Fluorescence spectra of Nd:YVO4 under excitation of a continuous wave (CW) diode laser and a femtosecond laser at 800nm were investigated. It was found that Nd:YVO4 shows different upconversion and downconversion luminescencent behaviors when excited by the diode laser and the femtosecond laser. The dependence of the upconversion luminescence intensity on the pump power of the femtosecond laser was discussed. The populations of the upper energy levels for upconversion and downconversion luminescence were calculated based on the Bloch equations. The calculations agree well with the experimental results.
©2007 Optical Society of America
There has been a growing interest in the study of rare-earth doped materials for frequency upconversion of infrared radiation into shorter wavelengths for a variety of applications such as infrared-pumped visible laser [1–2], three-dimensional optical data storage and display [3–4]. Intensive research efforts are therefore devoted to increase the efficiency of frequency upconversion. In the past decades, it has been extensively investigated for the designing and tuning of the upconversion luminescent properties of the rare-earth doped materials. A very broad range of unprecedented optical properties can be observed by changing the host lattice, dopant concentration, and codopant elements [5–6]. But there are few investigations on increasing the frequency upconversion efficiency by using the femtosecond laser as a pump source to the best of our knowledge. We observed experimentally that the efficiency of frequency upconversion of Nd:YVO4 pumped by a femtosecond laser is higher than that by a CW diode laser with the same wavelength. This result is consistent with our theoretical simulation based on the Bloch equations.
0.5atm% Nd-doped yttrium orthvanadate (Nd:YVO4) crystal is a zircon tetragonal, positive uniaxial crystal, whose space group is D4h, a=b=7.12, c=6.29. It was grown by the Czochralski method. The crystal sample was cut (a-cut) and polished to a size of 10×10×3mm3. No inclusions or other light scattering centers were observed by the optical microscope. A CW diode laser at 800nm and a regeneratively amplified 800nm Ti: sapphire laser that emits 120 femtosecond, 1 kHz, mode-locked pulses were used as the excitation sources. The laser beams were focused into samples by a lens with 100mm focal length. The focus was inside the sample. The fluorescence spectra excited by diode and femtosecond laser were recorded by a spectrophotometer of ZOLIX SBP300, while that excited by a 267nm monochromatic light from a xenon lamp were measured by a JASCO FP6500 spectrophotometer. All the measurements were preformed at room temperature.
3. Results and discussion
Visible upconversion luminescence was observed in Nd:YVO4 under the diode laser and the femtosecond laser excitation. Fig. 1 shows the emission spectra excited by the femtosecond laser with an average power of 26mW and the diode laser with a power of 250mW. The emission spectrum excited by 267nm monochromatic light is also shown in Fig. 1 for comparison. There are four emission peaks at 1338, 1064, 895 and 754nm for the fluorescence spectra excited by the diode laser, and only the peak at 754nm belongs to upconversion luminescence. These emissions can be assigned to 4F3/2→4I13/2 (1328nm), 4F3/2→4I11/2 (1064nm), 4F3/2→4I9/2 (895nm) and 2G9/2+2K13/2→4I15/2 (754nm) transitions of Nd3+, respectively. In the emission spectrum excited by the femtosecond laser, there are seven sharp emission peaks and they can be assigned to 4F3/2→4I13/2 (1328nm), 4F3/2→4I11/2 (1064nm), 4F3/2→4I9/2 (875nm), 2G9/2+2K13/2→4I15/2 (731nm), 4G7/2→4I13/2 (684nm), 4D3/2→4I13/2 (424 nm) and 4D3/2→4I11/2 (393nm) transitions of Nd3+, respectively. Five emission peaks belong to upconversion luminescence. The emission band peaking at 393 and 424nm is similar to the one peaking at 422nm excited by the 267nm monochromatic light. This result reveals that the emission bands peaking at 393 and 424nm are probably multi-photon absorption (MPA) induced photoluminescence.
