Plasmon enhanced photoacoustic generation from glass nanopillar arrays with silver nanoislands

Abstract

Ultrasonography is a noninvasive real time in vivo medical imaging method. Conventional ultraso-nography employs piezoelectric material (PZT) for ultrasound generation of 10 - 20 MHz in frequency. However, operating frequency over 50MHz is required to obtain few micron resolution in medical imaging. Photoacoustic effect is a local thermal expansion process due to optical absorption resulting a pressure wave. Photoacoustic effect can generate high frequency pressure wave in the order of tens of MHz by using the short optical pulse illumination. There has been some report that generates tens of MHz of ultrasound using photoacoustic effect in highly absorptive thin materials, such as a black PDMS, photonic crystal and plas-monic absorber. Plasmonic structure can absorb the optical energy in the specific wavelength at nanoscale. However, in plasmonic structure, not only the plasmonic structure itself can absorb the optical energy but also can enhance the optical energy nearby plasmonic structure due to local field enhancement between the nano-gaps produced by plasmonic structure. Therefore, plasmonic structure can work as photoacoustic gen-eration enhancement factor as well as absorber. Previous research on photoacoustic generation using plas-monic absorber has not taken full benefit of plasmon effect. Thereby, this work presents the localized surface plasmon effect as the enhancement factor for photoacoustic generation in polymeric absorber with full bene-fit of plasmon effect. The plasmon enhancement in 2D Ag nanoislands arrays is studied to verify the plas-mon enhancement in photoacoustic generation with high frequency. The results show that photoacoustic generation enhanced more than 2times through 420 nm to 680 nm in wavelength and especially 19 times at 680nm in excitation wavelength compare to the normal absorber without plasmonic structure. The substrate with 3D Ag nanoislands arrays is fabricated and measured to study the effect of local field enhancement...