Ultrafast processes in Solids. Two general categories of processes:. 1) Electronic processes How (fast) do “hot” electrons relax? How (fast) do electrons and holes recombine? How quickly is coherence lost? 2) Structural deformations/changes Phonons Melting Electron-phonon interactions .
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Ultrafast processes in Solids
Two general categories of processes:
Loosely classify solids into 5 categories:
The discrete atomic states in isolated atoms transform into bands in a crystalline solid as the interatomic distance decreases
Optical transitions typically correspond to an electron moving from one band to another (up for absorption, down for emission).
When are such transitions allowed?
Taking the full crystal structure into account yields a complicated band structure
Optical excitation adds the energy & momentum of a photon to that of an electron
A short pulse tuned well above gap will create a non-thermal distribution of electrons
Lin, Schoenlein, Fujimoto and Ippen, JQE 24, 267 (1988)
and realistic broadening
Photon Energy (eV)
Exciton formation II
However, it later became a topic of interest and controversy
Does a bound exciton actually have to form for luminescence to occur at exciton energy?
For a different interpretation see Chatterjee, et al., Phys. Rev. Lett. 92, 067402 (2004)
Exciton Dephasing I
Exciton Dephasing II
Exciton Dephasing: Negative Delay signal
Explanations ultimately included:
1) Local Fields
2) Excitation induced dephasing
4) Excitation induced shift
Distinguishing these was problematic
Solution: 2D spectroscopy (!?!)
It is also possible to form wavepackets of excitons
Co-circular polarized excitation pulses
[X. Li, T. Zhang, C.N. Borca, and S.T.C., Phys. Rev. Lett. 96, 057406 (2006)]
[C.N. Borca, T. Zhang, X. Li, and S.T.C., Chem. Phys. Lett. 416, 311 (2005)]
Only the full theory reproduces the experiment
T. Zhang, I. Kuznetsova, T. Meier, X. Li, R.P. Mirin, P. Thomas and S.T.C., Proc. Nat. Acad. Sci. 104, 14227-14232 (2007)
J.D. Koralek, et al., Rev. Sci. Instr. 78, 053905 (2007)
We discussed the observation of phonons in solids using impulsive stimulated Raman scattering
Melting observed with x-ray diffraction
Initial non-thermal melting
Acoustic effects: shifting of diffraction peaks
Generate electron pulses as for molecule studies
Sufficient resolution to see pair correlations