Near-field optical mapping of single gold nano particles using photoinduced polymer movement of azo-polymers

Significance Statement

Light-induced mass movement in azo-polymers has been investigated for a number of applications such as direct nanofabrication, however, the mechanism of light-induced polymer movement is still not well understood.

A number of previous research works indicate that direction of mass movement is influenced by the state of the incident light polarizations as well as intensity distributions of light. Above all, the various results have revealed that photo selective isomerization along with molecular reorientation yielded anisotropic fluidic force in the polymeric material under light-induced softening of the polymers by photo-isomerization.

Sub-diffraction imaging of optical near-field appears to be a potential application in view of the several applications implementing light-induced mass movement in azo-polymers. In light of this application, optical near-field distribution in the vicinity of nanostructures is shifted to surface deformations of azo-polymers via light-induced mass movement where spatial resolution may surpass the diffraction limit of light. Above all, intensity distribution and polarization state of the near-fields may be adequately determined.

Professor Hidekazu Ishitobi and colleagues reported polymer movement in azo-polymer films induced by a plasmonically enhanced near-field near gold nanoparticles. They focused on the effects of the gold nanoparticles diameters on the deformation patterns. Their work is now published in Optics Communications.

The authors adopted gold nanoparticles with 50 and 80 nm diameters for their study. They fixed the nanoparticles onto a glass substrate by silane coupling chemical reaction. They then prepared thin films of poly (Disperse Red 1 methacrylate) by spin coating from a chloroform solution onto the gold nanoparticles that were fixed on the glass substrates. 30nm and 50nm thick films were prepared for, respectively, 50nm and 80nm nanoparticles. The diameters of the nanoparticles and the thickness of the films were selected to get the same ratio of the nanoparticle diameter to film thickness to curtail large differences in the field distributions.

The research team measured in the vicinity of a 50nm gold nanoparticle, the deformations initiated by plasmonically enhanced near fields. The gold nanoparticle was covered by an azo-polymer on a glass substrate. The outcomes revealed that the polymer movement was not induced in the absence of the gold nanoparticles in view of the irradiation condition. This suggested that the effect of the far-field component of the irradiated light on the polymeric material could be neglected.

Image analysis indicated that the polymer situated at both sides of a gold nanoparticle moved along the polarization direction and then formed two dips near the nanoparticle and protrusions in the outer position. Polymer deformation showed that polymer situated on both sides of the nanoparticle along the polarization direction moved towards outer position along the polarization direction by anisotropic fluidic force. The polymers were observed to move from a region of high light intensity to a region of low light intensity along the incident light polarization. Gold nanoparticle diameters led to varying deformation patterns.

Near-field optical mapping of single gold nanoparticles using photoinduced polymer movement of azo-polymers-Advances in Engineering

About The Author

Dr. Hidekazu Ishitobi received his PhD in Engineering from Osaka University in 2002. After postdoctoral research at Osaka University and RIKEN, he joined Graduate School of Frontier Biosciences at Osaka University in 2011 as an assistant professor, and was promoted to an associate professor (2015). His research interests are focused on molecular nanophotonics.

Reference

Hidekazu Ishitobi, Taka-aki Kobayashi, Atsushi Ono, Yasushi Inouye. Near-field optical mapping of single gold nano particles using photoinduced polymer movement of azo-polymers. Optics Communications, volume 387 (2017), pages 24–29.

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