New strategy for designing promising mid-infrared nonlinear optical materials: narrowing the band gap for large nonlinear optical efficiencies and reducing the thermal effect for a high laser-induced damage threshold

Article abstract of DOI:10.1039/C8SC01210E

To circumvent the incompatibility between large nonlinear optical (NLO) efficiencies and high laser-induced damage thresholds (LIDTs) in mid-infrared NLO materials, a new strategy for designing materials with both excellent properties is proposed. This strategy involves narrowing the band gap for large NLO efficiencies and reducing the thermal effect for a high LIDT. To support these proposals, a series of isostructural chalcogenides with various tetrahedral center cations, Na₂Ga₂MQ₆ (M = Ge, Sn; Q = S, Se), were synthesized and studied in detail. Compared with the benchmark AGS, these chalcogenides exhibit significantly narrower band gaps (1.56-1.73 eV, AGS: 2.62 eV) and high NLO efficiencies (1.6-3.9 times that of AGS at 1910 nm), and also outstanding LIDTs of 8.5-13.3 × those of AGS for potential high-power applications, which are contrary to the conventional band gap view but can be attributed to their small thermal expansion anisotropy, surmounting the NLO-LIDT incompatibility. These results shed light on the search for practical IR NLO materials with excellent performance not restricted by NLO-LIDT incompatibility.

Full text of DOI:10.1039/C8SC01210E

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New Strategy for Designing Promising Mid-Infrared Nonlinear
Optical Materials: Narrowing Band Gap for Large Nonlinear
Optical Efficiency and Reducing Thermal Effect for High Laser-
induced Damage Threshold
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Shu-Fang Li,a Xiao-Ming Jiang,a,* Yu-Hang Fan,b Bin-Wen Liu,a Hui-Yi Zeng,a and Guo-Cong Guoa,*
www.rsc.org/
To circumvent the incompatibility between large nonlinear optical (NLO) efficiency and high laser-induced damage
threshold (LIDT) in mid-infrared NLO materials, a new strategy for designing both excellent properties is proposed. This
strategy involves narrowing band gap for large NLO efficiency and reducing thermal effect for high LIDT. To support of
these proposals, a series of isostructural chalcogenides with various tetrahedral center cations, Na2Ga2MQ6 (M = Ge, Sn; Q
= S, Se), were synthesized and studied in detail. Compared with the benchmark AGS, these chalcogenides exhibit
significantly narrower band gaps (1.56–1.73 eV, AGS: 2.62 eV) and high NLO efficiencies (1.6–3.9 times that of AGS at 1910
nm), and also outstanding LIDTs of 8.5–13.3 × AGS for potential high-power applications, which are contrary to the
conventional band gap view but can be attributed to their small thermal expansion anisotropy, surmounting the NLO-LIDT
incompatibility. These results shed light on the exploration of practical IR NLO materials with excellent performance not
restricted by the NLO–LIDT incompatibility.
power applications.11 Therefore, developing of new mid-IR
NLO materials with excellent comprehensive performance
(large NLO efficiency, high LIDT, wide transparency window,
Introduction
Nonlinear optical (NLO) materials are at the research front-
lines of laser science and technology because of their ability to
produce coherent and tunable light from solid-state lasers in
the fields of medical, military and communication network.1-4
For decades, various NLO materials such as β-BaB2O4 (BBO),5
LiB3O5 (LBO),6 KH2PO4 (KDP),7 KBe2B2O6F2 (KBBF),8 and KTiOPO4
(KTP)9 have been dis-covered and used commercially. Although
these materials are applicable for generating of coherent light
from the ultraviolet region to the near-infrared (IR) region, the
direct generation of mid-IR (2-20 µm) coherent light by second
harmonic generation (SHG) remains particularly challenging,
but crucial to various communication and optoelectronic
devices.10 For decades, numerous mid-IR NLO materials with
obvious SHG responses, mainly including pnictides,
chalcogenides, and halogenides, have been report-ed.11-12
However, only a few of them, namely, AgGaS2 (AGS),13-14
AgGaSe2 (AGSe),15-16 and ZnGeP2 (ZGP),17 can be used.
Unfortunately, these materials suffer from defects such as low
laser-induced damage thresholds (LIDT) that hinder their high-
and sufficient birefringence to achieve phase matchability)18 is
technologically and scientifically important.