All above upconversion luminescence due to transitions of Nd3+ may have three possible mechanisms: 1) three-photon excitation process. In this process, Nd3+ at ground state is firstly pumped into a real intermediate level (4F5/2) by absorption of one pump photon, then, simultaneously absorbs two pump photons again and is promoted to the upper excited states of YVO4 (1T2). After the interaction of femtosecond laser pulse with Nd:YVO4 is over, Nd3+ at 1T2 will nonradiatively relax to the lower excited states of 4D3/2, 4G7/2 or 2G9/2+2K13/2, from which, the upconversion emission occurs, while Nd3+ at 4F5/2 will nonradiatively relax to the lower excited states of 4F3/2, from which, the downconversion occurs. During the interaction of femtosecond laser pulse with Nd: YVO4, the up- and down-conversion emission will hardly occur because the 1T2 and 4F5/2 possess a much longer lifetime than the femtosecond pulse duration(120fs). 2) excited-state absorption process. In this case, the Nd3+ at ground state is firstly pumped into 4F5/2 same as the three-photon excitation process and nonradiatively relaxes to the lower excited states of 4F3/2 rapidly, from which, the electron absorbs one pump photon again and is promoted to the upper excited states of 2D5/2, finally, the electron probably nonradiatively relaxes to the lower excited states of 4G7/2 or 2G9/2+2K13/2, from which, characteristic emission at 684, 731 or 754nm occurs. In fact, within the time frame of a single femtosecond pulse, Nd3+ excited from the ground state into the 4F5/2 level cannot relax “rapidly” to the 4F3/2 level because the 4F5/2 level possesses a lifetime of a few ns. Possibly, there is an accumulation of excitation in the 4F3/2 level, and subsequent femtosecond pulses lead to excited-state absorption. But owing to the 1 kHz repetition rate of the femtosecond laser, the population at the 4F3/2 level, which possesses a lifetime of about 90μs, has been depleted when the subsequent femtosecond pulse comes. Therefore, excited-state absorption resulting from subsequent femtosecond pulses is not possible, and the emission bands peaking at 684 and 731nm excited by femtosecond laser do not result from excited-state absorption. 3) cooperative energy-transfer process. The cooperative energy-transfer process is a particular case of cross relaxation. For Nd:YVO4 crystal, it is described as the energy transfer between two neighboring ions in the excited state 4F3/2. The donor ion gives part of its energy to the acceptor ion by decaying to the excited level 4I13/2 or 4I11/2. The acceptor ion is then promoted to the excited state 4G7/2 or 4G9/2+4G11/2+2K15/2 and emits a photon of 684nm from 4G7/2 or nonradiatively relaxes to the lower excited states of 2G9/2+2K13/2, from which, the upconversion emission of 731 or 754nm occurs. Hence the energy-transfer process is observed as the direct, nonradiative decay of the donor ion, due to the long lifetime of metastable level 4F3/2. By the way, although Nd+ excited from the ground state into the 4F5/2 level by the femtosecond laser cannot relax “rapidly” to the 4F3/2 level within the time frame of a single femtosecond pulse, the cooperative energy-transfer process may still occur after a single femtosecond pulse.
All above analyses indicate that the emission bands peaking at 393 and 424nm excited by femtosecond laser may be due to three-photon absorption (3PA) process and the one peaking at 684 and 731nm are probably 3PA or cooperative energy-transfer process. As for the emission band peaking at 754nm excited by the diode laser, it may be due to excited-state absorption or cooperative energy-transfer process. The energy level diagrams for these upconversion processes are shown in Fig. 2 based on the data provided by Kaminskii .
Generally, conversion of infrared radiation to the visible emission can be ascribed to a multiphoton absorption process. The upconversion emission intensity I u depends upon the pump power density I p. Their relationship can be described as :
Where m may range from n, the number of pump photons required to excite the emitting state, in the limit of infinitely small upconversion rates down to 1 for the upper state and less than 1 for the intermediate state in the limit of infinitely large upconversion rates. According to the reference , there are eight situations of the slope of the luminescence intensity from the excited-states in double-logarithmic representation versus absorbed pump intensity as shown in Table 1.
