Optical Trapping of Quantum Dots Based on Gap-Mode-Excitation of Localized Surface Plasmon J. Phys. Chem. Lett. 1, (2010) Ashida Lab. Shinichiro Bando

surface plasmon Contents Introduction Samples Motivation Experimental setup Results Summary optical trapping a substrate quantum dots

Introduction Incident light is resonant with plasmon due to coherent oscillation of conduction band electrons. Then, an electromagnetic field is enormously enhanced at a junction of the nanoaggregate. Surface plasmon is coherent electron oscillation. Optical trapping Surface plasmon gradient force kT<U

Gold nanodimer arrays on a plasmonic glass substrate SEM image AFM image in 3D The NSL substrate has two types of nanotrapping gap site. nanogap nanovalley Optical absorption spectrum 200nm 40nm

A fluorescent quantum dot (Qdot) 10nm CdSe/ZnS core-shell nanoparticles 11nm 5nm CdSe ZnS

Q dots photoluminescence (PL) intensity as a function of detection position Q dot photoluminescence was promptly quenched when the Q dot locates in the vicinity of the gold.

Motivation ・ The conventional technique requires (kT<U) intense focused laser light (MW/cm 2 ). ・ The spatial resolution is limited to more than several hundreds of nanometers. We demonstrate the plasmon-based optical trapping of a very small semiconductor Q dot in a nanospace with considerably weak light irradiation.

Experimental setup 488nm 808nm The 808nm irradiation off The 808nm irradiation ON Photoluminescence quenchingPhotoluminescence

Optical trapping behavior of Q dots Before irradiation at 808nm During irradiation at 808nm Modulation of the photoluminescence intensity by repeatedly swiching the 808nm irradiation on and off. One possible explanation for this is optical trapping of Q dots at the nanogaps of the NSL gold structure.

488nm 808nm 488nm 808nm Photoluminescence quenching

Optical trapping behavior of Q dots in the presence of poly ethylene glycol (PEG) Black: before irradiation at 808 nm Color: during irradiation at 808 nm Modulation of the photoluminescence intensity by repeatedly swiching the 808nm irradiation on and off. The photoluminescence of the Q dot clusters increases markedly on exposure to the 808 nm irradiation. d=50nm d=30nm d=>70nm d=70-80nm

Enhancement factor F e (F e = I on /I off ) as a function of PEG concentration with varying The enhancement factor F hardly depends on the 808 nm laser intensity, I. I on : in the presence of 808 nm irradiation I off : in the absence of 808 nm irradiation

Optical micrographs of the temporal behavior of Q dots trapping by 808 nm irradiation The trapping behavior can be readily visualized. We clearly detected short time intervals.

Theoretical calculation of trapping potential The zy map of log(U/kT) Regions of U>kT are near the edges and gaps of metal blocks. The Q dot is trapped under an energetic condition of U<kT. This is partially due to a contrast in refractive index (n). This is partially due to the van der waals force.

・ In the absence of PEG, Q dot PL is quenched by 808 nm irradiation beyond a certain threshold. summary ・ In the presence of PEG, Q dot PL is enhanced by 808 nm irradiation beyond a certain threshold. ・ The enhancement factor Fe increases with increasing size of the Q dot cluster, whereas it scarcely depends upon laser intensity.

My work Ablation laser Manipulation laser CuCl 1 µm We can’t manipulate in nano space.

Future plan 1 µm +

Photoluminescence quenching Photoluminescence