Hung-Pin Chen

Constrained modeling of domain patterns in rhombohedral ferroelectrics

A nonconventional phase-field model is developed to predict ferroelectric domain structures. It employs a set of field variables motivated by multirank laminates to represent energy-minimizing domain configurations, giving rise to an explicit expression of the energy-well structure. The framework is applied to domain simulation in the rhombohedral phase assuming that polarization is close to the ground states. An electromechanical self-accommodation pattern consisting of eight rhombohedral variants and an engineered domain configuration are predicted and found in good agreement with those observed in experiment.

Magnetoelastic domains and magnetic field-induced strains in ferromagnetic shape memory alloys by phase-field simulation

Magnetoelastic domains in ferromagnetic shape memory alloys evolve through either variant rearrangement or magnetization rotation, resulting in a large or a small magnetic field-induced strain depending on the magnitude of applied compressive stress. These phenomena are simulated in this letter using an unconventional phase-field model motivated by energy-minimizing multirank laminated domain structures. The results agree well with experiments, and confirm the analysis of Ma and Li [Appl. Phys. Lett. 90, 172504 (2007)] based on an energy minimization theory.

Domain pattern and piezoelectric response across polymorphic phase transition in strained bismuth ferrite films

A model is developed to investigate the domain pattern and piezoelectric response across the polymorphic phase transition in strained epitaxial bismuth ferrite films. The orientations of stripelike pattern of the mixed rhombohedral and tetragonal phases are predicted as the consequence of competition between elastic and depolarization energies. The abnormally large piezoelectric response is attributed to the strain-driven softening in dielectric stiffness. The results are in good agreement with recent experimental observations and provide a guidance for developing similar strain-driven polymorphic phase transition in other related perovskite material systems.

Femtosecond ultrasonic spectroscopy using a piezoelectric nanolayer: Hypersound attenuation in vitreous silica films

We report ultra-broadband ultrasonic spectroscopy with an impedance-matched piezoelectric nanolayer, which enables optical generation and detection of a 730-fs acoustic pulse (the width of ten lattice constants). The bandwidth improvement facilitates THz laser ultrasonics to bridge the spectral gap between inelastic light and x-ray scatterings (0.1-1 THz) in the studies of lattice dynamics. As a demonstration, this method is applied to measure sound attenuation α in a vitreous SiO2 thin film. Our results extend the existing low-frequency data obtained by ultrasonic-based and light scattering methods and also show a α∝ f2 behavior for frequencies f up to 650 GHz.

Femtosecond excitation of radial breathing mode in 2-D arrayed GaN nanorods

Radial breathing oscillation of 2-D arrayed GaN nanorods was successfully excited in rods with different diameters by using femtosecond transient reflectivity measurement. Through analyzing thus measured diameter dependent oscillation frequency, we discovered that modification of the mechanical property appeared in the 2-D arrayed piezoelectric GaN nanorods, fabricated on top of a bulk substrate, when the rod diameter was on the order of or less than 50 nm. Our measurement observed a much reduced elastic stiffness constant (C11) of 193 ± 24 GPa in 35nm diameter nanorods, compared with the 365 ± 2 GPa in bulk GaN. This size-reduction induced mechanical modification would be a critical factor to be considered for future sensing and energy applications. Our study also provides a new spectroscopic method to explore the size-reduction-induced softening effect through the measurement of the radial breathing oscillations.

Femtosecond laser-ultrasonic investigation of plasmonic fields on the metal/gallium nitride interface

By using femtosecond laser-ultrasonic, we demonstrate an approach to study the surface plasmon field optically excited in the interface between metal and a semiconductor thin film. By femtosecond impulsive excitation on gallium–nitride (GaN), different optical probe signals were observed when the impulse-excited nanoacoustic pulse propagated through the metal film and metal nanoslits. By analyzing the shape and temporal response of thus induced acousto-optical signals, our femtosecond laser-ultrasonic study not only reveals the plasmonic field distribution optically excited in the metal/substrate interface but also confirms that the penetration depth of surface plasmon field into the substrate agrees well with a simulation result.

Effects of hydration levels on the bandwidth of microwave resonant absorption induced by confined acoustic vibrations

We found the hydration levels on the capsid surface of viruses can affect the bandwidth of microwave resonant absorption (MRA) induced by the confined acoustic vibrations (CAV). By decreasing the pH value of solution down to 5.2 or inactivating the capsid proteins, we enhanced the surface hydrophilicity and increased the magnitude of surface potentials. Both of these surface manipulations raised the surface affinity to water molecules and narrowed the bandwidths of CAV-induced MRA. Our results validate the viscoelastic transition of hydration shells.

Efficient excitation of guided acoustic waves in semiconductor nanorods through external metallic acoustic transducer

We demonstrate that guided acoustic waves inside a nanorod can be excited through an external metallic acoustic transducer. By attaching gold nanodisks on top of GaAs nanorods, the femtosecond optical excitation on the external acoustic transducer enables the generation of guided acoustic waves in the rods. The propagation behavior and mode shape of the observed guided acoustic waves are analyzed. These observations would not only lead to the development of superior external transducers for acoustic imaging, but also provide an experimental system for the study of the acoustic phonon transport behavior in nanorods and nanowires.

Phase-field modeling of martensitic microstructure with inhomogeneous elasticity

A phase-field model accounting for elastic inhomogeneity is established for microstructure study in martensitic materials. It is motivated by Hashin-Shtrikman variational formulation by introducing a homogeneous comparison medium and a polarized stress field. As a result, the driving force due to stress can be computed in the equivalent homogeneous medium since it is formally identical to that in the actual inhomogeneous solid. The model is applied to the simulations of three-dimensional self-accommodation patterns of microstructure for tetragonal and trigonal martensite. The results show that the former is an atypical pattern while the latter exhibits a common herringbone structure. Finally, the proposed framework also offers advantages of modeling other phase-transforming materials with ability in domain simulations together with effective properties as byproduct.

Highly Directed Radiation Pattern From a THz Photonic Transmitter With a Two-Dimensional Rampart Slot Array Antenna

In this study, we investigate the directivity of the terahertz (THz) radiation pattern from a newly designed two-dimensional rampart slot array antenna integrated in an edge-coupled membrane photonic transmitter. The antenna design is based on the array theory which is well-developed in the microwave regime. By means of the array arrangement of rampart slot antennas, we demonstrate that the 3-dB beam width of the THz radiation pattern at 907 GHz is successfully confined within 30 in both and planes.