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Isotopically modulated silicon and germanium multilayers are analyzed by means of femtosecond spectroscopy and pulsed X-ray scattering for determining thermal conductivity and phonon modes. Isotopic modulation decreases thermal conductivity stronger than expected from a band bending model in the coherent phonon transport regime, in particular for silicon. Femtosecond spectroscopy and X-ray scattering resolve zone folded vibration modes, which are located at the edge of the new, smaller Brillouin zone due to the multilayer periodicity. These modes can contribute to the reduction of thermal conductivity by Uniklapp processes within the zone-folded mini-bands.[GRAPHICS]Color-coded increase in ultrafast X-ray scattering in vicinity to the mini-zone boundary of a germanium multilayer. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The use of beam interference in combination with large-area photoconductive emitters for the generation of pulsed terahertz radiation is presented. An interference pattern with a periode twice that of the terahertz emitter is generated with a transmission phase grating, placed directly in front of the photoconductive emitter. This way, efficient terahertz generation is achieved with a single metallization layer and a single lithography step in the fabrication technology of the THz emitter.

The status quo of the research on Bloch oscillations in semiconductor superlattices is outlined. Bloch oscillations excited by femtosecond laser pulses have been investigated by different optical techniques. They have allowed measurement of the internal electric field changes associated with the charge oscillations, the emitted terahertz electromagnetic radiation and the nonlinear coherent optical response, all directly in the time domain. The main results are summarized and critically reviewed. The potential of Bloch oscillations for practical applications in terahertz radiation sources is assessed.

We have investigated the time-resolved vibrational properties of terahertz quantum cascade lasers by means of ultra-fast laser spectroscopy. By the observation of the acoustic folded branches, and by analyzing the involved phonon modes it is possible to extract accurate structural information of these devices, which are essential for their design and performance., Comment: 8 pages, 5 figures

We propose sub-harmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a sub-harmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the system's quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

Intersubband (ISB) transitions in semiconductor quantum wells (QWs) can be employed for various mid-infrared optoelectronic devices. Presently there is strong interest to extend the available wavelength range into the near infrared, by using materials with a large conduction band offset. To achieve such short wavelengths thin QWs are required, where the first excited state inside the QW may lie higher than some state related to indirect valleys. Examples for such material systems are strained InGaAs/AlAs or lattice matched InGaAs/AlAsSb, both grown on InP. We have studied the ISB relaxation dynamics in multi QWs of both material systems by femtosecond pump-probe measurements. The transient transmission as a function of the pump-probe delay does not show a single-exponential decay, indicating a more complicated relaxation dynamics. This can be caused by transfer of electrons to X- or L- states in the QWs or the barriers. We will show results on samples with different QW thicknesses and compare them to simulations based on rate equations.

We present an approach of photoconductive terahertz (THz) generation providing a broad bandwidth and exceptional electric field amplitude. A large-area interdigitated two-electrode structure is applied to a GaAs substrate to offer high electric fields. Photocarriers excited by a Ti:Sapphire oscillator laser with MHz repetition rate are accelerated there, yielding an intense THz output. An appropriate binary mask covers every second electrode interval and carriers are excited in uniform electric field areas only. Hence contrary carrier acceleration and destructive interference is avoided. Due to the periodic nature of the electrode structure, the size of the excitation spot on the photoconductive THz emitter can be varied. This results in a THz beam of variable divergence. By electro-optic sampling the THz radiation is detected and characteristic properties of the THz source are measured. For an excitation spot diameter of about 300 μm, which corresponds to the central wavelength of the THz pulses, the THz generation is most efficient. THz radiation with an average power of 145 µW is generated with an efficiency of 2 × 10^-4 for the conversion from NIR to THz power. Furthermore, the THz radiation has excellent focusing properties. A Gaussian beam profile with a diameter (FWHM) of less than 1.4 of the central wavelength of the THz pulses is found. The THz field amplitude in the center of the focused THz beam is 1.5 kV/cm, which is almost one order of magnitude more of what is achieved with conventional semi-large aperture photoconductive emitters and a similar excitation spot diameter. Exceptionally large signal-to-noise ratios are achieved by modulating the bias voltage in the kHz range and using lock-in technique. We suggest that the THz power can be further improved by a sufficient cooling system, e.g. water cooling. Furthermore the use of LT GaAs instead of semiinsulating GaAs can result in larger THz bandwidth.

The coherent interaction of femtosecond laser pulses in the pump–probe regime has been experimentally studied in the time domain by monitoring light reflection from a tellurium single crystal. The optical response of the probed medium exhibits periodic variations at a frequency equal to that of the exciting laser radiation. Experimental dependences of the observed "coherent artifact" on the pump/probe intensity ratio, the number of accumulated pulses, and the mutual orientation of the polarization vectors of electromagnetic fields and the crystallographic axes are well described by the proposed phenomenological model.

