Nonlinear optical properties of Ca5(BO3)3F crystal - OSA Publishing

Nonlinear optical properties of Ca5(BO3)3F crystal Ke Xu,1, Pascal Loiseau,1 Gerard Aka,1* Regine Maillard, 2 Alain Maillard,2 and Takunori Taira3 1

Laboratoire de la Chimie de la Matière Condensée de Paris, CNRS-UMR 7574, ENSCP, 11 rue Pierre et Marie Curie, Paris, 75005 Cedex 05,France Laboratoire Matériaux Optiques, Photonique et Systèmes, CNRS-UMR 7132, Université de Metz et Supelec, 2 rue E. Belin, Metz, 57070, France 3 Laser Research Center for Molecular Science, Institute for Molecular Science, 38 Nishigonaka, Myodaiji, Okazaki, 444-8585, Japan * Corresponding author: [email protected]

2

Abstract: A high quality Ca5(BO3)3F (CBF) crystal, grown by flux method, was investigated for its non linear optical properties. This material is non hygroscopic and possesses a moderate birefringence suitable for UV light generation. On the basis of its refractive index dispersion curves, it is inferred that CBF has great potential applications as non linear optical material, notably for UV light generation at 355 and 266nm. In order to characterize its non linear optical properties, CBF samples were first cut and oriented in phase matching conditions for second harmonic generation of 1064nm. Experimental results demonstrate that CBF has the moderate NLO coefficients. The optical conversion efficiency from 1064nm to 532nm was investigated for the first time: up to 54% were achieved. Non linear deff coefficients were also estimated as well as the external angular acceptance bandwidth of SHG at 1064nm. ©2008 Optical Society of America OCIS codes: (160.4430) Nonlinear optical materials; (190.2620) Harmonic generation and mixing; (140.3515) Lasers, frequency doubled.

References and links 1. 2. 3.

4. 5.

6.

7. 8.

9. 10.

T. Sasaki, Y. Mori, M. Yoshimura, Y. K. Yap, and T. Kamimura, “Recent development of nonlinear optical borate crystals : key materials for generation of visible and UV light,” Mater. Sci. Eng. R. 30, 1-54 (2000). S. R. Lei, Q. Z. Huang, Y. F. Zheng, A. D. Jiang, and C. T. Chen, “Structure of Calcium Fluoroborate, Ca5(BO3)3F,” Acta Cryst. C45, 1861-1863 (1989). K .Xu, P. Loiseau and G. Aka, “Elaboration, cristallogenèse et caractérisation de nouveaux matériaux non linéaires. Applications à l’autodoublage de fréquence de l’ytterbium et à la conversion de fréquence dans l’ultraviolet” in Master Report, University of Paris VI. 2004; Chapter 3. G. Chen, Y. Wu, and P. Fu, “Growth and characterization of a new nonlinear optical crystal Ca5(BO3)3F,” J. Cryst. Growth 292, 449 (2006). K .Xu, P, Loiseau, and G. Aka, “New nonlinear optical crystal for UV light source: Calcium Fluoroborate,” presented at ASSP-2007, Advanced Solid-State Photonics Topical Meeting, Vancouver, Canada, 28-31, Jan. 2007. F. Mougel and G. Aka, “Les oxoborates de calcium et de terres rares (TR) Ca4TRO(BO3)3. Une nouvelle famille de matériaux à fonctions multiples pour l'optique: Croissance cristalline, propriétés non linéaires et laser” in PhD Report, University of Paris VI. 1999; Chapter 1. All the results are calculated by the software “SNLO_version 38,” http://www.sandia.gov/imrl/X1118/xxtal.htm. G. Aka, A. Kahn-Harari, F. Mougel, D. Vivien, F. Salin, P. Coquelin, P. Colin, D. Pelenc, and J. P. Damelet, “Linear- and nonlinear-optical properties of a new gadolinium calcium oxoborate crystal, Ca4GdO(BO3)3,” J. Opt. Soc. Am. B 14, 2238 (1997). P. Segonds, B. Boulanger, J.-P. Fève, B. Ménaert, J. Zaccaro, G. Aka, and D. Pelenc, “Linear and nonlinear optical properties of the monoclinic Ca4YO(BO3)3 crystal,” J. Opt. Soc. Am. B 21, 765 (2004). R. S. Klein, G. E. Kugel, A. Maillard, A. Sifi, and K. Polgar, “Absolute non-linear optical coefficients measurements of BBO single crystal and determination of angular acceptance by second harmonic generation,” Opt. Mater. 22, 163 (2003).

