Report on Lasers

Modern Physics, Spring 2015

Laura Vicuna. May 15, 2015.

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I have narrowed down my question to "What properties of lasers make them different from other light sources in that they can be used in laser cutting and lithography?"

Own explanation (original hypothesis)

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When an electron is excited from a lower to a higher energy level, it will eventually want to drop back to its lower energy state. When it does this, it will emit a photon. This process is called spontaneous emission. Stimulated emission is the process by which an incoming photon can interact with an excited atomic electron. This interaction creates a new photon of the same energy with identical phase, frequency, polarization and direction.

Figure 1. Stimulated emission

This process however is exceeded by absorption, in which the energy of an absorbed photon causes an identical but opposite atomic transition. This is so because there are more electrons with lower energy states than in higher energy states. When population inversion is present, meaning there are more electrons in excited states than in lower energy states, the rate of stimulated emission exceeds that of absorption and optical amplification is possible. This is the precondition for light amplification in lasers. This is what gives lasers the property of coherence and the ability to continuously emit light.

Figure 2. Light coherency

A laser makes many of atoms release trillions of photons all at once so they can line up to form a concentrated light beam. When a photon hits an already excited atom, the excited atom gives off two photons of light. This is called stimulated emission which amplifies the photon.

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The light from a laser is released as a thin beam concentrated on one spot. We say that the beam is highly collimated. This occurs because the cavity of the laser has two nearly parallel mirrors on the front and the back from which the atoms bounce back and forth. The back mirror is nearly perfectly reflected, while the front mirror is about 99% reflecting, letting out 1% of the beam. This 1% is the beam that we see. While the light is bouncing back and forth between the mirrors, it is gaining being amplified due to stimulated emission. This is displayed in the figure below.

Figure 3. Reflection of atoms in beam

Summary (New explanation)

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