GaBiAs layers have been grown by molecular beam epitaxy at low (270–330°C) temperatures and were characterized by several experimental techniques. It was shown that the spectral photosensitivity cutoff wavelength reaches ∼1.4μm when the growth temperature is as low as 280°C. Optical pump–terahertz probe measurements made on these layers have evidenced that the electron trapping time decreases with decreasing growth temperature from 20 to about 1ps. GaBiAs layers were used for manufacturing photoconductive terahertz emitters and detectors, which, when excited with Ti:sapphire laser pulses, have demonstrated a signal bandwidth of 3THz.
We report significant differences in the temperature-dependent and time-resolved photoluminescence ͑PL͒ from low and high surface density In x Ga 1Ϫx As/GaAs quantum dots ͑QD's͒. QD's in high densities are found to exhibit an Arrhenius dependence of the PL intensity, while low-density ͑isolated͒ QD's display more complex temperature-dependent behavior. The PL temperature dependence of high density QD samples is attributed to carrier thermal emission and recapture into neighboring QD's. Conversely, in low density QD samples, thermal transfer of carriers between neighboring QD's plays no significant role in the PL temperature dependence. The efficiency of carrier transfer into isolated dots is found to be limited by the rate of carrier transport in the In x Ga 1Ϫx As wetting layer. These interpretations are consistent with time-resolved PL measurements of carrier transfer times in low and high density QD's. ͓S0163-1829͑99͒04748-7͔
We report photo- and electroluminescence from an alternating conjugated polymer consisting of fluorene units and low-band gap donor-acceptor-donor (D–A–D) units. The D–A–D segment includes two electron-donating thiophene rings combined with a thiadiazolo-quinoxaline unit, which is electron withdrawing to its nature. The resulting polymer is conjugated and has a band gap of 1.27 eV. The corresponding electro- and photoluminescence spectra both peak at approximately 1 μm. Light-emitting diodes, based on a single layer of the polymer, demonstrated external quantum efficiencies from 0.03% to 0.05%.
A coherent absorption dip in pump-probe experiment performed on a ten layer optically thin InGaAs∕GaAs quantum dot (QD) structure has been observed. Measurements performed for different wavelengths, polarizations, pulse widths, and temperatures allow assigning the dip to electromagnetically induced transparency (EIT). The EIT scheme is based on coupling of excitons with different spins in asymmetric QDs. Using spectrally narrow pulses, detrimental effect of the inhomogeneous broadening is reduced since only the QDs with transitions resonant with the pulse wavelength are addressed and participate in the EIT. The effect has been observed at temperatures up to 140K.
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