crystals
Article
Properties of LiGa0.5In0.5Se2: A Quaternary
Chalcogenide Crystal for Nonlinear Optical
Applications in the Mid-IR
Ludmila Isaenko 1,2 , Alexander Yelisseyev 1 , Sergei Lobanov 1 , Vitaliy Vedenyapin 1 ,
Pavel Krinitsyn 1 and Valentin Petrov 3, *
1
2
3
*
Laboratory of Crystal Growth, Institute of Geology and Mineralogy, SB RAS, 3 Ac. Koptyuga Ave.,
630090 Novosibirsk, Russia; [email protected] (L.I.); [email protected] (A.Y.);
[email protected] (S.L.); [email protected] (V.V.); [email protected] (P.K.)
Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, 2 Pirogova Str.,
630090 Novosibirsk, Russia
Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, 2A Max-Born-Str.,
D-12489 Berlin, Germany
Correspondence: [email protected]; Tel.: +49-30-6392-1272
Academic Editor: Ning Ye
Received: 22 June 2016; Accepted: 22 July 2016; Published: 28 July 2016
Abstract: LiGaSe2 (LGSe) and LiInSe2 (LISe) are wide band-gap nonlinear crystals transparent in
the mid-IR spectral range. LiGa0.5 In0.5 Se2 (LGISe) is a new mixed crystal, a solid solution in the
system LGSe–LISe, which exhibits the same orthorhombic structure (mm2) as the parent compounds
in the same time being more technological with regard to the growth process. In comparison with
LGSe and LISe its homogeneity range is broader in the phase diagram. About 10% of the Li ions
in LGISe occupy octahedral positions (octapores) with coordination number of 3. The band-gap of
LGISe is estimated to be 2.94 eV at room temperature and 3.04 eV at 80 K. The transparency at the
0-level extends from 0.47 to 13 µm. LGISe crystals exhibit luminescence in broad bands centered near
1.7 and 1.25 eV which is excited most effectively by band-to-band transition. From the measured
principal refractive indices and the fitted Sellmeier equations second-harmonic generation from
1.75 to 11.8 µm (fundamental wavelength) is predicted. The nonlinear coefficients of LGISe have
values between those of LGSe and LISe. 6 LGISe crystals are considered promising also for detection
of thermal neutrons.
Keywords: nonlinear optical crystals; ternary and quaternary chalcogenides; solid solution; mid-IR
1. Introduction
The orthorhombic lithium ternary chalcogenides with the chemical formula LiBC2 where B=In and
Ga, C=S and Se, occupy a special position among the non-oxide nonlinear optical crystals because they
are characterized by the widest band-gaps [1–7]. Thus, their main advantage consists in the possibility
to pump them at rather short wavelengths (in the near-IR), without two-photon absorption (TPA),
generating tunable mid-IR radiation by frequency down-conversion nonlinear optical processes [8].
The wider band-gap, in comparison e.g., to their chalcopyrite type AgBC2 analogues, also leads to
increased damage threshold and relatively weak refractive index dispersion in the infrared which is
important for frequency conversion of short laser pulses. In addition, the thermal conductivity of the
four LiBC2 compounds is also higher compared to their AgBC2 counterparts, which is an essential
advantage at high average powers. Whereas the chalcopyrites AgBC2 exhibit strong anisotropy of
the thermal extension with opposite sign of the coefficients along different crystallographic directions
(parallel and perpendicular to the optical axis c), these coefficients show only slight anisotropy in the
Crystals 2016, 6, 85; doi:10.3390/cryst6080085
www.mdpi.com/journal/crystals
Crystals 2016, 6, 85
2 of 10
LiBC2 compounds. The latter makes it possible to select any desirable orientation for the seed in the
process of crystal growth.
In the chalcopyrite family of nonlinear crystals AgBC2 , all four types of solid solutions have been
studied but in the limit of AgInC2 , the birefringence is too low for phase-matching. Ternary sulfides
and selenides with the general formula AgBC2 and chalcopyrite structure readily form continuous
series of isovalent substitutional solid solutions and this is true in particular for the AgGaSe2 –AgInSe2
system [9–11]. Mixed crystals of AgGa1´x Inx Se2 with In content up to 42% were grown [9] and it
was shown that AgGa1´x Inx Se2 nonlinear crystals possess considerable potential for the realization of
noncritically phase-matched three-photon interactions [9,11]. In contrast to the AgInC2 family, all four
wurtzite type LiBC2 compounds possess sufficient birefringence and obviously all their properties
can be continuously tuned by engineering the composition. The growth of two such solid solutions,
LiIn(S1´x Sex )2 [12] and LiGa1´x Inx Se2 [13], has been reported for x = 0.5. The present work is devoted
to the characterization of LiGa0.5 In0.5 Se2 (LGISe).
Crystal engineering in the system LiGaSe2 (LGSe)´LiInSe2 (LISe) is potentially interesting because
although most of the properties of the two parent compounds are quite similar and interpolation
is expected to provide a reliable estimate of the expected characteristics, the difference for a given
three-wave interaction process pumped by a specific laser system can be rather large. Thus, e.g.,
amplified Ti:sapphire laser systems, the most advanced sources of high energy femtosecond pulses,
can be used for pumping parametric amplifiers based on LGSe but not LISe although the latter exhibits
the highest nonlinearity among the LiBC2 compounds. This limitation is a consequence of the different
band-gaps: as a result of this the TPA coefficient measured at 820 nm is <0.07 cm/GW for LGSe but as
large as 0.6 cm/GW for LISe [2].
The situation is similar in another important application of the Li-containing chalcogenides,
namely for detection of thermal neutrons, where also a trade-off of properties is required.
This application is based on the high thermal neutron cross section (938 barns) of the 6 Li isotope
which converts thermal neutrons to highly ionizing particles through the nuclear reaction 6 Li(n,α)3 H,
while the LiBC2 semiconductors detect the electron-hole pairs induced by either tritium ions or
α-particles [14]. Among the LiBC2 crystals maximum response has been demonstrated with LISe
which exhibits the minimum band-gap. On the other hand, thermal neutrons can be detected by
recording the light pulses generated as a result of the interaction of the crystal with ionizing particles:
in this case LISe operates as a scintillator [15]. The problem is the natural abundance of 6 Li which
amounts to only 7.5%. To increase the detection efficiency LISe crystals with lithium as 6 Li can be
grown [16]. However, 115 In (with natural abundance of 95.7%) present in LISe also captures neutrons,
which results in gamma decay rather than charge creation. From this point of view, replacing In- by
Ga-ions can potentially increase the efficiency of the neutron detection [17]. Compositions with 25%,
50%, and 85% substitution were grown in [17].
The present paper presents information on the growth procedure, crystal structure, and the
linear and nonlinear optical properties (including photoluminescence, dispersion of refractive indices,
phase-matching conditions) of LGISe.
2. Crystal Growth and Structure
LGISe was obtained from the two compounds LISe and LGSe, taken in stoichiometric ratio.
