Linear and nonlinear optical properties of terbium ... - OSA Publishing

Linear and nonlinear optical properties of terbium calcium oxyborate single crystals Dongsheng Yuan,1 Zeliang Gao,1 Shaojun Zhang,1 Zhitai Jia,1,2,3 Jun Shu,1 Yang Li,1 Zhengping Wang,1,2 and Xutang Tao1,2,* 1

State Key Laboratory of Crystal Materials, Shandong University, No.27 South Shanda Road, Jinan, 250100, China Key Laboratory of Functional Crystal Materials and Device (Shandong University, Ministry of Education), No.27 South Shanda Road, Jinan, 250100, China

2

3

[email protected] * [email protected]

Abstract: The linear and nonlinear optical properties of TbCa4O(BO3)3 (abbreviated as TbCOB) single crystals were investigated for the first time. The refractive indices of TbCOB at several wavelengths were measured by using the minimum deviation method and the parameters of Sellmeier’s dispersion equation were determined from the experimental data. The complete set of six second-order nonlinear optical (NLO) coefficients of TbCOB single crystals were obtained using the Maker fringe (FM) technique, with the largest d32 being on the order of 1.65 pm/V. Moreover, the phase-matching (PM) configurations of second-order harmonic generation (SHG) in the principal planes were calculated, and the largest effective NLO coefficient is deff = 0.86 pm/V along (22.56°, 180°) PM direction. The SHG conversion efficiency from 1064 nm to 532 nm of 8 mm long crystal samples without AR coating along this direction was achieved 57.1% at 28.2 mW input power, and it has a small walk-off angle of 13.8 mrad. In addition, the comparison and discussion with GdCOB and YCOB were carried out. ©2014 Optical Society of America OCIS codes: (160.4330) Nonlinear optical materials; (190.2620) Harmonic generation and mixing; (140.3515) Lasers, frequency doubled.

References and links 1. 2.

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#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27606

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1. Introduction Nowadays, high power visible lasers have been widely used in various fields such as laser display, medicine, optical storage, bio-photonics, undersea communication, marking and precision micro fabrications [1–3]. The frequency doubling of solid-state lasers operating in the near-infrared range by nonlinear optical (NLO) crystals is one of the most available methods to obtain visible laser sources with compactness, efficiency and reliability. So far, many NLO materials have been reported including KH2PO4, β-BaB2O4, LiB3O5 and KTiOPO4 etc [4–7], as well as newly developed LuAl3(BO3)4 [8] and Ca5(BO3)3F [9] crystals in the last few years. Borate-based NLO crystals have been widely used in devices and modern laser systems due to their excellent properties such as high laser damage threshold, high optical quality and high transparency [10]. Among them, some have realized commercialization, such as BBO (β-BaB2O4) and LBO (LiB3O5). However, the growth period of these crystals is relatively long, normally lasting for one to two months [11]. The rare-earth calcium oxyborates ReCa4O(BO3)3 (ReCOB, Re: rare-earth elements) have been investigated for NLO applications since 1996 [12]. During the following years, it was confirmed that most of these crystals possess congruent melting behaviors and can be readily #223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27607

