Laser Physics and Spectroscopy

Pradip Narayan Ghosh

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781138588271
• eBook ISBN: 9780429960765
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Electrical Engineering & Telecommunications

In this book emphasis is laid on laser including its operation, different types, properties like coherence and monochromaticity, beam propagation, theoretical treatment of atom?field interaction, semi?classical laser theory, non?linear effects, quantum properties, photon concept and coherent states etc.

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Chapter 1

Introduction to Laser

Laser is an intense monochromatic source of electromagnetic radiation. It can generate coherent beam of photons having widely varying frequencies and intensities. LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. A laser can emit radiation over a large range of power. It can be very small having the size of a micrometer; it can also be as large as the size of a football ground. Laser can emit the light beam continuously in time and it can also operate with pulses of extremely short duration.

1.1 Historical background

The idea of MASER (M stands for microwave), predecessor of laser, was first conceived by C H Townes in 1954. Townes was sitting on a bench in Franklin Park in Washington DC in a mood of contemplation. He was toying with the idea that Einstein’s theory of stimulated emission proposed in 1917 might lead to a multiplication of photon number. He visualized that if the process could be effectively continued or sustained there might be an enormous amplification of photon number and hence the radiation intensity. He scribbled something on the back of a used envelope and went back to the laboratory. In a stimulated process as proposed by Einstein an incident photon, incident on an atom with energy level difference that resonates with the frequency v of the radiation beam, induces or stimulates an atom in the upper energy level to come down to the lower energy level. In this process the atom gives up its additional energy, ℏω = E₂ – E₁, by emitting a photon that will have exactly the same energy as that of the incident radiation ℏν. Additionally, the phase of the emitted photon will be the same as that of the incident photon thus leading to an efficient multiplication of the energy. The process needs the atoms to be in the upper or excited state and it needs an arrangement so that the process is sustained. These were the basic challenges to Townes and with the help of his students he could design a device that produced amplification of microwave radiation in ammonia molecule. This was the invention of MASER. The idea of light amplification was also conceived by the Russian scientists Basov and Prochorov almost at the same time. Subsequently Townes and Schawlow showed that the same principle could be extended to electromagnetic radiation in the optical region and hence LASER became a plausible source of radiation. The first laser source was developed in 1960.

1.2 Spontaneous and Stimulated Emission and Absorption

An atom has a set of quantized energy levels. It interacts with electromagnetic radiation in a process of energy exchange. The lowest energy state of the atom is the ground state. Thermal distribution of atoms in different energy levels follows the Boltzmann thermal distribution law

$N=N_0{e}^{-E/k_{B}T}$

(1.2.1)

It describes the number N of atoms with energy E at a temperature T, where N₀ is the number of atoms in the ground state and k_B is the Boltzmann constant. Hence the atoms prefer to occupy the lower energy states. An atom in an excited state will have a tendency to move downwards to an available lower energy level and the extra energy it possessed will be emitted as a photon (Fig. 1.1(a...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Chapter 1 Introduction to Laser
• Chapter 2 Semi-classical Theory of Atom-Field Interaction
• Chapter 3 Density Matrix Equations
• Chapter 4 Saturation Absorption Spectroscopy
• Chapter 5 Semi classical Theory of Laser Action
• Chapter 6 Quantum Theory of Radiation
• Chapter 7 Quantum Theory of Atom Field Interaction
• Chapter 8 Doppler-free spectroscopy with laser
• Chapter 9 Laser Cooling and Bose Einstein Condensation
• Chapter 10 Lasers in Quantum Information Science