Long lifetimes of quantum-dot intersublevel transitions in the terahertz range

Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversion required for terahertz laser operation in quantum-well-based devices (quantum-cascade lasers) is fundamentally limited by efficient scattering between the laser levels, which form a continuum in the plane of the quantum well. In this context, semiconductor quantum dots are a highly attractive alternative for terahertz devices, because of their intrinsic discrete energy levels. Here, we present the first measurements, and theoretical description, of the intersublevel carrier relaxation in quantum dots for transition energies in the few terahertz range. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to approximately 2 ps at 30 meV (7 THz), in very good agreement with our microscopic theory of the carrier relaxation process. Our studies pave the way for quantum-dot terahertz device development, providing the fundamental knowledge of carrier relaxation times required for optimum device design.

Rapid sizing of short tandem repeat alleles using capillary array electrophoresis and energy-transfer fluorescent primers

Genetic typing of the short tandem repeat (STR) polymorphism HUMTHO1 has been performed using capillary array electrophoresis and energy-transfer fluorescent dye-labeled polymerase chain reaction primers. Target alleles were amplified by use of primers labeled with one fluorescein at the 5' end and another fluorescein at the position of the 15th (modified) base to produce fragments that fluoresce in the green (lambda max = 525 nm). Unknown alleles were electrophoretically separated together with a standard ladder made up of alleles having 6, 7, 8, and 9 four-base pair repeats, each of which was amplified with an energy-transfer primer having a donor fluorescein at the 5' end and a rhodamine acceptor at the position of the 7th (modified) base to produce standard fragments fluorescing in the red (> 590 nm). Separations were performed on arrays of hollow fused-silica capillaries filled with a replaceable sieving matrix consisting of 0.8% hydroxyethyl cellulose plus 1 microM 9-aminoacridine to enhance the resolution. The labeled DNA fragments were excited at 488 nm, and the fluorescence was detected with a two-color confocal fluorescence scanner. Separations are complete in less than 20 min and allow sizing with an average absolute error or accuracy of less than 0.4 base pair and an average standard deviation of approximately 0.5 base pair with no correction for mobility shift and cross-talk between the fluorescence channels. This work establishes the feasibility of high-speed, high-throughput STR typing of double-stranded DNA fragments using capillary array electrophoresis.