Dynamic measurement of femtometer-displacement vibrations in mechanical resonators at microwave frequencies is critical for a number of emerging high-impact technologies including 5G wireless communications and quantum state generation, storage, and transfer. However, the resolution of continuous-wave laser interferometry, the method most commonly used for imaging vibration wavefields, has been limited to vibration amplitudes just below a picometer at several gigahertz. This is insufficient for these technologies since vibration amplitudes precipitously decrease for increasing frequency. Here we present a stroboscopic optical sampling approach for the transduction of coherent super high frequency vibrations. Phase-sensitive absolute displacement detection with a noise floor of 55 fm/√Hz for frequencies up to 12 GHz is demonstrated, achieving higher bandwidth and significantly lower noise floor simultaneously compared to previous work. An acoustic microresonator with resonances above 10 GHz and displacements smaller than 70 fm is measured using the presented method to reveal complex mode superposition, dispersion, and anisotropic propagation.
Strained 2D materials with lattice deformation have the optimal band structure, lattice vibration, and thermal conductivity and various methods have been proposed to introduce strain into 2D materials. However, the creation of localized strain in arbitrary 2D materials in predesigned areas is difficult and challenging. Herein, a versatile approach to creating on‐demand nanobubbles on five different 2D materials using a functional atomic force microscopy (AFM) tip is described. Strain‐induced redshifts are observed from the Raman scattering and photoluminescence (PL) spectra of the 2D materials in the region with the nanobubble arrays. In addition, the localized exciton state is observed from the periphery of a steep WS2 nanobubble by high‐resolution nano‐photoluminescence and is supported by theoretical simulation. These results demonstrate a programmable and reliable method to create localized strain in different 2D materials and pave the way for nanoscale strain engineering of 2D materials to cater to different applications.
Magnetic levitation (MagLev) is a promising technology for density‐based analysis and manipulation of nonmagnetic materials. One major limitation is that extant MagLev methods are based on the static balance of gravitational‐magnetic forces, thereby leading to an inability to resolve interior differences in density. Here a new strategy called “dynamically rotating MagLev” is proposed, which combines centrifugal force and nonlinear magnetic force to amplify the interior differences in density. The design of the nonlinear magnetic force in tandem with centrifugal force supports the regulation of stable equilibriums, enabling different homogeneous objects to reach distinguishable equilibrium orientations. Without reducing the magnetic susceptibility, the dynamically rotating MagLev system can lead to a relatively large change in orientation angle (∆ψ > 50°) for the heterogeneous parts with small inclusions (volume fraction VF = 2.08%). The rich equilibrium states of levitating objects invoke the concept of levitation stability, which is employed, for the first time, to characterize the spatial density heterogeneity of objects. Exploiting the tunable nonlinear levitation behaviors of objects provides a new paradigm for developing operationally simple, nondestructive density heterogeneity characterization methods. Such methods have tremendous potential in applications related to sorting, orienting, and assembling objects in three dimensions.
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