7. Optical Spectroscopy at
Cryogenic Temperatures
• Zero-Phonon Line: transition without
creation or destruction of phonons
• Phonon Wing: at T = 0 K, creation of one or
more phonons
Mirror Image
Absorption and
fluorescence
spectra are related
by a mirror
symmetry around
the 0-0 transition
Intensity and Width of ZPL
• Intensity decreases steeply with T
1 





 
I ZPL  exp   2  tanh
2k BT  




• Width limited by excited-state lifetime and
dephasing (thermal fluctuations)
1 2
 hom   *
T1 T2
Inhomogeneous Broadening
Disorder and defects cause a spread of
electronic transition frequencies
Single-Molecule Spectroscopy
Spectral selection of
single molecules
Detection of single
molecules by fluorescence
excitation (M. Orrit and J.
Bernard, Phys. Rev. Lett.
65 (1990) 2716)
The first optical detection of a
single molecule, via absorption
(W. E. Moerner and L. Kador,
Phys. Rev. Lett. 62 (1989) 2535)
8. Two-Level System in a Laser Field
• Detuning from resonance
• Rabi frequency
    eg

eg  E0



Optical Saturation
Saturation of the
fluorescence excitation
line of a single
dibenzoterrylene molecule
in a naphthalene crystal
Maximum intensity and width as functions of the laser power
Transients: Optical Nutation
Nutation transients without (left) and with (right) coherence damping
Antibunching histograms
Antibunching at low temperature (left, pentacene in p-terphenyl) and
at room temperature (right, terrylene in p-terphenyl)
Quantum Optics
Correlation histograms
of a single-photon source
Light Shift of the optical transition
9. Triplet State(s)
• Only one triplet level: correlation function
g
( 2)
k13 k13  k31 
( )  1 
e
k31
• Two sublevels:
k23  2T1T2
k13 
2 1   2T1T2
On- and Off-time Statistics
From: Th. Basché, S. Kummer, Ch. Bräuchle, Nature 373 (1995) 132
Optically Detected Magnetic Resonance
• Microwave transfers populations between triplet
sublevels, modifying the average fluorescence intensity
• … here for a pentacene molecule in a p-terphenyl crystal,
• … or changing the off-time statistics,
• here for terrylene in p-terphenyl, A. C. J. Brouwer et al.,
Phys. Rev. Lett. 80 (1998) 3944.
Single nuclear spins
ODMR of fully deuterated single pentacene molecules
containing only C12 atoms (left), or one C13 atom in two different
positions (center, right). The splitting is due to the nuclear spin
J. Köhler et al., Science 268, 1995,1457.
10. External Fields
• Stark effect
  1   
h    E  2 E    E
• quadratic
…or linear.
Shift of single terrylene molecule lines under modification of
the carrier gas in a semiconductor (ITO) by an applied
sawtooth voltage
Low-frequency localized acoustic modes
11. Spectral Diffusion
• Jumps or drift of the ZPL in spectrum
• Two-level Systems in Glasses
Evidence for a single
TLS in the correlation
of a terrylene molecule
in polyethylene
Spectral jumps in p-terphenyl crystals
a: p-terphenyl
b: terrylene
Crystal structure
4 spectroscopic
sites of terrylene
in p-terphenyl
Spectral diffusion close to domain walls
• Wall = 2D lattice of 2-level systems
• Random jumps
spectral diffusion
W. P. Ambrose et al.
J. Chem. Phys. 95
(1991) 7150.
12. Interacting Single Molecules
• Contact interactions
• Electron exchange
• Dipole-dipole coupling

3
J 
r
40
2
leads to ¨FRET, excitonic coupling
Exciton coupling in a dimer
1  cos A  sin  B
2   sin  A  cos B
J
tg 2 

Energies
2
  J
2
Bacterial Light-Harvesting Complex
B850 ring
B800 ring
Excitation spectra of single LH2’s
Ensemble
Individual Complexes
A. van Oijen et al.,
Science 285 (1999) 400.
Exciton coupling in the B850 ring
k= ± 1 excitons
split by distortion
k=0 exciton
Two Quasi-Resonant Molecules
C. Hettich et al., Science 298 (2002) 386.
• A new two-photon resonance appears at
high laser intensity between two singlemolecule lines
Two-photon resonance
Excitation of Molecule 1
Excitation of Molecule 2
Molecules are coupled!
13. Other Single Molecule Experiments
• Studies of soft matter and materials
• Other emitters, SC nanocrystals, color centers
• Blinking statistics
• Non-fluo. optical detection methods
• Photothermal detection
• Pump-probe and other nonlinear spectroscopies
Conclusion
• SM Microscopy at room T:
– biophysics
– material science
• SM Spectroscopy at room and low T:
– molecular physics
– quantum optics
– solid state physics