Red upconversion (UC) emission at 626 nm is obtained from a LiNbO3 crystal codoped with Er3+ and Eu3+ under 800 nm femtosecond laser excitation. Energy transfer from (2H11/2,4S3/2) levels of Er3+, which are excited by excited state absorptions, to 5D1 of Eu3+ followed by cascade to 5D0 nonradiatively leads to this red UC emission. The energy transfer efficiency of ~30% is obtained in LiNbO3:Er3+(1.0 mol%),Eu3+(0.1 mol%). These initial experimental results indicate that the red UC emission can be obtained from Er3+/Eu3+ codoped system under diode laser excitation.
© 2009 Optical Society of America
Besides the potential application in high brightness display of a crystal generating the three fundamental colors [1, 2, 3], red emission is also useful in excitation of photosensitive drugs in medicine  and in pumping solid state laser based on Cr3+, like LiCAF and LiSF [5, 6]. Under one-color diode lasers excitation, red upconversion (UC) emission is usually obtained from Er3+/Yb3+ or Ho3+/Yb3+ codoped systems [7, 8]. However, these codoped systems are quite prone to green UC emission, so red UC emission is not efficient nor pure. Eu3+ is an unsurpassable red emitting ion in fluorescence lamps, cathode-ray tubes, and projection television tubes under ultraviolet or vacuum ultraviolet excitation due to the high luminescent quantum yield, high resolution, and adjustable wavelength. But its scarce UC property excludes it as a red UC emitting ions in the past for a long time. Single Eu3+ UC emissions, which originate from three-photon simultaneous absorption of the Eu3+ charge transfer state or from second harmonic generation (SHG) of host under pulse laser excitation, are reported [9, 10]. These processes hardly occur under cw diode laser excitation. On the other hand, a red UC emission in Yb3+/Eu3+ codoped oxyfluororate glass under cw diode laser excitation is reported . In this work, a red UC emission in Er3+/Eu3+ codoped LiNbO3 crystal is investigated when excited with 800 nm femtosecond laser.
Er3+/Eu3+ codoped LiNbO3 crystal was grown along the ferroelectric c axis in air by Czochralski technique. 1.0 mol% of Er3+ and 0.1 mol% of Eu3+ were added to a congruent ([Li]/[Nb]=0.946) melt. The sample was cut from the middle of the boules without being polarized to y-cut slice, and then polished to optical grade. A regeneratively amplified 800 nm Ti:sapphire mode-locked laser (Spectra-Physics, Spitfire) delivering 1 kHz and ~130 fs pulses in duration was loosely focused onto the sample. The emission was collected by a fiber probe, whose other end was connected to the entrance port of a grating-spectrometer (Bruker Optics 250IS/SM), in a direction normal to the incident excitation beam. The spectrometer output was detected by a thermoelectrically cooled intensified CCD (ICCD) detector (Andor, iStar DH720). Power dependencies were measured by using neutral density filters. The displayed emission spectrum was corrected for the wavelength response of the ICCD detector according to its quantum efficiency curve.
3. Results and discussion
The UC emission spectrum of LiNbO3 crystal codoped with Er3+ and Eu3+ is shown in Fig. 1. The emission lines are labeled each with the respective electronic transition involved. The emission is mainly consisted of two emission bands around 550 and 626 nm. It is worth noting that the luminescence intensity of 0.1 mol%Eu3+ is comparable to that of 1.0 mol%Er3+.
To understand the UC mechanism the pump energy dependence of the UC fluorescent intensities are investigated. For an unsaturated UC process, the number of photons that are necessary to populate the upper emitting state can be obtained by 
where I UC is the integrated fluorescence intensity, P pump is the pump energy density, and n is the number of laser photons required to produce one UC photon. The pump energy dependence of UC emission intensities are plotted in Fig. 2. n are obtained from the slope of the graph of ln(I UC) versus ln(P pump). In the low pump energy side, n close to 2 indicate two-photon process dominating red and green UC emissions. In the high pump energy side, the slope of 1.06 means the occurrence of saturation for Eu3+.