The non-linear optical properties of solid state materials in the THz frequency region are to a large extent unexplored due to missing high-intensity coherent light sources in this frequency range. Although recent advances in table-top laser based THz systems have been enormous, free-electron lasers (FEL) are at present still the only tunable lasers which provide high peak intensities and a sufficient narrow spectral width to perform nonlinear spectroscopy at THz frequencies. Recently we investigated the dispersion of the second order nonlinear susceptibility (c(2)) in thin GaAs crystals below the optical phonon resonance via second harmonic generation (SHG) experiments with an FEL [1]. These experiments provide insight into the relative contributions of higher-order cohesive lattice forces to c(2). The nonlinear optical susceptibility in polar semiconductors in the THz frequency range is strongly influenced by the presence of optical phonons and should exhibit several peculiarities: A strong resonant enhancement of the SHG at half the frequency of the TO phonon (8.0 THz in GaAs) and at the TO phonon itself as well as a zero-crossing for frequencies between 4.0 THz and the TO phonon resonance due to the cancellation of higher order ionic and electronic contributions was predicted [2]. The experiments are performed with the THz FEL FELIX (Nieuwegein, Netherlands), which delivers picosecond pulses with µJ energy. We could observe both the resonance and the zero-crossing of c(2) below the Reststrahlen-band for the first time. From the value obtained for the zero-crossing of the nonlinear susceptibility we conclude that the contribution of the phonon interaction through the second-order lattice dipole moment has to be significantly smaller and the contribution from the third-order lattice potential anharmonicity has to be larger than determined previously [3]. Besides the relevance for the THz nonlinear susceptibility these terms are also important for two-phonon sidebands in the infrared absorption, phonon decay [3] and for a quantitative description of Raman spectra [4]. We propose that SHG below the optical phonon resonance is an elegant method to quantitatively determine the higher-order potential contributions to the nonlinear susceptibility. [1] T. Dekorsy et al., Phys. Rev. Lett. 90, 055508 (2003). [2] A. Mayer and F. Keilmann, Phys. Rev B 33, 6954 (1986). [3] C. Flytzanis, Phys. Rev. B 6, 1264 (1972); Phys. Rev. Lett. 29, 772 (1972). [4] S. Go, H. Bilz, and M. Cardona, Phys. Rev. Lett. 34, 580 (1975).

We have investigated intersubband transitions in strain-compensated AlAs/(In,Ga)As heterostructures, demonstrating both absorption and quantum-cascade laser emission at short wavelengths. Short-wavelength optical transitions in such structures are associated with a number of challenges in both the growth and design, including managing the internal strain and designing around indirect valleys. We achieve absorption peaks at wavelengths as short as 1.7 micrometer in fully strain-compensated AlAs/(In,Ga)As structures. Quantum cascade lasers based on similar heterojunctions exhibit laser emission as short as 3.7 micrometer. These lasers exhibit low-temperature threshold current densities of 860 A/cm2 in pulsed mode and output power as high as 6W per facet (12W total). At room temperature, the threshold current density is 4.5 kA/cm2 and the maximum power 240 mW per facet.

The optimization of emitters for THz radiation pulses has gained a lot of interest in recent years. They can be based on accelerating charge carriers, which are photoexcited by fs-laser pulses in the surface field of a semiconductor. We studied the THz emission from GaSb samples that were annealed at different temperatures in the range from 300 °C to 700 °C. Without annealing, no THz emission was observed under fs excitation at 800 nm. This is consistent with the fact, that for GaSb no surface states in the band gap have been found [1]. The THz emission was strongest for the annealing temperature of 450 °C with amplitudes comparable to that of an InGaAs emitter. We attribute the THz emission to a surface field caused by decomposition of the surface of the annealed samples. The decomposition is confirmed by measuring the surface stoichiometry using Auger electron spectroscopy. We suggest that the decrease of THz intensity for the higher annealing temperatures is due to a lowering of the carrier mobility. Our experiment demonstrates the possibility to modify materials for THz emission in a very simple way. [1] P.W. Chey, I.A. Babalado, T. Sukegawa, and W.E. Spicer, Phys. Rev. Lett. 35, 1602 (1975).

Ultrafast exciton dynamics in crescent-shaped GaAs quantum-wires have been investigated using pump-probe and Reflective-Electro-Optic-Sampling (REOS) methods. The differential-reflectivity shows the exciton energy-shift and the subband-relaxation. Also ultrafast coherent dynamics are observed in the REOS measurement.

We demonstrate a wavelength-tunable Ti:sapphire femtosecond laser with a repetition rate between 500 MHz and 1.5 GHz. A “half-Brewster-prism” is introduced into the cavity to achieve tuning from 733 nm to 840 nm by tilting a cavity mirror.