Abstract
A compact femtosecond dual-oscillator pump-probe setup with 48 MHz-repetition rate, relying on asynchronous optical sampling, is presented. The relative timing jitter between both lasers over the whole pump-probe delay range is of the order of or lower than 500 fs. We demonstrate that both a picosecond temporal resolution and a 48 MHz spectral resolution combined with the fast acquisition rate inherent to the asynchronous optical sampling allow performing broadband opto-acoustic imaging with a spectrum covering more than two decades from 300 MHz to 150 GHz. As an illustration, the opto-acoustic response of a supported thin film is investigated, revealing high frequency acoustic echoes close to the epicenter as well as low GHz surface acoustic waves propagating up to 40μm away from the epicenter. Semi-analytical calculations have been carried out and perfectly reproduce the dispersion of the surface acoustic waves experimentally observed.
Highlights
The understanding and control of elastic properties of matter at a nanometer scale as well as the investigation of the vibrational landscape of nanostructures in the GHz to THz range has considerably made progress since the emergence of the picosecond opto-acoustics in the mid 80’s [1, 2]
In this article we present an asynchronous optical sampling (ASOPS)-based setup with repetition rates close to 48 MHz and we demonstrate that this system is ideally suited to perform broadband opto-acoustic imaging with a 1 ps-temporal resolution and a 48 MHz-spectral resolution
We describe the structure of the dual-oscillator and we quantify the temporal resolution of the ASOPS pump-probe setup
Summary
The understanding and control of elastic properties of matter at a nanometer scale as well as the investigation of the vibrational landscape of nanostructures in the GHz to THz range has considerably made progress since the emergence of the picosecond opto-acoustics in the mid 80’s [1, 2]. The combined use of ASOPS with picosecond acoustics has emerged from the development of the dual-GHz femtosecond oscillator initiated by Janke et al [3, 12, 13] This high repetition rate is ideally suited for investigating the propagation of coherent acoustic phonons with frequencies ranging from few tens of GHz [14] to several THz [15]. In the picosecond acoustics field, ASOPS pump-probe setups with low repetition rate lasers [6, 8, 26,27,28] have taken benefit from the high temporal resolution of the system for investigating the propagation of sub-THz acoustic phonons [29,30]. The 48-MHz spectral resolution allows to illustrate the dispersion of the PRW in the spectral domain by analyzing the frequency dependence of the group velocity in the 0.3 - 1.2 GHz range
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