Laser physics and its applicationIntroductory ConceptThe word LASER is an acronym for Light Amplification by Stimulated Emission of RadiationLasers, broadly speaking, are devices that generate or

amplify light, just as transistors and vacuum tubes generate and amplify electronic signals at audio, radio, or microwave frequencies.

Lasers come in a great variety of forms, using many different laser materials, many different atomic systems, and many different kinds of pumping or excitation techniques.

• Here "light" must be understood broadly, since different kinds of lasers can amplify radiation at wavelengths ranging from the very long infrared region, merging with millimetre waves or microwaves, up through the visible region and extending now to the vacuum ultraviolet and even X- ray regions.

• The beams of radiation that lasers emit or amplify have remarkable properties of

1.directionality, 2.spectral purity, 3. and intensity. These properties have already led to an

enormous variety of applications, and others undoubtedly have yet to be discovered and developed.

• In general atoms interact with electromagnetic radiation. The outer electron or the valiant electron and some time called the spectrum electron interact with electromagnetic radiation which has the correct energy to transmit between energy levels. Three main processes control the electron transitions between energy levels:

• Stimulated absorption process• spontaneous emission• stimulated emission

• SPONTANEOUS EMISSIONlet us consider two energy levels, 1 and 2, of some

atom or molecule of a given material with energies E1 and E2 (E1 < E2), respectively.

The two levels can be any two of an atom's infinite set of levels.

It is convenient however to take level 1 as the ground level.

These electrons will remain in the excited state for a certain period of time, and then will return to lower energy states while emitting energy in the exact amount of the difference

between the energy levels (∆E)

• When this energy is delivered in the form of an electromagnetic (em) wave, the process is called spontaneous (or radiative) emission.

• The frequency ν0 of the radiated wave is then given by the well known expression:

• where h is Planck's constant

• Spontaneous emission is therefore characterized by the emission of a photon of energy hν0 = E2— E1 when the atom decays from level 2 to level 1

• Note that radiative emission is just one of two possible ways for the atom to decay.

• Decay can also occur in a nonradiative way.• In this case the energy difference E2 – E1 is

delivered in some form of energy other than em radiation (e.g., it may go into the kinetic or internal energy of the surrounding atoms or molecules).

• The properties of the emitted spontaneous radiation:

• The emission of the individual photon is random, being done individually by each excited atom, with no relation to photons emitted by other atoms.

• this emission is independent of external influence,

• Then ther is no preferred direction for different photons, and there is no phase relation between photons emitted by different atoms.

Example:Calculate the electron energy which has a frequency

4.74x1014Hz?Solution:

• ExampleThe visible spectrum wavelength range is: 0.4 - 0.7 [μm]

(400-700 [nm]). The wavelength of the violet light is the shortest, and the wavelength of the red light is the longest. Calculate:

a) What is the frequency range of the visible spectrum? b) What is the amount of the photon’s energy associated

with the violet light, compared to the photon energy of the red light

Solution: The frequency of violet light:

The frequency of red light:

The difference in frequencies

The energy of a violet photon

• The energy of a red photon

• difference in energies between the violet photon and the red photon is: 2.15*10-19 [J]

• This example shows how much more energy the violet photon have compared to the red photon.

• Stimulated emission of a photon• Let us now suppose that the atom is initially

found in level 2 and an em wave of frequency v = ν0 (i.e., equal to that of the spontaneously emitted wave) is incident on the material. Since this wave has the same frequency as the atomic frequency, there is a finite probability that this wave will force the atom to undergo the transition 2→ 1. In this case the energy difference E2— E1 is delivered in the form of an em wave that adds to the incident wave. This is the phenomenon of stimulated emission.

Stimulated emission of a photon: A photon with frequency ν12 hit an excited atom (left), and cause emission of two photons with frequency ν12 while the atom goes to a lower energy level (E1).

The incoming photon is an electromagnetic field which is oscillating in time and space. This field forces the excited atom to oscillate with the same frequency and phase as the applied force, which means that the atom cannot oscillate freely, but is forced to oscillate coherently with the incoming photon.

• Remember that two photons with the same wavelength (frequency) have the same energy: E = hν = hc/λ

The incoming photon does not change at all as a result of the stimulated emission process.

As a result of the stimulated emission process, we have two identical photons created from one photon and one excited state. Thus we have amplification in the sense that the number of photons has increased.

This is the process that was explained LASER

• There is a fundamental difference between the spontaneous and stimulated emission processes.

1. In the case of spontaneous emission, atoms emit an em wave that has no definite phase relation to that emitted by another atom.

2. the wave can be emitted in any direction.3. In the case of stimulated emission, since the

process is forced by the incident em wave, the emission of any atom adds in phase to that of the incoming wave and in the same direction.

Stimulated absorption processLet us now assume that the atom is initially lying in

level 1. If this is the ground level, the atom remains in this level unless some external stimulus is applied. We assume that an em wave of frequency v = v0 is incident on the material. In this case there is a finite probability that the atom will be raised to level 2. The energy difference E2 — E1 required by the atom to undergo the transition is obtained from the energy of the incident em wave. This is the absorption process.

• Photon Absorption: A photon with frequency ν12 hits an atom at rest (left), and excites it to higher energy level (E2) while the photon is absorbed.

• We saw that the process of photon absorption by the atom is a process of raising the atom (electron) from a lower energy level into a higher energy level (excited state), by an amount of energy which is equivalent to the energy of the absorbed photon.