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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