Wavelength dependence of refractive index in ...

Wavelength dependence of refractive index in scintillation (Lu0.9Y0.1)2SiO5 single crystal Huiling Hu, Chongjun He* College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China ABSTRACT The refractive indices of anisotropic (Lu0.9Y0.1)2SiO5 (LYSO) single crystal at different wavelengths have been measured by the minimum deviation method at room temperature. Its refractive indices decrease quickly with the increasing wavelength. Sellmeier dispersion equations were obtained by means of least square fitting, which can predict the refractive indices in transparent region. The dispersion behavior was also described by single-oscillator approximation with physical significance. Keywords: LYSO crystal, dispersion equation, refractive index

1. Introduction High energy and nuclear physics community is interested in fast bright heavy crystal scintillators, such as (Lu0.9Y0.1)2SiO5 (LYSO) [1-3]. Investigations have been carried out to explore the potential use of the LYSO crystal in future physics experiments [4,5]. LYSO crystal has been applied in high energy physics, nuclear physics, oil well drilling, safety inspection, and environmental inspection. It has been promising candidate for applications in medical imaging [6]. Optical and scintillation properties, including transmittance, emission and excitation spectra, light output, decay kinetics and light response uniformity, have been measured for LYSO crystal [1]. LYSO crystal possesses monoclinic structure. The anisotropic character of LYSO crystal is rarely considered in scintillator applications, since birefringence is irrelevant to light output properties of the crystal [7-9]. For laser studies and consistent spectroscopic evaluation of the crystal properties, the knowledge of the dependent refractive indices is required [10-12]. The character of wavelength-dependent parameters is also to be taken into account in simulations of absorption and scattering loss. The refractive index should be known so that transmittance data can be properly corrected for reflection at the crystal-air interfaces. Therefore, here we precisely determine the wavelength dependence of the missing refractive indices of LYSO crystal.

2. Experimental description LYSO crystal sample was prepared from a LYSO crystal grown by the Czochralski technique [13]. The refractive indices were recorded with KALNEW KPR-2000 precision refractometer. To reduce error in measuring refractive index, LYSO crystal samples were coupled to V-prism with contact liquid. Finally, crystal refractive index at different wavelengths and dispersion relation are obtained. According to the minimum deviation principle, the refractive indices can be calculated by the equation

n = sin

A+ D A / sin 2 2

where A is the vertex angle of crystal prism, and D is the minimum deviation angle [14].

*[email protected]; phone 86 15951966230

Pacific Rim Laser Damage 2015: Optical Materials for High-Power Lasers, edited by Jianda Shao, Takahisa Jitsuno, Wolfgang Rudolph, Proc. of SPIE Vol. 9532, 95320W · © 2015 SPIE CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2185408 Proc. of SPIE Vol. 9532 95320W-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/16/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

(1)

3. Results and discussion 3.1 Sellmeier dispersion equation Through the minimum deviation method, the refractive indices were measured precisely at the wavelength of 404.7 nm, 546.1 nm, 589.0nm and 671.0nm, which are listed in table 1. The n1 and n2 values changing with wavelength are shown in figure 1. LYSO single crystal has large refractive indices and obvious dispersion relation. Its refractive indices decrease quickly with the increasing wavelength. Typical Sellmeier dispersion equation of crystal is 2

ni = Ai +

Bi − Diλ2 λ − Ci

(2)

2

where i denotes 1 or 2, Ai, Bi, Ci and Di are all constants and λ is wavelength in micrometers [15]. These constants can be obtained by a least square fitting of equation (2). The curves in figure 1 are the fitting results. Table 1. Refractive indices of (Lu0.9Y0.1)2SiO5 single crystal at room temperature.

Wavelength (nm)

n1

n2

404.7

1.826

1.839

546.1

1.809

1.820

589.0

1.805

1.815

671.0

1.799

1.806

1.840

1.835

1.830

1.825

1.805

1.800

1.795

400

450

500

550

600

650

700

Wavelength(nm)

Figure 1. The refractive indices of (Lu0.9Y0.1)2SiO5 single crystal measured at different wavelength. The curves are the fitting results of Sellmeier equation.

