Biophotonics_Concepts_to_Applications

at the plasma threshold? (c) What is the ablation depth per pulse at the plasma

threshold?

Solution: (a) The irradiance is Eab=Hab/τ= 7.3× 106 W/cm^2

(b) The irradiance is Eplasma=Hplasma/τ= 4.7× 107 W/cm^2

(c) zab¼l^1 alnEEplasma

ab

¼ 16  1031 cm 1 ln^4 :^7 ^10

7

7 : 3  106



¼ 1 : 16 lm:

6.5.5 Plasma-Induced Photoablation

In moving up the irradiance versus time curve shown in Fig.6.19, a plasma con-
sisting of ionized molecules and free electrons is produced through a phenomenon
calledoptical breakdown. This breakdown starts at the point where the irradiance
exceeds 10^11 W/cm^2 and occurs when there are free electrons in an electricfield.
When such high irradiances impinge on biological tissue in pulses of less than one
nanosecond, molecules are torn apart and the plasma is formed. In plasma-induced
photoablation a free electron is accelerated by an intense electricfield that exists
around a tightly focused laser beam. As shown in Fig.6.27, when this energetic
free electron gets accelerated and collides with a molecule, the electron transfers
some of its energy to the molecule. If the transferred energy is enough to free a
bound electron in this molecule, the second released free electron also gets accel-
erated along with thefirst free electron and together they can initiate additional
energetic collisions. Thereby a chain reaction of similar collisions is initiated and
the plasma is created.
As the plasma expands it ablates tissue material in a clean and precise fashion.
Applications include the treatment of cataracts, refractive corneal surgery, and
caries treatments. Typical parameter values are 100-fs to 500-ps pulses with irra-
diances varying from 10^11 to 10^13 W/cm^2. Example parameters in ophthalmology
are irradiances of 10^11 W/cm^2 in 10-ps pulses.

An accelerated electron

(blue) collides with a

molecule and releases

another free electron (red)

Additional free

electrons are released

in each collision

Multiplication region

Molecule

Fig. 6.27 Cascading of free
electrons in an intense electric
field

186 6 Light-Tissue Interactions