Lasers PDF

LASERS: LASER stands for Light Amplification by Stimulated Emission of Radiation. Lasers are invented and developed between 1959 and 1962. In 1917, Einstein first predicted that the atom can emit radiation by the process of stimulated emission in addition to spontaneous emission and the idea of stim...

LASERS: LASER stands for Light Amplification by Stimulated Emission of Radiation. Lasers are invented and developed between 1959 and 1962. In 1917, Einstein first predicted that the atom can emit radiation by the process of stimulated emission in addition to spontaneous emission and the idea of stimulated emission was used to construct laser by Townes and Schawlov in USA. COMPARISON OF ORDINARY BEAM OF LIGHT AND LASER BEAM Sl. No. Ordinary beam Laser beam 1 It is not monochromatic It is monochromatic It is incoherent, i.e. the constituent waves are generally not It is coherent, i.e. the constituent waves are exactly in the 2 in the same phase same phase 3 It does not travel as a concentrated and parallel beam It travels as a concentrated parallel beam 4 It is produced by spontaneous emission It is produced by stimulated beam INTERACTION OF EXTERNAL ENERGY WITH THE ATOMIC ENERGY STATES There are three kinds of interactions of the external energy with the atomic energy states. First is known as absorption, in which suitable amount of energy is absorbed by the atoms of the ground state to get excited to the higher energy states. Second is known as spontaneous emission, in which the excited atoms emit photon to come back in the lower energy state without any external impetus. Third is known as stimulated emission, in which atom in the excited state need not wait for the spontaneous emission to occur, but with the influence of suitable energy impetus, excited atom is triggered to the lower energy state, with the release of appropriate energy. STIMULATED ABSORPTION OR ABSORPTION In any process of absorption or emission, at least two energy states are involved. In order to describe the process of absorption, let us consider an atomic system with energy states – E1 (lower) and E2 (higher). At ordinary temperature most of the atoms will be in the lower energy state. When we allow photons to interact with the system, an atom residing in the lower energy state E1 may absorb the incident photon and jump to the excited state E 2. This process of transition is known as induced or stimulated absorption or simply absorption. Corresponding to each transition made by an atom one photon disappears from the incident beam. Symbolically absorption is represented as Photon + Atom Atom* Fig 1: Illustration of absorption process Usually, the atoms can remain in the excited state for a limited time of the order of known as 10 -8 sec, which is known as lifetime of excited state. SPONTANEOUS EMISSION The excited state of atom with higher energy is highly unstable. When the excited atom in the state E 2 returns to the lower state E1 after the end of its life time in the excited state without the influence of any external impetus due to the tendency to attain minimum potential energy, the excess energy is released as a photon of energy hν= E2 - E1. This type of process in which photon emission occurs without any interaction with external radiation is called spontaneous emission, which is represented as Atom* Atom + Photon Fig 2: Illustration of spontaneous emission process Photon emitted spontaneously will not be in phased and hence light from an ordinary light source is incoherent. STIMULATED EMISSION An atom in the excited state need not wait for spontaneous emission to occur. There exists an alternative mechanism by which an excited atom can make a downward transition even before the end of its life time in the excited state and emit a photon. If a photon having suitable energy (hν= E2 - E1) is allowed to interact with it then the incident photon can trigger the excited atom to make a downward transition releasing the energy in the form of a photon. This process in which the emission is triggered by the external photon is called stimulated of induced emission and symbolically it is represented as Photon + Atom* Atom + 2 Photons Fig 3: Illustration of stimulated emission process The two photons will be moving in the same direction having the same frequency and will be in same phase. Hence light output due to stimulated emission will be completely coherent. In other words, stimulated emission process amplifies the intensity of the radiation (as input is 1hν whereas output is 2hν). DIFFERENCE BETWEEN SPONTANEOUS AND STIMULATED EMISSIONS SPONTANEOUS EMISSION STIMILATED EMISSION It is a natural transition in which an atom is de-excited after the It is an artificial transition which occurs due to de-excitation of an atom end of its life time in the higher energy state before the end of its life time in the higher energy state The probability of spontaneous emission depends only on the The probability of stimulated emission depends on the properties of properties of the two energy states between which the transition the two energy states involved in the transition as well as on the occurs energy density of incident radiation The emitted radiation is incoherent The emitted radiation is coherent The emitted radiation is less intense The emitted radiation is highly intense Less directionality, so more angular spread during propagation High directionality, so less angular spread during propagation Emitted light is not monochromatic Emitted light is nearly monochromatic RADIATIVE AND NON-RADIATIVE TRANSITIONS Transitions between energy states that occur with the absorption or emission of radiation are called radiative transitions. Transitions that occur without the absorption or emission of radiation are called non-radiative transitions. Non-radiative transitions occur mainly because of exchange of energy between the system and its surroundings. They are very common in laser materials. METASTABLE SATES An atom in an excited state remains there for a certain time called the lifetime of that state before making a transition to a lower state. Most of the states have a short lifetime of the order of 10 -8 s. However, some energy states have very long lifetime, which is of the order of 10-6 to 10-3 s. Energy states with such long lifetimes are called metastable states and the metastable state allows accumulation of a large number of excited atoms at that level. It would be impossible to create the state of population inversion without a metastable state. The occurrence of metastable state may be explained as follows- In certain cases, regular selection rules do not permit transitions from a particular state to a lower state. In such cases, the system has to remain in that state for longer time until weak perturbations and stimulating radiations induce transitions to lower state. Existences of metastable state are of fundamental importance in lasers. PRINCIPLE OF LASER We know that the photon emitted by a stimulated emission process and the photon that triggered the emission will be in the same phase and will travel in the same direction. In a system having a large number of atoms, this process can occur many times, giving rise to a substantial amplification of the incident radiation. Lasers are devices that work on this principle of amplification by stimulated emission. If we have a collection of atoms in the excited state, the build up of an intense beam is illustrated in Fig.4. For one photon interacting with an excited atom, there are two photons emerging. The two photons travelling in the same direction interact with two more excited atoms and generate a total of four photons. These four photons in turn stimulate four excited atoms and generate eight photons, and so on. The number of photons builds up in an avalanche like manner as shown in the figure below.  