Spontaneous emission from resonant cavities
14.1 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. The changes in spontaneous emission characteristics in resonant cavity (RC) and photonic crystal (PC) structures were reviewed by Joannopoulos (1995).
Microcavity structures have been demonstrated with different active media and different microcavity structures. The first microcavity structure was proposed by Purcell (1946) for emission frequencies in the radio frequency (rf) regime. Small metallic spheres were proposed as the resonator medium. However, no experimental reports followed Purcell’s theoretical publication. In the 1980s and 1990s, several microcavity structures were realized with different types of optically active media. The emission media included organic dyes (De Martini et al., 1987; Suzuki et al., 1991), semiconductors (Yablonovitch et al., 1988; Yokoyama et al., 1990), rare-earth-doped silica (Schubert et al., 1992b; Hunt et al., 1995b), and organic polymers (Nakayama et al., 1993; Dodabalapur et al., 1994). In these publications, clear changes in spontaneous emission were demonstrated including changes in spectral, spatial, and temporal emission characteristics.
At the beginning of the 1990s, current-injection resonant-cavity light-emitting diodes (RCLEDs) were first demonstrated in the GaAs material system (Schubert et al., 1992a) and subsequently in organic light-emitting materials (Nakayama et al., 1993). Both publications reported an emission line narrowing due to the resonant cavities. RCLEDs have many advantageous properties when compared with conventional LEDs, including higher brightness, increased spectral purity, and higher efficiency. For example, the spectral power density in RCLEDs was shown to be enhanced by more than one order of magnitude (Hunt et al., 1992, 1995a).
The changes in optical gain in VCSELs due to the enhancement in spontaneous emission was analyzed by Deppe and Lei (1992). The comparison of a macrocavity, in which the cavity is much longer than the emission wavelength (X << Lcav), with a microcavity (X « Lcav) revealed that the gain can be enhanced by factors of 2-4 for typical GaAs emission linewidths at room temperature (50 nm). Thus laser threshold currents can be lower in microcavity structures due to the higher gain.
It is important to distinguish between emission inside the cavity and emission out of the cavity. The enhancement of the spontaneous emission inside the cavity and emission through one of the mirrors out of the cavity can be very different. At moderate values of the cavity finesse, the spontaneous emission inside and out of the cavity is enhanced. However, for very high finesse cavities (see, for example, Jewell et al., 1988), the overall emission out of the cavity decreases (Schubert et al., 1996). In the limit of very high reflectivity reflectors (R1 = R2 ^ 100%), the emission out of the cavity becomes zero. This effect will be discussed in detail below.
A device in which all the spontaneous emission occurs into a single optical mode has been proposed by Kobayashi et al. (1982, 1985). This device has been termed a zero-threshold laser (Yokoyama, 1992) and a single-mode LED (Yablonovitch, 1994). In a conventional laser, only a small portion of the spontaneous emission couples into a single state of the electromagnetic field controlled by the laser cavity. The rest is lost to free-space modes that radiate out of the side of
the laser. The idea of a thresholdless laser is simple. It assumes a wavelength-size cavity in which only one optical mode exists. Thus spontaneous as well as stimulated emission couples to this optical mode. The thresholdless laser should lack a threshold, i. e. the clear distinction between the spontaneous and the lasing regime which is observed in the light-output versus current characteristic of conventional lasers. Clearly, the prospects of such a device are intriguing. Even though several attempts to demonstrate a thresholdless laser have been reported (Yokoyama et al., 1990; Yokoyama 1992; Numai et al., 1993), a thresholdless laser has not yet been demonstrated.