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High-quality n-type continuously graded Ge-rich Si1−xGex parabolic quantum wells with different doping levels were grown by using ultrahigh-vacuum chemical vapor deposition on Si(001) substrates. A thorough structural characterization study highlights an ideal parabolic compositional profile. THz intersubband absorption has been investigated in modulation-doped samples and samples directly doped in the wells. The comparison of experimental absorption data and theoretical calculations allowed us to quantify the impact of electron correlation effects on the absorption resonances in the different doping conditions and for electron sheet densities in the (1 ÷ 6) × 10 11 cm−2 range. A single optical resonance is present in modulation doped samples. Its peak energy and line shape are independent of temperature-induced variations of the electron distribution in the subbands up to 300 K, in agreement with the generalized Kohn theorem. This achievement represents a relevant step forward for the development of CMOS compatible optoelectronic devices in the THz spectral range, where thermal charge fluctuations play a key role. [ABSTRACT FROM AUTHOR]

We present a plasma switch based on gold-implanted germanium (Ge:Au) as a potential candidate for efficient cavity dumping of a free-electron laser (FEL). Ge:Au has a sub-nanosecond carrier lifetime—much shorter than the FEL pulsing period of 77 ns—and demonstrates a high photoinduced reflectivity in a broad range of infrared wavelengths from 6 to 90 μm. The Ge:Au plasma switch exhibits negligible absorption of the FEL radiation in the "off " state and requires only moderate thermoelectric cooling for incident FEL power of several Watts. A reflectivity level of more than 50% in the "on" state is achieved over the entire spectral range of this study. The corresponding optical pump fluence exhibits a linear relationship with the FEL frequency. This scaling is corroborated by our simulations highlighting the role of a finite sub-μm thickness of the photoinduced reflecting plasma layer. The demonstrated device is promising for the realization of the FEL cavity dumping for experiments that simultaneously require higher pulse energy and lower average power. [ABSTRACT FROM AUTHOR]

Terahertz imaging has the potential to identify and decipher portions of ancient manuscripts, which may be unreadable at infrared and visible wavelengths. We use a scanning confocal terahertz microscope to scan a medieval parchment with music notes and pentagrams written with different inks. The microscope is based on a continuous-wave solid-state source at 0.3 THz, emitting in the free space with a horn antenna, and a high numerical-aperture ellipsoidal reflector. We present terahertz images with diffraction-limited lateral resolution of approximately 0.5 mm, where the different inks all give similar high contrast. Symbols written on the 'verso' side of the parchment, barely glimpsed in the near-infrared photograph, leave a clear imprint in the terahertz images. Artifacts due to imperfect flatness of the parchment are also briefly discussed. [ABSTRACT FROM AUTHOR]

We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δ f / f ≈ 0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition, and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with a similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency. [ABSTRACT FROM AUTHOR]