1 Second harmonic generation in ZnO nanorods M. Das, S. Rana, † and P. Sen †∗ Department of Applied Physics, Shri G S Institute of Technology & Science, Indore - 452 003 India. † Laser Bhawan, School of Physics, Devi Ahilya University, Khandwa Road, Indore - 452 017 India. * Email: pratimasen@gmail.com Abstract—A comparative study of second harmonic generation in ZnO micro particles and nanorods has been experimentally studied by Kurtz technique using nanosecond pulsed Nd:YAG laser. The results have been theoretically explained considering higher order effects in the dipole moment. I. I NTRODUCTION Zinc oxide (ZnO) is considered as one of the potential candidate materials for optoelectronic devices because of its large band gap and exciton binding energy. An enhancement in the second harmonic (SH) response of ZnO thin films with decreasing film thickness was experimentally observed by Wang et al [1]. Apart from this, second-order susceptibility (SOS) was reported to depend on the aspect ratio of the ZnO nanorods by Chan et al [2] showing that the ZnO nanorods exhibit size dependent NLO coefficients. Resonant exciton second harmonic generation (SHG) in self assembled ZnO thin films was observed by Zhang et al [3], where they found that d 333 of thin films at resonance was nearly 14 times larger than its value in bulk ZnO in the off resonant regime. The angular dependence of SHG in ZnO nanolayers as observed by Neumann et al [4] suggest that the role of bulk property dominates over surface effects. The angularly resolved SH signals displayed a maxima at 43 ◦ to the angle of incidence. Broad band frequency doubling property of c-axis oriented ZnO nanorods was also observed by Das et al [5] where they reported large nonlinearity as well as angular dependence of SHG which confirms that bulk property dominates over the surface effects. In the present article, we report the experimental observation of second harmonic generation (SHG) in c-axis oriented ZnO nanorods using Kurtz powder method. The SHG in ZnO nanorods was compared with the micro particles of ZnO resulting in enhanced SHG in nanorods. We have measured highest scattered SH intensity at an angle of 45 ◦ from the incident fundamental radiation.The experimental findings are explained by taking into account the quadruple moment along with the dipole moment in the radiation- matter interaction. II. EXPERIMENT DETAILS The ZnO nanorods were prepared at Electronic Materials and Devices, National Physical Laboratory, New Delhi and their NLO property was examined in our laboratory. The average length of the nanorods ranged from 50 to 100 µm and diameter 25 to 30 nm. From transmission electron microscopy (TEM) diagram (Fig. 1), we find that the lattice constant of ZnO nanords are a = 3.2512 ±×10 −10 m and c = 5.2078 ± 0.0018×10 −10 m. The lattice constants of wurtzite ZnO bulk Fig. 1. Transmission electron microscopy (TEM) image of the ZnO nanorods. sample mostly range from 3.2475 to 3.2501 ×10 −10 m for the ‘a’ parameter and from 5.2042 to 5.2075 ×10 −10 m for the ‘c’ parameter showing modification of lattice constants. SH efficiency of ZnO nanorods was compared with their bulk counter part of particle size >150µm. We employed Kurtz powder technique to study SHG efficiency of nanorods and microparticles of ZnO. SHG measurement was performed using 7 ns pulsed Nd: YAG laser at 1064 nm wavelength. The incident beam was split into 95:5 ratios with 95 percent fundamental focused on the sample, while the 5 percent intensity of the fundamental was used as a reference beam to normalize the fluctuations of the incident laser. The scattered SH output was collected using a large aperture lens (numerical aperture = 2.9) and was filtered through a SH separator. The output of the separator was given to photo multiplier tube which is connected to 100 MHz digital storage oscilloscope (DSO) [as shown in Fig. 2]. III. THEORETICAL CONSIDERATIONS ZnO is a II-VI compound semiconductor whose ionicity re- sides at the borderline between covalent and ionic semiconduc- tors, obtained in wurtzite, zinc blende and rocksalt structures. The crystal structure of the ZnO nanorods under consideration is wurtzite and belongs to the noncentrosymmetric (NCS) space group 6mm with a basic geometry of AB 4 tetrahedron. We assume the ZnO particle to comprise of a collection of charges. The potential near the surface of the particle is given by [6] φ = 1 4πǫ 0 R 2  i  1+( d i R )  q i d i , (1) ICOP 2009-International Conference on Optics and Photonics CSIO, Chandigarh, India, 30 Oct.-1 Nov. 2009