Laser sources: Light Amplification & Stimulated Emission of Radiation

-First described in 1960.-Works on the basis of quantum theory of radiation Energy of each photon= hυ.-Highly useful sources in analytical instrumentation due to a)high intensities, b)narrow bandwidths, c)coherent nature of outputs,

-The laser sources are used in several routine analytical methods e.g. Raman spectroscopy,

Emission spectroscopy,

Fourier transform infrared spectroscopy (as a part of instruments)

-Produces a beam of highly monochromatic bandwidth of 0.01nm or less.-Used important sources in UV/Vis. & IR regions of spectrum.

Components:Principal components of laser source:

i)Lasing medium ii)Pumping medium

Lasing medium & pumping source:-Solid crystals ,e.g ruby, a semiconductor(gallium arsenide),solution of organic dye or a gas such as argon or krypton. -It is activated by radiation from an external source.-A few photons obtained from the pumping source trigger the formation of a cascade of photons of the same energy.

-Pumping can also be accomplished by an electrical current or by an electrical device.-The radiation produced by the lasing action is caused to pass back & forth through the medium numerous times by means of a pair of mirrors:

a)Mirror b)Partially transmitting mirror.

-Repeated passage causes a highly parallel beam.-Additional photons are generated with each passage -Consequence is enormous amplification.

-The repeated passage causes about the escape of non- parallel radiation from the sides of the medium.-A mirror is coated with a thin layer of reflecting material, so that a fraction of the beam is transmitted rather than reflected.-A lasing medium may be composed of many layers of atomic, ionic or molecular nature.

-Mechanism is same for all. Mechanism in details: -Let us consider only two molecules of molecular layer.-Each having two electronic energy levels:

Ground state (Ex) & Excited state(Ey)

-Ey is composed of various vibration levels Ey , Ey’ , Ey’’, Ey’’’ .

Mechanism of Laser action

-Vibration levels are not shown for (Ex).-Atomic or ionic lasers do not have vibrational levels but electronic levels.-Mechanism of laser action can be divided into four steps:

a)Pumping,b)Spontaneous emission(fluorescence),c)Stimulated emission,d)Absorption.

a)Pumping,

-The active species of the laser is excited by means of an

Electrical discharge

Passage of electrical current

Exposure to an intense radiant source

-In a molecular system, several of the higher electronic & vibrational energy levels of the active species are populated(a).

-Staying of the electrons at vibrational level is very brief & relaxes to first excited electronic state.-Some excited electronic states of laser materials enjoy life time considerably longer(often 1ms or more) than their vibrational counter part.-The long-lived state is called ‘Metastable excited state.’

b)Spontaneous emission(fluorescence),-A species in an excited state may lose all or part of its excess energy by spontaneous emission of radiation(fig.b).-Spontaneous process is random . So that, the radiation produced by different excited species, differs in direction & phase-Hence, the result is incoherent monochromatic radiation.

c)Stimulated emission,-It is the basis of laser behaviour(fig.c).-The excited species are struck by photons having the same energy (Ey - Ex) as the photons produced by spontaneous emission.-Collision of this type cause the excited species to relax immediately to the lower energy state.-Relaxation emits a photon of exactly the same energy & travels in the same direction as the photon that stimulated the process.

-Hence, the emission is exactly in phase.-Stimulated emission is totally coherent with the incoming radiation.

d)Absorption-The absorption process competes with the stimulated emission(fig.d).-Two photons with energies exactly equal to (Ey - Ex) are absorbed to produce the metastable excited state.-This step is identical to the last step of fig.a.

Population Inversion and Light Amplification

-To achieve light amplification in a laser, The number of photons produced by the stimulated emission must exceeds the number lost by absorption.

-This condition prevails only when: the number of particles in the higher excited state exceeds the number in the lower.-There must be a state of ‘population inversion’ from the normal distribution of energy state. -Pumping creates this state.-In the figure, nine molecules of the laser medium are in the two states Ey & Ex .-Non-inverted system with three molecules are in excited state & six are in the lower energy level(fig a). -The medium absorbs three of the incoming photons to produce three additional excited molecules.-But relaxes rapidly to ground state with out attaining the steady state population inversion-state .-The incoming radiation may stimulate emission of two photons from the excited molecules.-Hence, incoming beam gives three photons for absorption & gains two photons by stimulated emission(Net loss by one photon.-In fig.b pumping two molecules into higher energy states, creates a population inversion between Ey & Ex.-Six electrons in Ey & three electrons in Ex.-In the inverted system, stimulated emission prevails over absorption to produce a net gain in emitted photons.

-Light amplification, or lasing then occurs.