Mao-Kuen Kuo

Purcell effect of nanoshell dimer on single molecule’s fluorescence

The Purcell effect of a nanoshell dimer on the fluorescence of a single molecule placed within the dimers gap is studied. The numerical results show that the nanoshell dimer acts as an antenna, making the energy transfer from an excited molecule to the dimer more efficient, and as a lowpass filter for the radiation of fluorescence to the far field. Moreover, the enhancement factor of a nanoshell dimer on the fluorescence is much higher than that of a solid Au dimer in the longer-wavelength regime.

Determination of elastic constants of a concrete specimen using transient elastic waves

This paper presents the results of measuring the Rayleigh wave as well as longitudinal wave velocities in a concrete specimen using transient elastic waves. The Rayleigh surface wave generated by a steel ball impact on a concrete material is studied in detail and then a method for the determination of Rayleigh surface wave velocity based on the cross‐correlation method is proposed. The longitudinal wave velocity of a concrete specimen is determined by measuring the wavefront arrival of a longitudinal wave. A special triggering device for the accurate determination of the impact time origin is utilized. The measured velocities of both the Rayleigh wave and the longitudinal wave in the concrete specimen are in good agreement with those measured by utilizing the conventional ultrasonic method. It is noted that with the Rayleigh wave and the longitudinal wave velocities measured, the Young’s modulus as well as the shear modulus of a concrete specimen can be obtained in a straightforward way.

Mode-III crack kinking under stress-wave loading

Abstract A horizontally polarized step-stress wave is incident on a semi-infinite crack in an elastic solid. At the instant that the crack tip is struck, the crack starts to propagate in the forward direction, but under an angle κπ with the plane of the original crack. In this paper a self-similar solution is obtained for the particle velocity of the diffracted cylindrical wave field. The use of Chaplygins transformation reduces the problem to the solution of Laplaces equation in a semi-infinite strip containing a slit. The Schwarz-Christoffel transformation is employed to map the semi-infinite strip on a half-plane. An analytic function in the half-plane which satisfies appropriate conditions along the real axis, can subsequently be constructed. The Mode-III stress-intensity factor at the tip of the kinked crack has been computed for angles of incidence varying from normal to grazing incidence, for angles of crack kinking defined by -0.5⩽κ⩽0.5, and for arbitrary subsonic crack tip speeds.

Scan of surface-opening cracks in reinforced concrete using transient elastic waves

This study develops a method to scan the surface cracks of reinforced concrete using transient elastic wave tests. In the tests, an impact is applied at constant intervals along one side of the crack opening, and the surface response of the concrete due to each impact is measured and recorded. Then the procedure is repeated with the source applied on the opposite side. The method of ellipse intersection is adopted to process the surface response of the concrete structure. An image is then constructed that shows the three-dimensional image of the crack. The image can be used to determine the location of the reinforcing steel bars, the thickness of the covering, and to judge whether the crack penetrates through the rebars. Numerical examples and a model test are presented to verify the effectiveness of this method.

Plasmon resonances of spherical and ellipsoidal nanoparticles

Surface plasmon resonance (SPR) of a single metallic nanoparticle is analyzed and simulated via multi-multipole method for 3D problems. The excitation, at optical frequencies, of the SPR leads to an extremely strong field in the vicinity of the nanoparticle. Numerical results indicate that a red shift of SPR is induced for an ellipsoidal nanoparticle, compared to a spherical one. Two structures of core-shelled spherical nanoparticles are also studied; one is a nanosphere of Ag shell with an oxide core, and the other one is Ag core with an oxide shell (ZrO2 or SiO2). Numerical results illustrate the SPR of these two core-shelled structures are quite different from each other and different from that of a solid one. It suggests that one can manipulate the optical response on demand by tuning the core/shell ratio and the permittivity of shell or core.

Locating the crack tip of a surface-breaking crack. Part I. Line crack

Four distinct algorithms to locate the crack tip of a surface-breaking crack using only the arrival time information of the first diffracted waves are described and compared. To illustrate these algorithms, a line crack in a half-plane is considered. The first two algorithms are based mainly on elementary geometric arguments, where the crack tip is formulated as the intersecting point of two ellipses (algorithm 1) and/or three circles (algorithm 2). The other two algorithms are formulated as optimization problems, where cost functions based upon the arrival time data of diffracted waves are constructed. The unknown crack tip coordinates are then determined by minimizing the cost functions through the Lagrange multiplier method (algorithm 3) or the simplex method (algorithm 4). In the numerical experiments, the exact arrival times are superimposed by Gaussian error with different levels to simulate the real extracted arrival times from experimental signals. The numerical optimization method (algorithm 4) is found to have the best performance with respect to noise, as well as for accuracy. Moreover, the recovery of the crack length is much more robust than the orientation and depth.

