Part 2: “How Laser work?  Stimulated emission, Types, and mind-blowing applications”

Author:

Prof. Kali Chandrakant

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

50+ Years of experience in Physics teaching

 1. Introduction:

     Welcome back! In Part 1, we unpacked LASER’s origins and basics like population inversion. Now, see photons dance to create coherent beams—and transform industries.

 2. Nature of Radiation:

    “The energy which is propagating in the form the electromagnetic wave traveling at speed of 3×10⁸ m/s is called radiation.”

Based on range of frequency or wave length the radiation can be classified as: radio waves, micro waves, infrared, visible light, ultraviolet, X-rays, γ-rays, and cosmic rays.

Visible light lies between 4000 Å – 7000 Å.

Light shows dual nature:

• Wave
• Particle (Photon)

Each photon carries energy:    E = hν
 3. The effect of incident photon on the collection of atoms:

   3.1. Daily life exaple:         

To understand the effect of an incident photon on a collection of atoms, let us consider a common experience. Suppose there is a mango tree full of mangoes. To detach a mango from the tree, a stone is thrown toward it. If the kinetic energy of the stone overcomes the binding energy of the mango, the mango will fall down.

   Different outcomes are possible when the stone is thrown toward the mango:

1. The stone does not hit the mango.
2. The stone hits the mango, but the mango does not detach due to insufficient supplied energy.
3. The stone hits the mango and detaches it when the kinetic energy of the stone matches or exceeds the binding energy of the mango.

   In view of this example, let us now consider the effect of an incident photon on a collection of atoms.

 3.2. Photon passes away:

Fig F

Let E1​ represent the energy of the ground state with population N1, and ​ E2 represent the energy of the excited state with population N2 (N1>N2).  If a photon of energy hν≠E2−E1 is incident on a collection of atoms, it passes through the collection without interaction. (Fig F).

  3.3. Stimulated Absorption:

   “The process in which the atom absorbs photon energy and get excited is called stimulated absorption or simply absorption”

Fig G

A photon of energy hν=E2−E1 is incident on a collection of atoms with populations N1>N2 . There is a maximum probability that the photon will interact with atoms in the ground state, because the population of the ground state is higher. The photon energy matches the energy difference between the atomic levels. Hence, when the photon collides with an atom in the ground state, it is completely absorbed by that atom. As a result, the atom gets excited and shifts to the energy level E2 (Fig. G).

  3.3.1. The conditions required for stimulated absorption:

a.The population of lower state (N₁) must be greater than the population of excited state (N₂) i.e. N₁ ˃N₂.

 b.The energy of the incident photon must match the energy difference between the atomic levels; that is     hν=E₂-E₁.

    This is similar to absorbing kinetic energy of stone by stem in our mango tree example.

                         Atom + hν = excited atom

 The rate of absorption transition R_ab is the number of atoms per unit volume per second which are shifted from lower level to higher level. R_ab is proportional to the population N₁ of lower state and the energy density of photons ρ(ν). 

R_ab = B₁₂  ρ(ν) N₁

  Where, B₁₂ is known as the Einstein coefficient for stimulated absorption. B₁₂ represents the probability of stimulated transition from state 1 to state 2.

 3.4. Spontaneous Emission:

   “The emission of photon by an excited atom spontaneously is called spontaneous emission”

Fig H

After photon absorption, the atom in the excited state has a lifetime of excitation of the order of 10-8 second. Therefore, the time duration for which the atom remains in the excited state E2 is approximately 10-8 second. The atom then spontaneously returns to the ground state E1 . During this process, the electron of the atom jumps from energy level E2​ to E1​ by spontaneously emitting a photon of energy hν = E2 –E1 spontaneously. (Fig. H)

  Spontaneous emission is random in nature and is a completely disordered and uncontrolled process. The light emitted by usual sources such as the Sun, stars, electric bulbs, fire, etc., is due to spontaneous emission. Hence, the light from these sources is incoherent and un-polarized. Spontaneous emission is similar to people moving randomly in a bazaar.          

              Excited atom= atom + hν

The rate of spontaneous transition R_sp is the number of atoms per unit volume per second which are emitted from higher level to lower level. R_sp is proportional to the population N₂ of excited state only.

R_sp= A₂₁  N₂

Where, A₂₁ is known as the Einstein coefficient for spontaneous emission. A₂₁ represents the probability of spontaneous transition from state 2 to state 1.