To further clarify the UC mechanism, the decay kinetics of Eu3+ and Er3+ are measured, and the decay curve of Eu3+ around 626 nm are shown in Fig. 3. The decay curve shows a rise at initial stage followed by an exponential decay. The rise can be ascribed to ET  or to population from higher energy level. Whereas the energy gap between 5D1 and 5D0 is just 2 quantum of cut-off phonon energy (880 cm-1) in LiNbO3 crystal, which leads to very effective multiphonon relaxation from 5D1 to 5D0. The effective multiphonon relaxation from 5D1 to 5D0 is demonstrated by the absence of obvious emission band between 500 and 580 nm in the Eu3+:LiNbO3 emission spectrum under 393 nm excitation (Fig. 2 of Ref. ) and the direct exponential decay in Eu3+:Sr0.61Ba0.39Nb2O6 under 532 nm excitation into 5D1 level of Eu3+ . So the assignation of the rise to population from higher energy level is ruled out. Based on instant decay under excitation in the 5D1 or higher, the UC mechanisms of two-photon simultaneous absorption or SHG can be ruled out. If the UC mechanisms of two-photon simultaneous absorption or SHG is operative, Eu3+ will be excited to a higher excited state directly, and then cascade to 5D0 level without delay, which cannot explain the delay of emission signal in experiment. The UC mechanism of SHG can also be excluded by the different temperature dependence between SHG and red UC emission, as shown in Fig. 4. The spectra obtained at 45 and 160 °C are shown in the insert of Fig. 4.
So the delay in the emission signal is ascribed to ET. ET from Er3+ to Eu3+ must occur because the energy level structure of Eu3+ excludes the possibility of ET UC emission within Eu3+ ions. In view of energy levels matching condition, two channels of ET from Er3+ to Eu3+ can lead to Eu3+ red UC emission under 800 nm excitation: cooperative sensitization (Er3+:4I11/2 + Er3+:4I11/2 → Eu3+:5D2), as occurred in Yb3+/Eu3+ system , and ET [Er3+:(2H11/2,4S3/2) → Eu3+:5D1]. The cooperative sensitization from 4I11/2 level under 4I9/2 excitation is possible because the population from 4I9/2 to 4I13/2 will stay at 4I11/2 level for 210 μs. If the cooperative sensitization is operative, the decay curve of Eu3+ can be expressed as I(t) = k[exp(-2t/τ Er(4I11/2)) − exp(−t/τ)] . But using this expression to fit experimental points, the obtained τ Er(4I11/2) (50 μs) is much shorter than the 4I11/2 excited state lifetime of Er3+ (210 μs ). This antinomy result excludes the possibility of cooperative sensitization, and the only remained possibility is ET.
where k is a coefficient related to the efficiency of the emission process, γ 6 is the ET parameter, and τ A and τ D are the luminescent lifetimes of acceptor and donor ions, respectively. The obtained lifetimes from the fitted line in Fig. 3 are τ D = 25 ± 1 μs and τ A = 420 ± 20 μs. τ D is close to the thermally coupled (2H11/2,4S3/2) excited states lifetime (24.5 μs) (see the inset of Fig. 3). This coincides with ET of [Er3+:(2H11/2,4S3/2) → Eu3+:5D1]. τ A is ascribed to the 5D0 excited state lifetime of Eu3+.
Based on the above discussions, our UC emission mechanism is shown in Fig. 5. The Er3+ UC emission mechanisms are extensively investigated, and excited-state absorption (ESA) is adopted here due to the absence of delay in the Er3+ UC emission decay curve at 550 nm (see the inset of Fig. 3). The photon required for the ESA from 4I13/2 is from the posterior pulse after n (n=1, 2, 3…) ms . The ET from Er3+:(2H11/2,4S3/2) to Eu3+:5D1 excites Eu3+. Populations of Eu3+ at 5D1 level nonradiatively relax to 5D0, from where red UC emission occurs.
According to Ref. , we calculate the ET efficiency under Er3+ excitation based on emission spectrum from the expression
According to available literature [22, 23], the radiative quantum efficiency of the 5D0 state of Eu3+ (η Eu) is 0.43, the effective fluorescence quantum efficiency of the (2H11/2,4S3/2) states of Er3+ (η Er) is 0.25, the effective fluorescence branching ratio of (2H11/2,4S3/2) → 4I15/2 (β Er) is 72%. α is the coupling constant, which includes the mean emission wavelength and the absolute spectral response of the experimental setup. α1 and α2 are assumed to be the same since the red emission wavelength is in the proximity of green. The estimated transfer efficiency is about 30%. This value can be enhanced via increasing the concentration of acceptor ion (Eu3+) since the ET efficiency increases with the concentration of acceptor ion .
In summary, red UC emission is observed in a LiNbO3 crystal codoped with Er3+ and Eu3+ under 800 nm femtosecond laser excitation. This red UC emission is a two-photon process and can be ascribed to ET of Er3+:(2H11/2,4S3/2) → Eu3+:5D1. The ET efficiency of about 30% is obtained in the LiNbO3 crystal codoped with 1.0 mol % of Er3+ and 0.1 mol % of Eu3+. These results imply that a red UC emission from Eu3+ can occur in Eu3+/Er3+ codoped or Eu3+/Er3+/Yb3+ triply doped systems after Er3+ ions are excited to 4S3/2 level by cw diode laser.
This work was partially supported by National Natural Science Foundation of China (Grant No. 10774034).
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