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ORIGINAL PAPER

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Stimulated Raman Scattering in Melilite-Type Crystals

Ca2MgSi2O7and Ca2Ga2SiO7

Alexander A. Kaminskii, Ladislav Bohatý, Hans Joachim Eichler, Oliver Lux, Hanjo Rhee,

Manfred Burianek, and Petra Becker*

𝝌(3)-nonlinear optical interactions in two melilite-type stimulated Raman

scattering (SRS)-active non-centrosymmetric crystals, Ca2MgSi2O7and

Ca2Ga2SiO7, formerly known as Nd3+-laser media, are presented. Under

picosecond pumping at 1.064 and 0.532 µm cascaded and cross-cascaded

eﬀects occur in these tetragonal silicates. Besides the SRS-promoting phonon

modes with energy of 𝝎SRS1 ≈908 cm−1and 𝝎SRS2 ≈668 cm−1for

Ca2MgSi2O7,and𝝎SRS1 ≈720 cm−1and 𝝎SRS2 ≈550 cm−1for Ca2Ga2SiO7,

respectively, combined phonon modes are observed. For Ca2MgSi2O7new

data in a broad wavelength range of refractive indices and their dispersion are

given as well. The observed 𝝌(3)-nonlinear properties expand the functionality

of the studied silicates and foreshadow their use in self-frequency Raman

laser converters (self-SRS lasers).

Prof.A.A.Kaminskii

[+]

Institute of Crystallography

Federal Scientiﬁc Center “Crystallography and Photonics,”

Russian Academy of Sciences

Moscow 119333, Russia

Prof.L.Bohatý,Dr.M.Burianek,Prof.P.Becker

Section Crystallography

Institute of Geology und Mineralogy

University of Cologne

Zülpicher Str. 49 b, Köln 50674, Germany

E-mail: [email protected]

Prof.H.J.Eichler,Dr.O.Lux,Dr.H.Rhee

Institute of Optics and Atomic Physics

Technical University of Berlin

Berlin 10623, Germany

Dr. O. Lux

Institute of Atmospheric Physics

German Aerospace Center (DLR)

Münchener Str. 20, Oberpfaﬀenhofen, Wessling 82234, Germany

Dr. M. Burianek

Faculty Geosciences

University Bremen

Klagenfurter Straße 2-4, Bremen 28359, Germany

The ORCID identiﬁcation number(s) for the author(s) of this article

can be found under https://doi.org/10.1002/crat.202000038

[+]The author passed away during the preparation of the publication.

KGaA, Weinheim. This is an open access article under the terms of the

Creative Commons Attribution-NonCommercial-NoDerivs License,

which permits use and distribution in any medium, provided the original

work is properly cited, the use is non-commercial and no modiﬁcations

or adaptations are made.

DOI: 10.1002/crat.202000038

1. Introduction

Among the modern trends in the devel-

opment of the physics of laser crystals

the identiﬁcation of new physical prop-

erties of these crystals, which can en-

rich their application potential, is of topi-

cal interest. In recent years, these trends

were supported by fundamental studies

of the numerous manifestations of non-

linear cascaded interactions in 𝜒(3)-and

𝜒(2)+𝜒(3)-active laser crystals doped with

trivalent lanthanide (Ln3+) lasant ions.

Among them are crystals with melilite-

type structure, with a history that dates

back to 1968, when Ba2MgGe2O7was

ﬁrst described as a laser crystal.[1] In the

following 50 years, a large number of melilite-type laser crys-

tals have been discovered and investigated (see, e.g., reviews by

Weber[2] and Kaminskii[3]). A new stage in the study of the phys-

ical properties of this class of crystals was marked in 2008, when

the 𝜒(3)-nonlinear eﬀect of stimulated Raman scattering (SRS)

was discovered just in the same laser “melilite” Ba2MgGe2O7.[4]

Over the past decade, 𝜒(3)-nonlinear optical properties of a num-

ber of tetragonal (space group P

42m−D3

2d) “melilite” laser crys-

tals have been discovered (see Table 1), giving them the status

of SRS-active Ln3+-laser crystals with enhanced application po-

tential. Here, the realization of self-SRS-laser converters (also

known as self-Raman lasers), where stimulated emission (SE) of

the Ln3+-ions is coherently converted into Stokes laser output ra-

diation by SRS in the host crystal, oﬀers the generation of new

laser wavelengths and is thus of high interest.

The brief compilation of melilite-type 𝜒(3)-nonlinear crystals

given in Table 1 also provides an overview of observed 𝜒(3)-and

𝜒(2)+𝜒(3)-nonlinear photon–phonon processes. As follows from

this table, non-centrosymmetric melilite-type crystals show di-

verse nonlinear optical properties and many of them are also

laser-active media. The combination of these unique functional

potentials allows for the creation of novel self-frequency con-

version lasers. This prospect has motivated the present search

for new SRS-active melilite-type crystals. For the melilite-type

crystal Ca2MgSi2O7, doped with Nd3+(and codoped with Na+),

laser activity was investigated in ref. [19] and in a recent short

communication[21] the SRS potential of the (undoped) crystal was

preliminarily demonstrated.

In the present work, linear optical properties and a detailed

investigation of 𝜒(3)-nonlinear optical properties of Ca2MgSi2O7

are reported, together with SRS properties of a further melilite-

type crystal, Ca2Ga2SiO7.

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Table 1. Known SRS- and Ln3+-laser active tetragonal (space group P

42m−D3

2d) melilite-type crystals.

Crystal Observed 𝜒(3)-and𝜒(2)+𝜒(3)-nonlinear

optical interactions

SRS-promoting vibrational

modesa) [cm−1]

Known SE-channels of

Ln3+-lasant ions

Ba2MgGe2O7SRS, SHG,b) THG,c) “conical SHG,”d)

self-SHG(SRS),e) SFG,f)

self-SFG(SHG),g)

𝜒(3)-comb,h)

self-SRS(SFG),i)

self-SFG(SHG,SRS),j) ,[4]

771.5[4] (Nd3+): 4F3/2→4I11/2 [1]

Sr2MgGe2O7SRS, SHG, THG, 𝜒(3)-comb,

SFG, self-SFG(SHG),

self-SFG(SHG,SRS)[5]

≈779[5]

Sr2ZnGe2O7SRS, SHG, THG, 𝜒(3)-comb,

SFG, self-SFG(SHG),

self-SFG(SHG,SRS)[6]

≈778[6]

Ba2ZnGe2O7SRS, SHG, THG, 𝜒(3)-comb,

SFG, self-SHG(SRS),

self-SRS(SHG), 𝜒(3)-cr-casck) ,[5]

