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Lithium niobate nanoparticle-coated Y-coupler optical fiber for enhanced electro-optic sensitivity Ch. N. Rao,1 S. B. Sagar,1 N. G. Harshitha,1 Radhamanohar Aepuru,2 S. Premkumar,3 H. S. Panda,2 R. K. Choubey,1 and S. N. Kale1,* 1 2
Department of Applied Physics, Defence Institute of Advanced Technology, Girinagar, Pune 411025, India
Department of Materials Engineering, Defence Institute of Advanced Technology, Girinagar, Pune 411025, India 3 Armament Research & Development Establishment, Pashan Road, Pune, India *Corresponding author: [email protected] Received October 30, 2014; revised January 1, 2015; accepted January 1, 2015; posted January 7, 2015 (Doc. ID 225741); published February 5, 2015
Single crystals of lithium niobate (LiNbO3 ), possessing high birefringence and anisotropic properties have been explored, for a long time, to harness their excellent electro-optic properties. However, their nanoforms are comparatively less explored. In this context, dielectric constant and polarization (P) versus electric-field (E) characteristics of LiNbO3 nanomaterials have been studied. A nonideal P-E loop and a dielectric constant of 20 at the onset of 1 kHz were seen. The electro-optic sensitivity was found to be 4 times as compared to the bulk LiNbO3 crystals. The results are attributed to oxygen vacancies, antisite defects, and grain boundary effects in an already congruent structural matrix of LiNbO3 . © 2015 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (160.4330) Nonlinear optical materials; (060.2390) Fiber optics, infrared. http://dx.doi.org/10.1364/OL.40.000491
Lithium niobate (LiNbO3 ) is a conventional material with immense applications in opto-electronic industry, as an electro-optic and photorefractive material. Its unique set of properties include high birefringence, anisotropy, electro-optic coefficient, Curie temperature (1210°C), large nonlinear optical coefficient, and good piezoelectric and acousto-optic properties. Most of the properties have been harnessed in the applications such as surface acoustic waves (SAW) devices, optical couplers, optical isolators, optical modulators, and optical waveguides [1–3]. Appreciable research has been done with single crystals of LiNbO3 , but nanopowders have not been explored much. Few of these efforts, especially on thin films, were directed toward obtaining good quality films for integrated optics applications. Conventionally, polycrystalline ferroelectric thin films were grown on various substrates. Various deposition techniques, such as epitaxial growth by melting, chemical vapor deposition, liquid phase epitaxy, sputtering, and solgel, have been tried [4,5]. By conventional processing, LiNbO3 was prepared by calcinations of mixture of Li2 O and Nb2 O5 powders [6]. In recent years, solgel processing and coprecipitation methods have become popular for producing ceramic materials with improved compositional homogeneity and with lower sintering temperatures [7]. Nanomaterials have been synthesized and studied, but their effects on the said applications is still ongoing. The effects of oxygen-vacancy trap states, grain boundaries add to the already complex property range of such materials, which expand the application domain of these materials. Recently, in one of our works [8], it has been reported that the nanopowders show magneto-optic effect as well. Multiferroic properties are documented by Moreno et al. [9]. Enhancement of conventional bulk properties and exploration of new properties of this material in its nanoform is the motivation of this work. Any optically active material is normally characterized using fundamental property parameters such as its dielectric constant, refractive index, and polarization 0146-9592/15/040491-04$15.00/0
properties (mostly ferroelectric property studies). As these properties get modified, the response of the material will also get altered. LiNbO3 , conventionally, due to its structural anisotropy and mobility of Li ions near the phase transition temperature, is well known to be an optically active and structurally congruent material. Its high dielectric constant value and polarization (P) characteristics, with an applied electric field (E) (P-E measurements), shows its ferroelectric properties. Ferroelectric hysteresis curves of such materials do show remanence and coercivity, due to which they exhibit magnetic hysteresis as well. It is due to this that, in recent times, the material is projected as a multiferroic. Tuning the synthesis process and enhancing these properties can be a good motivation to study the nanoforms of such materials. In this context, through this manuscript, we report on synthesis of LiNbO3 nanomaterials using solgel method to yield polycrystalline nanopowders. These have been characterized using x-ray diffraction (XRD) studies, transmission electron microscopy (TEM) imaging, and x-ray photoelectron spectroscopy (XPS). 