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Longitudinal acoustic modes in planar thin gold films are excited and detected by a combination of ultrafast pump-probe photoacoustic spectroscopy and a surface plasmon resonance (SPR) technique. The resulting high sensitivity allows the detection of acoustic modes up to the 7th harmonic (258 GHz) with sub-pm amplitude sensing capabilities. This makes a comparison of damping times of individual modes possible. Further, the dynamics of the real and imaginary part of the dielectric function and the film’s thickness variation are separated by using the dependence of the amplitudes of the acoustic modes on the detection angle and the surface plasmon resonance. We find that longitudinal acoustic modes in the gold films mainly affect the real part of the dielectric function and highlight the importance to consider thickness related effects in acousto-plasmonic sensing. published

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We present a thin-disk multipass amplifier as a power scaling architecture for ultrafast lasers. 1950 W output power at a repetition rate of 800 kHz were achieved with pulses pre-compressed to 10 ps.

International audience; Picosecond acoustic waves at ~365 GHz in a semiconductor saturable absorber mirror (SESAM) structure are optically controlled by femtosecond time-resolved spectroscopy. Picosecond acoustic waves are excited and detected in the In0.27Ga0.73As multiple quantum wells of the SESAM structure by femtosecond lasers operating at around ~1 �m wavelength. A second pump pulse with an adjustable time delay and intensity with respect to the first one, is used to exert control over the acoustic waves, which yields various acoustic patterns. The coherent acoustic spectral components can be manipulated in terms of enhancement, selective suppression, comb-spacing doubling, and tunable profiles. By the aid of a simplified model with identical generation and detection sensitivity functions in the quantum well stacks, the periodicity of modulation, the phase jump, the selection of suppressed modes, and the upper boundary in the time-delay or intensity-ratio dependent variations are well interpreted. The modelling agrees well with experimental results in a wide tunable range of the time delay and the intensity ratio. The results show promising applications in arbitrary acoustic wave synthesis and acoustic field filters. Additionally, a prism is inserted in the pump path to induce a subharmonic-like frequency component. By controlling the rotating angle of the prism, the amplitude of subharmonic components can be adjusted. The potential explanation of the induced acoustic change can be the excitation of subharmonic frequencies or the interference of acoustic waves induced by different pump spectral components with varied time delays.

International audience; Semiconductor saturable absorber mirrors (SESAMs) with N-ion irradiated In0.3Ga0.7As quantum wells are investigated by both degenerate and non-degenerate phonon spectroscopy. We found that the N-ion irradiation on quantum wells (QWs) has impact not only on the recovery time of the SESAM structures, but also on the laser-induced coherent acoustic phonons. The recovery time is shortened when more quantum wells are irradiated by N ions, down to 1.6 ps (for all-QW-irradiated sample) from 19.9 ps (for sample without N-ion irradiation). The reflectivity change induced by coherent acoustic phonons is enhanced in the sample with partially irradiated QWs, but further enhancement does not occur in the all-QW-irradiated sample. The modification of carrier concentration induced by N-ion implantation potentially accounts for the improvement of phonon-induced reflectivity change, while further enhancement is hindered by the suppression of the deformation potential effect due to the fact that the recovery time is shorter than the acoustic wave period. In the multiple quantum well region, overall consistent acoustic results are obtained in the degenerate and non-degenerate configuration, with slight differences caused by different pump photon energies or excitation functions. In the distributed Bragg reflector region, up to seven orders of the center-modes of folded acoustic phonons starting from ~31 GHz are observed in the non-degenerate configuration. The detected first-order component is evidently enhanced and detected higher- order modes are overall slightly suppressed in the all-QW-irradiated sample compared to those in the partial-QW-irradiated sample. Our results imply that N- ion irradiation distribution can be used to tailor coherent acoustic phonons, thus this can be directly applied to monitor the optimization of SESAM characteristics for mode-locking lasers.

International audience; Mechanical micro- and nanoresonators show intriguing size dependent properties regarding their elastic behavior. The elastic modulus derived for wire- and pillar-resonators from dynamic flexural mode measurements often deviates significantly from their bulk counterpart and thus makes them interesting for a wide range of applications. This behavior has been mainly assigned to the increasing influence of surface stresses on the resonance frequency when resonators become smaller. However, despite showing also considerable different elastic moduli, some experiments on pillar resonators do not show a distinct size dependence. In order to test different models and the actual origin of the observed deviations, we started to combine nanomechanical flexural mode measurements with picosecond ultrasonics to obtain not only information about the effective elastic modulus but spatially resolve the local elastic properties inside pillar resonators. The pillars under investigation have been recently used to study mechanical coupling phenomena between nanoresonators and have previously shown strong deviations from the bulk modulus in flexural mode measurements. Resolving the elastic modulus along the pillars long axis becomes possible utilizing time resolved Brillouin scattering. Here, a pump-probe approach tracks a moving acoustic pulse inside the pillars via the interference caused by the light portions reflected off the interfaces and the moving acoustic pulse. This allows to monitor the local sound velocity inside the pillar and subsequent comparison with the results from flexural mode measurements become possible.

