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Two-Step Excitation and Blue Fluorescence Under Continuous-Wave Pumping in Nd:YLF

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Two-Step Excitation and Blue Fluorescence Under Continuous-Wave Pumping in Nd:YLF T. Y. Fan and R. L. Byer Vol. 3, No. 11/November 1986/J. Opt. Soc. Am. B 1519 Two-step excitation and blue fluorescence under continuous-wave pumping in Nd:YLF T. Y. Fan and Robert L. Byer Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305 Received April 18, 1986; accepted July 24, 1986 4 4 Near-UV and blue fluorescence from the D3/2 and D5/2 manifolds in Nd:YLF has been observed at room tempera- ture under cw pumping by a Rhodamine 590 dye laser. Excitation to these manifolds is attributed to two-step excitation involving excited-state absorption from the 4F3/2 metastable level. A similar phenomenon has also been observed in Nd:YAG and Nd:glass. The effective excited-state absorption cross section 2 is measured to be (2 1) X 10-20 cm at 587.4nm in the ir polarization, and the peak effective stimulated emission cross section 2 is measured to be 5 X 10-20 cm at 411.7nm, also in the r polarization. Estimated laser threshold at 411.7nm for two-step pumping at 587.4 nm is 70 mW. INTRODUCTION level lifetimes. In the case of Nd:YAG, the pump wave- length is different from that in this paper, and consequently There are a number of processes besides fluorescence and the energy levels in two-step excitation are also different. one-photon absorption that can occur in rare-earth-ion- and In the case of LaF , ETU is the dominant process, but two- transition-metal-ion-doped solids. Two-step excitation 3 step excitation is noted at temperatures approaching that of and energy transfer are examples of processes that can be liquid helium. Upconverted blue fluorescence has also been important in laser operation. For example, concentration noted in some fluoride crystals.7 In Nd:YLF, two-step exci- quenching in Nd3+-doped laser materials is an energy trans- tation and subsequent blue fluorescence have been used as a fer phenomenon that reduces the lifetime of the 4F / upper 3 2 technique to investigate Nd3+ ion pairs in YLF at low tem- laser level. Energy transfer upconversion (ETU) and two- peratures (<40 K),8 and the blue fluorescence has been ob- step excitation have also been used to create visible sources, served under ruby-laser pulsed pumping by a two-photon both incoherent and coherent, from infrared pump pho- 9 1 process. In this paper, blue and UV fluorescence is investi- tons. ,2 In two-step excitation, a single photon is absorbed gated, and the excitation mechanism is determined. followed by relaxation to an excited metastable energy level. This is followed by absorption of a second photon from the excited state, excited-state absorption (ESA), to an even EXPERIMENTAL APPARATUS higher lying level. This is followed by relaxation, either Nd:YLF is a uniaxial solid-state laser material. Its one- radiative or nonradiative. In ETU, two nearby ions in excit- photon absorption spectra, infrared fluorescence spectra, ed states interact simultaneously, causing one of the pair to and concentration quenching were previously character- be excited to an even higher-lying level and causing the other ized.'( From previously published data, the dopant concen- to relax. Both of these processes are interesting techniques tration was 2 at. % Nd given the measured fluorescence 4 0 for obtaining higher-energy photons from lower-energy lifetime of 440 Asec for F3/2 manifold in our sample.' How- pump photons because of their large effective nonlineari- ever, more recent measurements of concentration indicate ties.' that the r-polarized absorption line at 872.1 nm has an We have recently noted two-step excitation as well as absorption coefficient of 1.2 cm-' for 1 at. % Nd doping." ETU in Nd:LiYF4 (YLF).3 This paper will discuss two-step By this standard, the concentration of our sample is 1.7 at. % excitation that induces near-UV and blue emission under cw Nd. resonant pumping. These processes were discovered while The experimental apparatus used to record the emission Nd:YLF cw resonantly-pumped miniature lasers were being and excitation spectra consisted of an argon-ion-laser- 4 investigated. Two-step excitation and ETU have been pre- pumped cw Rhodamine 590dye laser for excitation, a lens to dominantly studied in rare-earth ions such as Er3+, which focus the pump beam into the sample, a 31034photomulti- have long (>1-msec) metastable level lifetimes, but similar plier tube for detection, a Chromatix CT-103 1-m monochro- processes have also been noted in Nd3+-doped materials. mator with a 1800-line/mm grating