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

• Phonon Wing: at T = 0 K, creation of one or more phonons

7. Optical Spectroscopy at Cryogenic Temperatures

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

• Width limited by excited-state lifetime and dephasing (thermal fluctuations)

1

22

tanhexpTk

IB

ZPL

*21

hom21

TT

Inhomogeneous Broadening

Disorder and defects cause a spread ofelectronic transition frequencies

Single-Molecule Spectroscopy

Spectral selection of single molecules

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)

8. Two-Level System in a Laser Field

• Detuning from resonance• Rabi frequency

0Eeg

eg

Optical Saturation

Saturation of the fluorescence excitationline of a single dibenzoterrylene moleculein 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

Light Shift of the optical transition

Correlation histogramsof a single-photon source

9. Triplet State(s)• Only one triplet level: correlation function

• Two sublevels:

311331

13)2( 1)( kkekkg

212

212

2313

12 TTTTkk

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

• quadratic …or linear.

EEEh�

21

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 singleTLS in the correlationof a terrylene moleculein polyethylene

Spectral jumps in p-terphenyl crystals

a: p-terphenyl

b: terrylene

Crystal structure 4 spectroscopic sites of terrylenein p-terphenyl

Spectral diffusion close to domain walls

W. P. Ambrose et al.J. Chem. Phys. 95(1991) 7150.

• Wall = 2D lattice of 2-level systems• Random jumps spectral diffusion

12. Interacting Single Molecules• Contact interactions• Electron exchange

• Dipole-dipole coupling

3

0

2

4 rJ

leads to ¨FRET, excitonic coupling

Exciton coupling in a dimer

BA sincos1

BA cossin2

Jtg 2

22 J

Energies

Bacterial Light-Harvesting Complex

B800 ring

B850 ring

Excitation spectra of single LH2’s

Ensemble

Individual Complexes

A. van Oijen et al., Science 285 (1999) 400.

Exciton coupling in the B850 ringk=0 excitonk= ± 1 excitons

split by distortion

Two Quasi-Resonant Molecules

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

C. Hettich et al., Science 298 (2002) 386.

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