1. IntroductionThe trivalent ytterbium (Yb) ion has become one of the most useful active ions that can be doped into host media to constitute various laser materials. It has a very simple electronic configuration, possessing only two electronic states in the optical transition regime, that is, the ground state
and an excited state
. This feature makes it advantageous over the traditional Nd ion that has been widely used in the 1-
spectral region, because detrimental effects, such as concentration quenching, energy transfer and up-conversion, can be largely diminished.
Among the wide varieties of host crystals that have been developed thus far for the Yb ion, those of tetragonal zircon structures belonging to the space group I41/amd and point group 4/mmm are of particular significance. These include orthovanadates and orthophosphates. The orthovanadates, including YVO4, GdVO4 and LuVO4, have been recognized as a class of most important host crystal for trivalent lanthanide active ions, in particular for the most common Nd ion.[1] As host medium for Yb ion, these orthovanadate crystals prove to be also unique in that the laser action of Yb ion exhibits optical bistability.[2,3] Unlike orthovanadates that can be easily grown from melt by the conventional Czochralski method, the orthophosphates are not congruently melting and can only, up to now, be grown by the high-temperature solution method. The difficulty in growing large-size crystal of high optical quality has greatly hampered the development of laser materials based on orthophosphates, although they have long been known to be promising host crystals for trivalent lanthanide active ions.[4] In fact, it was not until 2014 that the first laser action of Yb ion in orthophosphates was demonstrated with a Yb:LuPO4 crystal plate.[5] Since then, progress has been made in growing miniature columnar crystals of Yb:LuPO4, as well as in studies of its continuous-wave (CW) and Q-switched laser performance.[6–9]
Given the isomorphic crystal structures of LuPO4 and LuVO4 and the identical lattice site having a local symmetry of point group
at which the Yb ion enters, it will be interesting and instructive to compare the lasing properties of Yb ion in these two host crystals in order to clarify how and to what extent the host crystal structure and ligand field (crystal field) can determine the basic lasing properties of an active ion.
In this paper, we report on our results from such a comprehensive investigation into the basic lasing properties of Yb ion in the two host crystals. We first make a detailed comparison of the spectroscopic properties. Then the saturation behavior of absorption for pumping radiation is discussed in some detail. After that, the lasing properties are compared for two cases with emitting laser beam parallel to the a or c crystallographic axis.
2. ExperimentThe crystal of Yb ion-doped LuVO4 (Yb:LuVO4) was grown by the Czochralski method, with a Yb ion concentration of 1.5 at.% (
. The crystal samples utilized were cut along a or c crystallographic axis (a-cut or c-cut), each with a square aperture of 3.3 mm×3.3 mm and a thickness of 2.0 mm. The Yb ion-doped LuPO4 (Yb:LuPO4) was grown in high-temperature solution from spontaneous nucleation, with a Yb ion concentration of 5.0 at.% (
). Two crystal samples were used in this experiment: a 0.6 mm thick crystal plate with thickness along its crystallographic a axis (aperture of 3.0 mm×5.0 mm) and a 2.0 mm long miniature columnar crystal rod (along c axis) with a transverse dimension of about 0.7 mm. The end faces of the Yb:LuVO4 samples and the Yb:LuPO4 miniature rod were carefully polished, whereas the Yb:LuPO4 crystal plate was unpolished. For all the crystal samples, no antireflection coatings were placed on their end faces.
To study the lasing properties of the Yb ion in these two host crystals, we employed a simple plano-concave resonator, as illustrated schematically in Fig. 1. The plane mirror, M1, was coated for high transmittance (
) at a pumping wavelength of 976 nm and for high reflectance at 1010–1100 nm (
) where lasing occurs. The concave mirror, M2, having a radius of curvature
or 25 mm, was used as an output coupler of the laser oscillator and its transmittance (output coupling T) could be chosen in a wide range from T = 0.5% to T = 60%. To achieve an efficient laser action, the crystal plate sample was fixed on a copper heat-sink, whereas other crystal samples were fitted into a copper holder. Both copper heat-sinks were maintained at a temperature of 5 °C by cooling water. In order to examine the lasing properties of the two crystals under strong excitation conditions, we utilized as a pump source a high-brightness, high-power, fiber-coupled diode laser that had a wavelength of 976 nm (bandwidth less than 0.5 nm), a maximum output power of 50 W, a fiber core diameter of
and a numerical aperture (NA) of 0.22. The pump radiation from the fiber end was first focused by an optical re-imaging unit (ORU) and then was delivered through M1 into the laser crystal, with a pump beam spot radius of about
. With the help of a calibrated beam splitter, the laser emission spectrum could be monitored by use of a spectrometer, while the output power was measured with a power meter.
