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Optics Express

  • Editor: Michael Duncan
  • Vol. 10, Iss. 16 — Aug. 12, 2002
  • pp: 832–839
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Powerful visible (530–770 nm) luminescence in Yb,Ho:GGG with IR diode pumping

A.V. Kir’yanov, V. Aboites, A.M. Belovolov, M.I. Timoshechkin, M.I. Belovolov, M.J. Damzen, and A. Minassian  »View Author Affiliations


Optics Express, Vol. 10, Issue 16, pp. 832-839 (2002)
http://dx.doi.org/10.1364/OE.10.000832


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Abstract

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

1. Introduction

In recent years, Gadolinium Gallium Garnet (Gd3Ga5O12, - GGG) has been the subject of interest as an alternative host for Yb [6–8

6. M.I. Belovolov, E.M. Dianov, M.I. Timoschechkin, LV. Barashov, A. M. Belovolov, M.A. Ivanov, N.P. Morozov, A.M. Prokhorov, and K.M. Timoschechkin, “Room temperature CW Yb:GGG laser operation at 1,038-μm,” Conference on Lasers and Electro-Optics Europe1996, Technical Digest, p.43.

]. 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 [9

9. R. Walti, W. Luthy, H.P. Weber, S.Ya. Rusanov, A.A. Yakovlev, A.I. Zagumennyi, I.A. Shcherbakov, and A.F. Umyskov, “Yb3+ / Ho3+ energy exchange mechanisms in Yb,Ho:YAG crystals for 2 μm or 540 nm lasing,” J. Quant. Spectrosc. Radiat. Transfer, 54, 671–681 (1995).

].

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

Fig. 1. Absorption spectra of Yb,Ho:GGG at room temperature. (a) overall view; (b,c) – insets for UV and visible / near-IR spectral ranges, respectively.

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 (DILASDiodenlaser 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 results of luminescence measurements are shown in Figs.2 and 3.

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+.

Fig. 2. Main luminescence band (Yb3+) of Yb,Ho:GGG (T = 300 K). Pump -17W(λ ≈ 938 nm.

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 [12

12. A. Kaminskii, Crystalline Lasers: Physical Properties and Operation Schemes (CRC, Boca-Raton, FL, 1996).

] (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+ [11

11. E. Osiak, I. Sokolska, and S. Kuck, “Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,” J. of Alloys & Compounds , 323–324, 283–287 (2001). [CrossRef]

] and competition between linear decay and up-conversion for the depletion of the intermediate excited states [13

13. M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, and M.P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B , 61, 3337–3346 (2000). [CrossRef]

]. 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 [14

14. W. Ryba-Romanowski, P.J. Deren, S. Golab, and G. Dominik-Dzik, “Conversion of red light into green light in LiTa3:Ho,” J. of Appl. Phys. , 88, 6078–6080 (2000). [CrossRef]

].

Fig. 3. Luminescence spectra of Yb,Ho:GGG in visible (T = 300 K, Pump at λ ≈ 938 nm). Curves 1–3 correspond to different levels of pump -4(1), 12.5 (2), and 17 (3) W.

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

15. E. Montoya, O. Espeso, and L.E. Bausa, “Cooperative luminescence in Yb3+:LiNbO3,” J. of Luminescence , 87–89, 1036–1038(2000). [CrossRef]

,16

16. M.A. Noginov, G.B. Loutts, C.S. Steward, B.D. Lucas, D. Fider, V. Peters, E. Mix, and G. Huber, “Spectroscopic study of Yb doped oxide crystals for intrinsic optical bistability,” J. of Luminescence , 96, 129–140(2002). [CrossRef]

] 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

9. R. Walti, W. Luthy, H.P. Weber, S.Ya. Rusanov, A.A. Yakovlev, A.I. Zagumennyi, I.A. Shcherbakov, and A.F. Umyskov, “Yb3+ / Ho3+ energy exchange mechanisms in Yb,Ho:YAG crystals for 2 μm or 540 nm lasing,” J. Quant. Spectrosc. Radiat. Transfer, 54, 671–681 (1995).