70 nm of particles were synthesized and dielectric as well as ferroelectric properties were studied. The nonideal P-E loop was observed, and dielectric constant of 20 at the onset of 1 kHz was seen. The electro-optic properties exhibited by these nanopowders show sensitivity (w.r.t. voltage) twice (2.07 pm/V) as compared to the single crystal of LiNbO3 (1.04 pm/V), while the sensitivity (w.r.t. electric field) was found to be (12.4 nm/kV/ mm), which is 4 times higher as well. The results are correlated to the congruent nature of the material, its grain boundary effects (due to nanoform) and the optical anisotropy generated due to oxygen vacancies and antisite defects. Synthesis of lithium niobate nanoparticles was done using solgel method, which is mentioned in earlier [8]. The prepared samples were characterized for structure, morphology, particle size, and ferroelectric and dielectric studies. © 2015 Optical Society of America
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For the coating of LiNbO3 on the tip of the optical fiber coupler, Poly vinyl Alcohol (PVA) was used as binder. Here, 1 gm of PVA was dissolved in 40 ml DI water at 100°C. The mixture was continuously stirred to yield a viscous gel at the same temperature. 20 mg of LiNbO3 powder was added to 2 ml of PVA gel and grinded well to form the uniform mixer of LiNbO3 and PVA. This gel was dip coated (thickness is 25 μm) to deposit on the cleaved mirror-surface of the optical coupler end part. PVA was specifically chosen, since it has been reported that it does not influence the optical signal in the tested range [10], which has been confirmed by our studies as well (detailed below). The schematic diagram of electro-optic sensor is shown in Fig. 1(a). A laser source (C-band, ALS-10-B-FC) was connected to one side of the Y-coupler (SMF −28, 3 dB, 1 × 2 coupler) and the other to the polarizer. From end of the Y-coupler, the optical power was directed to the mirror surface, which gets reflected and goes back to the detector. LiNbO3 was deposited on this mirror surface of the optical fiber. The polarized beam of light was expected to reflect from the multiple layers of the LiNbO3 -modified mirror surface and interfere with the reflected beam going toward the optical spectral analyzer (OSA-86146B). The observed interference pattern is shown in Fig. 1(b), which shows the clear fringe pattern of interference of reflected modes after coating the LiNbO3 at the end of the fiber tip. The flat response indicated the characteristic reflected spectrum of the SMF coupler with respect to the C-band source (without coating). Then the LiNbO3 -modified tip of the optical fiber was subjected to the applied electric field, and distance between electric plates was kept as 6 mm. Results were also compared with the fiber tip coated with only PVA. Hence, the ability and performance of the fiberoptic sensor for electric field measurement as electrooptic modulator was tested. The variation of electric field was regulated using DC power supply. The sensor response was recorded by the OSA with applied electric field. Figure 2 shows the XRD pattern of the synthesized LiNbO3 nanoparticles exhibiting the desired phase
Fig. 1. (a) Schematic diagram of experimental setup, in which optical fiber Y-coupler is used. (b) Optical response after and before coating.
Fig. 2. XRD pattern of the synthesized LiNbO3 nanopowders. (Inset shows the TEM micrograph of LiNbO3 .)
formation. The TEM micrograph showed average particles of ∼70 nm. XPS measurements were recorded for final LiNbO3 sample (annealed at 700°C). Figure 3(a) shows the total scan of the LiNbO3 sample surface. The constituent elements, i.e., Li-1s, O-1s, Nb-4s, Nb-4p, Nb3d doublet, and C-1s could be clearly observed from the scanned XPS spectrum, which were in well agreement with the earlier reported literature [11]. Figures 3(b) and 3(c) show the fitting curves for the elements Li and O, which gave an idea of the binding energies of the crystal composition on the sample surface. As has been mentioned in the studies depicted in Refs. [11–15], these signatures could give us an idea of the defects in the nanocrystalline structure, which could link to the amount of Li and/or O escaping from the stoichiometric structure. As is known, in an already congruent structure of lithium niobate, Nb ions could show richer contribution in the XPS scans because of the mobility of Li and O ions. This has been observed and documented earlier [12]. The peaks of Li-1s at 54.8 eV and O-1s at 530.1 eV as shown in Figs. 3(b) and 3(c), and the fitted curves, were almost in-line with the reported findings [13–15], wherein they attribute these values to the vacancies of Li and O and effective richer contribution of Nb on the surface. These samples were further characterized using dielectric property measurement and polarization electric field (P-E) measurement system. P-E loop, which exhibits
Fig. 3. (a) Complete XPS spectrum of LiNbO3 , (b) fitted data for Li B.E. value in LiNbO3 , and (c) fitted data for oxygen in LiNbO3 .
