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Nonlinear optical properties of TeO2-P2 O5- ZnO-LiNbO3 glass doped with Er3+ ions
Optical Materials 60 (2016) 456e461
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nonlinear optical properties of TeO2-P2 O5- ZnO-LiNbO3 glass doped with Er3þ ions R. Miedzinski a, *, I. Fuks-Janczarek a, Y. El Sayed Said b a b
Institute of Physics, Faculty of Mathematics and Natural Science, J. Dlugosz University in Czestochowa, Al. Armii Krajowej 13/15, Czestochowa, Poland Physics Department, Faculty of Science, King Khalid University, P. O. Box 9004, Abha, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2016 Received in revised form 26 August 2016 Accepted 29 August 2016
A series of lithium niobate LiNbO3 (LN) single crystals doped with Er3þ were grown under the same conditions by melt-quenching method. The distribution coefficients of rare-earth (RE) elements in the ”crystal-melt” system of LN were determined at the beginning of the crystal growth. Their dependence on the dopant concentration in melt for 0.4 and 0.8 wt % was investigated. The procedure is applied to RE-doped lithium niobate (LiNbO3), a material of great interest for optoelectronic applications. We have ð3Þ ð3Þ obtained the real cR and imaginary parts cI of the third-order, nonlinear optical susceptibility to the nonlinear refractive index n2 and the nonlinear absorption coefficient b that are valid for absorbing systems. We show that nonlinear refractive or absorptive effects are the consequence of the interplay between the real and imaginary parts of the third-order susceptibilities of the materials. The method for measuring non-linear absorption coefficients and nonlinear refractive index based on well-known Z-scan is presented. © 2016 Published by Elsevier B.V.
Keywords: Optical materials Glasses Z-Scan method Melting quenching technique Two-photon absorption Er3þ Yb3þ
1. Introduction
2. Material and methods
Latest investigation of tellurite lithium niobate glasses has given very promising results with regard to nonlinear optical properties of these materials. Lithium niboate LiNbO3 is an important ferroelectric and it has been used in various applications such as fast ionic conductors, semiconductors, photonic material and rare earth ion host solid state lasers, surface acoustic wave devices and phase modulator wave-guide in integrated optics [1e4]. While activenonlinear LN crystals are intensively studied and used for manufacturing of nonlinear optical devices, there still remains a lot of unsolved problems associated with the production of high quality crystals. It is known that pure LiNbO3, as well as Er:LiNbO3 and Yb:Er:LiNbO3, show a low threshold of photorefractive damage. When LiNbO3 devices are used in high intensity laser, their performance is severely limited by the optical damage effect, which induces birefringence change and distorts the laser beams [5].
2.1. Glasses
* Corresponding author. E-mail addresses: [email protected] (R. Miedzinski), [email protected] (I. Fuks-Janczarek). http://dx.doi.org/10.1016/j.optmat.2016.08.033 0925-3467/© 2016 Published by Elsevier B.V.
Lithium niobate (LiNbO3) is a compound of niobium, lithium, and oxygen. It has trigonal crystal system, which the lacks of inversion symmetry and displays ferroelectricity, Pockels effect, piezoelectric effect, photoelasticity and nonlinear optical polarizability. It is transparent for wavelengths between 350 and 5200 nm [6,7]. Lithium niobate crystal doped with rare-earth (RE) ions have been investigated intensively during the last decade [8e11] because they exhibit good combination of active (laser) and nonlinear properties (”active-nonlinear” crystals). The tellurite glasses of the TeO2-P2 O5-ZnO-LiNbO3 (TPZL) system were obtained by melting 50 g batches in gold crucibles in an electric furnace at temperature of 850 C in the air atmosphere. The powder mixture was given in a covered a platinum crucible to avoid vaporization losses and heated in a melting furnace to a temperature of 900 C for 30 min; the melt was stirred from time to time. The highly viscous melt was poured out at 850 C onto a steel plate forming a layer with thickness of 2e5 mm. Subsequently, the sample was transferred to an annealing furnace and kept for 2 h at 380 C (below 20 C of the transition temperature). Then the furnace was
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457
Fig. 1. Photographs of examined samples.
switched off and the glass sample was allowed to cool. The chemicals employed with their high (99.99%) purity were as follows: TeO2 (51 mol%), P2 O3 (9 mol%), ZnO (15 mol%) and LiNbO3 (25 mol%). In order to introduce the Erbium rare earth elements ions into the TPZL glass matrix, the respective amounts of RE were added to the batches. The melting process, crystallization kinetics and nanocrystallization are precisely described in Ref. [12]. As a result, three different samples were produced and prepared in the following scheme: Sample A - pure glasses with composition 51%TeO2-9%P2 O5-15% ZnO-25%LiNbO3 Sample B - pure glasses doped with 0.4% Er2 O3 Sample C - pure glasses doped with 0.8% Er2 O3 The photographs of examined samples are presented on Fig. 1. As is shown, the samples were polished on both sides for measurements. The obtained glasses were transparent and had very good quality.
