Controlling nanoscale acoustic strains in silicon using chirped femtosecond laser pulses

Abstract

The influence of femtosecond laser pulse chirp on laser-generated longitudinal acoustic strains in Si (100) monocrystal substrates is studied. Degenerate femtosecond pump-probe transient reflectivity measurements are performed using a layered structure of thin Ti transducer film on an Si substrate. Experimental results show that acoustic strains, manifested as strong Brillouin oscillations, are more effectively induced when negatively chirped femtosecond laser pulses pump the transducer. These results are theoretically supported by a modified thermo-mechanical model based on the combination of a revised two-temperature model and elasticity theory that takes into account the instantaneous frequency of the chirped femtosecond laser pump pulses.

Ultrafast dynamics of non-equilibrium electrons and strain generation under femtosecond laser irradiation of Nickel

“…Similar results for the strain pulse propagation have been presented in previous works for picosecond light pulses [55,56]. Furthermore, reflectivity changes due to strain generation after ultrashort-pulsed laser irradiation of thin films on silicon surfaces have been also recently investigated [38,57]. Comparing the strain values at z=0 with the results predicted in previous works ( [55,56]), the non-constant ε zz (z=0,t) at all times is attributed to the fact that in the current simulations, lattice temperature rise after irradiation neither occurs instantaneously nor remains constant.…”

“…An experimental validation of the proposed mechanism is required to test the adequacy of the theoretical model, however, the scope of this work is primarily related to the introduction of a consistent theoretical framework that will take into account: (i) the energy balance between the thermal electron and the lattice baths enriched with the contribution of nonthermal electrons, (ii) the influence of the DOS around the Fermi energy (Ni is characterised by a nonconstant DOS around E F ), (iii) a potential variation of the optical parameters during the pulse duration, (iv) the thermomechanical response of the material due to material heating. Nevertheless, theoretical investigation of the thermomechanical response of the material and comparison of the simulation results with experimental observables in previous works where a simpler version of the model was used [38,57] confirm the primary importance and validation of the proposed underlying physical mechanism of laser matter interaction and associated processes. On the other hand, by ignoring the mechanical response of the system, previous works that assume a revised TTM based FIG.…”

The influence of dynamical change of optical properties on the thermomechanical response and damage threshold of noble metals under femtosecond laser irradiation

“…damage threshold estimation). Nevertheless, a similar description of the temporal variation of the dielectric constant and the optical properties has been performed in previous studies in Ag, Cu, Au (where the dbands ~2-3eV are located below the energy Fermi, well larger than the photon energy (~1.55eV) which show a good agreement with experimental observations 39,40,46,60 . In addition, in the present study, the (low) fluences that are used towards highlighting the thermomechanical response of the system lead to relatively small thermal excitation energies (less than 0.3eV) (~k B T e , k B : Boltzmann constant, T e : electron Temperature) which suggests that the region of the electron Density of States (eDOS) affected by thermal excitations is similar to that of the free electron gas model with only s electrons being excited, in principle.…”

“…However, the scope of this work is primarily related to the introduction of a consistent theoretical framework that will estimate the influence of the temporal evolution of the optical parameters to the thermomechanical response of the system due to laser heating which is expected to set the basis for an experimental confirmation. Nevertheless, a theoretical investigation of the thermomechanical response of the material and comparison of the simulation results with experimental observables in previous works where a simpler version of the model was used 46,60 appear to confirm the adequacy of the proposed underlying physical mechanism of laser matter interaction and associated strain-generation related processes. (TTM)…”

“…works for picosecond light pulses 75, 76 . Furthermore, reflectivity changes due to strain generation after ultrashort-pulsed laser irradiation of thin films on silicon surfaces have been also recently investigated 46,60 Comparing the strain values at z=0 with the results predicted in previous works ( 75, 76 ), the nonconstant ε zz (z=0,t) at all times is attributed to the fact that lattice temperature rise after irradiation neither occurs instantaneously nor remains constant. It is evident by estimating the spatial position of the lowest strain or stress values at different times (Figs.9,10 ('Multimedia view')) that the strain and stress pulses propagate at a speed equal to approximately,   L 2μ+λ /ρ =3651m/sec which is close to the experimentally measured longitudinal sound velocity in Au (~3360m/sec 77 ).…”