Spontaneous and Stimulated Radiation

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Spontaneous Emission

When an atom jumps from a higher energy stated to a lower energy state it emits light in the form of photons. In any source of light, the light that is emitted is incoherent, i.e., different photons have different phases and different wavelengths. The direction of light emitted is also not the same. Such a type of emission happening spontaneously is called spontaneous radiation.

In contrast to this, there is energy states one lower (E₁) and another higher (E₂). If a photon of light having energy exactly equal to the difference of E₁ and E₂ is made to incident on the atom, the atom in the lower state E₁ would absorb energy and reach the level E₂. This process is called as stimulated absorption of light.

Since the atom is not in the ground state, but in the excited state, if left to itself the atom reaches the ground state by emitting a photon with energy = E₂ - E₁ spontaneously.

If the atom is in the higher (E₂) energy and if the photon having an energy (E₂ - E₁) is incident on it, then something different happens. The incident photon interacts with the atom and may cause the atom to come to lower state along with an emission of a photon. This process is called stimulated emission. Here the emitted photon has the same energy phase and the direction of the incident photon.

LASER

It is an acronym for Light Amplification By Stimulated Emission Of Radiation. It is a process by which we get a coherent, highly monochromatic and perfectly parallel beam of light. Lasers may also be called as optical masses. It has been invented and developed in 1959. The laser provides the first opportunity to generate high intensity coherent monochromatic light. In 1917, Albert Einstein predicted the possibility of stimulated emission of radiation, which formed the basis for Masers and Lasers.

Laser works

in the following way. The system can be a solid, liquid or gas but is taken in the form of a cylinder. A system is chosen which has a lower energy E₁, a higher energy E₂. Using some technique the number of atoms in state E₂ is made higher than that in state E₁.

In this situation a photon with energy (E₂ - E₁) is incident and the atom at E₂ drops to E₁ emitting a photon in the same phase, energy and direction as that of the incident one. If these photons interact with two more atom, the process can go on like a chain reaction and emitting lot of photons leading to amplification of light.

What is so special about this amplified light: It is highly directional, coherent, and monochromatic and sharply focused, highly tunable and very bright.

Requirements to achieve amplification are

(i) Metastable states, where the lifetime of the atoms are of the order of milliseconds.

(ii) More number of atoms should stay at metastable state and less number in the lower state. In such a situation we say population inversion has taken place. To attain this the metastable state should continuously get atoms and atoms must be removed from the lower states. Such a process is named as 'pumping'.

(iii) The emitted photons should stay longer in the system so that they can interact with the system to release large number of photons. This can be achieved by keeping parallel mirrors, which reflect light back to the system.

(iv) The photons have to be made highly directional, which can be achieved by throwing out the photons at random directions through the sides of the laser and only those parallel to the axis of the cylinder is amplified.

Helium Neon Lasers Construction

Construction

This laser consists of a discharge tube in the cylindrical shape containing helium Neon gas in the ratio 9:1 at a low pressure. Two mirrors M₁ and M₂ are placed at the ends to facilitate the longer availability of emitted photons. A large potential difference is applied between the anode (A) and the cathode(C).

Working

When a high tension is established between the anode and the cathode, the gas in the discharge tube gets ionized and releases some electrons. These electrons are accelerated which collide with helium atoms and take them to excited level E₃, a Metastable state for helium. These helium atoms on collision transfer their energy to neon atoms which attain an energy level E₂, and helium atoms themselves to ground state. When this process continuous, pumping takes place and more and more Neon atoms are available in state E₂. Stimulated emission takes place and Laser light comes out of the partially transmitting mirror.

Lasers are used to send telephone signals over long distance. Lasers are used in the field of microsurgery. It is used to drill sharp holes in metals and diamond. Lasers are used to produce holograms, which record 3 dimensional image of an object.