Wavelength-dependent longitudinal polarizability of gold nanorod on optical torques

This study theoretically investigates the wavelength-dependent longitudinal polarizability of a gold nanorod (GNR) irradiated by a polarized laser beam. The resultant optical torque in terms of the Maxwell stress tensor was analyzed quantitatively using the multiple multipole method. Our results indicate that the real part of the longitudinal polarizability of GNR can be either positive or negative, leading to the parallel or perpendicular modes, respectively. For the parallel and perpendicular modes, the long axis of GNR is rotated to align parallel and perpendicular, respectively, to the polarization direction of the illuminating light. The turning point between these two modes, depending on the aspect ratio (AR) and the size of GNR, nearly coincides with the longitudinal surface plasmon resonance (LSPR). The perpendicular mode ranges from the transverse SPR to LSPR, and the range of the parallel mode is broadband from LSPR to the near infrared regime. Owing to that a larger optical torque and less plasmonic heating are of concern, an efficiency of optical torque is defined to evaluate the performance of different wavelengths. Analysis results indicate that lasers with wavelength in the perpendicular mode are applicable to rotate and align a GNR of a higher AR. For example, the laser of 785 nm (the perpendicular mode) is superior to that of 1064 nm (the parallel mode, off-resonant from LSPR of 955 nm) for rotating a GNR of AR = 4 and radius 20 nm with an orientation of 45° with respect to the laser polarization.

Average enhancement factor of molecules-doped coreshell (Ag@SiO 2 ) on fluorescence

Average enhancement factor (AEF) of a coreshell (Ag@SiO(2)) on the fluorescence of molecules doped within the silica shell is proposed and studied to estimate the overall performance of a large number of coreshells. Using Mie theory and dyadic Greens functions, the enhancement factor (EF) of a coreshell is first calculated for any arbitrarily oriented and located electric dipole embedded in the shell. AEF is then obtained by averaging the individual EF over all possible orientations and positions of the electric dipoles. AEF of a FITC-doped coreshell (radius of Ag core: 25 nm, thickness of shell: 15 nm) irradiated by a laser of 488 nm for FITCs emission at 518 nm is 2.406. It is much smaller than the maximum EF (30.114) of a coreshell containing a single molecule with a radial orientation at its optimal position. For Alexa 430-doped coreshell excited at 428 nm, AEF is 12.34 at the emission of 538 nm.

Plasmonic Fano resonance and dip of Au-SiO2-Au nanomatryoshka

This study theoretically investigates Fano resonances and dips of an Au-SiO2-Au nanomatryoshka that is excited by a nearby electric dipole. An analytical solution of dyadic Greens functions is used to analyze the radiative and nonradiative power spectra of a radial dipole in the proximity of a nanomatryoshka. From these spectra, the plasmon modes and Fano resonances that accompany the Fano dips are identified. In addition, the scattering and absorption spectra of a nanomatryoshka that is illuminated by a plane wave are investigated to confirm these modes and Fano dips. Our results reveal that a Fano dip splits each of the dipole and quadrupole modes into bonding and anti-bonding modes. The Fano dip and resonance result from the destructive interference of the plasmon modes of the Au shell and the Au core. The Fano factors that are obtained from the nonradiative power spectra of the Au shell and the Au core of a nanomatryoshka are in accordance with those obtained from the absorption cross section spectra. Moreover, these Fano factors increase as the plasmonic coupling of the Au shell with the core increases for both dipole and quadrupole modes.

Elastodynamic responses due to anti-plane point impact loadings on the faces of an interface crack along dissimilar anisotropic materials

Abstract The transient elastodynamic fields and the stress intensity factor of a semi-infinite interface crack lying between dissimilar anisotropic half planes are analyzed. The crack is subjected to a pair of suddenly-applied anti-plane concentrated point loadings on its faces, at a distance l away from the crack tip. The problem is first transformed into one with the interface crack lying between dissimilar isotropic half planes by a transformation of relevant coordinates and parameters. The crucial steps in the analysis are then the direct application of integral transforms together with the Wiener-Hopf technique and the Cagniard-de Hoop method, which were previously believed not to be appropriate. Exact expressions are obtained for the resulting mode-III stress intensity factors. KIII, and radiated stress fields as functions of time. The numerical results are presented for both anti-symmetric and symmetric loadings. The results show that KIII is almost constant with time, but experiences discontinuity at a time t = s2l, which is the arrival time of the cylindrical wave emitted from the point loading of the slower medium on the crack tip. Moreover. KIII jumps to the appropriate static value (at t = s2l) for the case of anti-symmetric loading but not for the case of symmetric loading. The results also show that the transient radiated stress component σθẑ has fewer discontinuities and fewer singularities than other stress components. On the other hand, the order of magnitude of σθẑ is one less than that of σrẑ.