  3.5. Stimulated Emission:

     “The emission of a photon by an excited atom due to the stimulus (or induction) provided by an incident photon of matching energy is called stimulated emission.”

   Now consider the situation in the collection of atoms where in the population inversion (N₂ ˃ N₂) is brought between meta-stable level (E₂) and ground level (E₁).

Fig I

The photon of energy hν = E2 – E1  is incident on the collection of atoms. There is a maximum probability that the photon will interact with atoms in the meta-stable level, since the population of this level is higher. The energy of the incident photon is in resonance with the excess energy possessed by atoms in level E2​. This resonance triggers the excited atom to emit a photon. Due to the stimulus provided by the incident photon, the atom jumps from energy level E2​ to E1​ by emitting a photon of energy      hν = E2 – E1.  (Fig I)

     The bosonic nature of photons ensures that the incident photon and the induced photon occupy the same quantum state. In other words, the induced photon finds itself in the same state as the incident photon. As a result, the stimulated photon has the same frequency, direction, phase, and polarization as the incident photon.

     In this process, the number of incident photons is one, while the number of output photons is two. Thus, stimulated emission leads to amplification of radiation. Since these two photons have the same phase, they emerge together as coherent radiation. The stimulated emission process is therefore an ordered and controlled process.

    This process was theoretically discovered by Albert Einstein in 1917 while re-deriving Planck’s law of radiation using the concept of probability coefficients, known as Einstein coefficients, for absorption, spontaneous emission, and stimulated emission. Stimulated emission is analogous to a military march, where motion is highly ordered and synchronized.

    Stimulated emission is the basic principle of laser action, and Albert Einstein laid the foundation for the invention of the laser.

The conditions required for stimulated emissions are:

1. There should be population inversion (N₂ ˃ N₁)
2. To bring the population inversion, the upper level E_2,must be meta-stable state.
3. The incident photon must have energy hν = E₂-E₁which brings resonance.  

                                Excited atom + hν = Atom + 2hν

 The rate of stimulated emission R_st is the number of atoms per unit volume per second which are shifted from excited level to lower level. R_st is proportional to the population N₂ of excited state and the energy density of photons ρ(ν). 

R_st = B₂₁  ρ(ν) N₂

    Where, B₂₁ is known as the Einstein coefficient for stimulated emission. B₂₁ represents the probability of stimulated emission from state 2 to state 1.

 4. Pumping schemes:

         Laser action takes place through the process of stimulated emission. The energy levels involved in laser action are:

a.Ground level
b. Pumping level
c. Upper lasing level (usually a meta-stable state)
d. Lower lasing level (usually the ground state)

   However, in some cases, the same energy level may serve more than one purpose. Accordingly, there are three pumping schemes:

1. Two-level system:
   The ground level also acts as the lower lasing level, whereas the pumping level and the upper lasing level together form the upper level.
2. Three-level system:
   Atoms are pumped to a higher excited level, from which they quickly drop to a meta-stable state. Laser action then takes place between this meta-stable level and the ground level.
3. Four-level system:
   In this system, all four energy states mentioned above are distinct and separate.

 5. Active medium and active centers:
    The material used in a laser is called the active medium, and the atoms or ions responsible for the actual laser action are called active centers.

          The active medium may be solid, liquid, or gaseous. In the case of a ruby laser, ruby is the active medium and the Cr⁺³ ions are the active centers.

6. Resonator:

Fig J

The resonator required for laser action consists of an active medium in the form of a long cylinder. One end of this cylinder is perfectly polished to serve as a perfect reflector, while the other end is partially polished to act as a partial reflector. (Fig J) In this case, the term partial reflector has a different meaning. The reflector reflects light of intensity below a certain threshold value. If the intensity of the light exceeds this threshold, the light is transmitted completely.

   7. Characteristics of a Laser:

    A laser possesses all the properties of light such as reflection, refraction, diffraction, and interference. However, it also has some special properties. A laser is
a. highly intense,
b. nearly perfectly monochromatic,
c. highly coherent,
d. highly directional, and
e. polarized light.

 7.1. Highly Intense light:
    Laser light is highly intense or bright because the energy of the laser source is concentrated in a single direction. A large number of photons travel in the same direction, whereas in an ordinary light source the photons are distributed in all possible directions.

 7.2. Perfectly Monochromatic light:
     Laser light is highly monochromatic because the resonant cavity allows only a selected wavelength of light to participate in laser action.

                              E₂ – E₁ = hν = hc / λ

     In an ordinary light source, photons are produced due to transitions between different energy levels, resulting in radiation of different wavelengths.

