Introduction and history

The resonant-cavity light-emitting diode (RCLED) is a light-emitting diode that has a lightemitting region inside an optical cavity. The optical cavity has a thickness of typically one-half or one times the wavelength of the light emitted by the LED, i.e. a fraction of a micrometer for devices emitting in the visible or in the infrared. The resonance wavelength of the cavity coincides or is in resonance with the emission wavelength of the light-emitting active region of the LED. Thus the cavity is a resonant cavity. The spontaneous emission properties from a lightemitting region located inside the resonant cavity are enhanced by the resocant-cavity effect. The RCLED is the first practical device making use of spontaneous emission enhancement occurring in microcavities.

The placement of an active region inside a resonant cavity results in multiple improvements of the device characteristics. Firstly, the light intensity emitted from the RCLED along the axis of the cavity, i.e. normal to the semiconductor surface, is higher compared with conventional LEDs. The enhancement factor is typically a factor of 2–10. Secondly, the emission spectrum of the RCLED has a higher spectral purity compared with conventional LEDs. In conventional LEDs, the spectral emission linewidth is determined by the thermal energy kT. However, in RCLEDs, the emission linewidth is determined by the quality factor (Q factor) of the optical cavity.

Modification of spontaneous emission

Radiative transitions, i.e. transitions of electrons from an initial quantum state to a final state and the simultaneous emission of a light quantum, are one of the most fundamental processes in optoelectronic devices. There are two distinct ways by which the emission of a photon can occur, namely by spontaneous and stimulated emission. These two processes were first postulated by Einstein (1917).

Stimulated emission is employed in semiconductor lasers and superluminescent LEDs. It was realized in the 1960s that the stimulated emission mode can be used in semiconductors to drastically change the radiative emission characteristics. The efforts to harness stimulated emission resulted in the first room-temperature operation of semiconductor lasers (Hayashi et al., 1970) and the first demonstration of a superluminescent LED (Hall et al., 1962).

Spontaneous emission implies the notion that the recombination process occurs spontaneously, that is without a means to influence this process. In fact, spontaneous emission has long been believed to be uncontrollable. However, research in microscopic optical resonators, where spatial dimensions are of the order of the wavelength of light, showed the possibility of controlling the spontaneous emission properties of a light-emitting medium. The changes of the emission properties include the spontaneous emission rate, spectral purity, and emission pattern. These changes can be employed to make more efficient, faster, and brighter semiconductor devices.

Introduction

The history of the laser dates back to at least 1951 and an idea of Townes. He wanted to use ammonia molecules to amplify microwave radiation. Townes and two students completed a prototype device in late 1953 and gave it the name maser or microwave amplification by stimulated emission of radiation. In 1958 Townes and Schawlow published results of a study showing that a similar device could be made to amplify light. The device was named a laser which is an acronym for light amplification by stimulated emission of radiation. In principle, a large flux of essentially single-wavelength electromagnetic radiation could be produced by a laser. Independently, Prokhorov and Basov proposed related ideas. The first laser used a rod of ruby and was constructed in 1960 by Maiman.

In late 1962 lasing action in a current-driven GaAs p-n diode maintained at liquid nitrogen temperature (77 K) was reported. Room-temperature operation and other improvements followed.

Soon, telephone companies recognized the potential of such components for use in communication systems. However, it took some time before useful devices and suitable glass-fiber transmission media became available. The first fiber-optic telephone installation was put in place in 1977 and consisted of a 2.4 km-long link under downtown Chicago.

Another type of laser diode suitable for use in data communication applications was inspired by the work of Iga published in 1977. By the late 1990s, these vertical-cavity surface-emitting lasers (VCSELs) had appeared in volume-manufactured commercial products.

The thermal and zero-point motion of electrically charged particles inside materials gives rise to a fluctuating electromagnetic field. Quantum theory tells us that the fluctuating particles can only assume discrete energy states and, as a consequence, the emitted fluctuating radiation takes on the spectral form of blackbody radiation. However, while the familiar blackbody radiation formula is strictly correct at thermal equilibrium, it is only an approximation for non-equilibrium situations. This approximation is reasonable at larger distances from the emitting material (far-field) but it can strongly deviate from the true behavior close to material surfaces (near-field).

Because fluctuations of charge and current in materials lead to dissipation via radiation, no object at finite temperature can be in thermal equilibrium in free space. Equilibrium with the radiation field can only be achieved by confining the radiation to a finite space. However, in most cases the object can be considered to be close to equilibrium and the non-equilibrium behavior can be described by linear response theory. In this regime, the most important theorem is the fluctuation–dissipation theorem. It relates the rate of energy dissipation in a non-equilibrium system to the fluctuations that occur spontaneously at different times in equilibrium systems.

The fluctuation–dissipation theorem is of relevance for the understanding of fluctuating fields near nanoscale objects and optical interactions at nanoscale distances (e.g. van der Waals force). This chapter is intended to provide a detailed derivation of some important aspects in fluctuational electrodynamics.

The scope of this chapter is to discuss optical interactions between nanoscale systems and the properties of the emitted radiation. This is different from Chapter 3 where we considered the focusing and confinement of free propagating radiation. To link the two topics it is also necessary to understand how focused light interacts with nanoscale matter. This is a difficult task since it depends on the particular material properties, the shape of the investigated objects, and also on the strength of interaction. Nevertheless, there are issues that can be discussed from a more or less general point of view.

At the heart of nano-optics are light–matter interactions on the nanometer scale. Optical interactions with nanoscale matter are encountered in various fields of research. For example: the activity of proteins and other macromolecules is followed by optical techniques; optically excited single molecules are used to probe their local environment; and optical interactions with metal nanostructures are actively investigated because of their resonant behavior important for sensing applications. Furthermore, various nanoscale structures are encountered in near-field optics as local light sources.

To rigorously understand light–matter interactions we need to invoke quantum electrodynamics (QED). There are many textbooks that provide a good understanding of optical interactions with atoms or molecules and we especially recommend the books in Refs. [1–3]. Since nanometer-scale structures are often too complex to be solved rigorously by QED we prefer to stick to classical theory and invoke the results of QED in a phenomenological way.