≈778, ≈257[5] (Nd3+): 4F3/2→4I11/2[7,8]

Ca2ZnSi2O7SRS, SHG, THG, 𝜒(3)-comb,

self-SFG(SRS), 𝜒(3)-cr-casc[9]

≈906, ≈663,

≈614, ≈243C[9]

SrLaGa3O7SRS[10] ≈523[10] (Nd3+): 4F3/2→4I11/2;[11]

(Pr3+):

3P0→3H4,

3P0→3F2[12]

SrGdGa3O7SRS[10] ≈523, ≈225[10] (Nd3+): 4F3/2→4I9/2[13]

Ca2Ga2SiO7SRS,l) self-SFG(SRS),m)

𝜒(3)-comb, 𝜒(3)-cr-casc

(this work)

≈720, ≈550,

≈170C,

≈1270C

(Nd3+): 4F3/2→4I11/2,[14–18]

4F3/2→4I13/2[14,15]

Ca2MgSi2O7SRS,n) self-SFG(SRS),

𝜒(3)-comb, 𝜒(3)-cr-casc

(this work)

≈908, ≈668,

≈240C,

≈1576C

(Nd3+): 4F3/2→4I11/2[19]

a)The character “C” denotes combined SRS-active vibrational modes b)SHG: Second harmonic generation c)THG: Third harmonic generation d)“Conical SHG”: Conical

harmonic generation, see, e.g., ref. [20] e)Self-SHG(SRS): Self-frequency doubling, i.e., SHG of the (anti-)Stokes components f)SFG: Sum-frequency generation (mixing) g)Self-

SFG(SHG): Sum-frequency generation involving SHG and SRS components h)𝜒(3)-comb: a frequency comb consisting of Stokes and anti-Stokes components that span at

least one octave, i.e., the highest frequency (energy) component must be at least double of the lowest frequency component i)Self-SRS(SFG): SRS originated from intense SFG

of the pump wave and (anti-)Stokes components j)Self-SFG(SHG,SRS): Sum-frequency generation involving SHG and (anti-)Stokes components k)𝜒(3)-cr-casc: Cascade of one

or many-step 𝜒(3)-processes involving diﬀerent SRS-active vibrational modes of the crystal for the generation of high-order Stokes and anti-Stokes components l)Preliminary

information on SRS is given in ref. [9] m)Self-SFG(SRS): Sum-frequency generation involving the pump radiation and its (anti-)Stokes components n)Preliminary information

on SRS is given in ref. [21].

2. Crystals of Ca2Ga2SiO7and Ca2MgSi2O7and

Linear Optical Properties

The parent crystal structure of melilite-type crystals with an over-

allchemicalcompositionM

2T(1)T(2)

2O7, which can host a multi-

tude of tetrahedrally coordinated (T(1) and T(2)) and eightfold coor-

dinated (M) cations, crystallizes with tetragonal space group sym-

metry P

42m−D3

2d(for an overview see, e.g., refs. [4,22,23]). The

structure consists of corner-sharing double-tetrahedra [T(2)

2O7]

and single tetrahedra [T(1)O4] that form layers parallel to (001).

These layers alternate with layers of the eightfold coordinated M

cations.[24] This parent melilite-type crystal structure is adopted

by Ca2Ga2SiO7, where Ga occupies the tetrahedrally coordinated

T(1) position, while the T(2) position is shared among Ga and

Si with an occupation factor of 0.5 for both kinds of atoms.

The M position is occupied by Ca.[14] The parent (“normal,” N)

melilite-type structure is also found for Ca2MgSi2O7at temper-

atures above ≈355 K (high temperature phase at ambient pres-

sure), with tetrahedrally coordinated Mg and Si on positions T(1)

and T(2), respectively, and Ca occupying the M position.[25] Be-

low ≈355 K Ca2MgSi2O7shows a 2D incommensurately mod-

ulated (IC) structure with modulation vectors q1=𝛽(a*+b*)

and q2=𝛽(−a*+b*)(witha*,b*=tetragonal reciprocal axes

of the basic unit cell, and 𝛽=temperature-dependent compo-

nent of the modulation vectors),[26,27] which, after,[27] origins in

a misﬁt between the layers of T(1)O4+T(2)

2O7groups and the

layers of Ca2+cations and cause a slight distortion and rota-

tion of the tetrahedral structural units and a distortion of the Ca

coordination surrounding.[27] The structure of IC-Ca2MgSi2O6

can be described with a 5D model with the superspace group

P

421m:p4mg.[25,28,29]

Crystals of Ca2Ga2SiO7had been grown by the Czochralski

technique at ≈1738 K from stoichiometric melt in a weakly

oxidizing atmosphere as described in ref. [17]. The crystals

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Figure 1. Example of a grown crystal of Ca2MgSi2O6, together with one

of the prepared oriented samples for the SRS investigations in the present

work.

of Ca2MgSi2O7used in the present work were grown by the

Czochralski technique from a melt with a slight surplus of 1 wt%

MgO and of 3 wt% SiO2in order to avoid the formation of

Ca3MgSi2O8as a parasitic phase during the growth process (as re-

ported in ref. [37]). Growth was performed at ≈1703 K in air using

seed crystal orientation along [001], Pt crucibles, crystal pulling

rate of 1–1.5 mm h−1and crystal rotation of 20–30 rpm. Applying

a rather ﬂat temperature gradient of less than 20 K cm−1large sin-

gle crystals of optical quality and dimensions up to 70 mm length

and 15 mm diameter were obtained, see Figure 1.

For SRS investigations two parallel-epipedal samples with

face normals along the tetragonal axes a,b,cand dimen-

sions 9.92 ×6.99 ×27.10 mm3(sample I, see Figure 1)

and 8.66 ×7.86 ×18.60 mm3(sample II) were prepared for

Ca2MgSi2O7, and one sample of the same orientation with di-

mensions 9.0 ×10.4 ×11.5 mm3for Ca2Ga2SiO7.Forallsam-

ples, the sample faces were polished but without antireﬂection

coating.

Using a plate-shaped sample with faces {001} of Ca2MgSi2O7

of 0.79 mm thickness the optical transparency range was mea-

sured with a Perkin Elmer Lambda 19 UV/vis/NIR spectropho-

tometer. The UV transmission border (at 50% transmission level)

is at ≈0.25 µm, the IR transmission range exceeds the IR wave-

length range accessible with our equipment and thus is higher

than 3.2 µm, see Figure 2.