dependent on the first-order susceptibility. For example, in a centrosymmetric molecule, one- and two-photon allowed transitions are mutually exclusive. The Beer's law for one photon absorption shows that the intensity of light decreases exponentially with depth z in the material (I(z) ¼ I0eaz). If nonlinear (multiphoton) effects are to be included then this differential equation must be extended and includes higher order intensity terms [15]:
dI ¼ aðuÞI þ bðuÞI 2 þ gðuÞI3 dz
(2)
so that:
bðuÞ ¼
2Zu 〈2〉 N WT ðuÞ ¼ s〈2〉 E I2
(3)
where: - b is the two-photon absorption coefficient - a is the absorption coefficient - W<2> is the transition rate for TPA per unit T volume - I is the irradiance - u is the photon frequency and the thickness of the slice is dz - N is the number density of molecules per cm3 - E is the photon energy (J) - s<2> is the TPA cross section (cm4 s/molecule).
2.2. Non-linear measurements The nonlinear optical properties of the matter are caused by the fact that the electric polarization vector P of the sample is a nonlinear function of the electric field E of the electromagnetic wave which propagate through the medium [13,14]. In general, the quantum mechanical analysis based on the microscopic model of the matter constituting the medium is necessary to analyze the response of a medium to an external electromagnetic field. Linear and nonlinear optical properties of glasses in the visible region where, ε0 is the free-space permittivity and the P can be expressed in terms of the E and the susceptibility c:
! ! <2> PNL;i ð r ; tÞ ¼ ε0 ðcijk $Ej Ek ð r ; tÞ ! <3> $Ej Ek El ð r ; tÞ þ … þ cijkl
(1)
In Equation (1), the indices i, j, k and l indicate the components of the electromagnetic wave polarization in the laboratory arbitrary setup. The terms beginning from the second describe the nonlinear phenomena of the corresponding order. The coefficients c are defined as the electrical (optical) susceptibilities of order n and their detailed description can be found in classical textbooks on the subject. For us, particular interest presents the third terms corresponding to the third-order susceptibilities. The susceptibility tensor characterizes the nonlinear optical response of the dielectric material on the macroscopic level. In particular, the imaginary part ð3Þ cð3Þ of the third-order nonlinear susceptibility cR is related to the I extent of TPA in a given molecule. The selection rules for TPA are therefore different from for one-photon absorption (OPA), which is
3. Experiment In this paper, Z-scan technique for measuring NLO coefficient was used. The Z-scan method was previously described in the work [16,17]. Generally in Z-scan experiment, a sample is moved along the optic axis through the focus of a single laser beam, while the energy transmitted through an aperture in the far field is recorded as a function of sample position. The Z-scan has many possible configurations (e.g. EZ-Scan, White Light Z-scan, Excite-Probe Zscan). In this note, both the ”open” and ”close aperture” Z-scan will be discussed. The measurable quantities connected with the Z-scan are nonlinear refraction index n2 and nonlinear absorption b.
Fig. 2. Z-scan experimental setup: BS - beamsplitter, S - sample, A - aperture, RD reference detector, SD - signal detector.
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Fig. 3. Absorption spectra of examined materials.
Fig. 4. Z-scan results for glass without Er2 O3 ions.
ð3Þ
Nonlinear refraction index is associated with the real part cR of the third-order nonlinear susceptibility and nonlinear absorption b ð3Þ is related to imaginary part cI . The third order nonlinear susceptibility is related to n2 and b by the following formulas [18]:
cð3Þ ¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 . m2 V2 cRð3Þ þ cIð3Þ
(4)
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459
Fig. 5. Z-scan results for glass with 0.4% wt of Er2 O3.
cRð3Þ ¼ 2n2 n20 ε0 c
(5)
cIð3Þ ¼ n20 ε0 cl=3p b
(6)
where c - speed of light, ε0 ¼ 8.85$1012 (F/m) - electric permittivity, l - wavelength and n0 - linear refractive index. The purpose of this kit is to describe a simple implementation of the Z-scan technique that can be used to characterize relatively thin (< 5 mm) optical materials. The experimental setup is shown on Fig. 2. The sample was moved along the axis of the focused laser beam with the increment of 50 mm. The interaction of the medium with the laser light changes as the sample is moved. This is because the sample experiences different intensities, dependent on the sample position (z) relative to the focus (z ¼ 0). By measuring the power transmitted through the sample as a function of sample position on z axis, information about the light-matter interaction can be extracted. A maximum transmittance through the aperture will occur when the sample is just in front of the focus. This maximum in transmittance will drop to a minimum as the sample is moved further and the beam diverges as a result of the negative lensing by the sample. The transmittance through the aperture will again return to the linear value as the sample is moved further from the focus. The measured transmittance is then independent of nonlinear refraction and only dependent on nonlinear absorption. It should be clear that the transmittance versus sample position graph of such an open aperture Z-scan should be symmetric around
the focus since the intensity distribution of a Gaussian beam is symmetric around the focus. The nonlinear absorption coefficient can be determined unambiguously by fitting function below to the transmittance data obtained form an open aperture Z-scan. For linear transmittance of the aperture S ¼ 1 the two photon absorption coefficient b can be quickly estimated by fitting:
q
DTðZÞz p0ffiffiffi
2 2
1 1þ
!
