zero-phonon line: transition without creation or destruction of phonons phonon wing: at t = 0 k,...
TRANSCRIPT
• 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)
12
2tanhexp
TkI
BZPL
*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:
3113
31
13)2( 1)( kkek
kg
212
212
2313
12 TT
TTkk
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
J
tg 222 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