ENHANCEMENT OF NANO AND MICRO PHOTONICS LITHIUM NIOBATE VIA SPIN- COATING TECHNIQUE FOR OPTICAL WAVEGUIDE
APPLICATIONS
Makram A. Fakhri, Y.Al-Douri, U.Hashim
Institute of Nano Electronic Engineering,
University Malaysia Perlis UniMAP, 01000 Kangar, Perlis, Malaysia Corresponding author: [email protected]
ABSTRACT
Lithium niobate (LiNbO3) nano and micro photonics structures are deposit on glass substrate by spin-coating technique. LiNbO3 are deposited at 3000 rpm for 30 sec and annealed at 400, 500, 600 oC, respectively. They are characterized and analyzed by scanning electron microscopy (SEM) and Ultre-Violet visible (UV-vis). The measured results have showed that at increasing of annealing temperature, the structures start to be more crystallized and be more homogenize until optimum structure applicable for optical waveguides then start to be less in crystallization and not homogeneous.
Keywords: Component; Lithium Niobate; Nanophotonic device; Optical waveguide
INTRODUCTION
Lithium Niobate (LN) is a very important optical material, which is widely used by the photonics industry mainly due to its excellent electro/acousto-optical properties [1- 3]. Lithium niobate (LiNbO3) is an important ferroelectric material because of its excellent piezoelectrical, electrooptical, pyroelectrical and photo-refractive properties [4,5]. It’s a widely used as polar material for photonic applications [6,7], in addition its employed in nonlinear optics for frequency conversion and in telecommunication for electro- optic modulation [8,9]. It is a very attractive material for the fabrication of optical wave-guide devices [10,11]. From experimental estimation, a direct and indirect energy band gap of LiNbO3 is reported to be in the range of 3.5 - 4.7 eV basing on LiNbO3 concentrations. These changes are attributed to several parameters like grain size, composition and defects [12-14]. There are different approaches known to synthesize undoped LiNbO3
nanocrystals using soft-chemistry [15], pulsed laser deposition [16,17], RF sputring [18] and hydrothermal methods [19,20]. LiNbO3 is a widely used polar material for photonic applications [21,22]. It is employed in nonlinear optics for frequency conversion, telecommunication for electro-optic modulation [23] and fabrication of optical waveguide devices [24,25], and Surface acoustic wave device (SAW) [26].
LiNbO3 waveguides are already widely used in many functional electro-optic and
acousto-optic devices [27] Waveguide structures are essential for many integrated- optic devices.
This work reports the preparation of LiNbO3 nanostructures by utilizing the spin- coating technique. The characterization and analysis have been elaborated as a function of molarity concentration. The refractive index is the main work on optical waveguides for LiNbO3 because of refraction coefficients between the base and deposit samples will ensure access to total internal reflection that give us better realizing of optical waveguides. The refractive index is measured and calculated to fit the best application of optical waveguides, then we prepared the thin film and enhanced it to get a high quality film have an optical properties exactly like single crystal LiNbO3 wafers.
EXPERIMENTAL
The preparation procedure for LiNbO3 films by using Nb2O5 (ultra-pure, 99.99%), and citric acid (CA.) are used without further purification. The solution is prepared by mixing Li2CO3, Nb2O5, citric acid and Ethylene Glycol. The molar ratio between Li2Co3 and Nb2O5 was 1:1 in order to maximize the formation of LiNbO3
stoichiometry phase. Firstly, the Li2Co3, Nb2O5, and citric acid were dissolved in Ethylene Glycol with heating and stirring at the 90 °C for 48 hours, then mixed all together with continued heating and stirring at the 90 °C for 48 hours. To obtain homogeneous and crack-free films of LiNbO3, the precursor was deposited by spin coating technique on quartz substrates at a spinning speed of 3000 RPM for 30 Sec and 0.5 Ml/L molarity concentration. Seven layers were prepared, the film was dried at the 120 °C for 5 min and calcined at the 250 °C for 30 min in static air and oxygen atmosphere to remove the organics then it was annealed at 400, 500, 600
°C, respectively. The structural evolution of the as-prepared thin films was examined using a high-resolution X-ray diffraction (HR-XRD), (X’Pert Pro MRD PW3040 system diffractometer, PANalytical Company, Netherlands) system equipped with Cu-K α-radiation of wavelength λ = 0.15418 nm at 40 kV and 30 mA. The thickness of the annealed samples was studied using Scanning Optical Reflectometer model (Filmetrics F20, China). The Scanning Electron Microscopy (JOEL JSM-6460LV, Oxford instruments, Analytical Ltd., Japan) used to investigate the surface morphology of LiNbO3, and Atomic Force Microscopy (AFM) (SPM-9600, Scanning Probe Microscope, Shimadzu, Japan) for investigating the roughness of LiNbO3. The optical properties were investigated using the double-beam Ultr-Violet (UV-vis) spectrophotometer (Shimadzu UV- Vis 1800, Japan).