The two ternary lithium selenides were first synthesized by the methodology described in previous
work [3,5]. The melting temperature of the quaternary LGISe is lower (830 ˝ C) than the melting
temperatures of both LISe (905 ˝ C) and LGSe (915 ˝ C). The synthesis of the quaternary compound
was carried out in a glass graphite crucible located inside a silica container, for 4 h at T = 970 ˝ C in Ar
atmosphere. The obtained charge of the quaternary compound was backfilled into a glass graphite
crucible. A conical bottom of the crucible provides a separation of the single crystal nucleus during the
growth process. The crucible was then inserted into a silica ampoule. The ampoule was evacuated to
10´3 Pa, then filled with dry Ar to provide some overpressure (about 1000 Pa) and finally sealed.
Crystals 2016, 6, 85
3 of 10
Crystals 2016, 6, 85
3 of 10
In [18] the process of incongruent evaporation was studied in detail for both LISe and LGSe
In [18] the process of incongruent evaporation was studied in detail for both LISe and LGSe
compounds at temperatures near the melting point. The perfection of a bulk crystal of certain
compounds at temperatures near the melting point. The perfection of a bulk crystal of certain
composition depends on the growth conditions such as the maximum temperature and the time
composition depends on the growth conditions such as the maximum temperature and the time of
of melt overheating. According to our data, an overheating exceeding 50 ˝ C results in strong shift
melt overheating. According to our data, an overheating exceeding 50 °C results in strong shift of the
of the compound composition away from the stoichiometric one. Volatile components with a high
compound composition away from the stoichiometric one. Volatile components with a high partial
partial pressure of Ga(In)2 Se3 escape from the reaction area and migrate into the free area of the reactor.
pressure of Ga(In)2Se3 escape from the reaction area and migrate into the free area of the reactor. Ar
Ar overpressure in the reactor solves partly the problem. The lower synthesis and growth temperatures
overpressure in the reactor solves partly the problem. The lower synthesis and growth temperatures
of LGISe suppress the effect of incongruent sublimation and thus from the point of view of crystal
of LGISe suppress the effect of incongruent sublimation and thus from the point of view of crystal
growth, the quaternary compound seems quite promising for achieving good optical quality.
growth, the quaternary compound seems quite promising for achieving good optical quality.
The
growth of LGISe single crystals was carried out by a modified vertical Bridgman-Stockbarger
The growth of LGISe single crystals was carried out by a modified vertical
technique.
The ampoule technique.
was quiescently
located inwas
thequiescently
furnace, in alocated
position
temperature
gradient
Bridgman-Stockbarger
The ampoule
inwith
the furnace,
in a position
˝ C/cm. The furnace was heated at a rate of 100 ˝ C/h to a temperature exceeding the melting
of
15
to
30
with temperature gradient of 15 to 30 °C/cm. The furnace was heated at a rate of 100 °C/h to a
point
by 50 ˝ C.
This temperature
was
maintained
10 h
and then the
furnace
was slowly
temperature
exceeding
the melting
point
by 50 °C.for
This
temperature
was
maintained
for 10 cooled
h and
down
following
program
which
ensured
crystal growth
rate ofwhich
0.1 to ensured
1.0 mm/day
during
the entire
then the
furnacea was
slowly
cooled
downafollowing
a program
a crystal
growth
rate
growth
process.
Finally,during
the furnace
was switched-off.
The Finally,
as-grown
had
a diameter
of 20 The
mm
of 0.1 to
1.0 mm/day
the entire
growth process.
thecrystals
furnace
was
switched-off.
and
a length
of up to
50 amm
but were
milky
1a). This
required
additional
annealing
in vacuum
as-grown
crystals
had
diameter
of 20
mm(Figure
and a length
of up
to 50 mm
but were
milky (Figure
1a).
˝ C for a period of 3 h to obtain high transparency. The resulting color varied from
or
in
Se
vapor
at
750
This required additional annealing in vacuum or in Se vapor at 750 °C for a period of 3 h to obtain
yellow
to pink depending
on the deviation
fromfrom
stoichiometry.
In Figure
1b, twoonsemiprisms
of LGISe
high transparency.
The resulting
color varied
yellow to pink
depending
the deviation
from
are
shown
in
transmitted
light:
they
were
used
to
study
the
dispersion
of
the
refractive
indices
and
stoichiometry. In Figure 1b, two semiprisms of LGISe are shown in transmitted light: they were used
the
birefringence.
The
annealed
LGISe
crystals
show
a
dark
red
luminescence
under
UV
excitation
at
to study the dispersion of the refractive indices and the birefringence. The annealed LGISe crystals
365
nma(Figure
1c).
show
dark red
luminescence under UV excitation at 365 nm (Figure 1c).
(a)
(b)
(c)
Figure1. 1.
(a) As-grown
LGISe (b)boule;
LGISe insemiprisms
in transmitted
light; (c)
Figure
(a) As-grown
LGISe boule;
LGISe (b)
semiprisms
transmitted light;
(c) Photoluminescence
Photoluminescence
(PL)
pattern
of
annealed
LGISe
under
UV
excitation.
(PL) pattern of annealed LGISe under UV excitation.
Thecrystal
crystalstructure
structureof
of LGISe
LGISe was
was studied
studied on
on single
single crystals
crystals with
with Oxford
Oxford Diffraction
Diffraction Gemini
The
Gemini R
R
Ultra(Oxford
(Oxford Diffraction
Diffraction Ltd.,
Ltd., Abingdon,
Abingdon, UK)
UK) and
and Bruker
Bruker P4
P4 (Bruker
(Bruker Advanced
Ultra
Advanced X-Ray
X-Ray Solutions,
Solutions,
Madison, WI, USA), MoKα λ = 0.71073 Å diffractometers, using the SHELXL-97 program
Madison, WI, USA), MoKα λ = 0.71073 Å diffractometers, using the SHELXL-97 program (George
(George Sheldrick, Göttingen, Germany). The LGISe structure is similar to that of the two parent
Sheldrick, Göttingen, Germany). The LGISe structure is similar to that of the two parent ternary
9
2 structure with space group Pna219or C 2 v (Table 1). The structure of
ternary compounds:
β-NaFeO
compounds:
β-NaFeO
1). The structure of the
2 structure with space group Pna21 or C2v (Table
2
compounds
is
formed
by
LiSe
4 and
BSe4 tetrahedrons,
the
orthorhombic
(point
group
mm2)
LiBSe
orthorhombic (point group mm2) LiBSe2 compounds is formed by LiSe4 and BSe
4 tetrahedrons, and the
and
the
Se-ions
are
arranged
in
hexagonal
packing
with
tetragonal
and
octahedral
cavities
Se-ions are arranged in hexagonal packing with tetragonal and octahedral cavities (tetrapores
and
(tetraporesOnly
and octapores).
Only half of
tetrapores
are normally
occupied
Li- and
B-ions while
octapores).
half of the tetrapores
arethe
normally
occupied
by Li- and
B-ionsby
while
all octapores
are
all
octapores
are
normally
empty.
The
structure
of
the
parent
components
LISe
and
LGSe
exhibits
normally empty. The structure of the parent components LISe and LGSe exhibits some characteristic
some characteristic
features.
ions occupy two
(in tetrafeatures.
Li ions occupy
two Li
nonequivalent
sitesnonequivalent
(in tetra- andsites
octapores)
inand
LISeoctapores)
but there in
is LISe
only
but such
theresite
is only
one such site
(in tetrapores)
in LGSe.
structure
the mixed
LGISe
crystal
one
(in tetrapores)
in LGSe.