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Here, the effective refractive index, L is the interferometer physical length and λ is the wavelength of the source. The intensity of the reflected beam reaches maximum when φ 2m 1π, where m 0; 1; 2…. Since we need to analyze the effect of applied electric field on the total system, one should consider the effect on the basic system and the effect after coating the nanomaterial on the tip of the fiber. In this context, the net change in wavelength of interferometer will comprise of two components; one is due to the clad-core region, and the other is due to the core-mirror region. The change in wavelength corresponding to maximum in interference pattern is given by:
Fig. 4. Dielectric measurement study of the LiNbO3 nanomaterial at room temperature. The inset shows the P-E loop for the LiNbO3 nanoparticles at room temperature.
hysteresis behavior at room temperature for LiNbO3 thin films, is shown in the inset of Fig. 4. It exhibits the nonideal behavior of the ferroelectric loop. Low value of the remnant polarization (Pr 0.0481 μC∕cm2 ) for a coercive field of Ec 43 kV∕cm was observed. The switching range of the coercive field was around 43 kV/cm, which was very high as compared to micro and bulk materials (≈20 kV∕cm) [9,16]. Such nonideal behavior of P-E loop has also been reported previously in nanoconfined BaTiO3 material [17]. The nonideal behavior is probably due to the antisite defects, dislocations at the grain boundaries, and induced dipole moment. It is due to these behaviors that interfacial polarization arises, which could cause high leakage in the medium, resulting in low value of remnant polarization. Figure 4 shows the variation of dielectric constant of LiNbO3 thin film as a function of frequency. It can be noticed that dielectric constant decreases on increasing the frequency. The observed value of dielectric constant was found to be 20 at 1 kHz. The value of dielectric constant in nanopowder is much smaller and different than the bulk LiNbO3 [18]. Several possible causes such as the influence of a barrier layer between the film and leaky grain boundaries may be responsible for such behavior. Similar phenomena have also been previously reported for dielectric thin films by Pontes et al. [19]. To understand this data better, the basic principle of Fabry–Perot system was reviewed. The multiple layers of the coated film on the tip of the fiber forms multiple reflection mirrors. The reflectance is given by: 2Ri 1 − cos ϕ Rl ; 1 R2i − 2Ri cos ϕ
(1)
where Rl is the reflectivity of the multiple layers (which is dependent upon the scattering and absorption losses, as well), and ϕ is the optical phase shift and is incident power [20]. The relation between phase difference and path difference within the mirrors is given by:
φ
4πΔneff L : λ
(2)
Δλm
4Δneff L : 2m 1
(3)
The coupled component (core-mirror region) of multiple reflections possesses an additional phase retardation arising due to the Pockels effect, which is given by: δE; λ
4πne;o E; λL ; λ
(4)
where E is the applied electric field and ne;o is the birefringence (no − ne ) term of the material. This suggests that the shift in wavelength of interferometer due to this second component is a function of applied electric field. Now, especially for lithium niobate, it is important to understand the exact reason for its sensitivity to the applied electric field. As is known, the electro-optic (EO) LiNbO3 crystal is negative uniaxial (i.e., no > ne ), when light passes through the EO material, the effective refractive index (neff ) changes linearly with electric field. Hence, two birefringence terms arise, i.e., Δne;o ne − no n3e r 33 − n3o r 13 E∕2 [21]. The first and second terms are the natural and electrically induced birefringences terms, respectively. It is this second term, which gets modified upon subjecting the material to electric field, thereby contributing to the change in birefringence and hence the shift in wavelength. The optical path gets altered in the EO sensor medium due to the application of electric field, relative to the optical frequency of the incident light beam. Further, the optical phase retardation experienced by electric field when the light is linearly polarized along the e-axis is: δe E; λ
4πne L 2πn3e r 33 EL : λ λ
(5)
Since light experiences only one refractive index (ne ) in the crystal, Δδe E; λ 2πn3e r 33 LE∕λ controls the E-field-induced phase modulations. Figure 5 shows the interferometric fringe patterns response to the applied electric field. It is important to mention here that first, the pattern was also observed using only a PVA coat, which did not show any fringence pattern and effect on the applied fields; indicating that the contribution to the further documented results was mainly due to the nanomaterial. The sensitivity of the sensor was found to be 2.07 pm/V (12.4 nm/kV/mm) in
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defects, vacancy sites, and space charge limiting fields. Higher sensitivity nanolithium niobate-based electrooptic sensor is hence proposed. The authors acknowledge Dr. Prahlada, ViceChancellor, DIAT for the support. The authors also thank to Dr. A. Abhyankar, Dr. Sandip Dhobale, and Dr. H.H. Kumar for their scientific help.