3. Results and discussion3.1. Comparison of spectroscopic propertiesThe intrinsic lasing properties of an active ion in a host medium are determined ultimately by its spectroscopic properties in that medium, including absorption spectrum, emission spectrum and fluorescence lifetime (or radiative lifetime). It is therefore necessary to compare the spectroscopic properties of these two crystals, for properly understanding their basic lasing behaviors. The tetragonal LuPO4 and LuVO4 are uniaxial crystals, each with an optic axis along the crystallographic c axis (the four-fold symmetry axis). Consequently, the spectroscopic properties of the Yb ion in these crystals can be fully characterized by two spectra with
(π-polarized) and
(σ-polarized). The fluorescence lifetimes, according to previous work,[10,11] is determined to be 0.256 ms for Yb:LuVO4 and 0.830 ms for Yb:LuPO4.
Figure 2 shows a direct comparison of polarized absorption cross-section spectra (σabs(λ)) for the Yb ion in the two crystals, which are measured at room temperature. The π-polarized spectrum for Yb:LuVO4 has been reduced by a factor of 4, in order to facilitate the comparison of spectral features. For the π polarization, the strongest absorptions for both crystals occur at 985.0 nm, corresponding to the zero-phonon transition between the lowest Stark levels of the ground state
and the excited state
. It can be seen that the peak absorption cross-section for Yb:LuVO4 is more than four times greater than that for Yb:LuPO4. In addition to the distinct magnitudes of peak absorption, the spectrum for Yb:LuVO4 is dominated by the zero-phonon transition, whereas for Yb:LuPO4, two additional absorption sidebands centered at 963.4 and 1000.0 nm are also pronounced. For the σ polarization, the absorption spectrum differs largely from that for π polarization, which is true for both crystals, implying the presence of strong anisotropy. In the case of Yb:LuVO4, the maximum absorption still occurs at 985.0 nm (zero-phonon line), but a much wider absorption band, with its peak located at 969.2 nm, proves to be predominant. For Yb:LuPO4, the absorption spectrum exhibits entirely different features, where the strongest absorption occurs at 975.0 nm and even the short-wavelength sideband peaked at 953.3 nm is also stronger than the zero-phonon absorption at 982.7 nm.
A direct comparison of Yb ion emission spectra (σem(λ)) in these two host crystals is presented in Fig. 3. Like the absorption spectra, one sees that for the π polarization, the primary emission peaks occurring at 985.0 nm for Yb:LuPO4 and Yb:LuVO4 coincide. However, more features or detailed structures are found in the emission spectrum of Yb:LuPO4, among which the emission bands peaked at about 1000, 1010, 1025 and 1035 nm prove to be of particular importance, because actual laser action is expected to occur within these emission bands. It is worth pointing out that the main emission band around 985 nm, which overlaps the strongest zero-phonon absorption band, is of little significance for practical laser operation due to the existence of extremely high resonant absorption loss. Similarly, for Yb:LuVO4, what turns out to be of practical importance is also the long-wavelength emission sideband covering a range from roughly 1020 to 1055 nm, which will be seen evidently below. The σ-polarized emission spectrum of Yb:LuVO4 consists of a predominant, strongest central band peaked at 985.0 nm, accompanied by two weak short- and long-wavelength sidebands, the latter one being responsible for the actual laser action. In contrast to these characters, the σ-polarized spectrum of Yb:LuPO4 is made up of more separated emission bands, which are peaked at 975.3, 983.1, 1002.0 and 1037.0 nm. The two long-wavelength emission bands prove to relate to the laser action achieved under usual excitation conditions.
For a quasi-three-level laser like an Yb ion one, the effective gain cross-section
, with β denoting the fraction of Yb ions that have been excited to the upper manifold (
), proves to be more useful for evaluating the lasing properties of a laser material. Figure 4 shows a group of
curves corresponding to different values of β, calculated for Yb:LuPO4 and Yb:LuVO4.
