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

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

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

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

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 ~ 10-100 Ao Core-shell quantum dots E.g. CdSe coated with ZnS or CdS, etc. Particle size ~10-100 Ao Epitaxially deposited Usually III-V semiconductors. E.g. GaAs, InGaAs, AlGaAs, etc. Particle size ~ 10-40 nm

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

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

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)

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

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

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

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

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

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

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]

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

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

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])

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

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

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, 1243-1256 (1999) [3] A. Chavez-Pirson, J. Temmyo, H. Kamada, H. Gotoh, and H. Ando, Applied Physics Letters 72, No. 6, 3494-3496 (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, 802-804 (1996) [5] Y. Toda, S. Shinomori, K. Suzuki and Y. Arakawa, Physical Review B 58 No. 16, R10 147-R10 149 (1998)

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