Highlights Of The Chapter

• A laser, which is an acronym for /ight amplification by stimulated emission of radiation, is an intense source of highly directional, coherent, and monochromatic source of light with low beam divergence, capable of being focused into a very small spot.

• The two important steps to generating laser action are (i) excitation to create a population inversion between two energy levels of molecules in an active medium and (ii) optical feedback between two reflectors that form an optical cavity where stimulated emission and amplification occur.

• If the active medium involves three energy levels, also known as a three-level structure, it produces only a pulse laser action.

• If the active medium involves four energy levels, also known as a four-level structure, it can produce a pulse as well as a continuous laser action.

• Different ranges of the laser pulse width and pulse repetition rates can be achieved by a variety of optical gating or switching techniques such as (a) Q-switching to produce nanoseconds pulses and (b) mode-locking to produce picoseconds or femtoseconds pulses.

• Lasers can be classified on the basis of the pumping (excitation) process (electrical, optical, etc.), the lasing medium (gas, liquid, or solid), and the temporal features (continuous wave, also abbreviated as CW, Q-switched pulses, mode-locked pulses).

• Radiometry quantitates the level and amount of light exposure that a material is subjected to effect light-matter interactions.

• When the intensity of light is high, such as from a laser, new types of processes called nonlinear optical processes take place. They are dependent on the electric field (hence, intensity) of light in higher order.

• Two important types of nonlinear optical processes are higher-harmonic generation and intensity-dependent multiphoton absorption.

• Second-order nonlinear optical processes depend quadratically on the intensity, an important example being second-harmonic generation. Here, an input beam of frequency v generates a new beam of frequency 2v, but no energy is absorbed or emitted by the medium (nonresonant).

• Third-order nonlinear optical processes depend cubically on the intensity, an important example being third-harmonic generation. Here, an input beam of frequency v generates a new input beam of 3v, again through a nonresonant interaction (no absorption or emission from the medium).

• For second-order effect to be manifested in a medium, it must be non-centrosymmetric (not an amorphous solid or a liquid). No such symmetry restrictions apply for a third-order effect.

• Examples of multiphoton processes are two-photon and three-photon absorption in which two photons or three photons, respectively, are simultaneously absorbed to reach an excited state of a molecule.

• An up-converted emission at a higher energy (higher frequency or shorter wavelength) is generated when a molecule is excited by multiphoton absorption of lower energy (longer wavelength) photons.

• Coherent anti-Stokes Raman scattering, abbreviated as CARS, is another nonlinear optical frequency conversion process. Here, two beams of frequencies vi and v2 generate a new output beam of frequency 2vi - v2 under the condition that the difference v1-v2 corresponds to the frequency of a Raman active vibrational mode of the molecule.

• Pulse lasers are capable of monitoring the formation of intermediates or products in real time. Biophysical and biochemical processes using time-resolved studies, such as transient spectroscopy or pump-probe experiments, can be studied this way.

• Laser safety guidelines must be followed while operating lasers, because laser light poses hazards to eyes and skin.

0 0

Post a comment