While refractive indices and their wavelength dispersion

of Ca2Ga2SiO7in the wavelength range 0.308–1.064 µm are

reported in literature[30] (see also Table 2), refractive indices of

Ca2MgSi2O7are given in literature only in the wavelength range

of visible light.[31,32] Therefore, we determined the refractive

indices and their wavelength dispersion of Ca2MgSi2O7in a

broad wavelength range between 0.36502 and 2.32531 µm. The

measurement of the refractive indices was performed by the

prism method using a prism with refracting edge [001] and faces

(hk0), and a high-precision goniometer-spectrometer system

Figure 2. Measured transmission spectrum (refers to the right axis) and refractive indices and their wavelength dispersion (left axis) of uniaxial positive

Ca2MgSi2O7. Refractive indices were measured by the prism method. Dots represent measured and corrected (see text) data, lines give the Sellmeier

ﬁts of the data. Error bars of the measured data are smaller than the data markers. Refractive indices of uniaxial negative Ca2Ga2SiO7, taken from ref.

[30], are given in comparison, together with the Sellmeier ﬁt curves (lines) given in ref. [30], see also Table 2.

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Table 2. Selected crystallographic and physical properties of Ca2MgSi2O7

and Ca2Ga2SiO7silicate crystals.

Characteristics Ca2MgSi2O7Ca2Ga2SiO7

Space group[25] P

42m−D3

2d(No. 113)a)

Unit cell parameters [Å] a=b=7.8348(3);

c=5.0087(2)[25]

a=b=7.793(3);

c=5.132(2)[14]

Formula unit per unit cell Z=2[25]

Melting temperature [K] ≈1727[35] ≈1738[14]

Method of crystal growth Czochralski[15,19,36,37]

Hardness (Mohs scale) 5–6[38]

Density [g cm−3] 2.945[25] 3.828[14]

Nonlinearities 𝜒(2) +𝜒(3)

Linear optical character Uniaxial positive

(no<ne)[31]

Uniaxial negative

(no>ne)[17]

Refractive index

(modiﬁed Sellmeier

equations)

n2(𝜆)=D1+D2

(𝜆2−D3)

−D4𝜆2b)

n2(𝜆)=1+A⋅𝜆2

(𝜆2−B2)

c) ,[30]

Nonlinear refractive

index [m2W−1]

n2=(6.5 ±1) ×10−20[39]

Phonon spectra

extensiond) [cm−1]

≈1030[46] ≈720[14]

Observed SRS-promoting

vibrational modese)

[cm−1]

𝜔SRS1 ≈908;

𝜔SRS2 ≈668;

𝜔SRS3C ≈240;

𝜔SRS4C ≈1576

𝜔SRS1 ≈720;

𝜔SRS2 ≈550;

𝜔SRS3C ≈170;

𝜔SRS4C ≈1270

a)Ca2MgSi2O7crystals possess an incommensurate–commensurate phase transi-

tion at ≈355 K,[26,27] see text b)Sellmeier coeﬃcients (𝜆in µm; 𝜉2is the sum of the

squares of the residuals):

D1D2D3D4𝜉2

no2.6159(3) 0.0176(2) 0.0154(10) 0.00810(9) 7.8×10−9

ne2.6440(4) 0.0173(2) 0.0150(14) 0.00810(11) 1.4×10−8

c)Sellmeier coeﬃcients for noand ne(𝜆in µm): Ao=1.8879(6), B2

o=0.015389(5)

(µm2), Ae=1.8570(7), B2

e=0.014139(6) (µm2)[30] d)Data taken from room-

temperature spontaneous Raman scattering spectra e)The letter “C” denotes

combined SRS-active vibrational modes.

(Möller-Wedel, for instrumental details see, e.g., ref. [33]). Re-

fractive indices were measured at 14 discrete wavelength and

the measured data were corrected with the refractive index of air

using the Edlén equation and data for air given in ref. [34]. The

corrected refractive indices for noand newere ﬁtted with a modi-

ﬁed Sellmeier equation (see Table 2). In Figure 2, the (corrected)

refractive index data are given, together with the Sellmeier ﬁts,

the Sellmeier parameters are listed in footnote b of Table 2. The

latter also contains selected further known relevant crystallo-

graphic and physical properties of the two melilite-type crystals.

In Figure 2, the refractive indices and the Sellmeier ﬁt curves

of Ca2Ga2SiO7given in ref. [30] are included for comparison as

well.

Based on the refractive indices and their dispersion phase

matching possibilities for collinear SHG were calculated us-

ing a well-tested home-made program. Both melilite-type

crystals, Ca2MgSi2O7and Ca2Ga2SiO7, do not allow collinear

SHG phase matching between their UV absorption edge and

3.5 µm.

3. 𝝌(3) Nonlinear Lasing

The spectroscopic investigation of 𝜒(3)-nonlinear optical pro-

cesses in Ca2MgSi2O7and Ca2Ga2SiO7single crystals was per-

formed using a mode-locked Nd3+:Y3Al5O12 master oscillator

power ampliﬁer system in combination with a spectrometric

setup, as described in previous publications (see, e.g., refs.

[40,41]). The pump laser system operated at 1 Hz repetition rate,

generating single pulses at 𝜆f1 =1.06415 µm wavelength with

pulse energy of up to 40 mJ and pulse duration of about 80 ps.

The pump beam was guided to the registration part of the exper-

imental setup which is shown in Figure 3a.

After propagation through an attenuation stage consisting of

a revolving half-wave-plate (𝜆f1/2) in combination with a Glan-

laser polarizer (P), the linearly polarized and collimated pump

beam could optionally be frequency-doubled using a KTiOPO4

(KTP) crystal. The SHG process generated ≈60 ps pulses at 𝜆f2 =

0.53207 µm wavelength. Suppression of the residual infrared

radiation was accomplished by inserting a Schott BG39 ﬁlter

glass behind the KTP crystal, which shows a transmission of

0.015% at 1.06415 µm and 96% at 0.53207 µm wavelength. The

nearly Gaussian beam was then focused into the melilite crystal

by using a plano-convex lens with a focal length of fL1 =250 mm.

A lens system consisting of a spherical bi-convex lens (fL2 =

100 mm) and a plano-convex cylindrical lens (fL3 =100 mm)

collimated the divergent output radiation and imaged it onto

the variable entrance slit of a Czerny-Turner monochromator

(McPherson Model 270, 6.8 Å pixel−1dispersion, 150 lines mm−1

grating). The spectral composition of the scattered emission

was ﬁnally recorded by a silicon CCD sensor (Hamamatsu

S3924-1024Q with 1024 pixels) for the UV and visible spectral

region and an InGaAs-CMOS sensor (Hamamatsu G9204-

512D with 512 pixels) for the range between 0.9 and 1.7 µm,

respectively (see spectral sensitivity of the two detectors in

Figure 3b).

