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Laser - @ktm network chat, Jokes, SMSs, Free classified ads 激光- @ KTM网络聊天，开玩笑，短信，免费分类广告 9. LASER Full form: Light Amplification by Stimulated Emission of Radiation. Laser: The device, which is used to produce laser light, is called Laser (torch). *Properties of laser light: [Difference between the Laser and Ordinary light] i) It is highly coherent. All the light waves in it are in same phase. ii) It is highly monochromatic. The light beam consists of same colour of light (same frequency i.e. same energy) iii) Intensity is higher than the ordinary light. All the waves are in same phase so, amplitude will be 28 higher, A = n a. So, intensity [, , A] is also higher. {Ordinary light , 10photons/sec. In laser 18 light 10photons/sec.} iv) All the rays are unidirectional, highly focused and non-diverging. So, it can travel a very long distance (Earth to moon, moon to earth) with out diverging same polarization. v)The principal of laser light production is stimulated emission but not the spontaneous. So, it cannot be produced ordinary light source. Some terms: A) Ground state:(valance state or shell): An atom or, a molecule is said to be in ground state if no energy levels above valance state are set up. In an atom, if all the electrons are contained within valance shell, then the atom is said to be in ground state. B) Excited state: In an atom or, a molecule if some energy levels are set up above the ground state, then the atom or, molecule is said to be in excited state. An atom or, a molecule is said to be in excited state if electrons are the in energy levels above the ground state. C) Excitation of an atom: The process of raising an atom from ground state to one of the excited state is called excitation of the atom. During excitation of an atom, electrons in ground state are raised to upper state. We know the lower energy states are those associated with lesser energy and the higher energy states are those associated with more energy. To raise an electron from lower energy state to any of the higher energy state, the electron is to be given some energy equal to the difference in energies of the two levels considered. If E, be energy associated with an energy level (1) and E that associated with energy level (2), 2 then energy required to raise the electron from E, level to E level is ,E = E – E 1221 Fig. 1. a. Ground state b. Excited state. Excitation of an atom , rising of an electron from lower to higher energy state. * Process of excitation of an atom An atom gets excited when it absorbs an energy equals to the difference in energies of the two levels. i) Collision During collision of an atom by other particle (ion or, electron) mutual transfer energy takes place and the atom gets excited. When, the accelerated particle moving nearby atom has energy sufficient to excite that atom only then it collides with the atom and gives off energy to the atom there by pick up the atom to excited state. 1 ii) Absorption of photon: The energy of a photon is E = h, where , is the frequency of the radiation which the photon associates to. When a photon of energy equal to the difference in two level of an atom incidents on that atom, the photon is absorbed by the atom and gets excited to higher energy level. , E were two energy levels in an atom, then the amount of energy that a photon should If E12 have so that it can excite the atom is E – E= h, 21 Fig. 2. a. Ground state b. Excited state. , Excitation energy = Energy difference between two levels. D) Emission of radiation: When an atom in higher energy level comes down to lower energy level, it gives out the energy equal to the difference in energies between the two levels considered, in the form of radiation of certain frequency. This amount of energy is called quanta and the particle or, packet carrying this amount of energy is called photon. Thus the photon emitted when an excited atom comes down from energy level E to E 21 (Electron jumps from E level to E level) is EEh,, Where,, is frequency of radiation (light) 212 - 1 = emitted? Fig 3. a. Excitation of an atom b. Excited state c. Emission of Radiation. ,Energy emitted = Energy given out by an electron during its transition from higher to lower energy state. ,Energy required to excite an atom from E to E = Energy emitted during transition from E to E. 1221 *Types of Emission: 1) Spontaneous emission: The emission of radiation, which takes place in absence of any external agents, is called spontaneous emission. The ground is most stable state. When an atom is picked up to an excited state, it becomes unstable and tends quickly to return to the ground state. Normally, the atom has lifetime in excited -7state equal to 10s. When the time is over it itself comes down