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This chapter describes the optical properties of planar microcavities in the weak coupling regime, and reviews the emission of light from microcavities in the linear regime. The derivation of the Purcell effect and stimulated emission of radiation by microcavities are presented, and the concept of lasing is introduced. Finally, the nonlinear optical properties of weakly coupled microcavities are considered, and the functionality of vertical cavity surface emitting lasers (VCSELs) is described.

Starting from the absorption of radiation by a classical electron oscillator, Einstein's A and B coefficients for spontaneous and stimulated transitions are introduced. Stimulated transition rates are derived by time-dependent perturbation theory. Relations between the Einstein B coefficients and oscillator strengths are also determined. The atomic frequency response for stimulated transitions is explained.

This chapter begins with a review of the semi-classical model of the interaction of charged particles with light, and then proceeds to the full quantum theory by replacing the classical fields with the corresponding field operators. A presentation of the quantum Maxwell equations and their behavior under parity and time reversal transformations is followed by a discussion of stationary density operators. The notion of positive- and negative-frequency parts of field operators is extended to interacting fields and used to define multi-time correlation functions that describe experimental results. The perturbation expansion is formulated by means of the interaction picture and combined with the dipole approximation to calculate the Einstein A- and B-coefficients. The chapter ends with a discussion of spontaneous emission in a cavity and Raman scattering.

This chapter discusses the reasons why a population inversion is required to obtain optical frequency amplification by stimulated emission. The effects of homogeneous and inhomogeneous broadening are considered. Transient and steady state population inversion are explained. Population inversion mechanisms in representative gas laser systems are also discussed.

This chapter treats fully quantum mechanically the interaction of charged particles with a fluctuating electromagnetic field using time-dependent perturbation theory. It discusses absorption, spontaneous emission, and stimulated emission in atomic physics.

Optical phase conjugation (OPC) is a nonlinear optical technique for the generation of phase conjugate waves (PCWs) and their application. For a coherent and monochromatic light wave (signal beam) with a given wavefront shape, one can generate a new light wave with a reversed wavefront shape (with respect to the propagation direction) via nonlinear optical methods. This newly generated optical wave is termed the PCW of the input signal beam. Three major nonlinear optical methods can be employed to generate various PCWs: nonlinear four- or three-wave mixing, backward stimulated scattering, and backward stimulated emission (lasing). PCWs can overcome disturbing and aberration influences from the propagation medium. Therefore, this technique is especially useful in high brightness laser oscillator/amplifier systems, laser aiming and targeting systems, and long-distance and high bit-rate optical fiber communication systems. It also provides an effective approach to achieve lasing or stimulated scattering from a randomly disturbed gain medium.

This chapter describes basic optical processes in semiconductor crystals, including interband optical absorption, gain, and emission. The consideration is performed for the two types of crystal structures, zinc-blende and wurtzite, in the framework of a semi-classical approach when the electromagnetic field is treated classically while the electrons are described by the quantum mechanical Hamiltonian and wave functions. Einstein coefficients are introduced in order to define the connection between absorption, stimulated emission, and spontaneous emission. Optical selection rules are obtained in the framework of the k⋅p theory by calculating the interband momentum matrix elements. The concepts of Wannier–Mott excitons and exciton polaritons are discussed in the framework of the effective mass approximation for the case of the direct band-gap semiconductors. The chapter establishes the symmetry classification of excitonic states in the semiconductor crystals in terms of the theory of irreducible group representations.

The quantum mechanical description of the interaction of light with atoms treats absorption, gain, and recombination as upward and downward induced transitions of electrons between two states by perturbation due to the optical electric field. This provides the basis for a quantitative theory of diode lasers. The chapter begins with Einstein’s description of absorption, stimulated emission, and spontaneous emission, setting the scene for calculation of the transition probability by solving the time-dependent Schrödinger equation for a coherent superposition of the wavefunctions of two states. The resulting absorption spectrum has a Lorentzian lineshape due to dephasing of these wavefunctions. Expressions for the optical absorption cross section and Einstein coefficients are derived in terms of the dipole matrix element, and the dipole is illustrated using a hypothetical one-dimensional atom. Fermi’s Golden Rule for transitions to a continuum of states (as in quantum wells) is introduced in terms of the momentum matrix element.

Field or second quantization is carried through for electromagnetism, giving creation and annihilation operators for photons. Vacuum energy arises from field fluctuations, which causes the Casimir force and the Lamb shift of spectral lines. The connection between absorption, spontaneous emission and the stimulated emission of radiation is shown to emerge naturally. This yields Einstein’s equations for radiation in thermal equilibrium. The prerequisites for lasing, the operation and the properties of lasers are described. Fully coherent (Laser) states are expressed in terms of Fock states. The first and second order coherence of lasers and thermal sources are worked out. The Hanbury Brown and Twiss experiment is described and the application of the principle to determining stellar sizes and interaction regions in particle collisions from meson correlations are described.

This chapter provides an introduction to optical gain and to the requirements for a semiconductor structure that enables this gain to be translated into laser action. The basis of laser action is the coherent amplification of light by stimulated emission, made possible when the population of states is inverted. Although such a system is far from thermal equilibrium, the energy distributions of carriers in a semiconductor laser are in quasi-equilibrium at room temperature described by Fermi functions. The amplification process is quantified by the gain coefficient and the gain spectrum. The gain is positive when the separation of the quasi-Fermi energies for electrons and holes exceeds the photon energy; the material is transparent where the photon energy is equal to the quasi-Fermi level separation. The chapter ends with the key requirements for a laser diode structure.

This chapter begins by identifying various kinds of imaging techniques. It then examines the role played by the amount of light and presence of fluorophore in the imaging process. the latter part of the chapter identifies the features of confocal microscopy and stimulated emission depletion (STED) microscopy. Sample problems are also provided at the end of the chapter.

In 1968, Drs Pravin M. Shah and Raymond Gramiak at the University of Rochester, New York, were conducting a study with the ultimate goal to investigate whether heart stroke volume could be estimated from the extent and duration of cusp separation of the aortic valve, as measured with M-mode ultrasound. Simultaneously, as the reference, they also measured cardiac output with the indicator dilution technique. Here, a bolus of a dye (indocyanine green) is injected and blood is sampled downstream to determine the rate at which the indicator has been transported from the injection site. In Dr Shah’s own account of the experiments, he explains that the routine at his university then was to place a catheter in the left atrium with the trans-septal technique, i.e. inserting the catheter in a vein and penetrating into the left atrium via the right atrium. During the injections of the dye, somewhat to their surprise, they observed a striking echo enhancement across the aorta. The enhancement also appeared when saline and dextrose in water was flushed through the catheter. Dr Gramiak reminded himself of a comment from Dr Claude Joyner, that a temporary echo-enhancement could be observed during saline injections, and they speculated that miniature bubbles produced by gaseous cavitation upon rapid injection of the fluid gave rise to the enhancement, and raised the idea that this could be used as a contrast agent. An in vitro study by Frederick Kremkau provided strong evidence that gas bubbles were actually responsible for the echo enhancement. It is interesting to note how discoveries are made independently around the world, when the time is ripe. At the same time in Lund, Drs Inge Edler and Kjell Lindström performed studies to measure blood flow in the heart. At this point no ultrasound Doppler signals had been recorded from the inside of the heart, and they used a calf heart in an in vitro model to verify that signals could be obtained when water and blood was led through the model.