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Homogeneous spectrum broadening caused by temperature increasing and inhomogeneous broadening caused by ions random replacement in Nd3+-doped LuxY1-xVO4 (x = 0, 0.26, 0.41, 0.61, 0.8 and 1) series crystals were investigated. The results revealed that random replacement had greatest influence on the Nd:Lu0.61Y0.39VO4 crystal which showed the widest inhomogeneous spectrum broadening among these crystals. With a semiconductor saturable absorber mirror (SESAM), the passively mode-locking of this series crystals was carried out under the same condition. The shortest pulse was obtained also by the Nd:Lu0.61Y0.39VO4 crystal, which indirectly manifested the widest line-width of this proportion and its excellent properties for pulse laser applications.
©2013 Optical Society of America
1. Introduction
Ultrafast mode-locking lasers with ultrashort pulse duration, high repetition rate, broad spectrum and high peak intensity, have progressed over the past few decades from picosecond to femtosecond [1, 2]. The rare-earth ion doping (Nd3+, Yb3+, Er3+, Tm3+ and Ho3+) crystalline gain media are in the spotlight of scientific interest, due to the low operational threshold, high efficiency and ability to be configured into extremely compact configurations [3]. As one of the most prominent laser materials, Nd3+-doped vanadates have been researched since 1966 [4], and have been commercial applications over the years. Unfortunately, the narrow emission spectrum has limited the shortening of the pulse width in mode-locking that favors a broad emission spectrum. Actually, the past research progresses were mainly focused on the modulation method [5, 6]. While, the research on the nature of the gain material, especially on how to broaden the emission spectrum, is relatively scarce. In the case of the emission spectrum, we distinguish between homogeneous and inhomogeneous broadening. Direct (one phonon) processes and Raman phonon scattering processes, which are temperature dependent, give rise to homogeneous broadening [7–9]. The inhomogeneous broadening of the fluorescence line is mostly generated by lattice defects, dislocations, and different local crystal fields surrounding the Nd ions [8, 10, 11]. In fact, the presence of lattice defects and dislocations is not readily apparent in high-quality crystals. Therefore, the disordered structure provides flexibility in broadening the spectra, and, at the same time, the eigen properties of the host are conserved.
Recently, random replacement of the cations at the same lattice site in the host materials has been demonstrated to be an available method to achieve the disordered structure and generate inhomogeneous spectrum broadening. As a typical material, Nd:LuxY1-xVO4 crystal was produced by using Nd:YVO4 and Nd:LuVO4 single crystals as starting points [12]. Previous studies have indicated its outstanding characteristics in Q-switching and mode-locking laser operations [13, 14]. For different crystals with typical components, the variation of the crystal field and inhomogeneous broadening are different. To the best of our knowledge, the best component can be determined directly by researching the spectrum or indirectly by use the laser and nonlinear optics. However, the laser operation of these crystals only focused on certain kind of proportions [14–16], and direct investigation of inhomogeneous spectrum broadening has not yet been reported. It is very important for understanding the dependence of inhomogeneous spectrum broadening on disordered structures since laser performance is not solely dependent on the design of the cavity. In this paper, the fluorescence lifetime and emission spectra of Nd3+-doped LuxY1-xVO4 (x = 0, 0.26, 0.41, 0.61, 0.8 and 1) crystals with 0.1 at% Nd3+ concentration (0.5 at% for Nd:YVO4 and Nd:LuVO4) were measured in the temperature range of 77 K to 300 K. The homogeneous spectrum broadening caused by temperature increasing, and the inhomogeneous spectrum broadening caused by random replacement of Lu and Y ions were studied comprehensively. Using a “V-shape” cavity, the continue-wave (cw) mode-locking laser was carried out and the best component was obtained, which is corresponding to the widest spectrum line-width.
