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We investigate the effect of mid-infrared (MIR) pumping on the transport properties of GaAs/(Al,Ga)As terahertz (THz) quantum lasers (TQLs), which rely on quantum coherence effects of intersubband transitions. Aiming at THz lasing at elevated temperatures, we extend the concept of THz gain with and without population inversion of a single, MIR-pumped, electrically driven THz stage proposed by Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)] to an entire TQL. However, experiments using a CO2 as well as a free-electron laser and numerical simulations show that this resonant MIR pumping causes a negative differential conductivity (NDC) in addition to the NDC caused by sequential tunneling. Lasing of these TQLs is prevented by the formation of electric-field domains below the resonance field strength for gain of each single THz stage. [ABSTRACT FROM AUTHOR]

We demonstrate silicon-based ultrafast metal-semiconductor-metal (MSM) photodetectors for near infrared optocommunication wavelengths. They show a response time of 12.5 ps full width at half maximum (FWHM) at both 1300 and 1550 nm wavelengths. The overall external quantum efficiencies are 13% at 1320 nm and 7.5% at 1550 nm. The sensitive volumes are 270 nm thick Ge films, grown on Si(111) by molecular beam epitaxy. Interdigitated Cr metal top electrodes with 1.5-5 µm spacing and identical finger width form Schottky contacts to the Ge film. A Ti-sapphire femtosecond laser with an optical parametric oscillator and an electro-optic sampling system are used to evaluate the temporal response, which is limited by the transit time of the carriers between electrodes. In addition, results on Si-Ge MSM heterostructure detectors with plate capacitor geometry are presented. At 1550 nm an ultrafast response of 9.4 ps FWHM and an overall quantum efficiency of 0.9% are measured. [ABSTRACT FROM AUTHOR]