The structure
of theThe
mixed
LGISe of
crystal
inherits
this feature
inherits
this component:
feature from
the 10%
LISeofcomponent:
about octapores
10% of the
Li ions
from
the LISe
about
the Li ions occupy
(Tables
2 andoccupy
3, Figureoctapores
2). Thus,
(Tables
2
and
3,
Figure
2).
Thus,
the
LGISe
structure
is
considered
to
contain
the
two subsystems.
the LGISe structure is considered to contain the two subsystems. The unit cell parameters
of LGISe are
unit cellÅ,
parameters
of LGISe
= 7.0376(2)
b =unit
8.3401(3)
Å, and is
c =392.4
6.6855(2)
Å. The
unit cell
aThe
= 7.0376(2)
b = 8.3401(3)
Å, andare
c =a 6.6855(2)
Å.Å,
The
cell volume
Å3 , larger
compared
3, larger compared to LGSe and smaller compared to LISe. The composition of the
volume
is
392.4
Å
to LGSe and smaller compared to LISe. The composition of the bulk crystals was determined by a
bulk crystals was determined by a high-accuracy chemical analysis, and the phase state was
controlled by X-ray diffraction. Assuming linear change of the lattice parameters with composition
Crystals 2016, 6, 85
4 of 10
high-accuracy chemical analysis, and the phase state was controlled by X-ray diffraction. Assuming
linear change of the lattice parameters with composition of the LiGax In1´x Se2 solid solutions, the
experimentally measured lattice parameters would correspond to a composition with x = 0.42–0.43.
Table 1. Crystal data and structure refinement details for LiGa0.5 In0.5 Se2 .
Formula
LiGa0.5 In0.5 Se2
formula weight (g/mol)
crystal system
space group
257.13
orthorhombic
Pna21
a = 7.0376(2),
b = 8.3401(3),
c = 6.6855(2)
392.40(2)/4
4.352
0.1 ˆ 0.2 ˆ 0.2
24.809
3.79–32.57
´10 ď h ď 10, ´12 ď k ď 11, ´5 ď l ď 10
3883/1213 (R(int) = 0.0250)
1213/1/39
0.663
R(F) = 0.0129, wR(F2 ) = 0.0486
R(F) = 0.0183, wR(F2 ) = 0.0594
0.040(19)
0.0107(8)
0.338/´0.424
unit cell dimensions (Å)
V (Å3 ); Z
calculated density (g/cm3 )
crystal size (mm)
µ (MoKα) (mm´1 )
θ range (deg) for data collection
Miller index ranges
reflections collected/unique
data/restraints/parameters
goodness-of-fit on F2 (GOF)
final R indices (I > 2σ(I))
R indices (all data)
absolute structure parameter
extinction coefficient
largest difference peak/hole (e¨Å´3 )
Table 2. Atomic coordinates and equivalent isotropic atomic displacement parameters for LiGa0.5 In0.5 Se2 .
Atom
x/a
y/b
z/c
Ueq (Å2 )
In/Ga *
Se(1)
Se(2)
Li(1) **
Li(2) ***
0.57623(3)
0.41137(5)
0.57352(4)
0.5841(10)
0.257(10)
0.62606(2)
0.87170(4)
0.62421(4)
0.1209(7)
0.114(6)
0.38637(16)
0.50826(6)
0.01222(8)
0.378(4)
0.142(11)
0.0157(1)
0.0207(2)
0.0204(2)
0.024(3)
0.03(3)
* refined site occupancy factor (sof): 0.52(1)/0.48(1) for In/Ga, respectively; ** sof: 0.90(5); *** sof: 0.10(5).
Table 3. Selected interatomic distances (Å) in LiGa0.5 In0.5 Se2 .
In-Tetrahedron
Li(1)-Tetrahedron
Li(2)-Pyramid
In–Se(2 a ) 2.4850(6)
In–Se(1) 2.4915(5)
In–Se(1 b ) 2.4954(5)
In–Se(2) 2.5015(13)
<In–Se> 2.4934
Li(1)–Se(2 a ) 2.56(1)
Li(1)–Se(1 c ) 2.47(3)
Li(1)–Se(2 d ) 2.57(1)
Li(1)–Se(1e ) 2.56(1)
<Li(1)–Se> 2.54
Li(2)–Se(1 c ) 2.50(7)
Li(2)–Se(1 f ) 2.52(6)
Li(2)–Se(2 g ) 2.61(6)
<Li(2)–Se> 2.54
Symmetry codes: a ´x + 1, ´y + 1, z + 1/2; b x + 1/2, ´y + 3/2, z; c ´x + 1, ´y + 1, z ´ 1/2, ´z + 1; d ´x + 3/2,
y ´ 1/2, z + 1/2; e x, y ´ 1, z; f x ´ 1/2, ´y + 1/2, z; g ´x + 1/2, y ´ 1/2, z.
Crystals 2016, 6, 85
Crystals
Crystals
2016,2016,
6, 85 6, 85
5 of 10
5 of 10
5 of 10
0.5Se
In20.5Se
2 possesses
β-NaFeO
2-type
structure
as the
ternary
2
Figure
2. LiGa
Figure
2. LiGa
0.5In0.5
possesses
the the
samesame
-NaFeO
2-type
structure
as the
fourfour
ternary
LiBCLiBC
2
Figure 2. LiGa0.5 In0.5 Se2 possesses the same β-NaFeO2 -type structure as the four ternary LiBC2
compounds.
At
room
temperature
and
normal
pressure
the
major
part
of
the
Li
ions
occupy
compounds. At room temperature and normal pressure the major part of the Li ions occupy
compounds. At room temperature and normal pressure the major part of the Li ions occupy tetrapores
tetrapores
in LGISe
the LGISe
structure
about
of Li
theions
Li ions
are localized
inside
octapores.
tetrapores
in the
structure
and and
onlyonly
about
10%10%
of the
are localized
inside
octapores.
in the LGISe structure and only about 10% of the Li ions are localized inside octapores.
Transmission
and
Luminescence
3. Transmission
andand
Luminescence
3.3.Transmission
Luminescence
The
transmission
ofthe
the
as-grown
yellow
LGISe
crystals
was
measured
using
conventional
The
transmission
of of
the
as-grown
yellow
LGISe
crystals
was
measured
using
a conventional
The
transmission
as-grown
yellow
LGISe
crystals
was
measured
using
a aconventional
spectrophotometer
in
the
UV
to
near-IR
and
a
Fourier-transform
spectrophotometer
in
the
mid-IR,
spectrophotometer
in
the
UV
to
near-IR
and
a
Fourier-transform
spectrophotometer
in
the
mid-IR,
spectrophotometer in the UV to near-IR and a Fourier-transform spectrophotometer in the
mid-IR,
Figure
extends
roughly
from
0.47
to13.5
13.5
µm
atthe
the
0-level
mm
thick
LGISe
samples.
Thus,
Figure
3. 3.
It3.extends
roughly
from
0.47
to to
13.5
µ mµm
at at
the
0-level
forfor
7for
mm
thick
LGISe
samples.
Thus,
Figure
ItItextends
roughly
from
0.47
0-level
7 7mm
thick
LGISe
samples.
Thus,
LGISe
can
be
pumped
at
1064
nm
without
TPA.