One sees that for the π polarization, the maximum gain cross-section in the case of Yb:LuVO4 shifts toward short-wavelength side with increasing the magnitude of β, and similar behavior is found for Yb:LuPO4 under low excitation levels (small values of β). With the excitation increasing to a sufficiently high level, however, multiple peaks of comparable magnitudes will appear for Yb:LuPO4 crystal as indicated by the curve for β = 0.36. This implies a severe gain competition. Besides this, it is also seen that under an identical excitation level, the amount of
for Yb:LuPO4 is much greater than that for Yb:LuVO4. In contrast to the situation for the π polarization, the σ-polarized maximum gain cross-sections for the two crystals turn out to be quite comparable to each other under the same excitation level.
From the comparison made above, one sees clear distinctions in spectroscopic property between Yb:LuPO4 and Yb:LuVO4 crystals. Considering the same crystal structure and lattice site of Yb ion, such distinctions must be attributed to the differences in the nature of ligand, crystal field strength, covalence effect, electron–phonon interaction, phonon spectrum and density of states.[12]
3.2. Absorption of pump radiation without lasingTo characterize the lasing properties of an active ion in a specific host medium, one needs first to evaluate its capability of absorbing pump radiation. The absorption capability of a sample is usually described by its optical density
, with α denoting the absorption coefficient and l the sample length.[13] For anisotropic crystals, the magnitude of α depends on the crystal orientation with respect to the pump beam. For the a-cut Yb:LuVO4 or the Yb:LuPO4 plate with pump beam traveling along the a axis, the absorption coefficient for un-polarized pump radiation can be calculated according to
. For the c-cut Yb:LuVO4 or the Yb:LuPO4 miniature rod with pump beam traveling along the c axis,
. In the expressions
is the Yb ion concentration. From Fig. 2, one can read the values for σabs at the pumping wavelength of 976 nm: for Yb:LuPO4,
,
; for Yb:LuVO4,
,
. Using these data, one can calculate the values of α and O.D. and the results are listed in Table 1. One can notice that except for the Yb:LuPO4 rod, the values of O.D. for the other three crystal samples are comparable to each other.
Table 1.
Table 1.
 Table 1.
Parameters relating to absorption of pumping radiation for different crystal samples.
.
| Crystal samples |
/1020 cm−3
|
l/cm |
α/cm−1
|
O.D. |
ηa0
|
|
| Yb:LuPO4 plate |
6.15 |
0.06 |
10.33 |
0.27 |
0.46 |
7.62 |
| Yb:LuPO4 rod |
6.15 |
0.2 |
15.44 |
1.34 |
0.95 |
5.14 |
| Yb:LuVO4, a-cut |
1.96 |
0.2 |
3.51 |
0.30 |
0.50 |
23.24 |
| Yb:LuVO4, c-cut |
1.96 |
0.2 |
2.70 |
0.23 |
0.42 |
28.50 |
| Table 1.
Parameters relating to absorption of pumping radiation for different crystal samples.
. |
We measure the absorption efficiency (ηa), Which is defined as a fraction of incident pump power (
absorbed by the sample, over a low power range of
, for all the crystal samples. The measurement is accomplished under non-lasing conditions by removing the output coupler (M2) from the resonator (Fig. 1). The variations of
with Pin are illustrated in Fig. 5. The data for Yb:LuPO4 with pump beam
are measured with the crystal plate, those for pump beam
are measured with the Yb:LuPO4 miniature rod, while the data for Yb:LuVO4 with pump beam
or
are measured with the a- or c-cut crystal samples.
