Lasers

Have you heard from your parents or read the labels on lasers to not point them in the eyes of others and yourselves? I find this physics tuition concept so mysterious. Although lasers seem like harmless, coloured light, but it is actually a wolf in sheep’s clothing. Lasers, other than being an extremely efficient tool to point out certain words for emphasis in class, are also used in the surgery of the eyes. For laser treatments to work, it must be absorbed into the tissues of our body and our eye has tissues that can absorb lasers efficiently, such as the retina.

Why are lasers used specifically for the surgery of the eyes? Lasers produce intense, monochromatic, highly directional and coherent beams of light through the mechanism of stimulated emissions. Stimulated emission occurs when an incoming photon of the exact energy between the difference of 2 energy levels stimulates the electron in the excited state to return to ground state and thereby emitting a second photon. The goal of the laser is to create as many stimulated emissions as possible, which is only possible when there are more atoms in excited states as compared to ground states (aka Population Inversion), and when the number of photons of the correct energy are increased, which will trigger stimulated emissions. Although we can shine photons of correct energy on to these atoms to observe atoms undergoing excitation from their ground states, these atoms also quickly undergo de-excitation via spontaneous emission since an excited state usually have very short lifetimes. Hence, very little emission will occur as the atom is in an excited state only for a very brief period of time.

For population inversion to occur, the atoms must be able to stay in special excited states with long lifetimes, so that stimulated emissions can occur before spontaneous emissions. Such long life-time states are known as Metastable states.

A laser pointer is one such device where many stimulated emissions happen. Inside a laser pointer, a tube containing an active material is placed between 2 mirrors. The atoms of the active material are ‘pumped’ to excited states by an external source of energy in order to create a population inversion so as to induce even more stimulated emission within the active material. One of the mirror is very slightly transparent, so as to allow a small fraction of photons to escape as laser output. These beams are very intense, highly directional, and are monochromatic.

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