Daniel Issenmann

Subharmonic Resonant Optical Excitation of Confined Acoustic Modes in a Free-Standing Semiconductor Membrane at GHz Frequencies with a High-Repetition-Rate Femtosecond Laser

We propose subharmonic 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 subharmonic 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 systems quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

Thermal conductivity of isotopically controlled silicon nanostructures

Nanostructured semiconductors open the opportunity to independently tailor electric and thermal conductivity by manipulation of the phonon transport. Nanostructuring of materials is a highly promising strategy for engineering thermoelectric devices with improved efficiency. The concept of reducing the thermal conductivity without degrading the electrical conductivity is most ideally realized by controlled isotope doping. This work reports on experimental and theoretical investigations on the thermal conductivity of isotopically modulated silicon nanostructures. State-of-the-art pump-and-probe experiments are conducted to determine the thermal conductivity of the different nanostructures of isotopically enriched silicon layers epitaxially grown on natural silicon substrates. Concomitant molecular dynamics calculations are performed to study the impact of the silicon isotope mass, isotope interfaces, and of the isotope layer ordering and thickness on the thermal conductivity. Engineering the isotope distribution is a striking concept to reduce the thermal conductivity of silicon without affecting its electronic properties. This approach, using isotopically engineered silicon, might pave the way for future commercial thermoelectric devices.

Reduced thermal conductivity of isotopically modulated silicon multilayer structures

We report measurements of the thermal conductivity of isotopically modulated silicon that consists of alternating layers of highly enriched silicon-28 and silicon-29. A reduced thermal conductivity of the isotopically modulated silicon compared to natural silicon was measured by means of time-resolved x-ray scattering. Comparison of the experimental results to numerical solutions of the corresponding heat diffusion equations reveals a factor of three lower thermal conductivity of the isotope structure compared to natural Si. Our results demonstrate that the thermal conductivity of silicon can be effectively reduced with isotopically modulated structures. This offers a promising approach to optimize silicon for thermoelectric applications.

Modification of vibrational damping times in thin gold films by self-assembled molecular layers

The mechanical contact between a thin gold film and a silicon substrate is investigated by ultrafast pump-probe spectroscopy providing quantitative values on the damping time of coherent longitudinal vibrations of the gold film. A distinct increase in damping times is observed when a self-assembled molecular layer is introduced between the gold film and the substrate. We deduce the frequency dependence of the damping times by varying the thickness of the gold films. Experimental results are compared to numerical simulations based on a visco-elastic model and the acoustic mismatch model.

Asynchronous sampling for ultrafast experiments with low momentum compaction at the ANKA ring.

A high-repetition-rate pump-probe experiment is presented, based on the asynchronous sampling approach. The low-α mode at the synchrotron ANKA can be used for a time resolution down to the picosecond limit for the time-domain sampling of the coherent THz emission as well as for hard X-ray pump-probe experiments, which probe structural dynamics in the condensed phase. It is shown that a synchronization of better than 1 ps is achieved, and examples of phonon dynamics of semiconductors are presented.

Structural study of near-field ablation close to plasmon-resonant nanotriangles

The optical near fields in close vicinity to plasmonic nanoscale objects show a considerable enhancement of the electrical field and are localized to dimensions much less than the wavelength of light. The authors show that an ablation process caused by the near-field enhancement of femtosecond laser pulses pattern the substrate below gold nanotriangles is a way to image the near-field distribution with a resolution below 20 nm. The mechanism of ablation studied by pulsed x-ray scattering reveals the nonthermal nature of the process.

Ultrafast laser pump X-ray probe experiments by means of asynchronous sampling

A high time resolution in the picosecond range is required for the time-domain investigation of phonon dynamics in crystalline systems. Following a recently developed scheme in the visible spectrum, this resolution can be achieved by a method called asynchronous optical x-ray sampling (ASOXS). A pulsed femtosecond laser with high repetition rate is synchronized to the electron bunches in a storage ring. A slight frequency detuning changes the mutual delay continuously, resulting in a time-domain x-ray sampling of the laser-excited system. At the synchrotron radiation source ANKA a machine mode with low momentum compaction factor αc is available, which delivers ultra-short x-ray pulses in the picosecond range.

Ultrafast X-ray scattering on nanoparticle dynamics

Pulsed X-ray scattering is used for the determination of structural dynamics of laser-irradiated gold particles. By combining several scattering methods such as powder scattering, small angle scattering and diffuse wide angle scattering it is possible to reconstruct the kinetics of structure evolution on several lengths scales and derive complementary information on the particles and their local environment. A generic structural phase diagram for the reaction as function of delay time after laser excitation and laser fluence can be constructed.

Modification of coherent acoustic phonon lifetimes in thin gold films by self-assembled molecular monolayers

We report on the modification of coherent acoustic phonon lifetimes in thin gold films induced by insertion of self-assembled monolayers as interface layers between gold films and substrates. The lifetimes of coherent acoustic phonons are investigated by asynchronous optical sampling (ASOPS) [1], a high speed pump-probe method which utilizes two Ti:sapphire lasers of ∼1 GHz repetition rate. The time delay between pump and probe pulse is obtained by a repetition rate offset of 5 kHz. The absence of moving mechanical parts allows for fast scanning times and high signal to noise ratios which are hard to achieve by conventional pump-probe methods [2].