2. Experiment
The Nd:LuxY1-xVO4 (x = 0, 0.26, 0.41, 0.61, 0.8 and 1) crystals were grown by the Czochralski method [12]. They belong to ZrSiO4 structure with space group I41/amd. The dimensions of the crystals were 3 × 3 × 10 mm3 and the Nd-doping concentration was 0.1 at.% (3 × 3 × 6 mm3 and 0.5 at.% for Nd:YVO4 and Nd:LuVO4). The two 3 × 3 mm2 end faces were polished and high-transmission (HT)-coated at 808 nm and 1.06 μm. The sample was fixed in a sample holder with the temperature varied from 77 K to 300 K. The temperature-dependent lifetime and fluorescence emission spectrum were recorded by an Edinburgh FS920 High Sensitive Fluorescence Spectrometer. In the lifetime measurements, the pump source is an Opolette HE 355 ΙΙ tunable laser system whose emission wavelengths include the Signal range (410-710 nm), Idler range (710-2400 nm) and 355 nm. We chose 355 nm as the excitation wavelength, the laser pulse width is about 5 ns and the repetition rate is 20 Hz. The pump energy used in the present work is ~250 μJ. The lifetime decay curve was recorded with a 100 MHz Tektronix digital oscilloscope. For the emission spectrum measurements, the pump source was a xenon lamp with an emission wavelength of 200-900 nm. And an 800-1700 nm InGaAs stable-state detector with a resolution of <0.09 nm (with 1200g/mm grating) was used to obtain the light emission data.
In the laser experiments, the pump source was a fiber-coupled 808 nm diode laser (ϕradius = 100 µm, NA = 0.22). The pump light with 100 µm beam radius was focused on the crystal by a 1:1 collimating system. The input mirror was a flat mirror, which was high transmission (HT) coated at 808 nm on one face, and high-reflection (HR) coated at 1064 nm on the other face. The output mirror was a concave mirror with a curvature radius of 500 mm and PR of 95% at 1064 nm. In our experiment, laser crystals were wrapped with indium foil and mounted on a water-cooled copper block, the temperature of cooling water was controlled to be 20 °C. The pulse train was detected by a fast response photo-detector connected to a DPO7104 digital oscilloscope (1GHz bandwidth and 10 Gs/s sampling rate, Tektronix Inc.), and the spectral parameters were monitored by an optical spectrum analyzer with a resolution of 0.05 nm (Yokogawa, AQ6370C).
3. Fluorescent lifetime
The decay curves of Nd:YVO4 at different temperature were plotted in a log scale and shown in Fig. 1. According to previous studies [17, 18], if excitation migration is severe, the curves will take on obvious, rapid nonlinear decay at the initial times (~dozens of microsecond). Since the decay curves in Fig. 1 were approximately linear, we considered that the excitation migration was relatively weak in present work. In fact, though not shown in the figure, the measured decay curves of all samples have single-exponential profiles. As shown in the inset of Fig. 1, the fluorescence lifetime of the Nd3+ 4F3/2 level decreased obviously when the crystal temperature was lowered, which has been observed in some previous reports [19, 20]. At the same time, comparing with the 0.5 at.% doped Nd:YVO4 and Nd:LuVO4, Nd:LuxY1-xVO4 series crystal with lower Nd3+ concentration (0.1 at.%) exhibit less dependence between fluorescence lifetime and temperature. A detailed analyzing on such phenomenon for Nd3+ doped vanadates can be found in reference [20]. At room temperature, the measured lifetime of Nd:YVO4 was 92 μs, which is same to previous result in [21] and much closer to the value of 90 μs [22]. Compared to Nd:YVO4 and Nd:LuVO4, the mixed crystals presented longer lifetime which is due to two possible reasons: Firstly, concentration quenching effect for high Nd3+ doped materials; Secondly, by random replacement of Y ions by Lu ions, the crystal field around Nd3+ ions varied and be not uniform, which strengthen the radiative processes [23]. For the mixed Nd:LuxY1-xVO4 (x = 0.26, 0.41, 0.61 and 0.8) crystals, as calculated by L. Guo with Judd-Ofelt (J-O) theory [12], the lifetimes of Nd:Lu0.26Y0.74VO4, Nd:Lu0.41Y0.59VO4 and Nd:Lu0.61Y0.39VO4 were nearly the same but longer than the value of Nd:Lu0.8Y0.2VO4. This was conformed again by our experimental results as shown in the inset of Fig. 1.