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Received 4 Aug 2008; revised 8 Sep 2008; accepted 8 Sep 2008; published 17 Oct 2008

27 October 2008 / Vol. 16, No. 22 / OPTICS EXPRESS 17735

11. 12.

R. S. Klein, G. E. Kugel, A. Maillard, K. Polgar, and A. Peter, “Absolute non-linear optical coefficients of LiNbO3 for near stoichiometric crystal compositions,” Opt. Mater. 22, 171 (2003). M. V. Pack, D. J. Armstrong, A. V. Smith, and G. Aka, “Measurement of the χ (2) tensor of GdCa4O(BO3)3 and YCa4O(BO3)3 crystals,” J. Opt. Soc. Am. B 22, 417 (2005)

1. Introduction Nonlinear optical (NLO) borate crystals usually possess a wide transparency, good chemical stability and high damage threshold. It is why borate crystals are often employed for highpower UV light generation. The first borate crystal discovered for UV generation was KB5O8.4H2O (KB5) [1]. Then, there were a lot of new borate materials NLO developed for this kind of application, for instance, β-BaB2O4 (BBO), CsLiB6O10 (CLBO), LiB3O5 (LBO) and K2Al2B2O7 (KAB) [1]. Until now, LBO is considered as one of the best choices for UV light generation at 355nm because of its small walk-off angle, its large angular acceptance and the large size of the as grown crystal. But, one of the disadvantages of LBO is its hygroscope. Recently, fluoroborate crystals have drawn considerable attention, such as KBe2BO3F (KBBF) [1], because fluorine could contribute to widen the transparency of borates in UV. Calcium fluoroborate, Ca5(BO3)3F (CBF) [2,3,4,5], is an interesting and new nonlinear optical crystal belonging to the fluoroborate family. CBF can be grown by flux method, using LiF as a flux. This material crystallizes in the same acentric space group Cm (Z=2) [2] as YCa4O(BO3)3 (YCOB). Cell dimensions of the CBF crystals are a=8.134(3) Å, b= 16.062(5) Å, c= 3.5422(9) and β= 100.88(2)° [5]. Its structure is built from [BO3]3- anionic groups, which are the most suitable for the design of nonlinear crystals aimed to have UV applications. CBF can be considered as derived from YCOB since Y3+ is replaced by Ca2+ and O2- is substituted by F- to preserve the electroneutrality of its structure. The CBF crystals are chemically stable and are not hygroscopic. The UV absorption edge of CBF is around 190nm [3,4,5], about 15nm shorter than the one of YCOB. By using the Kurtz’s powder technique, second-harmonic generation efficiency of CBF was measured about two times as large as that of KDP [4]. In this paper, we report the optical characterization of CBF crystals grown by flux method. The CBF crystal with a large size (Diameter=17mm and Length=35mm) has been obtained by using 20wt% LiF as flux in air. This experiment allowed us to determine the refractive indices and to cut the crystals at phase matching angles for SHG of 1064nm Nd:YAG. Thus, the nonlinear optical properties for SHG process, such as the effective NLO coefficient, the optical conversion efficiency from 1064nm to 532nm and the external angular acceptance bandwidth, were measured. 2. Crystal growth Contrarily to YCOB, CBF does not present a congruent melting: it can be grown by TSSG (Top Seeded Solution Growth) with 20 wt% of LiF as flux in air. The CBF crystal growth was performed from a YCOB seed oriented along b with a platinum crucible of 40mm in height and diameter by using an inductive furnace. Several growth rates and rotation rates were envisaged, from 0.1 to 0.5 mm/h and 5 to 20 rpm respectively. Good quality crystals were thus prepared (0.1 mm/h and 20 rpm) in spite of a few LiF inclusions in their core. Their good mechanical properties and their non hygroscopic character allow easy cutting and polishing. 3. Optical characterization 3.1 Refractive indices measured First, two prisms were cut and oriented in order to measure the refractive indices by the minimum deviation method, in the range from 355nm to 1064nm. Then, the Sellmeier equations (Eq. (1)) were derived from refractive index dispersion curves (Fig. 1). CBF is a negative biaxial optical crystal with a birefringence Δn=0.053 at 589nm which is larger than the one of YCOB (Δn=0.043 at 589nm) [6].