to ground state emitting photon of energy E –E= h,. It is random emission and the photons are not having same phase, same direction 21 or, characteristics of photons are arbitrary. ii) Stimulated emission: The emission of radiation, which takes place after triggering or, stimulating or, inducing an excited atom by an external agent, is called stimulated emission. Consider an atom in excited state E2 which has absorbed the photon of energy h, = E –E 21 When the next photon of energy h, = E –Eis incident on the atom, it triggers the atom to 21 decay to the ground state E. Here, two photons one-inducing photon and the next produced due to 1 transition, will be emitted. The photons emitted have same phase, same frequency, same energy, and same direction as the inducing photon has. It means the secondary photons are identical with primary photon. 2 Fig. 4. a. Ground state b. Excitetation of atom c. Excited state d. Stimulated Emission. As the emission starts after inducing it, it can be controlled. So, it is controlled emission and hence not random in nature. Moreover, the triggering photon acts as an inducing agent and by resonance action it stimulates the excited atom to come down to ground state emitting photon exactly similar to inducing photon. E) Metastable state: -7 Generally, the life time of an atom in excited state is very short i.e. 10 s. i.e. an electron when -7raised to an excited state remains there for a very short period of time 10 s. -4-3 Metastable state is that state in which the atom has longer lifetime i.e. (10 – 10) sec, i.e. an electron when put in this state remains for that period of time. None state will be metastable state initially but an energy level can be designed as a metastable state. It is that level from which the probability of transitions of an electron to other level is less and electron remains there for a longer period of time. ,Metastable state is that in which population inversion is established. ,For, the stimulated emission to make possible, metastable state is to be set up. ,Lasing action is possible only when metastable state is set up. And stimulating mechanism is built up. F) Population Inversion The medium, which is used to make a laser tourch, is called active medium. The atoms, ions or, molecules of the medium, which take part in lasing action, are called active centers. Generally the number of atoms (active centers) in ground state will be more than number of atoms in excited state. If in an assembly of atoms, the number of atoms in excited state is more than that in the ground state, the condition achieved is called population inversion. Let N ,N be the number of atoms in ground state 1 and excited state 2 respectively at any 12 instant of time, then i) if N > N , normal condition. 12 ii) if N > N , Population inversion. 21 Fig. 6. a. Normal condition b. Population inversion = Population inversion is the heart of lasing action. G) Lasing action or, principle of laser: Consider an active medium. Let, E be the metastable state and E the ground state. Let’s pump 21 the medium sop as to establish the population inversion at the energy level Esuch that N > N where, 221 N,no. of atoms in ground state E and 1 1 N ,no. of atoms in excited state E 22 Fig. 7 a. Atom in excited state b. Inducing the atom c. Stimulated emission 3 After some time, when the life time of one of the atoms in excited state is over, it comes to the EE,21. This is just spontaneous emission. This ground state emitting photon of frequency , = h photon, now trigger the other atom to come down to the ground state. The photon emitted in this transition will be similar in all aspect to the primary (triggering) photon. As a result two photons will be produced. These photons also trigger the other atoms and 4 photons will be produced. The process ncontinuous the photons remain within the active medium and number of photons increases as 2. As a result, the beam of non-diverging, highly monochromatic, highly intense light will be produced which is called laser light. H) Pumping: The process of achieving the population inversion is called pumping. Depending on the type of active medium used different process is there to achieve population inversion such as: i) Optical pumping , a flash of light is used to excite atoms, ii) Electrical pumping,an electric pulse is used to excite atoms, iii) Chemical pumping ,atoms are excited by chemical treatment. iv) Thermal pumping , atoms are excited by heating process *Pumping schemes: For the lasing action, there must at least one metastable state which is called upper lasing level. The scheme in which two energy levels are considered is called two levels scheme. Which cannot be used for lasing action. Three or four levels pumping schemes are possibly used. They contain following levels. 