Photoacoustic Drive of Microfluid Using Au/SiO2 Heterostructured Nanorods Array
Authors: Namura Kyoko , Nakajima Kaoru , Kimura Kenji , Suzuki Motofumi 
Published: February 2020

There is a growing interest in photoacoustic emission from noble metal nanoparticles (NPs) because of their potential applications in the field of manipulation of fluid or particles by convection flow and thermophoresis. Especially, two-dimensional noble metal NPs arrays are suitable for microfluid manipulation because they can establish a spatial temperature gradient at nanometer scale. However, their optical absorption is only a few tens of percent of the incident light even at the wavelength of the local plasma resonance, so that they are inefficient from the viewpoints of photothermal conversion efficiency.
Recently, we demonstrated the self-assembly of Au NPs/ SiO2 nanocolumns/ SiO2/ Ag mirror structures, i.e., local plasmon resonators, by taking advantage of dynamic oblique angle deposition technique. Since they form an optical cavity, the optical absorption of Au NPs array can be modulated from 3% to 97% by varying the thickness of the SiO2 layer due to the interference. In our previous study [1], we reported the photothermal conversion efficiency of the local plasmon resonators is proportional to their optical absorption. This result suggests that their heat generation can be spatially controlled through the patterning of the SiO2 layer thickness. If the local plasmon resonator with a high optical absorption can produce strong photoacoustic emission, they are expected to realize flexible fluid manipulation by a spatio-temporally controlled temperature gradient at nanometer scale. In this study, we investigate their potential for the fluid manipulation through photoacoustic measurements and simple fluid manipulation experiments.
The photoacoustic spectra are obtained by irradiating the samples with a laser (wavelength 785 nm) whose intensity was modulated sinusoidally (1–100 kHz) and recorded the amplitude of the acoustic signal as a function of the frequency. In order to demonstrate the fluid manipulation with local plasmon resonators, we created a PDMS cell, which was filled with water in which polystyrene microspheres were dispersed, on a sample. Then, we observed the motion of the polystyrene microspheres under laser illumination.
The photoacoustic amplitude of a local plasmon resonator with a high optical absorption (A = 97%) is much larger than the amplitude not only of a high reflective Ag film (A ≤ 5%) but also of a graphite (A = 85%). In addition, the photoacoustic amplitude of the local plasmon resonators remains at high value even in the high frequency region from 20 kHz and 100 kHz, while those of the graphite and Ag thin film attenuated significantly. These results suggest that the heat generation in the local plasmon resonators is well localized in the thin Au NPs layer (∼ 10 nm) and contributes to the efficient photoacoustic emission. In addition, the sample with a high optical absorption rapidly drive the polystyrene microspheres dispersed in water under laser illumination, whereas that with a low optical absorption dose not drive the microspheres at all. Consequently, the local plasmon resonators are suitable for flexible microfluid manipulation.
[1] K. Namura, et al., Opt. Lett. 36, 3533 (2011).