Stimulated Emission or Induced Emission

If the atom is in the excited state E2 and a photon of energy exactly equal to E2 – E1 = hυ is incident on it, then the incident photon interacts with the atom in the excited state and then it stimulates or induces the atom to come down to the ground state E1. A fresh photon is emitted in this process. Therefore, when an atom ejects a photon due to its interaction with a photon incident on it, the process is called stimulated emission (or induced emission).

Characteristics of stimulated emission:

a)      For each incident photon, there are two outgoing photons moving in the same direction.

b)      As the emitted photon has exactly the same energy, phase and direction as the incident photon, we will achieve an amplified and unidirectional coherent beam.

Note: The laser is based on the principle of stimulated emission.

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Spontaneous Emission

If the atom in the excited state automatically decays to the ground state by emitting a photon of energy (E2 – E1 = hυ), then this process is called spontaneous emission. Generally an atom/electron in excited state can stay for

10-9 – 10-8 seconds.

Characteristics of spontaneous emission:

a) The emitted photon of energy E2 – E1 = hυ can move in any random direction.

b) There will not be any phase relationship between the photons emitted from various atoms.

Hence, the radiations coming out due to spontaneous emissions are incoherent.

Reference: The content of all the articles of lasers posted here is referred from my book: Optics and Lasers by Amit Sarin and Sameer Kalia, published by kalyani publishers.

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Note from winnerscience: If you want the e-notes of all the laser  articles that will include the basics of lasers like stimulated absorption, difference between spontaneous and stimulated emission, Einstein  Coefficients, properties and applications of lasers, complete construction and working of lasers like Ruby laser, He-Ne laser, Carbon dioxide laser, Nd:YAG laser, dye laser, semiconductor laser, holography and additional articles of Q-switching and mode locking, then please contact You can post your queries also there.

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Stimulated Absorption in Laser

Stimulated Absorption is the starting point to achieve laser.

It happens when a photon of light having energy E2 – E1 = hυ (as shown in the figure) is incident on an atom in the ground state, the atom in the ground state E1 may absorb the photon and jump to a higher energy state E2. This process is called stimulated absorption or induced absorption. This is called so because the incident photon has stimulated the atom to absorb the energy.


The above is the diagram to show the stimulated absorption in laser. In the next articles, when will learn about stimulated emission and spontaneous emission. These processes are a must to achieve the laser that is light amplification by stimulated emission of radiation.

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Motorala split into two entities

Motorala split into two entities:

a) Motorala mobility which will deal with handsets and set-top boxes,

b) Motorala Solutions which will sell police radios and bar code scanners to government and business customers.

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CDAC-Admissions six months industrial training

Admissions in six months full time industrial training in following courses at centre for development of advances computing (CDAC), Mohali:

a) Networking and system security,

b) Wireless & broadband communication networks,

c) Embedded system design,

d) Bio-medical equipment technology and maintenance

Also visit:

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Five year integrated BA-LLB (Honors)-2011

The National Law University, Delhi invites applications for a Five year integrated B.A., LL.B. (Hons.) degree programme for the academic year 2011-2012 commencing from August, 2011.

Last Date for Receipt of Applications: April 11, 2011

Admission will be on merit through All India Law Entrance Test (AILET) to be conducted at Ahmedabad, Bangalore, Chandigarh, Chennai, Cochin, Delhi, Guwahati, Hyderabad, Jaipur, Jammu, Kolkata, Lucknow, Mumbai and Patna on May 1, 2011.


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