Fig. 5. Response of the sensor assembly in terms of the wavelength shift w.r.t. the applied electric field.
the effective range from 0 to 1.5 kV and which was reversible. Here, the change of refractive index of the LiNbO3 was a function of applied electric field, as discussed above. The optical phase shift and refractive index both depends on the strength of the electric field (E) and medium interaction length (L). Further, since the results showed that there is a shift in the wavelength toward higher values upon application of electric field, it implied that the effective refractive index gets modified to a higher value upon application of electric field. This has also been documented before [22]. In the present communication, we observed sensitivity two times higher for nano LiNbO3 as compared to the reported bulk LiNbO3 crystals [23,24], while sensitivity in terms of electric field is 4 times higher than the LiNbO3 crystal [25]. The probable reasons of high sensitivity in nano LiNbO3 may be large surface area in nanoforms, effective trap densities, and so on [26]. On the other hand, Gopalan and Mitchell [27] have reported that the switching fields required for domain reversal in nonstoichimetric or defected crystals are 4–5 times larger than the stoichiometric crystals. Poberaj et al. [24] have observed a wavelength shift at 1555 nm upon applying a field of 100 V in LiNbO3 crystal. This field is at least 3 times lower than the present reported values in nano LiNbO3 , which is direct indication of high defect densities in nano LiNbO3 as compared to bulk LiNbO3 crystals. Hence, combining the results of dielectric constant and net polarization and clubbing the observations made in refs, one can envisage that the nano LiNbO3 comprises of various defects, either due to oxygen vacancies, grain boundaries, high surface area, and antisite defects. These defects give rise to a congruent crystal that shows lower remenance, high coercive field, high applied electric fields, and higher sensitivity. In conclusion, the enhancement of electro-optic sensitivity with fiber optic end coated LiNbO3 nanomaterial on the tip of the SMF coupler is documented. The materials demonstrate higher electric fields, nonideal behavior of P-E loop, and the dielectric constant in congruent LiNbO3 nanoforms as compared to the ideal bulk LiNbO3 . The results are attributed to the probably large surface area,
References 1. Y. Jiang-Hong, X. Jing-Jun, Z. Guang-Yin, C. Xiao-Jun, L. Bing, and L. Guan-Gao, Chin. Phys. Lett. 17, 513 (2000). 2. V. Samuel, A. B. Gaikwad, A. D. Jadhav, S. A. Mirji, and V. Ravi, Mater. Lett. 61, 765 (2007). 3. M. Liu, D. Xue, S. Zhang, H. Zhu, J. Wang, and K. Kitamura, Mater. Lett. 59, 1095 (2005). 4. K. Kaigawa, T. Kawaguchi, M. Imaeda, H. Sakai, and T. Fukuda, J. Cryst. Growth. 191, 119 (1998). 5. W. S. Hu, Z. G. Liu, and D. Feng, Solid State Commun. 97, 481 (1996). 6. D. P. Birnie, J. Am. Ceram. Soc. 74, 988 (1991). 7. S. Hirano and K. Kato, Adv. Ceram. Mater. 2, 142 (1987). 8. Ch. N. Rao, A. Bharadwaj, S. Datar, and S. N. Kale, Appl. Phys. Lett. 101, 043102 (2012). 9. C. D. Moreno, R. Farias, A. H. Macias, J. E. Galindo, and J. H. Paz, J. Appl. Phys. 111, 07D907 (2012). 10. R. Kumar, A. P. Singh, A. Kapoor, and K. N. Tripathi, Opt. Eng. 43, 2134 (2004). 11. P. Kumar, S. Moorthy Babu, S. Perero, R. L. Sai, I. Bhaumik, S. Anesamoorthy, and A. K. Karnal, J. Phys. 75, 1035 (2010). 12. R. Courths, P. Steiner, H. Hdchst, and S. Hfifner, Appl. Phys. 21, 345 (1980). 13. M. K. Kuneva and V. I. Krastev, J. Mater. Sci. Mater. Electron. 11, 629 (2000). 14. M. Aufray, S. Menuel, Y. Fort, J. Eschbach, D. Rouxel, and B. Vincent, J. Nanosci. Nanotechnol. 9, 4780 (2009). 15. V. V. Atuchina, T. I. Grigorievaa, I. E. Kalabina, V. G. Kesler, L. D. Pokrovskya, and D. I. Shevtsovc, J. Cryst. Growth 275, e1603 (2005). 16. A. Z. Simoes, M. A. Zaghete, B. D. Stojanovic, C. S. Riccardi, A. Ries, A. H. Gonzalez, and J. A. Varela, Mater. Lett. 57, 2333 (2003). 17. A. Lamberti, N. Garino, K. Bejtka, S. Bianco, S. Stassi, A. Chiodoni, G. Canavese, C. F. Pirri, and M. Quaglio, New J. Chem. 38, 2024 (2014). 18. B. Jayaprakash and S. Buddhudu, Indian J. Pure Appl. Phys. 50, 320 (2012). 19. F. M. Pontes, E. R. Leite, E. Longo, E. B. Araujo, J. A. E. Varela, B. Araujo, and J. A. Eiras, Appl. Phys. Lett. 76, 2433 (2000). 20. M. Jiang and E. Gerhard, Sens. Actuators A 88, 41 (2001). 21. A. Yariv and P. Yeh, Optical Waves in Crystals (Wiley, 1984). 22. J. Pamulapati, J. P. Loehr, J. Singh, and P. K. Bhattacharya, J. Appl. Phys. 69, 4071 (1991). 23. A. Guarino, G. Poberaj, D. Rezzonico, R. DeglInnocenti, and P. Gunter, Nat. Photonics 1, 407 (2007). 24. G. Poberaj, M. Koechlin, F. Sulser, A. Guarino, J. Hajfler, and P. Günter, Opt. Mater. 31, 1054 (2009). 25. Y. Q. Lu, J. J. Zheng, Y. L. Lu, N. B. Ming, and Z. Y. Xu, Appl. Phys. Lett. 74, 123 (1999). 26. M. Luennemann, U. Hartwig, and K. Buse, J. Opt. Soc. Am. B 20, 1643 (2003). 27. V. Gopalana and T. E. Mitchell, Appl. Phys. Lett. 72, 16 (1998).
