Volume 91, Issue 5, 01 March 2002
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
Experimental determination of the internal quantum efficiency of AlGaInP microcavity light-emitting diodes91(2002); http://dx.doi.org/10.1063/1.1433938View Description Hide Description
Detailed study of external quantum efficiency is reported for AlGaInP-based microcavitylight-emitting diodes (MCLEDs). Unlike conventional light-emitting diodes(LEDs) the extraction efficiency and far field profile depend on the linewidth of the intrinsic spontaneous emission and wavelength detuning between cavity mode and peak electroluminescence. This dependence makes it difficult to estimate the intrinsic spectrum, hence the performances of MCLEDs. By using a nondestructive deconvolution technique, the intrinsic spectra of a MCLED and a reference LED (with the same active regions) could be determined at different current densities. This allowed precise calculation of for both devices (values close to 11% were found for the MCLED), and hence of their apparent internal quantum efficiencies At 55 A/cm2, values of 90% and 40% were determined for the LED and MCLED, respectively. In order to explain this difference, we measured for devices with different sizes. From a fitting procedure based on a simple model taking into account the device size, we found that the radiative efficiencies of LEDs and MCLEDs were close to 90%. We concluded that the low of MCLED was due to a bad current injection, and especially to electron leakage current, as confirmed by numerical simulations.
91(2002); http://dx.doi.org/10.1063/1.1433188View Description Hide Description
In recent years, it has been shown that one-dimensional photonic crystals such as superlattices can exhibit an omnidirectional reflection of light for a given frequency range. However, this property requires that the incident wave be launched from vacuum or from a low refractive indexmaterial. In this article, we show that the limitation about the choice of the substrate can be removed by adding a clad layer of low refractive index at one boundary of the superlattice. The additional layer acts like a barrier for the propagation of light. An alternative solution based on a combination of two multilayerstructures is also mentioned.
Coupling of defect pairs and generation of dynamical band gaps in the impurity bands of nonlinear photonic crystals for all-optical switching91(2002); http://dx.doi.org/10.1063/1.1446225View Description Hide Description
We investigate the coupling of two single defects in two-dimensional photonic crystals (PCs) with the same frequency but different field distributions. The defect pair like this is generally present in PCs as a combination of a reduced-size defect and an increased-size defect. In spite of the significant difference in field distribution, quasiflat impurity bands suitable for the transmission of ultrashort pulses can be achieved by properly choosing defect pairs. More importantly, the coupled cavity waveguide constructed with defect pairs offers an opportunity to establish a periodic modulation of defect modes with a control light. The dynamical band gap generated by the periodic modulation of defect modes suggests a high-efficiency all-optical switching operation in nonlinear PCs.
91(2002); http://dx.doi.org/10.1063/1.1447329View Description Hide Description
The transient diffraction response of a low-temperature-grown (LTG) photorefractive multiple quantum well(MQW)spatial light modulator (SLM) is studied experimentally as a function of the frequency and the on/off ratio of a single-sided applied voltage when a grating is written using a continuous-wave sinusoidal illumination. LTG MQW modulators have high spatial resolution, but their diffraction efficiency and speed are reduced. By varying the on/off ratio of the applied voltage, it is shown that the frequency response of the SLM can be optimized while maintaining its high spatial resolution. The flatness of the frequency response with respect to the peak diffracted signal is thus shown to increase by an order of magnitude. By temporally gating the diffracted signal, low and bandpass filtering capabilities of the SLM are experimentally demonstrated for applications such as speckle elimination and novelty filtering. When the diffracted signal is measured over an adjustable time window, it is shown that high frame rate operations in integrated optoelectronic systems can be achieved. A numerical model that includes the effect of high trap densities on the space-charge dynamics is developed. It is shown that the space-charge field grows nonuniformly along the MQW growth direction with the largest modulation in lateral field occurring only near the entrance face of the incident light, thus accounting partially for the low diffraction efficiency of LTG materials.