For both crystals, Ca2MgSi2O7and Ca2Ga2SiO7, SRS mea-

surements were performed using three diﬀerent excitation

geometries, namely, a(cc)a,c(aa)c,andb(aa)b(notation used in

analogy to that in ref. [42], see also footnote a of Table 3). SRS

and Raman-induced four-wave mixing (RFWM) spectra obtained

for Ca2MgSi2O7with pump wave of 𝜆f2 =0.53207 µm are shown

in Figures 4–6. In addition, pumping at 𝜆f1 =1.06415 µm

was applied for a(cc)aand c(aa)cgeometries (see Figures 7

and 8) and for b(aa)bgeometry in Figure 5b. The analysis of

the observed spectra is given in Table 3. For Ca2Ga2SiO7a

pump wave with 𝜆f2 =0.53207 µm was applied exclusively. The

resulting SRS and RFWM spectra are visualized in Figure 9a

for geometry b(aa)b, Figure 9b for geometry a(cc)a,andFigure

10 for excitation geometry c(aa)c. Their analysis is listed in

Table 3.

The analysis of the SRS and RFWM spectra of the tetragonal

melilite-type silicate crystals revealed a number of 𝜒(3)-nonlinear

optical cascaded and cross-cascaded processes, which are related

to two original vibrational modes with energies of 𝜔SRS1 ≈908

cm−1and 𝜔SRS2 ≈668 cm−1for Ca2MgSi2O7. These modes

correspond to the strongest lines in the spontaneous Raman

scattering spectrum.[46] For Ca2Ga2SiO7the two original vibra-

tional modes have energies 𝜔SRS1 ≈720 cm−1and 𝜔SRS2 ≈550

cm−1, respectively, and, analogously, correspond to pronounced

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Figure 3. a) Schematic diagram of the experimental setup used for the spectroscopic analysis of SRS and nonlinear mixing interactions in Ca2MgSi2O7

or Ca2Ga2SiO7single crystals (P: polarizer; L1–L3: lenses; see also text). b) Spectral sensitivity of the used Si-CCD and an InGaAs-CMOS line sensor

(data are taken from Hamamatsu Photonics K.K. technical data sheets).

lines of the spontaneous Raman spectrum.[14] As shown in

Figures 5 and 7, for Ca2MgSi2O7pumping with incidence

perpendicular to the optic axis of the crystal allows the sepa-

ration of the SRS-active vibration modes and, for both modes,

broadband Stokes and anti-Stokes comb generation via SRS

and RFWM. This mode separation is also observed for SRS

in Ca2Ga2SiO7and is demonstrated in Figure 9. Pumping

with incidence along the optic axis of the tetragonal crystals

results in activation of both SRS-promoting modes 𝜔SRS1 and

𝜔SRS2 with the emergence of cross-cascaded 𝜒(3)-nonlinear

transitions and, in addition, the occurrence of two SRS-active

“combined modes,” see Figures 6 and 8 for Ca2MgSi2O7and

Figure 10 for Ca2Ga2SiO7, see also Table 3. As explained in

footnote e of Table 3, the “combined” vibrational modes with

energies 𝜔SRS3C ≈240 cm−1and 𝜔SRS4C ≈1576 cm−1for

Ca2MgSi2O7, respectively, 𝜔SRS3C ≈170 cm−1and 𝜔SRS4C ≈

1270 cm−1for Ca2Ga2SiO7, are absent from the respective

spontaneous Raman scattering spectrum, as they result from

𝜒(3)-nonlinear coherent interaction of the two modes 𝜔SRS1

≈908 cm−1and 𝜔SRS2 ≈668 cm−1(for Ca2MgSi2O7), and

𝜔SRS1 ≈720 cm−1and 𝜔SRS2 ≈550 cm−1(for Ca2Ga2SiO7),

respectively.

The vibrational nature of the modes of the studied melilite-

type silicates can be derived based on investigations of the

spontaneous Raman scattering spectra of several melilite-type

silicates (see, e.g., refs. [9,46]). Their tetragonal (P

42m−D3

2d)

unit cell comprises 24 atoms that have 72 zone-center degrees of

freedom, which characterize the lattice vibrations as follows: Γ72

=10A1+6A2+7B1+11B2+19E. Among them the 69 phonons ΓO

=10A1+6A2+7B1+10B2+18E correspond to the optical modes.

The unit cells of the studied crystals contain two Si2O7units

(Ca2MgSi2O7), resp. two T(2)

2O7units (Ca2Ga2SiO7,withT

(2) =Si,

Ga). An analysis[46] shows for Ca2MgSi2O7that the observed SRS

spectral lines are related to fully symmetric A1optical modes

involving three oxygen atoms and one silicon atom. The lines at

≈908 cm−1and at ≈668 cm−1for Ca2MgSi2O7can be assigned to

the 𝜈s(SiO3) and to the 𝜈s(SiOSi) vibrations (A1-modes), respec-

tively. Shift dynamics of the atoms of these modes are explained

by the atomic displacement vectors of these phonons, which have

been studied in ref. [46] (see Table 3 therein). An assignment of

the lines at ≈720 cm−1and at ≈550 cm−1for Ca2Ga2SiO7(see also

ref. [14]) in analogy to that for Ca2MgSi2O7gives the 𝜈s(T(2)O3)

and the 𝜈s(T(2)OT(2)) vibrations (A1-modes; T(2) =Si, Ga),

respectively.

4. Discussion

The comparison of the linear optical properties of Ca2MgSi2O7

with those of further melilite-type crystals[33,47] shows that the

properties of Ca2MgSi2O7ﬁt well into the earlier observed

tendencies of the inﬂuence of cation substitution in the melilite-

type structure on 𝜒(1) optical properties.[47] Here, particularly

the inﬂuence on the birefringence of the crystals and thus on

phase matching for 𝜒(2)-processes is of interest. A variation of

the eightfold coordinated M-site occupation of the melilite-type

structure in a series Ba→Sr→Ca leads to a general reduction

of birefringence, as well as the T(1)-site substitution Mg↔Zn,

which aﬀects optical birefringence in a pronounced way with

frequently larger birefringence for the Mg compounds (with

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Table 3. Spectral composition of SRS and RFWM generation as well as of cascaded and cross-cascaded 𝜒(3)↔𝜒(3) nonlinear lasing in Ca2MgSi2O7and

Ca2Ga2SiO7crystals, recorded at room temperature with picoseconds Nd3+:Y3Al5O12-laser pumping at fundamental wavelengths 𝜆f1 =1.06415 µm

and 𝜆f2 =0.53207 µm (SHG).