Variations
of
LGISe
color
from
yellow
to
pink
are
LGISe
can
be
pumped
at
1064
nm
without
TPA.
Variations
of
LGISe
color
from
yellow
to
pink
are
LGISe can be pumped at 1064 nm without TPA. Variations of LGISe color from yellow to pink are
possible:
the
corresponding
transmission
spectra
are
given
in
Figure
3.
Yellow
and
pink
crystals
possible:
the
given in
inFigure
Figure3.3.Yellow
Yellow
and
pink
crystals
possible:
thecorresponding
correspondingtransmission
transmissionspectra
spectra are
are given
and
pink
crystals
have
have
similar
transmission
spectra
with
broad
absorption
bands
around
2.6
and
10.3
in the
have
similar
transmission
spectra
with
broad
absorption
bands
around
2.6
and
10.3
μm
in
the
similar transmission spectra with broad absorption bands around 2.6 and 10.3 µm in theµm
infrared.
infrared.
In
the
visible
region
there
is
a
broad
band
around
0.6
µm.
In
pink
samples
the
fundamental
infrared.
In
the
visible
region
there
is
a
broad
band
around
0.6
µ
m.
In
pink
samples
the
fundamental
In the visible region there is a broad band around 0.6 µm. In pink samples the fundamental absorption
absorption
shifts
to 0.515
an additional
absorption
feature
appears
inrange.
the
0.6–0.8
µm
absorption
edge
shifts
to and
0.515
m µm
andand
an absorption
additional
absorption
feature
appears
inµm
the
0.6–0.8
μm
edge
shifts
toedge
0.515
µm
anµadditional
feature
appears
in the
0.6–0.8
For
thin
range.
For
thin
LGISe
plates
of
light
yellow
color,
the
fundamental
absorption
edge
shifts
to
0.42
µm
range.
For
thin
LGISe
plates
of
light
yellow
color,
the
fundamental
absorption
edge
shifts
to
0.42
μm
LGISe plates of light yellow color, the fundamental absorption edge shifts to 0.42 µm at 300 K and to
at 300
K at
and
to
K (Figure
at 0.395
300
Kµm
and
to800.395
µ m µm
at3).
80atK80(Figure
3). 3).
K 0.395
(Figure
80
transmission [%]
1
60
40
20
2a 2b
1a 1b
0
0.4
0.6
0.8
2 4 6
wavelength [µm]
8 10 12
Figure 3. Unpolarized transmisson spectra of LiGa0.5 In0.5 Se2 crystals, 7 mm (1) and 60 µm (2)
Figure
3. Unpolarized
transmisson
spectra
of LiGa
0.5Se
In20.5crystals,
Se2 crystals,
7 mm
(1) and
(2) thick.
Figure
3. Unpolarized
transmisson
spectra
of LiGa
0.5In0.5
7 mm
(1) and
60 µ60
m µm
(2) thick.
thick. Fragments 1a and 1b were recorded at 300 K in areas of yellow and pink color, respectively.
Fragments
1a and
1b were
recorded
at 300
in areas
of yellow
color,
respectively.
Fragments
1a and
1b were
recorded
at 300
K inK areas
of yellow
and and
pinkpink
color,
respectively.
Fragments 2a and 2b were recorded for a 60 µm thick light yellow plate at 80 and 300 K, respectively.
Fragments
2a and
2b were
recorded
60 thick
µm thick
yellow
80 and
300respectively.
К, respectively.
Fragments
2a and
2b were
recorded
for afor
60 aμm
lightlight
yellow
plateplate
at 80atand
300 К,
The band-gap was estimated from measurements of the absorption coefficient α with a 60 µm thin
band-gap
estimated
from
measurements
of the
absorption
coefficient
α with
60 µm
TheThe
band-gap
waswas
estimated
from
measurements
of the
absorption
coefficient
α with
a 60aμm
plate of LGISe, Figure 4. From linear fits of the (αhν)2 dependence
on hν (the case of direct allowed
2 dependence on hν (the case of direct
2 dependence
plate
of LGISe,
Figure
4. From
linear
of the
(αhν)
thinthin
plate
of LGISe,
Figure
4. From
linear
fits fits
of the
(αhν)
on hν (the case of direct
transitions between parabolic bands) one arrives at 2.94 eV at 300 K which corresponds to 0.42 µm.
allowed
transitions
between
parabolic
bands)
arrives
at 2.94
eV300
at 300
К which
corresponds
allowed
transitions
between
parabolic
bands)
one one
arrives
at 2.94
eV at
К which
corresponds
to to
At 80 K, the band-gap Eg is 3.04 eV. For comparison, the band-gaps of LGSe and LISe are 3.34 and
AtK,
80the
K, the
band-gap
is 3.04
comparison,
band-gaps
of LGSe
0.420.42
μm.µm.
At 80
band-gap
Eg isEg3.04
eV. eV.
For For
comparison,
the the
band-gaps
of LGSe
andand
LISeLISe
are are
2.86 eV, respectively [1,6]. It is necessary to note that the Eg value for LGISe is closer to LISe than to
respectively
[1,6].
is necessary
to note
Eg value
for LGISe
is closer
to LISe
3.343.34
andand
2.862.86
eV, eV,
respectively
[1,6].
It isItnecessary
to note
thatthat
the the
Eg value
for LGISe
is closer
to LISe
LGSe. This means that the electric conductivity of LGISe should be closer to that of LISe and as a result
to LGSe.
means
electric
conductivity
of LGISe
should
be closer
to that
of LISe
thanthan
to LGSe.
ThisThis
means
thatthat
the the
electric
conductivity
of LGISe
should
be closer
to that
of LISe
andand
their response to thermal neutrons should be similar. Moreover, as mentioned before, substituting
a result
response
to thermal
neutrons
should
be similar.
Moreover,
as mentioned
before,
as aasresult
theirtheir
response
to thermal
neutrons
should
be similar.
Moreover,
as mentioned
before,
roughly half of the In-ions by Ga-ions improves the efficiency of the neutron detection.
substituting
roughly
half
of
the
In-ions
by
Ga-ions
improves
the
efficiency
of
the
neutron
detection.
substituting roughly half of the In-ions by Ga-ions improves the efficiency of the neutron detection.
Crystals 2016, 6, 85
Crystals 2016, 6, 85
6 of 10
6 of 10
3.4
15
LGSe
6 of 10
band-gap [eV]
2
2
2
0 5
2.8
2.9
3.0
3.1
3.2
band-gap [eV]
2
1
10
6
-2
5
3.2
3.4
2.94 eV (300 K)
3.04 eV (80 K)
15
(αhν) [10 cm eV ]
2
1
10
6
-2
2
(αhν) [10 cm eV ]
Crystals 2016, 6, 85
2.94 eV (300 K)
3.04 eV (80 K)
LGSe
3.23.0
LGISe
LISe
3.02.8
LISe0.0
0.2
photon energy hν [eV]
0
2.8
2.9
(a)
3.0
3.1
0.4 LGISe
0.6
0.8
1.0
x, Ga fraction
3.2
2.8
0.0
photon energy hν [eV]
0.2
0.4
(b)
0.6
0.8
1.0
x, Ga fraction
Figure4. (a)
4. Fundamental
(a) Fundamental
absorption edge for a 60-µm-thick LGISe plate,as represented
Figure
(αhν)2 = f(hν),as
(a)absorption edge for a 60-µm-thick LGISe plate, represented
(b)
2 = f(hν), at T = 300 K (1) and 80 K (2); (b) Band-gap values for LiInSe2 (LISe), LiGaSe2 (LGSe),
(αhν)
at T = 300 K (1) and 80 K (2); (b) Band-gap values for LiInSe2 (LISe), LiGaSe2 (LGSe), and LGISe, derived
(a) Fundamental
absorption edge
for a 60-µm-thick
LGISe plate, represented as
andFigure
LGISe, 4.
derived
from
room-temperature
absorption
measurements.
from
room-temperature
absorption
measurements.
