Hi Kebbab,

I am not sure to perfectly understand your question.

In case you are speaking about semiconductors or insulators, their atomic outer electrons are locked binding the atoms together: they have energy values that correspond to the energy that is necessary to break one valence bond between two atoms. These electrons cannot move, the material is an electrical insulator. On a solid state point of view the energy of the electrons in a solid lay in bands of values separated by energy gaps. In an insulator the electrons do fully fill the lowest energy bands. The last occupied one, the one with the highest energy, is named the valence band. Above this last full band is an empty band that is called the conduction band because it cannot be filled and therefore electrons that have its energy range are mobil electrons.

- Electrical conduction.

You can put some impurities in the material that needs one more or one less valence bond with the bulk atoms. Then an electron or a hole from these impurities is not involved in the valence binding, it is free to move. These free electrons associated to impurities make the material a (poor) conductor. In a solid state point of view one says that the impurities do create localized states close in energy to the conduction for the electrons so that room temperature thermal energy (1/40 eV) is enough to put them in the conduction band. The reasoning also stands for a missing electron (hole) that can move when having energies lying in the valence band.

- Optical conduction.

As electronic conduction is a matter of putting electrons in the conduction band, one other way to achieve this goal is to give an electron bound to the atoms enough energy as to break the bond and set it free to move. This can easily be performed by shining the material with light which photons do have an energy allowing the breaking of the bonds. In a solid state language, photons can promote electrons from the valence to the conduction band leaving a hole in the valence band. The freed electron and hole can then contribute to an electrical conduction of the material.

Hope it helps

Please follow the link :

https://www.researchgate.net/post/What_is_the_basic_difference_between_optical_band_gap_and_electrical_band_gap

You can also find some important material here :

http://en.wikipedia.org/wiki/Band_gap

Hi, find a paper related to electrical and optical band gap.

Hope it will help you.

Electrical conductivity can be simply explained on the basis of drude model itself. Using band theory of solids, valence band and conduction band overlaps leading to have more free electrons in the conduction band. This conduction electrons are responsible for conduction, however due to so-called scattering mechanism, material possesses resistivity. Optical conductivity is rather the measure of electrical conductivity in the alternating field. Hence dielectric constant comes into play, which rules allowed propagation of light into matter. Here the term plasma frequency and relaxation time are used. Optical propagation is depend on this two important physical quantities. If the radiation is incident on matter, it drives the electron, and if there is no scattering, light gets reflected totally and hence metals are shiny. Above plasma frequency, reflection declines and transmission start to dominate. By understand this, we can simply differentiate electrical and optical conductivity. The color of the metals is also dependent on this factor.

Band-gap itself is an universal formation for all kind of waves in periodic materials, and physically has the same properties for optical waves (photons) for electron waves, even for acoustic waves, even the water surface waves.

Hi Kebbab,

I am not sure to perfectly understand your question.

In case you are speaking about semiconductors or insulators, their atomic outer electrons are locked binding the atoms together: they have energy values that correspond to the energy that is necessary to break one valence bond between two atoms. These electrons cannot move, the material is an electrical insulator. On a solid state point of view the energy of the electrons in a solid lay in bands of values separated by energy gaps. In an insulator the electrons do fully fill the lowest energy bands. The last occupied one, the one with the highest energy, is named the valence band. Above this last full band is an empty band that is called the conduction band because it cannot be filled and therefore electrons that have its energy range are mobil electrons.

- Electrical conduction.

You can put some impurities in the material that needs one more or one less valence bond with the bulk atoms. Then an electron or a hole from these impurities is not involved in the valence binding, it is free to move. These free electrons associated to impurities make the material a (poor) conductor. In a solid state point of view one says that the impurities do create localized states close in energy to the conduction for the electrons so that room temperature thermal energy (1/40 eV) is enough to put them in the conduction band. The reasoning also stands for a missing electron (hole) that can move when having energies lying in the valence band.

- Optical conduction.

As electronic conduction is a matter of putting electrons in the conduction band, one other way to achieve this goal is to give an electron bound to the atoms enough energy as to break the bond and set it free to move. This can easily be performed by shining the material with light which photons do have an energy allowing the breaking of the bonds. In a solid state language, photons can promote electrons from the valence to the conduction band leaving a hole in the valence band. The freed electron and hole can then contribute to an electrical conduction of the material.

Hope it helps

When we measure conductivity for a given material (a semiconductor for example) , we can use electrical or optical experiment to get this parameter. Often the results are different, why ?

Is there a physical explanation ?

Optical and electrical measurements use different wavelengths of spectroscopy and hence results are expected to be different. May be this is not only one reason.

In my opinion, the term optical conductivity means the conductivity which arise under optical excitation without the presence of applied electrical field. Such conductivity appears in result of the presence of the gradient in concentration of carriers in different regions of semiconductor material. Thus the optical conductivity is caused with the diffusion current in the material. The value of optical conductivity depends on the power of the excited light.

The photodiffusion current spectra measuremens allow to determine both the band gap energy and the energy of different defects in semiconductor ( see, for example, the next : https://www.researchgate.net/publication/234074025_Optical_and_photoelectric_spectroscopy_of_photorefractive_Sn2P2S6_crystals?ev=prf_pub ). The term electrical conductivity related to the conductivity which arise in the presence of external electric field. In this case the drift current appears. The value of electrical conductivity increases with increasing applied electric field.

Article Optical and photoelectric spectroscopy of photorefractive Sn...

I agree with the opinion of Dr Yuriy Gnatenko. He is absolutely right.If a system is subjected to an external electric field then, in general, a redistribution of charges occurs and currents are induced. For small-enough fields, the induced polarization and the induced currents are proportional to the inducing field.

I think it should exist other explanation, but unfortunately, i haven't any response. In electrical or optical experiment, material is subject to electrical field (because of light Is an electromagnetic wave). Also the conductivity should be the same, that is not the case !!!!

Well, think about in this way. When you apply an electric field, conductance occurs. Atoms present in the material polarizes and the resultant field is sum of applied field and induced field. But if you measure optical conductivity, ie. electromagnetic nature of wave, atoms not only gets polarized, but also magnetized due its spin nature. Thus the resultant field you measure (using any convenient device) may be different when compared to electric field measurements. This may be reason you get different values of conductivity for electric and optical fields

Electrical conductivity is the response of the "free" electrons (electrons which can move) of a material to an applied electic field. Optical phenomena arise from the response of a material to an electromagnetic wave (e.g. light). In the second case not only the "free" electrons but all the charges (electons and nuclei) interact with the electromagnetic field.

Maxwell equations apply for both static electic fields and electromagnetic waves so somebody would expect the same response for static fields or waves. However, the electric and magnetic permittivities depend on the frequency and are quite different for zero frequency (static) and high frequencies (e.g. light). The physical reason for that difference, in a simple way, is given above.

The difference between optical and electrical conductivity can be simply explained by imaginary part of dielectric function,epsilon2 (as a function photon energy) of the investigated material.

Electrical conductivity is the same that optical conductivity but at frequency (pulsation) equal to zero (or at very low frequency compared to optoical frequencies if you make measurements with alternative current)

Electrical conductivity is in general measured with DC current. As explained by Ravi, optical conductivity is a complex number. Optical conductivity can be mesured directly with an ellipsometer.

Semiconductor optical conductivity is the change in conductivity caused by illumination, either an increase or a decrease, the electrical conductivity is due to a free electrons in the conduction band.

please give any mathematical relation between electrical conductivity and optical conductivity