1. Laser operation in Nd:Sc2SiO5 crystal based on
transition 4
F3/2→4
I9/2 of Nd3+
ions
X. Li,1,2
G. Aka,3,*
L. H. Zheng,2,3,4
J. Xu,2,5
and Q. H. Yang1
1
Department of Electronic Information Materials, Shanghai University, Shanghai 200444, China
2
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 588 Heshuo Road, Shanghai 201899, China
3
Laboratoire de Chimie de la Matière Condensée de Paris, ENSCP, 11 Rue P. et M. Curie, 75231 Paris Cedex 5,
France
4
zhenglihe@gmail.com
5
xujun@mail.shcnc.ac.cn
*gerard-aka@chimie-paristech.fr
Abstract: Laser operation based on energy transition 4
F3/2→4
I9/2 of Nd3+
ions in 1at.% Nd:Sc2SiO5 (Nd:SSO) crystal is reported. By using output
coupler of Toc = 2.5% and 808 nm laser diode pump source, laser operation
at 914 nm was preliminarily obtained with output power of 581 mW.
©2014 Optical Society of America
OCIS codes: (140.3480) Lasers, diode-pumped; (160.3380) Laser materials.
References and links
1. G. Aka, D. Vivien, and V. Lupei, “Site-selective 900,” Appl. Phys. Lett. 85(14), 2685–2687 (2004).
2. S. F. A. Kettle, Physical Inorganic Chemistry: a Coordination Chemistry Approach (Spektrum Academic
Publishers, 1996), p. 163.
3. L. H. Zheng, P. Loiseau, and G. Aka, “Diode-pumped laser operation at 1053 and 900 nm in
Sr1−xLax−yNdyMgxAl12−xO19 (Nd:ASL) single crystal,” Laser Phys. 23(9), 095802(6pp) (2013).
4. P. B. W. Burmester, T. Kellner, E. Heumann, G. Huber, R. Uecker, and P. Reiche, “Blue laser emission at 465
nm by type-I noncritically phase-matched second harmonic generation in Gd1- xYxCa4O(BO3)3,” Laser Phys. 10,
441–443 (2000).
5. V. Lupei, N. Pavel, and T. Taira, “Highly efficient continuous-wave 946-nm Nd:YAG laser emission under
direct 885-nm pumping,” Appl. Phys. Lett. 81(15), 2677–2679 (2002).
6. L. H. Zheng, J. Xu, L. B. Su, H. J. Li, Q. G. Wang, W. Ryba-Romanowski, R. Lisiecki, and F. Wu, “Estimation
of low-temperature spectra behavior in Nd-doped Sc2SiO5 single crystal,” Opt. Lett. 34(22), 3481–3483 (2009).
7. L. D. DeLoach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Evaluation of
absorption and emission properties of Yb3+
doped crystals for laser applications,” IEEE J. Quantum Electron.
29(4), 1179–1191 (1993).
8. L. H. Zheng, G. Aka, A. Ikesue, Y. L. Aung, P. Loiseau, and J. Xu, “Blue laser generated in Nd:YAG core
ceramics composites,” 9th Laser Ceramics Symposium, Daejeon, Korea, Dec. 02–06 (2013).
9. G. A. Kumar, L. R. Lu, A. A. Kaminskii, K. Ueda, H. Yagi, T. Yanagitani, and N. V. Unnikrishnan,
“Spectroscopic and stimulated emission characteristics of Nd3+
in transparent YAG ceramics,” IEEE J. Quantum
Electron. 40(6), 747–758 (2004).
10. T. Taira, “RE3+
-Ion-Doped YAG ceramic lasers,” IEEE J. Sel. Top. Quant. 13(3), 798–809 (2007).
11. M. Pokhrel, N. Ray, G. A. Kumar, and D. K. Sardar, “Comparative studies of the spectroscopic properties of
Nd3+
: YAG nanocrystals, transparent ceramic and single crystal,” Opt. Mater. Express 2(3), 235–249 (2012).
