Phononic crystals (PnCs) control the transport of sound and heat similar to the control of electric currents by semiconductors and metals or light by photonic crystals. Basic and applied research on PnCs spans the entire phononic spectrum, from seismic waves and audible sound to gigahertz phononics for telecommunications and thermal transport in the terahertz range. Here, we review the progress and applications of PnCs across their spectrum, and we offer some perspectives in view of the growing demand for vibrational isolation, fast signal processing, and miniaturization of devices. Current research on macroscopic low-frequency PnCs offers complete solutions from design and optimization to construction and characterization, e.g., sound insulators, seismic shields, and ultrasonic imaging devices. Hypersonic PnCs made of novel low-dimensional nanomaterials can be used to develop smaller microelectromechanical systems and faster wireless networks. The operational frequency, compactness, and efficiency of wireless communications can also increase using principles of optomechanics. In the terahertz range, PnCs can be used for efficient heat removal from electronic devices and for novel thermoelectrics. Finally, the introduction of topology in condensed matter physics has provided revolutionary designs of macroscopic sub-gigahertz PnCs, which can now be transferred to the gigahertz range with advanced nanofabrication techniques and momentum-resolved spectroscopy of acoustic phonons.

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.

A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively.

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.

We present a novel contactless technique for thermal conductivity determination and thermal field mapping based on creating a thermal distribution of phonons using a heating laser, while a second laser probes the local temperature through the spectral position of a Raman active mode. The spatial resolution can be as small as 300 nm, whereas its temperature accuracy is ±2 K. We validate this technique investigating the thermal properties of three free-standing single crystalline Si membranes with thickness of 250, 1000, and 2000 nm. We show that for two-dimensional materials such as free-standing membranes or thin films, and for small temperature gradients, the thermal field decays as T(r) ∝ ln(r) in the diffusive limit. The case of large temperature gradients within the membranes leads to an exponential decay of the thermal field, T ∝ exp[ − A·ln(r)]. The results demonstrate the full potential of this new contactless method for quantitative determination of thermal properties. The range of materials to which this method is applicable reaches far beyond the here demonstrated case of Si, as the only requirement is the presence of a Raman active mode.