.

Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007. 117 CASSANJES, F. C.; GOMES, A. S. L.; MACIEL, G. S.; GOSNELL, T. R.; NURMIKKO, A. V.; RISK, W. MENEZES, L. DE S.; MESSADDEQ, Y.; POIRIER, G.; P. Compact Blue-Green Lasers. 1. ed. New York: RAKOV, N.; RIBEIRO, S. J. L.; SUNDHEIMER, M. Cambridge University Press, 2003. L. Blue upconversion enhancement by a factor of 200 in JOUBERT, M.-F. Photon avalanche upconversion in rare Tm3+-doped tellurite glass by codoping with Nd3+ ions. earth laser materials. Optical Materials, Netherlands, Journal of Applied Physics, Argonne, vol. 92, no. 10, pp. vol. 11, no. 2-3, pp. 181 – 203, 1999. 6337-6339, 2002. MEIJERINK, A.; REID, M. F.; SOVERNA, S.; VAN CHADEYRON, G.; FRUKACZ, Z.; JOUBERT, M. PIETERSON, L.; WEGH, R. T. 4fn T 4fn-15d transitions -F.; KACZKAN, M.; MALINOWSKI, M.; PRACKA, of the light lanthanides: experiment and theory. Physical I.; WNUK A. Infra-red to visible up-conversion in Review B, Orlando, vol. 65, no. 4, pp. 045113, 2002. holmium-doped materials. Journal of Alloys and Compounds, Netherlands, vol. 341, no. 1-2, pp. 353- 357, 2002. LIBRANTZ, A. F. H.; GOMES, L.; RANIERI, I. M.; TARELHO, L. V. G. Investigation of the multiphoton 2 excitation process of the 4f 5d configuration in LiYF4

and LiLuF4 crystals doped with trivalent neodymium ion. Journal of Applied Physics, Argonne, vol. 95, no. 4, pp. 1681-1691, 2004.

Recebido em 5 abr. 2007 / aprovado em 22 maio 2007 Para referenciar este texto LIBRANTZ, A. F. H.; COURROL, L. C.; GOMES, L. RANIERI, I. M.; BALDOCHI, S. L. The Role of Nd3+ 1 3 in the Population Inversion Between G4 and H6 States of Tm3+ in Yb:Nd:Tm:YLF Crystal. Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007.

118 Exacta, São Paulo, v. 5, n. 1, p. 113-118, jan./jun. 2007. Artigos