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nx = 2.5313 +

0.01739 − 0.00467λ2 λ − 0.00632

n y = 2.6596 +

0.01802 − 0.01628λ2 λ − 0.01475

nz = 2.7041 +

0.01824 − 0.01555λ2 λ − 0.01737

2

2

2

2

(1)

2

2

1,74

Refractive index

1,71

1,68

1,65

nz ny

1,62

nx

1,59 0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

1,1

λ (µm)

Fig.1. Refractive index dispersion curves for CBF.

3.2 Calculated phase matching conditions Table. 1 indicates the phase matching angles, walk-off angle and angular acceptance for SHG (Second Harmonic Generation), THG (Third Harmonic Generation) and FHG (Fourth Harmonic Generation). These properties were calculated from Sellmeier equations (Eq. (1)) with the help of the software “SNLO_version 38” [7]. In our case, THG and FHG are realized by sum frequency generation of ω+2ω and ω+3ω respectively. CBF can achieve the SHG for type I and II in XY or ZX planes. As compared to YCOB, Fig. 2 and Fig. 3 give phase matching angle curves for type-I and type-II SHG as a function of the wavelength of the fundamental wave in CBF and YCOB. The noncritically phase matching (NCPM) wavelengths of CBF shift significantly toward UV as compared to YCOB. This strong decrease of the NCPM wavelengths can be attributed to the greater birefringence of CBF. Therefore, CBF is more interesting than YCOB for UV light generation. As a result, CBF offers opportunities to realize type-II THG and type-I FHG of fundamental IR laser at 1064nm, whereas YCOB does not.

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Table 1. Properties of CBF for SHG, THG and FHG of 1064nm Nd: YAG laser

Phase matching type

Principal planes

Phase matching angle (°)

Angular acceptance (mrad.cm)

Walk-off angle (mrad)

SHG: 1064nm+1064nm=532nm XY Plane θ=90°, φ

I

21.7

17.4

1.9

II

49.4

23.9

2.5

ZX Plane φ=0°, θ

I

38.2

31.5

1.0

II

15.6

16.3

3.8

THG: 1064nm+532nm=355nm XY Plane θ=90°, φ

I

44.9

20.1

0.8

II

66.8

18.4

1.6

YZ Plane φ=90°, θ

II

45.0

8.4

3.8

ZX Plane φ=0°, θ

I

21.4

23.1

0.9

FHG: 1064nm+355nm=266nm XY Plane θ=90°, φ

I

66.4

22.2

0.7

YZ Plane φ=90°, θ

I

42.2

8.8

1.8

a)

3000

CBF YCOB

2000

1500

λ

fundamental

(nm)

2500

1000

829nm

723nm

673nm

577nm

500

0 0

30

60

ϕ(°) XY Plane

90

60

30

θ(°) YZ Plane

0

30

60

90

θ (°) ZX Plane

Fig. 2. Phase-matching angle curves for type-I SHG as a function of the wavelength of the fundamental wave in CBF by comparison with YCOB.

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b)

3000

λ

fundamental

(nm)

2500

CBF YCOB

2000

1500

1216nm

1024nm 1000

989nm

824nm 500

0 0

30

60

ϕ(°)

XY Plane

90

60

30

0

30

60

θ (°)

θ (°)

YZ Plane

ZX Plane

90

Fig. 3. Phase-matching angle curves for type-II SHG as a function of the wavelength of the fundamental wave in CBF by comparison with YCOB.

A practical technique for 355nm UV light generation is type II sum frequency ω+2ω since the beam at 2ω, produced by type I SHG, has a polarization perpendicular to the fundamental beam at ω. Until now, LBO (LiB3O5) is regarded as the best choice for producing UV light at 355nm because of its small walk-off angle (9.30 mrad) and its large angular acceptance (ΔθL=3.54 mrad.cm) for type II [7]. From table. 1, the best THG configuration type II for CBF is found at θ=45° φ=90° in YZ plane: it is characterized by a small walk-off angle (8.40 mrad) and a relatively large angular acceptance (ΔθL= 3.8 mrad.cm), which could make this material a good alternative to LBO. Unfortunately, CBF does not allow realizing 266nm UV light by simple frequency doubling of the second harmonic of the fundamental laser at 1064nm. The shortest type I SHG wavelength of CBF is indeed calculated at 289nm, which is a little greater than 266nm. However, it is possible to achieve the UV fourth harmonic @ 266nm by sum frequency ω+3ω in XY and YZ planes, but only in type I configuration (Fig. 4). According to the calculations of walk-off angle and angular acceptance (Tab. 1), the phase matching configuration is better in YZ plane than XY plane, even if the deff non linear coefficients remain to be measured. Finally, CBF is more interesting than YCOB for UV light generation because YCOB does not allow the FHG by sum frequency ω+3ω.

