Axel Bruchhausen

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.

Spatial-temporally resolved high-frequency surface acoustic waves on silicon investigated by femtosecond spectroscopy

Various types of surface acoustic waves are generated by femtosecond pulses on bulk silicon with aluminium stripe transducers. Rayleigh and leaky longitudinal surface acoustic wave modes are detected in the time domain for various propagation distances. The modes are identified by measuring on various pitches and comparing the spectra with finite element calculations. The lifetimes of the modes are determined quantitatively by spatially separating pump and probe beam, showing a significant difference in the lifetimes of both modes. We were able to excite and measure Rayleigh modes with frequencies of up to 90 GHz using a 100 nm period grating.

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.

Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy

The dynamics of acoustic vibrations in terahertz quantum cascade laser structures (THz-QCLs) is studied by means of femtosecond pump-probe spectroscopy. The phonon modes are characterized by the folding of the acoustic dispersion into an effective reduced Brillouin zone. An accurate identification of this dispersion allows the sample structure and periodicity to be determined with high precision on the order of 0.1%. By temperature tuning the energy of the electronic levels of the system and performing wavelength dependent measurements, we are able to study the impulsive resonant generation and detection of coherent acoustic phonon modes. These results are supported by simulations of the electronic system that well explain the experimental observations. The effects of interface (IF) roughness on coherent acoustic phonon spectra are clearly observed for equal nominal THz-QCL structures but with different interface qualities.

Ultrafast spectroscopy of super high frequency mechanical modes of doubly clamped beams

We use ultrafast pump-probe spectroscopy to study the mechanical vibrations in the time domain of doubly clamped silicon nitride beams. Beams with two different clamping conditions are investigated. Finite element method calculations are performed to analyse the mode spectra of both structures. By calculating the strain integral on the surface of the resonators, we are able to reproduce the effect of the detection mechanism and identify all the measured modes. We show that our spectroscopy technique combined with our modelling tools allow the investigation of several different modes in the super high frequency range (3-30 GHz) and above, bringing more information about the vibration modes of nanomechanical resonators.

Imaging of a patterned and buried molecular layer by coherent acoustic phonon spectroscopy

A molecular layer of aminopropyltriethoxysilane is patterned with a focused ion beam and subsequently covered by a gold film. The gold-polymer-substrate structures are afterwards imaged by ultrafast coherent acoustic phonon spectroscopy in reflection geometry. We demonstrate that the lateral structure of the covered polymer layer can be detected via the damping time of the vibrational mode of the gold film. Furthermore, we utilize Brillouin oscillations originating from the silicon substrate to map the structures and to estimate the molecular layer thickness.

Viscoelastic properties and efficient acoustic damping in confined polymer nano-layers at GHz frequencies

We investigate the viscoelastic properties of confined molecular nano-layers by time resolved optical pump-probe measurements. Access to the elastic properties is provided by the damping time of acoustic eigenmodes of thin metal films deposited on the molecular nano-layers which show a strong dependence on the molecular layer thickness and on the acoustic eigen-mode frequencies. An analytical model including the viscoelastic properties of the molecular layer allows us to obtain the longitudinal sound velocity as well as the acoustic absorption coefficient of the layer. Our experiments and theoretical analysis indicate for the first time that the molecular nano-layers are much more viscous than elastic in the investigated frequency range from 50 to 120 GHz and thus show pronounced acoustic absorption. The longitudinal acoustic wavenumber has nearly equal real and imaginary parts, both increasing proportional to the square root of the frequency. Thus, both acoustic velocity and acoustic absorption are proportional to the square root of frequency and the propagation of compressional/dilatational acoustic waves in the investigated nano-layers is of the diffusional type, similar to the propagation of shear waves in viscous liquids and thermal waves in solids.

Optical generation of a broadband acoustic frequency comb in the 100 GHz-regime

Acoustic properties in the GHz frequency range are not very well understood for most semiconductors although intrinsic attenuation and scattering at interfaces of acoustic waves is of great interest for both fundamental and applied science. Additionally adhesion properties of very thin films are both hard to control and to evaluate. A thin silicon membrane with an aluminum transducer on top is used to gain knowledge of these crucial properties. The two-layer system is investigated by asynchronous optical sampling, a pump-probe technique employing 2 fs lasers [1]. The pump pulse generates strain fronts in the membrane, which are propagating and thereby defining a pulse. The Si/air-interface and the Al/air interface act as mirros and reflect the acoustic pulse. As a result an acoustic cavity is formed.

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].

High speed pump-probe spectroscopy of Si 3 N 4 -based micromechanical systems

In this work we present first time domain all optical investigations of a single double-clamped Si3N4 cantilever resonator, using high speed asynchronous optical sampling (ASOPS). This method is based on two asynchronously linked femto-second unidirectional ring lasers of ∼1 GHz repetition rate. One laser provides the pump pulse while the other provides the probe pulse. The time delay is realized by introducing a 5 kHz offset in the repetition rates of the two lasers without requiring a mechanical delay line [1,2].