Laser| Stimulated emission | Spontaneous emission| Stimulated Emission

                                            Laser

Laser is a device that emits a collimated, monochromatic, highly intense unidirectional beam of light. The full form of lase is Light Amplification due to stimulated emission of radiation. Laser light is different form ordinary light as it has 4 following special features.

• Monochomaticity A Laser emits light beam of single frequency & single wavelength.

• Directionality  A Laser emits light beam in one particular direction.

• Intensity         A Laser beam can be focus on a very small area A. 1 watt laser is thousand time more intense than 100 watt ordinary lamp.

• Coherence     Laser beam is highly coherent. for example: There is definite phase correleration between photon is a laser beam.

In 1917 Einstein show the intraction of radiation with the mater with the help of following process.

• Stimulated Absorption

Let us consider that initially an atom is in laser energy state having energy`E_{1}` If a photon of energy h`\nu` is incident on an atom in lower state, then the atom absorb this much ammount of energy and jump to appear state having energy `E_{2}` such that `h\nu=E_{2}-E_{1}`. This process is known as induced absorption. we may express it as`A+h\nu → A^{*}` Where A is an atom of ground state and `A^{*}` is same atom in excited state.

• Spontaneous Emissiion

Consider that intially an atom is higher energy state `E_{2}`. Since normal life time of atom in excited state is `10^{-8}` sec. So the atom will make a transition to lower energy state `E_{1}`.  During this transition, atom will release a photon of energy h`\nu` where  `h\nu=E_{2}-E_{1}` The emission of photon occur on its own. This process is known as Stimulated Emission. Thus we can say that the process of emission of a photon by an atom withot any external agency is called Spontaneous Emission `A^{*} →A+h\nu`

• Stimulated Emission

As suggested by Einstein in 1917, an atom in excited state  `E_{2}` can also make transition to lower energy state `E_{1}`. when triggered by photon of energy  h`\nu` . During this transition, emision of second photon take place. The emitted photon has same frequency, same direction, same phase as that of incident photon. This process may be express `A^{*}+h\nu→ A+2h\nu`