Fig. 1 Lifetime decay profile of Nd:YVO4 at a) 77 K, τ = 75 μs, b) 230 K, τ = 87 μs, c) 300 K, τ = 92 μs. Inset: fluorescence lifetime of Nd:LuxY1-xVO4 (x = 0.26, 0.41, 0.61, 0.8 and 1) at different temperatures.
4. Emission spectrum
4.1 Spectrum variations dependence on temperature
In present experiment, we measured the π- and σ-polarization 4F3/2→4I11/2 emission spectra of all the samples at the following temperatures: 77, 150, 230 and 300 K. As representative examples, the π-polarization emission spectra of Nd:Lu0.26Y0.74VO4, Nd:Lu0.61Y0.39VO4 and Nd:Lu0.8Y0.2VO4 with intensity normalized are shown in Fig. 2. The temperature dependencies of widths, positions and shifts of the fluorescence lines are evident. Because the relatively low concentration of Nd3+ ions and the short radiative lifetimes (~100 μs) of the Nd3+ 4F3/2 states, natural broadening and dipole broadening can be considered regligible [24]. Therefor, the temperature broadening mainly caused by the direct phonon and Raman phonon scattering processes [7–9, 25–27], and can be expressed by [9]:
Where ΔΓD(T) is the width due to direct phonon process between the nearby energy levels, ΔΓR(T) represents the width for the Raman multiphonon process associated with phonon scattering by impurity ions. As the temperature increased from 77 to 300 K, the π-polarization spectra broadening caused by the temperature was 0.4 nm for Nd:YVO4 and Nd:LuVO4, and ~1 nm for the mixed crystals. However, the broadening was relative not obvious for the σ-polarization spectra (~0.3 nm in the investigated temperature range), which manifests that the direct phono and Raman phonon scattering processes are relative weak in the σ-polarization emission. This is clearly shown in the left part of Fig. 3.Fig. 2 4F3/2→4I11/2, π-polarization emission spectra of a) Nd:Lu0.26Y0.74VO4, b) Nd:Lu0.61Y0.39VO4, c) Nd:Lu0.8Y0.2VO4 at different temperatures.
Fig. 3 Full width at half maximum (left) and the peak wavelength (right) of Nd:LuxY1-xVO4 (x = 0, 0.26, 0.41, 0.61, 0.8 and 1) crystals verses Lu composition for π- and σ-polarization at different temperatures.
Both π and σ-polarization emission lines for the 4F3/2→4I11/2 transition shifted toward the longer wavelength (red shift, as shown in the right part of Fig. 3) with increasing the temperature, which mainly caused by the stationary effects of the phonon-ion interaction [25, 26]. According to previous studies [20, 28], the peak wavelength should vary quasi-linearly over a given temperature range. In our observation, for the π-polarization, the increase rates are 2.2, 2.2, 1.8, 2.2, 1.8 and 1 pm/K for Nd:LuxY1-xVO4 (x = 0, 0.26, 0.41, 0.61, 0.8 and 1) crystals in the temperature range of 77 to 300K. And the corresponding values are 1.8, 1.8, 1.8, 2.2, 1.8 and 1.4 pm/K, respectively, for the σ-polarization.
