High acoustic strains in Si through ultrafast laser excitation of Ti thin-film transducers

Abstract

The role of thin-film metal transducers in ultrafast laser-generated longitudinal acoustic phonons in Si (100) monocrystal substrates is investigated. For this purpose degenerate femtosecond pump-probe transient reflectivity measurements are performed probing the Brillouin scattering of laser photons from phonons. The influence of the metallic electron-phonon coupling factor, acoustical impedance and film thickness is examined. An optical transfer matrix method for thin films is applied to extract the net acoustic strain relative strength for the various transducer cases, taking into account the experimental probing efficiency. In addition, a theoretical thermo-mechanical approach based on the combination of a revised two-temperature model and elasticity theory is applied and supports the experimental findings. The results show highly efficient generation of acoustic phonons in Si when Ti transducers are used. This demonstrates the crucial role of the transducer's high electron-phonon coupling constant and high compressive yield strength, as well as strong acoustical impedance matching with the semiconductor substrate.

Large non-thermal contribution to picosecond strain pulse generation using the photo-induced phase transition in VO2

“…When VO 2 is initially in an insulating phase, the PIPT is found to provide a contribution to the photo-generated strain which is as large as 0.45% while the estimated strain pulse generated by thermoelastic effect alone has five times smaller amplitude for the same laser fluence of 8 mJ cm −2 . Finally, total strain observed in our experiments reaches~1.5% at W~12 mJ cm −2 and is comparable with maximal strain amplitudes reported so far [21][22][23][24][25] . Owing to a pronounced nonthermal contribution from PIPT, the transducer heating accompanying such a strain generation is significantly reduced.…”

“…Even upon excitation of a semiconducting transducer by photons near the edge of the bandgap, heating still occurs at high fluences because of the Auger process 18 . As a result, it is still a challenge to generate picosecond strain pulses with reduced heating in a transducer with amplitude higher than 1% [21][22][23][24][25][26] . Therefore, the full practical potential of picosecond ultrasonic applications cannot be realized unless mechanisms, and related materials, for photoinduced strain generation are found which outperform existing ones in both efficiency and tunability, and allow for significant reduction of transducer heating.…”

Generation mechanism of terahertz coherent acoustic phonons in Fe

“…1 seems convincing, the validity of the 2TM is questionable. The assumption of a thermal electron system for early times (∼100 fs) is likely invalid [16] and previous works have speculated how this might affect the strain [32]. We have performed ab initio calculations which suggest that the phonon lifetimes exceed 10 ps (see Fig.…”

Μελέτη υψίσυχνων ελαστικών κυμάτων σε διαστρωματωμένα υλικά μετάλλου/πυριτίου με τη χρήση υπερβραχέων παλμών laser

“…[85][86][87][88][89], ο υψηλής ανάλυσης μικροσκοπικός χαρακτηρισμός διεπιφανειών [90,91], η μελέτη των δονητικών ρυθμών νανοσωματιδίων και νανουλικών [92][93][94], η μέτρηση της ακουστικής εξασθένησης σε γυαλιά [95,96] και η μελέτη βιολογικών υλικών [97,98]. Η γένεση γιγάντιων νανοακουστικών τάσεων σε πολυστρωματικά συστήματα υλικών, έχει ήδη δημοσιευθεί, για παράδειγμα σε Au-Co bilayers, τα οποία ανιχνεύονται με συμβολομετρία πλασμονίων [99] καθώς επίσης και σε Ti:Si υλικά τα οποία ανιχνεύονται με μετρήσεις άντλησης-ελέγχου της διαφορικής ανακλαστικότητας [30].…”

“…Επειδή ο σχηματισμός του ακουστικού παλμού εξαρτάται ισχυρά από την τοπική διαφορά θερμοκρασίας πλέγματος, γίνεται φανερό γιατί οι παλμοί αρνητικού chirp παράγουν τελικά το ισχυρότερο ελαστικό κύμα μέσα στο μέταλλο. Επιπλέον, λόγω της πολύ καλής προσαρμογής της ακουστικής εμπέδησης των δύο υλικών (Ti και Si), ποσοστό ~85%, του παραγόμενου ελαστικού κύματος στο Ti, υπολογίζεται να διαδίδεται μέσα στο Si [30].…”