Based on Table 1, the measurement of the slopes of multiphoton-excited luminescence enables an interpretation of the underlying upconversion mechanism. In order to interpret the upconversion mechanism of Nd:YVO4 crystal, the luminescence emission intensities of the 424, 684 and 731nm signals against the femtosecond laser power were investigated. The log-log relationship between pumping power density of the femtosecond laser and fluorescence intensity of Nd:YVO4 is shown in Fig. 3. At low pump power, the slope coefficient of the fitted line is 2.99 for the 424nm signal, 2.69 for the 731nm signal and 2.82 for the 684nm signal, but at high pump power it is reduced to 0.89, 0.72 and 0.46, respectively. According to Table 1, if the upconversion emissions at 684 and 731nm result from ETU, N i~I p i/n(i=n=2), that is to say, the slope coefficient should be 1, which is inconsistent with the experimental results; if the upconversion emissions at 424, 684 and 731nm result from ESA, N i~I p i(i=n=3) when pump power is low and N i~I p i/n(i=n=3) when pump power is high. These theoretic results agree well with the experimental results at low pump power, but not well at high pump power. We think that four- or five-photon excitation process may occur when pump power is high. If so, the slope coefficient m=i/n=0.75 or 0.6(i=3, n=4 or 5), which is approximately consistent with the experimental results.
Comparing the upconversion luminescences with downconversion luminescences in Fig. 1, it can be obviously seen that the downconversion luminescence such as 1064nm is much stronger than the upconversion luminescence for the diode laser excitation, by contraries the upconversion luminescence is much stronger for the femtosecond laser excitation. This shows that the upconversion efficiency pumped by the femtosecond laser is higher than that by the CW diode laser.
Theoretically, the interaction of laser with matter can be described by Bloch equation :
Where ρ is the density matrix which describes the system behavior, γ is the attenuation coefficient of the density matrix component, H=H 0+V is the Hamiltonian operator, H 0 is the unperturbed Hamiltonian operator, V is the interacting operator between the system and the field. For 3PA process of Nd:YVO4, we adopted three-level model. The first, second and third energy level are 4I9/2, 4F5/2 of Nd3+ and 1T2 of YVO4, respectively. In the off-diagonal components of matrix for the interaction of laser with Nd:YVO4, we can think that V 13=V * 31=0. Within the time frame of a single femtosecond pulse, the influence of the relaxation and decoherence between two different energy levels can be negligible. So the Bloch equations for the three-level model can be expressed as follows:
Where Ẽ(t) is the slowly varying envelope of the field, ω is its frequency, μ ij is the electric dipole matrix component. ω ij is the transition frequency between the energy level i and j and ρ ij=ρ̃ijexp(±iωt), Δ 12=ω 12-ω, Δ 13=ω 13-3ω, Δ 23=ω 23-2ω. Taking the femtosecond laser pulse as a Gaussian pulse, we calculated the values of ρ ii(i=1, 2, 3) at any moment according to the equations (3)–(9) for the parametric values P=26mW (average pump power), D=0.5mm (the laser spot diameter at the focus of lens), μ 12=μ 21=μ 23=μ 32=10-28C·M. Fig. 4 shows the calculation results. Obviously, the upconversion fluorescence intensity of Nd3+ is proportional to the population of 1T2 level and the downconversion one is proportional to the population of 4F5/2 level, so we can compare the relative intensity of upconversion and downconversion fluorescence according to the values of ρ 22 and ρ 33. As shown in Fig. 4, the populations of 1T2 is much more than that of 4F5/2 after the interaction of femtosecond laser pulse with Nd:YVO4. This reveals that the upconversion fluorescence intensity of Nd3+ excited by the femtosecond laser is higher than the downconversion one, which is consistent with the experimental results, while the downconversion luminescence is much stronger than the upconversion one for the diode laser excitation according to Fig.1. This shows that the upconversion efficiency pumped by the femtosecond laser is higher than that by the CW diode laser.
In summary, fluorescence spectra of Nd:YVO4 crystal under excitation of the CW diode laser and femtosecond laser have been investigated. The log-log relationship between pump power density of the femtosecond laser and fluorescence intensity of Nd:YVO4 shows that the upconversion processes for the 424, 731 and 684nm signals are 3PA processes at low pump power, while at high pump power, four- or five-photon excitation process may occur. We also find that the upconversion fluorescence intensity of Nd3+ excited by the femtosecond laser is higher than the downconversion one, which is consistent with the calculation result based on Bloch equations, and that the upconversion luminescence efficiency pumped by the femtosecond laser is higher than that by the diode laser. With the development of the femtosecond laser technique, the frequency upconversion pumped by the femtosecond laser will be more widely applied.
This work is supported by National Basic Research Program of China (Grants No. 2006CB806000).
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