One reason may be the increase of the oscillator strength due to quantum confinement effect.
Pathik may be right. Another possible explanation for indirect gap materials is that due to the spatial localisation of the exciton, its wavevector k becomes undetermined due to Heisenbergs uncertainty principle, permitting transitions with no, or a wider range, of phonons.
The velocity of propagation of a electromagnetic wave through a solid is given by the frequency-dependent complex refractive index N = n - ik where the real part, n is related to the velocity, and k, the extinction coefficient is related to the decay, or damping of the oscillation amplitude of the incident electric field. The optical properties of the solid are therefore governed by the interaction between the solid and the electric field of the electromagnetic wave which becomes high in quantum dots due to increase in field strength.
I would like to propose another simple answer. Quantum dots are zero-dimensional systems, thus the density of states (DOS) become delta-fuction like. If the energy of light which you consider is in the resonance of DOS, the extinction ratio might be larger than the bulk value. Note, however, if the energy of light is outside the resonance, it must be smaller, and the average value integrating over whole wavelengths must be the same with that of bulk - this is due to the strict sum rule.
@Takashi ..If average of extinction coefficient of QDs is same as bulk then from where the question of large extinction coefficient of QDs has arrived?
Dear Ashi - sorry I cannot catch your question very correctly. Instead of answering, let me explain what we call the sum rule more in detail.
When we study isolated atoms, the optical spectrum should look like the ensemble of split lines, as predicted by quantum mechanics. If these atoms form a crystal, the spectrum becomes a set of spectral bands - this is the heart of band theory. With changing from atoms to a crystal, the spectrum changes from lines to bands, but we can imagine that the summation of all optical transitions is kept unchanged, and it depends only on the number of atoms. This is the sum rule.
Changing from bulk to quantum dots, the spectral signature can heavily change, but the total, i.e., spectrally-integrated, spectrum must be constant. Consequently, you can observe the enhancement of extinction coefficient at particular wavelengths, but not at every wavelengths. Further questions are welcome.
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