Pumping condition 𝜒(3)-nonlinear lasing components SRS-promoting vibration modes [cm−1]

𝜆f[µm] Excitation geometrya) Wave-lengthb)

[µm]

Spectral linec) Attributiond) 𝜔SRS1 𝜔SRS2 𝜔SRS3

e) 𝜔SRS4

Ca2MgSi2O7crystal

0.53207 a(cc)a(see Figure 4) 0.4647 ASt3-1 *𝜔f2+3𝜔SRS1 =𝜔ASt3-1 ≈908

0.4852 ASt2-1 *𝜔f2+2𝜔SRS1 =𝜔ASt2-1 ≈908

0.5076 ASt1-1 *𝜔f2+𝜔SRS1 =𝜔ASt1-1 ≈908

0.53207 𝜆f2 𝜔f2 –

0.5591 St1-1 𝜔f2−𝜔SRS1 =𝜔St1-1 ≈908

0.5890 St2-1 *𝜔f2−2𝜔SRS1 =𝜔St2-1 ≈908

0.6228 St3-1 *𝜔f2−3𝜔SRS1 =𝜔St3-1 ≈908

≈b(aa)≈b(see Figure 5a) 0.4518 ASt5-2 *𝜔f2+5𝜔SRS2 =𝜔ASt5-2 ≈668

0.4658 ASt4-2 *𝜔f2+4𝜔SRS2 =𝜔ASt4-2 ≈668

0.4808 ASt3-2 *𝜔f2+3𝜔SRS2 =𝜔ASt3-2 ≈668

0.4968 ASt2-2 *𝜔f2+2𝜔SRS2 =𝜔ASt2-2 ≈668

0.5138 ASt1-2 *𝜔f2+𝜔SRS2 =𝜔ASt1-2 ≈668

0.53207 𝜆f2 𝜔f2 –

0.5517 St1-2 𝜔f2−𝜔SRS2 =𝜔St1-2 ≈668

0.5728 St2-2 *𝜔f2−2𝜔SRS2 =𝜔St2-2 ≈668

0.5956 St3-2 *𝜔f2−3𝜔SRS2 =𝜔St3-2 ≈668

0.6202 St4-2 *𝜔f2−4𝜔SRS2 =𝜔St4-2 ≈668

c(aa)c(see Figure 6) 0.4647 ASt3-1 *𝜔f2+3𝜔SRS1 =𝜔ASt3-1 ≈908

0.4852 ASt2-1 *𝜔f2+2𝜔SRS1 =𝜔ASt2-1 ≈908

0.4909 ASt1-4 **𝜔f2+𝜔SRS4 =𝜔ASt1-4 ≈1576

0.4968 ASt2-2 *𝜔f2+2𝜔SRS2 =𝜔ASt2-2 ≈668

0.5014 ASt1-3{ASt1-1} **[(𝜔f2+𝜔SRS1)+𝜔SRS3]=

=𝜔ASt1-3{ASt1-1}

≈908 ≈240

0.5076 ASt1-1 *𝜔f2+𝜔SRS1 =𝜔ASt1-1 ≈908

0.5138 ASt1-2 *𝜔f2+𝜔SRS2 =𝜔ASt1-2 ≈668

0.5254 ASt1-3 **𝜔f2+𝜔SRS3 =𝜔ASt1-3 ≈240

0.53207 𝜆f2 𝜔f2 –

0.5390 St1-3 **𝜔f2–𝜔SRS3 =𝜔St1-3 ≈240

0.5517 St1-2 𝜔f2–𝜔SRS2 =𝜔St1-2 ≈668

0.5591 St1-1 𝜔f2–𝜔SRS1 =𝜔St1-1 ≈908

0.5667 St1-3{St1-1} **[(𝜔f2−𝜔SRS1)−𝜔SRS3]=

=𝜔St1-3{St1-1}

≈908 ≈240

0.5728 St2-2 *𝜔f2−2𝜔SRS2 =𝜔St2-2 ≈668

0.5808 St1-4 **𝜔f2−𝜔SRS4 =𝜔St1-4 ≈1576

0.5890 St2-1 *𝜔f2−2𝜔SRS1 =𝜔St2-1 ≈908

0.6228 St3-1 *𝜔f2−3𝜔SRS1 =𝜔St3-1 ≈908

1.06415 a(cc)a(see Figure 7) 0.5692 ASt9-1 *𝜔f1+9𝜔SRS1 =𝜔ASt9-1 ≈908

0.6002 ASt8-1 *𝜔f1+8𝜔SRS1 =𝜔ASt8-1 ≈908

0.6348 ASt7-1 *𝜔f1+7𝜔SRS1 =𝜔ASt7-1 ≈908

0.6736 ASt6-1 *𝜔f1+6𝜔SRS1 =𝜔ASt6-1 ≈908

0.7175 ASt5-1 *𝜔f1+5𝜔SRS1 =𝜔ASt5-1 ≈908

0.7675 ASt4-1 *𝜔f1+4𝜔SRS1 =𝜔ASt4-1 ≈908

0.8250 ASt3-1 *𝜔f1+3𝜔SRS1 =𝜔ASt3-1 ≈908

0.8918 ASt2-1 *𝜔f1+2𝜔SRS1 =𝜔ASt2-1 ≈908

0.9385 Y3Al5O12:Nd3+Luminescencef)

0.9460

(Continued)

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Table 3. Continued.

Pumping condition 𝜒(3)-nonlinear lasing components SRS-promoting vibration modes [cm−1]

𝜆f[µm] Excitation geometrya) Wave-lengthb)

[µm]