From Fig. 5, one notes first a saturation behavior, i.e., a reduction in the magnitude of ηa with increasing Pin. The saturation degrees are different for different samples. At very low level of Pin where no saturation occurs, the small-signal or unsaturated absorption efficiency, ηa0, can be calculated by
. Using the parameters given in Table 1, one can calculate the values of ηa0 for different cases: 0.46 (Yb:LuPO4, pump beam
, 0.95 (Yb:LuPO4, pump beam
, 0.50 (Yb:LuVO4, pump beam
) and 0.42 (Yb:LuVO4, pump beam
). From Fig. 5, one can see good agreement between the measured data and the calculated results. The saturation behavior is characterized by the parameter of absorption saturation intensity (Isat). When the intensity of incident radiation is increased such that I becomes comparable to Isat, the absorption saturation will occur. For a quasi-three-level system like the Yb ion, due to the overlap of absorption and emission spectra, Isat depends on both σabs and
, where τf is the fluorescence lifetime andhν is the photon energy of the radiation.[14] The value for σabs at the pumping wavelength (976 nm) has been given above and the emission cross-sections can also been read from Fig. 3:
,
for Yb:LuPO4;
,
for Yb:LuVO4. Given the values of
(pumping wavelength of 976 nm) and
(Yb:LuPO4),[11] 0.256 ms (Yb:LuVO4),[10] one can calculate the magnitudes of Isat for different cases and the results are also listed in Table 1. One notes that the saturation intensities for Yb:LuVO4 prove to be much higher than those for Yb:LuPO4, which explains the much less degree of saturation in the absorption of Yb:LuVO4 sample. For the pump beam produced from a fiber-coupled diode laser, its transverse intensity distribution can be assumed to be roughly Gaussian and the on-axis intensity is
, where wp is the pump beam spot radius. In our experiment,
, so that
(kW/cm2). According to the calculated values for Isat listed in Table 1, one sees that the incident pump power required for causing saturation is roughly 0.5 W (Yb:LuPO4) and 2 W (Yb:LuVO4), which are in agreement with the measurements. From Fig. 5, one may also notice the much more pronounced absorption saturation of Yb:LuPO4 with pump beam
than that with pump beam
, which is seemingly contrary to the fact that
(Table 1). The physical reason for this contradiction is attributed to the divergence of the pump beam traversing in the crystal sample. In the 2 mm long crystal rod, the beam divergence would be more significant than in the 0.6 mm thick plate, leading to lower intensity away from the beam waist and hence less degree of saturation.
3.3. Lasing properties with an emitting beam along the a axisThe lasing properties of the Yb ion in LuPO4 and LuVO4 crystals are investigated, with an emitting laser beam parallel to the a or the c crystallographic axis. The resonator configuration is optimized individually for each crystal sample. The output characteristics are described in terms of absorbed pump power, which is determined by
. Under lasing conditions, the absorption saturation will greatly diminish because of the presence of intra-cavity circulating laser intensity resulting in stimulated emission from the upper level, giving rise to the so called “population recycling” effect. Due to this effect, the magnitude of ηa could be close to or even reach the value of ηa0.[15]
Firstly, the lasing properties of Yb:LuPO4 with emitting beam
are investigated by use of the crystal plate. The optimal laser performance is achieved by employing a resonator of 23 mm in length, which is formed with an output coupler having a curvature radius of 25 mm. Laser action is realized with output coupling ranging from T = 0.5% to T = 20%, with T = 3% being optimal. The emitting radiation is linearly polarized with
(π-polarized). Figure 6(a) shows the plots of output power versus Pabs for various output couplings. Under the optimal condition of T = 3 %, lasing threshold is reached at Pabs,th = 0.22 W, above which the output power scales linearly with a slope efficiency of 56%, reaching 3.30 W at Pabs = 6.50 W, the optical–optical conversion efficiency being 50.8%. One sees that even under a relatively high output coupling of T = 20%, fairly efficient laser action can still be achieved, producing an output power of 1.35 W at Pabs = 5.86 W, the optical–optical and slope efficiencies being 23.0% and 28%, respectively.
We next investigate the lasing properties of Yb:LuVO4 with emitting beam
and the crystal sample utilized is the a-cut one. In this case, efficient laser action can only be realized with output coupling not greater than T = 15% and with the resonator (
) adjusted in the hemispherical configuration (cavity length of about 26 mm). This seems to be a common character for Yb ion-doped orthovanadate crystal.[3] Figure 6(b) shows the output power as a function of Pabs for different output couplings. The polarization state of the emitting laser radiation is also found to be π-polarized. Much higher lasing threshold is observed for Yb:LuVO4 than for Yb:LuPO4. For instance, in the case of T = 5%, which proves to be optimal, the lasing threshold is measured to be Pabs,th = 1.80 W, in contrast to Pabs,th = 0.28 W measured under the same output coupling for Yb:LuPO4 (Fig. 6(a)). It should be noted that the results depicted in Fig. 6(b) are obtained in the hemispherical resonator configuration leading to the lowest lasing threshold and the value of Pabs,th will even be greatly increased under the same resonator configuration as employed in the case of Yb:LuPO4. The high lasing threshold is believed to be due to the strong resonant absorption loss at the lasing wavelength, which also results in optical bistability.[2] As can be seen in Fig. 6(b), very efficient laser action can be achieved under the optimal output coupling conditions: an output power of 8.63 W is produced at Pabs = 15.2 W, with an optical–optical conversion efficiency 56.8%. The slope efficiency determined for an pump power range of
reaches to 68%. Above this pumping level, the laser action is found to be less efficient, owing to the increasingly strengthened, thermally induced loss.
