Powerful visible luminescence in a Gadolinium Gallium Garnet (GGG) crystal, co-activated with Yb3+(~15 at.%) and Ho3+(~0.1 at.%) ions, is investigated under CW laser diode pumping (λ = 938 and 976 nm). The main visible emission band is observed in the green with its peak at λ ~540 nm) and measured to be about 10% with respect to Yb3+IR luminescence (λ ~1000 nm). Red (λ ~650 nm) and near-IR (λ ~755 nm) emission bands are also observed but are weaker (about 3–5%). Analysis of the crystal absorption and luminescence spectra allows one to conclude that Yb3+ -Ho3+ stepwise up-conversion is the mechanism explaining the phenomenon. Ho3+ ions embedded in the crystal in small concentration are shown to form an effective reservoir for energy transferred from the excited Yb3+ subsystem and to be an efficient source of the visible emission.
© 2002 Optical Society of America
Ytterbium (Yb) doped crystals such as Yb:YAG, Yb:KGW, Yb:FAP, etc. are promising materials for high-power solid state lasers in the λ ~1 μm spectral range. Yb doped lasers provide maximal output powers (at minimum thermal load), high brightness, and the best overall efficiencies [1,2]. Low Stokes losses in Yb3+ system (due to near spectral neighboring of the Yb3+ absorption and luminescence bands) and absence of excited-state absorption (ions of Yb3+ can be characterized by a single pair of levels: 2F7/2 (ground state) and 2F5/2 (excited state)) make, in certain cases, Yb-doped materials superior over traditional Neodymium ones. Therefore, a search for new crystals activated with Yb is of great importance. On the other hand, it is known that co-activation of Yb-doped crystals with Holmium (Ho) ions leads to appearance of visible luminescence, which is explained by the Yb3+ - Ho3+ stepwise upconversion mechanism [3–5].
In recent years, Gadolinium Gallium Garnet (Gd3Ga5O12, - GGG) has been the subject of interest as an alternative host for Yb [6–8]. Yb:GGG has a number of attractive features such as higher emission and absorption cross sections in comparison to other Yb-doped crystals (e.g. Yb:YAG and Yb:FAP) while its thermo-optical properties are comparable to Yb:YAG.
In this letter, we present experimental features of luminescent properties of novel crystal GGG co-doped with Yb and Ho, concentrating attention on investigating its luminescence in the visible. High-intensity visible emission in Yb,Ho:GGG observed under CW IR diode pumping is believed to allow lasing by the up-conversion scheme.
2. Preparation and characterization of Yb,Ho:GGG samples
A number of GGG crystals with high Yb doping (5 at.% up to 50 at.%) and relatively weak Ho concentrations (~0.1 at.%) were grown by the Czochralski method. In the experiments, samples with 15 at.% (Yb) / 0.1 at.% (Ho) concentrations were used. This choice of the ratio for the co-dopants was based on investigations on preliminary samples yielding maximum visible (green) luminescence and in agreement with earlier reported optimum percentages of co-doping for Yb,Ho:YAG .
The Yb,Ho:GGG samples were polished disks of size Ø10 × 0.9 mm. The non-saturated transmission coefficient of the samples was measured to be 26% at λ = 938 nm. The measured basic spectroscopic parameters of the crystals are as follows: room temperature Yb3+ excited state (2F5/2) relaxation time τ ≈ 0.85 ms and peak (1025 nm) Yb3+ absorption cross-section σa ≈ 3.4*10-19cm2 in the 2F5/2 - 2F7/2 band. The parameters τ and σa were evaluated from the data on Yb3+ luminescence decay curves and absorption spectra respectively. The latter was also cross-checked by experimentally estimating the saturating intensity, Is = hν/σaτ ≈ 0.7 kW/cm2.
3. Absorption and luminescence spectra of Yb,Ho:GGG
3.1. Absorption spectra
Yb,Ho:GGG absorption spectra are shown in Fig.1. The spectra were collected with a Perkin Elmer spectrometer (resolution of 1 nm). As it is seen from the overall spectrum (Fig.1a), the main absorption band in Yb,Ho:GGG lies between 900 and 1000 nm (Yb3+, transition 2F7/2-2F5/2), which is characteristic for all Yb3+-activated materials. There are also additional absorption bands in the UV, visible, and near-IR spectral regions, as seen in Figs. 1b and 1c. The high-intensity band in the near UV (Fig.1b) corresponds to the f-d configuration transition of Yb3+ ions. The single-peak at λ ≈275 nm is, most probably, related to the Yb2+ ions , which can accompany Yb3+ ions during Yb garnet growth. The triple-peak band near λ ≈300–320 nm is attributed to Gd3+ ions, whose presence is the difference of the GGG crystal from, say, the YAG crystal host. In Fig.1c the weak absorption peak near λ ≈370 nm is also identified as a feature of Yb2+ ions, while the transitions at λ ≈450–550 nm and λ ≈850–900 nm are attributed to Ho3+ ions’ absorption . The presence of extremely weak absorption peaks in the IR region, near λ ≈1150, 1205, and 1400 nm are also attributed to Ho3+ ions’ absorption.
