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The UV/visible polarized excited – state absorption (ESA) spectra from the 1D2 and 3Pj manifolds of 4f2-configuration of Pr3+ ions doped into LiY0.3Lu0.7F4 mixed crystal were studied at room temperature. The data about ESA cross-sections are necessary to estimate an efficiency of stepwise excitation of 4f5d-states of Pr3+ ions in these crystals.
© 2016 Optical Society of America
1. Introduction
The praseodymium doped wide-band gap dielectric crystals are well known UV and visible phosphors emitting from 220 nm to 730 nm due to inter- 4f5d → 4f2 and intraconfigurational 4f2→4f2 transitions of Pr3+ ions [1–4]. The most interesting are dipole allowed 4f5d-4f2 transitions that are generally characterized by intense broad fluorescence bands with the high quantum yield [5,6]. However now the 4f5d-4f2 UV laser action based on any solid-state material doped by Pr3+ ions is not still realized because of color center formation under resonant 4f2-4f5d pumping. As it is clear from the studies of Ce-doped UV active media it is most likely due to excited state photoionization of impurity ions by pumping radiation [7]. To avoid this problem the up-conversion stepwise excitation by visible or appropriately chosen UV photons was proposed and the selection criteria of pumping quantum energies were elaborated [8]. Moreover, results of [1,8,9] demonstrate the opportunity of such kind pumping of well-known UV active media based on Ce3+ ions doped double fluoride crystals with sheelite type structure (LiLnF4, where Ln = Y, Gd-Lu). There are two discussed and tested approaches which provide excitation of 5d-states of Ce3+ ions in Pr,Ce:LiYF4 and Pr,Ce:LiLuF4 single crystals by use of two-photon 3H4(Pr)→1D2(Pr)→4f5d(Pr) or 3H4(Pr)→3Pj(Pr)→4f5d(Pr) stepwise pumping of 4f5d-states of Pr3+ ions together with nonradiative energy transfer from these states to 5d-state of Ce3+ ions 4f5d(Pr)→5d(Ce). Both of them are based on excited state absorption from 1D2 or 3Pj manifolds of Pr3+ ions to its 4f5d-configurational states. This absorption has to be maximized by appropriately choosing of pumping wavelengths. At the same time impurity ions photoionization should be excluded [8]. Also it is necessary to provide high concentrations of Ce3+ and Pr3+ ions in these crystals [9]. Authors of [9] reports that optical gain was not observed in Pr,Ce:LiYF4 and Pr,Ce:LiLuF4 due to low concentrations of Ce3+ and Pr3+ ions and hence low 4f5d(Pr)→5d(Ce) energy transfer. The Ce3+:LiY0.3Lu0.7F4 mixed crystals allow to increase Ce3+ ions contents up to 5 times as compared with LiYF4 and LiLuF4 crystals [10] and simultaneously enhance 4f5d(Pr)→5d(Ce) energy transfer efficiency at least in 1.5 times [11]. Recently this crystal double-doped by Pr3+ and Ce3+ ions have demonstrated UV optical gain under the 3H4(Pr)→1D2(Pr)→4f5d(Pr)→5d(Ce) pumping and it seems to be possible that further pumping optimization will allow to design a first diode pumped UV solid-state laser [12]. Here we are reporting about the excited state absorption spectra from 1D2 and 3Pj manifolds of Pr3+ ions doped in LiY0.3Lu0.7F4 crystal.
2. Sample
The Pr3+:LiY0.3Lu0.7F4 crystal was grown in Kazan University by Bridgman technique in carbon crucible. The dopant concentration in the melt was 1 at%. The sample had a cylindrical shape with 5 mm in diameter and 6.4 mm length with polished bases.
As the LiY0.3Lu0.7F4 is uniaxial crystal, it was preliminary oriented, cut and polished in order to study spectra with light polarization parallel (π) and perpendicular (σ) to the c optical axis.
3. Experiments and results
The ESA spectra were recorded at room temperature in 200-420 nm spectral range using a pump-probe technique. The experimental setup is depicted on the Fig. 1.
The pumping was provided by optical parametric oscillator (OPO) pumped by the third harmonic of Nd:YAG laser. The pumping beam was π-polarized and its wavelengths were tuned around 479 nm and 469 nm or 595 nm corresponding to the most intense absorption bands of 3H4 - 3P0, 3H4 - 3P1 or 3H4 - 1D2 transitions, respectively [13].
The pumping pulse energy was about 5 mJ with 7 ns pulse duration. The laser was operated at 10 Hz pulse repetition rate. The pumping beam was focused at the sample providing energy density per pulse between 0.15 and 0.75 J/cm2.
To study ESA spectra, the, the broadband emission from laser spark in xenon at high pressure induced by fundamental harmonic of the same Nd:YAG laser was used as the probe.
The pulse duration of the probe beam was about 10 ns at pulse repetition rate 10 Hz and it was delayed by about 10 ns after pumping moment (Fig. 2). The probe radiation was focused on the sample by fused silica lens and propagated through the sample in the opposite direction than the pumping beam. It was done to avoid the blinding of the photodetector by pumping radiation.