12. M. R. Gaume, Ph.D. thesis, Universite Pierre et Marie Curie-Paris VI, Paris (2002).
1. Introduction
Diode pumped laser devices based on neodymium-doped laser materials operating on energy
transition 4
F3/2→4
I9/2 of Nd3+
ions is a promising approach to meet the growing demands for
biology, medical detection, optical sensor and communication [1]. Moderate energy gap
between the 4
F3/2 level and the fundamental 4
I9/2 level in Nd3+
ions is crucial in selecting
appropriate laser hosts. The terminal laser energy level should be high enough to avoid strong
re-absorption owing to the thermal population of the Stark levels. Various factors influencing
the positions of the electronic levels are taken into accounts. Firstly, hosts with ionic matter
presenting a weak nephelauxetic effect are preferred. Secondly, oxides or fluorides matrices
containing low covalence and high electron-electron interactions are beneficial with increased
nephelauxetic effect and anionic co-ordination number due to long distance of Nd – O [2].
Thirdly, Nd doped matrices with moderate crystal field splitting could limit the competitive
#199523 - $15.00 USD Received 15 Oct 2013; revised 12 Jan 2014; accepted 13 Jan 2014; published 14 Feb 2014
(C) 2014 OSA 1 February 2014 | Vol. 4, No. 2 | DOI:10.1364/OME.4.000458 | OPTICAL MATERIALS EXPRESS 458

2. splitting between fundamental and excited state levels. Finally but not the least, suitable
thermal conductivity and moderate splitting on energy level 4
F3/2 are advantageous.
Laser operations based on transition 4
F3/2→4
I9/2 of Nd3+
ions have been investigated in
aluminum hosts [3–5]. In this letter, we report a diode pumped laser operation at 914 nm in
monoclinic silicate family of neodymium doped scandium silicate (Nd:Sc2SiO5, Nd:SSO)
crystal.
2. Gain cross section on energy transition 4
F3/2→4
I9/2 of Nd3+
ions in Nd:SSO crystal
1 at.% Nd:SSO single crystal was grown by Czochralski technique. The orientation of the
crystal used for the absorption and emission spectra measurement is b-cut sample parallel to
Y axis where b represents for crystallographic axis and Y represents for optical indicatrix
axis. Branching ratio for energy transition 4
F3/2→4
I9/2 of Nd3+
ions with central wavelength at
914 nm was calculated to be 30% by Judd-Ofelt theory [6]. The room temperature absorption
spectra were carefully measured with light changeover at 861 nm in order to get rid of
covering the band signals of the related energy transfer. The absorption cross section σabs and
emission cross section σem is shown in Fig. 1. The absorption cross section σabs at 803 nm was
calculated to be 1.69 × 10−20
cm2
and that at 808 nm was calculated to be 0.95 × 10−20
cm2
.
The stimulated emission cross section σem at 914 nm was estimated to be 1.13 × 10−20
cm2
by
using the Füchtbauer-Ladenburg equation [6].
Fig. 1. Room-temperature absorption and emission cross section in Nd:SSO crystal.
Gain cross section, denoted as σg and measured in units of area, is an important parameter
in a laser design and operation. σg determines the transition populations from the upper levels
to lower levels caused by a particular flux of photons. σg can be estimated with spectroscopic
measurements and fluorescence lifetime. The gain cross section could be given by Eq. (1) [7].
(1 )g em absσ β σ β σ= × − − × (1)
It can be noted that β is the inversion coefficient defined as the population ratio on 4
F3/2
level over the total Nd3+
ion population densities. Figure 2 shows the gain cross section σg
around 914 nm obtained for different inversion ratio β. For β = 1, gain cross-section σg is
equal to the emission cross section σem. It can be indicated from Fig. 2 that laser inversion at
914 nm may happen at β = 0.25 with obtained σg of 1.42 × 10−21
cm2
.
#199523 - $15.00 USD Received 15 Oct 2013; revised 12 Jan 2014; accepted 13 Jan 2014; published 14 Feb 2014
(C) 2014 OSA 1 February 2014 | Vol. 4, No. 2 | DOI:10.1364/OME.4.000458 | OPTICAL MATERIALS EXPRESS 459

3. Fig. 2. Gain cross section of energy transition 4
F3/2→4
I9/2 in Nd:SSO crystal.
3. Spectroscopic parameters of energy transition 4
F3/2→4
I9/2 of Nd3+
in Nd:SSO crystal
Laser performance at 900 nm has been reported in Nd doped Sr1-xLaxMgxAl12-xO19 (Nd:ASL)
crystal due to the favorable spectroscopic parameters [3]. Laser operation at 946 nm in
Nd:YAG core ceramics composites has recently been realized [8]. Frequency doubling blue
laser based on the transition 4
F3/2→4
I9/2 of Nd3+
ions has been demonstrated in Nd:ASL crystal
and Nd:YAG ceramics [1,8]. Infrared laser emission based on energy transition 4
F3/2→4
I9/2 of
Nd3+
ions in SSO crystal was performed. Accordingly, the spectroscopic parameters in
Nd:SSO crystal are compared with those in Nd:ASL crystal and Nd:YAG ceramics as shown
in Table 1.
