Optical Trapping of Quantum Dots Based on Gap-Mode-Excitation of Localized Surface Plasmon

1. 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

2. Contents optical trapping • Introduction • Samples • Motivation • Experimental setup • Results • Summary surface plasmon a substrate quantum dots

3. Introduction Optical trapping Surface plasmon gradient force Surface plasmon is coherent electron oscillation. kT<U 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 .

4. Gold nanodimer arrays on a plasmonic glass substrate 40nm 200nm 200nm AFM image in 3D Optical absorption spectrum SEM image The NSL substrate has two types of nanotrapping gap site. nanogap nanovalley

5. 10nm A fluorescent quantum dot (Qdot) CdSe/ZnS core-shell nanoparticles ZnS CdSe 5nm 11nm

6. 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.

7. 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.

8. 808nm 488nm 488nm Experimental setup Photoluminescence Photoluminescence quenching The 808nm irradiation off The 808nm irradiation ON

9. Optical trapping behavior of Q dots Before irradiation at 808nm Modulation of the photoluminescence intensity by repeatedly swiching the 808nm irradiation on and off. During irradiation at 808nm One possible explanation for this is optical trapping of Q dots at the nanogaps of the NSL gold structure.

10. 808nm 488nm Photoluminescence quenching 808nm 488nm

11. Optical trapping behavior of Q dots in the presence of poly ethylene glycol (PEG) The photoluminescence of the Q dot clusters increases markedly on exposure to the 808 nm irradiation. d=>70nm d=70-80nm d=50nm d=30nm Modulation of the photoluminescence intensity by repeatedly swiching the 808nm irradiation on and off. Black: before irradiation at 808 nm Color: during irradiation at 808 nm

12. Enhancement factor Fe (Fe = Ion/Ioff) as a function of PEG concentration with varying Ion: in the presence of 808 nm irradiation Ioff: in the absence of 808 nm irradiation The enhancement factor F hardly depends on the 808 nm laser intensity, I.

13. 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.

14. 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.

15. ・In the presence of PEG, Q dot PL is enhanced by 808 nm irradiation beyond a certain threshold. summary ・In the absence of PEG, Q dot PL is quenched by 808 nm irradiation beyond a certain threshold. ・The enhancement factor Fe increases with increasing size of the Q dot cluster, whereas it scarcely depends upon laser intensity.

16. 1 µm My work CuCl Ablation laser Manipulation laser We can’t manipulate in nano space.

17. 1 µm Future plan +

18. Photoluminescence Photoluminescence quenching