Among all the requirements for high-power applications,
large NLO efficiency and high LIDT are the most challenging to
achieve concurrently. Previous theoretical and experimental
studies on IR NLO materials show that the wider band gaps are
beneficial to high LIDTs,1, 19-20 while also resulting in small NLO
efficiency, which implies the incompatibility between large
NLO efficiency and high LIDT. Two main methods have been
adopted to balance the conflict. One is increasing the band gap
by replacing the Ag+ in classical IA-IIIA-VIA2 type of IR NLO
materials with strong electropositive alkali or alkaline-earth
metal. Thus, LiBQ2 (B = Ga, In; Q = S, Se, Te) family and BaGa4S7
were observed,21-25 their band gaps up to 4 eV lead to LIDTs
exceeding those of the Ag analogs, while their NLO efficiencies
are slightly lower. For example, the band gap of LiGaS2 is 3.65
eV, which is significantly larger than that of AGS (2.62 eV);26
nevertheless, the SHG coefficient of LiGaS2 is approximately a
quarter of that of AGS.27 The other method involves combing
two or more NLO-active motifs such as GaQ4, InQ4, GeQ4, and
SiQ4 (Q = S, Se) tetrahedra in a single IR NLO chalcogenide
compound to modulate the LIDT and NLO efficiency.28-44 The
combination of different NLO-active motifs can optimize the
NLO efficiency and LIDT with high flexibility. For example, by
incorporating GeS2 into AgGaS2, quaternary AgGaGeS4 was
developed with improved LIDT, which makes it a promising
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alternative to the widely used AGS.45-47 Additionally, a series of expansion coefficient may be modulated in a relatively easy
Li-containing quaternary compounds have been reported, manner to enhance LIDT. The expansion coefficient of a
including Li2Ga2GeS6, LiGaGe2Se6, Li2In2GeSe6, and Li2In2SiSe6, compound depends on the expansion coefficients (dR/dT) of
the combination of different microscopic NLO-active units, all chemical bonds in it, which can be evaluated through dR/dT
namely, GaQ4, InQ4, GeQ4, SiQ4, in these compounds = 1.35k/G based on bond-valence theory,55 where k is the
strengthen their nonlinear susceptibilities compared with the Boltzmann constant and G is the force constant of the
48-49
Although many chemical bonds. Normally, for the same type of chemical
corresponding ternary compounds.21,
attempts have been made using these two main methods, bonds, G is larger for shorter bond lengths. However, for the
excellent IR NLO materials with both high LIDT and strong NLO same types of center and coordinated ions, according to the
efficiency are rare.
bond-valence-sum rule, center ions that have fewer
Theoretically, SHG is a two-photon process, and the value coordinated atoms are likely to form shorter bond lengths and
of NLO efficiency is determined by the optical transition matrix larger force constants. In such circumstances, relatively smaller
elements, which strongly depend on the band gap of an IR NLO thermal expansion coefficients and higher LIDT can be
material.50 Normally, a small band gap can easily result in predicted.
strong optical transition and is therefore beneficial to large
For the case in which thermal LIDT dominates the laser-
NLO efficiency. Compared with SHG, the laser damage on the induced damage process in the practical application of mid-IR
NLO crystals is a highly complicated process that involves NLO materials, NLO efficiency and LIDT are relatively
pitting, erosion, melting, delamination, fracture, and independent of each other, and almost no incompatibility
discoloration.51 The mechanisms of laser damage process can exists between them. We propose the new strategy of
be mainly categorized into thermal processes and dielectric designing IR NLO materials by narrowing the band gap for
processes. For NLO crystals with high laser transparency and large NLO efficiency and reducing the thermal effect for high
few defects for optical absorption, dielectric process laser-induced damage threshold. These approaches are
mechanism is applied, and dielectric LIDT is proportional to effective in achieving high NLO efficiency and LIDT
atomic density/(n2-1), where n is refractivity and has large concurrently.
values for materials with narrow band gaps. Therefore, the
Recently, metal chalcogenides are highly attractive in
narrow band gaps simultaneously lead to large NLO efficiency exploring new mid-IR NLO materials. Some alkali metal-
and low LIDT. This result is also the reason for the so-called contained quaternary chalcogenides were reported to exhibit
incompatibility between large NLO efficiency and high LIDT in large NLO efficiencies, such as K4GeP4Se12,56 Na2Ge2Se5,57
some NLO compound systems, such as well-known oxides and K3TaAsS11,10 and CsZrPSe6,58 which possess NLO coefficients of
IA-IIIA-VIA2 type of IR NLO materials.7
14.02, 12.01, 9.91, and 9.92 times that of AGS, respectively.