grown using the Czochralski (Cz) method. YCOB and GdCOB, the most widely studied crystals of ReCOB family, were found to have a melting temperature of about 1510 and 1490 °C, respectively [10]. These negative biaxial crystals have relatively larger deff values (about four to five times those of KDP and DKDP), and show highly stable mechanical and chemical properties [13]. Besides, compared to the commercial LBO and BBO crystals, ReCOB crystals were reported to possess small walk-off angles (about half of that of LBO for type-I phase matching), high damage threshold (comparable to that of LBO), and relatively large angular acceptance angles (approximately two times that of BBO) [14–17]. In 2014, Tu et al. obtained 4-inch YCOB crystals without inclusions and cleavages by conventional Cz technology, and the nonlinear optical property had been investigated by non-collinear OPCPA (optical parametric chirped-pulse amplification) and nanosecond OPA (optical parametric amplification) experiments [18]. The demand of NLO crystals in solid-state lasers keeps inspiring the synthesis, growth and characterization of new NLO single crystals. TbCa4O(BO3)3 is a new multifunctional crystal belonging to the rare-earth calcium oxyborate family. This material crystallizes in the same acentric space group Cm (Z = 2) as Ca5(BO3)3F [19]. Recently, the bulk crystals with diameter of 20-25 mm had been grown by using the Cz method [20]. The TbCOB crystal is chemically stable and not hygroscopic, and it exhibits a high transparency ranging from 490 nm to 1500 nm. The Mohs hardness value of TbCOB crystal is 5.58. Concerning its potential application as a nonlinear optical crystal, there is no report on the fundamental optical properties, like optical axis orientation, refractive indices, second-order nonlinear coefficients, phase-matching properties, and SHG laser experiments, and so on. In this paper, we present the optical characterization of TbCOB crystals grown using the Cz method. The refractive indices were measured accurately over the high transmission range, and relative dispersion curves were fitted for the first time. The complete set of second-order NLO coefficients of TbCOB single crystals were obtained using the MF technique. Moreover, the numerical calculations of the optimum phase-matching (PM) conditions in principle planes were carried out. The SHG conversion experiments from 1064 nm to 532 nm were also performed by using a Q-switched Nd:YAG laser. 2. Refractive indices and dispersion curves The accurate measurements of refractive indices and the determination of the dispersion curves in the entire transparency range are needed for further calculations of the PM directions. Herein, both the calculations and experimental verification were adopted. As shown in Fig. 1, the left prism with crystallographic c-axis parallel to the bisector of the transverse section apex, and crystallographic b-axis normal to the transverse section, was cut and polished. The right prism was designed with both (010) plane and crystallographic c-axis normal to the transverse section, and then processed as the other one.

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27608

Fig. 1. Schematic prisms for the measurements of refractive indices, through which ny can be got from the left sample, meanwhile, nx and nz can be obtained from the right one.

The refractive indices of TbCOB were measured by using the minimum deviation method in [21]. The right prism permits measurements of the refractive indices nx and nz, meanwhile ny and n’ (a vector sum of nx and nz in the bc plane) can be identified through polarizing microscope and some calculations. The prisms were mounted upon a goniometer stage and kept at 21°C during the measurement. Several monochromatic sources, in the visible and the near-IR range, were used to measure the values of nx, ny, and nz as function of wavelength. The accuracy of the measurements is estimated to be 1 × 10−5. TbCOB is a negative biaxial optical crystal with a birefringence Δn = 0.036 at 589 nm, which is little smaller than that of YCOB (Δn = 0.043 at 589nm) [10]. The refractive indices were fit to the Sellmeier Equation [22]: ni 2 = Ai +

Bi

λ 2 − Ci

− Di λ 2 , (i = x, y, z )

(1)

Based on the measured values, the Sellmeier constants Ai, Bi, Ci and Di can be determined through nonlinear curve fits, as shown in Table 1. Table 1. Sellmeier Equation Coefficients for TbCOB crystals

A B C D

nx

ny

nz

2.80878 0.02233 0.01613 0.00589

2.90000 0.02334 0.01729 0.01104

2.92981 0.02373 0.01568 0.01198

Table 2. Comparison of the refractive indices between the experimental and calculated values λ(nm)

ny

nx

nz

Exp.

Cal.

Exp.

Cal.

Exp.

Cal.

546.075

1.69888

1.69888

1. 72621

1.72621

1.73500

1.73500

587.562

1.69547

1.69546

1. 72260

1.72260

1.73139

1.73138

706.519

1.68881

1.68881

1. 71549

1.71550

1.72422

1.72421

768.194

1.68647

1.68648

1. 71297

1.71296

1.72163

1.72164

852.110

1.68403

1.68403

1. 71023

1.71024

1.71888

1.71887

1529.580

1.67470

1.67470

1. 69830

1.69830

1.70646

1.70646

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27609

The measured results of refractive indices and the values calculated by Eq. (1) are all given in Table 2. The calculated and the measured values agree well, with the deviation no more than 1 × 10−5. It is therefore concluded that the values of refractive indices generated by the Sellmeier Equation in the region from 500 nm to 1550 nm are reliable, and the corresponding dispersion curves (blue: nz, red: ny, black: nx) are shown in Fig. 2. The birefringence values (nz -nx) are located at 0.0318-0.0361 from the near-IR to the green visible regions.

