WELL COME Lecture On LASER B Sc III

WELL - COME

Lecture On LASER B. Sc. III, Paper XIX ( VI Semester) Presented By Dr. Mrs. Maheshmalkar Pradnya Ramdas Assi. Professor, Dept. of Physics

Light Amplification by Stimulated Emission of Radiation (LASER). (device that generates an intense beam of coherent light, or other electromagnetic radiations, in one direction)

In 1958, Charles Townes and Arthur Schawlow theorized about a visible laser, an invention that would use infrared and/or visible spectrum light.

Characteristics of Laser Light 1. Coherence. Laser beam is perfectly coherent in nature. The emitted photon after getting triggered is exactly in phase with the incident photon. 2. Monochromaticity. Laser light consists of essentially single colour or single wavelength.

3. Extreme brightness. Laser light is much more brighter and intense than that from any of the conventional (usual) source. This is due to emitted and incident photons are perfectly in phase. 4. Strong unidirectionality (Collimated). The laser emits light in one direction while a conventional light source emits light in all directions. The width of laser beam is extremely narrow. Hence a laser beam can travel to long distances without spreading. They can be brought to an extremely sharp focus.

Quantum Properties of Light Quantum processes dominate (control) the fields of atomic and molecular physics. The treatment here is limited to a revise of the characteristics of absorption, emission, and stimulated emission which are essential to an understanding of lasers and their applications. Atomic transitions which emit or absorb visible light are generally electronic transitions, which can be pictured in terms of electron jumps between quantized atomic energy levels.

Production of Laser

The interaction of light with matter There are three types of possible interactions between a system of atoms and light that are of interest: Absorption. If light (photons) of frequency ν 12 pass through the group of atoms, there is a possibility of the light being absorbed by atoms which are in the ground state, which will cause them to be excited to the higher energy state. The probability of absorption is proportional to the radiation intensity of the light, and also to the number of atoms currently in the ground

Spontaneous emission. The process in which an excited atom emits a photon all by itself and without any external impetus is known as spontaneous emission. An excited atom can stay at the excited level for an average lifetime. It is not stimulated by any other agent during it’s lifetime. After life time the excited atom undergoes a transition to the lower energy state on it’s own, by emitting a photon of energy E= hn, is called as spontaneous emission.

Spontaneous emission

Spontaneous(automatic) emission

Stimulated emission. The process of emission of photon in which an excited atom which is already in the excited state, it may be perturbed (disturbed)by an external agent (the passage of a photon). A forced transition occurring under the influence of an external agent is known as induced or stimulated emission. In this case, the excited atom relaxes to the ground state, and is induced to produce a second photon.

Terms related to Laser Lifetime The time during which the particle can exists in the excited state for a limited time. Then it come back to the ground,

population inversion The concentration of active atoms occupying an energy state is known as population inversion. It occurs when a system (such as a group of atoms or molecules) exists in a state with more members in an excited state than in lower energy states. The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser.

population inversion Let N 1 and N 2 are the populations of lower energy state E 1 and higher energy state E 2 respectively. In normal or equilibrium state the lower energy level is densely populated than the higher energy level i. e. N 1 >>N 2. Two produce stimulated emission , accumulation (growth) of excited atoms at the excited state( i. e. population of higher excited state) must be greater than that of lower energy state. Hence making N 2 >> N 1 is called as N 1 population inversion.

Optical pumping : It is the process of raising of atoms from lower energy level to higher energy level through the process of a absorption of light. This process does not necessarily always directly involve light absorption; other methods of exciting the laser medium, such as electrical discharge or chemical reactions or by operating flash tube or by other some techniques, may be used.

Metastable state: It is the state where the atoms remains excited for longer time than the normal state. This state allows concentration of large number of atoms at the excited energy state. This state plays an important role in the laser operation.

Energy level diagram • The possible energies which electrons in the atom can have is depicted in an energy level diagram.

Laser finds many applications in Science, industry, Medicine, Communication, Military, Information technology for ……. . • Eye surgery, Cancer treatment • Holography • Spectro-chemistry • Optical imaging • Semiconductor processing, • Printing, copying, scanning • welding , cutting, drilling

Doctor removes a brain tumor by aiming a laser beam at the tumor. The exposed brain is visible on surrounding monitor screens.

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