Fig. 2 The time profiles and delay between pumping and probing radiation beams registered by Alphalas photodiodes with 50 ps rise time coupled to Aktakom 2282 oscilloscope with bandwidth of 200 MHz.
To provide ESA measurements the spot of probe beam was centered to pump spot on the sample. The spot diameter of probe beam was less than the pumping one.
The probe radiation passed through the sample was collimated by the lens, analyzed by Glan-Taylor polarizer and focused to the waveguide of Stellarnet CCD spectrometer with spectral resolution better than 0.5 nm.
ESA cross-section spectra (σESA(λ)) were calculated by the formula:
Here: I0(λ), I(λ) – spectra of the probe radiation passed through the unpumped and pumped sample, respectively, Ilum(λ) – the fluorescence spectrum of the sample, pumped at the same condition as it was done for I(λ) spectrum, L – the sample length, N – the population density on the excited 1D2 and 3P0 states of Pr3+ ions, which was calculated by the Eq. (2):
where E – the absorbed pumping energy, λ – the pump wavelength, h – the Planck’s constant, c – the speed of light and S – the pumping area. The absorbed energy of pumping radiation was determined as the difference between the pumping energy incident to the sample and output one. Measurements of pumping energy were realized by Ophir multichannel power meter.The ESA cross-section spectra are presented on Fig. 3 for two polarizations of probing radiation. The ESA cross-section values are in order of 10−18 cm2 that is typical for 4fn-4fn-15d parity-allowed electric-dipole transitions.
Fig. 3 Room temperature ESA cross-sections spectra of Pr3+:LiY0.3Lu0.7F4, Pr3+:LiLuF4 and Pr3+:LiYF4 [1] single crystals from the 1D2 (a,b), 3Pj (c,d) manifolds of Pr3+ ions. The shaded area corresponds to stimulated emission cross-section spectra of Ce3+ ions. To populate 1D2 and 3Pj manifolds of Pr3+ ions the pump radiation at 595 nm and 479 nm was used, correspondently.
Comparing with the results of recent ESA measurements with ESA in Pr3+:LiLuF4 and Pr3+:LiYF4 [1,14] we observe some differences for 3Pj – 4f5d and 1D2 – 4f5d transition cross-sections. Generally the 3Pj – 4f5d transitions are less intensive in Pr3+:LiY0.3Lu0.7F4 crystals than in Pr3+:LiYF4 or Pr3+:LiLuF4 ones at about 1.3 times. On the other hand 1D2 – 4f5d transition cross-sections are at about 1.5 times higher. This difference can be associated with the distinct in experimental technique between our measurement and measurements in [1,14]. Here we have studied the time-resolved ESA spectra using 10 ns duration pulse of probing radiation in contrast to [1] where the light of continuous xenon lamp was used. Therefore, the results of this paper seems to be more correct because it allowed us to avoid artifacts associated with redistribution of energy between 4f2-manifold assemblies of the Pr3+ ions during pumping pulse-repetition interval. This redistribution can lead to origin of ESA from the other manifolds.
It is clear from the Fig. 3, that 3H4(Pr)→ 3Pj(Pr)→4f5d(Pr)→5d(Ce) pumping scheme is more preferable for getting of UV lasing in Ce3+,Pr3+:LiY0.3Lu0.7F4 crystals than 3H4(Pr)→1D2(Pr)→4f5d(Pr)→5d(Ce) one because it demonstrates the lowest 4f2-4f5d ESA coefficient (π polarization spectrum) in spectral range of 5d-4f fluorescence of Ce3+ ions. Besides this scheme can be realized using well-developed blue laser diode operated at 444 nm to pump 3Pj manifolds of Pr3+ ions and the third harmonic radiation from DPSS lasers operated at about 350-360 nm. The last one opens an opportunity to design a first tunable up-conversionally pumped UV solid-state laser based on 5d-4f transitions of Ce3+ ions.
4. Conclusion
Here the room temperature 4f2 – 4f15d1 time-resolved ESA spectra from 3Pj and 1D2 manifolds of Pr3+ ions doped in LiY0.3Lu0.7F4 mixed crystal were studied for the first time. The corresponded cross-sections were calculated and compared with the ones for Pr3+:LiLuF4 and Pr3+:LiYF4 crystals. The partial difference between our results and results of [1] was revealed and explained. The advantages of 3H4(Pr)→3Pj(Pr)→4f5d(Pr)→5d(Ce) up-conversion pumping scheme in comparison with 3H4(Pr)→1D2(Pr)→4f5d(Pr)→5d(Ce) one were demonstrated and discussed. The experimental data allow us to estimate an efficiency of stepwise excitation of 4f5d-states of Pr3+ ions in these crystals and optimize future optical gain or laser test experiments.
Acknowledgments
This work was mainly funded by Russian Foundation for Basic Research grant 15-02-05309. Improvements of the crystal growth facilities were performed due to financial support by Russian Scientific Foundation grant (project Nº15-12-10026). Semashko V.V. and Korableva S.L. are also grateful for financial support by the subsidy of the Russian Government (agreement No.02.A03.21.0002) to support the Program of Competitive Growth of Kazan Federal University among World’s Leading Academic Centers and the subsidy allocated to Kazan Federal University for the state assignment in the sphere of scientific activities.
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