Jones T. K. Wan

Atomic-scale magnetometry of distant nuclear spin clusters via nitrogen-vacancy spin in diamond

The detection of single nuclear spins is an important goal in magnetic resonance spectroscopy. Optically detected magnetic resonance can detect single nuclear spins that are strongly coupled to an electron spin, but the detection of distant nuclear spins that are only weakly coupled to the electron spin has not been considered feasible. Here, using the nitrogen-vacancy centre in diamond as a model system, we numerically demonstrate that it is possible to detect two or more distant nuclear spins that are weakly coupled to a centre electron spin if these nuclear spins are strongly bonded to each other in a cluster. This cluster will stand out from other nuclear spins by virtue of characteristic oscillations imprinted onto the electron spin decoherence profile, which become pronounced under dynamical decoupling control. Under many-pulse dynamical decoupling, the centre electron spin coherence can be used to measure nuclear magnetic resonances of single molecules. This atomic-scale magnetometry should improve the performance of magnetic resonance spectroscopy for applications in chemical, biological, medical and materials research, and could also have applications in solid-state quantum computing.

Interparticle force in polydisperse electrorheological fluids

We have developed a multiple image method and an integral equation approach to compute the interparticle force for a polydisperse electrorheological (ER) fluid in which the suspending particles can have various sizes. As an illustration, we apply the formalism to a pair of dielectric spheres of different radii and calculate the force as a function of the separation. The results show that the point-dipole approximation errs considerably because many-body and multipolar interactions are ignored. The approximation becomes even worse when the particle sizes differ too much in polydisperse ER fluids. From the results, a dipole-induced-dipole (DID) model is proposed for computer simulation. The DID model accounts for many-body and multipolar interactions partially, but is simpler than the coupled-dipole model because a self-consistent solution is not needed.

Dependence of surface plasmon lifetimes on the hole size in two-dimensional metallic arrays

Two-dimensional metallic arrays with different hole sizes have been fabricated by using interference lithography and their optical properties have been studied by using angle-dependent reflectivity measurements. The lifetimes of different surface plasmon polariton (SPP) modes have been determined from the linewidths of reflectivity spectra for different resonance wavelengths and hole sizes. It is found that the lifetimes display an λn dependence, where n is strongly dependent on hole radius. In particular, n is found to be ∼4 for small hole, as expected from Rayleigh dipole scattering, and linearly increases to ∼11 when the hole radius is increased to 177 nm. By plotting n with hole radius, the relationship between n and hole radius is found.

Effective non-linear optical properties of metal-dielectric composites of spheroidal particles

Non-linear optical properties of randomly oriented non-linear spheroidal metal particles in a dielectric host are investigated. Two different Maxwell-Garnett-type approximations are derived based on whether a self-consistency condition on the net polarization is invoked. These two methods lead to quite different spectral density functions. Moreover, we show that the shape of particles has a large influence on the spectral function through the depolarization effects and thereby has a pronounced effect on the optical absorption and non-linear optical susceptibility. We suggest that the self-consistent formalism can be used for large volume fractions, as mutual interaction between different polarizations is included in the theory.

Interparticle force in polydisperse electrorheological fluids: Beyond the dipole approximation

We have developed a multiple image method to compute the interparticle force for a polydisperse electrorheological (ER) fluid. We apply the formalism to a pair of dielectric spheres of different dielectric constants and calculate the force as a function of the separation. The results show that the point-dipole (PD) approximation errs considerably because many-body and multipolar interactions are ignored. The PD approximation becomes even worse when the dielectric contrast between the particles and the host medium is large. From the results, we show that the dipole-induced-dipole (DID) model yields very good agreements with the multiple image results for a wide range of dielectric contrasts and polydispersity. The DID model accounts for multipolar interaction partially and is simple to use in computer simulation of polydisperse ER fluids.

Studies of the plasmonic properties of two-dimensional metallic nanobottle arrays

Two-dimensional metallic nanobottle arrays with different aperture sizes have been fabricated by using interference lithography and their corresponding dispersion relations have been mapped by angle-dependent reflectivity measurement. Verified by finite-difference time-domain simulation, features arising from Bragg scattered surface plasmon polaritons (SPPs), localized SPPs, and Wood’s anomalies are clearly observed from the dispersion relations. In addition, it is found that changing the aperture size of the nanobottle can strongly modify the field strength and pattern of the SPPs. It is expected that nanobottle arrays can find applications in areas such as surface-enhanced Raman scattering and thermovoltaic devices.

The plasmonic properties of elliptical metallic hole arrays

The dispersion relations of two-dimensional Au elliptical hole arrays have been studied by angle-dependent reflectivity. Other than the propagating surface plasmon polaritons (SPPs), both experiment and simulation show a strong local cavity mode emerges when the electric field of the incident light is parallel to the minor axis of the holes. In addition, the calculated field patterns show the field strength increases by five times when the hole shape changes from circular to elliptical. Finally, the interaction between the propagating SPPs and cavity mode has been found and it can be controlled systemically by changing the geometry of the arrays.

Nonlinear ac response of anisotropic composites.

When a suspension consisting of dielectric particles having nonlinear characteristics is subjected to a sinusoidal (ac) field, the electrical response will in general consist of ac fields at frequencies of the higher-order harmonics. These ac responses will also be anisotropic. In this work, a self-consistent formalism has been employed to compute the induced dipole moment for suspensions in which the suspended particles have nonlinear characteristics, in an attempt to investigate the anisotropy in the ac response. The results showed that the harmonics of the induced dipole moment and the local electric field are both increased as the anisotropy increases for the longitudinal field case, while the harmonics are decreased as the anisotropy increases for the transverse field case. These results are qualitatively understood with the spectral representation. Thus, by measuring the ac responses both parallel and perpendicular to the uniaxial anisotropic axis of the field-induced structures, it is possible to perform real-time monitoring of the field-induced aggregation process.

Tunable thermal emission at infrared frequencies via tungsten gratings

The author investigates the manipulation of thermal emission by using one-dimensional tungsten gratings with different groove depths. It is found that, by systematically increasing the depth of the groove, the linearly polarized emission at particular frequencies can be substantially enhanced to achieve that of the blackbody radiation limit, whereas the emission in other frequency ranges shows no noticeable changes. The results can provide useful insights into the design of thermovoltaic applications.

Effects of highly conducting interface and particle size distribution on optical nonlinearity in granular composites

The optical nonlinearity has been investigated in granular metal/dielectric composites taking the effects of highly conducting interfaces between the constituent phases as well as the distribution of particle sizes into account. We compute analytically the spectral function for composites with a binary distribution of particle sizes. For a log-normal distribution of width σ, numerical results show that the spectral density m(s) changes from a delta function for zero width to a prominent peak, accompanied by a broad spectrum for a finite width σ. As a result, the locations of the nonlinearity enhancement peak and the absorption spectrum shift to small frequencies with the increase of the interfacial factor I. The strength of the absorption and the optical nonlinearity are always decreased near resonance with the increase of σ, while the absorption peak and optical nonlinearity peak shift to smaller volume fraction f as I is increased. Moreover, the variation of I will further increase the optical absorptio...