4.2 Spectrum variations dependence on Lu ion composition
As shown by the previous reports [9, 26, 29], the Nd3+ line-width is nearly the same at the temperature range of 0-80 K, which reveals the little effect of the direct phonon and Raman phonon scattering processes (ΔΓT≈0) in this temperature range. Therefore, the temperature independent broadening plays a major role in this temperature range. For the Nd:YVO4 single crystal, the temperature independent broadening mainly caused by the crystal strains, spontaneous one-phonon and multi-phonon emission processes [9], which can be expressed by:
Where ΔΓS, ΔΓO, and ΔΓM represent the broadening caused by the crystal strains, spontaneous one-phonon and multi-phonon emission processes, respectively. In present experiment, the value of ΔΓI in 4F3/2→4I11/2 transition of Nd:YVO4 single crystal is 0.4 nm for π-polarization and 0.5 nm σ-polarization, at the temperature of 77 K. The corresponding spectrum is shown in Figs. 4(a) and 4(c) with dotted line. However, if Y is random replaced by Lu ions at a lattice site in YVO4, an inhomogeneous spectrum broadening generates and give rise a Guassian line shape. Then, the superposition of all the contributions for the mixed crystal can be written as:Here, ΔΓr is the broadening caused by the random replacement and it is obviously shown in Fig. 4. At each researched temperature, the most wide line-width is obtained by the Nd:Lu0.61Y0.39VO4 crystal among all of the six samples. For all the mixed crystals, the line-width of σ-polarization is much broader than that obtained in the π-polarization. All of them can be attributed to the stronger inhomogeneous spectrum broadening caused by the random replacement. According to the detailed data shown in the left part of Fig. 3, inhomogeneous broadening (ΔΓr) caused by this mechanism is 2, 2.1, 2.3 and 2.1 nm for π-polarization spectra and 4.3, 4.4, 4.6 and 4.3 nm for the σ-polarization spectra, with x = 0.26, 0.41, 0.61 and 0.8, respectively. If we used Nd:LuVO4 as the reference, the crresponding value is: 1.9, 2, 2.2 and 2 nm for π-polarization spectra and 4.3, 4.4, 4.6 and 4.3 nm for the σ-polarization. All the results above directly show that the random replacement plays an very important role in the emission spectrum and leads to much wider line-width, which is favorable for producing a short pulse suration for mode-locked laser. At the same temperature, the emission line for the 4F3/2→4I11/2 transition has an red shift with the Lu composition increased. This phenomenon is also depicted by Fig. 4 and the detail data is shown in the right part of Fig. 3. As the x changed from 0 to 1, the shifted wavelength is about 0.4-0.5 nm for both π and σ-polarization.Fig. 4 Emission spectra of Nd3+ 4F3/2→4I11/2 transition in Nd:LuxY1-xVO4 (x = 0, 0.26, 0.61, 0.8 and 1) for: a) π-polarization at 77 K, b) π-polarization at 300 K, c) σ-polarization at 77 K, and d) σ-polarization at 300 K.
5. cw mode-locking (CWML) laser performance
With a “V–shape” cavity (as shown in Fig. 5(a)), the π-polarization mode-locking laser of the Nd:LuxY1-xVO4 (x = 0, 0.26, 0.61 and 0.8) crystal was demonstrated. According to ABCD matrix formalism, the distance between the input mirror (M1) and the output mirror (M2) was set to be 490 mm, and the SESAM, acting as a flat mirror, was positioned 520 mm away from M2. After taking into consideration the experimentally measured thermal focal length in the laser crystal, mode radius on the gain medium and the SESAM were calculated to be about 100 and 90 µm, respectively. Variation of the CWML output power versus the absorbed pump power is shown in Fig. 5(b), the maximum output power is 4.8 W obtained by Nd:YVO4, corresponding to a conversion effect of 38.8%. To prevent Q-switching instabilities, the intracavity energy intensity must be large enough [30]. For the Nd:Lu0.61Y0.39VO4 crystal, stable CWML realized at the absorbed pump power of 7.2 W. The energy intensity on the SESAM was calculated to be 420 µJ/cm2, which is 8 times larger than the saturation fluence (50 µJ/cm2). The corresponding pulse train was shown in Fig. 5(c), the pulse to pulse intensity fluctuation was estimated to be less than 2% and indicated that the relaxation oscillations were well suppressed. Typical autocorrelation signal of the Nd:Lu0.61Y0.39VO4 CWML laser and the Sech2 fitting is depicted by Fig. 5(d). The inset is the laser spectrum, which is smooth and no additional peaks. The time-bandwidth product of the pulses is 0.5, indicating the existence of the pulse chirp. It is believe that if the chirp can be effectively suppressed by optimizing the cavity parameters, even shorter pulse can be obtained.