0 , Ground level 1 , The lower lasing level 2 , The upper lasing level 3 , The pumping level i. Three levels pumping scheme: In this scheme, there are 3 levels, out of which two levels are lasing levels upper and lower. Fig. 8 (a)Case 1. (b)Case 2. Two cases can be considered in this scheme. ndCase. 1. Fig. (a) The level-3, the upper most level is pumping level, 2 level is the upper lasing level and the lower lasing level and ground level coincide. Case. 2. Fig. (b) The pumping level–3 and upper lasing level coincide, level –2 is lower lasing level, which is slightly above the ground level. st* Lasing action in 1 case: The atoms are raised to the pumping level –3, but it is unstable and the atom transit to upper lasing level –2. The level is metastable state and the atoms have longer lifetime. Due to the continuous process of excitation, the number of atoms in this level will be more than that in ground level. Thus population inversion is achieved. After certain time, when the lifetime of any one of the atoms in this level is over, it undergoes spontaneous emission. The photon thus emitted triggers the other atoms. Thus it initiates the lasing action. 4 nd* Lasing action in 2 case: In this case, the pumping level is designed as the metastable state. So, the atoms pumped to the pumping level will be in metastable state. The lower lasing level lies slightly above the ground level. The atoms in lower lasing level transit to the ground level soon. Due to the continuous process of pumping, population inversion is easily achieved. The same process described above will be responsible for the lasing action. stnd Here, in both of the case i.e., in 1 case transition from level 3 to level 2 and in 2 case transition from level 1 to level ‘0’ are non- radiative. The energy released is dissipative and imparted to the lattice vibration. ii. Four levels pumping scheme: In this scheme, there are 4 distinct levels. There are two auxiliary i.e. non-radiative transitions. Which are from pumping level (3) to upper lasing level (2) and from lower lasing level (1) to ground level (0). Level (2) is designed to be metastable state. Due to continuous process of excitation, the population inversion will be achieved on the upper lasing level (2). The same process described above is responsible for the lasing action. Fig. 9. I) Resonance Cavity: To intensify the light, the active medium is placed in between the two mirrors M1, M2 where one M1 is perfectly (96%) reflecting and the next M2 is partially reflecting (50%). The cavity between the mirrors is called resonance cavity. The sides of cavity are open. Due to the presence of two mirrors multiple reflections of light rays takes place i.e. light moves in both directions and in each of its course trigger the atoms in metastable state. Thus, this helps to intensify the light. The distance between the two mirrors is chosen so that it is integral multiple of wavelength of light. Due to this the light always resonates with the cavity, standing waves are produced that causes the continuous emission of radiation. The sides are open. Due to this diverging rays, if present are removed. The output is taken from the partially reflecting mirror. Fig. 10 Types of laser: a) Solid state laser b) Liquid laser c) Gas laser d) Semiconductor laser a) Solid state laser (Ruby laser): A ruby is the transparent crystal of Al Odoped with approximately 0.05% of chromium ions 23 3+Cr in the form of CrO. 2 33+ Here, the aluminum oxide AlO acts as an active medium and the chromium ion cris 23 responsible for lasing action. So it is active center. Ruby crystal of length 4cm and 0.1cm in diameter is prepared and placed in between two mirrors M perfectly reflecting (96%) and the next M partially reflecting (50%) 12 The pumping process used here is optical pumping, which is carried out with the use of a helical xenon flash tube placed around the ruby crystal. 5 Fig. 11 Action of Ruby Laser: The electric discharge in Xenon flash tube produces a flash of intense light. The light is 3+ ions and they are picked up to pumping level E (Energy band) The atoms transit absorbed by Cr3 quickly non-radiating to the two metastable states E, E in which the lifetime of atoms is longer. Due 12 to the continuous process of excitation, the population inversion is established in these levels. In fact the two levels E, Eare two upper lasing levels and the lower lasing level coincide 1 2 with ground state. So, the pumping scheme established here is 3-levels pumping scheme. When the lifetime of one of the atoms is over, it transits to the ground state emitting the radiation of frequency EEEE,2,010 , = (Transition from E to E , = (transition from E to E) 120)210hh The photon thus emitted triggers the other atoms to