[14a-2C-1~10] 4.1 Plasmonics

▲ [14a-2C-5] Plasmonic-Nanoparticle-Enhanced Hyper-Raman Spectroscopy

〇(DC)Bikas Ranjan¹, Mamoru Hashimoto², Yuika Saito¹, Prabhat Verma¹ (1.Department of Applied Physics, Suita, Osaka University, Japan, 2.Department of Physics, Toyonaka, Osaka University, Japan)

キーワード：Hyper-Raman Scattering,Nanoparticles,Plasmonics

Hyper-Raman scattering (HRS) is a weak, nonlinear optical process. It measures the molecular vibration and gives information about the intrinsic properties of the molecules. In HRS, incident photons and scattered photons are separated by almost half of the wavelength, where as, in normal Raman scattering (RS), incident photons and scattered photons wavelengths are very close. As excitation and scattering mechanism of RS and HRS are different, therefore, selection rules differ for both. This allows HRS to observe some of the molecular vibrations that were not allowed in normal RS. Though, HRS is important molecular spectroscopy technique, but signal is weak, therefore, enhancement is needed. Plasmonic nanoparticles, which have surface plasmon resonance (SPR) band, could be used for enhancement. Several research groups have reported the SPR technique to enhance the incident photons or scattered photons by using single SPR band of nanoparticles. However, single band enhancement is still not promising; therefore, we propose double band enhancement. Here, we used plasmonic nanoparticles with large aspect ratios, such as nanorods. It is possible to tune the wavelengths of SPR bands by varying the length and the diameter of the nanorods. The nanorods show two distinct surface plasmon resonance (SPR) bands, which originate from the plasmons propagation in two perpendicular directions. When the incident photons and scattered photons of HRS are in match with the both SPR bands of nanorods, it is possible to enhance the incident photons and scattered photons of HRS. In our work, first we did finite-difference time-domain (FDTD) simulation of silver nanorods and nanospheres with different lengths and diameters, respectively. The silver nanorod of diameter 45 nm and length 160 nm gives two SPR bands, close to 400 nm and 800 nm. For experimental observation of the particle-enhanced HRS, we utilized the silver nanorods that show two SPR bands ~400 nm and ~800 nm, and demonstrated the doubly enhanced HRS signal from crystal violet dye molecules. Here, the length and the diameter of the silver nanorods are selectively chosen in such a way that the longer wavelength SPR (~800 nm) is tuned to the laser excitation wavelength and the shorter wavelength SPR (~400 nm) is tuned to the wavelength of scattered photons. Thus, the selective enhancements of both the incident photons and the scattered photons allow us to enhance the weak HRS signal.