Quantum effects in colloidal nanoparticles. From the beauty of single quantum dot physics to ensemble applications...

Transcription

1 Quantum effects in colloidal nanoparticles From the beauty of single quantum dot physics to ensemble applications... Prof Pavlos Lagoudakis Ioffe 10/06/2013

2 Colloidal nanocrystals: nano-engineering low cost chemical synthesis optical tunability through size shape versatility CdSe CdS electron hole hole confined to core, electron spreads over whole length ~16nm nanorods of mixed dimensionality highly polarised luminescence Talapin et al., Nano Letters, 3,

3 Single Particle Spectroscopy 3

4 Single Particle Spectroscopy 10-5 mg/ml 10-6 mg/ml 10-7 mg/ml 10 µm Changing particle concentration 4

5 Manipulate brightness with external electric field V 8 µm 18 nm PL Energy (ev) Integrated Intensity (arb. u.) a) b) E E Electric Field (kv/cm) Manipulating oscillator strength 5

6 Tune photoluminescence energy a) Energy (ev) E b) Electric Field (kv/cm) 2.10 The wave functions are solved iteratively using a finite element method with a sequential optimization of V e,h following the Hartree self-consistent potential approach Muller et al Nano Letters Vol. 5, No Kraus et al Phys. Rev. Lett. 98, Energy (ev) 2.05 E Electric Field (kv/cm) enhanced Quantum Confined Stark Shift (up to 100 mev) at 4K 6

7 Experimental Setup Goal: Control the emission properties with applying an electric field excitation amplifier system 400 nm 5 µj/cm² trigger E-field generator delay spectrometer MCP PL 630 nm CCD glass + ITO SiO (10 nm) Ag (100nm) SiO (10 nm) polystyrene + nanorods (400 nm) 7

8 anipulating the wavefunctions in the ensemble 40 V QCSE in ensemble of nanorods E peak (mev) 8 4 E Electric field (MV/cm) Quadratic Stark effect of up to 14 mev 8

9 Gated Photoluminescene laser gate E-Field laser E-Field gate zoom: 10x PL quenching Kraus et al Phys. Rev. Lett. 98, PL burst at room temperature 9

10 Photoluminescence Quenching and Burst 40 V laser E-Field (1 µs) gate (50 ns) 0 V

11 Exciton Storage laser E-Field (1 µs 100 µs) gate 1 µs 100 µs 10 % Kraus et al Phys. Rev. Lett. 98, Sufficient exciton storage up to 100 µs! Device! 11

12 Burst E-field Dependence laser E-Field (1 µs, 10 V 40 V) gate 40 V 10 V Kraus et al Phys. Rev. Lett. 98, Controlling amount of exciton storage! 12

13 The interface to Life Sciences Hi-spots: used by pharmaceuticals Brain 80,000 cell/mm 2 c.a. 161,000 cell/mm 2

14 Neurophotonics V 8 µm 18 nm PL Energy (ev) Voltage (V) 500 Electric field Time (s) Reversely: E-field sensors at the nanoscale 14

15 Controlling alignment of nanocrystal networks 15

16 Controlling alignment of nanocrystal networks PC12 cells Use of nanocrystals underneath the cell layer Seeding of Pc12 cells Li-Niobate substrate Interrogating Neuronal Activity Electrical activity (action potential) Cellular activity (axonal transport) 16

17 Control growth localisation of cells 17

18 Intercepting Neural Signalling (+) Spatial alignment of nanocrystals Interrogating Neuronal Activity Electrical activity (action potential) Cellular activity (axonal transport) Phd project on Neuro/Nano-science Postdoctoral position available School of Physics and Astronomy School of Biological Sciences cell/neuritis-growth Prof Pavlos Lagoudakis 18

19 Intercepting Neural Signalling (+) PC12 cells Seeding of Pc12 cells Li-Niobate substrate Intercepting Neuronal Activity Electrical activity (action potential) Cellular activity (axonal transport) cell/neuritis-growth 19

20 Single mode, single exciton lasing in NQRs From the beauty of single quantum dot physics to ensemble applications...

21 Whispering gallery mode resonators Light confinement inside a microsphere Geometric optic Wave optic Schematic of a spherical microcavity showing distribution and spatial orientation of the mode Rakovich et al., Laser & Photon. Rev. 4, No. 2, (2010)

22 Single-mode single-exciton Laser from quasi-type II Colloidal Quantum Rods Schematic of the experimental arrangement used for demonstration of fiber-coupled laser operation of CdSe/CdS core/shell nanorods in silica microspheres. 22

23 Fabrication procedures flame brushing technique MODIFIED flame brushing technique CLEO 2012 May 6-11, San Jose, California, USA

24 Whispering gallery mode fluorescence 25

25 Single-mode laser emission: INTENSITY (arb. units) CdSe/CdS fluoresence WGM laser emission WAVELENGTH (nm) Laser (black line) and fluorescence emission (red line) spectra from a 9.2-µm-diameter hybrid sphere and the CdSe/CdS nanorods attached to the sphere, respectively. 26

26 Output intensity vs absorbed power: OUTPUT POWER (µw) single-mode laser operation P th = 67.5 µw / η=6.4% ABSORBED PUMP POWER (µw) Laser output power as function of pump power for the single-mode laser operation of a 9.2-µm-large hybrid microsphere. 27

