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

2. 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 Contents

3. Quantum Dots (QDs) 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

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

5. Single QD’s Property – Spectral Diffusion Single QD’s Property – Spectral Diffusion Lisa Faye Marshall, Doctor of Philosophy Spectral 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.

6. Core/Shell Structure (CdSe/ZnS) Core/Shell Structure (CdSe/ZnS) 1.Introduction CdSe ZnS shell core Energy Conduction Band Valence Band CdSeZnS 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

7. Radiation Force 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

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

9. Previous Work 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)

10. Experimental Purpose 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).

11. Sample 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/materi alscience/nano-materials/lumidots.html

12. Experimental Set Up Experimental Set Up 2.Experiments Ti; Sapphire laser Dichroic B.S. Objective lens Trap region filter Objective lens Trap 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

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

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

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

16. A single QD ? 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 !!

17. Image of the Trapped QD(s) 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

18. Summary 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.

19. Future Plan 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.