Biomedical Engineering Reference
In-Depth Information
Fig. 1.29.
One-photon versus two-photon absorption
Of particular interest to the biomedical field is that this damage can be
restricted to a small focal zone, making possible a highly accurate method of
cutting biological tissue. The amount by which damage can be restricted to
the irradiated region depends on various parameters, including the illumina-
tion wavelength, pulsewidth, intensity, focusing conditions, and absorption
properties of the sample.
The mechanism by which the sample absorbs light changes with increasing
intensity. At a suciently high photon density, photons can be absorbed in
simultaneous pairs or triplets (two- or three-photon absorption). Figure 1.29
illustrates the fundamental principle of two-photon absorption. The two pho-
tons must both be absorbed within 10
−
15
sandinthesameregionofthe
sample - a process that is low in probability [34]. This ensures that signifi-
cant levels of two-photon absorption are only seen in regions of high photon
density. The effect of this is to reduce the volume in which light is absorbed
and therefore to decrease the volume in which laser-induced damage appears.
Two-photon absorption can be applied in laser and biological material inter-
actions by using near-infrared laser illumination. Many biological materials
do not absorb strongly in the near-infrared, allowing transmission of laser
power through the specimen except for the region in focus, where absorption
of two or more photons occurs [35].
The peak power provided by pulsed laser illumination is suciently high
to enable two-photon absorption. Pulsed illumination does, however, have
