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Data files for the article "Engineering nanoscale hypersonic phonon transport". Published in Nature Nanotechnology.

Controlling vibrations in solids is crucial to tailor their elastic properties and interaction with light. Thermal vibrations represent a source of noise and dephasing for many physical processes at the quantum level. One strategy to avoid these vibrations is to structure a solid such that it possesses a phononic stop band, that is, a frequency range over which there are no available elastic waves. Here we demonstrate the complete absence of thermal vibrations in a nanostructured silicon membrane at room temperature over a broad spectral window, with a 5.3-GHz-wide bandgap centred at 8.4 GHz. By constructing a line-defect waveguide, we directly measure gigahertz guided modes without any external excitation using Brillouin light scattering spectroscopy. Our experimental results show that the shamrock crystal geometry can be used as an efficient platform for phonon manipulation with possible applications in optomechanics and signal processing transduction., This project has received funding from the European Union’s H2020 FET Proactive project TOCHA (No. 824140) and Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement (No. 754558). The ICN2 authors acknowledge funding from the Severo Ochoa programme from Spanish MINECO (No. SEV-2019-0706), Plan Nacional (RTI2018-093921-A-C44 - SMOOTH) and MCIN project SIP (PGC2018-101743-B-100), as well as by the CERCA Programme Generalitat de Catalunya. O.F. and G.A. are supported by BIST PhD Fellowships, R.C.N. by a Marie Sklodowska-Curie fellowship (No. 897148) and P.D.G. by a Ramon y Cajal fellowship (No. RyC-2015-18124). M.A. and S.S. gratefully acknowledge funding from the Villum Foundation Young Investigator Programme (No. 13170), the Danish National Research Foundation (No. DNRF147 – NanoPhoton), Innovation Fund Denmark (No. 0175-00022 – NEXUS) and Independent Research Fund Denmark (No. 0135-00315 – VAFL).

Optical forces can set tiny objects in states of mechanical self-sustained oscillation, spontaneously generating periodic signals by extracting power from steady sources. Miniaturized self-sustained coherent phonon sources are interesting for applications such as mass-force sensing, intra-chip metrology and intra-chip time-keeping among others. In this paper, we review several mechanisms and techniques that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical cavities, namely stimulated emission, dynamical back-action, forward stimulated Brillouin scattering and self-pulsing., This work was supported by the European Comission project TAILPHOX (ICT-FP7-233883), the Spanish Severo Ochoa Excellence program and the MINECO project PHENTOM (FIS2015-70862-P). DNU and MFC gratefully acknowledge the support of a Ramón y Cajal postdoctoral fellowship and a Severo Ochoa studentship, respectivelyn.

We present and validate a reformulated Akhieser model that takes into account the reduction of thermal conductivity due to the impact of boundary scattering on the thermal phonons' lifetime. We consider silicon nanomembranes with mechanical mode frequencies in the GHz range as textbook examples of nanoresonators. The model successfully accounts for the measured shortening of the mechanical mode lifetime. Moreover, the thermal conductivity is extracted from the measured lifetime of the mechanical modes in the high-frequency regime, thereby demonstrating that the Q-factor can be used as an indication of the thermal conductivity and/or diffusivity of a mechanical resonator., The authors acknowledge the financial support from the FP7 project MERGING (grant no. 309150), as well as from the Spanish MINECO projects nanoTHERM (grant no. CSD2010-0044) and TAPHOR (MAT2012-31392). E.C.A. gratefully acknowledges the Becas Chile 2010 CONICYT fellowship from the Chilean government.

Phoxonic crystals can exhibit dual phononic/photonic band gaps. Therefore, the confinement of both acoustic and optical waves in a phoxonic cavity can allow the enhancement of their interaction. In this paper, we discuss our recent theoretical works on the strength of the optomechanic coupling, based on both photoelastic and moving interfaces mechanisms, in nanobeam phoxonic crystals cavities.

Under the terms of the Creative Commons Attribution (CC BY) license to their work., We report on the modification of the optical and mechanical properties of a silicon 1D optomechanical crystal cavity due to thermo-optic effects in a high phonon/photon population regime. The cavity heats up due to light absorption in a way that shifts the optical modes towards longer wavelengths and the mechanical modes to lower frequencies. By combining the experimental optical results with finite-difference time-domain simulations, we establish a direct relation between the observed wavelength drift and the actual effective temperature increase of the cavity. By assuming that the Young's modulus decreases accordingly to the temperature increase, we find a good agreement between the mechanical mode drift predicted using a finite element method and the experimental one., This work was supported by the EU through the project TAILPHOX (ICT-FP7-233883) and the ERC Advanced Grant SOULMAN (ERC-FP7-321122) and the Spanish projects TAPHOR (MAT2012-31392).