However, for many practical mid-IR NLO materials, Highly electropositive alkali metals in these chalcogenides
intrinsic and defect-induced optical absorptions are non- usually play the role of dimensional reduction agents to reduce
negligible, which may lead to a temperature increase when the the coordinated atoms of host chalcogenide frameworks.
laser beam is incident on the surface of an NLO crystal, Based on the preceding analysis, besides having high NLO
thereby resulting in thermal expansion, strain, distortion, efficiencies, alkali-metal-contained chalcogenides may exhibit
cracking, catastrophic shattering and so on. In this condition, high LIDTs due to the strengthening of force constants and the
the thermal process mechanism can be applied;51 reduction of thermal expansion coefficients of metal-
corresponding thermal LIDTs scale proportionally to CκS/αβ, chalcogen bonds.
where C is the heat capacity of the material and can seem
close to each other for any solid with a similar atomic density compounds in the alkali metal sodium-contained
above room temperature, κ is thermal conductivity, S is chalcogenides, namely, Na2Ga2GeSSe5 ), Na2Ga2GeSe6 ),
damaging stress and is positively correlated with Young’s Na2Ga2SnSSe5 ), and Na2Ga2SnSe6 ). All these materials
To verify our new strategy, we found four new
(
1
(2
(
3
(4
modulus and shear modulus of the material, β is expansion exhibit phase-matching behavior with large NLO efficiency of
coefficient and α is absorption coefficient. These results show 1.6–3.9 × AGS. Their band gaps are obviously narrower than
that LIDT can be enhanced by reducing the thermal effects, those of AGS, which should result in relatively smaller LIDTs
i.e., by increasing the thermal conductivity and damaging based on the conventional view. However, the four
stress, and decreasing the expansion and absorption compounds exhibit extremely large LIDTs of 8.5–13.3 × AGS,
coefficients of the IR NLO materials.
which can be attributed to their smaller thermal expansion
The intrinsic physical parameters of NLO crystals, thermal anisotropy compared with those of AGS. In the following
conductivity, damaging stress, and absorption coefficient section, we report synthesis, characterization, optical and
strongly depend on the microscopic structural features of thermal properties of these compounds. Theoretical
practical crystals, such as number and size of localized calculations of electronic band structures and NLO efficiencies
absorbing defects and impurities.52-54 Nevertheless, the are also conducted for these compounds.
expansion coefficient of a particular type of NLO crystal does
not change significantly with different crystal growing
conditions. Therefore, from the perspective of crystal
structural design of mid-IR NLO materials, the parameter of
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Single-crystal Structure Determination. Block single crystals of
1–4 were selected for single-crystal diffraction. Diffraction data
were collected by using graphite monochromated Mo Kα
Experimental Section
Synthesis. The chemicals used in this study are the following:
Na (99.7%), Ga (99.99%), Ge (99.99%), Sn (99.9%), S (99.5%),
and Se (99.9%). All of these chemicals were from Aladdin
Chemistry Co. Ltd. and were used as received without further
purification. Na2S and Na2Se starting materials were prepared
through stoichiometric reaction of Na and S or Se in liquid
radiation (λ
= 0.71073 Å) on a Rigaku Pilatus CCD
diffractometer at 293 K. The intensity data sets were collected
with a ω-scan technique and reduced using CrystalClear
software.59
The structures of 1–4 were solved by direct methods and
refined by full-matrix least-squares method on F2 with
anisotropic thermal parameters for all atoms. All calculations
were performed with SHELXL package of crystallographic
software.60 The formulas collectively consider the
crystallographically refined compositions and requirements of
charge neutrality. The Addsym/Platon program was used to
check the final structures for additional symmetry, and no
other missed or higher symmetry element was found.61 Crystal
data and structural refinement information for 1–4 are
summarized in Table 1. Atomic coordinates and equivalent
isotropic displacement parameters are listed in Table S1.
Selected bond distances are reported in Table S2.
ammonia.30 For the synthesis of target compounds,
stoichiometric mixture of starting materials Na2S, Ga, Ge (Sn),
and Se in a molar ratio of 1:2:1:5 for and , and Na2Se, Ga,
Ge (Sn), and Se in a molar ratio of 1:2:1:6 for and , were
a
1
3
2
4
loaded into a graphite crucible and placed in quartz tubes. The
tubes were flame-sealed under vacuum (~10−4 Torr) and then
placed into a temperature controlled muffle furnace, heated
from room temperature to 800 oC in 40 h, kept at that
temperature for 96 h, and then cooled to room temperature at
4 °C/h. The products were washed with degassed DMF and
dried with ethanol. Yellow crystals of
of , and red crystals of , with respective yields of 60%, 60%,
70%, and 50% based on Ga for , were obtained. All the
1 and 2, orange crystals
3
4
1–4
compounds are stable in air and water.