(7)
Z2 Z02
where
. q0 ¼ bI0 1 ea0 L a0
(8)
to the experimental points. In equations above I0 denotes the laser light intensity, L is a sample thickness and a0 is the sample linear absorption coefficient. In a presence of nonlinear absorption the division of closed/open scans gives the curve that approximate the closed aperture scan for a material without nonlinear absorption but heaving the same Dn. When the closed aperture scan is done the nonlinear refractive index can by estimated by measure the peak to valley (DTpv) value which is linearly related to the phase distortion DF0:
460
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Fig. 6. Z-scan results for glass with 0.8% wt of Er2 O3.
DTpv ¼ 0:406ð1 SÞ0:25 DF0
(9)
where:
DF0 ¼ kn2 I0 Leff
(10)
The k parameter visible in Eq. (10) is the wave number and the Leff ¼ ð1 expða0 LÞÞ=a0 is effective thickness. The Z-scan experiment for each sample was carried out for three different light intensity I0 and the nonlinear refractive index n2 was estimated by linear regression of the function:
DnðI0 Þ ¼ n2 I0
(11)
In the experiment the Continuum MiniLite II Nd:YAG laser was used with 532 nm wavelength. The laser light intensity I0 was in range 0.91e4.25 (GW/cm2) and the thickness of the samples were 3.75 mm, 3.74 mm and 3.76 mm for pure, 0.4% and 0.8% of Er2 O3 concentration. In closed aperture mode the linear transmittance of the aperture was S ¼ 0.4. The UVeviseNIR absorption spectra of the glasses were determined at wavelengths from 200 to 1100 nm with
an error of < 5%, using an OceanOptics HR4000CG-UV-NIR spectrometer and deuterium tungsten halogen source lamp DT-MINI-2GS Mini also from Ocean Optics. It is also worth pointing out that second harmonic generation experiment was carried out with the setup and other parameters exactly like in our previous work [19]. Nonetheless for the glasses described in this paper, the second harmonic generation did not observed. 4. Result and discussion The absorption spectra of examined glasses are presented in Fig. 3. The UVeviseNIR spectra of the examined glasses (0.4% ÷ 0.8 Er2 O3) show a few typically absorption peaks for Er3þ doped glass [20,21] corresponding to the transitions: 4 I / 15/2 4 I11/2(978 nm), 4 I / 15/2 4 I9/2(800 nm), 4 I / 15/2 4 F9/2(652 nm), 4 I / 15/2 4 S3/ 4 2 4 4 4 2(542 nm), I / 15/2 H11/2(521 nm), I / 15/2 F7/2(488 nm), 2 4 4 I / 15/2 H9/2(407 nm) and I / 15/2 G11/2(378 nm). The 378 nm wavelength peak is covered by absorption of the host glass. Therefore, in the UV region we can observed only one peak at 407 nm. In the visible range we have four peaks at: 488 nm, 521 nm, 542 nm and 652 nm. Finally, in the NIR we have two last peaks
Table 1 Liner and nonlinear optical properties of examined glasses. Sample
n [1]
n2 [106 cm2/GW]
B [cm/GW]
c<3> [1021 m2/V2] R
c<3> [1021 m2/V2] I
c<3> [1021 m2/V2]
Pure 0.4% Er2 O3 0.8% Er2 O3
1.92± 5% 2.01 ± 5% 2.00 ± 5%
3.96 5.37 5.66
1.15 ± 8% 1.35 ± 8% 1.70 ± 8%
7.76 11.5 12.02
6.36 8.18 10.20
10.03 14.14 15.76
R. Miedzinski et al. / Optical Materials 60 (2016) 456e461
corresponding to 800 nm and 978 nm wavelength. These absorption curves confirm the presence of erbium ions in glass structure. In the point of view the Z-scan experiment, very important is the absorption coefficient at the wavelength of used laser. For the examined samples the linear absorption a0 for 532 nm wavelength are 0.097, 0.239 and 0.511 cm1 respectively for pure and doped by 0.4% and 0.8% of Er2 O3 rare earth ions. These value and linear refractive index are necessary to calculate the third-order nonlinear susceptibility c<3> according to Z-scan technique. The results of Zscan are presented in Figs. 4e6. Each group of figures show the scans for open aperture, closed aperture, division of the closed/ open scans and changes in refractive index. The Z-scan measurements shows good agreement between experimental results and theoretical approximation. We also present the graphs which show the change in refractive index in the function of peak on-axis irradiance at the focus. The nonlinear refractive index n2 were calculated as a linear fit of the function 11. The calculated third order nonlinear optical coefficients