The authors describe how acoustic phonons can directly tunnel through vacuum and, therefore, transmit energy and conduct heat between bodies that are separated by a vacuum gap. This effect is enabled by introducing a coupling mechanism, such as piezoelectricity, that strongly couples electric field and lattice deformation. The electric field leaks into the vacuum as an evanescent field, which leads to finite solid-vacuum-solid transmission probability. Because of strong resonances in the system, some phonons can go through the vacuum gap with (or close to) unity transmission, which leads to significant thermal conductance and heat flux.
Holograms-diffractive elements-are designed and fabricated for shaping millimetre-wave radio fields. Methods for the synthesis of hologram elements are discussed and several beam shapes are tested: plane waves, radio-wave vortices and Bessel beams. Here we present an overview of the methods applied and results obtained with quasi-optical hologram techniques using both amplitude and phase holograms.
A computer-generated binary amplitude hologram is used to transform an initial Gaussian electromagnetic field with spherical phase front at 310 GHz into a non-diffracting Bessel beam. The beam profile is measured with the help of a near field scanner. In contrast to the situation in the optical region, both amplitude and phase information is readily obtainable from the generated field.
Acoustic wave fields both within the active electrode area of a solidly mounted 1.8 GHz bulk acoustic wave resonator, and around it in the surrounding region, are measured using a heterodyne laser interferometer. Plate-wave dispersion diagrams for both regions are extracted from the measurement data. The experimental dispersion data reveal the cutoff frequencies of the acoustic vibration modes in the region surrounding the resonator, and, therefore, the energy trapping range of the resonator can readily be determined. The measured dispersion properties of the surrounding region, together with the abruptly diminishing amplitude of the dispersion curves in the resonator, signal the onset of acoustic leakage from the resonator. This information is important for verifying and further developing the simulation tools used for the design of the resonators. Experimental wave field images, dispersion diagrams for both regions, and the threshold for energy leakage are discussed.
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