Resonant-cavity light-emitting diodes
The resonant-cavity light-emitting diode (RCLED) is a light-emitting diode that has a light - emitting 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 light - emitting region located inside the resonant cavity are enhanced by the resonant-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. As a result, the spectral emission width of the RCLED is a factor of 2-5 narrower compared with conventional LEDs. For the same reason, the wavelength shift with temperature is determined by the temperature coefficient of the optical cavity and not by the energy gap of the active material. This results in a significantly higher temperature stability of the RCLED emission wavelength compared with conventional LEDs. Thirdly, the emission far-field pattern of the RCLED is more directed compared with conventional LEDs. In conventional LEDs, the emission pattern is lambertian (i. e. cosine-function-like). In an RCLED, the emission pattern is directed mostly along the optical axis of the cavity.
These characteristics of RCLEDs are desirable for local-area, medium bit rate optical communication systems. LEDs play an important role in local-area (< 5 km) medium bit rate (< 1 Gbit/s) optical communication networks. In particular, plastic optical fibers are increasingly used for optical communication over short distances. The higher emission intensity and the more directed emission pattern afforded by the RCLED increase the power coupled into the optical fiber. As a result, the RCLED can transmit data over longer distances. Furthermore, the higher spectral purity of RCLEDs results in less chromatic dispersion allowing for higher bit rates.
Light-emitting diodes are the transmitter device of choice for medium bit rate optical communication over distances less than 5 km. Compared with lasers, LEDs are less expensive, more reliable, and less temperature sensitive. The RCLED has improved characteristics compared with conventional LEDs while maintaining the inherent advantages of LEDs. The reflectivity of the RCLED reflectors is lower compared with vertical-cavity surface-emitting lasers (VCSELs), thereby allowing for a lower RCLED manufacturing cost compared with VCSELs. At 650 nm, the preferred communication wavelength for plastic optical fibers, VCSELs are difficult to manufacture due to the lack of high-reflectivity reflectors.
RCLEDs are also used for high-brightness applications (Streubel et al., 1998; Wirth et al., 2001, 2002). In these devices, the resonance wavelength is designed to be at the long-wavelength end of the spontaneous emission spectrum of the semiconductor. This ensures that the emission intensity, integrated over all spatial directions, is maximized.
The enhanced spontaneous emission occurring in resonant-cavity structures can be beneficially employed in semiconductor and polymer LEDs. Resonant-cavity light-emitting diodes were first realized in 1992 (Schubert et al., 1992a) in the GaAs material system. About a year later, RCLEDs were demonstrated in organic materials (Nakayama et al., 1993).
Resonant-cavity structures with enhanced spontaneous emission also include Er-doped microcavities (Schubert et al., 1992b). Owing to the inherently narrow luminescence line of intra-atomic Er radiative transitions, there is a very good overlap between the cavity optical mode and the Er luminescence line. At the present time, no Er-doped current-injection devices exist. However, the great potential of Er-doped resonant cavities makes the realization of Er - doped RCLEDs likely in the future.