(αhν)2 = f(hν), at T = 300 K (1) and 80 K (2); (b) Band-gap values for LiInSe2 (LISe), LiGaSe2 (LGSe),
and
LGISe, derived from
absorptionatmeasurements.
Photoluminescence
(PL)room-temperature
spectra were recorded
300 and 80 K using a diffraction luminescence
Photoluminescence (PL) spectra were recorded at 300 and 80 K using a diffraction luminescence
spectrometer SDL1
SDL1 (LOMO
JSC,
St.St.Petersburg,
Russia) andand
a cooled
FEU-83
photomultiplier
(Ekran
spectrometer
(LOMO
JSC,
Petersburg,
cooled
FEU-83
photomultiplier
Photoluminescence
(PL)
spectra
were
recorded Russia)
at 300 and 80 Kausing
a diffraction
luminescence
Optical
Systems,
Novosibirsk,
Russia).
By means
ofand
anofaMDR2
monochromator
(LOMO
JSC, St.
(Ekran
Optical
Systems,
Novosibirsk,
Russia).
By
means
an
MDR2
monochromator
spectrometer
SDL1 (LOMO
JSC, St. Petersburg,
Russia)
cooled
FEU-83
photomultiplier(LOMO
(Ekran JSC,
Petersburg,
Russia),
different
spectral
ranges
were
selected
from
the
1
kW
Xe
lamp
spectrum
Optical Systems,
Novosibirsk,
Russia). ranges
By means
of an
MDR2from
monochromator
(LOMO
JSC, St. toto
St. Petersburg,
Russia),
different spectral
were
selected
the 1 kW Xe
lamp spectrum
excite
PL
in
LGISe.
Typical
PL
spectra
recorded
at
80
K
for
excitation
with
3.1,
2.75
2.48 eV
Russia),
different
spectral
ranges at
were
from the
1 kW
lamp
to
excitePetersburg,
PL in LGISe.
Typical
PL spectra
recorded
80 Kselected
for excitation
with
3.1,Xe
2.75
andspectrum
2.48and
eV photon
photon
energy
450
and respectively)
500
respectively)
shown
in5.Figure
5. Emission
occurs
excite
PL450
in (400,
LGISe.
Typical
PL nm,
spectra
recorded
at are
80inK
for excitation
with
3.1,
2.75 basically
and
2.48 basically
eV
energy
(400,
and 500
nm,
are shown
Figure
Emission
occurs
in two
photon
energy
(400, 450
and
500eV
nm,
respectively)
are shown in Figure 5. Emission occurs basically
in
two
spectral
ranges:
1.5
to
2.0
and
1.1
to
1.4
eV.
spectral ranges: 1.5 to 2.0 eV and 1.1 to 1.4 eV.
in two spectral ranges: 1.5 to 2.0 eV and 1.1 to 1.4 eV.
1.0
1.0
1.0
1.0
A
Eg
0.5
C
PL [a.u.]
0.5
PL [a.u.]
PL [a.u.]
PL [a.u.]
A
C
Eg
0.5
0.5
2
B B
0.0 0.0
1.0 1.0
2.0
1.51.5
2.0
photon
energy
h
ν
[eV]
photon energy hν [eV]
(a)(a)
1
1
0.00.0
1 1
3
2
3
2 2
3
4
3
4
photon
energy
h
ν
[eV]
photon energy hν [eV]
5
5
(b) (b)
Figure 5. (a) PL spectra recorded with LiGa0.5In0.5Se2 crystals at 80 K for excitation at 3.1 eV (400 nm,
Figure
LiGa
InIn
Figure5.5.(a)
(a)PL
PLspectra
spectrarecorded
recordedwith
with
LiGa
0.5Se
Se22 crystals
crystals at
at 80
80KKfor
forexcitation
excitationatat3.1
3.1eV
eV(400
(400nm,
nm,
0.50.5
0.5
curve A), 2.75 eV (450 nm, B), and 2.48 eV (500 nm, C). The arrows in (a) indicate the photon energies
curve
A),
2.75
eV
(450
nm,
B),
and
2.48
eV
(500
nm,
C).
The
arrows
inin(a)
indicate
the
photon
energies
curve
A),
2.75
eV
(450
nm,
B),
and
2.48
eV
(500
nm,
C).
The
arrows
(a)
indicate
the
photon
energies
(hν = 1.265, 1.38, and 1.65 eV) for which the photoluminescence excitation (PLE) spectra were
(hν
1.265,
1.38,
and 1.65 eV)
for
which
the
photoluminescence
excitationexcitation
(PLE) spectra
were
measured;
(hν= measured;
= 1.265,
1.38,
1.65
eV)
forLiGa
which
the photoluminescence
(PLE)
spectra
were
(b) 80and
K PLE
spectra
of
0.5In0.5Se2 recorded for emission at 1.65 eV (750 nm, curve 1),
(b)
80
K
PLE
spectra
of
LiGa
In
Se
recorded
for
emission
at
1.65
eV
(750
nm,
curve
1),
1.38 eV
0.5 eVof
0.5
2 nm,
measured;
(b) 80
K2),
PLE
LiGa
0.5In
Se2 arrow
recorded
emission
at 1.65 eVEg(750
1),
1.38 eV (900
nm,
andspectra
1.265
(980
3).0.5The
in (b)for
indicates
the band-gap
at 80nm,
К. curve
(900
and
1.265
eV (980
The
arrow
in (b)
indicates
the band-gap
Eg at 80 K.
1.38nm,
eV 2),
(900
nm,
2), and
1.265nm,
eV 3).
(980
nm,
3). The
arrow
in (b) indicates
the band-gap
Eg at 80 К.
Table 4. Results from a Gaussian deconvolution analysis of the PL spectra of LiGa0.5In0.5Se2.
Table
4. 4.
Results
from
a Gaussian
deconvolution analysis
of of
thethe
PLPL
spectra
of of
LiGa
InIn
Se
Table
Results
from
a Gaussian
analysis
spectra
LiGa
Se
0.5 0.5
0.50.5
2 .2.