Fig. 2. Refractive indices dispersion curves for TbCOB crystals. Note that the dot symbols are experimental data, and the curves present the Sellmeier fits.

3. Determination of optical axis orientation Monoclinic crystals such as TbCOB display three different refractive indices along the optical axis (X, Y, Z) that do not correspond to crystallographic reference axis (a, b, c). Based on the above experimental refractive indices, we can work out the degree between the crystallographic c-axis and optical X-axis. At the same time, a polarizing microscope was used to identify the relative orientation of X-axis corresponding to c-axis. With the help of Xray analysis, we have chosen (0 1 0), (−2 0 1), (1 0 1) as the reference planes, because our crystal was grown along (0 1 0) direction, and (−2 0 1), (1 0 1) are two exposure planes during the growing course. Finally, this orientation was determined as b∥Y: (a, Z) = 26.12°, (c, X) = 14.86°, β = 101.26°. In Fig. 3 (the blue lines indicate the profile of grown TbCOB crystals), the relative orientation of the optical principle axis to the crystallographic axis is similar to the one encountered in GdCOB [23] and YCOB [24]. (a, Z) and (c, X) angles are 26.12° and 14.86° in TbCOB, whereas they are 26° and 15° in GdCOB, and 24.7° and 13.5° in YCOB, respectively.

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27610

Fig. 3. Relative orientation of optical axis (X, Y, Z) with regard to crystallographic axis (a, b, c) for the monoclinic TbCOB single crystals.

4. Measurement of the NLO coefficients Under the Kleinman symmetry, TbCOB has six independent second-order NLO coefficients, d11, d12, d13, d31, d32, and d33. The magnitude of the above coefficients was determined by the FM technique. Meanwhile, considering the comparability of crystal symmetry, we selected YCOB crystals as the standard, and the accurate measurement of d33 for YCOB is easy to obtain through FM method. In our experiment, a Q-switched Nd:LYF4 laser (Sunlight 200 SGR-10) at 1053 nm with a repetition frequency of 1 Hz was used as the fundamental light source. The second-order harmonic signals from the samples were selectively detected by a photomultiplier tube (PMTH-S1V1-CR131), averaged by a fast-gated integrator and boxcar averager (Stanford Research Systems), and then recorded by data acquisition software. The crystal samples were uncoated and cut along X, Y and Z principal axis (thick direction) with sizes of 6 × 9 × 0.6 mm3. The experimental MF platform was produced according to the procedure in [25], and all the measurements were performed at room temperature. An X-cut YCOB crystal with dimension of 6.5 × 8.6 × 0.6 mm3 was selected as the calibrated sample. Considering the repeatability of the measurements, we take the determination of d33 of TbCOB as example. Figure 4(a) shows the orientation of the X-cut TbCOB crystal sample, E is the fundamental light and P is the SHG light. The type-I MF of d33 is shown in Fig. 4(b), where the solid and dashed curves represent the experimental values and envelope curve, respectively. In the experiments, the intensity of SHG (I2) can be expressed as Eq. (2). According to the MF theory [26], with the relative measurement of d33 in YCOB, the dij values can be calculated from the ratio of the central envelope values measured on TbCOB and YCOB samples as Eq. (3). I 2 ∝ f (θ ) × dij2 × T dij (TbCOB ) d33(YCOB )

=

I 2(TbCOB ) I 2(YCOB )

×

f (θ )(YCOB ) f (θ )(TbCOB )

(2) ×

T(YCOB ) T(TbCOB )

|θ = 0o

(3)

where f(θ) is a function about incident angle θ, T is the transmittance (the samples of both TbCOB and YCOB have a nearly close transmittance at 1053 nm and its frequency doubling

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27611

wavelength), and d33(YCOB) = 1.20 pm/V [27] is the standard. After obtaining the SHG power and the values of f(0) for each sample, the coefficients of TbCOB can be obtained.