Spectral linec) Attributiond) 𝜔SRS1 𝜔SRS2 𝜔SRS3

e) 𝜔SRS4

0.9704 ASt1-1 *𝜔f1+𝜔SRS1 =𝜔ASt1-1 ≈908

1.06415 𝜆f1 𝜔f1 –

1.1780 St1-1 𝜔f1−𝜔SRS1 =𝜔St1-1 ≈908

1.3191 St2-1 *𝜔f1−2𝜔SRS1 =𝜔St2-1 ≈908

1.4985 St2-1 *𝜔f1−3𝜔SRS1 =𝜔St3-1 ≈908

≈b(aa)≈b(see Figure 5b) 0.5531 ASt13-2 *𝜔f1+13𝜔SRS2 =𝜔ASt13-2 ≈668

0.5743 ASt12-2 *𝜔f1+12𝜔SRS2 =𝜔ASt12-2 ≈668

0.5972 ASt11-2 *𝜔f1+11𝜔SRS2 =𝜔ASt11-2 ≈668

0.6220 ASt10-2 *𝜔f1+10𝜔SRS2 =𝜔ASt10-2 ≈668

0.6490 ASt9-2 *𝜔f1+9𝜔SRS2 =𝜔ASt9-2 ≈668

0.6784 ASt8-2 *𝜔f1+8𝜔SRS2 =𝜔ASt8-2 ≈668

0.7106 ASt7-2 *𝜔f1+7𝜔SRS2 =𝜔ASt7-2 ≈668

0.7460 ASt6-2 *𝜔f1+6𝜔SRS2 =𝜔ASt6-2 ≈668

0.7851 ASt5-2 *𝜔f1+5𝜔SRS2 =𝜔ASt5-2 ≈668

0.8286 ASt4-2 *𝜔f1+4𝜔SRS2 =𝜔ASt4-2 ≈668

0.8771 ASt3-2 *𝜔f1+3𝜔SRS2 =𝜔ASt3-2 ≈668

0.9317 ASt2-2 *𝜔f1+2𝜔SRS2 =𝜔ASt2-2 ≈668

0.9935 ASt1-2 *𝜔f1+𝜔SRS2 =𝜔ASt1-2 ≈668

1.06415 𝜆f1 𝜔f1 –

1.1456 St1-2 𝜔f1−𝜔SRS2 =𝜔St1-2 ≈668

≈c(aa)≈c(see Figure 8) 0.6348 ASt7-1 *𝜔f1+7𝜔SRS1 =𝜔ASt7-1 ≈908

0.6736 ASt6-1 *𝜔f1+6𝜔SRS1 =𝜔ASt6-1 ≈908

0.6847 ASt1-2{ASt5-1} **[(𝜔f1+5𝜔SRS1)+𝜔SRS2]=

=𝜔ASt1-2{ASt5-1}

≈908 ≈668

0.7054 St1-2{ASt6-1}**[(𝜔f1+6𝜔SRS1)−𝜔SRS2]=

=𝜔St1-2{ASt6-1}

≈908 ≈668

0.7175 ASt5-1 *𝜔f1+5𝜔SRS1 =𝜔ASt5-1 ≈908

0.7301 ASt1-2{ASt4-1} **[(𝜔f1+4𝜔SRS1)+𝜔SRS2]=

=𝜔ASt1-2{ASt4-1}

≈908 ≈668

0.7536 St1-2{ASt5-1}**[(𝜔f1+5𝜔SRS1)−𝜔SRS2]=

=𝜔St1-2{ASt5-1}

≈908 ≈668

0.7675 ASt4-1 *𝜔f1+4𝜔SRS1 =𝜔ASt4-1 ≈908

0.7819 ASt1-2{ASt3-1} **[(𝜔f1+3𝜔SRS1)+𝜔SRS2]=

=𝜔ASt1-2{ASt3-1}

≈908 ≈668

0.8090 St1-2{ASt4-1}**[(𝜔f1+4𝜔SRS1)−𝜔SRS2]=

=𝜔St1-2{ASt4-1}

≈908 ≈668

0.8250 ASt3-1 *𝜔f1+3𝜔SRS1 =𝜔ASt3-1 ≈908

0.8417 ASt1-2{ASt2-1} **[(𝜔f1+2𝜔SRS1)+𝜔SRS2]=

=𝜔ASt1-2{ASt2-1}

≈908 ≈668

0.8731 St1-2{ASt3-1}**[(𝜔f1+3𝜔SRS1)−𝜔SRS2]=

=𝜔St1-2{ASt3-1}

0.8918 ASt2-1 *𝜔f1+2𝜔SRS1 =𝜔ASt2-1 ≈908

0.9113 ASt1-2{ASt1-1} **[(𝜔f1+𝜔SRS1)+𝜔SRS2]=

=𝜔ASt1-2{ASt1-1}

≈908 ≈668

0.9385 Y3Al5O12:Nd3+Luminescencef)

0.9460

0.9483 St1-2{ASt2-1}**[(𝜔f1+2𝜔SRS1)−𝜔SRS2]=

=𝜔St1-2{ASt2-1}

≈908 ≈668

0.9704 ASt1-1 *𝜔f1+𝜔SRS1 =𝜔ASt1-1 ≈908

(Continued)

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Table 3. Continued.

Pumping condition 𝜒(3)-nonlinear lasing components SRS-promoting vibration modes [cm−1]

𝜆f[µm] Excitation geometrya) Wave-lengthb)

[µm]

