Holmium doping concentration influence
on Ho:YAG crystal spectroscopic properties
Jan Šulca , Michal Němeca , David Vyhlídala , Helena Jelínkováa , Karel Nejezchlebb , Jan Polákb
a Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical EngineeringBřehová 7, 115 19 Prague 1, Czech Republicb Crytur, Ltd. Turnov, Na Lukách 2283, 511 01 Turnov, Czech Republic

ABSTRACT
We present the dependence of the main spectroscopic properties of Ho:YAG on Ho doping concentration estimated at room temperature. Five Ho:YAG crystals with various Ho-doping concentrations were grown by
Czochralski method: 0.34 at.% Ho/Y, 0.51 at.% Ho/Y, 0.90 at.% Ho/Y, 1.61 at.% Ho/Y, and 1.98 at.% Ho/Y.
Detailed absorption spectra with a high resolution were measured in the range from 185 up to 6500 nm. The
emission spectra were measured in the range from 500 up to 3500 nm under excitation in UV (450 nm) and IR(1862 nm) range. The 5 I7 upper-laser-level lifetime was measured using the confocal method. From the measured
transmission data, the absorption cross-sections in all investigated spectral ranges were determined. Using theabsorption spectrum data, the Judd-Ofelt analysis was performed to reach nett spontaneous radiative lifetime of
the 5 I7 level. Based on these data and measured fluorescence spectra, the emission cross-sections for 2.1 µm laserband were determined together with non-radiative relaxation rates. It was found that the Ho-doping concentration significantly influenced mainly the upper laser level lifetime, which drops from 7 ms for the lowest doping
to 5.6 ms for the 1.98 at.% doped Ho:YAG sample.
Keywords: Ho:YAG laser crystal, room temperature spectroscopy, laser crystal spectroscopy, mid-infrared laser,
Judd-Ofelt analysis, fluorescence decay time measurement.

1. INTRODUCTION
Diode-pumped solid state lasers, operating in the 2.1 µm region, are very attractive due to the overlapping of
their emission with absorption bands of molecules such as H2 O and CO2 . Such laser sources have a variety ofscientific, medical, and technical applications, including atmospheric remote sensing, high harmonic generation,
etc..1, 2 One possibility how to reach this spectral region is to use quasi-3-level holmium lasers operating at the5
I8 → 5 I7 transition in thulium co-doped crystals, enabling the direct 0.8 µm pumping using commercial laser
diodes. Efficient high-power Ho-laser operation based on this scheme was previously obtained in different hosts
such as YLF, YAP, LuAG, and GdVO4 .3–5 The main advantage of this solution is the availability of pumpradiation sources. On the other hand, the interaction of Tm3+ and Ho3+ ions presents some problems, limiting
the possibilities of this approach and designing a suitable active medium composition can be very complicated. In
addition, incomplete energy transfer from Tm3+ ionts (at room temperature 60 %) reduces short pulse generationand increases the thermal loading of Tm/Ho lasers.6 Absence of Tm3+ ions in the matrix simultaneously with
Ho3+ decreases deleterious energy transfers and leads to an increase in the Ho3+ upper laser 5 I7 lifetime.7, 8
Thanks to development of new powerful 2 µm laser source like Tm-doped fibres and laser diodes, the resonant
Ho3+ excitation directly to the 5 I7 is now possible.
Further author information: (Send correspondence to J.Š.)J.Š.: E-mail: jan.sulc@fjfi.cvut.cz, Tel.: +420 224 358 672, Fax: +420 222 512 735
M.N.: E-mail: michal.nemec@fjfi.cvut.cz, Tel.: +420 224 358 672, Fax: +420 222 512 735
D.V.: E-mail: david.vyhlidal@fjfi.cvut.cz, Tel.: +420 224 358 672, Fax: +420 222 512 735
H.J.: E-mail: helena.jelinkova@fjfi.cvut.cz, Tel.: +420 224 358 538, Fax: +420 222 512 735
K.N.: E-mail: karel.nejezchleb@crytur.cz, Tel.: +420 481 319 511, Fax: +420 481 322 323
J.P.: E-mail: jan.polak@crytur.cz, Tel.: +420 481 319 511, Fax: +420 481 322 323