transit to the ground state. In this process, these atoms also produce photons of similar properties. The process continues. As a result, an intense beam of light will be coming out of the crystal through partial reflecting mirror. Since thee are two distinct metastable states, the laser light produced from ruby laser consists of two wavelengths. and 6943A:6920A: b) Gas laser (He-Ne-laser): He-Ne-laser is an example of gas laser, which consists of a glass tube containing a mixture of Helium , Neon gases in the ratio of 10:1 at a low pressure [1 torr]. The tube is kept in the space between two mirrors, M perfectly reflecting (96%) next M, partially reflecting (50%) creating a 12 resonance cavity. The pumping process used in this case is electrical pumping which is carried out by electric discharge through the mixture of the gas with two electrodes provided. Fig. 12. a. He-Ne Laser. b. Energy Diagram for He and Ne atoms. Action of He-Ne-Laser: Here, the mixture of He and Ne-gas is active medium and the Ne-atoms are active centers. He-atoms play an important role in excitation of Ne-atoms, it is also called assistant pumping agent. Since excitation potential of He-atom is less than Ne-atom, He-atoms are easier to excite than Ne-atoms. So, the He- atoms are excited first and then Ne-atoms. ’ During excitation process, He-atoms get excited to one of the state E’,E. While Ne-atoms 12 remain in ground state. The excited He-atoms collide and give energy to Ne-atoms and pull them up to excited states E ,E. This process of collision and energy transfer is called resonant collision energy 46 transfer process. This is possible to occur here because the energy of levels E and E in Ne atoms is 46’nearly equal to E of E’ of He-atoms. 12 The levels E4, E. in Ne-atoms are metastable states and called upper lasing levels where as the 5 levels E, E are called lower lasing levels. In the continuous process of excitation, the number of Ne 3 4 6 atoms in the upper lasing levels will be more that in lower energy level and the population inversion is established. When the lifetime of one of the atoms in upper lasing level is over, it comes down to lower lasing level and emits a photon of certain frequency. The photon thus emitted triggers the other atom to transit to the lower lasing level. The atom emits the photon exactly similar to the triggering photon. The process continues. As a result an intense beam of laser light is produced. There are three possible transitions from upper to lower lasing levels as ,E ,3.39,m [ invisible] 1) E54 2) E ,E,1.15,m [ invisible] 32 3) E ,E ,0.6328,m (6328 A:) [ Red] 52 So, the laser light may contain 3 wavelengths. Out of these the wavelength 6328 A:only lies in visible range, which is red in colour. The laser is designed to produce this colour of light. Uses of laser light 1) Industrial use: Due to its high intensity and non-diverging property, it can be focused to a small area of diameter of the order of 100,m. This helps to produce high temperature enough to melt and vaporize the metals and non-metals. This property can be used in a) Welding b) cutting c) drilling and preparing holes d) Heat treatment 2) Medical use: Due to the production of localized heat, laser light can cut and weld blood vessels. Highly focused laser beam is used in blood less surgery. The operation-using beam of laser is painless, because it takes very short time. Laser is used in ophthalmology, therapy and stomatology. Laser beam can be used for welding of detached retina in the eyeball. He-Ne laser has produced curing effects on tropic ulcers, poorly healing wounds and bone fractures. 3) Laser in communication: Laser light are used in radios and T.V because being coherent, these can be modulated to send hundreds of messages simultaneously. Also, they are not absorbed by water; they are useful in water communication between submarines. It is used in optical fiber communication process. 4)Photography and holography: In high speed photography, lasers are useful because these can detect fast moving bullets and missiles. Holography i.e. three dimensional photography is possible by the use of laser light. 5) In scientific research: Laser lights are used in the study of chemical and crystalline structures of various molecules. In Raman spectroscopy and various others related phenomena, these are extensively used. 6) Laser in space: Due to its non-diverging property, by the use of laser beam telescope, we can study the moon’s surface. 7) In Engineering: Laser light can be used to detect the defects on the products of heavy equipments like in aeroplane, ships. Engineers use laser light in surveying for critical alignment. 8) Laser in cooling: -9 The lowest attainable temperature is obtained by using laser, which is equal to 10 K 7