Silver Nanoparticles Substantially Enhance Fluorescence of Chlorophyll
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Frontside-Illuminated GaAs/AlGaAs Quantum Well Photodetector Yields 3 THz Peak Spectral Response and µA/cm2 Dark Current there have been increasing efforts in recent years to develop practical sources and detectors for the terahertz (thz) frequency range to take advantage of this largely unused spectral range for applications including detection of molecules for food inspection, homeland security, and astrophysics. despite recent progress, large-format array detectors required for practical imaging applications are still unavailable. M. Patrashin and I. hosako at the national Institute of Information and Communications technology, Japan, have made a significant step in that direction by developing an alternative device concept based on GaAs/AlGaAs quantum well (QW) structures and achieved effective intersubband absorption in wells with a suitable barrier layer composition. their detector achieved a peak response close to the targeted 3 thz spectral band with a low-temperature dark current in the range of 1 µA/cm2 because the latter was limited to thermally assisted tunneling through the barrier. As described in the January 15 issue of Optics Letters (p. 168), the researchers based their quantum well photodetector design on 18-nm GaAs QWs and 80-nm AlGaAs barriers with a 2% Al concentration. the intersubband absorption was activated by doping the center of the well with Si, which populates the QW’s ground subband. three sets of samples with 100 µm, 50 µm, and 25 µm period gratings were grown by molecular beam epitaxy (MBe) on semi-insulating GaAs substrates and consisted of a 20-period multiple QW structure sandwiched between 0.4 µm top and 0.8 µm bottom contact layers. the scientists used standard microelectronic processing to develop single element square-shaped mesas of different sizes. the reported test results are from 1×1 mm2 photodetectors with a 50 µm period grating. the detectors showed stable operation between ±60 mV down to a temperature of 3 K and did not show any hysteresis in the current–voltage (I–V) curve indicating good contact properties even at cryogenic temperatures. Flattening of the current density versus temperature response at 1 µA/cm2 at low temperature shows that resonance inter-well and impurity-assisted tunneling are suppressed by the 80-nm barriers. By comparing the I–V characteristics under different photon flux conditions, a responsivity of 13 mA/W was obtained at an electric bias of 40 mV
and an operating temperature of 3 K. the researchers state that by decreasing the doping concentration in the contact layer, its transmission can be improved by up to 60%, which would directly enhance the responsivity of the device. Future simulations and experiments will be conducted to clarify the mechanisms underlying the observed spectral response of the grating coupler. AlFred A. ZInn
dynamics governing flow of dilute particle suspensions are “dependent on the accumulated shear strain applied to the suspension.” Models of these systems would do well to incorporate chaotic dynamics and
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