Nanometric optical tweezers based on nanostructured substrates Miyasaka Lab. Hiroaki YAMAUCHI A. N. Grigorenko, N. W. Roberts, M. R. Dickinson & Y. Zhang Nature Photonics 2, (2008)

Contents Introduction ・ Optical tweezers ・ Localized plasmon resonance Experiment ・ Nanostructured substrate ・ Nanotweezers set-up Results and Discussion Summary

Optical tweezers radiation force Laser beam Laser beam is focused tightly Large intensity gradients are created Drawbacks of conventional optical tweezers Diffraction-limited Brownian motion T. Kusumi

Localized plasmon resonance Gold nanoparticle Optical near field Incident light The resonant interaction of the electromagnetic wave with the surface charges of the metal Nanometric optical tweezers rely on strongly enhanced electromagnetic fields near metallic nanoparticles.

Electron micrograph of the nanostructured sample Pair separation s=200nm Lattice constant c=500nm Height h=90nm Diameter d=100~140nm Arrays of gold nanopillars fabricated by high-resolution electron-beam lithography on a glass substrate s c Nanostructured substrate h Electromagnetic field intensity near the nanostructured substrate 30 times larger than laser light (1064nm)

Nanotweezers set-up 1064nm

200nm bead 14  m 0.7  m Scan speed 4  m s -1 Nanometric trapping

Histogram of particle displacement Nanometric trapping Conventional trapping An order of magnitude improvement 200nm bead a=0.7  m Time step 5ms 176nm 18nm

Escape speed F tr =  ・ 6  rv esc  : oil viscosity r : bead radius v esc : escape velocity K : correction coefficient trapping forceviscous drag force Modified Stokes law v esc = 150  ms -1 F tr = 2nN (1  m bead) Escape speed is the speed at which the particle is not able to follow the laser beam and escape it v1v1 v2v2 V 2 > V esc V 1 < V esc

: empty glass substrate: nanostructured substrate Escape speeds for trapped beads Escape speed near the surface of the empty glass decrease near the nanostructured surface 10 times increase

effective trapping quality factor Q = F tr c / nP c : speed of light n : refractive index of the oil P : laser power 30 times higher than that of conventional tweezers ( a = 1  m,1  m bead ) (P=440 mW, 1  m bead, a=2.5  m) best coupling of the symmetric plasmonic mode Escape speeds for trapped beads

Summary Nanometric optical tweezers has been realized using nanostructured substrate. The nanodot array provides almost an order of magnitude improvement of particle positioning with respect to conventional optical tweezers.