## 1 Introduction  
Raman spectroscopy, a spectroscopic analysis based on the inelastic scattering of light (laser as the excited light source) from excited molecules, reveals information on the vibration of functional chemical bonds in molecules and enables reliable identification of unknown species [1]. However,because of the extremely small scattering intensity and persistent problem of photoluminescence of Raman spectroscopy, which increases the background noise and results in a low signal-to-noise ratio. The surface-enhanced Raman scattering (SERS) technique, first observed by [PERSON] in 1974 [2], can efficiently enhance the Raman signal through localized surface plasma resonance (LSPR) from metals. When electromagnetic waves travel along the surface of a metal plate with a wave frequency smaller than the plasma frequency of electrons in the metal, the interaction between the wave and the electron clouds amplifies the electric field locally, leading to large-intensity Raman signals [1].  
Dielectrics--especially nanostructured semiconductors--exhibit enhanced Raman signals due to a series of optical effects, such as light absorption and trapping, photo-induced charge transfer (CT) and optical resonance, occurring in the dielectric or between the targeted molecules and dielectric [3]. The zinc oxide (ZnO) nanostructure, a unique material exhibiting semiconductor and piezoelectric dual properties [4], has proven to be a suitable SERS substrate [5; 6]. The enhancement of Raman signals is due to the inherent nanostructure with a large surface-to-volume ratio and the CT between the absorbed analyte and substrate [7]. A combination of metallic and ZnO nanostructures further amplifies the Raman signal because of the resulting effective CT and LSPR effect. Metallic nanostructures, such as silver (Ag) or gold (Au), facilitate not only additional absorbance due to LSPR, but also effective electromagnetic enhancement and CT [7].  
We fabricated a SERS substrate comprising ZnO nanorods (ZnO NRs) decorated with Au nanoparticles (AuNPs). Previous studies have demonstrated Ag-ZnO [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] or Au-ZnO [20; 21; 22; 23; 24; 25] hybrid nanostructures with enhanced Raman signals. However, the deposition process of metallic nanostructures is complex and time consuming. We introduced a pulsed-laser-induced photolysis (PLIP) method for fabricating AuNPs on ZnO NRs in the short time of 30 min without further treatment. The ZnO NRs fabricated using the sol-gel method on glass substrate were immersed in an auric acid (HAuCl4) and hydrogen peroxide (H2O2) aqueous solution, and AuNPs were grown on the surface of ZnO NRs. Under pulsed-laser irradiation of only eight minutes, H2O2 reduced Au ions, forming AuNPs solution. By controlling the surface morphology of ZnO NRs and growth conditions of AuNPs, a 10\({}^{-9}\) M concentration of rhodamine 6G (R6G) was feasibly detected. The experimental results indicate that thermal annealing promotes the growth of ZnO NRs in the (002) direction and enhances the Raman signal. However, ZnO NRs in the (002) direction are not preferred for the growth of individual AuNPs to enhance the Raman signal. ZnO NRs with the vertical c-axis (002) orientation are prone to AuNP aggregation, and the Raman signal is weakened because of the nonhomogeneous distribution of AuNPs on the ZnO NR surface. Under the same AuNP growth conditions, by using the PLIP method, ZnO NRs with the horizontal (100) and (101) orientations promote the growth of individual and homogeneous dispersed AuNPs, which offers the \"hot spot\" for enhancing the Raman signal. Moreover, the optimization of the SERS behaviors for different fabrication conditions of AuNPs using the PLIP method was investigated and found out short pulsed laser irradiation of eight minutes and chemical bath immersion time of 15-30 min are suitable for AuNP-decorated ZnO SERS substrate.