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We have studied terahertz absorption of samples containing two layers of self-aligned, self-assembled InAs quantum dots separated by a thin GaAs barrier. The vertically coupled dots were charged with electrons by applying a voltage bias between a metal gate and a doped layer beneath the dots. For a positive gate bias corresponding to flatband conditions, an absorption peak was observed near 10 meV (2.4 THz). The absorption is attributed to the inhomogeneously broadened transition between the quantum mechanically split levels (bonding and antibonding states) in the vertically coupled quantum dots. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated the far-infrared absorption in InAs/GaAs quantum dot molecules. The quantum dot molecules consist of two vertically coupled InAs self-assembled quantum dots separated by a GaAs barrier. The electronic coupling between the dot states results in an intraband absorption at THz frequencies. We show that this absorption can be bleached under high excitation intensity delivered by a free-electron laser. The saturation intensity is found to be on the order of 1 W cm-2. The electron relaxation time T[sub 1] is estimated from the saturation intensity. A lower limit for T[sub 1] of the order of 30 ps is deduced. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Cropping systems can influence the accumulation and distribution of plant nutrients in the soil profile, which can affect their utilization efficiency by crops and pollution potential in the environment. A field experiment was conducted on a Dark Brown loam soil at Scott, Saskatchewan, Canada to assess the effects of input level, cropping diversity and crop phase on the accumulation and distribution of nitrate–nitrogen (N) and extractable phosphorus (P) in the soil profile at the end of 1995 to 2000 growing seasons. The 54 treatments were combinations of three input levels (organic—ORG, reduced—RED and high—HIGH), three cropping diversities (low diversity—LOW, diversified annual grains—DAG, and diversified annual and perennials—DAP), and six crop phases chosen from fallow (tillage-fallow or chem-fallow), green manure [lentil—Lens culinaris Medicus or sweet clover—Melilotus officinalis (L.) Lam], spring wheat (Triticum aestivum L.), canola (Brassica napus L. and Brassica rapa L.), fall rye (Secale cereale L.), field pea (Pisum sativum L.), spring barley (Hordeum vulgare L.), flax (Linum usitatissimum L.), oats (Avena sativa L.), and bromegrass (Bromus inermis Leyss), alfalfa (Medicago sativa Leyss) mixture hay. Soil was sampled from the 0–15, 15–30, 30–60, and 60–90 cm depths in each crop phase from 1995 to 2000, with additional depths 90–120, 120–150, 150–180, 180–210, and 210–240 cm taken from the wheat phase in 2000. In general, there were greater amounts of nitrate–N with HIGH input compared to ORG or RED inputs, especially under LOW diversity. The nitrate–N in various soil depths suggested some downward movement of nitrate–N to the deeper soil depths when HIGH input was compared to ORG input. In LOW cropping diversity, green manure or fallow usually had more nitrate–N in soil than other crop phases. In DAG... [ABSTRACT FROM AUTHOR]