1. Single Quantum Dot Optical Spectroscopy
Presented by
Rohini Vidya Shankar
Amrita Urdhwareshe

2. Motivation Discrete atom-like states in 0 D quantum dots
Discrete exciton levels just below the bandgap
Quantum confinement effect for excitons
Ultra narrow transitions and spectra expected

3. Observed quantum dot emission
Optical spectra of 35 Ao CdSe nanocrystals: no discrete lines, even at low T
Ref [1]

4. Inhomogeneous broadening
Ensemble averaging of optical properties
Need to take single dot spectra

5. Experimental techniques
Samples of single quantum dots to look at
Chemically prepared and spin coated on substrates
Usually II-VI semiconductors. E.g. CdSe, PbS, CdS, etc.
Particle size ~ Ao
Core-shell quantum dots
E.g. CdSe coated with ZnS or CdS, etc.
Particle size ~ Ao
Epitaxially deposited
Usually III-V semiconductors. E.g. GaAs, InGaAs, AlGaAs, etc.
Particle size ~ nm

6. Experimental techniques (contd.)
Optical techniques used
Far-field epifluorescence microscopy/spectroscopy
Near-field optical spectroscopy

7. Far-field epifluorescence spectroscopy
Light focused and collected using the same objective
Both images and spectra obtained by switching between a mirror and a diffraction grating
Need low areal densities ~ one quantum dot per µm2

8. Far field images and spectrum
A) Image of single CdSe 45 Ao nanocrystals at 10 K (Ref [2])
B) Image of the same region as in (A) with narrowed entrance slit
C) Spectrally dispersed image of the entrance slit in (B)

9. Near field optical spectroscopy
Low temperature nano-probing system based on shear-force distance regulation.
Near field excitation of the sample and near-field collection of the luminescence
Useful for quantum dot areal densities of the order of 100/µm2

10. Near-field imaging
Near-field luminescence image of a single In0.4Ga0.6As/Al0.5Ga0.5As QD (T = 5 K) (Ref [3])
Quantum dot emits light in a narrow band centered at a wavelength of 733nm

11. Observations
Same 35 Ao CdSe spectra (Ref [1]): dotted lines show ensemble measurement. Solid lines: single quantum dot measurement
Narrow peakwidth at low T!

12. Observations Ensemble vs single CdSe nanocrystal spectra (Ref [2])
Ensemble spectrum: average of many single nanocrystal spectra
Shift in energy peaks with average nanocrystal size

13. Fluorescence blinking
On/off nature of fluorescence spectra (Ref [4])
Typical on-off timescale ~.5 sec.
Not observed for ensembles

14. Blinking (contd.) On times: dependent on excitation intensity
Vary inversely as excitation intensity
Off times: Independent of excitation intensity
Proposed explanation
Photo ionization of nanocrystals
Also possibly, thermally activated charge trapping

15. Spectral diffusion Different lineshapes for different nanocrystals
Excitation intensity and integration time dependent linewidths
Spectral diffusion: result of locally changing electric fields
Possibly correlated to fluorescence intermittency
Ref [2]

16. Spectra of capped nanocrystals
Capping materials: higher bandgap semiconductors
Highly enhanced quantum yield of spectra (as high as 50%)
Red shift of the emission peak
Decreases intermittency to a timescale ~several seconds to few minutes

17. Polarized photoluminescence studies
Narrower linewidth enables precise measurements of luminescence character
Information about the spin-related effects such as Zeeman splittings.
Relaxation processes in single GaAs/InAs quantum dots studied using polarized photoluminescence (PL) spectroscopy in an external magnetic field

18. Unpolarized and Polarized Spectra
Typical unpolarized photoluminescence spectra from a single GaAs quantum dot ~20nm at various magnetic fields (Ref [5])
Luminescence spectra for all polarization geometries at 8 T (Ref [5])

19. Summary Need to observe single quantum dot spectra
Techniques of sample preparation and spectrum acquisition
Salient features of the spectra
Narrow linewidths
Size dependence of emission peaks
Blinking/intermittency
Spectral diffusion
Polarization dependence

20. Potential applications
DNA and protein labeling
Highly luminescent single quantum dots can overcome the functional limitations encountered with chemical and organic dyes
Easily tunable emission wavelength by changing the particle size or composition
Optical coherence tomography using quantum dots
Quantum-dot-based super-luminescent light-emitting diodes
High-bandwidth high-power light sources
Spectra of these devices can be largely tuned

21. References
[1] U. Banin, M. Bruchez, A. P. Alivisatos, T. Ha, S. Weiss and D. S. Chemla, Journal of Chemical Physics 110 No. 2, 1195 – 1201 (1999)
[2] Stephen A. Empedocles, Robert Neuhauser, Kentaro Shimizu and Moungi G. Bawendi, Advanced Materials 11, No. 15, (1999)
[3] A. Chavez-Pirson, J. Temmyo, H. Kamada, H. Gotoh, and H. Ando, Applied Physics Letters 72, No. 6, (1998)
[4] M. Nirmal, B. O. Dabbousi, M. G. Bawendi, J. J. Macklin, J. K. Trautman, T. D. Harris and L. E. Brus, Nature 383, (1996)
[5] Y. Toda, S. Shinomori, K. Suzuki and Y. Arakawa, Physical Review B 58 No. 16, R R (1998)

22. Thank You!