Table 1. Crystal data and structure refinement parameters for 1–4
.
1
2
3
4
Chemical formula
Formula weight
Na2Ga2GeSSe5
684.87
Na2Ga2GeSe6
731.77
Na2Ga2SnSSe5
730.97
Na2Ga2SnSe6
777.87
Crystal size (mm3)
0.117 × 0.115 × 0.106
0.110 × 0.105 × 0.102
0.117 × 0.083 × 0.066
Orthorhombic
Fdd2
13.264(4)
0.071 × 0.063 × 0.046
Crystal system
Space group
a (Å)
12.987(10)
23.653(17)
7.519(5)
2310(3)
8
12.985(4)
23.880(8)
7.585(3)
2352.0(14)
8
13.329(3)
24.291(7)
7.621(2)
2467.5(11)
8
b (Å)
23.936(7)
7.514(2)
2385.6(12)
8
c (Å)
V (Å3)
Z
Dcalcd(g cm-3
μ(mm-1
)
3.939
4.133
4.070
4.188
)
23.170
3.25-25.46
1.049
25.671
3.23-25.46
0.977
22.007
3.40-25.46
1.114
24.058
3.19-25.47
1.101
θ range (°)
GOF on F2
R1a [I >2
wR2b [I >2
σ
(I)]
(I)]
0.0375
0.0990
0.0389
0.1009
0.00
0.0238
0.0486
0.0285
0.0496
0.00
0.0257
0.0661
0.0270
0.0668
0.00
0.0209
0.0549
0.0218
0.0551
0.00
σ
R1a (all data)
wR2b(all data)
Flack parameter x
Δρmax/Δρmin(e Å-3
)
1.288/-0.991
0.538/-0.986
0.662/-0.768
0.852/-0.528
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Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) with a PE50-DIF-C energy sensor and a vernier caliper were
measurements were performed on a Rigaku MiniFlex II respectively used to measure the power the of laser beam and
diffractometer using Cu Kα radiation in reflection mode at identify the damaged spots.
room temperature with a step size of 0.02° in the range of 2θ = Thermal Analyses. The thermal properties of 1–4 were
5−65°. The experimental and simulated PXRD paꢀerns of 1–4 investigated by differential scanning calorimetry (DSC) analysis
are shown in Fig. S2, indicating the purities of the as- using a Netzsch STA 449C thermal analyzer in a nitrogen
synthesized samples, despite the possible existence of a tiny atmosphere. Each sample of approximately 10 mg was placed
amount of NaGa3Se5 in phase
Energy-Dispersive X-ray
2.
in sealed silica tubes evacuated to 10−4 Torr, heated to 900 °C
Spectroscopy
Analysis. at 10 °C /min and cooled to 30 °C at 10 °C /min.
Semiquantitative microprobe element analyses of the crystals Electronic Structure Calculations. Electronic band structures
of the four compounds were performed with a field emission and densities of states (DOS) of 1–4 were calculated using the
scanning electron microscope (FESEM, JSM6700F) equipped structure models directly from single-crystal XRD analysis,
with an energy-dispersive X-ray spectroscope (EDX, Oxford which are calculated based on density functional theory (DFT)
INCA). The empirical formula of Na2.3Ga2.1Ge1.0S0.8Se5.1 for
Na1.9Ga2.3Ge1.0Se5.8 for , Na2.2Ga2.3Sn1.0S1.0Se5.1 for
, and The electrons in Na: 2s22p63s1, Ga: 3d104s24p1, Ge: 4s24p2, Sn:
Na1.8Ga2.0Sn1.0Se6.1 for
were obtained, and no other element 5s25p2, S: 3s23p4 and Se: 4s24p4 were treated as valence
was detected (Fig. S1), which were consistent with the results electrons. The plane-wave cutoff energy is set to 800 eV for 1–
determined from single-crystal XRD. . Moreover, the numerical integration of the Brillouin zone
1,
with CASTEP code provided by Materials Studio package.64-65
2
3
4
4
SHG Measurements. The SHG measurements of 1–4 were was performed by using 2 × 1 × 3 Monkhorst–Pack k-point
measured using a modified Kurtz-Perry powder technique with meshes for all compounds. The Fermi level (Ef = 0 eV) was
1910 nm laser radiation.62 Microcrystalline powder samples selected as reference.
were ground and sieved into several distinct particle size
Theoretical studies on optical properties regarding the
ranges (30–50, 50–75, 75–100, 100–125, 125–150, and 150– complex dielectric function ε(ω) = ε1(ω)+iε2(ω) are expressed
200 μm)for the SHG phase-matching measurements. AGS as follows:66-67
2e2
π
2
crystals with similar particle sizes were used as reference.