Very promising Ho-doped material for resonant Ho3+ excitation is Ho:YAG. The low quantum defect of
Ho:YAG together with the excellent thermo-mechanical properties of YAG, it makes the heat load small andgives the potential for scaling to high output power. The Ho:YAG can be advantageously resonantly pumped
with Tm-based fiber lasers in the broad absorption band peaking at 1907 nm. Using such pump sources for thismaterial, efficient, tunable, Q-switched, and ultra-short-pulse lasers operating in 2.1 µm spectral region wererealized.1, 2, 9–12 Using such a scheme, Ho:YAG based thin-disc lasers were also realized.13, 14 Current status of
1.9 µm laser diode development will allow high-power operations of directly diode pumped Ho:YAG laser innear future.15–17 To develop such system and to optimize Ho:YAG crystal design, the detailed spectroscopic
informations are necessary.
In our work, we present the dependencies of Ho:YAG main spectroscopic properties (absorption and emission
cross-sections in the laser band, and upper-laser-level lifetime) on Ho3+ doping concentration in the range from
0.3 up to 2 at.% Ho/Y.

2. MATERIALS AND METHODS
2.1. Ho:YAG crystal samples
Five Ho:YAG crystals with various Ho-doping concentrations were grown by Czochralski method from high purity
raw materials. The concentration of Ho in the melt was following: 0.34 at.% Ho/Y, 0.51 at.% Ho/Y, 0.90 at.%
Ho/Y, 1.61 at.% Ho/Y, and 1.98 at.% Ho/Y. The samples for spectroscopy properties investigation were preparedin the form of polished wafers 2 – 5 mm thick with the diameter up to 55 mm. The samples were perfectly clear
free of any defects. The samples summary is presented in Table 1.
Table 1. List of tested Ho:YAG samples

Sample SN
Ho3+ doping
Ho3+ concentration
Thickness
Diameter[at. % Ho/Y][10−20 ×cm−3 ]
[cm]
[cm]

13237
0.34
0.472
0.48
5.4

13256
0.34
0.472
0.25
5.1

13286
0.51
0.708
0.25
5.3

13304
0.9
1.249
0.25
5.3

13332
1.61
2.234
0.30
5.3

13355
1.98
2.749
0.29
5.4

2.2. Methods of characterization
The spectroscopic properties of Ho:YAG crystal samples were investigated at room temperature. The transmission spectra of the samples in the range from 185 up to 1450 nm were measured using SHIMADZU UV-3600
spectrophotometer (sampling interval 0.1 nm, slit width 1 nm). In the range from 1430 nm up to 6500 nm ThermoScientific Nicolet IS5 FTIR spectrometer was used to measure the transmission spectra (resolution 0.8 cm−1 ,sampling 0.241 cm−1 , 32 scans per measurement). The fluorescence spectra in the spectral range from 400 up

Table 2. Parameters of excitation sources

Radiation source
Model
Wavelength
Pulse duration
Pulse energy
Pulse rep. rate
Spot size?

Fibre coupled LD
Necsel Blue448 nm?
14 ms
20 mJ
5 Hz
400 µmDPSSL
Tm:GdVO4
1863 nm?
10 ms
10 mJ
5 Hz
40 µm

Single emitter LD
DILAS BA-1908
1907 nm?
10 ms
10 mJ
5 Hz
100 µmOPO
EKSPLA NT252
690 – 2600 nm
3 ns
0.2 – 1 mJ
5 Hz/1 kHz]
35 µm

Laser diodes were temperature tuned. Tm:GdVO4 DPSSL was tuned by SiO2 Lyot filter.18]

Burst mode with repetition rate 5 Hz was used – 10 pulses in the burst.

to 6000 nm were measured. The fluorescence signal was collected perpendicularly to the excitation beam using
the following fiber-coupled spectrometers: StelarNet Black-Comet C-50 in the range 200 – 850 nm, Ocean Optics
NIR 512 in the range 850 – 1700 nm, StelarNet Red-Wave NIRX-25 in the range 1500 – 2250 nm, and Arcoptix
FTMIR-L1-060-2TE-R4 in the range 2000–6000 nm. The Ho3+ ions fluorescence decay was observed by an InGaAs photodiode (Thorlabs, FD05D). To block the rest of the excitation radiation, a bandpass filter (Thorlabs,
FB2250-500) was used. The signal from photodiodes was registered using an oscilloscope (Tektronix, TDS 3052B,
500 MHz, 5 GS/s). For fluorescence excitation, several laser sources were used: fibre coupled (ID 400 µm) InGaN
laser diode emitting at 448 nm (Necsel Blue 445, 10W), laboratory build diode pumped Tm:GdVO4 laser operating at 1863 nm (3 W); AlGaIn/AsSb laser diode operating at 1907 nm (DILAS BA-1908-1000-CM, 1 W, C-mout,
150 µm single emitter); tunable OPO laser system covering spectral range from 670 up to 2600 nm (EKSPLANT252, 1kHz, 1 W @ 750 nm). Parameters of the excitation sources are summarized in Table 2.