,11

11. E. Osiak, I. Sokolska, and S. Kuck, “Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,” J. of Alloys & Compounds , 323–324, 283–287 (2001). [CrossRef]

,14

14. W. Ryba-Romanowski, P.J. Deren, S. Golab, and G. Dominik-Dzik, “Conversion of red light into green light in LiTa3:Ho,” J. of Appl. Phys. , 88, 6078–6080 (2000). [CrossRef]

,17–22

17. R.A. Hewes and J.F. Sarver, “Infrared excitation processes for the visible luminescence of Er3+, Ho3+, and Tm3+ in Yb3+-sensitized rare-earth trifluorides,” Phys. Rev. , 182, 427–436 (1969). [CrossRef]

], 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 [11

11. E. Osiak, I. Sokolska, and S. Kuck, “Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,” J. of Alloys & Compounds , 323–324, 283–287 (2001). [CrossRef]

] (compare with Fig.6 where data of Ref.11

11. E. Osiak, I. Sokolska, and S. Kuck, “Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,” J. of Alloys & Compounds , 323–324, 283–287 (2001). [CrossRef]

are shown).

Fig. 4. Dependencies of up-converted green and near-IR emission intensity in Yb,Ho:GGG on pump (λ ≈ 938 nm) power. Inset – the dependencies for the most intense spectral peaks in double-logarithm scale.

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 5F35I8 and 5I65I8 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

11. E. Osiak, I. Sokolska, and S. Kuck, “Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,” J. of Alloys & Compounds , 323–324, 283–287 (2001). [CrossRef]

).

Fig. 5. Energy level diagram explaining visible emission in Yb,Ho:GGG at stepwise up-conversion in Yb3+-Ho3+ mixture system.
Fig. 6. Emission spectra of Ho3+:BaY2F8 crystal at room temperature excited at λ = 888nm [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. Kinetics of green luminescence at λ = 538.3 nm (Ho3+) under short pump pulse (50μs) excitation in Yb,Ho:GGG crystal . 1 – Excitation pulse; 2 – Green luminescence signal (triangles – experimental data; solid line – their approximation by bi-exponential function).

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

21. W. Ryba-Romanowski, S. Golab, G. Dominik-Dzik, and P. Solarz, “Conversion of infrared into red emission in YVO4:Yb,Ho,” Appl. Phys. Lett. , 79, 3026–3028 (2001). [CrossRef]

,22

22. E. Osiak, I. Sokolska, and S. Kuck, “Avalanche-like mechanisms and up-conversion processes under infrared pumping in Ho3+,Yb3+:YAlO3,” J. of Luminescence , 94–95, 289–292 (2001). [CrossRef]

] 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

21. W. Ryba-Romanowski, S. Golab, G. Dominik-Dzik, and P. Solarz, “Conversion of infrared into red emission in YVO4:Yb,Ho,” Appl. Phys. Lett. , 79, 3026–3028 (2001). [CrossRef]

for Yb,Ho:YVO4, where similar case of up-conversion is realized.

5. Conclusion

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.

6. Acknowledgments

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.

References and links

1.

C. Stewen, M. Larionov, A. Giesen, and K. Contag, “Yb:YAG thin disk laser with 1 kW output power,” Advanced Solid-State Lasers , H. Injeyan, U. Keller, and C. Marshall, eds., Vol.34 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C.2000), pp. 35–41.

2.

E.C. Honea, RJ. Beach, S.C. Mitchell, J.A. Skidmore, M.A. Emanuel, S.B. Sutton, S.A. Payne, P.V. Avizonis, R.S. Monroe, and D.G. Harris, “Dual-rod Yb:YAG laser for high-power and high-brightness applications,” Advanced Solid-State Lasers ,H. Injeyan, U. Keller, and C. Marshall, eds., Vol.34 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C.2000), pp. 42–47.

3.

L.F. Johnson and H.J. Guggenheim, “Infrared-pumped visible laser,” Appl. Phys. Lett. , 19, 44–47 (1971). [CrossRef]

4.

Th. Rotacher, W. Luthy, and H.P. Weber, “Diode pumping and laser properties of Yb:Ho:YAG”, Optics Commun. , 155, 68–72 (1998). [CrossRef]

5.

I.R. Martin, V.D. Rodriguez, V. Lavin, and U.R. Rodriguez-Medoza, “Upconversion dynamics in Yb3+- Ho3+-doped fluoroindate glasses,” J. of Alloys & Compounds , 275–277, 345–348 (2001).

6.

M.I. Belovolov, E.M. Dianov, M.I. Timoschechkin, LV. Barashov, A. M. Belovolov, M.A. Ivanov, N.P. Morozov, A.M. Prokhorov, and K.M. Timoschechkin, “Room temperature CW Yb:GGG laser operation at 1,038-μm,” Conference on Lasers and Electro-Optics Europe1996, Technical Digest, p.43.