are presented in Table 1. The lowest value of both n2 and b coefficients has a pure sample. The nonlinear refractive index increases with a doping of the samples to a value of 5.37 [106 cm2/GW] and 5.66 [106 cm2/GW] from 3.96 [106 cm2/GW] value, for 0.4% Er2 O3 and 0.8% Er2 O3, respectively. It was also found that the nonlinear absorption coefficient b for both doped samples increase by a value 0.20 [cm/GW] and 0.55 [cm/GW] in relation to pure sample. Nonlinear refraction and absorption result are correlated with the real and imaginary parts of the third-order susceptibilities of the material. Accordingly, ð3Þ ð3Þ it is not surprising that the calculated values of cR and cI , which are obtained by 5 and 6, are also higher for doped samples. As a result, the greatest value of the nonlinear susceptibility was obtained for a sample of 0.8% Er2 O3 and it is 15.76 [1021 m2/V2]. 5. Conclusions Research has shown, that the preparation method of the glasses gives good quality optical materials. The absorption UVeviseNIR spectra confirm the presence of rare earth ions in the glass structure. Increasing the concentration of erbium ions causes increasing the optical density of the materials. The nonlinear optics research has shown, that base value of the real part of the third order susceptibility increased about 48% for concentration of erbium ions 0.4%. Double the Er2 O3 concentration (0.8%) increase the real part about next 4.5%. A regards the imaginary part of the c<3> we have received 29% increased for 0.4% Er2 O3 concentration and next 24% for double concentration (0.8%wt). The presence of erbium ions increase the third order nonlinear susceptibility significantly. Double the concentration has a more influence for the value of imaginary part of c<3> which is related to the two photon absorption. References [1] H. Sugita, T. Honma, Y. Benino, T. Komatsu, Formation of LiNbO3 crystals at the surface of TeO2 -based glass by YAG laser-induced crystallization, Solid State Commun. 143 (6e7) (2007), http://dx.doi.org/10.1016/j.ssc.2007.06.002,
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288e284. [2] T. Honma, K. Koshiba, Y. Benino, T. Komatsu, Writing of crystal lines and its optical properties of rare-earth ion (Er3þ and Sm3þ) doped lithium niobate crystal on glass surface formed by laser irradiation, Opt. Mater. 31 (2) (2008) 315e319, http://dx.doi.org/10.1016/j.optmat.2008.04.015. [3] T. Honma, T. Komatsu, D. Zhao, H. Jain, Writing of rare-earth ion doped lithium niobate line patterns in glass by laser scanning, IOP Conference Series, Mater. Sci. Eng. 1 (1) (2009) 012006, http://dx.doi.org/10.1088/1757-8981/1/ 1/012006. [4] Y. Tsukada, T. Honma, T. Komatsu, Self-organized periodic domain structure for second harmonic generations in ferroelastic b-(Sm,Gd)2 (MoO)43 crystal lines on glass surfaces, Appl. Phys. Lett. 94 (4) (2009), 10.1063/1.3076080. [5] M. Li, Y. Xu, R. Wang, X. Zhen, C. Zhao, Second harmonic generation in Zndoped Li-rich LiNbO3 crystals, Cryst. Res. Technol. 36 (2) (2001) 191e195, 10.1002/1521-4079(200102)36:2$<$191::AID-CRAT191$>$3.0.CO;2-H. [6] T. Volk, M. Wohlecke, Lithium Niobate, Vol. 115 of Springer Series in Materials Science, Springer-Verlag Berlin Heidelberg, 2009, http://dx.doi.org/10.1007/ 978-3-540-70766-0. [7] R. Weis, T. Gaylord, Lithium niobate: summary of physical properties and crystal structure, Appl. Phys. A 37 (4) (1985) 191e203, http://dx.doi.org/ 10.1007/BF00614817. [8] M. Palatnikov, I. Biryukova, N. Sidorov, A. Denisov, V. Kalinnikov, P. Smith, V. Shur, Growth and concentration dependencies of rare-earth doped lithium niobate single crystals, J. Cryst. Growth 291 (2) (2006) 390e397, http:// dx.doi.org/10.1016/j.jcrysgro.2006.03.022. [9] T. Bodziony, S. Kaczmarek, J. Hanuza, EPR and optical studies of LiNbO3:Yb and LiNbO3:Yb, Pr single crystals, J. Alloys Compd. 451 (12) (2008) 240e247, http://dx.doi.org/10.1016/j.jallcom.2007.04.189 the 6th International Conference on f-Elements (ICFE-6)The 6th International Conference on f-Elements (ICFE-6). [10] A. Majchrowski, M. Chrunik, I. Kityk, K. Ozga, E. Michalski, P. Morawiak, L. Jaroszewicz, Photoinduced optical effects in rare earth doped d-{BiB3O6}, Mater. Lett. 122 (2014) 114e116. http://dx.doi.org/10.1016/j.matlet.2014.01. 