During the measurements, all of the samples were pressed
between two glass microscope cover slides and secured in the
1 mm-thick plastic holders. After the mixed signals passed
through the monochromator, the doubled frequency signals
(955 nm) were detected by an Andor DU420A-BR-DD CCD.
Powder LIDT Measurements. The powder LIDTs of 1–4 and the
reference AGS were evaluated by single-pulse measurement
method using a focused high-power 1064 nm laser beam with
a pulse width τp of 10 ns and a repetition rate of 1 Hz.63 The
measurements were conducted by gradually increasing the
laser power until the damaged spot of samples was monitored
under a microscope after the irradiation. The Nova II sensor
∑
ε2(ω) =
ꢂ,ꢃ,ꢄ |< ΨKC |û · r |ΨKV > | δ ECK - EVK - E
ꢀ ꢁ, (1)
Ωε0
where ꢅꢀꢆꢂꢄ ꢇ ꢆꢂꢃ ꢇ ꢆꢁ indicates the energy difference
between the conduction and valence bands at k point with
absorption of energy E, û denotes the polarization of the
incident electric field, Ω is the volume of the primitive cell, e is
the electric charge, ꢈꢂꢄ and ꢈꢂꢃ are the vectors defining the
conduction and valence band wave functions at k, respectively,
and ε1(ω) can be obtained by using the dispersion relationship
of Kramers–Kronig as following:
∞
ε1(ω) = 1 + 2 P
ω'ε (ω') dω'.
(2)
2
ꢉ
ω'2- ω2
0
π
Fig. 1 (a) The 3-D structure of 1 viewed down to the c direction. Grey atoms: Na, pink tetrahedra: [Ga(S/Se)Se3)]5-, green tetrahedra: [Ge(S/Se)2Se2]4-; yellow/orange atoms: S/Se
atoms. (b) The infinite ∞1[Ga(S/Se)Se2)]3- chains represented as pink columns and isolated [Ge(S/Se)2Se2]4- connections (green tetrahedra) in the unit cell of 1. The Na+ cations are
left out for clarity.
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The P before the integral indicates the principal value. The intensities increase with the growth of particle sizes. The
first-order nonresonant susceptibility at the low-frequency results are consistent with type-I phase-matching behavior,
region is given by ꢊꢋꢌꢍ(ω) = ε1(ω)−1, and the second-order which is important in practical applications. Remarkably, the
susceptibilities were calculated using the anharmonic oscillator SHG intensities of 1–4 are about 2.3, 1.6, 3.9, and 2.1 times
model.50
that of commercial AGS with the particle size of 75−100 μm
(Fig. 2b). Generally, the SHG signal intensity measured by the
Kurtz − Perry powder method is proporꢁonal to the square of
the second-order NLO coefficient deff, and the reported deff of
Results and discussion
AGS is 12.5 pm/V26. Thus, the derived deff for
1−4 are 18.95,
Crystal Structure. Compounds
1~4 are isostructural and
15.81, 24.68, and 18.11 pm/V.
crystallize in the NCS space group of Fdd2 in the orthorhombic
system with unit cell parameters of a = 12.432(4) – 13.329(3)
Å, b = 22.584(7) – 24.291(7) Å, c = 7.239(2) – 7.261(2) Å, and Z
Interestingly, compound
3 Na2Ga2SnSSe5 has the highest
SHG intensity among these compounds, whereas compound
2
has the lowest. This result is reasonable according to structural
analysis. Both S/Se and Ga/Sn occupancy disorders are
=
8. In the asymmetric unit, two crystallographically
independent positions are observed for Ga, Ge, or mixed
Ga/Sn atoms; three positions for fully occupied S, Se, or mixed
S/Se atoms; and one position for Na.