Positiondeconvolution
of Peak Maximum
FWHM
#
eV
nm
eV
Position
Maximum
FWHM
Positionof
ofPeak
Peak Maximum
FWHM
#
1.247
994
0.164
#1
eV
nm
eV
eV
nm
eV
2
1.291
960
0.187
1
1.247
994
0.164
1.350
918
0.247
13
1.247
994
0.164
2
1.291
960
0.187
1.702
728
0.329
24
1.291
960
0.187
3
1.350
918
0.247
3
1.350
918
0.247
4
1.702
728
0.329
1.702
728of the PL spectra
0.329yielding four components
The results from a Gaussian deconvolution
analysis
are summarized in Table 4: they include the peak positions in photon energies (eV) and wavelengths
(nm)
as
wellfrom
as
thea full
width at
half-maximum
(FWHM).
recorded
for four
3.1 eV
excitation
Theresults
results
from
a Gaussian
deconvolution
analysis
of the
PLAspectra
yielding
four
components
The
Gaussian
deconvolution
analysis
of Spectrum
the
PL spectra
yielding
components
are
consists
of
components
4,
3,
2
from
Table
4,
whereas
spectra
B
and
C
are
constituted
mainly
by (nm)
are summarized
in Table
4: include
they include
the positions
peak positions
in photon
energies
(eV)wavelengths
and wavelengths
summarized
in Table
4: they
the peak
in photon
energies
(eV) and
(nm) as well as the full width at half-maximum (FWHM). Spectrum A recorded for 3.1 eV excitation
consists of components 4, 3, 2 from Table 4, whereas spectra B and C are constituted mainly by
Crystals 2016, 6, 85
7 of 10
as well as the full width at half-maximum (FWHM). Spectrum A recorded for 3.1 eV excitation consists
of components 4, 3, 2 from Table 4, whereas spectra B and C are constituted mainly by components 2
and 1, respectively. The PL intensity is rather low at room temperature but it increases by two orders
of magnitude as the crystal is cooled down to 80 K. Similar PL emission with a main peak near 1.8 eV
has been observed in LISe [16] and LGSe. Another PL band was observed in LISe near 2.4 eV [16]:
both bands, at 1.8 and 2.4 eV, were associated with a radiative recombination in donor-acceptor
pairs (DAPs) [16]. Photoluminescence excitation (PLE) spectra recorded at different photon energies
are shown in Figure 4b. One can see that the dominating red PL is excited most effectively in a
group of narrow bands at 2.998, 3.147 and 3.275 eV (413, 394 and 378 nm, respectively). The arrow
in Figure 4b marks the band-gap value of 3.04 eV for LGISe derived from the low-temperature
absorption measurements. Thus, the main band near 3.2 eV in the PLE spectra of LGISe agrees well
with the band-gap value and corresponds to band-to-band electronic transitions. There are no other
appreciable bands in the PLE spectra for 1.7 eV emission in LGISe and this agrees with the model of
DAP recombination [16]. DAP recombination takes place following band-to-band transitions when
free charge carriers of different sign (electrons and holes) are generated. Subsequently these free
electrons and holes are captured by traps and stay there for a long time at low temperature. At a later
stage, there may be a radiative recombination between such captured charge carriers (donors and
acceptors) [16]. The low energy group of PL bands near 1.25 eV is observed not only by band-to-band
excitation but also by excitation in a broad band near 2.2 eV (Figure 4b, curves 2, 3). This is an indication
of an intracenter PL mechanism while the 2.2 eV signal corresponds to an absorption band of some point
defect (presumably a cation antisite defect). The variety of PL emission and the fine structure of the PLE
spectra near 3.2 eV may be a result of several nonequivalent positions of the Li+ ions in LGISe.
4. Dispersion, Birefringence and Nonlinearity
Two semiprisms (inset Figure 6a), with a reflecting face parallel to a principal plane, were prepared
from LGISe (yellow color) for measurement of the three principal refractive indices by the
autocollimation method. The three refractive indices were measured by the autocollimation method at
23 wavelengths between 0.48 and 12 µm [13]. The data were fitted by one-pole Sellmeier equations
with quadratic IR correction term. The Sellmeier coefficients are summarized in Table 5. Figure 6a
shows the measured refractive index values and the calculated curves using the fitted Sellmeier
equations. Interestingly, ny and nz of LGISe exceed, in a substantial part of the mid-IR transmission
range, the corresponding values for LISe (which in turn are larger than those for LGSe).
Table 5. Sellmeier equations for LGISe, n2 = A + B/(λ2 ´ C) ´ Dλ2 , where λ is in µm, valid in the
0.48–12 µm range.
n
A
B
C
D
nx
ny
nz
5.03847
5.24012
5.26219
0.18748
0.21386
0.21331
0.07033
0.07033
0.07550
0.00204
0.00193
0.00209
The Sellmeier equations predict index crossing of ny and nz near the cut-off edges. However, in the
major part of the transmission window the correspondence between the dielectric and crystallographic
axes in the orthorhombic LGISe is xyz = bac and this relationship can be adopted for reporting the
phase-matching angles (defined in the xyz frame) and the nonlinear coefficients (defined in the abc
frame). This means that the two optic axes which determine the propagation directions where the
index of refraction is independent of polarization move from the x–z principal plane (in the major part
of the clear transmission) to the x–y plane (towards the edges). The angle Vz between the two optic axis
1{2
and the z-principal axis, is determined by sinVZ “
2 ´n2 q
n Z p nY
X
1{2
nY pn2Z ´n2X q
. It approaches 90˝ towards the crossing
points where absorption is already substantial but in the interesting for practical applications clear
transmission range the angle Vz is almost constant, around 75˝ . Thus the optically negative (Vz > 45˝ )
Crystals 2016, 6, 85
8 of 10
biaxial
LGISe crystal can be regarded as quasi-uniaxial, which is a consequence of the fact8 ofthat
the
Crystals 2016, 6, 85
10
two indices ny and nz are very close. Therefore no phase-matching can be expected for propagation
whichinisthe
a consequence
of x-principal
the fact thataxis.
the two indices ny and nz are very close. Therefore no
directions
vicinity of the
phase-matching can be expected for propagation directions in the vicinity of the x-principal axis.
90
2.6
ny
2.5
nz
fit nx
2.4
fit ny
fit nz
2.3
x-z
oo-e
60
45
eo-e
x-y
30
15
2.2
0
75
angle θ, ϕ [°]
refractive index
nx
2
4
6
8
10
wavelength [µm]
12
0
oe-o
2
y-z
oe-o
4
6
8
10
12
fundamental wavelength [µm]
(a)
(b)
Figure 6. (a) Measured refractive indices of LGISe (symbols) and calculated curves using the
Figure 6. (a) Measured refractive indices of LGISe (symbols) and calculated curves using the
Sellmeier coefficients from Table 5 (the inset shows the semiprisms fabricated for the measurements);
Sellmeier coefficients from Table 5 (the inset shows the semiprisms fabricated for the measurements);
and (b) Second harmonic generation (SHG) phase-matching curves in the three principal planes of
and (b) Second harmonic generation (SHG) phase-matching curves in the three principal planes of
LGISe. The symbols correspond to experimental SHG points at 4.63 µm in the x–z plane and at 4.42
LGISe.
The
symbols
correspond
µm
in the
x–y principal
plane.to experimental SHG points at 4.63 µm in the x–z plane and at 4.42 µm
in the x–y principal plane.
This can be seen from the second harmonic generation (SHG) phase-matching curves calculated
which
arebepresented
in the
Figure
6b. The
predicted
phase-matching
agree very curves
well with
the
This can
seen from
second
harmonic
generation
(SHG) angles
phase-matching
calculated
SHG
measurements
performed
in
the
x–y
and
x–z
planes
with
femtosecond
pulses.