3. RESULTS AND DISCUSSION
3.1. Ho:YAG transmission spectra and absorption coefficient
The overview room-temperature transmission spectra of all tested samples were measured in the range from 185
up to 6500 nm. An example of raw recorded spectra for the highly-doped sample is shown in Figure 1. The presenceof Ho3+ ions is well manifested by characteristic absorption lines. The spectra also show the characteristic YAG
matrix cut-off in UV and IR spectral regions.

Figure 1. The raw transmission spectra of highly-doped Ho:YAG crystal sample (SN 13355) measured by Shimadzu
UV-3600 spectrophotometer and (b) Nicolet IS5 FTIR spectrometer.

The measured transmission spectra of Ho:YAG samples were corrected for Fresnel losses and using data from
Table 1 and Lambert-Beer’s law, the absorption coefficient was calculated. In Figure 2 we present for all samples
these results in the spectral area 1800 – 2200 nm, which is the most important for the Ho3+ laser operation at
2.1 µm. The Ho3+ absorption in this region corresponds to the transition from the ground state 5 I8 to the upper
2.1 µm laser level 5 I7 . The measured Ho:YAG absorption in this region has the maximum at ∼ 1907 nm. Thisabsorption line is about 5 nm wide. As it is demonstrated in the inset of Figure 2b, the absorption coefficient islinearly dependent on the nominal Ho doping concentration in the melt.

Figure 2. The absorption spectra of Ho:YAG for 5 I8 →5 I7 transition (a), the detail for the pumping region (b), and forthe laser emission region (c). The inset of figure (b) shows the linear dependence of absorption on Ho doping concentrationin the melt.

3.2. Ho:YAG fluorescence spectra
Figure 3 shows the normalized room temperature fluorescence lines of Ho:YAG observed in the spectral range
480 – 6000 nm under excitation by a 448 nm laser diode (5 I8 → 5 G6 transition). The spectra were measured
in the direction perpendicularly to the excitation beam which was propagating through the sample close to its
surface, but for higher doping concentration these results could be partially affected by a radiation trapping, as
it is mostly visible in Figure 3c. Otherwise, the Ho3+ doping concentration has a minor influence on Ho:YAG
emission spectra and we did not observe significant differences between samples differing in Ho-doping underavailable spectrometer resolution. Is should be also noted that the spectra were not corrected for the detector
sensitivity. The spectrum, which is most important for Ho3+ lasing at 2.1 µm line corresponding to 5 I7 → 5 I8transition is plotted with the best resolution in Figure 3d. The observed Ho:YAG fluorescence from the upper
2.1 µm laser level 5 I7 has the maximum at 2090 nm and lasts up to 2040 nm. Most of the peaks very well correspondto the absorption peaks observed in the corresponding absorption spectra (see Figure 2c). The longest observed
emission lines around 3000 nm corresponded to 5 I6 → 5 I7 transition of Ho3+ – see Figure 3e. The intenzity ofthese emission lines are 500-times lower in compassion wit peaks at 2090 nm.

3.3. Ho:YAG decay time measurement
The decay time of 2 µm Ho3+ fluorescence in dependence on Ho doping was investigated. The fluorescence decay
time was measured on a thin samples at room temperature using confocal imaging of the fluorescence emitting
spot to decrease the influence of fluorescence reabsorption and reemission on the measured fluorescence decay
time. We used this method successfully sooner for the Yb:YAG crystal samples investigation.19 The experimental
arrangement is shown in Figure 4. The collimated excitation radiation from a particular source, operating in thepulsed regime, is transmitted to the focusing lens L1 (achromatic doublet lens, f = 75 mm, diameter 50 mm,
Thorlabs A508-075-B). The tested laser crystal sample LC is placed in the focus. The emitted fluorescence light
was collected by the collimating lens L2 , and next focused by lens L3 (L2 and L3 are identical to L1 ). The rest of theFigure 3. The normalized fluorescence spectra of Ho:YAG depicting in detail all observed fluorescence lines under 448 nm
Ho:YAG excitation in the range 480 – 6000 nm. (a) – measured by StelarNet Black-Comet with resolution 1.5 nm (sampling
0.5 nm), (b) – measured by Ocean Optics NIR 512 with resolution 2.5 nm (sampling 2 nm), (c) – measured by StelarNet
Red-Wave with resolution 5 nm (sampling 2 nm), (d), (e) – measured by Arcoptix FTMIR with resolution 2 nm (sampling
0.5 nm), the y-scale od subfigure (e) is multiplied by 1000 in comparison with (d).