7.

M. Shimokozono, N. Sugimoto, A. Tate, Y. Katoh, M. Tanno, S. Fukuda, and T. Ryouh, “Room-temperature operation of an Yb-doped Gd3Ga5O12 buried channel waveguide laser at 1.025 μm wavelength,” Appl. Phys. Lett. , 68, 2177–2179 (1996). [CrossRef]

8.

S. Chenais, F. Druon, F. Balembois, P. Georges, A. Brun, A. Brenier, and G. Boulon, “Diode-pumped operation of Yb:GGG laser”, Conference on Lasers and Electro-Optics 2001, Technical Digest, pp.170–171.

9.

R. Walti, W. Luthy, H.P. Weber, S.Ya. Rusanov, A.A. Yakovlev, A.I. Zagumennyi, I.A. Shcherbakov, and A.F. Umyskov, “Yb3+ / Ho3+ energy exchange mechanisms in Yb,Ho:YAG crystals for 2 μm or 540 nm lasing,” J. Quant. Spectrosc. Radiat. Transfer, 54, 671–681 (1995).

10.

M. Henke, J. Persson, and S. Kuck, “Preparation and spectroscopy of Yb2+- doped Y3Al5O12, YAlO3, and LiBaF3,” J. of Luminescence , 87–89, 1049–1051 (2000). [CrossRef]

11.

E. Osiak, I. Sokolska, and S. Kuck, “Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,” J. of Alloys & Compounds , 323–324, 283–287 (2001). [CrossRef]

12.

A. Kaminskii, Crystalline Lasers: Physical Properties and Operation Schemes (CRC, Boca-Raton, FL, 1996).

13.

M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, and M.P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B , 61, 3337–3346 (2000). [CrossRef]

14.

W. Ryba-Romanowski, P.J. Deren, S. Golab, and G. Dominik-Dzik, “Conversion of red light into green light in LiTa3:Ho,” J. of Appl. Phys. , 88, 6078–6080 (2000). [CrossRef]

15.

E. Montoya, O. Espeso, and L.E. Bausa, “Cooperative luminescence in Yb3+:LiNbO3,” J. of Luminescence , 87–89, 1036–1038(2000). [CrossRef]

16.

M.A. Noginov, G.B. Loutts, C.S. Steward, B.D. Lucas, D. Fider, V. Peters, E. Mix, and G. Huber, “Spectroscopic study of Yb doped oxide crystals for intrinsic optical bistability,” J. of Luminescence , 96, 129–140(2002). [CrossRef]

17.

R.A. Hewes and J.F. Sarver, “Infrared excitation processes for the visible luminescence of Er3+, Ho3+, and Tm3+ in Yb3+-sensitized rare-earth trifluorides,” Phys. Rev. , 182, 427–436 (1969). [CrossRef]

18.

L. Esterowitz, J. Noonan, and J. Bahler, “Enhancement in a Ho3+ - Yb3+ quantum counter by energy transfer,” Appl. Phys. Lett. , 10, 126–127 (1967). [CrossRef]

19.

A. Diening and S. Kuck, “Spectroscopy and diode-pumped laser oscillation of Yb3+,Ho3+-doped yttrium scandium gallium garnet,“ J. of Appl. Phys. , 87, 4063–4068 (2000). [CrossRef]

20.

L.F. Johnson and H.J. Guggenheim, “Infrared-pumped visible laser,” Appl. Phys. Lett. , 19, 44–47 (1971). [CrossRef]

21.

W. Ryba-Romanowski, S. Golab, G. Dominik-Dzik, and P. Solarz, “Conversion of infrared into red emission in YVO4:Yb,Ho,” Appl. Phys. Lett. , 79, 3026–3028 (2001). [CrossRef]

22.

E. Osiak, I. Sokolska, and S. Kuck, “Avalanche-like mechanisms and up-conversion processes under infrared pumping in Ho3+,Yb3+:YAlO3,” J. of Luminescence , 94–95, 289–292 (2001). [CrossRef]

OCIS Codes
(140.5680) Lasers and laser optics : Rare earth and transition metal solid-state lasers
(160.2540) Materials : Fluorescent and luminescent materials
(160.5690) Materials : Rare-earth-doped materials
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence

ToC Category:
Research Papers

History
Original Manuscript: June 10, 2002
Revised Manuscript: August 1, 2002
Published: August 12, 2002

