In this book, we explore the phenomenon of lasing from the ground-up. The quantum-mechanical model of the atom, supported by numerous experiments, dictates that the electrons in an atom are populated in quantized energy levels, with discrete gaps between the levels. Therefore, by adding or extracting the energy from them, an electron can be moved around the different energy states of an atom. Photoelectric effect, for which Albert Einstein won the Nobel prize, described how “packets” of light, called photons, at the right frequency and energy can move these electrons around.

 

If an incoming photon has just the right amount of energy as the gap between two energy levels in an atom, the electron can move from its lower “ground” state to an upper “excited” state by absorbing the photon. However, the electron always years to be in the lowest possible energy level. It achieves that through many events such as

 

1. Non-radiative decay, where it simply gives off the extra energy as heat,

2. Spontaneous emission, where it spontaneously and randomly emits another photon with energy equal to the gap between two energy levels, and

3. Stimulated emission, where it interacts with a photon while it is in the excited state and emits another identical photon, essentially a photocopy.

 

The goal of a laser is to have a beam of light that essentially consists of a stream of identical photons, such that the resulting energy is tremendously high. Therefore, a laser must suppress random events such as non-radiative decay and spontaneous emission, while amplifying the event of stimulated emission to generate a stream of identical photons. This is achieved by placing a crystal, such as titanium-doped sapphire, with atoms of the necessary energy levels, in a resonator that selectively amplifies light generated by stimulated emission. Thus, a laser is born.

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