In
the
x–z
plane
which are presented in Figure 6b. The predicted phase-matching angles agree very well with the SHG
the deviation
at 4.63 µm
within
accuracy
while
in the x–y pulses.
plane, atInathe
fundamental
measurements
performed
in was
the x–y
andthe
x–zcutplanes
with
femtosecond
x–z plane the
wavelength of 4.42 µm, the internal experimental phase-matching angle was roughly 1° larger than
deviation at 4.63 µm was within the cut accuracy while in the x–y plane, at a fundamental wavelength
the calculated one. The SHG range for the fundamental wavelength, covered by interactions in the
of 4.42 µm, the internal experimental phase-matching angle was roughly 1˝ larger than the calculated
principal planes that possess non-vanishing effective nonlinearity deff, extends from 1.75 to 11.8 µm.
one. The SHG
range for the
fundamental
wavelength,
coveredinteractions
by interactions
planes
Phase-matching
calculations
for general
type three-wave
in the in
x–ythe
andprincipal
x–z planes,
that possess
non-vanishing
effective
deff , extends
from 1.75
to 11.8and
µm.
where broad
tuning can be
realizednonlinearity
for optical parametric
generators,
amplifiers
oscillators were
Phase-matching
calculations
for
general
type
three-wave
interactions
in
the
x–yLGSe,
and x–z
planes,
presented in [13]. As already mentioned, similar to the two parent compounds LISe and
LGISe
wherewill
broad
can beTPA
realized
optical
parametric
generators,
and oscillators
not tuning
suffer from
at a for
pump
wavelength
of 1.064
µm andamplifiers
phase matching
for suchwere
down-conversion
is feasible
in the
principal
planes.
presented
in [13]. Asnonlinear
alreadyprocesses
mentioned,
similar
to x–y
theand
twox–z
parent
compounds
LISe and LGSe,
In not
the principal
planes,
theatexpressions
for the effective
deff ofphase
LGISematching
read:
LGISe will
suffer from
TPA
a pump wavelength
of nonlinearity
1.064 µm and
for such
eoe
2
down-conversion nonlinear processes
x–y
principal planes.
(x–yx–z
plane)
d eff
= d effoeeis=feasible
− ( d 24 sin 2 φin+ the
d15 cos
φ )and
(1a)
In the principal planes, the expressions for the effective nonlinearity deff of LGISe read:
deoe
ef f
“
deffoeo = deffeoo = −d24 sinθ (y–z plane)
“ooe´pd24 sin2 φ ` d15 cos2 φq px ´ y
d eff = d 31 sin θ (x–z plane, θ < Vz)
“
deoo
doeo
ef f
e f f “ ´d24 sinθ py ´ z planeq
doee
ef f
(1b)
planeq
(1c)
(1a)
(1b)
with superscripts “o” and “e” denoting the ordinary and extraordinary beams. LGISe behaves as an
optically negative uniaxial crystal
in the x–y and x–z (for θ < Vz) planes, and as an optically positive (1c)
dooe
e f f “ d31 sinθ px ´ z plane, θ ă Vz q
uniaxial crystal in the y–z plane. Under Kleinman symmetry, d15 = d31 is satisfied.
with superscripts
“o” and
the ordinary
andinvestigated
extraordinary
beams.
The elements
dil of “e”
the denoting
nonlinear tensor
have been
by SHG
usingLGISe
type-I behaves
or type-IIas an
optically
negative
crystal
in of
theLGISe.
x–y and
x–z
(for θ < Vzpulses
) planes,
and
as anthe
optically
positive
interactions
in uniaxial
the principal
planes
Since
femtosecond
were
applied,
use of very
uniaxial
inLGISe
the y–z
Under
Kleinman
symmetry,
d15 =itdpossible
thincrystal
oriented
andplane.
reference
samples
(LISe and
LGSe) makes
to neglect the effects of
31 is satisfied.
absorption,
beam
walk-off,
and focusing
with
thebeen
data analysis
in theby
low-depletion
derived
The
elements
dil of
the nonlinear
tensor
have
investigated
SHG usinglimit
type-I
or type-II
from
standard
plane-wave
SHG
theory.
The
conversion
efficiency
was
kept
low,
and
we
corrected
interactions in the principal planes of LGISe. Since femtosecond pulses were applied, the use of very
the relative
measurements
only for
the crystal
thickness
and the
slightly
of refraction
thin oriented
LGISe
and reference
samples
(LISe
and LGSe)
makes
it different
possibleindex
to neglect
the effects
and Fresnel reflections. Unfortunately, only for type-I interaction, the spectral acceptance of all three
of absorption, beam walk-off, and focusing with the data analysis in the low-depletion limit derived
samples was much larger than the spectral bandwidth of the femtosecond pulses near 4.6 µm.
from standard plane-wave SHG theory. The conversion efficiency was kept low, and we corrected
the relative measurements only for the crystal thickness and the slightly different index of refraction
and Fresnel reflections. Unfortunately, only for type-I interaction, the spectral acceptance of all three
samples was much larger than the spectral bandwidth of the femtosecond pulses near 4.6 µm.
Crystals 2016, 6, 85
9 of 10
The femtosecond source at 1 kHz repetition rate was a KNbO3 -based optical parametric amplifier.
The single pulse energy was in the 2–3 µJ range. The measurements were performed simultaneously
with the two reference (LISe and LGSe) samples.
The coefficient d31 was estimated by type-I SHG in the x–z plane from Equation (1c). The LGISe
sample for SHG in the x–z plane was cut at θ = 53.6˝ and had a thickness of 0.7 mm. The reference
LGSe (0.9 mm thick) and LISe (0.86 mm thick) samples were cut at θ = 56.4˝ and θ = 46.7˝ , respectively.
We obtained d31 (LGISe) = 0.94d31 (LISe) and d31 (LGISe) = 1.03d31 (LGSe). The ratio of the nonlinear
coefficients of LISe and LGSe was 1.1, close to the ratio of 1.19 estimated at 2.3 µm from previous
measurements [2,6]. Thus, it can be concluded that within the experimental accuracy, ˘10% in
the relative measurements, LGISe has nonlinearities of the same order of magnitude as the other
two selenide compounds. While similar measurements were also performed for type-II SHG in the
x–y plane to determine d24 from Equation (1a), the thickness of the samples did not permit reliable
estimation because the spectral acceptance was not sufficient in order to consider samples of different
thickness unaffected by this factor.
5. Conclusion
LGISe, a new quaternary nonlinear crystal for the mid-IR grown as a solid-solution in the system
of the two ternary lithium selenides LGSe and LISe, exhibits the same orthorhombic structure (mm2)
as the parent compounds but is more technological with regard to the growth process since its
homogeneity range is broader in the phase diagram. It is transparent between 0.47 and 13 µm with
a band-gap of 2.94 eV at room temperature. The Sellmeier equations, constructed on the basis of
refractive index measurements, reproduce well SHG phase-matching angles in the principal dielectric
planes. The fundamental wavelength range for the SHG process extends from 1.75 to 11.8 µm.
The second-order nonlinearity of LGISe has an intermediate value, between those of LGSe and LISe.
Since the band-gap of LGISe is much closer to that of LISe, the electrical conductivity and response to
thermal neutrons are expected to be also similar. Additional gain in efficiency for thermal neutron
detection is expected due to substitution of half of the 115 In ions by Ga and the resulting weakening of
the competing interaction mechanism with the neutrons. Intense DAP luminescence allows one to use
LGISe also as a scintillator for detection of thermal neutrons.