Table 1. Spectroscopic parameters of energy transition 4
F3/2→4
I9/2 of Nd ions in Nd:SSO
crystal and Nd:ASL crystal, Nd:YAG ceramics.
Host
X =
Ω4/Ω6
β 4
F3/2→4
I9/2
ΔE
cm−1
Thermal
Conductivity
W·m−1
·K−14
I9/2
4
I11/2
τexp
(μs)
σem ×
10−20
cm2
τexp × σem
(μs ×
10−20
cm2
)
Nd:SSO crystal [6]
0.3 0.30 0.57 215 1.16 249 526 4.1
Nd:ASL crystal [3]
0.95 0.45 0.45 380 0.23 87 553 4.4(c); 6.3(a)
Nd:YAG
ceramics [9–11] 0.67 0.37 0.51 250 9.32 2330 851 9
As shown in Table 1, the energy Stark splitting in Nd:SSO crystal is 526 cm−1
and the
value of τexp × σem is 2.49 × 10−18
μs·cm2
. The value of energy Stark splitting in Nd:ASL
crystal is 553 cm−1
but the value of τexp × σem is 8.7 × 10−19
μs.cm2
which is lower than that in
Nd:SSO crystal. The energy Stark splitting in Nd:YAG ceramic is the largest (851 cm−1
)
while the value of τexp × σem (2.33 × 10−17
μs.cm2
) is the largest [9,10]. Nd:SSO crystal
possess moderate value of τexp × σem when compared with those in Nd:ASL crystal and
Nd:YAG ceramic, while the energy splitting is close to that in Nd:ASL crystal. Taking
thermal property into account, SSO crystal possess the advantage of minus thermal-optics
coefficient (dn/dT = −6.3 × 10−6
K−1
) which is beneficial for releasing thermal lens effect [12].
With the favorable thermal conductivity (4.1 W·m−1
·K−1
) and moderate splitting (526 cm−1
) in
Nd:SSO crystal, efficient laser performance on energy transition 4
F3/2→4
I9/2 of Nd3+
ions in
SSO crystal can be expected.
#199523 - $15.00 USD Received 15 Oct 2013; revised 12 Jan 2014; accepted 13 Jan 2014; published 14 Feb 2014
(C) 2014 OSA 1 February 2014 | Vol. 4, No. 2 | DOI:10.1364/OME.4.000458 | OPTICAL MATERIALS EXPRESS 460

4. 4. Laser operation at 914 nm in Nd:SSO crystal
Laser setup for energy transition 4
F3/2→4
I9/2 in Nd:SSO crystal is shown in Fig. 3. Fiber
coupled laser diode (LIMO) with a numerical aperture of 0.22 and core diameter of 100 μm
was used as pump source with maximum output power of 35 W, where central emitting
wavelength could be tuned by a temperature controller. The coupling optics consists of two
identical plano - convex lenses with focal lengths of 100 mm used to reimage the pump beam
into the laser crystal at a ratio of 1:1. The resonator was consisted of an input coupler M1
(plane), Nd:SSO crystal and output coupler M2 (BK7). M1 is coated with HT @ 800 nm -
810 nm & 1083 nm and HR @ 914 nm. M2 is coated with HR @ 800 nm - 810 nm & Toc =
2.5% @ 914 nm with radius curvature of 100 mm. 1.0 at.% Nd:SSO sample with aperture of 5
mm × 5 mm and length of 10 mm was polished. The orientation of the polished crystal is b-
cut. The propagation of the light is along b direction. Filter RG850 (SHOTT company) was
used to cut the pump power left after the laser crystal. To further remove the generated heat
during laser oscillation, the Nd:SSO crystal was wrapped with indium foil and mounted in a
water-cooled copper heat sink. Temperature was controlled at 5 °C using a thermo-coupler
device.
Fig. 3. Laser setup for transition 4
F3/2→4
I9/2 in Nd:SSO crystal.
Fig. 4. Laser performance at 1083 nm in Nd:SSO crystal with different TOC.