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3000

2000

1500

1122nm

λ

fundamental

(nm)

2500

Type I Type II

1000

976nm 500

0 0

30

ϕ (°)

60

90

60

θ (°)

30

0

30

θ(°)

60

90

XY Plane ZX Plane YZ Plane Fig. 4. Phase-matching angle curves for type-I and type-II FHG as a function of the wavelength of the fundamental wave in CBF.

4. NLO performances 4.1 Effective NLO coefficients (deff) for SHG of 1064nm In order to characterize the nonlinear optical properties of CBF, the wavelength of 1064nm was chosen as the fundamental wave to perform the first SHG tests. For the present experiment, three uncoated samples of CBF (Fig. 5.) were available. They were cut, oriented by optical means and polished in the phase matching conditions for SHG of Nd:YAG laser at 1064nm (Tab. 1). The aperture and thickness of these crystals are reported in Table. 2. XY Plane Type II

ZX Plane Type I

XY Plane Type I

Fig. 5. Photographs of the uncoated CBF crystals oriented at phase matching conditions for SHG of 1064nm. Table 2. Orientation and dimensions of the CBF crystals oriented

Crystals

XY plane type II

XY plane type I

ZX plane type I

Aperture (mm )

5×6

5×5

5×5

Thickness (mm)

7.07

9.18

5.34

2

The relative orientation of the indicatrix axes to the crystallographic axes was estimated by indexing the Laue diffraction pattern of the CBF crystal shown in Fig. 5, optically oriented for type I SHG@1064 nm in XZ plane (θ = 38.2): the results are shown in Fig. 6. The relative #99750 - $15.00 USD

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orientation of the indicatrix axes to the crystallographic axes in CBF is similar to the one encountered in GdCa4O(BO3)3 (GdCOB) [8] and in YCa4O(BO3)3 (YCOB) [9]. (a, Z) and (c,X) angles are respectively 25.2° and 14.3° in CBF whereas they are respectively 26° and 15° in GdCOB, and 24.7° and 13.5° in YCOB.

X

c (3.54 Å) Z

14.3°±0.8°

β (100.9°) 25.2°±0.8° a (8.13 Å)

☼

Y b (16.06 Å)

Fig. 6. Relative orientation of the indicatrix axes to the crystallographic axes in CBF.

deff coefficients were measured for the three phase matching configurations reported in Tab. 2 by using the technique of the absolute measurements described in details in references [10,11]: the experimental set-up is showed in Fig. 7. A CW Nd: YAG laser emitting a vertically polarized TEM00 mode, with a mean power of 6 W, was used as the fundamental beam. Interference Separator Lens filter ω/2ω NLO Photo Crystal 2 ω Photodoide 2ω CBF

ω

Lens

ω

Polarizer

Separator

70%

cr

ω

30%

Half-wave plate Beam dump

Laser Nd- YAG

Thermopiles Electronic box

Electronic box électronique

Fig. 7. Experimental set-up used for the absolute measurements of deff.

The mean values measured of dXYeff II, dXYeff I and dZXeff I of CBF and YCOB are reported with their formula in Table. 3. It appears that deff of CBF depends strongly on the type of interaction: deff for type II is greater than for type I. The greatest deff of CBF is located in XY #99750 - $15.00 USD

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plane for type II: dXYeff II (CBF) = 0.63 pm/V. In the same experimental conditions, dZXeff I of GdCa4O(BO3)3 (GdCOB) was also measured: dZXeff I (GdCOB) is equal to 0.75 pm/V (Table 3), which indicates that GdCOB is actually more efficient than CBF in ZX plane for type I SHG@1064nm. Moreover, it can be noticed that our experimental measure of deff for GdCOB agrees with the value (0.74 pm/V) which can be calculated from the reference [12]. The deff coefficients of YCOB which can be calculated from this same reference [12] are also shown in Table 3 to establish a comparison with CBF. As compared to YCOB in the same configuration (XY plane type II), deff of CBF is larger than that of YCOB (0.17 pm/V) [12]. However, dXYeff I and dZXeff I of CBF are lower than those of YCOB in the same conditions (Tab. 3). In addition, the d13 coefficient of CBF can be deduced from the measure of deff in type I for XY plane: d13 (CBF) = 0.39 pm/V, this value being smaller than the one of YCOB (0.59 pm/V) [12]. More experiments on samples cut in different phase matching conditions will be performed in order to measure other non linear optical coefficients. Table 3. Values of the effective non linear coefficients dXYeff II , dXYeff I and dZXeff I for CBF (measured), GdCOB (measured) and YCOB (calculateda)

