O ptical trapping of quantum dots in air and helium gas

1. 2013/10/30M1 colloquium Optical trapping of quantum dotsin air and helium gas KAWAI Ryoichi Ashida Lab.

2. Contents • Introduction • Quantum dots (QDs) / Single QD’s property • Radiation force / 2 photon absorption • Previous work • Experiments • The results in air • The results in helium gas • Summary / Future Plan Contents

3. Quantum Dots (QDs) • 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 1.Introduction

4. Single QD’s Property– Blinking on off Lisa Faye Marshall, Doctor of Philosophy Spectral dynamics ofsingle quantum dots (2011) 1.Introduction

5. Single QD’s Property– Spectral Diffusion Spectral diffusion: Emission wavelength fluctuates. Lisa Faye Marshall, Doctor of Philosophy Spectral dynamics ofsingle quantum dots (2011) Blinking and spectral diffusion may be caused by environment fluctuation around QDs. 1.Introduction

6. Core/Shell Structure (CdSe/ZnS) Energy ZnS Conduction Band shell CdSe core Band gap shell Valence Band CdSe ZnS ⇧Quantum Dot 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. 1.Introduction

7. Radiation Force Optical axis • Gradient force • Dissipative force • Scattering force • Absorption force Even if the core is coated, the effects are not under control. Dissipative force (Scat.+Abs. force) Optical trapping technique Colloidal QD Trap region Gradient force Lens 1.Introduction

8. 2 Photons Absorption QD QD Photon Absorbing 2 photons at the same time. Excitation → Emission We can excite QDs even if the photon’s energy is lower than the band gap of the QDs. 1.Introduction

9. Previous Work They succeeded in trapping single quantum dot by the gradient force in the water. Theyconfirmed that through luminescence measurement by 2 photon absorption. LiselotteJauffrd,Lene B. Oddershede, Nano Lett. 10, 1927-1930 (2010) 1.Introduction

10. Experimental Purpose 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). 2.Experiments

11. Sample 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 2.Experiments

12. Experimental Set Up Ti; Sapphirelaser filter Pinhole (100µm) DichroicB.S. Spectrometer+ CCD Objective lens Trap region Trappinglaser(Ti; Sapphire laser) wavelength: 775 nm power: 2.1 W continuous wave Trap region Objective lens Nd; YAG laser (SHG) wavelength: 532 nm power: 0.1 mJ/pulse pulse width: 10 ns repetition frequency: 10 Hz Sample Nd; YAG laser filter 2.Experiments

13. Time-dependentP.L. Spectrum(in air) Spectral diffusion Blinking 2.Experiments

14. Time-dependent P.L. Spectrum (in helium) Blinking Slight blue shift 2.Experiments

15. Comparison of P.L. Spectra ⇦ in air Spectral diffusion or deterioration due to oxidation ⇦ inhelium gas Emission peak wavelength is approximately constant. 2.Experiments

16. A single QD ? 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!! 2.Experiments

17. Image of the Trapped QD(s) We succeeded in taking movies of trapping QD(s).These images are captured from the movies. ⇧Trapped QD(s) no QD 2.Experiments

18. Summary • We succeeded in trapping QD(s) in air and helium gas. • Inair, we found spectral diffusion or deterioration due to oxidation. • In helium gas, we succeeded in trapping QD(s) longer than in air. 3.Summary / Future Plan

19. 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. 3.Summary / Future Plan