for wavelength selectiv- For example, blue and near-UV emission has been observed ity, and a lock-in amplifier for signal extraction. The dye- by two-step excitation in Nd3+-doped compounds such as laser beam was typically chopped at -30 Hz with '30% duty Nd:YAG (Ref. 5) and Nd:LaF 3 (Ref. 6) under pulsed excita- cycle. A calcite polarizer was used to select the polarization tion. However, in pulsed excitation the dynamics of the for the various measurements. The subsequent signal from processes can be quite different from the cw regime because the lock-in amplifier was digitized and stored in a computer. peak pump powers can be higher and pulse times can be The spectral response of the detection system was normal- much shorter than characteristic times such as metastable ized by using a quartz-halogen lamp.12 A Perkin-Elmer 330 0740-3224/86/111519-07$02.00 © 1986 Optical Society of America 1520 J. Opt. Soc. Am. B/Vol. 3, No. 11/November 1986 T. Y. Fan and R. L. Byer spectrophotometer was used for measuring absorption spec- where v is the transition energy in inverse centimeters, n is tra. All measurements were performed at room tempera- the refractive index, Q,\'sare the experimentally determined ture. intensity parameters for the given ion in the given host, and the (fN(aSL)JII U()fN(a'S'L')J') are reduced matrix ele- 6 BLUE AND NEAR-UV FLUORESCENCE ments. Using the reduced matrix elements,' the Oxparam- eters for Nd:YLF,"7 and the assumption that the relative The 4 4 normalized fluorescence spectra for both r(E ||c) and populations of D3/2 and D5/2 are given by a Boltzmann a(E c) polarizations are shown in Fig. 1. There are four distribution, the calculated radiative transition rates and regions in which emission is observed that are near 358 nm, branching ratios to the 4I term are shown in Table 1; emis- 383 nm, 412 nm, and 450 nm, which correspond to transi- sion to other manifolds was ignored since none was observed. 4 tions from D3/2and 4D5/2 manifolds to 4I9/2, 4I11/2, 4I13/2, and While the Judd-Ofelt theory is correct in predicting strong 4I15/2, 9 3 4 4 respectively, based on energy-level positions. "1 The emission from D3/2 and D / , the branching ratios among 4 4 5 2 D3/2 and D5/2 manifolds should be thermally coupled at the 4I term are not accurate. The Judd-Ofelt theory also 4 room temperature. In other words, the relative populations predicts strong emission to F 5/2; however, this emission was are given by 4 a Boltzmann distribution. There is a signifi- not observed. In the case of emission from F3/2, the Judd- cant discrepancy of approximately 40 cm-' between the ob- Ofelt theory is more accurate in predicting branching ratios. served energy-level positions in Refs. 9 and 13 for these two This is most likely because one of the two fundamental manifolds; our data are more consistent with those in Ref. 9. The branching ratio flij from the upper manifold i to a I I I II I I lower manifold j is given by Nd:YLF POLARIZED FLUORESCENCE 7r A,. E f XIP(X)dX fi = A _ P (1) I-. z E Aik E E | >,ikP(X)d X k p k zF where Aik is the transition rate from the upper manifold i to a > lower manifold k, and the sum on k is over all lower mani- ;a 011I~~~0~-- folds. Note that the sum in the denominator is equal to 1-r, _ _ - _ _~~~~~~~0,__ I -J- where Tris the radiative lifetime. The second part of Eq. (1) w a: gives fij in terms of the polarized emission spectra, where IikP(X) is the polarized emission intensity per unit wave- length interval as a function of wavelength Xfrom manifold i to manifold k, and the sum over p is over polarization. The S. 4 4 results for fluorescence from the D3/2 + D5/2 manifolds are shown in Table 1, where emission to states other than the 4 Ij 350 360 -37 380 390 400 manifolds has been ignored since no emission was observed WAVELENGTH (nm) experimentally. Electric dipole transitions were assumed so (a) the axial spectrum is identical to that for the r polarization. In this case the sum over polarization becomes 2I(X) + IP(X). One difficulty with this measurement is that the emission to the ground-state manifold, 4I9/2, can be reab- sorbed, causing the measured branching ratio to that mani- fold to be too small. To minimize this problem, the sample C', was pumped near its surface. The radiative transition rates Aij, and therefore the branching ratios, can be calculated from the Judd-Ofelt theory.'4 "l5 The radiative transition rate for electric dipole transitions from a state fN(aSL)J) to state fN(IS'LS)J/), 2 2 -J which denote manifolds S+lLj and S+lLj,', is given by w a: A = [647r4e2] V3 (2J + 1)-' X , (2a) X=2,4,6 410 420 430 440 450 460 WAVELENGTH (nm) where (b) Fig.
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