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Lithium niobate nanoparticle-coated Y-coupler optical fiber for enhanced electro-optic sensitivity Ch. N. Rao,1 S. B. Sagar,1 N. G. Harshitha,1 Radhamanohar Aepuru,2 S. Premkumar,3 H. S. Panda,2 R. K. Choubey,1 and S. N. Kale1,* 1 2
Department of Applied Physics, Defence Institute of Advanced Technology, Girinagar, Pune 411025, India
Department of Materials Engineering, Defence Institute of Advanced Technology, Girinagar, Pune 411025, India 3 Armament Research & Development Establishment, Pashan Road, Pune, India *Corresponding author: [email protected] Received October 30, 2014; revised January 1, 2015; accepted January 1, 2015; posted January 7, 2015 (Doc. ID 225741); published February 5, 2015
Single crystals of lithium niobate (LiNbO3 ), possessing high birefringence and anisotropic properties have been explored, for a long time, to harness their excellent electro-optic properties. However, their nanoforms are comparatively less explored. In this context, dielectric constant and polarization (P) versus electric-field (E) characteristics of LiNbO3 nanomaterials have been studied. A nonideal P-E loop and a dielectric constant of 20 at the onset of 1 kHz were seen. The electro-optic sensitivity was found to be 4 times as compared to the bulk LiNbO3 crystals. The results are attributed to oxygen vacancies, antisite defects, and grain boundary effects in an already congruent structural matrix of LiNbO3 . © 2015 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (160.4330) Nonlinear optical materials; (060.2390) Fiber optics, infrared. http://dx.doi.org/10.1364/OL.40.000491
Lithium niobate (LiNbO3 ) is a conventional material with immense applications in opto-electronic industry, as an electro-optic and photorefractive material. Its unique set of properties include high birefringence, anisotropy, electro-optic coefficient, Curie temperature (1210°C), large nonlinear optical coefficient, and good piezoelectric and acousto-optic properties. Most of the properties have been harnessed in the applications such as surface acoustic waves (SAW) devices, optical couplers, optical isolators, optical modulators, and optical waveguides [1–3]. Appreciable research has been done with single crystals of LiNbO3 , but nanopowders have not been explored much. Few of these efforts, especially on thin films, were directed toward obtaining good quality films for integrated optics applications. Conventionally, polycrystalline ferroelectric thin films were grown on various substrates. Various deposition techniques, such as epitaxial growth by melting, chemical vapor deposition, liquid phase epitaxy, sputtering, and solgel, have been tried [4,5]. By conventional processing, LiNbO3 was prepared by calcinations of mixture of Li2 O and Nb2 O5 powders [6]. In recent years, solgel processing and coprecipitation methods have become popular for producing ceramic materials with improved compositional homogeneity and with lower sintering temperatures [7]. Nanomaterials have been synthesized and studied, but their effects on the said applications is still ongoing. The effects of oxygen-vacancy trap states, grain boundaries add to the already complex property range of such materials, which expand the application domain of these materials. Recently, in one of our works [8], it has been reported that the nanopowders show magneto-optic effect as well. Multiferroic properties are documented by Moreno et al. [9]. Enhancement of conventional bulk properties and exploration of new properties of this material in its nanoform is the motivation of this work. Any optically active material is normally characterized using fundamental property parameters such as its dielectric constant, refractive index, and polarization 0146-9592/15/040491-04$15.00/0