Acknowledgments: We thank Sergey F. Solodovnikov for the X-ray structural analysis and Vladimir L. Panyutin
for the useful discussions and his help in the SHG experiments and in the preparation of this manuscript.
This work was supported by the Russian Foundation of Basic Research (Grant No. 15-02-03408a).
Author Contributions: Ludmila Isaenko and Alexander Yelisseyev conceived and realized the structural
studies, the transmission and photoluminescence measurements; Sergei Lobanov and Pavel Krinitsyn grew
the crystals, Vitaliy Vedenyapin performed the refractive index measurements and analyzed the phase-matching;
Valentin Petrov studied the SHG and determined the nonlinear coefficients; Ludmila Isaenko, Alexander Yelisseyev
and Valentin Petrov wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
Abbreviations
The following abbreviations are used in this manuscript:
LGSe
LISe
LGISe
TPA
PL
PLE
FWHM
DAP
SHG
LiGaSe2
LiInSe2
LiGa0.5 In0.5 Se2
two-photon absorption
photoluminescence
photoluminescence excitation
full width at half-maximum
donor-acceptor pair
second harmonic generation
Crystals 2016, 6, 85
10 of 10
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Isaenko, L.; Yelisseyev, A.; Lobanov, S.; Titov, A.; Petrov, V.; Zondy, J.-J.; Krinitsin, P.; Merkulov, A.;
Vedenyapin, V.; Smirnova, J. Growth and properties of LiGaX2 (X = S, Se, Te) single crystals for nonlinear
optical applications in the mid-IR. Cryst. Res. Technol. 2003, 38, 379–387. [CrossRef]
Petrov, V.; Yelisseyev, A.; Isaenko, L.; Lobanov, S.; Titov, A.; Zondy, J.-J. Second harmonic generation and
optical parametric amplification in the mid-IR with orthorhombic biaxial crystals LiGaS2 and LiGaSe2 .
Appl. Phys. B 2004, 78, 543–546. [CrossRef]
Isaenko, L.; Vasilyeva, L.; Merkulov, A.; Yelisseyev, A.; Lobanov, S. Growth of new nonlinear crystals LiMX2
(M = Al, In, Ga; X = S, Se, Te) for the mid-IR optics. J. Cryst. Growth 2005, 275, 217–223. [CrossRef]
Isaenko, L.; Yelisseyev, A.; Lobanov, S.; Krinitsin, P.; Petrov, V.; Zondy, J.-J. Ternary chalcogenides LiBC2
(B = In, Ga; C = S, Se, Te) for mid-IR nonlinear optics. J. Non-Cryst. Sol. 2006, 352, 2439–2443. [CrossRef]
Zondy, J.-J.; Petrov, V.; Yelisseyev, A.; Isaenko, L.; Lobanov, S. Orthorhombic crystals of lithium thioindate
and selenoindate for nonlinear optics in the mid-IR. In Mid-Infrared Coherent Sources and Applications;
Ebrahim-Zadeh, M., Sorokina, I., Eds.; Springer: Dordrecht, The Netherlands, 2008; pp. 67–104.
Fossier, S.; Salaün, S.; Mangin, J.; Bidault, O.; Thenot, I.; Zondy, J.-J.; Chen, W.; Rotermund, F.; Petrov, V.;
Petrov, P.; et al. Optical, vibrational, thermal, electrical, damage and phase-matching properties of lithium
thioindate. J. Opt. Soc. Am. B 2004, 21, 1981–2007. [CrossRef]
Petrov, V.; Zondy, J.-J.; Bidault, O.; Isaenko, L.; Vedenyapin, V.; Yelisseyev, A.; Chen, W.; Tyazhev, A.;
Lobanov, S.; Marchev, G.; et al. Optical, thermal, electrical, damage, and phase-matching properties of
lithium selenoindate. J. Opt. Soc. Am. B 2010, 27, 1902–1927. [CrossRef]
Petrov, V. Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using
non-oxide nonlinear crystals. Progress Quantum Electron. 2015, 42, 1–106. [CrossRef]
Schunemann, P.G.; Setzler, S.D.; Pollak, T.M. Phase-matched crystal growth of AgGaSe2 and AgGa1´x Inx Se2 .
J. Cryst. Growth 2000, 211, 257–264. [CrossRef]
Badikov, V.V.; Kuz’micheva, G.M.; Panyutin, V.L.; Rybakov, V.B.; Chizhikov, V.I.; Shevyrdyaeva, G.S.;
Shcherbakov, S.I. Preparation and structure of AgGa1´x Inx Se2 single crystals. Inorg. Mater. 2003, 39,
1028–1034. [CrossRef]
Badikov, V.V.; Laptev, V.B.; Panyutin, V.L.; Ryabov, E.A.; Shevyrdyaeva, G.S. Study of nonlinear-optical
characteristics of AgGa1´x Inx Se2 crystals. Quantum Electron. 2005, 35, 252–256. [CrossRef]
Andreev, Y.M.; Atuchin, V.V.; Lanskii, G.V.; Pervukhina, N.V.; Popov, V.V.; Trocenko, N.C. Linear optical
properties of LiIn(S1´x Sex )2 crystals and tuning of phase matching conditions. Sol. State Sci. 2005, 7,
1188–1193. [CrossRef]
Vedenyapin, V.; Isaenko, L.; Yelisseyev, A.; Lobanov, S.; Tyazhev, A.; Marchev, G.; Petrov, V. New mixed
LiGa0.5 In0.5 Se2 nonlinear crystal for the mid-IR. Proc. SPIE 2011, 7917. [CrossRef]
Tupitsyn, E.; Bhattacharya, P.; Rowe, E.; Matei, L.; Groza, M.; Wiggins, B.; Burger, A.; Stowe, A. Single crystal
of LiInSe2 semiconductor for neutron detector. Appl. Phys. Lett. 2012, 101. [CrossRef]
Wiggins, B.; Groza, M.; Tupitsyn, E.; Lukosi, E.; Stassun, K.; Burger, A.; Stowe, A. Scintillation properties
of semiconducting 6 LiInSe2 crystals to ionizing radiation. Nucl. Instr. Meth. Phys. Res. A 2015, 801, 73–77.
[CrossRef]
Cui, Y.; Bhattacharya, P.; Buliga, V.; Tupitsyn, E.; Rowe, E.; Wiggins, B.; Johnstone, D.; Stowe, A.; Burger, A.
Defects in 6 LiInSe2 neutron detector investigated by photo-induced current transient spectroscopy and
photoluminescence. Appl. Phys. Lett. 2013, 103, 092104. [CrossRef]
Wiggins, B.; Bell, J.; Burger, A.; Stassun, K.; Stowe, A.C. Investigations of 6 LiIn1´x Gax Se2 semi-insulating
crystals for neutron detection. Proc. SPIE 2015, 9593. [CrossRef]
Isaenko, L.I.; Vasilyeva, I.G. Nonlinear LiIII CVI 2 crystals for mid-IR and far-IR: Novel aspects in crystal
growth. J. Cryst. Growth 2008, 310, 1954–1960. [CrossRef]
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).