Biexcitons and coherent states of excitons in crystals

The theory of the coherent pairing of excitons in the presence of a resonant electromagnetic field is generalized to describe the biexciton spectra of molecular crystals, which contain several molecules (atoms) in the unit cell. The excitation spectrum for crystals like anthracene having two molecules per unit cell is discussed in detail. It is found that, at high exciton densities and under resonance of near-resonance conditions and at temperatures below some value T c , the formation of four biexciton states is feasible provided that the dielectric gaps induced by the electromagnetic field are less than those of the corresponding biexciton states. Expressions for the absorption coefficient describing optical transitions to the biexciton states are derived.The theory of the coherent pairing of excitons in the presence of a resonant electromagnetic field is generalized to describe the biexciton spectra of molecular crystals, which contain several molecules (atoms) in the unit cell. The excitation spectrum for crystals like anthracene having two molecules per unit cell is discussed in detail. It is found that, at high exciton densities and under resonance of near-resonance conditions and at temperatures below some value T c , the formation of four biexciton states is feasible provided that the dielectric gaps induced by the electromagnetic field are less than those of the corresponding biexciton states. Expressions for the absorption coefficient describing optical transitions to the biexciton states are derived.

Journal of Physics C Solid State PhysicsApr 28, 19772 citations

Coherent states of excitons

Leonid V. Keldysh

Uspekhi Fizicheskih NaukOct 1, 201714 citations

Femtosecond dephasing in CdSx Se1−x mixed crystals: The role of localized biexcitons

J.‐Y. Bigot, A. Daunois, Jean Oberlé +1 more

The dephasing of excitonic localized states is investigated in ${\mathrm{CdS}}_{\mathit{x}}$ ${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ mixed crystals at 2 K with low intensity femtosecond optical pulses. We observe a fast relaxation process which is first reported here. It is shown to be related to the creation of localized biexcitons via the exciton-biexciton transitions which act as an additional dephasing mechanism with a time constant ${\mathit{T}}_{2}^{\mathrm{ex}\mathrm{\ensuremath{-}}\mathrm{bi}}$ of 700 fs. The coherence between excitonic and biexcitonic states, which manifests by 800 fs period beats, is studied in detail. A simple model is presented which describes both the fast initial decay and the beating behavior.The dephasing of excitonic localized states is investigated in ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ mixed crystals at 2 K with low intensity femtosecond optical pulses. We observe a fast relaxation process which is first reported here. It is shown to be related to the creation of localized biexcitons via the exciton-biexciton transitions which act as an additional dephasing mechanism with a time constant ${\mathit{T}}_{2}^{\mathrm{ex}\mathrm{\ensuremath{-}}\mathrm{bi}}$ of 700 fs. The coherence between excitonic and biexcitonic states, which manifests by 800 fs period beats, is studied in detail. A simple model is presented which describes both the fast initial decay and the beating behavior.

Physical Review LettersSep 20, 199335 citations

Macroscopic coherent states of excitons in semiconductors

Leonid V. Keldysh

WORLD SCIENTIFIC eBooksNov 1, 20233 citations

The dephasing of excitonic localized states is investigated in

{\mathrm{CdS}}_{\mathit{x}}

${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ mixed crystals at 2 K with low intensity femtosecond optical pulses. We observe a fast relaxation process which is first reported here. It is shown to be related to the creation of localized biexcitons via the exciton-biexciton transitions which act as an additional dephasing mechanism with a time constant ${\mathit{T}}_{2}^{\mathrm{ex}\mathrm{\ensuremath{-}}\mathrm{bi}}$ of 700 fs. The coherence between excitonic and biexcitonic states, which manifests by 800 fs period beats, is studied in detail. A simple model is presented which describes both the fast initial decay and the beating behavior.