properties (mostly ferroelectric property studies). As these properties get modified, the response of the material will also get altered. LiNbO3 , conventionally, due to its structural anisotropy and mobility of Li ions near the phase transition temperature, is well known to be an optically active and structurally congruent material. Its high dielectric constant value and polarization (P) characteristics, with an applied electric field (E) (P-E measurements), shows its ferroelectric properties. Ferroelectric hysteresis curves of such materials do show remanence and coercivity, due to which they exhibit magnetic hysteresis as well. It is due to this that, in recent times, the material is projected as a multiferroic. Tuning the synthesis process and enhancing these properties can be a good motivation to study the nanoforms of such materials. In this context, through this manuscript, we report on synthesis of LiNbO3 nanomaterials using solgel method to yield polycrystalline nanopowders. These have been characterized using x-ray diffraction (XRD) studies, transmission electron microscopy (TEM) imaging, and x-ray photoelectron spectroscopy (XPS). 70 nm of particles were synthesized and dielectric as well as ferroelectric properties were studied. The nonideal P-E loop was observed, and dielectric constant of 20 at the onset of 1 kHz was seen. The electro-optic properties exhibited by these nanopowders show sensitivity (w.r.t. voltage) twice (2.07 pm/V) as compared to the single crystal of LiNbO3 (1.04 pm/V), while the sensitivity (w.r.t. electric field) was found to be (12.4 nm/kV/ mm), which is 4 times higher as well. The results are correlated to the congruent nature of the material, its grain boundary effects (due to nanoform) and the optical anisotropy generated due to oxygen vacancies and antisite defects. Synthesis of lithium niobate nanoparticles was done using solgel method, which is mentioned in earlier [8]. The prepared samples were characterized for structure, morphology, particle size, and ferroelectric and dielectric studies. © 2015 Optical Society of America
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For the coating of LiNbO3 on the tip of the optical fiber coupler, Poly vinyl Alcohol (PVA) was used as binder. Here, 1 gm of PVA was dissolved in 40 ml DI water at 100°C. The mixture was continuously stirred to yield a viscous gel at the same temperature. 20 mg of LiNbO3 powder was added to 2 ml of PVA gel and grinded well to form the uniform mixer of LiNbO3 and PVA. This gel was dip coated (thickness is 25 μm) to deposit on the cleaved mirror-surface of the optical coupler end part. PVA was specifically chosen, since it has been reported that it does not influence the optical signal in the tested range [10], which has been confirmed by our studies as well (detailed below). The schematic diagram of electro-optic sensor is shown in Fig. 1(a). A laser source (C-band, ALS-10-B-FC) was connected to one side of the Y-coupler (SMF −28, 3 dB, 1 × 2 coupler) and the other to the polarizer. From end of the Y-coupler, the optical power was directed to the mirror surface, which gets reflected and goes back to the detector. LiNbO3 was deposited on this mirror surface of the optical fiber. The polarized beam of light was expected to reflect from the multiple layers of the LiNbO3 -modified mirror surface and interfere with the reflected beam going toward the optical spectral analyzer (OSA-86146B). The observed interference pattern is shown in Fig. 1(b), which shows the clear fringe pattern of interference of reflected modes after coating the LiNbO3 at the end of the fiber tip. The flat response indicated the characteristic reflected spectrum of the SMF coupler with respect to the C-band source (without coating). Then the LiNbO3 -modified tip of the optical fiber was subjected to the applied electric field, and distance between electric plates was kept as 6 mm. Results were also compared with the fiber tip coated with only PVA. Hence, the ability and performance of the fiberoptic sensor for electric field measurement as electrooptic modulator was tested. The variation of electric field was regulated using DC power supply. The sensor response was recorded by the OSA with applied electric field. Figure 2 shows the XRD pattern of the synthesized LiNbO3 nanoparticles exhibiting the desired phase
Fig. 1. (a) Schematic diagram of experimental setup, in which optical fiber Y-coupler is used. (b) Optical response after and before coating.