Fig. 4. (a) Orientation of the X-cut TbCOB crystal sample used to measure the Maker fringes of d33: E is the fundamental light and P is the SHG light. (b) Experimental Maker fringe (typeI) of d33 (solid curve) and the envelope curve (dashed curve).

The values of all independent NLO coefficients, and the configurations used to carry out the MF experiments (including sample orientation, the direction of the polarization for both fundamental and harmonic waves, and the axis of rotation for their determination) are listed in Table 3. Simultaneously, we decide the relative plus-minus signs of dij of TbCOB according to that of YCOB crystal (in [27]), since both of them belong to the same crystal system. Table 3. The values of six independent NLO coefficients, and the configurations used to carry out the MF experiment (including sample orientation, the direction of the polarization for both fundamental and harmonic waves, and the axis of rotation for their determinations). Note that all the dij have an error of ± 10% of itself NLO coeff. d11 d12 d13 d31 d32 d33

Values 0.52 0.25 −0.69 −0.36 1.65 −1.27

Sample orientation Z Z Y

Rotation axis X Y Z

E

P

X Y Z

X X X

Y

X

X

Z

X X

Y Z

Y Z

Z Z

5. Phase-matching properties for SHG For the negative biaxial crystals, the determination of all possible phase-matching directions for SHG is a rather complicated project. However, it is easier to determine the phasematching directions for the particular case of light propagation in the principal planes XY, YZ, and ZX, and it is the case most frequently used in practice. Dmitriev et al. proposed general equations for calculating phase matching angles of a biaxial crystal in the principal planes [22]. Using the Sellmeier fitting of the refractive indices, one can find all the phase-matching configurations in the TbCOB principal planes for SHG of fundamental radiation in the nearIR and IR spectral regions. The following two formulas can decide the accurate PM angles for type-I, and there are no other PM configurations can be realized in the principal planes. The angle θ and φ correspond, respectively, to the polar and azimuthal angles of the lightpropagation direction. For ZX principal plane (Type-I (o + o→e)):

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27612

 n (2 w)  n 2 (2 w) − n 2 ( w) 1/ 2    x y θ = arc tan  z  2   2 (2 ) n w ( ) (2 ) − n w n w  y   z  x For XY principal plane (Type-I (o + o→e)):

(4)

 n (2 w)  n 2 (2w) − n 2 ( w) 1/ 2    y z (5) ϕ = arc tan  x  2   2 (2 ) ( ) (2 ) − n w n w n w x  y  z   Figure 5 shows the PM angles at different wavelengths for type I (PM-I) in the first octant of a TbCOB crystal, and we can see that TbCOB is phase-matchable in the range 1000-1500 nm for PM-I. As far as frequency doubling at 1064 nm is concerned, only type-I SHG phase matching can be realized, where the phase-matching angle was found to be (90°, 44.35°) in the XY principal plane, and (22.56°, 0°) in the ZX principal plane, respectively. In addition, (22.56°, 180°) is also the PM direction, if we take into consideration of the symmetry of refractive indices ellipsoid.

Fig. 5. Phase-matching angle curves for type-I SHG as a function of the wavelength of fundamental wave in the first octant.

As calculated, TbCOB crystal can only realize type-I PM in the principal planes. To completely investigate the SHG properties of crystal devices in laser experiments, the according effective NLO coefficients in PM directions were calculated through the expressions listed in Table 4. The largest NLO coefficient (deff = 0.86 pm/V) is along (22.56°, 180°), which is the optimum PM direction in the principal planes. Furthermore, we need to evaluate the outer-cavity SHG properties in details with 1064 nm pumping sources. Table 4. Values and expressions [23,28] of deff in principal planes for TbCOB crystals Principal plane XY ZX ZX

PM direction

deff expressions

Values

(90°, 44.35°) (22.56°, 0°) (22.56°, 180°)

d13 sinφ d12 cosθ −d32 sinθ d12 cosθ + d32 sinθ

0.48 0.40 0.86

5. Second harmonic generation properties We have designed and prepared some crystal devices to test its SHG conversion efficiency and reliability. The outer-cavity SHG experiments were performed using a PY61 Nd:YAG picosecond laser (Continuum Corp. USA) as the pumping fundamental source, and the working wavelength, repeat frequency and pulse width of the laser were 1064 nm, 10 Hz and 35 ps, respectively. In order to improve the beam quality of the incident light, an aperture was set before the crystal sample, thus, the beam diameter was minimized to 2 mm in the TbCOB

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27613

crystal samples. The filter placed between the crystal and the power meter has a total reflection at 1064 nm, and a transmittance of 82% at 532 nm, as shown in Fig. 6.