 7.3. Perfectly Coherent light:
     Due to the process of stimulated emission, laser light is perfectly coherent. That is, all the waves travel in the same phase because of the selective nature of stimulated emission. Laser light exhibits both spatial coherence (coherence with respect to space) and temporal coherence (coherence with respect to time). Hence, all the emitted photons are in the same phase.

 7.4. Highly Directional light:
      A laser beam is emitted in the form of an almost parallel beam and travels in a specific direction through a very small cross-sectional area. The divergence of laser light is very small. Therefore, a laser beam can travel long distances without significant spreading.

  7.5. Plane Polarized light:
      Laser light is plane-polarized. All the waves of the laser vibrate in only one plane of electric field oscillation. The stimulated emission process and the resonant cavity are responsible for the polarization of laser light.

 8. Types of Lasers:

    Lasers are commonly classified based on the type of active medium used. There are four main types of lasers:

  1. Solid-state lasers

 2. Gas lasers

 3. Liquid dye lasers

 4. Semiconductor diode lasers

Some lasers operate in continuous wave (CW) mode, while others operate in pulsed mode.

8.1. Solid-State Laser:

    In a solid-state laser, the active medium is a solid material containing active centers. The first historically successful laser was the ruby laser, built by Theodore Maiman in 1960. It is a pulsed laser based on a three-level pumping scheme and operates using optical pumping. The ruby laser emits red light with a wavelength of 6943 Å.

      Another popular solid-state laser is the Nd:YAG laser, developed in 1964. It operates on a four-level pumping scheme using optical pumping and emits radiation in the infrared region.

8.2. Gas Laser:

      In 1961, Ali Javan, Bennett, and Herriott developed the first helium–neon (He–Ne) gas laser. It operates on a four-level pumping scheme using electrical discharge. It is a continuous-wave laser with a wavelength of 6328 Å.

       The carbon dioxide (CO₂) laser is one of the most powerful lasers used in industry. It also operates on a four-level pumping scheme and emits radiation in the infrared region.

8.3. Liquid Dye Laser:

      Dye lasers are liquid lasers that use organic dye solutions as the active medium and operate using optical pumping.

8.4. Semiconductor Diode Laser:

      A semiconductor laser is a forward-biased p–n junction diode. The first semiconductor laser was developed by Hall and Nathan in 1962 using Ga-As as the active medium. These lasers are small in size and have high efficiency.

 9. Applications of Lasers:

    Lasers find applications in a wide variety of fields such as fundamental sciences, electronics, civil and mechanical engineering, medicine, and industry. The number of applications of lasers is continuously increasing.

1. Holography is a technique for producing three-dimensional (complete) images of objects or scenes and is made possible using lasers.
2. In the medical field, lasers are used for bloodless and painless surgeries, retinal welding, destruction of malignant tumors, dentistry, and ophthalmology.
3. The intense laser beam is useful for material processing, as its energy density can be controlled precisely. Lasers are used in industry for welding, cutting, soldering, drilling holes, and heat treatment of various materials.
4. Lasers have greatly increased the information-carrying capacity of optical communication systems through the use of optical fibers, revolutionizing modern communication.
5. Other applications of lasers include laser printers, optical computing and signal processing, playing video discs, reading bar codes, measuring distances to remote objects, laser scanning, missile guidance, and laser weapons.

10. Summary of LASER – Part 2 (Working, Types & Applications):

     Part 2 focuses on the working principle of LASER, based on the interaction between photons and atoms. It explains the three fundamental processes involved: stimulated absorption, spontaneous emission, and stimulated emission. Among these, stimulated emission is the most important, as it produces photons that are identical in phase, frequency, direction, and polarization, leading to light amplification.

        The concept of population inversion and the presence of a meta-stable state allow stimulated emission to dominate over absorption. An optical resonator, consisting of two mirrors, reflects photons back and forth through the active medium, causing repeated amplification and producing a powerful laser beam through the partially reflecting mirror.

   This part also discusses different pumping schemes, such as two-level, three-level, and four-level laser systems, along with the role of the active medium. Various types of lasers—solid-state, gas, liquid (dye), and semiconductor lasers—are described with examples like Ruby laser, He-Ne laser, CO₂ laser, and semiconductor lasers.

     Finally, Part 2 highlights the wide range of applications of LASER in medicine, industry, communication, science, defense, and everyday life. It concludes that LASER technology has become an indispensable tool in modern science and technology due to its unique properties and versatility.