Proc. of SPIE Vol. 9532 95320W-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/16/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

The Sellmeier dispersion equations of n1 and n2 for LYSO single crystal are 2

n1 = 3.1190 +

0.0719 + 0.000003λ2 λ + 0.7102 2

2

n2 = 3.0667 +

0.1498 + 0.000028λ 2 λ + 0.3114

(3)

2

According to equation (3), the refractive index of LYSO single crystal can be calculated at other wavelengths in the transparent region. For example, at 578 nm wavelength, n1=1.806, n2=1.816; at 633 nm wavelength of He-Ne laser, n1=1.801, n2=1.810. 3.2 Single-oscillator dispersion behavior Although equations (3) can precisely predict refractive indices at different wavelengths, the constants in it do not have special physical significance. By a single-oscillator approximation, Wemple and Didomenico developed a single-term Sellmeier relation,

n2 −1 =

S 0 λ0

2

1 − λ0 / λ 2

2

=

E d E0

(4)

2

E0 − E 2

where n is refractive index, So is an average oscillator strength, λo is an average oscillator position, Eo is the single oscillator energy, and Ed is the dispersion energy, λ and E are the wavelength and energy of incident light [16-20]. The parameters in equation (4) can be obtained by plotting 1/(n2-1) versus λ-2 and E2 (as shown in figure 2 and figure 3). For n1, S1 =1.524×1014 m-2, λ1=0.119 µm, Eo=25.58 eV and Ed=56.46 eV; for n2, S2=1.201×1014 m-2, λ2=0.134 µm, Eo=22.62 eV and Ed=50.39 eV.

0.448 0.447 0.446 0.445 0.444

0.443 0.442 0.441

0.440

rc1 0.439 0.438 0.437 0.436 0.435 0.434 0.433 0.432 2.2

2.4

2.6

2.8

3.0

3.2

3.4

x-20.0.115

Figure 2. The linear fitting curves of (n2-1)-1 dependent on λ-2 for (Lu0.9Y0.1)2SiO5 single crystal.


0.448 0.447 0.446 0.445 0.444

0443 0442 0.441

0.440 0.439 0.438

0437 0.436 0.435 0.434 0.433

0432 8

10

12

14

16

18

20

E2(eV2)

Figure 3. The linear fitting curves of (n2-1)-1 dependent on E2 for (Lu0.9Y0.1)2SiO5 single crystal.

4. SUMMARY We have measured refractive indices of LYSO single crystals by the minimum angle of deflection at room temperature. LYSO single crystals have large refractive indices and obvious dispersion relation, refractive indices decrease fast with the increasing wavelength. Sellmeier dispersion equation was obtained by means of least square fitting, which involves four parameters. Single-oscillator dispersion equation, which involves only two parameters, is also important because it has physical significance. The oscillator parameters are related directly to the energy band structure. It should be pointed out that refractive index anisotropy has a negligible effect on the calculated light output of a scintillator crystals made from LYSO. However, for accurate determination of the reflection loss at the crystal surfaces the anisotropy has to be taken into consideration.

5. ACKNOWLEDGEMENT This work has been financially supported by the NUAA Fundamental Research Funds, NO. NS2014079; NUAA graduate student innovation base (laboratory) open fund, NO. kfjj20150802.

REFERENCES [1] Chen, J. M., Zhang, L. Y. and Zhu, R. Y., "Large size LSO and LYSO crystal scintillators for future highenergy physics and nuclear physics experiments," Nucl. Instrum. Methods Phys. Res. Sect. A 572(1), 218-224 (2007). [2] Mao, R. H., Wu, C., Dai, L. E. and Lu, S., "Crystal growth and scintillation properties of LSO and LYSO crystals," J. Cryst. Growth 368, 97-100 (2013). [3] Aogaki, S. and Takeutchi, F., "Performance of a 511 keV Gamma-Ray Imager Using a LYSO (Ce) Crystal Array With Wavelength Shifter," IEEE Trans. Nucl. Sci. 57(3), 1502-1511 (2010). [4] Ding, D. Z., Liu, B., Wu, Y. T., Yang, J. H., Ren, G. H. and Chen, J. F., "Effect of yttrium on electron–phonon coupling strength of 5d state of Ce3+ ion in LYSO:Ce crystals," J. Lumin. 154, 260-266 (2014).