Fig. 6. The experiment set-up for the SHG conversion from 1064 nm to 532 nm.

The bulk crystal was cut into several samples with the dimension of 3 × 3 × 8 mm3 for the PM-I directions of (θ, φ) = (22.56°, 0°), (90°, 44.35°) and (22.56°, 180°), respectively. All the samples had the same length of 8 mm, and their transparent ends were polished but uncoated. The experiments were performed at room temperature. Figure 7 shows the SHG conversion efficiency versus the fundamental power for TbCOB crystals. It was observed that the values of SHG conversion efficiency were generally increased with increasing fundamental energy from 3.5 mW to 28.5 mW for (22.56°, 180°) and (90°, 44.35°)-cut samples. The SHG conversion capacity along (22.56°, 180°) PM direction was larger than that along (22.56°, 0°) and (90°, 44.35°) PM directions, and the latter two had a maximum conversion efficiency of less than 35%. In the principle planes, the maximum conversion efficiency was obtained along (22.56°, 180°) PM direction, with the result of as high as 57.1% at the fundamental average power of 28.2 mW, and it was the best results obtained for TbCOB crystal from the three PM directions in the principal planes. Furthermore, the results of the outer-cavity SHG experiments at 1064 nm are in accord with the effective NLO coefficients along different PM directions. Because of its small birefringence, TbCOB exhibits a small walk-off. Based on the theory model in [29, 30], the calculated value along this PM direction is 13.8 mrad (13 mrad for GdCOB [14]) through numerical method, and it is almost an order of magnitude smaller than that of BBO. In addition, the angular acceptance was calculated to be 0.9 mrad·cm.

Fig. 7. The SHG conversion efficiency versus the fundamental power along PM directions in principal planes for TbCOB crystals at 1064 nm.

6. Discussion In the ReCOB family of crystals, gadolinium calcium oxyborate (GdCOB) and yttrium calcium oxyborate (YCOB) have been studied thoroughly, and they have been assessed as #223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27614

competitive candidates as good SHG (or SFD) generators and host materials for lasing applications. The basic linear and nonlinear optical properties of TbCOB crystals were studied and compared with the above two crystals. These crystals melt congruently and are easy to grow through the Czochralski and Bridgman techniques. Being non-linear optical crystals with low-symmetry, they are constructed from the basic structure units: (BO3)3- anionic groups. While, the differences from trivalent rare-earth cations may also produce an effect on the crystal structure and NLO properties of ReCOB series such as refractive index, PM angle, effective nonlinear coefficients (deff), etc. According to the special “Lanthanide contraction effect”, the rare-earth elements with the ionic radius sorted in descending order are: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu. From the view of crystal structure, six oxygen atoms form a distorted octahedron surrounding the Re3+ ions in the ReCOB structure, and it seems that the Gd3+ ion with radius of 0.939 Å, and Y3+ ion with radius of 0.900 Å have appropriate fit to these sites with the evidence of excellent properties of GdCOB and YCOB crystals. While for large-sized ReCOB crystals, the harmful cleavage planes along (101) and (−201) usually occur during the Cz growth period. As for TbCOB, Tb3+ ion has the medial ion radius, r(Gd3+) > r(Tb3+) > r(Y3+), which indicates medium bonding length, bonding strength and polyhedral distortion in the crystal structure of ReCOB family, and indeed the occurrence of cleavage can hardly be observed during the crystal growth of TbCOB. At the same time, the TbCOB crystals still remain comparable optical properties. As we can see from Table 5, the birefringence of TbCOB at 1064 nm is 0.0340, while it is 0.0332 and 0.0411 for GdCOB and YCOB. The relative values of [dij] of TbCOB crystal are consistent with those of ReCOB crystals, especially closer to GdCOB. Besides, the optimum PM direction in principal planes and the corresponding deff at 1064 nm are (22.56°, 180°) and 0.86 for TbCOB crystals. Though the medium deff between that of GdCOB and YCOB, the SHG conversion efficiency from 1064 nm to 532 nm of 8 mm long TbCOB crystal samples along (22.56°, 180°) PM direction can reach 57.1% at 28.2 mW input power by using a Q-switched Nd:YAG laser, which is competitive in ReCOB crystals. Table 5. List and comparison of the fundamental linear and non-linear optical parameters for TbCOB, GdCOB and YCOB crystals. Note that the data of GdCOB and YCOB are from [27] by Michael V. Pack et al., and the deff @ 1064 nm were calculated by using the values of corresponding PM angle and dij Crystal