observed in
3, which can lead to structural distortion and
enhancement of noncentrality and SHG intensity, compared
with the corresponding perfect structure. Moreover, the
cation size effect may also have effects on the formation of
these compounds and ultimately their NLO properties.74,75
In compound 1, the 3-D framework consists of two
different building blocks, namely, 1D infinite 1[Ga(S/Se)Se2)]3-
∞
chains (pink tetrahedral chains in Fig. 1a) and isolated
[Ge(S/Se)2Se2]4- tetrahedra (green tetrahedra in Fig. 1a). Each
Ga atom is coordinated by one mixed S/Se and three Se atoms
forming the [Ga(S/Se)Se3)]5- tetrahedra with Ga−S/Se distances
ranging from 2.359 Å to 2.406 Å, which then connect with each
other by sharing common S/Se atoms to form infinite linear
1[Ga(S/Se)Se2)]3- chains. Similar to Ga atoms, each Ge atom is
∞
tetrahedrally coordinated by two mixed S/Se and two Se
atoms to form the [Ge(S/Se)2Se2]4- tetrahedra with Ge−S/Se
distances ranging from 2.352 Å to 2.374 Å. These two different
building blocks, namely, infinite 1[Ga(S/Se)Se2)]3- chains and
∞
isolated [Ge(S/Se)2Se2]4- tetrahedra, are assembled to form the
3D anionic (Ga2GeSSe5)2- framework of
1 (Fig. 1b). Two groups
of 1[Ga(S/Se)Se2)]3- chains along the a+c and a-c directions
∞
intersect each other with [Ge(S/Se)2Se2]4- tetrahedra acting as
the connections between them and forming tunnels along the
c direction, which are embedded with the counter cations Na+.
All positions in
besides the S/Se mixing, the Ga and Sn positions are mixed
with a molar ratio of 2:1. The Sn positions in , similar to the
Ge positions in , are mixed with Ga with half occupancy. The
2 are fully occupied. In compound 3,
4
1
mixed occupancy of S/Se and Ga/Sn positions are determined
by the structural refinements that consider charge balance.
The Ga−S(Se), Ge−S(Se), (Ga/Sn)−S, and Na−S(Se) bond lengths
in 1–4 (Table S2) are close to those in BaGa4S7,25 BaGa2GeS6,36
Ba2Ga8GeS16,41 PbGa2GeSe6,18 Na4MgGeSe6,68 Ba5Ga2Se8,69
KCd4Ga5Se12,70 Na3Mo15Se19,71 and Na4Sn3S8.72
Interestingly, modifying the chalcogen ratio and mutual
replacement of Ga, Ge, and Sn atoms does not lead to
structural changes in the Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se)
system. Such structure retention is important when
conducting chalcogen or metaldoping to optimize the physical
performances of these materials, similar to the case of
nonlinear optical materials.73
Fig. 2 (a) Phase-matching curves of 1–4 and reference AGS. (b) SHG intensities of 1–4
with AGS as the reference in the particle size of 75−100 μm.
NLO Properties. The SHG of 1–4 has been investigated with a
1910 nm laser and AGS as reference. The SHG signals of 1–4 as
a function of particle size are shown in Fig. 2. Their SHG
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By contrast, no mixing position can be observed in 2, and
no additional SHG enhancement from the structural distortion
is present. For cases and , either S/Se ( ) or Ga/Sn ( ) is
present in their structures, and their SHG intensities are
between those of and . Given the structure retention of the
Table 3. Thermal expansion coefficients αL (×10-5 K-1) of
and thermal expansion anisotropy of 1–4 and reference AGS.
a, b and c axis lengths
1
4
1
4
1
2
3
4
AGS
2
3
Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se) system under chalcogen
and metal variation, new compounds with SHG intensity even
a
5.58
-5.54
-8.81
1.59
5.86
6.85
-8.75
1.49
-5.46
-2.71
2.99
2.01
3.68
-4.09
5.52
1.51
3.08
3.08
-9.158
2.97
b
higher than that of
metal variation.
3 may be obtained by the chalcogen and
c
The IR transmission spectra show that compounds 1−4
have no obvious intrinsic absorption of chemical bonds in
δ
abroad spectral range from 0.25 to 25 μm (Fig. S3), indicating
that compounds 1−4 may be good candidates for a variety of
NLO applications in the mid- and far-IR regions. The optical
dominate the LIDT of
the LIDT order of LIDT (
accordance with the opposite of NLO efficiency order of deff
< deff ) < deff ) < deff ). This issue is discussed in the following
section.
1
−
4
, rather than the band gap, although
diffuse reflectance spectra indicate optical band gaps (Eg) of
3
) < LIDT ( ) < LIDT ( ) < LIDT ( ) is in
1
4
2
1.56, 1.61, 1.63 and 1.73 eV for 1−4 (Fig. S4), which are
significantly smaller than that of AGS (2.65 eV).1 The NLO
efficiency is positively associated with the band gap, and the
(2)
(
4
(
1
(3
band gaps of
with their NLO efficiencies larger than that of AGS. The band-
gap order of Eg( ) < Eg( ) < Eg( ) < Eg( ) is not in accordance
with the opposite of NLO efficiency order of deff ) < deff ) <
deff ) < deff ), which can be attributed to the role of structure
1−4 narrower than that of AGS are consistent
Thermal Properties. Thermal effects are non-negligible in the
practical application of mid-IR NLO materials; the large
temperature increase caused by optical absorptions lead to
thermal expansion, strain, distortion, cracking, catastrophic
shattering, and damage of crystals.51 Thermal expansion
anisotropy (TEA, δ) of NLO materials is identified as one of the
most important intrinsic parameters that influence the
ultimate experimental derived LIDTs. NLO materials with
smaller TEA tend to sustain a larger thermal shock under laser
irradiation and exhibit higher LIDTs. To study the TEA of 1–4
and the reference AGS, the temperature dependence of the
lattice parameters was measured using an X-ray diffractometer
from 300 K to 500 K with a step of ~20 K (Fig. S6). The thermal
expansion coefficients (TEC, αL = R0-1[dR(T)/dT], R0 is the value
1
2
3
4
(
2
(4
(
1
(3
distortion induced by the atom mixing. Overall, narrowing the
band gap of an NLO material is an effective way to achieve
large NLO efficiency.