transmitted excitation radiation was blocked by filter F (IR band pass filter, Thorlabs BF2250-500). The pinhole
aperture A (clear aperture diameter 100 µm) was placed at the focal plane of L3 . This spatial filtering was used
to eliminate out-of-focus fluorescence radiation to reduce the laser crystal emitting volume. Behind the pinhole,
high speed, large active area, InGaAs PIN diode D was placed (Thorlabs FD05D). The photodiode together withthe pinhole and filter were placed on XYZ translation stage to reach the highest signal corresponding to theexact focal spot. The fluorescence intensity decay was recorded by digital oscilloscope (Tektronix, TDS 3052B,
500 MHz, 5 GS/s). The recorded data were fitted by single-exponential decay curve and by this procedure thedecay time of 5 I7 level was determined.
Fluorescence decay time in the spectral region 2 – 2.2 µm was measured for all six available samples (seeTable 1). For excitation, four various laser sources (see Table 2) and six particular wavelengths were used mostly
for all samples. All results are summarized in Figure 5a. The lifetime obtained from this measurement for theFigure 4. Measurement of fluorescence decay time using confocal method in low temperature. L1 – excitation radiation
focusing lens, LC – laser crystal, L2 – fluorescence radiation collimation lens, L3 – fluorescence radiation focusing lens, F– excitation radiation blocking filter, A – pinhole aperture, D – detector.

Figure 5. The measured 5 I7 level fluorescence decay time of Ho:YAG in dependence on the excitation source and Ho3+
concentration (a). The measured 5 I7 level fluorescence decay time of Ho:YAG sample with the highest Ho3+ -doping(SN 13355) in dependence on the absorption coefficient at noted excitation wavelength (b).

lowest Ho-doped Ho:YAG (0.34 at.% Ho/Y) estimated as an average of all measurements is (7.1 ± 0.1) ms. It
was found that the fluorescence decay time for this transition decreases with the increase of Ho3+ content in
the crystal, which can be caused by Ho3+ concentration quenching. We also found that the particular excitation
source parameters (emission wavelength, spot size in the focus) could significantly affect the results obtained by
the described method of fluorescence decay time measurement. This is probably caused by insufficient suppressionof the fluorescence reabsorption effect by used method. It can be seen that the reabsorption influence (decay
time prolongation) is stronger for bigger excitation spot size and smaller absorption of excitation radiation.
This increases the excited volume of Ho:YAG sample which increases the probability that reabsorption, leading
to decay time prolongation, occurs. The influence of excitation radiation absorption for fixed excitation spot
size was studied in more detail using OPO excitation of Ho:YAG sample with the highest Ho3+ -doping (SNFigure 6. Ho3+ 5 I7 level fluorescence decay time of Ho:YAG – comparison of measured data with literature (Zverev7120 ,Walsh0621 , Payne9222 , Wang1923 , Kwiatkowski1410 , Walsh1024 , Nabors9425 ).

13355). We used ten absorption peaks of Ho:YAG with absorption coefficient from 0.18 up to 2.85 cm−1 and wemeasured the corresponding fluorescence decay time. The results are shown in Figure 5b. One can see that theabsorption of excitation radiation strongly influences the measured fluorescence decay time. We can conclude
that the real lifetime of 5 I7 level will be even lower than what we have obtained with 1907 nm OPO excitation,
but the difference will be probably low. It is interesting to compare these data with the literature as can be seenin Figure 6.

3.4. Ho:YAG absorption cross-section and Judd-Ofelt analysis
The room-temperature absorption spectra of Ho:YAG samples were used to calculate Ho:YAG absorption crosssection just dividing by Ho3+ volumetric concentration (see Table 1). The final Ho:YAG absorption cross-section
spectrum, presented in Figure 7 together with the denotation of the upper states of the absorption transition
from ground state 5 I8 , was obtained as an average over particular samples spectra. In fact, not all spectra wereused for all observed absorption bands: due to a low signal-to-noise ratio, the low-doped samples (SN 13237,
13256, 13286) data were excluded for week bands absorption cross-section calculation (5 I5 , everything below
400 nm); due to signal saturation the highly-doped samples (SN 13332, 13355) were excluded for strong bands
absorption cross-section calculation (from 5 G5 up to 5 F5 ).

