Stimulated vs. Spontaneous Emission of Radiation: Principles and Applications, Summaries of Physics

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Metastable State

The key to the laser is the presence in many atoms of one or more excited energy levels whose lifetimes may be 10^3 s or more instead of the usual 10^8 s. Such relatively long-lived states are called metastable (temporarily stable) An atom can exist in a metastable energy level for a longer time before radiating than it can in an ordinary energy level. An atom can be excited to a higher level by supplying energy to it. Normally, excited atoms have short life times and release their energy in a matter of 10 ^8 seconds through spontaneous emission. It means atoms do not stay long to be stimulated. As a result, they undergo spontaneous emission and rapidly return to the ground level; thereby population inversion could not be established. In order to do so, the excited atoms are required to ‘wait’ at the upper energy level till a large number of atoms accumulate at that level. In other words, it is necessary that excited state have a longer lifetime. A Meta stable state is such a state. Metastable can be readily obtained in a crystal system containing impurity atoms. These levels lie in the forbidden gap of the host crystal. There could be no population inversion and hence no laser action, if metastable states don’t exist.

Pumping

The process to achieve the population inversion in the medium is called Pumping action. It is essential requirement for producing a laser beam. The methods commonly used for pumping action are:

1. Optical pumping (Excitation by Photons)
2. Electrical discharge method (Excitation by electrons)
3. Direct conversion
4. In elastic atom – atom collision between atoms

1. Optical pumping: When the atoms are exposed to light radiations energy h, atoms in the lower energy state absorb these radiations and they go to the excited state. This method is called Optical pumping. It is used in solid state lasers like ruby laser and Nd-YAG laser. In ruby laser, xenon flash lamp is used as pumping source. 2. Electrical discharge method (Excitation by electrons) In this method, the electrons are produced in an electrical discharge tube. These electrons are accelerated to high velocities by a strong electrical field. These accelerated electrons collide with the gas atoms. By the process, energy from the electrons is transferred to gas atoms. Some atoms gain energy and they go to the excited state. This results in population inversion. This method is called Electrical discharge method. This method of pumping is used in gas lasers like argon and CO 2 Laser 3.Direct Conversion In this method, due to electrical energy applied in direct band gap semiconductor like Ga As, recombination of electrons and holes takes place. During the recombination process, the electrical energy is directly converted into light energy.

Characteristics of Laser Laser light has four unique characteristics that differentiate it from ordinary light: these are;  Coherence  Directionality  Monochromatic  High intensity

1. Coherence

We know that visible light is emitted when excited electrons (electrons in higher energy level) jumped into the lower energy level (ground state). The process of electrons moving from higher energy level to lower energy level or lower energy level to higher energy level is called electron transition. In ordinary light sources (lamp, sodium lamp and torch light), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths. Therefore, photons emitted by an ordinary light source are out of phase. In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have single wavelength or color. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light.

Thus, light generated by laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.

2. Directionality In conventional light sources (lamp, sodium lamp and torchlight), photons will travel in random direction. Therefore, these light sources emit light in all directions. On the other hand, in laser, all photons will travel in same direction. Therefore, laser emits light only in one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading. If an ordinary light travels a distance of 2 km, it spreads to about 2 km in diameter. On the other hand, if a laser light travels a distance of 2 km, it spreads to a diameter less than 2 cm.

Components of Lasers

1. Active Medium: It is the material in which the laser action takes place. The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium / Neon, or semiconductors such as GaAs. This medium decides the wavelength of laser radiation. Active mediums contain atoms which can produce more stimulated emission than spontaneous emission and cause amplification they are called “Active Centers”. 2. Pumping Energy Source (Excitation Mechanism): Energy Source (Excitation mechanisms) pumps the active centers from ground state to excited state to achieve population inversion. The pumping by energy source can be optical, electrical or chemical depending on the active medium.

3. Resonance Cavity: Resonance cavity consists of active medium enclosed between two mirrors one is highly reflective mirror (100% reflective) and the other is partially transmissive mirror (99% reflective).

Different Type of lasers

Ruby Laser

The first working laser was built in 1960 by Maiman, using a ruby crytal and so called the Ruby laser. Ruby belongs to the family of gems consisting of Al2O3 with various types of impurities. For example, pink Ruby contains 0.05% Cr atoms. The schematic diagram of ruby laser can be drawn as: Construction of Ruby Laser The ruby laser consists of a ruby rod. which is made of chromium doped ruby material. At the opposite ends of this rod there are two silver polished mirrors. Whose one is fully polished and other is partially polished. A spring is attached to the rod with fully polished end for adjustment of wave length of the laser light. Around the ruby rod a flash light is kept for the pump input. The whole assembly is kept in the glass tube. Around the neck of the glass tube the R.F source and switching control is designed in order to switch on and off the flash light for desired intervals. Operation of Ruby Laser: When we switch on the circuit the R.F operates. As a result, the flash of light is obtained around the ruby rod this flash causes the electrons within ruby rod to move from lower energy band towards higher/rod. this flash causes the electrons within ruby rod to move from lower energy band towards higher energy band. The population inversion take place at high energy band and electrons starts back to travel towards the lower energy band. During this movement the electron emits the laser light. This emitted light travels between the two mirrors where cross reflection takes place of this light. The stimulated laser light now escapes from partially

Disadvantages of Ruby Laser  In ruby lasers no significant stimulated emission occurs, until at least half of the ground state  electrons have been excited to the Meta stable state.  Efficiency of ruby laser is comparatively low.  Optical cavity of ruby laser is short as compared to other lasers, which may be considered a  disadvantage. Applications of ruby Laser  Due to low output power they are class-I lasers and so may used as toys for children’s.  It can be used in schools, colleges, universities for science programs.  It can be used as decoration piece & artistic display.

Helium - Neon (He-Ne) Laser

Construction: (i) Active medium: It is a gas laser, which consists of a narrow quartz tube filled with a mixture of helium and neon gases in the ratio 10:1 respectively, at low pressure (~0.1 mm of Hg). Ne atoms act as active centres and responsible for the laser action, while He atoms are used to help in the excitation process. The length of the quartz tube is about 50 cm and the diameter is about 1 cm. ii) Optical resonator: To construct the optical resonator cavity, two parallel mirrors are placed at the ends of the quartz tube one of them is partly transparent while the other is fully reflecting. The spacing between the mirrors is adjusted such that it should be equal to the integral multiple of half- wavelengths of the laser light. (iii) Pumping system: The pumping is done through electrical discharge by using electrodes that are connected to a high frequency alternating current source.