Zero-Phonon Line: transition without creation or destruction of phonons

1
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

2
Mirror Image
Absorption and fluorescence spectra are related
by a mirror symmetry around the 0-0 transition
3
Intensity and Width of ZPL

• Intensity decreases steeply with T
• Width limited by excited-state lifetime and
  dephasing (thermal fluctuations)

4
Inhomogeneous Broadening
Disorder and defects cause a spread of electronic
transition frequencies
5
Single-Molecule Spectroscopy
Spectral selection of single molecules
6
The first optical detection of a single molecule,
via absorption (W. E. Moerner and L. Kador, Phys.
Rev. Lett. 62 (1989) 2535)
Detection of single molecules by fluorescence
excitation (M. Orrit and J. Bernard, Phys. Rev.
Lett. 65 (1990) 2716)
7
8. Two-Level System in a Laser Field

• Detuning from resonance
• Rabi frequency

8
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
9
Transients Optical Nutation
Nutation transients without (left) and with
(right) coherence damping
10
Antibunching histograms
Antibunching at low temperature (left, pentacene
in p-terphenyl) and at room temperature (right,
terrylene in p-terphenyl)
11
Quantum Optics
Correlation histograms of a single-photon source
Light Shift of the optical transition
12
9. Triplet State(s)

• Only one triplet level correlation function
• Two sublevels

13
On- and Off-time Statistics
From Th. Basché, S. Kummer, Ch. Bräuchle, Nature
373 (1995) 132
14
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,

15

• or changing the off-time statistics,
• here for terrylene in p-terphenyl, A. C. J.
  Brouwer et al., Phys. Rev. Lett. 80 (1998) 3944.
  

16
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.
17
10. External Fields

• Stark effect
• quadratic or linear.

18
Shift of single terrylene molecule lines under
modification of the carrier gas in a
semiconductor (ITO) by an applied sawtooth voltage
19
Low-frequency localized acoustic modes
20
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
21
Spectral jumps in p-terphenyl crystals
a p-terphenyl b terrylene
4 spectroscopic sites of terrylene in p-terphenyl
Crystal structure
22
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.
23
12. Interacting Single Molecules

• Contact interactions
• Electron exchange
• Dipole-dipole coupling

leads to FRET, excitonic coupling
24
Exciton coupling in a dimer
Energies
25
Bacterial Light-Harvesting Complex
B850 ring
B800 ring
26
Excitation spectra of single LH2s
Ensemble
Individual Complexes
A. van Oijen et al., Science 285 (1999) 400.
27
Exciton coupling in the B850 ring
k 1 excitons split by distortion
k0 exciton
28
Two Quasi-Resonant Molecules
C. Hettich et al., Science 298 (2002) 386.

• A new two-photon resonance appears at high laser
  intensity between two single-molecule lines

29
Two-photon resonance
Excitation of Molecule 1
Excitation of Molecule 2
Molecules are coupled!
30
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

31
Conclusion

• SM Microscopy at room T
• biophysics
• material science
• SM Spectroscopy at room and low T
• molecular physics
• quantum optics
• solid state physics