Experimental configuration. The sensors are placed four sites from the actuator and at the end of the crystal. The sensor located four sites away from the actuator is used to measure the localized vibrations within the vicinity of the defect (without being in direct contact with it, so as to avoid affecting its dynamics). The sensor at the end of the crystal is used to measure the transmission through the crystal. For our rectifier geometry, the bifurcation-based rectification mechanism is only clearly evident with a defect placed two particles away from the actuator. For defect particles placed three or more particles away from the actuator, the high attenuation of the signal (with frequency within the band gap 1 ) does not allow sufficient energy from the actuator to arrive to the defect particle.For defect particles placed next to the actuator, we observe that the effect of the boundary is dominant, and the dynamics of the system becomes more chaotic. The chain length of 19 particles was selected as a balance between having high enough attenuation (arising from the band gap) to demonstrate the rectification effect, and having a small enough dissipation of the signal to maximize the experimental tractability. In our numerical simulations, we observe that decreasing the dissipation in the system can increase the transmission efficiency in the forward configuration.

We report the experimental observation of discrete breathers in a one-dimensional diatomic granular crystal composed of compressed elastic beads that interact via Hertzian contact. We first characterize their effective linear spectrum both theoretically and experimentally. We then illustrate theoretically and numerically the modulational instability of the lower edge of the optical band. This leads to the dynamical formation of long-lived breather structures, whose families of solutions we compute throughout the linear spectral gap. Finally, we observe experimentally such localized breathing modes with quantitative characteristics that agree with our numerical results.

We study the interaction of surface acoustic waves (SAWs) with a contact-based vibrational resonance of 1 μm silica microspheres forming a two-dimensional granular crystal adhered to a substrate. The laser-induced transient grating technique is used to excite SAWs and measure their dispersion. The measured dispersion curves exhibit "avoided crossing" behavior due to the hybridization of the SAWs with the microsphere resonance. We compare the measured dispersion curves with those predicted by our analytical model and find excellent agreement. The approach presented can be used to study the contact mechanics and adhesion of micro- and nanoparticles as well as the dynamics of microscale granular crystals.

Reconfiguration of silicon photonic integrated circuits relying on the weak, volatile thermo-optic or electro-optic effect of silicon usually suffers from a large footprint and energy consumption. Here, integrating a phase-change material, Ge 2 Sb 2 Te 5 (GST) with silicon microring resonators, we demonstrate an energy-efficient, compact, non-volatile, reprogrammable platform. By adjusting the energy and number of free-space laser pulses applied to the GST, we characterize the strong broadband attenuation and optical phase modulation effects of the platform, and perform quasi-continuous tuning enabled by thermooptically-induced phase changes. As a result, a non-volatile optical switch with a high extinction ratio, as large as 33 dB, is demonstrated.

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Contact-based vibrations play an essential role in the dynamics of granular materials. Significant insights into vibrational granular dynamics have previously been obtained with reduced-dimensional systems containing macroscale particles. We study contact-based vibrations of a two-dimensional monolayer of micron-sized spheres on a solid substrate that forms a microscale granular crystal. Measurements of the resonant attenuation of laser-generated surface acoustic waves reveal three collective vibrational modes that involve displacements and rotations of the microspheres, as well as interparticle and particle-substrate interactions. To identify the modes, we tune the interparticle stiffness, which shifts the frequency of the horizontal-rotational resonances while leaving the vertical resonance unaffected. From the measured contact resonance frequencies we determine both particle-substrate and interparticle contact stiffnesses and find that the former is an order of magnitude larger than the latter. This study paves the way for investigating complex contact-based dynamics of microscale granular crystals and yields a new approach to studying micro-to nanoscale contact mechanics in multiparticle networks.

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We investigate the tunable vibration filtering properties of one-dimensional diatomic granular crystals composed of arrays of stainless steel spheres and cylinders interacting via Hertzian contact. The arrays consist of periodically repeated three-particle unit cells (steel-cylinder-sphere) in which the length of the cylinder is varied systematically. We apply static compression to linearize the dynamic response of the crystals and characterize their linear frequency spectrum. We find good agreement between theoretical dispersion relation analysis (for infinite systems), state-space analysis (for finite systems), and experiments. We report the observation of up to three distinct pass bands and two finite band gaps and show their tunability for variations in cylinder length and static compression.