175. [11] T. Lukasiewicz, A. Majchrowski, I. Kityk, J. Kroog, Influence of the rare-earth doping on the photoinduced {EOEs} in the {GdCOB}, Mater. Lett. 57 (1) (2002) 130e133. http://dx.doi.org/10.1016/S0167-577X(02)00718-8. [12] E.S. Yousef, B. Al-Qaisi, Kinetics and fabrication nanocrystallization of optical tellurophosphate glasses in system as: TeO2-P2 O5-ZnO-LiNbO3, J. Alloys Compd. 538 (2012) 193e200, http://dx.doi.org/10.1016/j.jallcom.2012.05.088. [13] S. Hoffmann-Eifert, D. Richter, S. Trolier-Mc Kinstry, Dielectric, Ferroelectric and Optical Properties, Wiley-VCH, 2012. [14] N. Bloembergen, Nonlinear Optics, 1996. [15] R.W. Boyd, Nonlinear Optics, third ed., Academic Press, Burlington, 2008. [16] M. Sheik-Bahae, A.A. Said, E.W.V. Stryland, High-sensitivity, single-beam n2 measurements, Opt. Lett. 14 (17) (1989) 955e957, http://dx.doi.org/10.1364/ OL.14.000955. [17] M. Sheik-Bahae, A. Said, T.-H. Wei, D. Hagan, E. Van Stryland, Sensitive measurement of optical nonlinearities using a single beam, Quantum Electron. IEEE J. 26 (4) (1990) 760e769, http://dx.doi.org/10.1109/3.53394. [18] Y. Hu, J. Zhang, Z. Li, T. Li, Crystal structure, z-scan measurements and theoretical calculations of third-order nonlinear optical properties of tetrachloro(1,10-phenanthroline-n,n) tin(iv), Synth. Met. 197 (2014) 194e197. http://dx.doi.org/10.1016/j.synthmet.2014.09.025. http://www.sciencedirect. com/science/article/pii/S0379677914003178. [19] I. Fuks-Janczarek, R. Miedzinski, L. Kassab, F. Alves, Nonlinear optical features on yb3þ/tm3þ codoped pbo-geo2 glasses with si nanoparticles, Mater. Res. Bull. 77 (2016) 8e14, http://dx.doi.org/10.1016/j.materresbull.2016.01.032 cited By 0, http://www.scopus.com/inward/record.url?eid¼2-s2.084955502235&partnerID¼40&md5¼170ccd243ebc7ae2d36cff009d1ea425. [20] I. Kityk, N. AlZayed, A. El-Naggar, M. Reben, J. Wasylak, G. Lakshminarayana, A.H. Reshak, M. Brik, Erpr doped tellurite glass nanocomposites for white light emitting diodes, Opt. Commun. 285 (5) (2012) 655e658. http://dx.doi.org/10. 1016/j.optcom.2011.10.088. http://www.sciencedirect.com/science/article/ pii/S0030401811012326. [21] T. Wei, Y. Tian, C. Tian, X. Jing, J. Zhang, L. Zhang, S. Xu, Optical spectroscopy and population behavior between 4i11/2 and 4i13/2 levels of erbium doped germanate glass, Opt. Mater. Express 4 (10) (2014) 2150e2165, http:// dx.doi.org/10.1364/OME.4.002150. http://www.osapublishing.org/ome/ abstract.cfm?URI¼ome-4-10-2150.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Nonlinear optical properties of TeO2-P2 O5- ZnO-LiNbO3 glass doped with Er3þ ions R. Miedzinski a, *, I. Fuks-Janczarek a, Y. El Sayed Said b a b
Institute of Physics, Faculty of Mathematics and Natural Science, J. Dlugosz University in Czestochowa, Al. Armii Krajowej 13/15, Czestochowa, Poland Physics Department, Faculty of Science, King Khalid University, P. O. Box 9004, Abha, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2016 Received in revised form 26 August 2016 Accepted 29 August 2016
A series of lithium niobate LiNbO3 (LN) single crystals doped with Er3þ were grown under the same conditions by melt-quenching method. The distribution coefficients of rare-earth (RE) elements in the ”crystal-melt” system of LN were determined at the beginning of the crystal growth. Their dependence on the dopant concentration in melt for 0.4 and 0.8 wt % was investigated. The procedure is applied to RE-doped lithium niobate (LiNbO3), a material of great interest for optoelectronic applications. We have ð3Þ ð3Þ obtained the real cR and imaginary parts cI of the third-order, nonlinear optical susceptibility to the nonlinear refractive index n2 and the nonlinear absorption coefficient b that are valid for absorbing systems. We show that nonlinear refractive or absorptive effects are the consequence of the interplay between the real and imaginary parts of the third-order susceptibilities of the materials. The method for measuring non-linear absorption coefficients and nonlinear refractive index based on well-known Z-scan is presented. © 2016 Published by Elsevier B.V.