Spectral linec) Attributiond) 𝜔SRS1 𝜔SRS2 𝜔SRS3

e) 𝜔SRS4

0.9935 ASt1-2 *𝜔f1+𝜔SRS2 =𝜔ASt1-2 ≈668

1.06415 𝜆f1 𝜔f1 –

1.1456 St1-2 𝜔f1−𝜔SRS2 =𝜔St1-2 ≈668

1.1780 St1-1 𝜔f1−𝜔SRS1 =𝜔St1-1 ≈908

Ca2GaSi2O7crystal

0.53207 ≈b(aa)≈b(see Figure 9a) 0.5027 ASt2-2 *𝜔f2+2𝜔SRS2 =𝜔ASt2-2 ≈550

0.5170 ASt1-2 *𝜔f2+𝜔SRS2 =𝜔ASt1-2 ≈550

0.53207 𝜆f2 𝜔f2 –

0.5481 St1-2 𝜔f2−𝜔SRS2 =𝜔St1-2 ≈550

0.5652 St2-2 *𝜔f2−2𝜔SRS2 =𝜔St2-2 ≈550

a(cc)a(see Figure 9b) 0.4942 ASt2-1 *𝜔f2+2𝜔SRS1 =𝜔ASt2-1 ≈720

0.5124 ASt1-1 *𝜔f2+𝜔SRS1 =𝜔ASt1-1 ≈720

0.53207 𝜆f2 𝜔f2 –

0.5533 St1-1 𝜔f2−𝜔SRS1 =𝜔St1-1 ≈720

0.5762 St2-1 *𝜔f2−2𝜔SRS1 =𝜔St2-1 ≈720

c(aa)c(see Figure 10) 0.4772 ASt3-1 *𝜔f2+3𝜔SRS1 =𝜔ASt3-1 ≈720

0.4942 ASt2-1 *𝜔f2+2𝜔SRS1 =𝜔ASt2-1 ≈720

0.4984 ASt1-4 **𝜔f2+𝜔SRS4 =𝜔ASt1-4 ≈1270

0.5027 ASt2-2 *𝜔f2+2𝜔SRS2 =𝜔ASt2-2 ≈550

0.5080 ASt1-3{ASt1-1} **[(𝜔f2+𝜔SRS1)+𝜔SRS3]=

=𝜔ASt1-3{ASt1-1}

≈720 ≈170

0.5124 ASt1-1 *𝜔f2+𝜔SRS1 =𝜔ASt1-1 ≈720

0.5170 ASt1-2 *𝜔f2+𝜔SRS2 =𝜔ASt1-2 ≈550

0.5215 St1-3{ASt1-2} **[(𝜔f2+𝜔SRS2)−𝜔SRS3]=

=𝜔St1-3{ASt1-2}

≈550 ≈170

0.5273 ASt1-3 *𝜔f2+𝜔SRS3 =𝜔ASt1-3 ≈170

0.53207 𝜆f2 𝜔f2 –

0.5369 St1-3 **𝜔f2−𝜔SRS3 =𝜔St1-3 ≈170

0.5431 ASt1-3{St1-2} **[(𝜔f2−𝜔SRS2)+𝜔SRS3]=

=𝜔St1-3{St1-2}

≈550 ≈170

0.5481 St1-2 𝜔f2−𝜔SRS2 =𝜔St1-2 ≈550

0.5533 St1-1 𝜔f2−𝜔SRS1 =𝜔St1-1 ≈720

0.5585 St1-3{St1-1} **[(𝜔f2−𝜔SRS1)−𝜔SRS3]=

=𝜔St1-3{St1-1}

≈720 ≈170

0.5652 St2-2 *𝜔f2−2𝜔SRS2 =𝜔St2-2 ≈550

0.5706 St1-4 **𝜔f2−𝜔SRS4 =𝜔St1-4 ≈1270

0.5762 St2-1 *𝜔f2−2𝜔SRS1 =𝜔St2-1 ≈720

a)Notation is used in analogy to that in ref. [42]. The characters between parentheses are (from left to right) the polarization of the pump and of the nonlinear radiation, respec-

tively, while the characters to the left and to the right of the parentheses are the propagation direction of the pump and the nonlinear radiation, respectively b)Measurement

accuracy is ±0.0003 µm c)For example: ASt1-3{ASt1-1}forCa

2MgSi2O7crystal: RFWM involving the ﬁrst-order anti-Stokes component related to the vibrational mode 𝜔SRS3 ≈

240 cm−1and the third-order anti-Stokes component related to the vibrational mode 𝜔SRS1 ≈908 cm−1;St

1-3{St1-1}forCa

2Ga2SiO7crystal: the ﬁrst-order Stokes component

related to the vibrational mode 𝜔SRS3 ≈170 cm−1which has been originated from the ﬁrst-order Stokes component related to the vibrational mode 𝜔SRS1 ≈720 cm−1d)

square brackets, the most probable nonlinear optical generation process (SRS or RFWM) and the participating spectral components are given. Lines related to the cascaded

and cross-cascaded 𝜒(3)-nonlinear transitions are single- (*) and double- (**) asterisked, respectively e)Two vibrational modes, 𝜔SRS3 ≈240 cm−1and 𝜔SRS4 ≈1576 cm−1,

of the Ca2MgSi2O7crystal and two vibrational modes, 𝜔SRS3 ≈170 cm−1and 𝜔SRS4 ≈1270 cm−1,oftheCa

2Ga2SiO7crystal are considered as combined vibrational modes.

We refer to them as “phantom” or “masked” modes since they do not appear in the spontaneous Raman scattering spectra (see ref. [46] for Ca2MgSi2O7). We hypothesize

that they are the result of the cross-cascaded interactions between the coherently excited “original” phonons 𝜔SRS1 ≈908 cm−1and 𝜔SRS2 ≈668 cm−1for the Ca2MgSi2O7

crystal: 𝜔SRS1(≈908 cm−1)−𝜔SRS2(≈668 cm−1)=𝜔SRS3(≈240 cm−1), and 𝜔SRS1(≈908 cm−1)+𝜔SRS2(≈668 cm−1)=𝜔SRS4(≈1576 cm−1). Likewise for the Ca2Ga2SiO7crys-

tal: 𝜔SRS1(≈720 cm−1)−𝜔SRS2(≈550 cm−1)=𝜔SRS3(≈170 cm−1), and 𝜔SRS1(≈720 cm−1)+𝜔SRS2(≈550 cm1)=𝜔SRS4(≈1270 cm−1). Note that the presence of “combined”

vibrational modes related to cross-cascaded 𝜒(3)-nonlinear-optical interaction has already been observed previously in several crystals (see, e.g., refs. [9,40,43,44]) f)The lines

at 0.9385 and 0.9460 µm wavelength are caused by the emission from the Nd3+:Y3Al5O12 pump laser ampliﬁer being incident on the monochromator and detector. They

correspond to two Nd3+-ion luminescence inter-Stark transitions at 11 507 cm−14F3/2→4I9/2 852 and 11 423 cm−14F3/2→4I9/2 852 cm−1, respectively (see, e.g., ref. [45]). The

spectral lines could be used as reference lines.

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Figure 4. SRS and RFWM spectrum of the tetragonal Ca2MgSi2O7crystal

recorded with picosecond pumping at the wavelength 𝜆f2 =0.53207 µm

in excitation geometry a(cc)a. The wavelength of all lines (pump line is

asterisked) are given in µm, their intensities are shown without correc-

tion for the sensitivity of the used multichannel analyzing system with a

Si-CCD line sensor (Hamamatsu, model S3924-1024Q). The energy spac-

ing related to the SRS-promoting vibrational mode 𝜔SRS1 ≈908 cm−1

of the Stokes and anti-Stokes sidebands is indicated by the horizontal

scale brackets. The assignment of all recorded spectral lines is given in

Table 3.

same M and T(2) cations given). So, while the Zn compound

Ca2ZnSi2O7is uniaxial negative with small birefringence ne−no

around −0.012[47] Ca2MgSi2O7is uniaxial positive with a rather

small birefringence ne−noaround +0.008, and the refractive

indices of both compounds do not allow phase matching for, e.g.,

SHG. In contrast, the refractive indices of germanate melilites

with M =Ba, Sr and T(1) =Mg show larger birefringence of up

to 0.04 and phase-matched SHG and SFG is possible in a broad

wavelength range. This diﬀerence also aﬀects the observed

SRS and RFWM spectra of melilite-type crystals, where cross-

cascaded 𝜒(3)↔𝜒(2)-processes are observed for Ba2MgGe2O7,

Ba2ZnGe2O7,andSr

2MgGe2O7(see Table 1), but not for

Ca2ZnSi2O7and Ca2MgSi2O7. On the other hand, however,

cross-cascaded 𝜒(3)↔𝜒(3)-processes are observed in the latter two

crystals due to the presence of several SRS-promotoing vibration

modes.

