Carrier saturation in multiple quantum well metallo-dielectric semiconductor nanolaser: is bulk material a better choice for gain media?

Although multi quantum well (MQW) structure is frequently suggested as the appropriate medium for providing optical gain in nanolasers with low threshold current, we demonstrate that in general bulk gain medium can be a better choice. We show that the high threshold gain required for nanolasers demands high threshold carrier concentrations and therefore a highly degenerate condition in which the barriers between the quantum wells are heavily pumped. As a result, there occurs spontaneous emission from the barrier in very dissipative low Q modes or undesired confined higher Q modes with resonance wavelengths close to the barrier bandgap. This results in a competition between wells and barriers that suppresses lasing. A complete model involving the optical properties of the resonant cavity combined with the carrier injection in the multilayer structure is presented to support our argument. With this theoretical model we show that while lasing is achieved in the nanolaser with bulk gain media, the nanolaser with MQW gain structure exhibits well emission saturation due to the onset of barrier emission.

Spectroscopic determination of protein concentrations from proteinase K digests.

A method to determine protein concentrations and absorptivities based on absorbance measurements of proteinase K digests has been developed. Molar absorptivities of proteinase K digests at 56 degrees C can be predicted by using the following equation: epsilon (M)(280)=5318 x (No.of Trp) + 1227 x (No.of Tyr) + 133 x (No.of Cys-Cys). Protein concentration in the digest can be determined by dividing the corrected digest solution absorbance by the calculated epsilon(M)(280). The absorptivity of a native protein can then be calculated by dividing the absorbance of the intact protein solution by the concentration value obtained for the digest solution. Precision of the experimental data is within +/-3%, and the error of the method does not exceed 4.5%. The accuracy of determination does not depend on the size of the protein, Trp/Tyr ratio, presence or absence of certain chromophores, or other structural factors. The method requires amounts of protein routinely used for absorbance measurements.