Thus, the analysis of the absorption spectra allows us to conclude that the samples under investigation are indeed Yb3+,Ho3+:GGG (any additional possible doping, say, with Er3+ or Tm3+ ions, is not observed in the spectra). The latter conclusion is important for understanding the nature of the visible (green-to-near-IR) emission in the crystals.
3.2. Luminescence spectra
Luminescence spectra of Yb,Ho:GGG were collected using a setup in which a high-power semiconductor laser diode array (DILAS – Diodenlaser GmbH, λ = 938 nm / 30 W and ATC Semiconductor Devices, λ = 976 nm / 3 W) was used as the pump source. The pump radiation was focused onto the sample using a pair of cylindrical lenses with focal lengths of 2.5 cm. The pump intensity on the sample was estimated to be ~1 kW/cm2. Luminescence signal from the samples was recorded by a spectrophotometer (resolution of 0.5 nm) equipped with a fiber head. In order to monitor any possible directionality effects the luminescence signal was measured both from the front and the back of the sample.
The main (IR) luminescence signal, around λ ≈ 1μm, is shown in Fig.2; its shape resembles that observed in other Yb-activated crystals and is attributed to 2F55/2-2F7/2 transition of Yb3+.
Luminescence spectra collected in the visible are shown in Fig.3. It is seen that a very strong visible signal is detected in the green (centered at λ ≈530–560 nm) and slightly less powerful signals – in the near-IR (λ ≈740–770 nm) and red (λ ≈635–670 nm) spectral ranges. No directionality effect in the visible emission was detected.
In our experimental conditions, the intensity of the visible luminescence in the green was notably high and was estimated to be 8–10% of the IR emission at λ ≈ 1025 nm. This fact supports the prospect of achieving lasing in Yb.Ho:GGG under IR diode pumping. The other visible emission bands in the samples (λ ≈635–670 nm and λ ≈740–770 nm), reported for the first time to our knowledge, are remarkably strong as well.
The visible green emission could be seen with as low as 10 mW of incidence diode pump power (λ =938 and 976 nm) and had the dependence versus pump shown in Fig.4. On the inset of Fig. 4, the log-log scale of the power curves, over the whole excitation range, has a slope of 1.57 (green signal) and 1.61 (near-IR signal). The similarity of the values of the slopes is not surprising, since both processes start from the same level of Ho3+ ion (5F4 / 5S2, see below). It can also be noted that if an up-conversion mechanism for excitation exists in a material under study then a slope is an indication of an effective number of photons involved in that mechanism. The energy-transfer up-conversion step-wise mechanism  (with two or even more sequential processes involved) is, most probably, responsible for the anti-Stokes visible emission in Yb,Ho:GGG. Thus, an expected value of the slope might be near 2; the observed slope ~1.6 can be explained by additional cross-relaxation processes within the Ho3+  and competition between linear decay and up-conversion for the depletion of the intermediate excited states . It is further noted that the red signal (not shown in Fig.4) presents more complicated dependence on pump (due to more complicated scheme of excitation, see below) and can even influence on the green up-conversion signal .
4. Mechanism explaining visible luminescence in Yb,Ho:GGG
The visible emission observed experimentally should be attributed to some specific mechanism. Below we offer possible explanations for this phenomenon.
First of all, any nonlinear-optical process (like SHG) should be rejected, since it is inconsistent with the high magnitude of the visible signal, its broad wavelength band and fixed spectrum with change of diode wavelength (between 976 nm and 938 nm diode wavelength) and lack of directionality.
Secondly, a few papers [15,16] comment on a energy up-conversion effect based on collective effects of a pair of Yb ions in pure Yb3+, but highly-doped, garnets to explain extremely weak green emission (≈10-6 – 10-5 with respect to IR); but in those cases the green emission is centered exactly at a half-wavelength of the main IR luminescence (i.e., at λ ≈480–500 nm, not at λ ≈ 540–550 nm). This mechanism does not explain the appearance of the visible emission centered at λ – 650 and 755 nm.
The comparison of the absorption spectra (Fig.1) and luminescence spectra (Figs.2,3) with the known data on spectral features of Ho3+ ions in other materials [9,11,14,17–22], and character of the dependences of the visible emission on pump (Fig.4) allows us to infer that the known Yb3+-Ho3+ stepwise up-conversion scheme (see Fig.5) is the only unique mechanism fully explaining the visible (green-to-near-IR) emission in Yb,Ho:GGG. Indeed the luminescence spectra obtained (Fig.3, a–c) are virtually the same as the ones given for a Ho3+:BaY2F8 crystal  (compare with Fig.6 where data of Ref.11 are shown).