Fig 4: Build up of an intense beam in a laser CHARACTERISTICS OF LASER LIGHT Coherence: The most important characteristics which distinguishes the laser from other sources of light is that it is highly coherent. Coherence means that the waves in a radiation field bear the same phase relationship to each other at all times. Coherence is of two types- Temporal coherence and Spatial coherence. Temporal coherence (also called longitudinal coherence) refers to the coherence of two waves at two different locations along the common direction of propagation of the two waves. Spatial coherence (also called transverse coherence) refers to the phase relationship between waves travelling in a plane perpendicular to the direction of propagation, that is, between waves travelling side-by-side. Monochromaticity: The light from a laser typically comes from one atomic transition with a single precise wavelength. So the laser light has a single spectral colour and is almost the purest monochromatic light available. It means the laser light is not exactly monochromatic, but it has high degree of monochromaticity. The deviation from monochromaticity is due to the Doppler effect of the moving atoms from which the  radiation originates. The light from normal monochromatic sources spread a wavelength range of the order of 100A0 to 1000 A0. But in case laser light the spread is of the order of few angstroms(> kT, e hν/kT -1 will be very large and spontaneous emission far exceeds stimulated emission. At ordinary temperature this happens in the visible region. Stimulated emission becomes important when hν = kT and may dominate when hν E1; N2 < N1, therefore if an electromagnetic radiation is incident on the system at the thermal equilibrium condition, then there is net absorption of radiation. Usually population decreases with the increase of the energy of the state. But it is observed that for emission processes and for laser action, it is essential that the number of excited atoms must be more than the atoms in the ground state. In other words, the number of atoms in higher energy level, E2 must be greater than the number of atoms in the lower energy state, E 1. The process by which this condition is achieved is known as the process of population inversion. From equation (2) it is clear that N 2 > N1 only when T assumes negative value and that is why the state of population inversion is also known as the negative temperature state. Here it should be clearly understood that the negative temperature is not a physical quantity but it is a convenient mathematical expression signifying the non-equilibrium state of the system. Fig.7 Normal population of a system (N 1>N2) Fig.8 Inverted population of a system (N 2>N1) If N1, N2 and N3 be the populations in energy states E1, E2 and E3 respectively such that E1N3. This situation is shown in Fig.7. If the process of stimulated emission dominates over the process of spontaneous emission, then it may possible that N2>N1. If this happens, the state is called the state of population inversion. To achieve population inversion the external energy is supplied to excite the atoms of the system and a system in which population inversion is achieved is called active system. PUMPING METHODS The process by which we can realise and maintain the state of population inversion is known as pumping. In this process, it is necessary that atoms must be continuously promoted from the lower level to the excited state. The pumping energy is to be supplied somehow to the atoms to raise them from the lower level to the excited level and to ensure that population at the excited level is more than that at the lower energy level. Thus in simple words we can say that the processes by which atoms are raised from the lower energy level to the upper energy level are called pumping. The commonly used pumping methods are Optical Pumping: In optical pumping, a light source is used to supply luminous energy. Most often this energy is given in the form of short flashes of light. This technique was first used by Maiman in the Ruby laser and is also widely used in solid-state-lasers. Electrical Discharge: The pumping by electric discharge is preferred in lasing materials whose higher energy levels have a narrow bandwidth e.g. Argon-ion laser. When a potential difference is applied between  cathode and anode in a discharge tube, the electrons emitted from cathode are accelerated towards anode. Some of these electrons collide with atoms of the active medium, ionise the medium and raise it to higher level. This produces the required population inversion. This is also called direct-electron excitation. This procedure is mainly used in gas lasers. Inelastic atom-atom collision: This procedure is suitable when we have two types of atoms in the active medium. An electric discharge raises one type of atom to their excited states. These excited atoms collide inelastically with the second type of atom, energy exchange takes place and the required population inversion is created in the later atoms. An example of this type is He-Ne laser. Direct conversion: A direct conversion of electrical energy into radiant energy occurs in light emitting diodes (LEDs). This method is used in semi conductor lasers. Chemical pumping: In chemical pumping, the energy for pumping is obtained from a chemical reaction. For example when hydrogen combines with fluorine to form hydrogen fluoride, enough heat is generated. H2 + F2 2HF This energy is enough to pump a CO2 laser. Chemical pumping usually applies to materials in the gas phase and it generally requires highly reactive and often explosive gas mixture. Active Medium An active medium is a medium which when excited reaches the state of population inversion and promotes stimulated emission leading to light amplification. Those atom which cause laser action through stimulated emission of light are called active centre.