I wrote Laser Fundamentals with the idea of simplifying the explanation of how lasers operate. It is designed to be used as a senior-level or first-year graduate student textbook and/or as a reference book. The first draft was written the first time I taught the course “Laser Principles” at the University of Central Florida. Before that, I authored several general laser articles and taught short courses on the subject, giving careful consideration to the sequence in which various topics should be presented. During that period I adjusted the sequence, and I am now convinced that it is the optimal one.

Understanding lasers involves concepts associated with light, viewed either as waves or as photons, and its interaction with matter. I have used the first part of the book to introduce these concepts. Chapters 2 through 6 include fundamental wave properties, such as the solution of the wave equation, polarization, and the interaction of light with dielectric materials, as well as the fundamental quantum properties, including discrete energy levels, emission of radiation, emission broadening (in gases, liquids, and solids), and stimulated emission. The concept of amplification is introduced in Chapter 7, and further properties of laser amplifiers dealing with inversions and pumping are covered in Chapters 8 and 9 [Chapters 8–10 in the Second Edition – Ed.]. Chapter 10 [11] discusses cavity properties associated with both longitudinal and transverse modes, and Chapters 11 and 12 [12 and 13] follow up with Gaussian beams and special laser cavities.

Atom traps

Light forces

It is well known that a light beam carries momentum and that the scattering of light by an object produces a force. This property of light was first demonstrated by Frish [139] through the observation of a very small transverse deflection (3 × 10–5 rad) in a sodium atomic beam exposed to light from a resonance lamp. With the invention of the laser, it became easier to observe effects of this kind because the strength of the force is greatly enhanced by the use of intense and highly directional light fields, as demonstrated by Ashkin [20, 21] with the manipulation of transparent dielectric spheres suspended in water. The results obtained by Frish and Ashkin raised the possibility of using light forces to control the motion of neutral atoms. Although the understanding of light forces acting on atoms was already established by the end of the 1970s, unambiguous demonstration of atom cooling and trapping was not accomplished before the mid 1980s. In this section we discuss some fundamental aspects of light forces and schemes employed to cool and trap neutral atoms.

The light force exerted on an atom can be of two types: a dissipative, spontaneous force and a conservative, dipole force. The spontaneous force arises from the impulse experienced by an atom when it absorbs or emits a quantum of photon momentum.

An exoergic collision converts internal atomic energy to kinetic energy of the colliding species. When there is only one species in the trap (the usual case) this kinetic energy is equally divided between the two partners. If the net gain in kinetic energy exceeds the trapping potential or the ability of the trap to recapture, the atoms escape; and the exoergic collision leads to trap loss.

Of the several trapping possibilities described in Chapter 3, by far the most popular choice for collision studies has been the magneto-optical trap (MOT). A MOT uses spatially dependent resonant scattering to cool and confine atoms. If these atoms also absorb the trapping light at the initial stage of a binary collision and approach each other on an excited molecular potential, then during the time of approach the colliding partners can undergo a fine-structure-changing collision (FCC) or relax to the ground state by spontaneously emitting a photon. In either case electronic energy of the quasimolecule converts to nuclear kinetic energy. If both atoms are in their electronic ground states from the beginning to the end of the collision, only elastic and hyperfine changing collisions (HCC) can take place. Elastic collisions (identical scattering entrance and exit states) are not exoergic but figure importantly in the production of Bose–Einstein condensates (BEC). At the very lowest energies only s-waves contribute to the elastic scattering, and in this regime the collisional interaction is characterized by the scattering length.