Optical Trapping of Quantum Dots Based on Gap-Mode-Excitation of Localized Surface Plasmon

J. Phys. Chem. Lett. 1, 2327-2333 (2010)

Ashida Lab. Shinichiro Bando

surface plasmon

Contents

• Introduction

• Samples

• Motivation• Experimental setup• Results • Summary

optical trapping

a substratequantum 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

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/cm2).・ 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 488nm

808nm

The 808nm irradiation off The 808nm irradiation ON

Photoluminescence quenchingPhotoluminescence

Optical trapping behavior of Q dots

Before irradiation at 808nmDuring irradiation at 808nm

Modulation of the photoluminescence intensity by repeatedly swiching the 808nm irradiation on and off.

nm. 808 at 5kW/cm3I value, threshold certain a discovered We 2th

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 nmColor: 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.

nm. 808 at 0.5kW/cmI value, threshold certain a discovered We 2th

d=50nm

d=30nm

d=>70nm

d=70-80nm

Enhancement factor Fe (Fe = Ion/Ioff) as a function of PEG concentration with varying

The enhancement factor F hardly depends on the 808 nm laser intensity, I.

Ion: in the presence of 808 nm irradiationIoff: 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. )0.5kW/cm (I 2

th

)5kW/cm (I 2th

・ 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