5. Conclusion

The current work extends our investigations during the preced-

ing decade of new SRS-active non-centrosymmetric tetragonal

(P

42m−D3

2d) melilite-type crystals (among which the ﬁrst

crystal was Ba2MgGe2O7,[4] see Table 1). As can be seen from

the overview in Table 1 most of these crystals, including the

two melilites of the present work, Ca2MgSi2O7and Ca2Ga2SiO7,

are Ln3+(here, Nd3+)-laser active media. Together with the

here discovered attractive SRS properties of Ca2MgSi2O7and

Ca2Ga2SiO7, this allows a simultaneous functionality as a host

for laser ions and as a Raman-shifting material, which opens the

possibility to realize self-SRS-laser converters. The development

of such lasers is currently of great interest in modern laser

physics as it allows for the generation of novel laser sources at

wavelengths which are not directly accessible with conventional

laser materials. With the two crystals of the present work the

currently available panoply of self-SRS-laser converter mate-

rials could be complemented. An overview of so far known

crystalline self-SRS media is given in Table S1 (Supporting

Information).

Figure 5. Two parts of the SRS and RFWM spectrum of the tetragonal Ca2MgSi2O7crystal recorded with picosecond pumping at the wavelength

𝜆f1 =1.06415 µm in excitation geometry a(cc)a. Part (a) was recorded with a Si-CCD line sensor (Hamamatsu, model S3924-1024Q) and part (b) with

an InGaAs-CMOS line sensor (Hamamatsu, model G9204-512D), see the corresponding spectral sensitivity in Figure 3b). The energy spacing related

to the SRS-promoting vibrational mode 𝜔SRS1 ≈908 cm−1of the Stokes and anti-Stokes sidebands is indicated by the horizontal scale brackets. The

assignment of all recorded spectral lines is given in Table 3.

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Figure 6. SRS and RFWM spectra of the tetragonal Ca2MgSi2O7crystal recorded in excitation geometry ≈b(aa)≈bwith picosecond pumping at the

wavelengths a) 𝜆f2 =0.53207 µm and b) 𝜆f1 =1.06415 µm, using Si-CCD and InGaAs-CMOS line Hamamatsu sensors (see Figure 3). The energy

spacing of recorded Stokes and anti-Stokes sidebands are related to SRS-promoting vibrational mode 𝜔SRS2 ≈668 cm−1is indicated by the horizontal

scale brackets. The used notations are analogous to that of Figure 4. The assignment of all recorded spectral lines is given in Table 3.

Figure 7. SRS and RFWM spectrum of the tetragonal Ca2MgSi2O7crystal recorded with picosecond pumping at the wavelength 𝜆f2 =0.53207 µm in

excitation geometry c(aa)cusing a Si-CCD line sensor (Hamamatsu, model S3924-1024Q). The energy spacing related to the SRS-promoting vibrational

modes 𝜔SRS1 ≈908 cm−1,𝜔SRS2 ≈668 cm−1,𝜔SRS3C ≈240 cm−1,and𝜔SRS4C ≈1576 cm−1of the Stokes and anti-Stokes sidebands is indicated by the

horizontal scale brackets. The used notations are analogous to that of Figure 4. The assignment of all recorded spectral lines is given in Table 3. Note

that this spectrum is also given in ref. [21], however, with a reduced wavelength range.

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Figure 8. SRS and RFWM spectrum of the tetragonal Ca2MgSi2O7crystal

recorded with picosecond pumping at the wavelength 𝜆f1 =1.06415 µm

in excitation geometry ≈c(aa)≈cusing a Si-CCD line sensor (Hamamatsu,

model S3924-1024Q). The energy spacing related to the SRS-promoting

vibrational modes 𝜔SRS1 ≈908 cm−1and 𝜔SRS2 ≈668 cm−1of the Stokes

and anti-Stokes sidebands is indicated by the horizontal scale brackets.

The used notations are analogous to the in Figure 4. The assignment of

all recorded spectral lines is given in Table 3.

Figure 9. SRS and RFWM spectra of the tetragonal Ca2Ga2SiO7crystal

recorded with picosecond pumping at the wavelength 𝜆f2 =0.53207 µm

in excitation geometry: a) ≈b(aa)≈band b) a(cc)awith a Si-CCD line sen-

sor (Hamamatsu, model S3924-1024Q). The energy spacing related to the

SRS-promoting vibrational modes: a) 𝜔SRS2 ≈550 cm−1and b) 𝜔SRS1 ≈

720 cm−1of the Stokes and anti-Stokes sidebands is indicated by the hori-

zontal scale brackets. The used notations are analogous to that of Figure 4.

The assignment of all recorded spectral lines is given in Table 3.

Figure 10. SRS and RFWM spectrum of the tetragonal Ca2Ga2SiO7crystal

recorded with picosecond pumping at the wavelength 𝜆f2 =0.53207 µm

in excitation geometry c(aa)cusing a Si-CCD line sensor (Hamamatsu,

model S3924-1024Q). The energy spacing related to the SRS-promoting

vibrational modes 𝜔SRS1 ≈720 cm−1,𝜔SRS2 ≈550 cm−1,𝜔SRS3C ≈170

cm−1,and𝜔SRS4C ≈1270 cm−1of the Stokes and anti-Stokes sidebands

is indicated by the horizontal scale brackets. The used notations are anal-

ogous to that of Figure 4. The assignment of all recorded spectral lines is

giveninTable3.

Supporting Information

Supporting Information is available from the Wiley Online Library or from

the author.

Acknowledgements

The authors wish to note that the investigations were considerably pro-

moted through mutual scientiﬁc help within the “Joint Open Laboratory for

Laser Crystals and Precise Laser Systems,” and were stimulated by funda-

mental research programs of the Institute of Crystallography, Federal Re-

search Center of “Crystallography and Photonics” of the Russian Academy

of Sciences, by the Institute of Optics and Atomic Physics of the Technical

University of Berlin and by the Section Crystallography, Institute of Geol-

ogy and Mineralogy of the University of Cologne. The authors dedicate this

manuscript to the memory of their colleague and friend, Prof. Alexander

A. Kaminskii, who passed away on October 29th, 2019. The authors are

grateful for his tremendous contributions to the study of stimulated Ra-

man scattering and other nonlinear optical eﬀects in various crystals over

the past three decades.

Conﬂict of Interest

The authors declare no conﬂict of interest.

www.advancedsciencenews.com www.crt-journal.org

Keywords

Ca2Ga2SiO7,Ca

2MgSi2O7, many-phonon 𝜒(3)-nonlinear lasing, stimu-

lated Raman scattering (SRS), tetragonal melilite-type crystals

Received: February 21, 2020

Revised: April 21, 2020

Published online: June 14, 2020

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