Keywords: Optical materials Glasses Z-Scan method Melting quenching technique Two-photon absorption Er3þ Yb3þ
1. Introduction
2. Material and methods
Latest investigation of tellurite lithium niobate glasses has given very promising results with regard to nonlinear optical properties of these materials. Lithium niboate LiNbO3 is an important ferroelectric and it has been used in various applications such as fast ionic conductors, semiconductors, photonic material and rare earth ion host solid state lasers, surface acoustic wave devices and phase modulator wave-guide in integrated optics [1e4]. While activenonlinear LN crystals are intensively studied and used for manufacturing of nonlinear optical devices, there still remains a lot of unsolved problems associated with the production of high quality crystals. It is known that pure LiNbO3, as well as Er:LiNbO3 and Yb:Er:LiNbO3, show a low threshold of photorefractive damage. When LiNbO3 devices are used in high intensity laser, their performance is severely limited by the optical damage effect, which induces birefringence change and distorts the laser beams [5].
2.1. Glasses
* Corresponding author. E-mail addresses: [email protected] (R. Miedzinski), [email protected] (I. Fuks-Janczarek). http://dx.doi.org/10.1016/j.optmat.2016.08.033 0925-3467/© 2016 Published by Elsevier B.V.
Lithium niobate (LiNbO3) is a compound of niobium, lithium, and oxygen. It has trigonal crystal system, which the lacks of inversion symmetry and displays ferroelectricity, Pockels effect, piezoelectric effect, photoelasticity and nonlinear optical polarizability. It is transparent for wavelengths between 350 and 5200 nm [6,7]. Lithium niobate crystal doped with rare-earth (RE) ions have been investigated intensively during the last decade [8e11] because they exhibit good combination of active (laser) and nonlinear properties (”active-nonlinear” crystals). The tellurite glasses of the TeO2-P2 O5-ZnO-LiNbO3 (TPZL) system were obtained by melting 50 g batches in gold crucibles in an electric furnace at temperature of 850 C in the air atmosphere. The powder mixture was given in a covered a platinum crucible to avoid vaporization losses and heated in a melting furnace to a temperature of 900 C for 30 min; the melt was stirred from time to time. The highly viscous melt was poured out at 850 C onto a steel plate forming a layer with thickness of 2e5 mm. Subsequently, the sample was transferred to an annealing furnace and kept for 2 h at 380 C (below 20 C of the transition temperature). Then the furnace was
R. Miedzinski et al. / Optical Materials 60 (2016) 456e461
457
Fig. 1. Photographs of examined samples.
switched off and the glass sample was allowed to cool. The chemicals employed with their high (99.99%) purity were as follows: TeO2 (51 mol%), P2 O3 (9 mol%), ZnO (15 mol%) and LiNbO3 (25 mol%). In order to introduce the Erbium rare earth elements ions into the TPZL glass matrix, the respective amounts of RE were added to the batches. The melting process, crystallization kinetics and nanocrystallization are precisely described in Ref. [12]. As a result, three different samples were produced and prepared in the following scheme: Sample A - pure glasses with composition 51%TeO2-9%P2 O5-15% ZnO-25%LiNbO3 Sample B - pure glasses doped with 0.4% Er2 O3 Sample C - pure glasses doped with 0.8% Er2 O3 The photographs of examined samples are presented on Fig. 1. As is shown, the samples were polished on both sides for measurements. The obtained glasses were transparent and had very good quality.
dependent on the first-order susceptibility. For example, in a centrosymmetric molecule, one- and two-photon allowed transitions are mutually exclusive. The Beer's law for one photon absorption shows that the intensity of light decreases exponentially with depth z in the material (I(z) ¼ I0eaz). If nonlinear (multiphoton) effects are to be included then this differential equation must be extended and includes higher order intensity terms [15]:
dI ¼ aðuÞI þ bðuÞI 2 þ gðuÞI3 dz
(2)
so that:
bðuÞ ¼
2Zu 〈2〉 N WT ðuÞ ¼ s〈2〉 E I2
(3)
where: - b is the two-photon absorption coefficient - a is the absorption coefficient - W<2> is the transition rate for TPA per unit T volume - I is the irradiance - u is the photon frequency and the thickness of the slice is dz - N is the number density of molecules per cm3 - E is the photon energy (J) - s<2> is the TPA cross section (cm4 s/molecule).