Fig. 2. XRD pattern of the synthesized LiNbO3 nanopowders. (Inset shows the TEM micrograph of LiNbO3 .)
formation. The TEM micrograph showed average particles of ∼70 nm. XPS measurements were recorded for final LiNbO3 sample (annealed at 700°C). Figure 3(a) shows the total scan of the LiNbO3 sample surface. The constituent elements, i.e., Li-1s, O-1s, Nb-4s, Nb-4p, Nb3d doublet, and C-1s could be clearly observed from the scanned XPS spectrum, which were in well agreement with the earlier reported literature [11]. Figures 3(b) and 3(c) show the fitting curves for the elements Li and O, which gave an idea of the binding energies of the crystal composition on the sample surface. As has been mentioned in the studies depicted in Refs. [11–15], these signatures could give us an idea of the defects in the nanocrystalline structure, which could link to the amount of Li and/or O escaping from the stoichiometric structure. As is known, in an already congruent structure of lithium niobate, Nb ions could show richer contribution in the XPS scans because of the mobility of Li and O ions. This has been observed and documented earlier [12]. The peaks of Li-1s at 54.8 eV and O-1s at 530.1 eV as shown in Figs. 3(b) and 3(c), and the fitted curves, were almost in-line with the reported findings [13–15], wherein they attribute these values to the vacancies of Li and O and effective richer contribution of Nb on the surface. These samples were further characterized using dielectric property measurement and polarization electric field (P-E) measurement system. P-E loop, which exhibits
Fig. 3. (a) Complete XPS spectrum of LiNbO3 , (b) fitted data for Li B.E. value in LiNbO3 , and (c) fitted data for oxygen in LiNbO3 .
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Here, the effective refractive index, L is the interferometer physical length and λ is the wavelength of the source. The intensity of the reflected beam reaches maximum when φ 2m 1π, where m 0; 1; 2…. Since we need to analyze the effect of applied electric field on the total system, one should consider the effect on the basic system and the effect after coating the nanomaterial on the tip of the fiber. In this context, the net change in wavelength of interferometer will comprise of two components; one is due to the clad-core region, and the other is due to the core-mirror region. The change in wavelength corresponding to maximum in interference pattern is given by:
Fig. 4. Dielectric measurement study of the LiNbO3 nanomaterial at room temperature. The inset shows the P-E loop for the LiNbO3 nanoparticles at room temperature.
hysteresis behavior at room temperature for LiNbO3 thin films, is shown in the inset of Fig. 4. It exhibits the nonideal behavior of the ferroelectric loop. Low value of the remnant polarization (Pr 0.0481 μC∕cm2 ) for a coercive field of Ec 43 kV∕cm was observed. The switching range of the coercive field was around 43 kV/cm, which was very high as compared to micro and bulk materials (≈20 kV∕cm) [9,16]. Such nonideal behavior of P-E loop has also been reported previously in nanoconfined BaTiO3 material [17]. The nonideal behavior is probably due to the antisite defects, dislocations at the grain boundaries, and induced dipole moment. It is due to these behaviors that interfacial polarization arises, which could cause high leakage in the medium, resulting in low value of remnant polarization. Figure 4 shows the variation of dielectric constant of LiNbO3 thin film as a function of frequency. It can be noticed that dielectric constant decreases on increasing the frequency. The observed value of dielectric constant was found to be 20 at 1 kHz. The value of dielectric constant in nanopowder is much smaller and different than the bulk LiNbO3 [18]. Several possible causes such as the influence of a barrier layer between the film and leaky grain boundaries may be responsible for such behavior. Similar phenomena have also been previously reported for dielectric thin films by Pontes et al. [19]. To understand this data better, the basic principle of Fabry–Perot system was reviewed. The multiple layers of the coated film on the tip of the fiber forms multiple reflection mirrors. The reflectance is given by: 2Ri 1 − cos ϕ Rl ; 1 R2i − 2Ri cos ϕ
(1)
where Rl is the reflectivity of the multiple layers (which is dependent upon the scattering and absorption losses, as well), and ϕ is the optical phase shift and is incident power [20]. The relation between phase difference and path difference within the mirrors is given by:
φ
4πΔneff L : λ
(2)
Δλm
4Δneff L : 2m 1
(3)
The coupled component (core-mirror region) of multiple reflections possesses an additional phase retardation arising due to the Pockels effect, which is given by: δE; λ
