2019-01-21, Chang, Ying, Yang, Dexing, Guo, Yuning

To improve the focused ultrasonic beam induced by laser phased array (LPA), the superiority of an improved LPA distribution with conjunction of geometric attenuation and directivity functions of the stimulated ultrasonic beams are investigated theoretically instead of only considering the directivity function. Numerical simulations for the generation of focused longitudinal waves in the thermoelastic regime were implemented to reveal the advantages of the improved LPA design. It is shown that the amplitude of the focused beam increased by 42.1%, and the rise time reduced by 25.0%, as well as spatial sizes narrowed by 50.6% and 41.9% in longitudinal and transverse directions, respectively. In addition, the thermal expansion superposition caused by adjacent laser pulses, which deteriorates the focusing features of the focused beam, should be avoided by setting proper spacing of LPA.

2017-05-11, He, Chuan, Ristow, Oliver, Grossmann, Martin, Brick, Delia, Guo, Yuning, Schubert, Martin, Hettich, Mike, Gusev, Vitalyi, Dekorsy, Thomas

A free-standing GaAs membrane is investigated by pump-probe reflectivity measurements with femtosecond laser pulses of 400-nm wavelength. It is found that the detected wide spectrum of laser-generated coherent strain waves in the membrane does not contain a specific hypersonic frequency. Theoretical analysis reveals that this effect is related to zero sensitivity of the acousto-optic detection at a particular frequency defined by the wavelength of the probe laser pulse on the mechanical free surface of the GaAs membrane. We predict that a similar behavior is expected in Si and Au membranes and films, indicating that the presence of zeros in the spectral transformation function of acousto-optic conversion is a rather general phenomenon in picosecond ultrasonics that has so far been neglected.

2017, Brick, Delia, Engemaier, Vivienne, Guo, Yuning, Grossmann, Martin, Li, Guodong, Grimm, Daniel, Schmidt, Oliver G., Schubert, Martin, Hettich, Mike, Dekorsy, Thomas

We present a detailed experimental and theoretical study of the acoustic phonon modes in rolled-up multilayers with thickness of the layers in the nanometre and diameters in the micrometre range. We compare our results to planar, unrolled multilayers grown by molecular beam epitaxy. For the planar multilayers the experimentally obtained acoustic modes exhibit properties of a superlattice and match well to calculations obtained by the Rytov model. The rolled-up superlattice tubes show intriguing differences compared to the planar structures which can be attributed to the imperfect adhesion between individual tube windings. A transfer matrix method including a massless spring accounting for the imperfect adhesion between the layers yields good agreement between experiment and calculations for up to five windings. Areas with sufficient mechanical coupling between all windings can be distinguished by their acoustic mode spectrum from areas where individual windings are only partially in contact. This allows the spatially resolved characterization of individual tubes with micrometre spatial resolution where areas with varying interface adhesion can be identified.

2018-11, Chang, Ying, Yang, Dexing, Guo, Yuning

This study explores the feasibility of using laser generated ultrasonic wave to detect the interfacial damage of coating-substrate structure. Two Pearson correlation coefficient based post processing algorithms are respectively modified and used in order to evaluate the damage-induced waveform variation and thus position the damage accurately. In addition, the ultrasonic signals of frequency-domain and time-domain are separately studied to optimize the damage visualization performance of the two approaches. The proposed method helps to facilitate nondestructive examination of laser ultrasonic technique.

2017-01-24, Guo, Yuning, Hettich, Mike, Dekorsy, Thomas

The guiding of elastic waves in a two-dimensional graded phononic crystal plate is investigated. This effect is induced by the resonance coupling of attachments and matrix in a silicon pillar-substrate system and the resonance frequencies of guided surface modes can be tuned by tailoring the geometry and material properties of the pillars. The resonance frequencies increase with radius and Young's modulus, and decrease with height and density of the pillars, which provides several possibilities for the guiding of elastic waves. These devices show the capability of spatially selecting different frequencies into designed channels, thus acting as a phononic multi-channel filter.

2017, Guo, Yuning

2017-12-22, Guo, Yuning, Dekorsy, Thomas, Hettich, Mike

A topological state with protected propagation of elastic waves is achieved by appropriately engineering a phononic metamaterial based on 2D pentamode structures in silicon. Gapless edge states in the designed structure, which are characterized by pseudospin-dependent transport, provide backscattering-immune propagation of the elastic wave along bend paths. The role of the states responsible for forward and backward transfer can be interchanged by design.

2017-01-19, Guo, Yuning, Brick, Delia, Grossmann, Martin, Hettich, Mike, Dekorsy, Thomas

The directional waveguiding in a 2D phononic crystal is simulated based on the analysis of equifrequency contours. This approach is utilized to investigate acoustic beam splitting in a defect-free nanostructure in the low GHz range. We find relaxed limitations regarding the source parameters compared to similar approaches in the sonic regime. Finally, we discuss the possibility to design an acoustic interferometer device at the nanoscale at GHz frequencies.

2016, Guo, Yuning, Schubert, Martin, Dekorsy, Thomas

The study of surface modes in phononic crystal waveguides in the hypersonic regime is a burgeoning field with a large number of possible applications. By using the finite element method, the band structure and the corresponding transmission spectrum of surface acoustic waves in phononic crystal waveguides generated by line defects in a silicon pillar-substrate system were calculated and investigated. The bandgaps are caused by the hybridization effect of band branches induced by local resonances and propagating modes in the substrate. By changing the sizes of selected pillars in the phononic crystal waveguides, the corresponding bands shift and localized modes emerge due to the local resonance effect induced by the pillars. This effect offers further possibilities for tailoring the propagation and filtering of elastic waves. The presented results have implications for the engineering of phonon dynamics in phononic nanostructures.