Module 2a - Lasers (Session 1) PDF

https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A Module 2a - LASERs Session 1 : Spontaneous emission, Stimulated emission, Light...

https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A Module 2a - LASERs Session 1 : Spontaneous emission, Stimulated emission, Light Amplification Pre-requisites LASER is Light Amplification by Stimulated Emission of Radiation. Laser as a light source Non-laser light source 1) Quantum mechanical electronic 1) Emission of light is a result of device which produces intense, spontaneous emission of photons by monochromatic and coherent beam thermally excited solids (filament lamps of light as a result of stimulated or electronically excited atoms, ions or emission. molecules (fluorescent lamps). 2) Light is emitted in one direction. 2) Light spreads in all directions. 3) Light is coherent. 3) Light is not coherent. 4) Light is highly intense. 4) Intensity is comparatively less. 5) Light is monochromatic. 5) Light may not be monochromatic. Laser beam is used as a high power electromagnetic beam in Engineering and Biological applications. Basic Properties of LASER 1. Directionality : Lasers emit light in one direction. 2. Divergence : Angular spread for laser is very small. 3. Coherence : Light waves emerging from laser maintain common phase relationship. 4. Intensity : Laser beam is very narrow and hence its intensity is very high 5. Monochromaticity : Laser beam is characterized by only one wavelength. How LASERs are different compared to X-rays? LASERs X-rays 1) LASERs are electromagnetic waves 1) X-rays are electromagnetic waves having wavelength of the order of few having wavelength of few thousand angstroms. angstroms. 2) LASERs are highly coherent. 2) X-rays are not highly coherent. 3) LASERs are obtained due to phenomenon 3) X-rays are given out when high called stimulated emission of radiation. speed electrons strike the target of high atomic number and melting point Prepared by Sanjiv Badhe Page 1 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A Interaction of light with matter - About Light :  Light consists of discrete bundles or chunks (quantum) of energy. Energy of each bundle is “h”. - Max Planck  Albert Einstein provided theoretical justification to this and introduced name photon to this quantum of light energy.  Photon represents minimum energy unit of light.  Each photon carries energy „h‟ where „‟ is frequency of light wave.  Light energy cannot have arbitrary values but must be multiple of „h‟. About Matter :  Electrons in an atom cannot have arbitrary amount of energy, but they take only discrete energies. – Bohr.  Electrons in an atom can have only discrete energy levels which are schematically represented by horizontal lines drawn to the energy scale – Nucleus n=5 n=4 n=3 Energy n=2 n=1 n=2 Ground State n=3 n=1 n=4 n=5 Energies of electrons Ground State of an atom :  The lowest stable state of the atom.  Electrons move in their respective orbits without emitting energy. Excited State of an atom :  If electrons get sufficient energy, they jump to the higher energy level and the atom is said to be excited state. Quantum Transition :  Passing of an atom from one energy state to the other state.  Whenever quantum transition occurs between energy states E1 and E2, energy E2 ~ E1 = h is absorbed or released as a radiation. Prepared by Sanjiv Badhe Page 2 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A ABSORPTION  Consider an atom residing in the energy level E1.  Let E2 be the allowed energy level of this atom.  When a photon of energy h = E2 – E1 is incident on this atom, electrons in atom absorb this energy and the atom is excited to the energy level E2.  This process is called Absorption. E2 E2 E2 h= E2- E1 E1 E1 E1 Before Transition During Transition After Transition Absorption SPONTANEOUS EMISSION :  Excited atom can stay at the excited level for a limited time known as (life-time of that state).  After the life-time of the state gets over, the atom is de-excited and come back to the lower energy level.  During the transition, Excess energy is given in the form of photon of energy h = E2 – E1.  This process is called Spontaneous Emission of Radiation.  It is independent of outside circumstances.  It is probabilistic in nature.  Light spreads in all directions around the source.  Light intensity decreases rapidly with distance from the source.  