Interaction between Laser Radiation and Biological Tissue

The interaction between electromagnetic radiation and biological tissue depends on
The wavelength of light, which determines the energy of each photon of light..
The intensity of radiation.
The shape of irradiation (continuous or pulsed).
For power levels are up to few Watts, the interaction is divided into 3 regions of wavelengths
Short UV region - the photons interact with the proteins, RNA and DNA, and usually kills the biological cells.
Near UV and Short visible range - photochemical reactions such as photosynthesis. Especially with the Excimer laser.
Visible and Near Infra-Red region - Thermal effects due to absorption of the radiation.

Eye Medical Applications with laser

Medical Applications

There are many medical applications of lasers, and there are different ways to classify them into groups:
· According to the organ to be treated by the laser, such as: Eye, General Surgery, Dentistry, Dermatology, Blood vessels, Cardiac, etc.
· According to the type of laser used for treatment, such as: CO2, YAG, and Argon.
· According to the type of treatment, such as diagnostic, surgery, connecting blood vessels.
The classification used here is basically according to the type of treatment, with comments on suitable lasers used for each application:
Lasers in medical surgery.
Lasers in diagnostic medicine, and in combination with drugs.
Lasers for specific applications: Soft lasers.
When using lasers for medical treatments, a good understanding of the interaction between specific laser radiation with specific biological tissue is required.

Applications of Lasers in Chemistry

From the variety of applications of lasers in chemistry, we shall mention:
· Excitation of molecules to specific levels, and examination of the emitted radiation.
· Measurements of the relaxation time of specific excited levels of molecules.
· Disruption of chemical bonds in molecules at specific region - When a laser beam is focused, a very high electric field is created at the focal point (up to 109 V/cm). Such electric fields are larger than the force which holds the valence electrons in an atom. Another possibility is to use wavelengths which are very short (which means that the photons are very energetic) to break the chemical bonds. This is usually done with the Excimer laser.
· Raman spectroscopy: Raman scattering is a process of inelastic scattering of the photon by the molecule. The photon is absorbed by the molecule, and another photon, with a different frequency is emitted. The change in frequency of the photon is connected to the energy transitions in the molecule which absorb the photon. The most important Raman scattering is connected to vibrational transitions of the molecule. By measuring the change in frequency, it is possible to identify the specific molecule.
There are two kind of Raman scattering processes:
o Stokes scattering - when the photon lose energy, and the molecule absorb this energy, and go into excited state. The frequency of the emitted photon is less than the frequency of the incident photon.
o Anti-Stokes scattering - when the photon receives energy from the molecule. The frequency of the emitted photon is higher than the frequency of the incident photon.

Spectral analysis.

the entire lasing process is based on absorption and emission of photons at certain specific wavelengths. The wavelength emitted from the laser is monochromatic, and its linewidth is very narrow.
Thus, the laser can be used for controlled excitation of molecules. Especially useful for this are the tunable lasers, whose wavelength can be precisely tune to excite specific molecule.

The mostly used lasers for material processing are:

CO2 laser - has high power and is highly absorbed in most materials.
Nd-YAG laser - has high power and can be transmitted through optical fibers.

Interaction Mechanism between the Laser Beam and matter

The mechanism of interaction between the laser beam and the processed material:
· Thermal Effects - Most of the applications of lasers in material processing were based on the absorption of the laser radiation inside the material, and the effects were thermal in nature. The absorption process transfers energy to the material. As a result, there is a rise in the temperature in that region to high temperatures.
· Photochemical Effects - Breaking the bonds between the molecules in the material. The Excimer laser emits in the Ultra-Violet (VU) part of the electromagnetic spectrum, and its photons are very energetic. It can be used to cut very delicate and accurate structures without causing thermal damage to surrounding areas.