Introduction

What is it?

The best way to describe what happens in multiphoton absorption is to explain first what happens in single photon absorption.


Einstein explained, using the photoelectric effect, that light is emitted and absorbed by matter only at very specific wavelengths¹. This means, for example, that medical X-rays are only absorbed by some parts of the body and not others, allowing imaging to take place. The emitted and absorbed wavelengths differ for different materials². This gave rise to the photon model of light, which in turn lead to quantum mechanics.


When light at an absorption wavelength hits a material, the atoms in that material react by being 'excited'. What this means is that the energy of the light is transferred to an electron in the atom, which 'excites' it to a higher energy level. At these wavelengths the material is opaque; however, absorption will occur at only specific wavelengths, and away from these the object will be transparent³. It should be noted that some materials that are usually thought to be opaque are transparent at different wavelengths, and vice versa⁴.


In this model, one photon of light will transfer its energy to one atom of the material. This is called single photon absorption. Until that atom 'relaxes', i.e. the electron drops back down to its lower energy state, the atom will not absorb any more photons at that wavelength, a phenomenon called bleaching or saturating the absorption⁵. This type of absorption is the most common form, and is the sort of absorption that occurs naturally.



However, when dealing with very intense light, the rules change a little. The energy levels in the material stay the same, but now it is possible to excite using photons of some fraction (half, third) of the single photon energy. If (in the case of two-photon absorption) two photons, each with half the energy needed to excite the material, hit the material within around one femtosecond⁶ then the material may act as if they were a single photon with the correct wavelength⁷. This is a X⁽³⁾nonlinear process and so is dependent on the intensity of the laser beam.

How can it be used?

Despite sounding like an excuse for research for research's sake, this process does actually have a use. It can be used to in very precise optical microscopes, although the design is quite different to the generic microscope illustrated in the linked page.


This form of microscopy is called multiphoton fluorescent microscopy. It allows three-dimensional imaging to take place. What happens is that the specimen that is to be examined is treated with a stain (called a fluorophore) which adheres to the areas of interest, but does not hold in the other areas. This stain absorbs light with twice the photon energy of the laser light, and emits at a different wavelength. Now, when the laser beam is focused onto the material, those places where the stain has been applied will fluoresce⁸. However, this will only occur in the most intense area of the laser beam, which will be at the focal point of the objective lens. The fluorescence is therefore restricted to the dimensions of the focal spot, and nowhere else does fluorescence occur. This is significantly different to the case of single photon absorption, where absorption and hence fluorescence would occur down the full length of the beam in the fluorophore, making 3-D imaging very difficult⁹

There are other advantages to using multiphoton absorption in this way. The most important is that most materials that are absorbent at a given wavelength are transparent at the two-photon absorption wavelength, thus allowing deep penetration of the material and looking at features inside a sample¹⁰.