LASER PHYSICS (essential components)

A laser comprises two essential components:

1. An amplifying or gain medium: This can be a solid, liquid, or gas, consisting of atoms, molecules, ions, or electrons with energy levels used to enhance the power of a light wave during propagation. This process is facilitated by stimulated emission.
2. A system to excite the amplifying medium, known as a pumping system: This provides the necessary energy for light amplification. Pumping systems can be optical (using sources like the sun, flash lamps, arc lamps, tungsten-filament lamps, diode lasers), electrical (utilizing gas discharge tubes or electric current in semiconductors), or chemical.

While these two components are sufficient for amplifying an existing light source, most lasers also include an optical resonator or cavity to generate a specific type of radiation. The complete device, incorporating the amplifying medium, pumping system, and optical resonator, is known as a laser oscillator. However, it is commonly referred to simply as a “laser.”

The laser oscillator employs reflecting mirrors to significantly amplify the light source by bouncing it back and forth within the cavity. Additionally, it features an output beam mirror that allows a portion of the light wave in the cavity to be extracted for practical use.

*tried to illustrate different functions in diagram below

The emission-absorption principle describes the fundamental processes by which atoms interact with electromagnetic radiation. These processes include absorption, spontaneous emission, and stimulated emission. Let’s delve into each mechanism:

1. Absorption:
   • In absorption, an atom in a lower energy level absorbs a photon with energy equal to the energy difference between the two levels.
   • This absorption process elevates the atom to a higher energy level.
   • The absorbed photon’s energy is converted into the internal energy of the atom, raising it to a higher energy state.
   • There is a change in population of level E1 and also change in level E2 during absorption .
   • -dN₁/dt = N₁
   • -dN₁/dt = W₁₂
   • cross -sectional area W₁₂ = σ ₁₂F
2. Spontaneous Emission:
   • When an atom is in an excited state (higher energy level), it can spontaneously transition to a lower energy state.
   • During this transition, the atom emits a photon without any external influence.
   • The emitted photon’s frequency and direction are random, and its phase can vary.
   • -dN₂ /dt = N₂
   • dN₂ /dt = A₂₁N₂
3. Stimulated Emission:
   • In stimulated emission, an incident photon interacts with an atom that is already in an excited state.
   • This interaction causes the excited atom to undergo a transition to a lower energy state, emitting a photon.
   • Importantly, the emitted photon has the same frequency, phase, polarization, and direction as the incident photon. Hence, it’s “stimulated” by the incident radiation.
   • The presence of the incident photon stimulates the emission process, leading to amplification of the radiation.
   • Stimulated emission is the principle behind the operation of lasers, where the emitted photons are coherent and highly directional due to the synchronization with the incident photons.
   • -dN₂ /dt = N₂
   • -dN₂ /dt =-W₂₁ N₂
   • W₂₁ = σ ₂₁F

In summary, absorption elevates atoms to higher energy levels, spontaneous emission results in the spontaneous decay of excited atoms, and stimulated emission is initiated by the presence of incident photons, leading to the emission of additional photons with identical properties. These processes play crucial roles in various phenomena such as atomic spectroscopy, laser operation, and the behavior of electromagnetic radiation in matter.

Population inversion and pumping are crucial concepts in the operation of laser systems, where achieving a state of non-equilibrium population distribution between energy levels is essential.

1. Population Inversion:
   • In a typical thermal equilibrium scenario described by Boltzmann’s Law, more atoms are found in lower energy levels than in higher ones. This is because as per the law, the population of a particular energy level decreases exponentially as the energy of the level increases.
   • However, in laser systems, achieving a population inversion is necessary. This means having more atoms or molecules in an excited state (higher energy level) than in the ground state (lower energy level).
   • Population inversion is a non-equilibrium condition and is crucial for the amplification of light in lasers.
2. Pumping:
   • To create a population inversion, energy must be added to the system through a process known as pumping.
   • Pumping methods vary depending on the type of laser. Common pumping mechanisms include optical pumping (using light), electrical pumping (using an electric current), or even chemical pumping (using chemical reactions).
   • The pumping process raises atoms or molecules from lower energy levels to higher ones, thereby increasing the population of the excited states.
   • Once a population inversion is achieved through pumping, it sets the stage for stimulated emission to dominate over absorption and spontaneous emission, leading to laser action.

This is known as population inversion and is given by 

 . Light is amplified when the population inversion is positive. Pumping may be electrical, optical or chemical.