Laser

Objective: The purpose of this lab is to understand how laser works. Laser stands for "light amplification by the stimulated emission of radiation". Laser are commonly used in many ways, such as military applications, CD, reading bar code, and astronomy applications. The properties of laser are monochromatic, coherent, directional, and is highly focused. There are mainly three ways for lasers to move from one state to another. They are absorption, spontaneously emission,and stimulated emission.


Absorption



spontaneous emission


stimulated emission


Conclusion
Laser commonly use these ways to emit energy. In absorption, no photon is emitted. For the other two, photon is emitted.The first one is absorption. The atom is initially at ground state. If the atom absorb enough energy from a photon, it can be excited to a higher state. The amount of energy needed is E=hf=E(excited)-E(ground). No photon is emitted at the end.

The second one is spontaneous emission. This time, the atom is initially at excited state. However, no external energy from photon is received. So, the atom has to move by itself to ground state. Once it moves down to ground state, different from absorption, it will emit a photon. Again, the energy of photon emitted is E=hf. In this case, no external aid is provided, so the atom has to decline to ground state spontaneously to emit a photon. Therefore, it is called spontaneous emission
 
The third one is stimulated emission. This time, the atom is initially at excited state. But there is a external photon that provide energy of hf to stimulate the atom to ground state. (The atom no longer needs to move to ground state spontaneously.) When the external photon stimulates the atom to go to ground state, another photon, of same energy, phase, direction, and polarization, is emitted. The additional photon also has energy of hf. For stimulated emission, many photons are stimulated and emitted.
The favorable situation is there are more excited atom than ground state.