2.2. Non-linear measurements The nonlinear optical properties of the matter are caused by the fact that the electric polarization vector P of the sample is a nonlinear function of the electric field E of the electromagnetic wave which propagate through the medium [13,14]. In general, the quantum mechanical analysis based on the microscopic model of the matter constituting the medium is necessary to analyze the response of a medium to an external electromagnetic field. Linear and nonlinear optical properties of glasses in the visible region where, ε0 is the free-space permittivity and the P can be expressed in terms of the E and the susceptibility c:
! ! <2> PNL;i ð r ; tÞ ¼ ε0 ðcijk $Ej Ek ð r ; tÞ ! <3> $Ej Ek El ð r ; tÞ þ … þ cijkl
(1)
In Equation (1), the indices i, j, k and l indicate the components of the electromagnetic wave polarization in the laboratory arbitrary setup. The terms beginning from the second describe the nonlinear phenomena of the corresponding order. The coefficients c
3. Experiment In this paper, Z-scan technique for measuring NLO coefficient was used. The Z-scan method was previously described in the work [16,17]. Generally in Z-scan experiment, a sample is moved along the optic axis through the focus of a single laser beam, while the energy transmitted through an aperture in the far field is recorded as a function of sample position. The Z-scan has many possible configurations (e.g. EZ-Scan, White Light Z-scan, Excite-Probe Zscan). In this note, both the ”open” and ”close aperture” Z-scan will be discussed. The measurable quantities connected with the Z-scan are nonlinear refraction index n2 and nonlinear absorption b.
Fig. 2. Z-scan experimental setup: BS - beamsplitter, S - sample, A - aperture, RD reference detector, SD - signal detector.
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Fig. 3. Absorption spectra of examined materials.
Fig. 4. Z-scan results for glass without Er2 O3 ions.
ð3Þ
Nonlinear refraction index is associated with the real part cR of the third-order nonlinear susceptibility and nonlinear absorption b ð3Þ is related to imaginary part cI . The third order nonlinear susceptibility is related to n2 and b by the following formulas [18]:
cð3Þ ¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 . m2 V2 cRð3Þ þ cIð3Þ
(4)
R. Miedzinski et al. / Optical Materials 60 (2016) 456e461
459
Fig. 5. Z-scan results for glass with 0.4% wt of Er2 O3.
cRð3Þ ¼ 2n2 n20 ε0 c
(5)
cIð3Þ ¼ n20 ε0 cl=3p b
(6)
where c - speed of light, ε0 ¼ 8.85$1012 (F/m) - electric permittivity, l - wavelength and n0 - linear refractive index. The purpose of this kit is to describe a simple implementation of the Z-scan technique that can be used to characterize relatively thin (< 5 mm) optical materials. The experimental setup is shown on Fig. 2. The sample was moved along the axis of the focused laser beam with the increment of 50 mm. The interaction of the medium with the laser light changes as the sample is moved. This is because the sample experiences different intensities, dependent on the sample position (z) relative to the focus (z ¼ 0). By measuring the power transmitted through the sample as a function of sample position on z axis, information about the light-matter interaction can be extracted. A maximum transmittance through the aperture will occur when the sample is just in front of the focus. This maximum in transmittance will drop to a minimum as the sample is moved further and the beam diverges as a result of the negative lensing by the sample. The transmittance through the aperture will again return to the linear value as the sample is moved further from the focus. The measured transmittance is then independent of nonlinear refraction and only dependent on nonlinear absorption. It should be clear that the transmittance versus sample position graph of such an open aperture Z-scan should be symmetric around
the focus since the intensity distribution of a Gaussian beam is symmetric around the focus. The nonlinear absorption coefficient can be determined unambiguously by fitting function below to the transmittance data obtained form an open aperture Z-scan. For linear transmittance of the aperture S ¼ 1 the two photon absorption coefficient b can be quickly estimated by fitting:
q
DTðZÞz p0ffiffiffi
2 2
1 1þ
!
(7)
Z2 Z02
where
. q0 ¼ bI0 1 ea0 L a0
(8)
to the experimental points. In equations above I0 denotes the laser light intensity, L is a sample thickness and a0 is the sample linear absorption coefficient. In a presence of nonlinear absorption the division of closed/open scans gives the curve that approximate the closed aperture scan for a material without nonlinear absorption but heaving the same Dn. When the closed aperture scan is done the nonlinear refractive index can by estimated by measure the peak to valley (DTpv) value which is linearly related to the phase distortion DF0:
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Fig. 6. Z-scan results for glass with 0.8% wt of Er2 O3.
