We have performed a variable stripe length method at 5 K to measure the optical gain of CdS nanowires. When the excited carrier density is lower than the Mott density, we found that various inelastic scatterings of excitons and LO-phonons are involved. As a consequence of inelastic exciton-exciton scattering, the excitons scattered down to the low polariton branch cause P-emission lasing, and the excited excitons also result in a population inversion through intra-relaxation. When LO-phonon scatterings are involved, a broad optical gain spectrum is observed resulting from exciton-LO and P-LO scatterings.
We have employed a variable stripe length method in order to measure the optical gain of GaAs/AlGaAs quantum rings. Although the large lateral diameter of quantum rings (∼50 nm) with a few nm size distribution is expected to cause a small spectral inhomogeneity (∼1%), a broad gain width (∼300 meV) was observed. This result was attributed to a variation of the vertical heights and variations in localized states that exhibit crescent shaped wavefunctions, whereby the energy levels are distributed over a broad spectral range. When the excitation intensity is decreased, irregular peaks appear in the gain spectrum gradually. Similar phenomena were also observed as the temperature increased. We conclude that excited carriers in quantum rings are distributed stochastically at various localized states and that the population inversion is sensitive to both excitation intensity and temperature.
We measured optical modal gain of a dye–virus hybrid structure using a variable stripe length method, where Alexa-fluor-488 dye was coated on a virus assembly of M13 bacteriophage. Inspired by the structural periodicity of the wrinkle-like virus assembly, the edge emission of amplified spontaneous emission was measured for increasing excited optical stripe length, which was aligned to be either parallel or perpendicular to the wrinkle alignment. We found that the edge emission showed a strong optical anisotropy, and a spectral etalon also appeared in the gain spectrum. These results can be attributed to the corrugated structure, which causes a similar effect to a DFB laser, and we also estimated effective cavity lengths.
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