Dynamics of Light

Abstract

A laser is a coherent light source, which employs induced emission from excited states of materials most effectively. Interactions between the laser light and nonlinear optical materials make it possible not only to generate ultrashort light pulses but also to propagate a soliton wave in optical fibers. On the other hand, superradiance and superfluorescence take out excited state energies of materials as a coherent spontaneous emission when the material system is described as a coherent superposition of dipole moments of atoms over the system. Superradiance and superfluorescence are emitted as intense, short light pulses, the intensities of which are gigantic and proportional to N², the square of the atom number N, and the pulse width is proportional to 1/N. In this chapter these two contrasting light pulses are studied. In Sect. 5.1 we discuss the generation of short light pulses from broadband lasers such as Ti-sapphire and dye lasers, using Q-switching (Sect. 5.1.1) and mode-locking (Sect. 5.1.2) techniques. We also study the mechanism of pulse compression and of soliton wave propagation in optical fibers (Sect. 5.1.3). These phenomena are closely related to the dispersion of group velocity of the light and optical Kerr effects in the medium. Over the late 1980s and early 1990s [41, 42], laser intensities have increased by more than four orders of magnitude to reach enormous intensities of 10²⁰ W/cm². The field strength at these intensities is on the order of a teravolt per centimeter (10¹² V/cm), or a hundred times the Coulombic field binding the ground state electron in the hydrogen atom. A laser interacting with matter – solid, gas, plasma – generates higher-order harmonics of the incident beam as short as the 3 nm wavelength range, energetic ions or electrons with mega-electron-volt (10⁶ eV) energies, giga-Gauss (10K tesla) magnetic fields, and violent accelerations of 10²¹ g (g is Earth’s gravity).