A Pedagogical Description of Channel Interference in Multiphoton Absorption Processes
Journal Title: Biomedical Journal of Scientific & Technical Research (BJSTR) - Year 2017, Vol 1, Issue 6
Abstract
In this mini-review, the author discusses a different view of two-photon absorption and in general any multi-photon absorption process in a molecular system in a very didactic way. This novel point of view is termed as "channel interference”. It connects the overall absorption cross section with the orientation of different transition dipole moment vectors that come into existence when a molecule undergoes two- or multiphoton absorption process. Therefore, channel interference provides an in-depth view of the process which can be used to control and fine tune the multi-photon activity in a molecular system for designing an efficient material. When light interacts with matter, the electric field ofthe incident light causes a distortion of the electron cloud in the molecule, which is measured in terms of polarization. A stronger electric field and loosely bound electron cloud give larger polarization than a weaker electric field and a tightly bound electron cloud. In most of the molecules, the said polarization can be expressed as a linear function of the electric field and hence the optical processes associated with this linear polarization are called linear optical processes. The normal UV-Vis or IR absorption falls into this category. Most of the molecules show this kind of absorption with UV-Vis or IR source of radiation. However, when LASER is used as incident light, polarization becomes a non-linear function of the corresponding electric field. The optical processes associated with such non-linear polarization are called non-linear optical processes. Two-photon or multi-photon absorption and hyper-polarizabilities are the two examples of non-linear optical processes. In this mini-review, we’ll discuss the multi-photon absorption process with an emphasis on two-photon absorption (2PA) caused by a linearly polarized light. Linear and non-linear absorptions Pictorially, a linear absorption process can be shown as in Figure 1 where, S0 and Sf represent the initial and final states respectively. E0 and Ef are the corresponding energies of the states and hν is the energy of the incoming photon. If an incident light of low intensity is used, absorption will occur if and only if hν matches the energy gap (ω0f = |Ef- E0|) between the two states. If the system is centrosymmetric, then an additional requirement for an allowed transition is that the two involved states should have the different symmetry. For example, if the absorption is taking place from ground state (having gerade symmetry), in a centro-symmetric molecule, then the final excited state should have gerade symmetry [1] this is the case of linear absorption, also called one-photon absorption (1PA). This is measured in terms of oscillator strength (δ1p) which is directly proportional to the product of «0f and square of transition dipole moment (TDM) vector (μof) between initial and final states [2,3].
Authors and Affiliations
Mehboob Alam
Keywords
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- EP ID EP582177
- DOI 10.26717/BJSTR.2017.01.000532
- Views 153
- Downloads 0
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