Powder LIDTs. The powder LIDTs of 1−4 and the benchmark
AGS measured by the single pulse method is summarized in
Table 2. During the measurement, small spot areas with size of
0.1590, 0.1662, 0.2463 and 0.1963 cm2 are selected for
1−4,
respectively. The measured damage energies of 1−4
(20.07−27.99 mJ) are much higher than that of AGS (11.02 mJ).
The derived powder LIDTs of 1−4 are 9.9, 13.3, 8.5, and 10.1
at T = 0 K) of the lattice parameters (a, b, c axis lengths) of 1~4
times that of AGS, supporting their potential for high-power
applications. In this study, the experimental LIDT of benchmark
are derived. TEA (δ), defined as the ratio of the maximum and
minimum of TECs, are 1.59, 1.49, 2.01 and 1.51 for 1–4 (Table
3). The TEC and TEA of AGS have been measured in our
previous work76 and are close to the other reported values.77
AGS is 1.27 MW⋅
cm-2, which is close to the reported values.18
According to the conventional view of incompatibility between
large NLO efficiency and high LIDT NLO materials, compounds
The measured TEA order of δ(AGS) > δ(
3) > δ(1) > δ(4) >
1−4 with band gaps of significantly narrower than that of AGS
δ(2) is in accordance with their experimental LIDTs that have
should have smaller LIDT.
the opposite order. According to the structural analysis of the
four compounds, atom mixing occurs in the structures except
However, the experimental LIDT values of 1−4 are much
higher than that of AGS, and there should be other factors that
2, where the structural defects are indispensable, as well as
the defect-induced optical absorptions during the
measurements. TEA is proposed to be the dominant factor of
LIDTs of the Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se) system, as
supported by the experimental results. Reducing the thermal
effect, especially by decreasing the thermal expansion
anisotropy of an NLO material, is reasonable to achieve high
LIDT. The TEA and LIDT of some typical IR NLO crystals, LiInS2,
ZnGeP2, GaSe and AgGaSe2 have been listed in Table S3. The
TEA of ZnGeP2 and GaSe are close to each other, resulting in
their similar LIDTs. The TEA of LiInS2 is much larger than that
of AGS. However, its LIDT is also much larger than that of
AGS, which can be ascribed to the shorter pulse width used
for the LIDT measurement of LiInS2.
Table 2. LIDTs of 1~4.
Compound
Damage
Energy
(mJ)
Spot Area
(cm2)
Damage
Threshold
[MW⋅
cm-2]
1
20.07
0.1590
0.1662
0.2463
0.1963
0.8659
12.62
16.84
10.79
12.82
1.27
2
27.99
26.59
25.19
11.02
3
4
AGS
The DSC results in Fig. S5 show that compounds 1–4
6 | J. Name., 2012, 00, 1-3
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undergo congruent melting upon heating and crystallization compounds 1–4 have three independent non-zero SHG tensors
upon cooling events with relatively lower melting d113 d223 and d333 under the restriction of Kleinman
,
,
temperatures (606, 571, 659, and 689 °C for 1–4). In symmetry.The calculated energy dependence of SHG tensors
comparison, the melting points of the known IR NLO materials, of 1–4 are shown in Fig. 3, at the wavelength of 1910 nm
such as AGS (998 °C), LiGaS2 (1050 °C), LiGaSe2 (915 °C), (0.6492 eV). The d113, d223, and d333 values are 18.72, 19.22 and
40
BaGa4S7 (1090 °C), and BaGa4Se7 (968 °C) are significantly 19.09 pm/V for
1
; 13.89, 13.39 and 13.84 pm/V for
; and 17.06, 16.83 and 16.21 pm/V
. The calculated average SHG coefficients dcal defined as
Theoretical Calculations. Theoretical band gaps of 1–4 are the arithmetic mean of all SHG tensors with a descending
calculated to be 2.01, 2.02, 2.00, and 1.95 eV, (Fig. S7). order of dcal , 22.69 pm/V) > dcal , 19.01 pm/V) > dcal , 16.70
Compared with the experimental results, the calculated band pm/V) dcal
2; 23.69,
higher. These results reveal that large crystals of 1–4 can be 22.10 and 22.28 pm/V for
grown using the Bridgman method at lower temperatures. for
3
4
(
3
(
1
(4
>
(
2
,
13.71 pm/V) are consistent with the
gaps of 1–4 have certain deviations due to the limitation of the experimental results.