The lasing wavelengths vary slightly only with pump power increasing, but depend strongly on the output coupling utilized. Figure 7 illustrates the dependence of laser emission spectrum on output coupling for Yb:LuPO4 measured at
(Fig. 7(a)) and for Yb:LuVO4 measured at Pabs = 4.1 W (Fig. 7(b)).
Through a comparison, one can easily recognize the difference in the evolution of lasing wavelength with output coupling between the two crystals. In the case of Yb:LuPO4, the laser emission spectra fall into two separate groups: one with lasing wavelengths ranging from about 1034 to 1042 nm for T = 5%, 3% and 0.5% and the other with lasing wavelengths ranging from about 1009 to 1016 nm for T = 20% and 10%. In contrast, the laser emission spectrum for Yb:LuVO4 is found to shift progressively, with the output coupling increasing from T = 0.5% to T = 15%, toward short-wavelength side, from 1051.0–1052.9 nm to 1020.7–1023.2 nm.
The varying behavior of the lasing spectrum with output coupling can be explained qualitatively in accordance with the gain cross-section curves plotted in Fig. 4. For a quasi-three-level laser operating in the free running modes, laser action usually occurs at wavelengths where the effective gain cross-section, and hence the overall gain, reaches its maximum. One sees that for Yb:LuPO4, the maximum values of the π-polarized
under low excitation levels of β = 0.06, 0.08, and 0.10, shift only very slightly, from about 1039 to 1035 nm. This coincides with the variation of lasing spectrum resulting from the output coupling increasing from T = 0.5% to T = 5%. One can also note that under strong-enough excitation,
, the short-wavelength emission band extending from about 1000 to 1010 nm will become dominant. This is the physical reason behind the “jumping” of lasing spectrum when the output coupling is changed from T = 5% to T = 10%. In the case of Yb:LuVO4, the progressive evolution of lasing spectrum with the output coupling as illustrated in Fig. 7(b), can similarly be understood from the gain maximum shifting with the excitation level. However, the long-wavelength laser emission obtained under low output couplings is not predicted by the σg(λ) curves for small values of β. This disagreement may result from the less accuracy of the emission spectrum in the long wavelength region, which is determined by the reciprocity method and will be severely limited by the extremely low absorption.
3.4. Lasing properties with an emitting beam along the c axisLaser action with an emitting beam parallel to the crystallographic c axis can be achieved with the Yb:LuPO4 miniature rod or with the c-cut Yb:LuVO4 sample. For Yb:LuPO4, the most efficient laser action is realized by employing a resonator that is formed with a coupler of
(cavity length of 13 mm). For Yb:LuVO4, the laser resonator utilized is the same as that in the case of the a-cut sample (a hemispherical cavity). Propagating along the c axis, the laser radiation generated is unpolarized, just as expected, given the fact that the spectroscopic properties for
laying in the plane perpendicular to the optic axis are isotropic.
For the Yb:LuPO4 miniature rod sample, laser action is realized, with output coupling changed from T = 0.5% to T = 60%. The output characteristics are depicted in Fig. 8(a) for several different output couplings. As long as only the output power and lasing efficiency are concerned, the laser performances prove to be very close to each other for output couplings ranging from T = 5% to T = 20% as can be seen in this figure. In the case of T = 20%, the output power increases almost linearly with pump power increasing, with a slope efficiency of 56%. It reaches a maximum value of 8.35 W at Pabs = 17.9 W, resulting in an optical–optical conversion efficiency of 46.7%. One also sees from Fig. 8(a) that even under an output coupling as high as T = 60%, fairly efficient laser action can still be achieved, with output power approaching nearly a level of 4 W.
As in the case of the a-cut Yb:LuVO4 crystal, laser action is attainable only with the c-cut Yb:LuVO4 sample under output coupling conditions of
. Figure 8(b) shows the output powers versus Pabs for different values of T. Once again, one sees much higher lasing thresholds than those for the laser action achieved with the Yb:LuPO4 rod. For example, under the condition of T = 5%, the lasing threshold is measured to be
, which is twice higher than that for the Yb:LuPO4 rod (0.80 W). The highest output power is generated under an optimal output coupling of T = 5% and it is measured to be 4.53 W at
. The optical–optical conversion efficiency is 41.2%.