Before moving to laser operation at 914 nm in Nd:SSO crystal, laser behavior at 1083 nm
on the 4
F3/2 → 4
I11/2 transition were recorded with different transparency of output coupler TOC
= 2%, TOC = 6% and TOC = 10% as shown in Fig. 4. The cavity mirrors were replaced with
coatings covering 1083nm accordingly. The highest output power of 2.54 W and slope
efficiency of 33.3% was obtained with TOC = 10% and absorbed pump power of 11.69W. In
the case of TOC = 6% and TOC = 2%, output power of 482 mW and 266 mW was obtained.
According to the absorption spectra shown in Fig. 1, the absorption band was centered at
811 nm and 803 nm. The commercially available laser diode pump with central wavelength of
#199523 - $15.00 USD Received 15 Oct 2013; revised 12 Jan 2014; accepted 13 Jan 2014; published 14 Feb 2014
(C) 2014 OSA 1 February 2014 | Vol. 4, No. 2 | DOI:10.1364/OME.4.000458 | OPTICAL MATERIALS EXPRESS 461

5. 808 nm was adopted. Unfortunately, the maximum pumping wavelength could not go further
to 811nm even with temperature adjustment. A second laser diode pump source with central
wavelength of 804 nm was employed. With the temperature adjustment on the laser diode
pump source, the best laser performance at 914 nm was obtained when the laser diode
emitting wavelength was centered at 803 nm. In the following discussion, Nd:SSO laser
pumped by different laser diode pump was compared with central wavelength of 803 nm and
808 nm.
The stable oscillation was maintained at cavity length of 11 cm. The relationships between
output power at 914 nm in Nd:SSO crystal and the absorbed pump power at 803 nm and 808
nm are shown in Fig. 5. The laser beam profile at 914 nm is also shown in Fig. 5. When laser
diode pump is centered at 803 nm, 271 mW of output power with 11.9 W of absorbed pump
power corresponding to a slope efficiency of 5.4% was obtained. On the other hand, the use of
laser diode pump centered at 808 nm led to 581 mW of output power with 13.3 W of absorbed
pump power, while the slope efficiency was found to be 8.6%. Laser diode pump source with
central wavelength of 808 nm is beneficial for higher laser output when compared to that of
803 nm, although the absorption cross section at 808 nm is lower than that at 803 nm
according to Fig. 1. The slope efficiency of 8.6% obtained could be affected by crystal
quality, crystal length, as well as the cavity conditions. Higher output power and slope
efficiency based on transitions of 4
F3/2→4
I9/2 of Nd ions could be expected by shortening
sample length, improving crystal quality and laser diode pump source with central wavelength
of 811 nm. These experiments are now under progress. Tracking back to the data in Table 1,
hosts with branch ratio higher than 30% for energy transition 4
F3/2→4
I9/2 of Nd ions seems to
be positive for laser performance.
Fig. 5. Laser operation at 914 nm in Nd:SSO crystal by laser diode pump centered at 803 nm
and 808 nm.
5. Conclusion
In conclusion, Nd:SSO with the advantage of minus refractive index versus temperature,
favorable thermal conductivity (4.1 W·m−1
·K−1
), moderate energy Stark splitting (526 cm−1
)
were firstly reported with laser performance at 914 nm. The realization of diode pumped laser
operation at 914 nm in monoclinic Nd:SSO crystal opens the way to prolific hosts for second
harmonic generation of blue lasers. Improvement of the optical quality of the Nd:SSO crystal
should help enhance the laser performances at 914 nm. This work is now in progress.
#199523 - $15.00 USD Received 15 Oct 2013; revised 12 Jan 2014; accepted 13 Jan 2014; published 14 Feb 2014
(C) 2014 OSA 1 February 2014 | Vol. 4, No. 2 | DOI:10.1364/OME.4.000458 | OPTICAL MATERIALS EXPRESS 462

6. Acknowledgments
We are grateful to the financial supports from Shanghai Municipal Natural Science
Foundation (Grant No. 13ZR1446100, 12JC1409100) and National Natural Science
Foundation of China (Grant No. 91222112, 61205171, 51272264).
#199523 - $15.00 USD Received 15 Oct 2013; revised 12 Jan 2014; accepted 13 Jan 2014; published 14 Feb 2014
(C) 2014 OSA 1 February 2014 | Vol. 4, No. 2 | DOI:10.1364/OME.4.000458 | OPTICAL MATERIALS EXPRESS 463