DTpv ¼ 0:406ð1 SÞ0:25 DF0
(9)
where:
DF0 ¼ kn2 I0 Leff
(10)
The k parameter visible in Eq. (10) is the wave number and the Leff ¼ ð1 expða0 LÞÞ=a0 is effective thickness. The Z-scan experiment for each sample was carried out for three different light intensity I0 and the nonlinear refractive index n2 was estimated by linear regression of the function:
DnðI0 Þ ¼ n2 I0
(11)
In the experiment the Continuum MiniLite II Nd:YAG laser was used with 532 nm wavelength. The laser light intensity I0 was in range 0.91e4.25 (GW/cm2) and the thickness of the samples were 3.75 mm, 3.74 mm and 3.76 mm for pure, 0.4% and 0.8% of Er2 O3 concentration. In closed aperture mode the linear transmittance of the aperture was S ¼ 0.4. The UVeviseNIR absorption spectra of the glasses were determined at wavelengths from 200 to 1100 nm with
an error of < 5%, using an OceanOptics HR4000CG-UV-NIR spectrometer and deuterium tungsten halogen source lamp DT-MINI-2GS Mini also from Ocean Optics. It is also worth pointing out that second harmonic generation experiment was carried out with the setup and other parameters exactly like in our previous work [19]. Nonetheless for the glasses described in this paper, the second harmonic generation did not observed. 4. Result and discussion The absorption spectra of examined glasses are presented in Fig. 3. The UVeviseNIR spectra of the examined glasses (0.4% ÷ 0.8 Er2 O3) show a few typically absorption peaks for Er3þ doped glass [20,21] corresponding to the transitions: 4 I / 15/2 4 I11/2(978 nm), 4 I / 15/2 4 I9/2(800 nm), 4 I / 15/2 4 F9/2(652 nm), 4 I / 15/2 4 S3/ 4 2 4 4 4 2(542 nm), I / 15/2 H11/2(521 nm), I / 15/2 F7/2(488 nm), 2 4 4 I / 15/2 H9/2(407 nm) and I / 15/2 G11/2(378 nm). The 378 nm wavelength peak is covered by absorption of the host glass. Therefore, in the UV region we can observed only one peak at 407 nm. In the visible range we have four peaks at: 488 nm, 521 nm, 542 nm and 652 nm. Finally, in the NIR we have two last peaks
Table 1 Liner and nonlinear optical properties of examined glasses. Sample
n [1]
n2 [106 cm2/GW]
B [cm/GW]
c<3> [1021 m2/V2] R
c<3> [1021 m2/V2] I
c<3> [1021 m2/V2]
Pure 0.4% Er2 O3 0.8% Er2 O3
1.92± 5% 2.01 ± 5% 2.00 ± 5%
3.96 5.37 5.66
1.15 ± 8% 1.35 ± 8% 1.70 ± 8%
7.76 11.5 12.02
6.36 8.18 10.20
10.03 14.14 15.76
R. Miedzinski et al. / Optical Materials 60 (2016) 456e461
corresponding to 800 nm and 978 nm wavelength. These absorption curves confirm the presence of erbium ions in glass structure. In the point of view the Z-scan experiment, very important is the absorption coefficient at the wavelength of used laser. For the examined samples the linear absorption a0 for 532 nm wavelength are 0.097, 0.239 and 0.511 cm1 respectively for pure and doped by 0.4% and 0.8% of Er2 O3 rare earth ions. These value and linear refractive index are necessary to calculate the third-order nonlinear susceptibility c<3> according to Z-scan technique. The results of Zscan are presented in Figs. 4e6. Each group of figures show the scans for open aperture, closed aperture, division of the closed/ open scans and changes in refractive index. The Z-scan measurements shows good agreement between experimental results and theoretical approximation. We also present the graphs which show the change in refractive index in the function of peak on-axis irradiance at the focus. The nonlinear refractive index n2 were calculated as a linear fit of the function 11. The calculated third order nonlinear optical coefficients are presented in Table 1. The lowest value of both n2 and b coefficients has a pure sample. The nonlinear refractive index increases with a doping of the samples to a value of 5.37 [106 cm2/GW] and 5.66 [106 cm2/GW] from 3.96 [106 cm2/GW] value, for 0.4% Er2 O3 and 0.8% Er2 O3, respectively. It was also found that the nonlinear absorption coefficient b for both doped samples increase by a value 0.20 [cm/GW] and 0.55 [cm/GW] in relation to pure sample. Nonlinear refraction and absorption result are correlated with the real and imaginary parts of the third-order susceptibilities of the material. Accordingly, ð3Þ ð3Þ it is not surprising that the calculated values of cR and cI , which are obtained by 5 and 6, are also higher for doped samples. As a result, the greatest value of the nonlinear susceptibility was obtained for a sample of 0.8% Er2 O3 and it is 15.76 [1021 m2/V2]. 5. Conclusions Research has shown, that the preparation method of the glasses gives good quality optical materials. The absorption UVeviseNIR spectra confirm the presence of rare earth ions in the glass structure. Increasing the concentration of erbium ions causes increasing the optical density of the materials. The nonlinear optics research has shown, that base value of the real part of the third order susceptibility increased about 48% for concentration of erbium ions 0.4%. Double the Er2 O3 concentration (0.8%) increase the real part about next 4.5%. A regards the imaginary part of the c<3> we have received 29% increased for 0.4% Er2 O3 concentration and next 24% for double concentration (0.8%wt). The presence of erbium ions increase the third order nonlinear susceptibility significantly. Double the concentration has a more influence for the value of imaginary part of c<3> which is related to the two photon absorption. References [1] H. Sugita, T. Honma, Y. Benino, T. Komatsu, Formation of LiNbO3 crystals at the surface of TeO2 -based glass by YAG laser-induced crystallization, Solid State Commun. 143 (6e7) (2007), http://dx.doi.org/10.1016/j.ssc.2007.06.002,
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