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 • Width limited by excited-state lifetime and dephasing (thermal fluctuations)
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
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 • Two sublevels:
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.
Shift of single terrylene molecule lines under modification of the carrier gas in a semiconductor (ITO) by an applied sawtooth voltage
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 4 spectroscopic sites of terrylene in p-terphenyl Crystal structure
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 leads to ¨FRET, excitonic coupling
Exciton coupling in a dimer Energies
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 single-molecule 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