Figure 7. Absorption cross-section of Ho:YAG crystal at 300 K (upper states corresponding to absorption transition fromground state 5 I8 are marked).

Using the obtained absorption cross-section spectrum data, the Judd-Ofelt (J-O) analysis27, 28 was performed.
We follow the procedure described in detail by Hehlen et al.29 Fitting the calculated barycenter energies of 18
levels, we obtained electrostatic (F(2) = 420.9 ± 4.4 cm−1 , F(4) = 62.4 ± 1.4 cm−1 , F(6) = 7.1 ± 0.1 cm−1 )and spin-orbit (ζ = 2186 ± 6 cm−1 ) parameters. Using these data we calculated reduced-matrix elements fortensor operators U(k) and L + gS optimized for Ho:YAG (see Table 3). Finally, the J-O intensity parametersΩk (k = 2, 4, 6) for Ho:YAG were derived from the measured absorption spectra using the least-square fitting
approach based on minimizing of the relative root-mean-square deviation. For these calculations, 12 absorption
bands corresponding to Ho3+ ion transitions from ground state 5 I8 to higher levels were used (5 I7 , 5 I6 , 5 I5 , 5 F5 ,5
F4 +5 S2 , 5 G5 , 3 K(2)7 +5 G4 , 3 H(4)6 –5 G2 , 3 M10 –5 G4 , 1 L(2)8 –3 I(1)7 , 3 D3 , and 3 D4 ; transitions to levels 3 K(2)7 ,5F2 , 5 F3 , and 5 G6 were excluded from J-O analysis due to overlapping with hypersensitive transitions 5 I8 →5 G6 ).
The obtained intensity J-O parameters were: Ω2 = 0.55 ± 0.08 × 10−20 cm2 , Ω4 = 1.95 ± 0.03 × 10−20 cm2 , andΩ6 = 1.42 ± 0.03 × 10−20 cm2 (oscillator strengths RMS error 18 %). The experimental fexp and calculated fclc
oscillator strengths are presented in Table 3. Comparison of obtained intensity J-O parameters with the datafrom the literature is in Table 4. Based on the calculated J-O intensity parameters, the spontaneous radiative
lifetime τr for the 5 I7 → 5 I8 laser transition has been calculated to be τr = 7.86 ± 0.15 ms, which is in good
agreement with the corresponding radiative lifetime 7.7 ms experimentally found by Zverev et al.20

Table 3. Input data for J-O analysis of Ho:YAG: barycenter energies determined from the measured absorption crosssection spectra; refractive index of YAG26 ; set of optimized squared reduced-matrix elements for tensor operators U(k)and L + gS; experimental fexp and calculated fclc oscillator strengths.

Transition
5

Barycenter

Refr.

I8 →

[cm

]

index

¯ (2) ¯2
¯hU i¯

5

5232
8770
11269
15648
18570
21990
23910
25899
27923
34673
35871
39965
41389

1.802
1.813
1.819
1.829
1.837
1.849
1.856
1.865
1.874
1.913
1.921
1.953
1.965

0.02480.00880001.542700.00690.235100.0194
0
0

−1

I7
I6
5
I5
5
F5
5
S2 , 5 F4
3
K(2)7 , 5 F3 , 5 F2 , 5 G6
5G53
K(2)7 , 5 G4
3
H(4)6 – 5 G2
3
M10 – 5 G4
1
L(2)8 – 3 I(1)7
5
D3
5
D4
5

fexp

¯ (4) ¯2
¯hU i¯¯ (6) ¯2
¯hU i¯|hL + gSi|

[100.13350.03910.01140.41200.24250.8772
0.52350.04230.34160.2706
0.291700.27001.51200.69690.08790.56980.92070.87750.00020.06940.3636
0.05280.02500.02720.014623.2428000067.032400.29340.02160
7.4147
0
0

14.0
7.9
1.7
30.0
40.8
80.1
26.7
5.3
42.3
26.7
22.3
2.2
27.6

2

−6

fclc
]

