“Basics of LASER: From Einstein’s prediction to magical light machines”

Author:

Prof. Kali Chandrakant

M.Sc., M.Ed., D.C.S.

50+ Years of experience in Physics teaching

 1. Introduction:

 1.1. Why LASER feels like magic:

Have you ever wondered how a single beam of light can cut hard metal, heal human eyes, read barcodes, or even power science-fiction weapons?
      That powerful beam is called LASER, an acronym for Light Amplification by Stimulated Emission of Radiation.

    Unlike ordinary light from the Sun or electric bulbs—which spreads in all directions—laser light is highly intense, narrow, and precise. Because of these unique properties, lasers have become one of the most revolutionary inventions of modern science.

 1.2. Ordinary light vs Laser light:

Ordinary light	Laser light
Incoherent	Highly coherent
Divergent	Highly directional
Multi-wavelength	Nearly monochromatic
Weak intensity	Extremely intense

Scientists always dreamed of producing bright, directional and coherent light—and lasers made this dream real.

  1.3. Einstein’s revolutionary idea:

In 1917, Albert Einstein predicted a process called stimulated emission, which later became the foundation of laser technology.
    However, it took 43 years of scientific effort before the first working laser was built.

 2. Historical development of LASER:

Important milestones in laser history:

• 1939 – Fabrikant (USSR) proposed radiation amplification
• 1954 –
  • Charles Townes & Arthur Schawlow (USA)
  • Basov & Prokhorov (USSR)
    Developed MASER (Microwave Amplification)
• 1960 – Theodore Maiman constructed the first Ruby Laser
• 1960 – Ali Javan developed the He–Ne gas laser
• 1962 – Robert Hall developed the semiconductor laser
• 1964 – Kumar Patel invented the CO₂ laser

Nobel Prize 1964 was awarded to Townes, Basov, and Prokhorov.

     These discoveries gave birth to photonics, merging light with modern technology.

 3. Basic atomic concepts behind LASER:

 Energy states of atoms:

Atoms exist in discrete energy levels:

 3.1. Ground State (E₁) :

   Lowest energy, most stable

• The electrons revolving in different orbits round the nucleus constitutes stable atom. Let us consider the sodium atom ₁₁Na²³.

Fig A

The electron distribution is 1S2 , 2S2 , 2P6 , 3S1 , 3P0 (Fig. A). In this case the entire atom has minimum energy, and the atom is said to be in ground state, which is stable state.  This ground state energy can be represented by E1.  In the ground state: 1. The atom is stable. 2. The electron distribution is normal. 3. The atom has minimum energy.

  3.2. Excited State (E₂) :

Higher energy, unstable

Fig. B

Consider one electron in 2S state, which is excited to 3P state by supplying energy. Along with the electron the atom also has excess energy and the atom is in excited state. This excited state of the atom is represented by E2.(Fig B) In the excited state: 1. The atom is unstable. 2. The electron distribution is not normal. 3. The atom has excess energy.

• In further discussion, the atom as whole is taken in to consideration, with two energy states: Ground state E₁and Excited state E_.  

At room temperature, most atoms remain in the ground state.

 3.3. Boltzmann distribution law:

• The number atoms in any atomic state are governed by Boltzmann’s distribution law.
• Let N₁ be the number of atoms (or population) in ground state of energy E_1, N₂ be the number of atoms (or population) in excited state of energy E_2, k be Boltzmann’s constant and T be the Kelvin temperature of the system.
• Boltzmann’s distribution law is stated in mathematical form:

N₂=N₁e^{−(E2−E1)/kT}

Fig. C

This law indicates that the population of ground state is very large as compared to the population of excited state.(Fig C) (N1 ˃˃ N2).

Hence, under normal conditions, excited atoms are very few.

 3.4. Why excited states don’t last long:

     Nature prefers stability. “Every system in nature is in stable condition when it possesses minimum energy”.
Ordinary excited states decay quickly with a lifetime of about:  10^-8 s.

3.5. Meta-stable state: The game changer:

    Some excited states—called meta-stable states—have much longer lifetimes: 10^-3    seconds.   

        “The energy level having longer duration of excitation life time (of the order

10^-3 second) is called meta-stable level or state”.

This is 100,000 times longer, allowing excited atoms to accumulate.

Example: Cr³⁺ ions in Ruby laser

 3.6. Pumping: Supplying energy:

   “The energy required to take atoms from lower energy state to higher energy state is pumping energy”.

Fig. D

Fig D represents the process of pumping by which atoms in lower energy level E1 is taken upper energy level E2 by proper pumping energy. Optical, electrical, chemical, thermal etc. energies are used for pumping. The pumping process is one of important process for laser action.

To excite atoms, external energy is supplied through:

• Optical pumping
• Electrical discharge
• Chemical reactions

This process is known as pumping, similar to filling water into an upper tank.

 3.7. Population inversion:

      “If the population of atoms in higher energy state is greater than the population of atoms in lower state then this situation is called Population Inversion. ( N₂ ˃ N₁ )”

Fig E

Fig E shows that when number of atoms (N2) of upper state E2 is greater than that of the number of atoms (N1) in lower state E1. The population inversion can be achieved by using pumping energy. By pumping large number of atoms to meta-stable state population inversion is brought. Laser operation requires this non-equilibrium condition of population inversion.

Laser action requires a special condition called population inversion,

     where:N₂ > N₁

This is opposite to natural conditions and is achieved using meta-stable states.

 3.8. Fermions vs Bosons:

• Fermions (electrons, proton,quarks): There is only one particle to occupy the same quantum state. 
  • Half-integer spin
  • Follow Pauli’s exclusion principle
• Bosons (photons, pions): There is no restriction on the number of them that occupy the same quantum state. 
  • Integer spin
  • Unlimited particles in the same state

 Photons being bosons makes laser amplification possible.

 4. Summary of LASER – Part 1 (Basics & Theory):

    LASER (Light Amplification by Stimulated Emission of Radiation) is a device that produces highly intense, coherent, monochromatic, and directional light. Unlike ordinary light sources such as bulbs or the Sun, laser light does not spread randomly and can be precisely controlled. The theoretical foundation of LASER was laid by Albert Einstein in 1917 through the concept of stimulated emission, although the first practical laser was developed much later in 1960.

      This part explains the historical development of LASER, beginning from Einstein’s prediction to the invention of MASER and the first ruby laser. It introduces essential atomic concepts such as ground state, excited state, and energy levels. Under normal conditions, most atoms remain in the ground state, as explained by the Boltzmann distribution.

      A key idea discussed is the meta-stable state, which has a much longer lifetime than ordinary excited states. This allows atoms to accumulate in higher energy levels. Using an external energy source called pumping, atoms are excited to these levels. When more atoms exist in the excited state than in the ground state, a condition called population inversion is achieved, which is essential for laser action.

Part 1 builds a strong conceptual foundation by explaining why ordinary light cannot become laser light and how special atomic conditions are created to enable laser operation.

  In Part 2, we will see how photons multiply like an avalanche and create a powerful laser beam.