Measured values deff

CBF

GdCOB

YCOB

deff formula

(pm/V)

0.63

-

0.17

d31sin2φ+d32cos2φ

dXYeff I (pm/V)

0.14

-

0.34

d13sinφ

0.48

0.75

0.69

d12cosθ-d32sinθ

XY

d

eff II

ZX

d

eff I

(pm/V) a

From J.Opt.Soc.Am.B, 22, 417 (2005)

4.2 Conversion efficiency for SHG @ 1064nm For the two CBF samples having the greatest deff as discussed before (Tab. 3), the SHG @ 1064nm conversion efficiency (Fig. 8) was measured by using a diode pumped Q-switch (100Hz) Nd:YAG laser. The maximum conversion efficiency in ZX plane for type I SHG is 25%: it corresponds to a maximum average output power of 0.35mJ with 1.4mJ of fundamental input power. The maximum average SHG power generated in XY plane for type II is higher: 0.81mJ for 1.5mJ of fundamental input power, which leads to a conversion efficiency of 54%. The phase matching configuration in XY plane for type II corresponds to a higher conversion efficiency than in ZX plane for type I, which is in accordance with the results of deff obtained previously (Tab. 3).

55

Conversion efficiency (%)

50 45

SHG in XY plane for type II

40 35 30 25 20 15

SHG in ZX plane for type I

10 5 0 0,4

0,6

0,8

1,0

1,2

1,4

1,6

Energy at 1064nm (mJ)

Fig. 8. SHG conversion efficiency of CBF @ 1064nm.

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4.2 Angular acceptance for SHG @ 1064nm One slab of CBF with 0.8mm in thickness (Fig. 9) was cut, oriented and polished in XY plane for type I SHG@1064nm in order to estimate the angular acceptance of CBF for this configuration. Fig. 10 shows that the measured FWHM external angular acceptance bandwidth is ΔφL = 2.0 mrad.cm.

Standardized SHG intensity (u.a.)

Fig. 9. Photograph of the polished CBF slab cut for angular acceptance experiment.

1,0

0,8

0,6

FWHM

0,4

0,2

0,0 -6

-4

-2

0

2

4

External angle (°)

Fig. 10. Measured external angular acceptance for type I SHG@1064nm in XY plane with 0.8mm CBF crystal.

4. Conclusion A new NLO crystal Ca5(BO3)3F (CBF) was grown in large size from the system CBF-LiF system in order to investigate its optical properties. CBF crystallizes in the same space group (Cm) as YCOB. This material is very interesting to achieve frequency conversion in UV because it not only possesses a UV absorption cut-off about 190nm, but also presents phase matching conditions for type-II THG and for type-I FHG of 1064nm. As a result, CBF can be regarded as a good competitor for LBO since it is chemically stable, and non hygroscopic. Moreover, it has a small walk-off angle (8.40 mrad) and a relatively large angular acceptance (ΔθL= 3.8 mrad.cm) for type-II THG in YZ plane. The nonlinear optical properties of CBF were characterized by SHG measurements at 1064 nm. Phase matching angles based on the Sellmeier equations for SHG of Nd: YAG laser are φ=21.7° for type-I, φ=49.4° for type-II in XY plane, and θ=38.1° for type-I in ZX plane. #99750 - $15.00 USD

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The highest SHG efficiency at 1064nm is found in XY plane for type-II: non linear effective coefficient deff = 0.63 pm/V and allows a conversion efficiency of 54%. The tensor d13 obtained is 0.39 pm/V. In XY plane for type-I SHG, external angular acceptance bandwidth (ΔφL) measured is 2.0 mrad.cm. Acknowledgments This work was supported by the French National Center for Scientific Research and the French National Research Agency (No. ANR-06-BLAN-0169-01). We thank M. Jacquet and M. Escudier, from Institut des Nanosciences de Paris (INSP), for cutting, orienting and polishing the CBF crystals.

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