photon energies are needed to break molecular bonds, which then leads to ablation.
A popular tool is a 193-nm ArF laser. For these lasers the photon energy of 6.4 eV
is sufficient to break the C–C (3.6 eV), C–O (3.6 eV), and C–N (3.1 eV) bonds of
polypeptide chains that are found in collagen. Although lasers with slightly lower
photon energies (longer wavelengths) also could be used, such as 2.48-nm KrF
lasers, the resulting photoablated surface is less smooth with a bit rougher edges
than in the photoablations carried out with ArF lasers.
In order to initiate photoablation, a minimum threshold intensity must be applied
to the tissue, as is shown in Fig.6.26. At this threshold the rate of molecular bond
disassociation must be greater than the rate of bond reformation. The rate of bond
disassociation is directly related to the number of photons falling on the tissue per
second, which is proportional to the irradiance.
Knowing the ablation threshold and assuming that absorption follows the
exponential relationship described in Eq. (6.3) then allows an estimate of the
photoablation depth in tissue for a given irradiance level E. Thus from Eq. (6.3) the
light absorption in the tissue can be written as
EðzÞ¼E 0 expðlazÞð 6 : 25 Þwhere E 0 is the irradiance incident on the tissue surface,μais the absorption
coefficient of the material, and z is the depth into the tissue from the surface. If Eab
is the irradiance at thephotoablation threshold(an irradiation level E 0 at which
photoablation starts), then the depth zabto which material is ablated as a function of
the incident irradiation E 0 is
zab¼1
laln E 0 1
laln Eab¼1
lalnE 0
Eabð 6 : 26 ÞA plot of the ablation depth zabas a function of the natural log of the incident
irradiance E 0 is shown in Fig.6.26. In addition to showing the irradiance level
E 0 =Eabat the photoablation threshold, thefigure also indicates that there is an
irradiance level E 0 =Eplasmaat which a plasma threshold is reached. As described
in Sect.6.5.5, high irradiances create a plasma at which point molecules are torn
apart. Further increases in irradiance levels beyond the plasma threshold do not lead
to deeper ablation depths, because now the plasma absorbs all the incoming
irradiation.
Example 6.13Consider the case in which laser pulses of durationτ=15ns
from an ArF laser are used to irradiate a tissue sample for which the
absorption coefficient is 16× 103 cm−^1. Suppose that the ablation threshold
occurs at a radiant exposure (energy density) of Hab= 110 mJ/cm^2 and that
the plasma threshold occurs at a radiant exposure of Hplasma= 700 mJ/cm^2.
(a) What is the irradiance at the ablation threshold? (b) What is the irradiance6.5 Light-Tissue Interaction Mechanisms 185