The German Aerospace Center (DLR) is the national aeronautics and space research centre of the Federal Republic of Germany. Its extensive research and development work in aeronautics, space, energy, transport, security, and digitalisation is integrated into national and international cooperative ventures. With more than 9000 co-workers in 50 institutes and research facilities it is the largest engineering research organizations in Germany. The Institute of Technical Physics of the German Aerospace Center located in Stuttgart and Lampoldshausen has a long standing tradition in the development of advanced laser technology, laser systems, and laser based instrumentation. Combined with the application portfolio in the areas of aerospace and security research the Institute is a partner in the development of laser based instrumentation for several applications. These range from directed energy systems, laser propagation in the atmosphere, laser based ranging and tracking of space debris, optical air data systems to laser based standoff detection of hazardous substances. In total 80 co-workers work on projects including the development realization of demonstrators with high technology readiness levels with a focus on serving the end users with solutions for today’s challenges in security and defence scenarios. The talk will cover our activities in the realization of laser systems for stand-off CBE detection as well as detection system carrier by UAVs.

Supplemental Document

International audience; We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a In0.27Ga0.73As/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.

We present an advancement in applications of ultrafast optics in picosecond laser ultrasonics - laser-induced comb-like coherent acoustic phonons are optically controlled in a In0.27Ga0.73As/GaAs multiple quantum well (MQW) structure by a high-speed asynchronous optical sampling (ASOPS) system based on two GHz Yb:KYW lasers. Two successive pulses from the same pump laser are used to excite the MQW structure. The second pump light pulse has a tunable time delay with respect to the first one and can be also tuned in intensity, which enables the amplitude and phase modulation of acoustic phonons. This yields rich temporal acoustic patterns with suppressed or enhanced amplitudes, various wave-packet shapes, varied wave-packet widths, reduced wave-packet periods and varied phase shifts of single-period oscillations within a wave-packet. In the frequency domain, the amplitude and phase shift of the individual comb component present a second-pump-delay-dependent cosine-wave-like and sawtooth-wave-like variation, respectively, with a modulation frequency equal to the comb component frequency itself. The variations of the individual component amplitude and phase shift by tuning the second pump intensity exhibit an amplitude valley and an abrupt phase jump at the ratio around 1:1 of the two pump pulse intensities for certain time delays. A simplified model, where both generation and detection functions are assumed as a cosine stress wave enveloped by Gaussian or rectangular shapes in an infinite periodic MQW structure, is developed in order to interpret acoustic manipulation in the MQW sample. The modelling agrees well with the experiment in a wide range of time delays and intensity ratios. Moreover, by applying a heuristic-analytical approach and nonlinear corrections, the improved calculations reach an excellent agreement with experimental results and thus enable to predict and synthesize coherent acoustic wave patterns in MQW structures.

Terahertz frequency metrology by radio frequency downconversion using femtosecond optical sampling relies on the harmonic factor retrieval between the terahertz frequency and the optical sampling rate. At typical femtosecond laser repetition rates, this imposes an ambiguity for frequency metrology. We report on a dual-comb sampling system for the unambiguous frequency measurement of terahertz quantum cascade lasers with hertz-level precision. Two Ti:sapphire oscillators with 10 GHz repetition rate are used for the electro-optic sampling of terahertz radiation at 2.5 THz emitted by actively mode-locked terahertz quantum cascade lasers with 9.7 GHz and 19.6 GHz repetition rates. By coherent downconversion, the emitted terahertz waveforms are measured in the radio frequency domain. The terahertz frequency comb is stabilized by employing a phase-locked loop on a radio frequency beat-note signal. A second infrared sampling comb is used to measure the absolute frequencies of the terahertz radiation. This method, which is based on the detuning of the sampling repetition rates, allows the direct retrieval of the quantum cascade laser’s absolute frequency in real time without using additional optical frequency references for calibration. In order to demonstrate the feasibility of the stabilization and readout technique, a high-resolution spectroscopy measurement on gaseous methanol is presented. published