Citation
Alexander Kir'yanov, V. Aboites, A. Belovolov, M. Timoshechkin, M. Belovolov, M. Damzen, and A. Minassian, "Powerful visible (530�??770 nm) luminescence in Yb,Ho:GGG with IR diode pumping," Opt. Express 10, 832-839 (2002)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-10-16-832


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References

  1. C.Stewen, M.Larionov, A.Giesen, and K.Contag, �??Yb:YAG thin disk laser with 1 kW output power,�?? Advanced Solid-State Lasers, H.Injeyan, U.Keller, and C.Marshall, eds., Vol. 34 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C. 2000), pp. 35-41.
  2. E.C.Honea, R.J.Beach, S.C.Mitchell, J.A.Skidmore, M.A.Emanuel, S.B.Sutton, S.A.Payne, P.V.Avizonis, R.S.Monroe, and D.G.Harris, �??Dual-rod Yb:YAG laser for high-power and high-brightness applications,�?? Advanced Solid-State Lasers, H.Injeyan, U.Keller, and C.Marshall, eds., Vol. 34 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C. 2000), pp. 42-47.
  3. L.F.Johnson and H.J.Guggenheim, �??Infrared-pumped visible laser,�?? Appl. Phys. Lett. 19, 44-47 (1971). [CrossRef]
  4. Th. Rotacher, W. Luthy, and H.P. Weber, �??Diode pumping and laser properties of Yb:Ho:YAG,�?? Opt. Commun. 155, 68-72 (1998). [CrossRef]
  5. I.R. Martin, V.D. Rodriguez, V. Lavin, and U.R. Rodriguez-Medoza, �??Upconversion dynamics in Yb3+- Ho3+- doped fluoroindate glasses,�?? J. of Alloys Compounds 275-277, 345-348 (2001).
  6. M.I. Belovolov, E.M. Dianov, M.I. Timoschechkin, L.V. Barashov, A.M. Belovolov, M.A. Ivanov, N.P. Morozov, A.M. Prokhorov, and K.M. Timoschechkin, �??Room temperature CW Yb:GGG laser operation at 1,038-µm,�?? Conference on Lasers and Electro-Optics Europe 1996, Technical Digest, p. 43.
  7. M. Shimokozono, N. Sugimoto, A. Tate, Y. Katoh, M. Tanno, S. Fukuda, and T. Ryouh, �??Room-temperature operation of an Yb-doped Gd3Ga5O12 buried channel waveguide laser at 1.025 µm wavelength,�?? Appl. Phys. Lett. 68, 2177-2179 (1996). [CrossRef]
  8. S. Chenais, F. Druon, F. Balembois, P. Georges, A. Brun, A. Brenier, and G. Boulon, �??Diode-pumped operation of Yb:GGG laser�??, Conference on Lasers and Electro-Optics 2001, Technical Digest, pp. 170-171.
  9. R. Walti, W. Luthy, H.P. Weber, S. a.Rusanov, A.A. Yakovlev, A.I. Zagumennyi, I.A. Shcherbakov, and A.F. Umyskov, �??Yb3+ / Ho3+ energy exchange mechanisms in Yb,Ho:YAG crystals for 2 µm or 540 nm lasing,�?? J. Quant. Spectrosc. Radiat. Transfer 54, 671-681 (1995).
  10. M. Henke, J. Persson, and S. Kuck, �??Preparation and spectroscopy of Yb2+- doped Y3Al5O12, YAlO3, and LiBaF3,�?? J. Luminescence 87-89, 1049-1051 (2000). [CrossRef]
  11. E. Osiak, I. Sokolska, and S. Kuck, �??Upconversion-induced blue, green and red emission in Ho3+:BaY2F8,�?? J. Alloys Compounds 323-324, 283-287 (2001). [CrossRef]
  12. A. Kaminskii, Crystalline Lasers: Physical Properties and Operation Schemes (CRC, Boca-Raton, FL, 1996).
  13. M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, and M.P. Hehlen, �??Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,�?? Phys. Rev. B 61, 3337-3346 (2000). [CrossRef]
  14. W. Ryba-Romanowski, P.J. Deren, S. Golab, and G. Dominik-Dzik, �??Conversion of red light into green light in LiTa3:Ho,�?? J. of Appl. Phys. 88, 6078-6080 (2000). [CrossRef]
  15. E. Montoya, O. Espeso, and L.E. Bausa, �??Cooperative luminescence in Yb3+:LiNbO3,�?? J. Luminescence 87-89, 1036-1038 (2000). [CrossRef]
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