Fig. 5 a) schematic of the laser setup, b) Variation of the CWML output power versus the absorbed pump power, c) pulse train of Nd:Lu0.61Y0.39VO4 crystal recorded in 20 ns and 1 µs per division (div) time scales, d) Normalized autocorrelation pulse trace for 8 ps duration. Insert: the corresponding laser spectrum of the CWML.
Table 1 shows the parameters of the spectrum and the results of the cw mode-locking. Compared to the Nd:YVO4, though the mixed crystals presented the lower output power, shorter pulse width and higher peak power can be obtained at the same condition. As we discussed above, the random replacement of the ions makes the emission spectrum has an inhomogeneous broadening, and such broadening in turn makes the spectrum useful for generating ultrashort CWML pulses over single crystal. Finally, with the widest spectrum line, the shortest pulse width of 8 ps and highest peak power of 1.25 kW were obtained by the Nd:Lu0.61Y0.39VO4 crystal. This pulse width is slightly longer than 5.1 ps that obtained in reference [14]. We thought the relative wide pulse duration was mainly caused by the small modulation depth of our SESAM [5], at the same time different Nd-concentration and cavity structure might have impacts either. Because of their much broader fluorescence emission line-width, we believed that, the performance of the laser could be further improved by optimizing the Nd-concentration, the modulation depth of the SESAM and the parameters of the cavity.
Table 1. Parameters of the spectrum and CWML laser for the Nd:LuxY1-xVO4 crystals.
6. Conclusion
The fluorescence lifetime and emission spectrum of Nd3+-doped LuxY1-xVO4 (x = 0, 0.26, 0.41, 0.61, 0.8 and 1) crystals were measured in the temperaure range of 77 to 300 K. Because of the different ligand field around the Nd3+ ions in the LuxY1-xVO4 host materials, the mixed crystal presented longger lifetime. As the temperature changed, the variation of the lifetime of the mixed crystals was not obvious, which can be attributed to the low Nd3+ concentration (0.1 at.%). Compared to Nd:YVO4, the line-width of the mixed crystals is obviously broadened, which is mainly caused by the random replacement of Lu and Y ions. At the temperature of 77 K, the inhomogeneous broadening caused by this mechanism was 2, 2.1, 2.3 and 2.1 nm for π-polarization spectra, and 4.3, 4.4, 4.6 and 4.3 nm for σ-polarization spectra, with x = 0.26, 0.41, 0.61 and 0.8, respectively. As the temperature increased to 300 K, the thermal broadening was about 1 nm in π-polarization spectra for all the crystals (0.4 nm for Nd:YVO4 and Nd:LuVO4), but not obvious in the σ-polarization. The CWML laser operation of these crystals was carried out under the same condition. With the widest line-width, the Nd:Lu0.61Y0.39VO4 presented the shortest pule width and the highest peak power among them. It manifested that the inhomogeneous spectrum broadening caused by the random replacement of Lu and Y ions make the mixed crystals could outperform both Nd:YVO4 and Nd:LuVO4 in the ultrashort pulse laser operation. According to our research, shorter pulse and higher peak power should be obtained by the c-cut crystal, because of their wider inhomogeneous spectrum broadening. The related work is underway.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (61178060), Program for New Century Excellent Talents in University (NCET-10-0552), Natural Science Foundation for Distinguished Young Scholar of Shandong Province (2012JQ18), and Independent Innovation Foundation of Shandong University (2012TS215),
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