Light is incoherent. E2 E2 E2 h= E2- E1 E1 E1 E1 Before Transition During Transition After Transition Spontaneous Emission Prepared by Sanjiv Badhe Page 3 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A STIMULATED EMISSION  If a photon can stimulate an atom to move from a lower energy state E1 to the higher energy state E2 by means of absorption, then a photon should also be able to stimulate an atom from the higher energy level E2 to the lower energy level E1 – Albert Einstein.  Consider an atom in the excited energy level E2.  External photon having energy h = E2 – E1 incident on this system, stimulates this atom to jump back to lower energy level E1 before its life time gets over.  During this transition, atom emits a photon with same energy as the energy of incident photon. This is called as stimulated emission. E2 E2 E2 h= E2- E1 h= E2- E1 E1 E1 E1 Before Transition During Transition After Transition Stimulated Emission Features of Stimulated Emission : 1. This process can be controlled from out side. 2. The emitted photon propagates in the same direction as that of the stimulating photon. 3. Emitted photon has same frequency, phase and plane of polarization as that of the incident photon. Light produced is monochromatic and coherent. 4. Light amplification : A h A h A h A h A A h h A h 20 21 22 23 2N Fig. 4.5 Light Amplification 5. High Intensity : Intensity of resultant light is proportional to the square of the umber of atoms emitting that light. Prepared by Sanjiv Badhe Page 4 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A POPULATION INVERSION Let N1 = Number of atoms in the lower energy level. N2 = Number of atoms in the higher energy level.  In Normal equilibrium, the lower energy level is more densely populated than the higher energy level. (N1 >> N2)  Stimulated emission to be effective for light amplification, it should be dominant over the process of absorption.  This is achieved by adjusting N2 >> N1  Getting more number of atoms in higher energy level than lower energy level (N2 >> N1) is called Population Inversion. PUMPING  To achieve population inversion, atoms must be continuously promoted from lower energy level to the higher energy level.  The process by which atoms are raised from the lower energy level to the higher energy level is called Pumping. Pumping Methods : 1. Optical Pumping :  Light energy is used for pumping.  Photons are made incident on the active medium.  E.g. Flash discharge tubes, continuously operating lamps, Spark gaps. 2. Electrical Pumping :  Electric current is passed through the active medium.  Electrons collide with atoms and excite them to higher energy states.  Used in gas lasers. 3. Direct Conversion :  Electrical energy is directly converted into light energy.  Electrical current is passed through active medium but atoms are not excited to higher states.  The current carriers themselves are excited to higher states to achieve population inversion. E.g. Semiconductor Laser. Prepared by Sanjiv Badhe Page 5 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A Active Medium : It is the medium, when excited, reaches the state of population inversion and eventually causes Light Amplification. Metastable State :  Excited states of atom have short life time ( 10 – 9 sec.)  Atoms do not stay at such excited state and fall down due to spontaneous emission after the life-time gets over.  Hence population inversion cannot be achieved.  To achieve population inversion, life-time of the excited state must be higher ( 10 – 3 sec.), so that number of atoms can gather at that state.  Such excited states are called as Metastable States.  If the metastable state does not exist, there could be no population inversion and hence, no stimulated emission and hence no laser operation. Role of resonant cavity :  To produce LASER beam, we must collimate the stimulated emission by properly designing a resonant cavity in which, the light waves can be used over again and again for amplification. Resonant cavity is made up of one fully reflecting mirror and one partially reflecting mirror.  Resonant cavity is useful for enhancing the light amplification.  Resonant cavity is used to get a laser beam in one direction. Laser Beam Partially reflecting Fully reflecting Mirror Mirror Resonant Cavity Prepared by Sanjiv Badhe Page 6 of 7 https://www.youtube.com/c/EngineeringPhysicsbySanjiv LASERs https://youtu.be/wOyVaF-DG4A N3 E3 Spontaneous Emission h E2 N2 Metastable State Stimulated h Emission E1 N1 N2 > N1 Three level laser System E4 N4 Spontaneous Emission E3 N3 Metastable State h Stimulated h Emission E2 N2 N3 > N2 Spontaneous Emission E1 N1 Four level system Prepared by Sanjiv Badhe Page 7 of 7