Ultrafast laser pulse chirp effects on laser-generated nanoacoustic strains in Silicon

M Bakarezos; E Tzianaki; S Petrakis; G Tsibidis; P A Loukakos; V Dimitriou; C Kosmidis; M Tatarakis; N A Papadogiannis

doi:10.1016/j.ultras.2018.01.008

Ultrafast laser pulse chirp effects on laser-generated nanoacoustic strains in Silicon

Ultrasonics. 2018 May:86:14-19. doi: 10.1016/j.ultras.2018.01.008. Epub 2018 Feb 4.

Authors

M Bakarezos¹, E Tzianaki², S Petrakis³, G Tsibidis⁴, P A Loukakos⁴, V Dimitriou³, C Kosmidis², M Tatarakis³, N A Papadogiannis⁵

Affiliations

¹ Centre for Plasma Physics and Lasers, School of Applied Sciences, Technological Educational Institute of Crete, Tria Monastiria, 741 00 Rethymno, Greece; Department of Music Technology & Acoustics Engineering, School of Applied Sciences, Technological Educational Institute of Crete, 1 E. Daskalaki Str., 741 33 Rethymno, Greece. Electronic address: bakarezos@staff.teicrete.gr.
² Physics Department, School of Sciences, University of Ioannina, 451 10 Ioannina, Greece.
³ Centre for Plasma Physics and Lasers, School of Applied Sciences, Technological Educational Institute of Crete, Tria Monastiria, 741 00 Rethymno, Greece.
⁴ Foundation for Research and Technology - Hellas (FORTH), Institute of Electronic Structure and Laser (IESL), N. Plastira 100, Vassilika Vouton, 700 13 Heraklion, Greece.
⁵ Centre for Plasma Physics and Lasers, School of Applied Sciences, Technological Educational Institute of Crete, Tria Monastiria, 741 00 Rethymno, Greece; Department of Music Technology & Acoustics Engineering, School of Applied Sciences, Technological Educational Institute of Crete, 1 E. Daskalaki Str., 741 33 Rethymno, Greece.

PMID: 29407277
DOI: 10.1016/j.ultras.2018.01.008

Abstract

Nanoacoustic strains are generated in Silicon by chirped femtosecond laser pulses using thin Titanium films as transducers. We investigate the effect that the generating laser pulse chirp has on the amplitude of the induced strains, manifested as Brillouin oscillations observed in degenerate femtosecond pump-probe transient reflectivity measurements. The strain amplitude is larger when negatively chirped pulses are used, which is attributed to the more efficient conversion of laser pulse light into acoustic strain in the Titanium transducer. Our present studies clearly show that the dependence of the Brillouin amplitude and the lattice strain is a non-monotonous function of the laser chirp parameter. An optimum negative laser pulse chirp is found for which the strain amplitude is maximized. A detailed thermomechanical model satisfactorily supports the experimental findings. In such a way, it is possible to suppress or enhance the induced nanoacoustic strain amplitude, thus all-optically controlling it by at least a factor of two.

Keywords: Laser pulse chirp; Laser ultrasonics; Nanoacoustic strains; Picosecond ultrasonics; Silicon; Ultrafast acoustics.