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.