27 Single mode laser Normalized intensity (a. u.) Output power (µw) nanocrystal fluoresence WGM laser emission Wavelength (nm) single-mode laser operation P th = 67.5 µw / η=6.4% Excitation in equatorial zone Microsphere diameter: 9.2 μm Q-factor after the coating 10 5 Laser emission at nm Laser line FWHM: 0.06 nm Lasing threshold: 67.5 μw Maximum output power: 5.5 μw Slope efficiency: 6.4% Absorbed pump power (µw)

28 Multimode laser emission Microsphere diameter: 29.4 μm Normalized intensity (a.u.) nanocrystal fluorescence WGM laser emission Wavelength (nm) Free spectral range The lasing modes distance: 2.4 nm.

29 Tunable laser emission Normalized intensity (a.u) 1.0 IR-laser power: 0 mw (no illumination) mw 40 mw 60 mw Wavelength (nm) Heating of the microsphere with 3.5-µm fs laser (Rep. Rate 80MHz) Shift of laser emission 2.1 nm Temperature variation of the microsphere: Calculation of the tuning range: thermal expansion of the CdSe/CdS nanocrystals thermal expansion of the silica microsphere template thermo-optic coefficient of the nanocrystals change of the CdSe bandgap manuscript submitted to Nature Photonics

30 IR tuning of laser emission: model vs experiment λ (nm) 3.0 microsphere thermal expansion (sim) NQR thermal expansion (sim) 2.5 NQR (dn/dt)-change (sim) band gap-change (sim) 2.0 total contribution (sim) total contribution (exp) IR-LASER POWER (mw) 31

31 Multimode laser emission Normalized intensity (a. u.) nanocrystal fluorescence WGM laser emission biexciton emission single-exciton emission Wavelength (nm) Excitation away from the equatorial zone Second laser line: nm Pump power thresholds of 100 μw and 122 μw

32 Multimode laser emission Intensity (a.u.) single exciton emission biexciton emission Absorbed pump power (µw) Excitation away from the equatorial zone Second laser line: nm Pump power thresholds of 100 μw and 122 μw

33 ħω Em. Abs. Em. Transparenc y Population Inversion Biexciton excitation Biexciton gain mechanism

34 E ħω Em. Δxx: Stark Shift Abs. Stark Shift: Δ ΧΧ =Ε ΧΧ -2Ε Χ Charge density: ρ X (r)=ρ h (r)+ρ e (r) ρ X (r)=e( Ψ h (r) 2 - Ψ e (r) 2 ) Gain

35 Optical gain on CdSe/CdS QRs Length: 27.7±2.2 nm Width: 4.1±0.6 nm electron CdSe CdS E ħω Em. Abs. hole Gain Conduction band offset: ±0.3 ev Valence band offset: 0.78 ev ρ X (r)=e( Ψ h (r) 2 - Ψ e (r) 2 )

36 Exciton dynamics in CdSe/CdS QRs Calculation of the wavefunctions and energies of electrons and holes The calculations have been done in collaboration with Dr. Chunyong Li

37 Exciton dynamics in CdSe/CdS QRs Single Exciton: Ee= ev-- Eh= ev EX=Ee+Eh+Eg=1.972 ev Energy (ev) CB λx= nm 0.3 ev CdSe 7 nm 28 nm ΔE = ev CdS 0.6 ev Biexciton: 2.46 ev Ee1= ev-- Eh1= ev ev 1.68 ev Ee2= eV-- VB Eh2= eV 4.34 nm EXX=Ee1+Eh1+Ee2+Eh2+2*Eg= CdSe CdS ev Stark Shift: ΔXX=Exx-2*Ex= ev λxx= nm CdS single exciton biexciton The calculations have been done in collaboration with Dr. Chunyong Li

38 Optical gain on CdSe/CdS QRs Length: 27.7±2.2 nm Width: 4.1±0.6 nm ħω Em. Abs. E ħω Em. Abs. electron CdSe CdS hole Transparency Gain Conduction band offset: ±0.3 ev Valence band offset: 0.78 ev

39 Hybrid Photonics group the people Dr Caryl Richards Microcavities Ultrafast spectroscopy PhD and post doc positions available Industrial collaborations/acknowledgements: Coherent Inc, IBM-Zurich, Luxtaltek, Leybold, Obducat, TSMC, PhotonSTAR, Q-cells, Solar World, Philips Martin Charlton (ECS, USoton) John Chad (School of Biology, USoton) Pavlos Savvidis (University of Crete) Ian Watson, Martin Dawson (Univ. Strathclyde) Mohamed Henini (University of Nottingham) David Lidzey (The University of Sheffield) Dmitry Talapin (University of Chicago) Horst Weller (University of Hamburg) Andrey Rogach (University of Hong Kong) Alexander Eychmuller, (Uni of Dresden) Jacqueline Bloch and Aristide Lemaître (LPN, Paris) Funding: EU-FP7: ITN-Icarus, ITN-spinoptronics, ITN- Clermont4, N4E NoE. EPSRC: EP/G063494/1, EP/G059268/1, EP/F026455/1, EP/F013876/1 Royal Society Industry in kind contribution: 900k University Southampton: 27k from central funds 40