4πne;o E; λL ; λ
(4)
where E is the applied electric field and ne;o is the birefringence (no − ne ) term of the material. This suggests that the shift in wavelength of interferometer due to this second component is a function of applied electric field. Now, especially for lithium niobate, it is important to understand the exact reason for its sensitivity to the applied electric field. As is known, the electro-optic (EO) LiNbO3 crystal is negative uniaxial (i.e., no > ne ), when light passes through the EO material, the effective refractive index (neff ) changes linearly with electric field. Hence, two birefringence terms arise, i.e., Δne;o ne − no n3e r 33 − n3o r 13 E∕2 [21]. The first and second terms are the natural and electrically induced birefringences terms, respectively. It is this second term, which gets modified upon subjecting the material to electric field, thereby contributing to the change in birefringence and hence the shift in wavelength. The optical path gets altered in the EO sensor medium due to the application of electric field, relative to the optical frequency of the incident light beam. Further, the optical phase retardation experienced by electric field when the light is linearly polarized along the e-axis is: δe E; λ
4πne L 2πn3e r 33 EL : λ λ
(5)
Since light experiences only one refractive index (ne ) in the crystal, Δδe E; λ 2πn3e r 33 LE∕λ controls the E-field-induced phase modulations. Figure 5 shows the interferometric fringe patterns response to the applied electric field. It is important to mention here that first, the pattern was also observed using only a PVA coat, which did not show any fringence pattern and effect on the applied fields; indicating that the contribution to the further documented results was mainly due to the nanomaterial. The sensitivity of the sensor was found to be 2.07 pm/V (12.4 nm/kV/mm) in
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defects, vacancy sites, and space charge limiting fields. Higher sensitivity nanolithium niobate-based electrooptic sensor is hence proposed. The authors acknowledge Dr. Prahlada, ViceChancellor, DIAT for the support. The authors also thank to Dr. A. Abhyankar, Dr. Sandip Dhobale, and Dr. H.H. Kumar for their scientific help.
Fig. 5. Response of the sensor assembly in terms of the wavelength shift w.r.t. the applied electric field.
the effective range from 0 to 1.5 kV and which was reversible. Here, the change of refractive index of the LiNbO3 was a function of applied electric field, as discussed above. The optical phase shift and refractive index both depends on the strength of the electric field (E) and medium interaction length (L). Further, since the results showed that there is a shift in the wavelength toward higher values upon application of electric field, it implied that the effective refractive index gets modified to a higher value upon application of electric field. This has also been documented before [22]. In the present communication, we observed sensitivity two times higher for nano LiNbO3 as compared to the reported bulk LiNbO3 crystals [23,24], while sensitivity in terms of electric field is 4 times higher than the LiNbO3 crystal [25]. The probable reasons of high sensitivity in nano LiNbO3 may be large surface area in nanoforms, effective trap densities, and so on [26]. On the other hand, Gopalan and Mitchell [27] have reported that the switching fields required for domain reversal in nonstoichimetric or defected crystals are 4–5 times larger than the stoichiometric crystals. Poberaj et al. [24] have observed a wavelength shift at 1555 nm upon applying a field of 100 V in LiNbO3 crystal. This field is at least 3 times lower than the present reported values in nano LiNbO3 , which is direct indication of high defect densities in nano LiNbO3 as compared to bulk LiNbO3 crystals. Hence, combining the results of dielectric constant and net polarization and clubbing the observations made in refs, one can envisage that the nano LiNbO3 comprises of various defects, either due to oxygen vacancies, grain boundaries, high surface area, and antisite defects. These defects give rise to a congruent crystal that shows lower remenance, high coercive field, high applied electric fields, and higher sensitivity. In conclusion, the enhancement of electro-optic sensitivity with fiber optic end coated LiNbO3 nanomaterial on the tip of the SMF coupler is documented. The materials demonstrate higher electric fields, nonideal behavior of P-E loop, and the dielectric constant in congruent LiNbO3 nanoforms as compared to the ideal bulk LiNbO3 . The results are attributed to the probably large surface area,
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