Giant oscillator strength is inherent in excitons that are weakly bound to impurities or defects in crystals.
The spectrum of fundamental absorption of direct-gap semiconductors such as gallium arsenide (GaAs) and cadmium sulfide (CdS) is continuous and corresponds to band-to-band transitions.
In a perfect crystal, this spectrum is preceded by a hydrogen-like series of the transitions to s-states of Wannier-Mott excitons.
Anomalously high intensity of the impurity-exciton lines indicate their giant oscillator strength of about
per impurity center while the oscillator strength of free excitons is only of about
Shallow impurity-exciton states are working as antennas borrowing their giant oscillator strength from vast areas of the crystal around them.
[4] Giant oscillator strengths of impurity excitons endow them with ultra-short radiational life-times
Interband optical transitions happen at the scale of the lattice constant which is small compared to the exciton radius.
Therefore, for large excitons in direct-gap crystals the oscillator strength
which is the value of the square of the wave function of the internal motion inside the exciton
for producing a bound exciton can be expressed through its wave function
, reflect the fact the exciton is created at a spatial scale small compared with its radius.
The integral in the numerator can only be performed for specific models of impurity excitons.
If the exciton is bound to a defect by a weak short-range potential, a more accurate estimate holds
Giant oscillator strength for shallow trapped excitons results in their short radiative lifetimes
[5] When quantum yield of radiative emission is high, the process can be considered as resonance fluorescence.
Similar effects exist for optical transitions between exciton and biexciton states.
An alternative description of the same phenomenon is in terms of polaritons: giant cross-sections of the resonance scattering of electronic polaritons on impurities and lattice defects.
are not universal and change within collections of specimens, typical values confirm the above regularities.
per a single impurity center should not be surprising because the transition is a collective process including many electrons in the region of the volume of about
High oscillator strength results in low-power optical saturation and radiative life times
[7][8] Similarly, radiative life times of about 1 ns were reported for impurity excitons in GaAs.
[9] The same mechanism is responsible for short radiative times down to 100 ps for excitons confined in CuCl microcrystallites.
It is an important property of typical molecular crystals with two or more symmetrically-equivalent molecules in the elementary cell, such as benzine and naphthalene, that their exciton absorption spectra consist of doublets (or multiplets) of bands strongly polarized along the crystal axes as was demonstrated by Antonina Prikhot'ko.
This splitting of strongly polarized absorption bands that originated from the same molecular level and is known as the 'Davydov splitting' is the primary manifestation of molecular excitons.
[11][12][13] As a result, the polarization ratio of an impurity exciton band depends on its spectral position and becomes indicative of the energy spectrum of free excitons.
[14] In large organic molecules the energy of impurity excitons can be shifted gradually by changing the isotopic content of guest molecules.
Building on this option, Vladimir Broude developed a method of studying the energy spectrum of excitons in the host crystal by changing the isotopic content of guest molecules.
[15] Interchanging the host and the guest allows studying energy spectrum of excitons from the top.
The isotopic technique has been more recently applied to study the energy transport in biological systems.