DFT method in semiconductors and insulators.78-79 As observed
seen from the partial DOS presented in Fig. S8, for all four
Conclusions
compounds, the valence band close to the Fermi level
originates predominantly from Se-4p and S-3p states for
and Se-4p for . The conductive bands (CB) of 1–4 close to
the CB bottom are mostly composed of Ga-4p, Ge-4p and Se-
4p states in , Ge-4p and Se-4s states in , and Sn-5p and Se-4s
states in and . Therefore, their optical absorptions can be
1, 3
A new strategy to circumvent the incompatibility between the
large NLO efficiency and high LIDT in mid-infrared NLO
materials, namely, narrowing band gap for large NLO efficiency
and reducing thermal effect for high LIDT, is proposed in this
study. To support this proposal, a series of isostructural
chalcogenides with different tetrahedral center cations,
Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se), were successfully
synthesized using the solid-state method. The 3D structures of
these chalcogenides were assembled by 1D infinite
2, 4
1
2
3
4
mainly attributed to the charge transfer from Se-4p (S-3p)
states to Sn-5p (Ga-4p, Ge-4p) and Se-4p (Se-4s) states, and
the electronic structure around the band edges is mainly
derived from the (Sn/Ga/Ge)(S/Se)4 tetrahedra units, which
provide the dominant states in the optical matrix elements
describing the virtual excitations of the SHG effect in 1–4. The
strongly isolated s and p orbitals of Na+ cations are localized
far from the band gaps and have a negligible influence on the
SHG effect.
1[Ga(Sn)Q3)]3- tetrahedral chains and [MQ4]4- tetrahedral
∞
connections. All of them exhibit large SHG responses of
approximately 1.6–3.9 × AGS with phase-matching ability,
which result from their relatively small band gaps (1.56–1.73
eV) compared with AGS (2.65 eV). They also exhibit
outstanding LIDTs of 8.5–13.3 × AGS for potential high-power
applications, which is contrary to the conventional view on
band gaps. However, the measured thermal expansion
anisotropy order of these compounds is in agreement with
their experimental LIDTs with the opposite order, indicating
that thermal expansion anisotropy is one of the main thermal
parameters that influence LIDTs of IR NLO materials and high
LIDT can be achieved by reducing the thermal effect. The
results of this study shed light on the exploration of practical IR
NLO materials with excellent performances not restricted by
the NLO–LIDT incompatibility.
To obtain
a
profound understanding of the NLO
the theoretical calculations of SHG
properties of 1–4
,
coefficients were performed. The real (ε1) and imaginary (ε2)
parts of optical dielectric constants along the x, y and z
ave
directions are shown in Fig. S9 and S10. The curves of ε2
defined as (εx+εy+εz)/3, reveal the strongest absorptions of
5.72, at 5.54, at 8.54, and
,
1
at
2
3
4
at 8.74 eV, which can be mainly
assigned to the electronic interband transitions according to
the DOS analysis. Owing to the NCS point group of mm2,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the NSF of China
(21701176), the National Postdoctoral Program for Innovative
Talents (BX201600163), the China Postdoctoral Science
Foundation (2016M600510), the National Key Laboratory
Development Fund (20180026) and the NSF of Fujian Province
(2018J05034).
Fig. 3 Calculated frequency-dependent SHG coefficients of 1–4.
Notes and references
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Chemical Science
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New Strategy for Designing Promising Mid-Infrared Nonlinear
Optical Materials: Narrowing Band Gap for Large Nonlinear
Optical Efficiency and Reducing Thermal Effect for High
Laser-induced Damage Threshold
Shu-Fang Li,a Xiao-Ming Jiang,a,* Yu-Hang Fan,b Bin-Wen Liu,a Hui-Yi Zeng,a and
Guo-Cong Guoa,*
Graphic Abstract
A new strategy is verified on the exploration of practical IR NLO materials not restricted by the
NLO–LIDT incompatibility.