The above evidence strongly supports the visible luminescence in Yb,Ho:GGG to stem from the presence of Ho3+ co-dopants and for energy transfer within the Yb3+ – Ho3+ system. Let us discuss in more detail the the proposed scheme of the up-conversion mechanism in Yb,Ho:GGG (Fig.5). In particular, owing to close energy match of the levels 2F5/2 (Yb3+) and 5I6 (Ho3+), an easy transfer of the absorbed energy is possible from the excited Yb3+ ion to a Ho3+ ion (5I6, ≈8,000 cm-1). The Ho3+ can, as the next step of up-conversion, be excited to the band lying at ≈18,000 cm-1 (the level 5S2). Excitation of this Ho3+ band is removed via emission of a near-IR photon at λ ≈ 755 nm (transition to 5I7 level, ≈5,000 cm-1) and green emission at λ ≈ 540 nm (transition to the ground-state, 5I8). These processes are shown in Fig.5 as (1) and (2), respectively. Red emission (shown in Fig.5 as (3)) is realized via the transition from the state 5F5 (after partial relaxation of excitation from the level 5S2) to the ground state, 5I8. Additionally, one more step of energy up-conversion can take place, when one more excited ion of Yb3+ transfers its energy to the Ho3+ ion in the 5S2 state, which results in excitation of the latter to the 3H6 state (≈28,000 cm-1). After a series of non-radiative processes, the level 5F3 of Ho3+ can be populated and from there excitation is radiatively removed via red emission (shown as 3’, dashed line) at λ ≈ 650 nm (the transition to the term 5I7).
Finally, weak blue (λ ≈ 480 nm) and IR (λ ≈ 1205 nm) luminescence signals observed in the crystal are also well explained by the model. These luminescence lines correspond to the transitions 5F3 – 5I8 and 5I6 – 5I8 of Ho3+, respectively.
Hence, the scheme sketched in Fig.5 seems to be the most adequate to explain the visible luminescence in Yb,Ho:GGG, including fine structure of the luminescence bands, which is analogous to level structure observed for Ho3+:BaY2F8 (see Fig.6 and Ref 11).
To further support the above assertions we performed kinetic studies of the green luminescence (λ = 539 nm) under short-pulse (50 μs) excitation by a Ti-Sapphire laser operating at λ ≈ 944 nm.
Fig.7.shows the green luminescence signal (curve 2) which is seen to grow after the end of the exciting pulse (curve 1). This indicates that the green emission is caused by transitions from real impurity levels (as concluded above, Ho3+). The green emission response is quite characteristic [21,22] for the mechanism of anti-Stokes luminescence (i.e., the stepwise up-conversion). Similar shape for the luminescence kinetics was observed at λ = 755 nm, indicating the emission to be due to 5S2 level population evolution. We deduced characteristic time constant of the green and near-IR luminescence and determined that the processes are well described by a bi-exponential dependence with the rates W1 ≈ 1.2×103/s (decay contribution of the main IR Yb3+ emission) and W 2 ≈ 5.5×103/s (contribution of the Ho3+ emission). The red luminescence (λ = 650 nm) has a different, more complicated, character and practically linearly follows the main IR Yb3+ luminescence; investigation of the latter feature is the subject of a separate investigation.
One more interesting observation is that, at short-pulse excitation, the red (λ ~ 650 nm) and near-IR (λ ~ 755 nm) signals decrease substantially, while the main green signal (λ ~ 540 nm) stays still powerful (the correspondent ratio of the luminescence signals becomes in the latter case ~1/3/10, respectively). This fact is in full agreement with observation of Ref.21 for Yb,Ho:YVO4, where similar case of up-conversion is realized.
We have demonstrated the absorption and luminescence features (at high-power IR laser diode excitation) of Yb,Ho:GGG. We have found that the presence in the crystal of Ho3+ ions, even at small concentration, is a source of notably powerful (≈10% with respect to IR) luminescence in the visible. The Yb3+-Ho3+ stepwise up-conversion in GGG is proved to be the mechanism explaining the phenomenon. The Ho3+ ions are shown to form an effective reservoir for energy stored in the Yb3+ sub-system and to be a source of the visible light.
At present time, work is in progress on developing an IR-pumped Yb,Ho:GGG laser oscillating in the visible (λ ~ 540 nm) spectral range.
This work was supported by CONACyT (Mexico) under Grant #32269-E. A.V.K. wishes to thank M.A.Noginov (USA), A.A.Kaminskii (Russia), N.N.Il’ichev (Russia), and L.A.Diaz-Torres (Mexico) for fruitful discussions.
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