Volume 97, Issue 8, 15 April 2005
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
97(2005); http://dx.doi.org/10.1063/1.1873035View Description Hide Description
We present temperature-dependent measurements of the local parameter, which reflects the dominant recombination processes. At room temperature, increases with current from 1.2 at low bias up to a threshold value, , of only 2.0, demonstrating the existence of significant amounts of monomolecular recombination. At elevated temperatures, rises above 2.5. We calculate the laser’s band diagram and estimate 35 meV of band-gap renormalization at threshold through polarization-resolved measurements of exciton peaks in the absorptionspectrum and inflection points of the absorption,spontaneous emission, and gain spectra. The small effective valence-band barrier height of 115 meV leads to hole leakage due to thermionic emission and carrier spillover into the barriers. This could explain the increase in at elevated temperatures and decrease in internal quantum efficiency from 57% at 15 °C to 47% at 75 °C. We also analyze the spontaneous emission and gain spectra below and above threshold. The spontaneous emission clamped rather weakly at threshold, which also partially explains the low internal efficiency at room temperature. We present initial reliability measurements of over 100 h at 10-mW continuous-wave power from a single facet. After 50 h of life testing, the threshold current increased by 79% and the external differential quantum efficiency decreased by 39% of their respective pretest values. By comparing the local parameter versus current-density curves before and after life testing, we have identified an increase in monomolecular recombination as the main source of degraded device performance.
97(2005); http://dx.doi.org/10.1063/1.1876578View Description Hide Description
We present a comprehensive study on femtosecond laser-induced refractive index modification in a wide variety of multicomponent glasses grouped as borosilicate, aluminum–silicate, and heavy-metal oxide glasses along with lanthanum–borate and sodium–phosphate glasses. By using high-spatial resolution refractive index profiling techniques, we demonstrate that under a wide range of writing conditions the refractive index modification in multicomponent glasses can be positive, negative, or nonuniform, and exhibits a strong dependence on the glass composition. With the exception of some aluminum–silicate glasses all other glasses exhibited a negative/nonuniform index change. We also demonstrate direct writing of waveguides in photosensitive Foturan® glass with a femtosecond laser without initiating crystallization by thermal treatment. Upon ceramization of lithium–aluminum–silicate glasses such as Foturan®, Zerodur®, and Robax® we observe switching of laser-induced refractive index change from being positive to negative. The measured transmission losses in the waveguides at agree with the index profile measurements in alkali-free aluminum–silicate glasses.
97(2005); http://dx.doi.org/10.1063/1.1863453View Description Hide Description
We propose a ray optics model to calculate the trapping force on a dielectric particle located on the interface between two media and illuminated by a focused evanescent field beam. Such a focused evanescent beam is produced by a high numerical aperture objective with a central obstruction whose size satisfies the total internal reflection condition on the interface. The dependence of the lifting force on the obstruction size, the particle size, and the distance of the particle from the interface is revealed.
Effect of strain relaxation and exciton localization on performance of quaternary light-emitting diodes97(2005); http://dx.doi.org/10.1063/1.1877816View Description Hide Description
The optical and structural properties of quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the excitonlocalization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs.