Confinement of phonon propagation in laser deposited tungsten/polycarbonate multilayers

Abstract

Nanoscale multilayer thin films of W and PC (Polycarbonate) show, due to the great difference of the components' characteristics, fascinating properties for a variety of possible applications and provide an interesting research field, but are hard to fabricate with low layer thicknesses. Because of the great acoustic mismatch between the two materials, such nanoscale structures are promising candidates for new phononic materials, where phonon propagation is strongly reduced. In this article we show for the first time that W/PC-multilayers can indeed be grown with high quality by pulsed laser deposition. We analyzed the polymer properties depending on the laser fluence used for deposition, which enabled us to find best experimental conditions for the fabrication of high-acoustic-mismatch W/PC multilayers. The multilayers were analyzed by fs pump-probe spectroscopy showing that phonon dynamics on the ps time-scale can strongly be tailored by structural design. While already periodic multilayers exhibit strong phonon localization, especially aperiodic structures present outstandingly low phonon propagation properties making such 1D-layered W/PC nano-structures interesting for new phononic applications.

High elastic impedance terasonic hybrid superlattices

“…This interesting direction-dependent elastic wave propagation blocking could be experimentally proved by observing the reflectivity signal from the top of the SL. Indirectly, Döring et al [8] have revealed this effect though the localization of the observed resonance mainly into the thick (20.2 nm) W layer on the top of the SL.…”

“…Writing in New Journal of Physics, Döring et al [8] report the growth of hard tungsten, (W)/soft (polycarbonate, PC) SL by pulsed laser deposition (PLD) with low interfacial roughness and sharp periodicity of the order of 20 nanometers. The utilization of the fabrication technique for polymer deposition is not trivial because of material degradation and ill-defined packing that can significantly impact the soft layer properties.…”

“…The utilization of the fabrication technique for polymer deposition is not trivial because of material degradation and ill-defined packing that can significantly impact the soft layer properties. As shown by Döring et al [8], laser fluence can impact on both surface roughness and elastic Young's modulus (E) of the PC layer, but the mechanism is still elusive; it is hardly conceivable that deposited PC can assume about twenty times higher mechanical strength than it is commonly accepted. Notably, this parameter is critical for the identification of the acoustic resonances in their pump-probe reflectivity experiment.…”

“…The full dispersion ω(k ) (frequency versus phonon wave vector) for transparent SLs can be recorded by Brillouin spectroscopy [4,6,7,9]. Using pump-probe spectroscopy, Döring et al [8] did not obtain ω(k ) of a periodic W/PC SL with a lattice constant of d=17.5 nm. Instead, only a single resonance frequency at 0.32 THz dominated the time-dependent reflectivity signal.…”

“…Driven by numerical FDTD simulations, Döring et al [8] have addressed the blocking of the resonance mode (0.32 THz) whilst penetrating into the last W layer (weak strain in the periodic W/PC in figure 1(b))of an aperiodic W/PC SL, schematically shown in figure 1(b). Again, a single resonance mode was experimentally resolved at 0.135 THz that theoretically can hardly reach the second W layer ( figure 1(b)).…”