In addition to the differences in lasing threshold, output power, and output coupling dependence, the nonlinear natures of the output–input curves for Yb:LuVO4 and Yb:LuPO4 also turn out to be different from each other. One sees from Fig. 8(b) that in particular for lower output couplings, output power increases very rapidly with Pabs increasing in the vicinity of lasing threshold, leading to very high local slope efficiency. In contrast, as can be seen in Fig. 8(a) for Yb:LuPO4, the output power just above lasing threshold increases more slowly and the slope efficiency is lower than the value measured at high pump level, a behavior that is typical of most quasi-three-level lasers. Such an anomalous output character results from the unique nature of resonant absorption in Yb:LuVO4 that also leads to bistable laser behavior.[2,16,17]
Figure 9 shows the variations of lasing spectrum with output coupling, measured at Pabs = 8.8 W for Yb:LuPO4 (Fig. 9(a)) and at Pabs = 3.6 W for Yb:LuVO4 (Fig. 9(b)). Certain similarities are observed, e.g., the wavelength region over which lasing occurs in the case of T = 0.5 % or T = 5% is seen to be overlapped largely for the two crystals. However, the shortest lasing wavelength attainable for the case of Yb:LuPO4 is found to be within a range of 1000–1005 nm, compared with that of 1015–1020 nm for the case of Yb:LuVO4. As in the case of emitting beam
, the dependence of lasing spectrum on the output coupling can also be understood by referring to the σ-polarized gain cross-section curves depicted in Fig. 4(b).
3.5. Comparative summaryFrom the discussion given in the previous subsections, we have seen significant distinctions between Yb:LuPO4 and Yb:LuVO4 in spectroscopic properties, absorption saturation behavior and laser action, despite the fact that they possess the same crystal structure as well as the same local site symmetry for the Yb ion.
The π-polarized absorption or emission spectra of the two crystals share a common feature that they are dominated by the zero-phonon transition band centered at 985.0 nm. However, the peak cross-sections for Yb:LuVO4 are four times greater than for Yb:LuPO4, suggesting much stronger transition strength of Yb ion in LuVO4 crystal. In contrast, the detailed spectral features of either absorption or emission for σ polarization turn out to be entirely different and the peak cross-sections for the Yb ion in the two crystals are comparable to each other.
The absorption for pumping radiation at 976 nm exhibits a considerably different saturation behavior and the degree of saturation proves to be much more pronounced for Yb:LuPO4. This distinction arises from the differences between the two crystals in absorption and emission cross-sections at the pumping wavelength, as well as in fluorescence lifetime.
Some of the lasing properties of the Yb ion in these two host crystals are more or less similar to each other to a certain extent, but many of them turn out to be fairly different. First of all, only in a hemispherical resonator configuration can the efficient laser action be achieved with Yb:LuVO4. In contrast, efficient laser operation can readily be realized with Yb:LuP O4 in a common stable resonator. The difficulty in realizing the efficient laser action is believed to be due to the sizable resonant absorption loss existing in Yb:LuVO4 crystal, which is also responsible for the high lasing threshold. Next, the output coupling range, over which laser action could be realized, proves to be much wider for Yb:LuPO4, making it advantageous over Yb:LuVO4 in some applications such as Q-switched lasers. Accompanying this advantage, lasing wavelengths as short as 1000 nm are attainable with Yb:LuPO4. Thirdly, anomalous nonlinearity in the output–input relation is noticed for Yb:LuVO4 with emitting beam
and it also results from the unique nature of resonant absorption loss, which can lead to optical bistability in laser action, in particular under very low output coupling.[2,16,17] Such a bistable laser behavior has not been observed with Yb:LuPO4. Very probably, the conditions required for optical bistability to occur, which are determined by the spectroscopic properties,[16,17] cannot be met in Yb:LuPO4. Finally, it is also worth mentioning that experimentally, Q-switching laser action seems to be very difficult to realize with Yb:LuVO4. Until now, little information has been known about passive or active Q-switching laser properties of this crystal. By comparison, both passive and active Q-switching laser action have been demonstrated with Yb:LuPO4.[7–9]