[10−6 ]
18.9
10.3
1.8
28.0
36.9
159.0
27.7
5.8
42.1
24.8
38.5
1.9
28.1

Table 4. Ho:YAG Judd–Ofelt intensity parameters Ωλ and calculated spontaneous radiative lifetime τr for 5 I7 → 5 I8

Ω2 [10−20 cm2 ]
1.2
0.101
0.04
0.1
0.55
0

Ω4 [10−20 cm2 ]
5.29
2.086
2.67
2.59
1.95
1.83
∗

Ω6 [10−20 cm2 ]
1.48
1.724
1.89
1.48
1.42
1.54

τr [ms]
6.75
7.82
6.36
7.5
7.87
7.53

References
Kaminskii30
Walsh et al.21
Malinowsk et al.31
Ryabochkina et al.32
this work
this work∗

Results, if transitions to levels 3 K(2)7 , 5 F2 , 5 F3 , and 5 G6were incorporated in the J-O analysis.

3.5. Ho:YAG 5 I7 → 5 I8 emission cross-section
The normalized averaged florescence spectrum corresponding to the 5 I7 → 5 I8 transition was used to calculate
the emission cross-section spectrum σe (λ) in 2.1 µm laser emission band with the help of Füchtbauer-Ladenburg
equation 33
λ5
I(λ)
R
σe (λ) =,(1)
8πfj τr cn2 band I(λ)λdλwhere c is the speed of light in vacuum, I(λ) is the fluorescence intensity at wavelength λ, and fj is fraction of
the pumped population in the particular upper state j given by the Boltzmann distribution. This Boltzmann
factor was set to 1. As the radiative lifetime τr , value 7.87 ms obtained from J-O analysis, was utilized as a best
estimate. The used value of refractive index n was 1.802. The result is shown in Figure 8. The observed Ho:YAG
emission cross-section peaks at 2091 nm. For comparison, the data calculated from the absorption cross-section
using the reciprocity method based on McCumber theory34 are shown in the same graph (together with theabsorption cross-section) and good agreement was obtained. As an approximation of the net thermodynamic freeenergy, required for the reciprocity method, the value 5151 cm−1 was used.

Figure 8. The room-temperature emission cross-section corresponding to 5 I7 → 5 I8 transition of Ho3+ for Ho:YAG
calculated using the Füchtbauer-Ladenburg formula and reciprocity method. Absorption cross-section is included.

4. CONCLUSION
Spectroscopic properties of Ho:YAG samples the influence of Ho3+ doping concentration (from 0.3 up to 2 at.%)were investigated at room temperature. From the measured transmission spectra, the absorption coefficient andabsorption cross-section spectra were calculated. It was found, that absorption coefficient is linearly dependent
on nominal Ho3+ doping. In the spectral area 1800 – 2200 nm, which is the most important for the Ho3+
laser operation at 2.1 µm, the measured Ho:YAG absorption has the maximum at ∼ 1907 nm. This absorption
line is about 5 nm wide (FWHM) which is enough for diode pumping. Using the calculated absorption crosssection spectrum data, the Judd-Ofelt analysis was performed.The obtained intensity J-O parameters were:Ω2 = 0.55 ± 0.08 × 10−20 cm2 , Ω4 = 1.95 ± 0.03 × 10−20 cm2 , and Ω6 = 1.42 ± 0.03 × 10−20 cm2 , which is mostly
in good agreement wit data published in literature. Based on the calculated J-O intensity parameters, thespontaneous radiative lifetime τr for the 5 I7 → 5 I8 laser transition has been estimated to be τr = 7.86 ± 0.15 ms.
The corresponding 2.1 µm fluorescence decay is single-exponential was also measured experimentally. It was
found, that the fluorescence at 2.1 µm is concentration quenched and drops from 7 ms for the lowest doping to
5.6 ms for the highly doped Ho:YAG sample. Also the fluorescence spectra of all investigated were measured andused for emission cross-section calculation in 2.1 µm laser band.
We can conclude, that the Ho3+ -doping concentration significantly influences mainly the upper laser level
lifetime, which drops from 7 ms to 5.6 ms for the highly doped Ho:YAG sample. However, for this sample theabsorption coefficient reached a value close to 3 cm−1 which is promising for construction of diode pumped
thin-disk Ho:YAG laser.

ACKNOWLEDGEMENTS
This research was supported by Technology Agency of the Czech Republic, project FW01010219: Multicomponent
single crystal materials for solid state lasers.

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