RESULTS AND DISCUSSION Morphological studies
It is a very interested parameter for integrated-optic and optoelectronic applications.
Figure 1 shows SEM images (5*5 µm) of LiNbO3 nanostructures deposited on quartz substrates at different molarity concentrations. Since the density of
nucleation for the LiNbO3 thin films was not uniform on the flat substrate with different molarity concentration. At least molarity concentrations note that the emergence of a high proportion of the pores and voids, these pores because of these impurities impacts like Nb2O5, the LiNbO3 thin films grew smoothly at 500 oC that is led to perfect distribution better than in this annealing temperature as shown in (Figure 1). On the other hand, the structure is more homogenous at higher molar concentration. As discussed earlier, this suggests that 500 oC leads to increase of the regular distribution of LiNbO3 nano and microstructures. A closer examination of these samples indicates that our synthesized film like Ice layers shape of morphology.
Figure 1: Surface morphology of LiNbO3 nanostructures with different Annealing temperatures. (a) 400 °C, (b) 500 °C and (d) 600 °C
Optical properties
The transmission spectra of LiNbO3 nanostructures at different annealing temperature is shown in Figure 2, it was found that the transmission increase as annealing temperature increases due to increasing the grain size and decreasing in structure thickness. These values of transmission are about 89.25 – 95.99% with annealing temperature (400 700) °C. The deposited samples were white to brown in color and show high transmittance; this is due to excessive LiNbO3 atoms in the structure [28]. The high value of transmittance is attributed to these excessive
(LiNbO3) ions existing at interstitial sites that probably transparent for light.
Figure 2: Transmission spectra of LiNbO3 nanostructures in different annealing temperatures
The wide direct band gap makes LiNbO3 good material for potential applications in optoelectronic and photonic devices such as multilayer dielectric filters, optical waveguides, SAW and solar cell due to decreasing the window absorption loses and that will improve the short circuit current of the cell. The energy band gap (Eg) is found by plot (αhv)2 vs hν as shown in Figure 3, as given in Table 1 in good accordance with the experimental value [29].
Table 1: The energy band gaps, refractive index and optical dielectric constant correspond to the maturity concentration of LiNbO3 nanostructures
Figure 3: Energy band gap of LiNbO3 nanostructures in different annealing temperatures
Figure 4: Reflection spectra of LiNbO3 nanostructures in different annealing temperatures
The optical reflectance (R%) of LiNbO3 nanostructures was measured using double-beam UV- Vis and it can be calculated from the absorption and the transmittance spectrum using the relation; R+T+A = 1. Figure 4 shows the
reflectance of LiNbO3 nanostructures in the wavelength range 250- 800 nm at room temperature.
The refractive index (n) was determined from a transmittance spectrum as a function of the wavelength in the range 250-700 nm. The n value could be determined from the transmission spectrum. There is a decreasing in the refractive index in the visible range; it was estimated 2.43 – 2.54 at 330 nm as shown in Figure 5 and given in Table 1. The refractive index changes slightly and steadily after 330 nm to 800 nm [32, 33] as given in Table 1. We can notice from that, the refractive index increases as annealing temperature increases. This behavior may be attributed to increasing of grain size and decreasing of thickness because of the annealing temperature. All the highest values of refractive index are a suitable for optical waveguide.
Figure 5: Refractive index of LiNbO3 nanostructures in different annealing temperatures
Figure 6 shows the relationship between n and annealing temperature that justifies the direct correlation between n and A to big grain size affects directly on the refractive index.
Figure 6: Refractive index with concentration of LiNbO3 nanostructures in different annealing temperature.
CONCLUSION
The LiNbO3 nanostructures have been chemically prepared by spin-coating technique. AFM shows diameter of gain size (from 64 to 167) nm, and roughness ranging between 10.6-16.0 nm. Optical properties give high values of transmission that is about 89 - 96% and the measured energy band gaps are 3.6, 3.85, 4, 4.2 eV, We found an approximate match between the energy band gap in the values calculated using the transmission. The refractive index is determined from the transmission spectrum and found that the highest value is an appropriate for optical waveguide.