[5] Correia, P. M. M., Castro, I. F. C. and Veloso J. F. C. A., "Development of a micro PET system with improved spatial resolution through depth-of-interaction measurement," Proc. SPIE 9286, 92864G (2014). [6] Lee, C. H. and Cho, G., "DOI resolution measurement and error analysis with LYSO and APDs," Nucl. Instrum. Methods Phys. Res. Sect. A 591(1), 84-87, (2008). [7] Ding, D. Z., Yang, J. H., Ren, G. H., Nikl, M., Wang, S., Wu, Y. T. and Mao, Z. Y., "Effects of anisotropy on structural and optical characteristics of LYSO:Ce crystal," Phys. Status Solidi B 251(6), 1202-1211 (2014). [8] Ding, D. Z., Weng, L. H., Yang, J. H., Ren, G. H. and Wu, Y. T., "Influence of yttrium content on the location of rare earth ions in LYSO:Ce crystals," J. Solid State Chem. 209, 56-62 (2014). [9] Dai, L, Xu, C., Zhang, Y., Li, D. Y. and Xu, Y. H., "Influences of Mg2+ ion on dopant occupancy and upconversion luminescence of Ho3+ ion in LiNbO3 crystal," Chin. Phys. B 22(9), 094201 (2013). [10] Li, N., Cong, Z. H., Qin, Z. G., Zhang, X. Y., Wang, W. T., Wang, C., Yu, H. H., Zhang, H. J., Xu, X. D. and Xu, J., "Continuous-wave and actively Q-switched laser operations at 1359.5nm of Nd:LYSO crystal," Opt. Laser Technol. 59, 43-46 (2014). [11] Dissertori, G., Luckey, D., Nessi-Tedaldi, F., Pauss, F., Quittnat, M., Wallny, R. and Glaser, M., "Results on damage induced by high-energy protons in LYSO calorimeter crystals," Nucl. Instrum. Methods Phys. Res., Sect. A 745, 1-6 (2014). [12] He, C. J., Zhang, Y. G., Sun, L., Wang, J. M., Wu, T., Xu, F., Du, C. L., Zhu, K. J. and Liu, Y. W., "Electrical and optical properties of Nd3+-doped Na0.5Bi0.5TiO3 ferroelectric single crystal," J. Phys. D: Appl. Phys. 46(24), 245104 (2013). [13] Qin, L. S., Li, H. Y., Lu, S., Ding, D. Z. and Ren, G. H., "Growth and characteristics of LYSO (Lu2(1−x−y)Y2xSiO5:Cey) scintillation crystals," J. Cryst. Growth 281(2-4), 518-524 (2005). [14] He, C. J., Yi, X. J., Wu, T., Wang, J. M., Zhu, K. J. and Liu, Y. W., "Wavelength dependence of refractive index in lead-free Na0.5Bi0.5TiO3-BaTiO3 single crystals," Opt. Mater. 36(12), 2023-2025 (2014). [15] He, C. J., Fu, X. D., Xu, F., Wang, J. M., Zhu, K. J., Du, C. L. and Liu, Y. W., "Orientation effect on bandgap and dispersion behavior of 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 single crystals," Chin Phys B 21(5), 054207 (2012). [16] Yao, B., Tian, H., Hu, C. P., Zhou, Z. X. and Liu, D. J., "Wavelength dependence of electro-optic effect in paraelectric potassium sodium tantalate niobate single crystal," Appl. Opt. 52(3-4) 8229-8232 (2013). [17] Tian, H., Jia, J. S., Cui, X., Yao, B., Zhou, Z. X. and Chen, D. Y., "Temperature influence on the voltagecontrolled diffractive property of Mn-doped potassium sodium tantalate niobate crystal," Opt. Mater. 35(12), 2425-2428 (2013). [18] He, C., Chen, H., Sun, L., Wang, J., Xu, F., Du, C., Zhu, K. and Liu, Y., "Effective electro-optic coefficient of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals," Cryst. Res. Technol. 47(6), 610-614 (2012). [19] Li, Y., Li, J., Zhou, Z. X., Guo, R. Y. and Bhalla, A. S., "Low-frequency–dependent electro-optic properties of potassium lithium tantalate niobate single crystals," Europhys. Lett. 102(3), 37004 (2013). [20] He, C. J., Hu, H. L., Jia, H., Wang, J. M., Zhu, K. J. and Liu, Y. W., "Optical properties of (1x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 single crystals," Proc. of SPIE 9270, 92701A (2014).
