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Single Quantum Dot Optical Spectroscopy

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Single Quantum Dot Optical Spectroscopy. Presented by. Rohini Vidya Shankar. Amrita Urdhwareshe ... Discrete atom-like states in 0 D quantum dots. Discrete ... – PowerPoint PPT presentation

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Title: Single Quantum Dot Optical Spectroscopy


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 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

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, 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)

22
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