Notes Bull. Korean Chem. Soc. 2010, Vol. 31, No. 9 2675 DOI 10.5012/bkcs.2010.31.9.2675 Nonlinear Optical Properties of ZnO Hyeong Seop Shim, Noh Soo Han, Joo Hee Seo, Seung Min Park, * and Jae Kyu Song * Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea * E-mail: smpark@khu.ac.kr (S. M. Park), jaeksong@khu.ac.kr (J. K. Song) Received June 11, 2010, Accepted July 12, 2010 Key Words: ZnO, Second-harmonic generation, Two-photon absorption, Three-photon absorption 350 400 450 500 550 600 Wavelength (nm) Intensity (a) 200 nm 350 360 370 380 390 400 Wavelength (nm) (b) 800 nm 710 nm 750 nm × 200 Intensity 400 500 600 700 800 Wavelength (nm) (c) 2 1 1/(n 2 - n 2 ) 2 ω 2 ω Refractive index 2.25 2.20 2.15 2.10 2.05 2.00 1.95 extraordinary ordinary Figure 1. (a) Photoluminescence spectrum of the ZnO powders ob- tained with a He-Cd laser (325 nm). Inset shows the transmission electron microscopy (TEM) image of the ZnO powders used for the measurements. (b) Emission spectrum of the ZnO powders obtained by intensive excitation of fundamentals, such as 800, 750, and 710 nm. The dotted line indicates the band gap of ZnO. (c) Refractive index and its difference related term as a function of the wavelength. Recent advances in material science have led to the produc- tion of high quality crystals for II-VI and III-V semiconductors. Among the many II-VI semiconductors available, zinc oxide (ZnO) has attractive properties, such as a wide band gap (3.37 eV) and a large exciton binding energy (60 meV). 1,2 However, compared to the linear optical properties, there is less under- standing of the nonlinear optical properties of ZnO. Some studies of the nonlinear optical responses, such as second-harmonic generation (SHG) and third-harmonic generation (THG), have been reported. 3-7 The nonlinear optical responses supply a deep penetration depth of the probe beam because long excitation wavelengths can be used in two-photon absorption (2PA) and three-photon absorption (3PA). 5-9 Despite the importance of competition between harmonic generation (SHG and THG) and multiphoton absorption (2PA and 3PA) for potential applica- tions, there are only a few reports on the simultaneous measure- ments of harmonic generation and multiphoton absorption. 5,6 In this Note, the nonlinear optical properties of ZnO were exa- mined by nonresonant optical excitation of femtosecond pulses, which revealed concurrent harmonic generation and multi- photon absorption. Figure 1(a) shows a typical photoluminescence spectrum of the ZnO bulk powders. The emission peak at 382 nm was assign- ed to the well-known exciton emission. 1,2 The absence of visible emission that normally results from defects suggests that the nonlinear optical properties of ZnO can be investigated sys- tematically without the perturbation of defects. Figure 1(b) shows the nonlinear optical responses of ZnO, which were ob- tained by intensive excitation of the fundamentals such as 800, 750, and 710 nm. The SHGs at 400 and 375 nm, which were twice the photon energy of 800 and 750 nm, were observed along with exciton emission at 382 nm. A small intensity of SHG at 355 nm was also found when the fundamental was tuned to 710 nm. Although the full width at half maxima (FWHMs) of the SHGs at 400, 375, and 355 nm appeared to be different on the wavelength scale, the FWHMs in the photon energy scale were almost identical. The intensities of the SHGs were not similar in Figure 1(b) because the intensity of SHG at a given time (I2ω) has a depen- dence on the frequencies of the incident fundamental beam from the following equation. 10 3 2 4 2 2 2 2 2 2 2 2 512 sin ( ( )) ( ) ( ) ( ) ( ) ( ) ( ) SHG eff I t T I d A n n ω ω ω ω ω ω π α α α α α α α   Ψ =       −   (1)