Two-photon absorption
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Two photon absorption (TPA) is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from its ground state to a two-photon allowed excited state.
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[edit] Background
The phenomeon was originally proposed by Maria Göppert-Mayer in 1931 in her doctoral dissertation. The first experimental verification was provided by Werner Kaiser in 1961, since that is when lasers started to exist. The first TPA process was observed in doped europium salts. In (PRL 97, 143903 (2006)) two photon absorption by sunlight is presented.
TPA is a third-order nonlinear optical process. In particular, the imaginary part of the third-order nonlinear susceptibility is related to the extent of TPA in a given molecule. The selection rules for TPA are different than for one-photon absorption (OPA). For example, for a centrosymmetric molecule, the one-photon allowed excited state is NOT allowed by TPA.
Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Both techniques require pulsed lasers because TPA is a third-order nonlinear optical process, and therefore is most efficient at very high intensities. Phenomenologically, this can be thought of as the third term in a conventional anharmonic oscillator model for depicting vibrational behavior of molecules. Another view is to think of light as photons. In TPA two photons combine to bridge an energy gap larger than the energies of each photon individually. If there were an intermediate state in the gap, this could happen via two separate one-photon transitions. In TPA it occurs even without the presence of the intermediate state. This can be viewed as being due to a "virtual" state created by the interaction of the photons with the molecule.
The "nonlinear" in the description of this process means that the strength of the interaction increases faster than linearly with the electric field of the light. This means that the efficiency of the process increases with increasing intensity, so TPA is best observed at very high intensities; much higher than those from conventional light sources. Further, in order to understand the TPA spectrum, monochromatic light is also desired in order to measure the TPA cross section at different wavelengths. Hence, pulsed lasers are the choice of excitation.
[edit] Development of the field and potential applications
Until the early 1980s, TPA was used as a spectroscopic tool. Scientists compared the OPA and TPA spectra of different organic molecules and obtained several fundamental structure property relationships. However, in late 1980s, applications were started to be developed. Peter Rentzepis suggested applications in three dimensional optical data storage. Watt Webb suggested microscopy and imaging. Other applications such as 3-D microfabrication and optical power limiting were also suggested.
[edit] Microfabrication
One of the most distinguishing features of TPA is that the rate of absorption of light by a molecule depends on the square of the light's intensity. This is different than OPA, where the rate of absorption is linear with respect to input intensity. As a result of this dependence, if material is cut with a high power laser beam, the rate of material removal decreases very sharply from the center of the beam to its periphery. Because of this, the "pit" created is sharper and better resolved than if the same size pit were created using normal absorption. This makes TPA attractive for 3D microfabrication.
[edit] Imaging
The human body is not transparent to visible wavelengths. Hence, one photon imaging using fluorescent dyes is not very efficient. If the same dye had good two-photon absorption, then the corresponding excitation would occur at approximately two times the wavelength at which one-photon excitation would have occurred. As a result, we can push the envelope from the visible or near infrared region into the far infrared region where the human body shows good absorption. Further, according to Rayleigh's scattering law, the amount of scattering is proportional to 1 / λ4, where λ is the wavelength. As a result, if the wavelength is reduced by a factor of 2, the Rayleigh scattering is reduced by a factor of 16. This improves the contrast dramatically. Hence TPA could be very valuable in imaging of biological species. Two-photon excitation microscopy is an emerging field.
[edit] Optical power limiting
Another area of research is optical power limiting. In a material with a strong nonlinear effect, the absorption of light increases with intensity such that beyond a certain input intensity the output intensity approaches a constant value. Such a material can be used to limit the amount of optical power entering a system. This can be used to protect expensive or sensitive equipment such as sensors.
[edit] Photodynamic therapy
Photodynamic therapy (PDT) is a method for treating cancer. In this technique, an organic molecule with a good triplet quantum yield is excited so that the triplet state of this molecule interacts with oxygen. The ground state of oxygen has triplet character. This leads to triplet-triplet annihilation, which gives rise to singlet oxygen, which in turn attacks cancerous cells. However, using TPA materials, the window for excitation can be extended into the infrared region, thereby making the process more viable to be used on the human body.
