My Title

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Time Space-Resolved Luminescence of Excitons
--Experiments in the MRL Laser Laboratory
Joon Jang and Jim Wolfe Department of
Physics Frederick Seitz Materials Research
Laboratory

• Layout of the land its a great facility
• Photoluminescence in space, time and energy
• Excitons in cuprous oxide (Cu2O)
• Future directions

Co-workers in the MRL Laser Laboratory Jeremy
Warren, James Kim (currently at Lumileds), Keith
OHara (currently at Carl Zeiss Meditec)
special assistance from the Laser lab staff
Jeff White, Ray Strange, Julio Soares and
students Research supported by U.S. Department
of Energy
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Typical photoexcitation of a Semiconductor
Conduction band
Photons hn gtgt Egap
Free electrons and holes
Near surface absorption
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The electrons and holes combine to form Excitons
at low temperatures
Conduction band
phonons
Binding energy
f
hfluminescence
hflaser
k
Valence band
Refs Dexter and Knox, Excitons (Interscience,
1965) Rashba and Sturge, Excitons (Elsevier,
1987)
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Photoluminescence of Excitons
Basic experiment
Enter the MRL Laser Laboratory
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The MRL Laser Lab
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The MRL Laser Lab
YLF Pump Laser
Dye Lasers
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The MRL Laser Lab
Dye lasers l 500-600 nm
(R6G) pulse width 5 ps
Mode-locked YLF Laser l 527 nm (doubled)
pulse width 200 ps repetition
rate 76 MHz average power 2 W
Autocorrelator resolution 100 ps
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Picosecond Photoluminescence System
Time- and space-resolved PL T 1.8K 300K
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Picosecond Photoluminescence System (contd)
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Picosecond Photoluminescence System (contd)
Laser light Luminescence
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Picosecond Photoluminescence System (contd)
Galvo
Scanning mirrors for imaging
Galvo
0.5-m Spectrometer grating 1200 lines/mm
dispersion 1.6 nm/mm blaze 1200 nm
Luminescence light. Image of crystal is
focused onto slit plane.
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Picosecond Photoluminescence System (contd)
Multichannel- plate phototube (Hammamatsu)
Spectrometer
Photon counting electronics Rate meter Photon
counter Discriminator Delay Time-to-amplitude
converter
Total time response 70ps
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Picosecond Photoluminescence System (contd)
Tool box
Computer (controls scanning and acquisition.)
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Crystals with long-lived excitons (gt1 ms)
(Excitonic lifetimes gtgt thermalization times.)
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Excitonic Luminescence in Cu2O
T 2 K cw excitation with Ar laser
(scattered laser light filtered out)
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Typical low-power luminescence spectra
(Snoke, Wolfe, and Mysyrowicz, PRL 59, 827 (1987))
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Bose-Einstein Condensation (BEC) of Excitons?
m particle mass
M.H. Anderson, J.R. Ensher, M.R. Matthews, C.E.
Wieman, and E.A. Cornell, Science 269, 198 (1995)
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Rapid Diffusion of Excitons (Cu2O)
one barrier to overcome
Time resolved images of exciton diffusion
200 ns 600 ns
At high densities and T the expansion is
non-diffusive due to shortened lifetimes.
D 1000 cm2/sec at T 1.3 K ! Excitons are
extremely mobile.
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Direct measurement of the gas density
OHara, OSuilleabhain, Wolfe, PRB 60, 10565
(2000) OHara thesis, UIUC (1999)
1) Measure radiative efficiency from exciton
absorption
-- agrees with previous measurements.
2) Measure absolute luminescence flux for our
setup
3) Measure effective volume by spatial scanning.
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Can we reach the BEC density??
Unfortunately, the gas heats up as the density is
increased (by increasing laser power).
What is causing the heating of the gas at high
density?
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Auger Recombination of Excitons?
Possible exciton loss mechanism
One e-h pair recombines, giving a band gap energy
to the remaining pair
dn/dt - An2
Two bad effects 1. Loss of excitons 2.
Heating gas
Compare to theoretical Auger rate.
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Strong density-dependent decay
Cu2O

• Our experiments with T 72 212 K show a strong
  density-dependent recombination rate that is
  inversely proportional to the temperature.
• Unless this exciton loss mechanism can be
  overcome, the prospect for BEC of excitons in
  Cu2O looks pretty bleak.
• Remaining Puzzle The measured constant A is
  several orders of magnitude larger than present
  theories.

A new explanation (just submitted
for publication)
Kavoulakis and Baym, PRB
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Formation of short-lived biexcitons?
? impurity-induced lifetime ?A biexciton Auger
lifetime C capture coefficient n mass action
constant ? biexciton binding energy
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Formation of short-lived biexcitons?
T 8 K
Crucial parameters obtained from our analysis
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Prospectus Can we detect the biexcitons?

• Our next Laser-lab experiments
• Create biexctions directly with 2-photon
  excitation.
• Detect their recombination products excitons
• Can we avoid biexciton formation with stress or
  external fields? Or possibly observe BEC of
  biexcitons at low temperatures?

Stay tuned.