Optical Properties, Excitons, and Light-Matter Interaction
Absorption and Emission in Crystals
Selection rules and optical matrix elements
Optical response near the band edge is governed by interband dipole matrix elements, symmetry selection rules, and the joint density of states. In direct-gap materials, radiative recombination is efficient because crystal momentum conservation can be satisfied without phonon assistance.
Seminal concept
The distinction between direct and indirect gaps is one of the most important ideas in semiconductor optoelectronics.
Excitons and Many-Body Effects
Bound electron-hole states
An exciton is a Coulomb-bound electron-hole pair with reduced mass . In the hydrogenic approximation, the binding energy scales as , so reduced screening and low dimensionality can dramatically enhance excitonic effects.
Excitonic regimes
| Regime | Key features | Where it appears |
|---|---|---|
| Wannier-Mott | Large radius, weak binding | Bulk inorganic semiconductors |
| Frenkel-like | Tight binding, localized | Some molecular and organic semiconductors |
| 2D excitons | Enhanced binding, reduced screening | Monolayer transition-metal dichalcogenides |
| Moiré excitons | Superlattice-modulated states | Twisted or heterostructure systems |
Why are exciton binding energies often larger in two-dimensional semiconductors?
Reduced dimensionality and weaker dielectric screening strengthen electron-hole attraction.
Correct answer: Because dielectric screening is reduced and Coulomb interactions are enhanced
Define an exciton in one sentence.
This quasiparticle dominates optical spectra near the absorption edge in many semiconductors.
Correct answer: A bound state of an electron and a hole held together by Coulomb attraction.
Nonlinear and Ultrafast Optical Response
Beyond linear optics
Strong fields and ultrafast excitation probe carrier cooling, coherent polarization, many-body renormalization, and nonequilibrium phase space filling. Techniques such as time-resolved ARPES, pump-probe spectroscopy, and four-wave mixing reveal dynamics inaccessible in steady-state measurements.
Common spectroscopies
Absorption/reflectance
- Band-edge energies and excitons
- Accessible and widely used
Photoluminescence
- Radiative recombination pathways
- Sensitive to defects and localization
What does a strong Stokes shift often indicate in a semiconductor photoluminescence spectrum?
A large Stokes shift usually signals structural relaxation, localization, or defect-mediated emission.
Correct answer: Significant relaxation, trapping, or disorder before emission
Why do time-resolved optical techniques matter for semiconductor science?
Many key processes are inherently nonequilibrium.
Correct answer: They expose carrier and exciton dynamics on femtosecond to nanosecond timescales that static spectra cannot resolve.