TbCOB

GdCOB

YCOB

Optical axis orientation

b∥Y: (a, Z) = 26.12°, (c, X) = 14.86°, β = 101.26°

b∥Y: (a, Z) = 26°, (c, X) = 15°, β = 101.3°

b∥Y: (a, Z) = 24.7°, (c, X) = 13.5°, β = 101.2°

0.0340

0.0332

0.0411

0.52 0.25 −0.69

0.28 0.212 −0.58

0.155 0.235 −0.59

−0.36

−0.32

−0.30

1.65 −1.27

1.67 −1.20

1.62 −1.20

(22.56°, 180°)

(19.7°, 180°)

(31.7°, 180°)

0.86

0.76

1.05

Birefringence @1064 nm d11 d12 [dij] @ d13 1064 nm d31 (Pm/V) d32 d33 Optimum PM direction in principal planes deff @ 1064 nm (Pm/V)

Moreover, the large paramagnetic moment of Tb3+ makes TbCOB single crystals different from the traditional NLO materials, some further work will be carried out from the

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27615

perspective of the interactions between magnetic and non-linear optical properties. ReCOB crystals were also reported to be excellent laser host materials for Nd3+, Yb3+, Er3+ laser active ions, and the doping of the crystals does not alter their congruent melting behaviors. The photoluminescence (PL) studies performed on the doped-TbCOB polycrystalline materials suggest that the doped TbCOB crystals can act as efficient laser crystals in addition to being NLO active. 7. Conclusion As a new optical crystal, the linear and nonlinear optical properties of TbCOB single crystals have been investigated for the first time. TbCOB was measured to be a negative biaxial crystal with a birefringence Δn = 0.034 at 1064 nm. All the NLO coefficients were determined, with the values being similar with those of GdCOB and YCOB. In the optical principal planes, the largest effective NLO coefficient is deff = 0.86 pm/V along (22.56°, 180°) configuration within the all PM directions. The outer-cavity SHG conversion efficiency of 8 mm long crystal samples without AR coating at 1064 nm along the optimum PM direction can reach 57.1% at 28.2 mW, and it has a small walk-off angle of 13.8 mrad. On the basis of its easy growth, moderate birefringence, high SHG conversion efficiency, and small walk-off angle, TbCOB crystal is one of good potential nonlinear optical materials. Maintaining comparable nonlinear optical properties with GdCOB and YCOB, TbCOB crystals can be further studied from the aspects of high quality crystal growth, magneto-optical interaction or laser host materials. Acknowledgments The authors greatly thank Qingming Lu for his kind help in the determination of optical axis orientation and crystal processing. This work is supported by the National Natural Science Foundation of China (Grant Nos. 51321091, 51202128, and 51323002), 973 Program of People's Republic of China (Grant No. 2010CB630702) and the Program of Introducing Talents of Disciplines to Universities in China (111 Program No. b06015).

#223423 - $15.00 USD Received 18 Sep 2014; revised 24 Oct 2014; accepted 24 Oct 2014; published 30 Oct 2014 (C) 2014 OSA 3 November 2014 | Vol. 22, No. 22 | DOI:10.1364/OE.22.027606 | OPTICS EXPRESS 27616