Two-photon absorption

As a two -photon absorption is defined as the simultaneous absorption of two photons by a molecule or atom, which thereby turns into an energetically excited state. The energy of these photons is not enough there to bridge the energy gap between the ground state (state 0 in the illustration ) and excited state ( state 2 in the adjacent figure).

Description

There are between state 0 and state 2 is not Allowed energy level, so the photons have to be absorbed almost simultaneously, ie within a time interval of the order of 0.1 femto = 10-16 s for the description of this process, use is made of a virtual intermediate levels, the service life approximately equal to the duration of the absorption process. In the adjacent figure, the absorption from state 0 takes place via the intermediate virtual level 1 to 2. Such a map is also called Jablonski diagram. Secondary processes from the excited state of 2, such as Fluorescence (shown with a dashed arrow down), are independent of the type of excitation.

For a two-photon absorption can take place, the sum of the energies of the photons absorbed the energy difference must match between the molecular states:

Here are the vibrational frequencies of the two photons and is Planck's constant. If necessary, must be ( to the angular momentum, for example ) meets other selection rules.

The process of two-photon absorption was first described in 1931 by Maria Goeppert- Mayer in her doctoral thesis theoretically. However, since such a process is very unlikely, and it therefore needs a very high temporal and spatial photon density for such an event, the two-photon absorption could be experimentally demonstrated by Wolfgang Kaiser and CGB Garrett shortly after the invention of the laser ( 1961). An impression of the likelihood of such an event is given in: In bright sunshine, a molecule of a good one-or two -photon absorber absorbs about one photon per second over a single-photon event. A two-photon absorption takes place only once every 10 million years under the same conditions. The probability of two-photon absorption is described by the two-photon action cross section. It is given in units of Goeppert- Mayer (GM).

Applications

Applications of two-photon absorption (for example, multi-photon microscopy) are based primarily on their quadratic dependence on the intensity of light (as opposed to the linear dependence in the one-photon absorption) and the possibility of longer wavelength (and thus lower energy ) light to use. Two -photon absorbers are also used in the 3d - lithography in 3D optical data storage as well as markers and probes in biology. Thanks to the non-linearity can be achieved in each case here that only a desired depth, a significant absorption occurs in the material. Another important application lies in the Doppler-free atomic saturation spectroscopy systems. The design of so-called two-photon dyes ( substances with high two-photon absorption), is an active area of ​​research. Materials from several GM ( instead of the usual GM) have already been produced.