crystals
Article
LiNbO
3
-Tm
3+
Crystal. Material for Optical Cooling
Ninel Kokanyan
1,2,
* , Nune Mkhitaryan
3
, Gagik Demirkhanyan
3,4
, Ajith Kumar
5
, Michel Aillerie
2
,
Dhiraj Sardar
5
and Edvard Kokanyan
3,4
Citation: Kokanyan, N.; Mkhitaryan,
N.; Demirkhanyan, G.; Kumar, A.;
Aillerie, M.; Sardar, D.; Kokanyan, E.
LiNbO
3
-Tm
3+
Crystal. Material for
Optical Cooling. Crystals 2021, 11, 50.
https://doi.org/10.3390/
cryst11010050
Received: 28 October 2020
Accepted: 6 January 2021
Published: 9 January 2021
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4.0/).
1
Chaire Photonique, LMOPS, CentraleSupélec, 2 rue Edouard Belin, 57220 Metz, France
2
Université de Lorraine, LMOPS, 2 rue Edouard Belin, 57070 Metz, France; michel.aillerie@univ-lorraine.fr
3
Armenian State Pedagogical University after Kh. Abovyan, TigranMetsi Ave., 17, Yerevan 0010, Armenia;
nune.leo.nm@gmail.com (N.M.); gdemirkhanyan@gmail.com (G.D.); edvardkokanyan@gmail.com (E.K.)
4
Institute for Physical Research, National Academy of Sciences of Armenia, Ashtarak-2 0203, Armenia
5
Department of Physics and Astronomy, The University of Texas at San Antonio, San Antonio, TX 78249, USA;
akgsh2005@gmail.com (A.K.); Dsardar@utsa.edu (D.S.)
* Correspondence: ninel.kokanyan@centralesupelec.fr
Abstract: The possibilities of LiNbO
3
-Tm
3+
crystals for optical cooling based on anti-Stokes lumines-
cence in the wavelength range of 1818–2200 nm are investigated. The concentration dependences
of the final temperature of the crystal have been determined under continuous (CW) excitation at
wavelengths of 1822–1977 nm with a pump intensity F
p
= 5 × 10
21
cm
-2
s
-1
. It was shown that
significant cooling with ΔT = 22 K, 19 K, and 16.4 K can be achieved, respectively, with excitation at
wavelengths 1977, 1967, and 1948 nm.
Keywords: Lithium Niobate crystals; optical cooling; rare-earth ions
1. Introduction
Lithium Niobate crystals, LiNbO
3
(LN), doped with trivalent rare earth ions (RE
3+
),
are good materials for small-sized multifunctional lasers in the infrared and visible spectral
regions [1,2]. These crystals can be promising for optical cooling based on anti-Stokes
radiation (ASL) of impurity ions, as well as for creating self-cooling lasers [3–6]. In [7],
the possibilities of an LN-Ho crystal for optical cooling were studied. In [5] is given a
review of studies on the cooling capabilities of crystalline (KGd(WO
4
)
2
, KY(WO
4
)
2
, YAG,
Y
2
SiO
5
, YLF, etc.) and amorphous (ZBLANP, ZBLANP, BIG etc.) materials doped with
Yb
3+
, Er
3+
and Tm
3+
ions.
The optical spectra of impurity absorption and radiation of the LN: Tm
3+
crystal in
the 600–2000 nm wavelength region were studied in [8,9]. In particular, in [8], the standard
Judd-Ofelt analysis of the absorption spectra from the
3
H
6
ground-state manifold is carried
out and the intensity parameters are defined: Ω
2
= 6.29 × 10
-20
, Ω
4
= 0.54 × 10
-20
and
Ω
6
= 0.79 × 10
-20
cm
2
. The energy levels scheme of ground,
3
H
6
, and first exited,
3
F
4
,
manifolds of Tm
3+
ion in LN is given on Figure 1. The main spectroscopic parameters with
taking into account the transitions between Stark sublevels of
3
H
6
and
3
F
4
are calculated in [9].
In this work, based on an analysis of the energy scheme of Stark levels and the results
of calculations of spectroscopic parameters, we consider the possibilities of an LN-Tm
3+
crystal for optical cooling based on ASL in the wavelength range 1818–2000 nm.
Crystals 2021, 11, 50. https://doi.org/10.3390/cryst11010050 https://www.mdpi.com/journal/crystals