REFERENCES
[1] M. A. Fakhri, Y. Al-Douri, U. Hashim, E. T. Salim, Australian Journal of Basic and Applied Sciences 9 128-133 (2015)
[2] E. Marenna, C.Aruta, E.rFanelli, M.Barra, P.Pernice, A.Aronne, Journal of Solid State Chemistry 182 1229–1234 (2009)
[3] P. Kumar , S.M. Babu , S Perero , R.M.L.Sai, I Bhaumik, S. Ganesamoorthy and A .K. Karnal, Pramana, J. Phys. 75 1035-1040 (2010)
[4] J. Zhang, X. Zhang, Optics Communications 320 56– 59 (2014)
[5] L. Cao, A. Aboketaf, Z. Wang, S. Preble, Optics Communications 330 40–44 (2014)
[6] H. Chen, , T. Lv, A. Zheng, Y. Han, Optics Communications 294 202–207 (2013) [7] P. Ganguly, Optics Communications, 285, 4347–4352 (2012)
[8] B. Knabe, D. S. Tze, T. Jungk, M. Svete, W. ssenmacher, W. Mader, K. Busem, phys. status solidi (a) 208 857–862 (2011)
[9] S. Mamoun , A.E. Merad , L. Guilbert, Computational Materials Science, 79 125- 131 ( 2013)
[10] C. Thierfelder, S. Sanna, A. Schindlmayr, and W. G. Schmidt, phys. status solid (C) 7 362–365 (2010)
[11] A. Tumuluri, K .L. Naidu, K.C.James Raju, International Journal of Chem. Tech.
Research CODEN (USA): IJCRGG 6 3353-3356 (2014)
[12] O. P. Nautiyal1, S. C. Bhatt, American Journal of Materials Science 1 1-4 (2011) [13] L. H. Wang, D. R. Yuan, X. L. Duan, X. Q. Wang, and F. P. Yu, Cryst. Res.
Technol 42 321– 324 (2007)
[14] K. Peithmann, M. Zamani-Meymian, M. Haaks, K. Maier, B. Andreas, K. Buse and H. Modrow, Appl.Phys. B 82 419–422 (2006)
[15] M.Liu, D Xue, K Li, Journal of Alloys and Compounds 449 28-31 (2013)
[16] X. Wang, Y.Liang, S. Tian, W. Man, J. Jia, Journal of Crystal Growth 375 73–77 (2013)
[17] X. Wang, Z. Ye , G. Wu, L. Cao, B. Zhao, Materials Letters 59 2994-2997 (2005) [18] V. Iyevlev, A. Kostyuchenko, Journal of Materials Science: Materials in
Electronics 22 1258- 1263 (2011)
[19] G. Isobe, P. Bornmann, T. Hemsel and T. Morita, Proceedings of Symposium on Ultrasonic Electronics 32 19-20 (2011)
[20] M. Aufray, S. Menuel, Y. Fort, J. Eschbach, D. Rouxel, B. Vincent, Journal of Nanoscience and Nanotechnology 9 4780–4785 (2009)
[21] J. Amet, F.I. Baida, G.W. Burr, M.-P. Bernal, Photonics and Nanostructures – Fundamentals and Applications 6 47–59 (2008)
[22] Y. Guo , Ch. Divin A. Myc, F. L. Terry, J.R. Baker, T. B. Norris, and J. Y. Ye, Opt Express 16 11741–11749 (2008)
[23] H. Lu, B. Sadani, G. Ulliac, N. Courjal, C. Guyot, J. -M. Merolla, M. Collet, F. I.
Baida, M.-P. Bernal, Opt Express 20 20884-20893 (2012)
[24] J. Son, S. S. Orlov, B. Phillips, L.Hesselink, Journal of Electroceramics 17 591- 595 (2006)
[25] J.W. Son, S. S. Orlov, B.Phillips, L. Hesselink, Journal of Electroceramics 17 591-595 (2006)
[26] K.S. Kao, C.C. Cheng, Y.C. Chen, Y.H. Lee, Applied Physics A 76 1125-1127 (2003)
[27] G-h. Shao, X-s. Song, F. Xu, and Y-q. Lu1, Opt Express 20 19343-19348 (2012) [28] I.-K. Jeong, Journal of the Korean Physical Society 59 2756-2759 (2011)
[29] E.T. Salim, Y. Al-Douri, M.S. Al Wazny, M.A. Fakhri, Solar Energy 107 523–
529 (2014)
[30] N.S.L.S. Vasconcelos, J.S. Vasconcelos, V. Bouquet, S.M. Zanetti, E.R. Leite, E.
Longo, L.E.B. Soledade, F.M. Pontes, M. Guilloux-Viry, A. Perrin , M.I.
Bernardi, J.A. Varela, Thin Solid Films 436 213–219 (2003)
[31] David E. Zelmon and D. L. Small, J. Opt. Soc. Am. B 14 3319 – 3322 (1997) [32] T.Ghosh, B.Samanta, P.C. Jana, and P. Ganguly, Fiber and Integrated Optics 31
1-10 (2012)
[33] D.L. Zhang , Q. Zhang, C.X. Qiu, W.H. Wong, E. Y. B. Pun, Optical Material Express 4 2215-2220 (2014)