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Optical trapping of quantum dots
in air and helium gas
KAWAI RyoichiAshida Lab.
2013/10/30 M1 colloquium
1. Introduction Quantum dots (QDs) / Single QD’s property Radiation force / 2 photon absorption Previous work
2. Experiments The results in air The results in helium gas
3. Summary / Future Plan
Contents
Contents
Quantum Dots (QDs)
1.Introduction
• Fluorescent semiconductor nanocrystals
• Narrow emission spectrum … its wavelength is dependent on the size of QDs.
• Broad excitation spectrum Application:• Solar batteries• Single photon sources• Markers to visualize biological
systems etc… http://www.evidenttech.com/technology
Single QD’s Property – Blinking
Lisa Faye Marshall, Doctor of PhilosophySpectral dynamics of single quantum dots (2011)
1.Introduction
on
off
Single QD’s Property – Spectral Diffusion
Lisa Faye Marshall, Doctor of PhilosophySpectral dynamics of single quantum dots (2011)
1.Introduction
Spectral diffusion: Emission wavelength fluctuates.
Blinking and spectral diffusion may be caused by environment fluctuation around QDs.
Core/Shell Structure (CdSe/ZnS)
1.Introduction
CdSe
ZnS
shell
shell
core
Energy
Conduction Band
Valence BandCdSe ZnS
Band gap
Coating the core with the shell whose band gap is higher than that of the core.
Confining the produced hole-electron pair in the core.
⇧Quantum Dot
Radiation Force
1.Introduction
• Gradient force
• Dissipative force Scattering force Absorption force
Optical axis
Trap region
Colloidal QD
Gradient force
Dissipative force(Scat.+Abs. force)
Lens
Even if the core is coated, the effects are not under control.
Optical trapping technique
2 Photons Absorption
1.Introduction
Photon
Absorbing 2 photons at the same time.
Excitation → Emission
QD QD
We can excite QDs even if the photon’s energy is lower than the band gap of the QDs.
Previous Work
1.Introduction
They succeeded in trapping single quantum dot by the gradient force in the water. They confirmed that through luminescence measurement by 2 photon absorption.
Liselotte Jauffrd, Lene B. Oddershede, Nano Lett. 10, 1927-1930 (2010)
Experimental Purpose
2.Experiments
Our goal is to separate QDs from the environment and find out the QD’s intrinsic physical property. As a first step, we demonstrate the optical trapping of QDs in air and inert gas (helium).
Sample
2.Experiments
CdSe/ZnS (Core/Shell)
average particle diameter: 6.3 nm emission peak wavelength: 640 nm
SIGMA ALDRICH http://www.sigmaaldrich.com/japan/materialscience/nano-materials/lumidots.html
Experimental Set Up
2.Experiments
Ti; Sapphire laser
Dichroic B.S.
Objective lens
Trap region
filter
Objective lensTrap region
Nd; YAG laser
Spectrometer + CCD
Trapping laser(Ti; Sapphire laser) wavelength: 775 nm power: 2.1 W continuous wave
Nd; YAG laser (SHG) wavelength: 532 nm power: 0.1 mJ/pulse pulse width: 10 ns repetition frequency: 10 Hz
Sample
Pinhole (100µm)
filter
Time-dependent P.L. Spectrum (in air)
2.Experiments
700680660640620600580wavelength [nm]
300
250
200
150
100
50
0
tim
e [s
ec.]
500
480
460
440
420
400
380
Blinking
Spectral diffusion
Time-dependent P.L. Spectrum (in helium)
2.Experiments
700680660640620600580wavelength [nm]
6000
5000
4000
3000
2000
1000
0
tim
e [s
ec.]
700
650
600
550
500
Blinking
Slight blue shift
Comparison of P.L. Spectra
2.Experiments
80
60
40
20
0
P.L
. [a.
u.]
700680660640620600580wavelength [nm]
ensemble trapped (t=19 sec.) trapped (t=57 sec.)
700
600
500
400
300
P.L
. [a.
u.]
700680660640620600580wavelength [nm]
ensemble trapped (t=171 sec.) trapped (t=355 sec.)
⇦ in airSpectral diffusion
ordeterioration due to oxidation
⇦ in helium gas Emission peak wavelength is approximately constant.
A single QD ?
2.Experiments
In helium gas, we could trap QD(s) longer and more stably than in air. In the air, we found spectral diffusion. In both air and helium gas, we could found blinking.
Indications of a single QD !!Indications of a single QD !!
Image of the Trapped QD(s)
2.Experiments
⇧Trapped QD(s)
We succeeded in taking movies of trapping QD(s).These images are captured from the movies.
no QD
Summary
3.Summary / Future Plan
• We succeeded in trapping QD(s) in air and helium gas.
• In air, we found spectral diffusion or deterioration due to oxidation.
• In helium gas, we succeeded in trapping QD(s) longer than in air.
Future Plan
3.Summary / Future Plan
• To confirm whether trapped QD is single.
• To study the details of the trapped QD(s).
• To expand many kinds of nanoparticles other than QDs.