Boyang Ding

Spectral and directional reshaping of fluorescence in large area self-assembled plasmonic-photonic crystals.

Spectral and directional reshaping of fluorescence from dye molecules embedded in self-assembled hybrid plasmonic-photonic crystals has been examined. The hybrid crystals comprise two-dimensional hexagonal arrays of dye-doped dielectric nanospheres, capped with silver semishells. Comparing the reshaped fluorescence spectra with measured transmission/reflection spectra and numerical calculations reveals that the spectral and directional reshaping of fluorescence is the result of its coupling to photonic crystal Bloch modes and to void plasmons localized inside the silver caps.

Manipulating light absorption in dye-doped dielectric films on reflecting surfaces

We experimentally and numerically developed a tunable absorbing nanoscale thin-film system, comprising of dye molecules doped dielectric coatings on reflecting surfaces, the absorption behaviors of which can be flexibly tuned by adjusting the system parameters, i.e. the coating thickness and the doping concentration of dye molecules. Specifically, with appropriate system parameters, our absorbing thin-film system exhibits very directional and polarization dependent absorption properties, which can be significantly altered if applied with different parameters. Calculations demonstrate the unique absorption behaviors are a result of coupling between molecular absorption and Fabry-Perot resonances in the thin-film cavity. In addition, we theoretically show that both the spectral and directional range of the absorption in the thin-film system can be intentionally regulated by doping dyes with different absorption band and setting proper excitation conditions of Fabry-Perot resonances.

Dye-doped spheres with plasmonic semi-shells: Lasing modes and scattering at realistic gain levels

Summary We numerically simulate the compensation of absorption, the near-field enhancement as well as the differential far-field scattering cross section for dye-doped polystyrene spheres (radius 195 nm), which are half-covered by a silver layer of 10–40 nm thickness. Such silver capped spheres are interesting candidates for nanoplasmonic lasers, so-called spasers. We find that spasing requires gain levels less than 3.7 times higher than those in commercially available dye-doped spheres. However, commercially available concentrations are already apt to achieve negative absorption, and to narrow and enhance scattering by higher order modes. Narrowing of the plasmonic modes by gain also makes visible higher order modes, which are normally obscured by the broad spectral features of the lower order modes. We further show that the angular distribution of the far-field scattering of the spasing modes is by no means dipole-like and is very sensitive to the geometry of the structure.

Mode Modification of Plasmonic Gap Resonances Induced by Strong Coupling with Molecular Excitons

Plasmonic cavities can be used to control the atom-photon coupling process at the nanoscale, since they provide an ultrahigh density of optical states in an exceptionally small mode volume. Here we demonstrate strong coupling between molecular excitons and plasmonic resonances (so-called plexcitonic coupling) in a film-coupled nanocube cavity, which can induce profound and significant spectral and spatial modifications to the plasmonic gap modes. Within the spectral span of a single gap mode in the nanocube-film cavity with a 3 nm wide gap, the introduction of narrow-band J-aggregate dye molecules not only enables an anticrossing behavior in the spectral response but also splits the single spatial mode into two distinct modes that are easily identified by their far-field scattering profiles. Simulation results confirm the experimental findings, and the sensitivity of the plexcitonic coupling is explored using digital control of the gap spacing. Our work opens up a new perspective to study the strong coupling process, greatly extending the functionality of nanophotonic systems, with the potential to be applied in cavity quantum electrodynamic systems.

Dynamic Response of a Multilayered Poroelastic Half-Space to Harmonic Surface Tractions

The propagator matrix method is developed to study the dynamic response of a multilayered poroelastic half-space to time-harmonic surface tractions. In a cylindrical coordinate system, a method of displacement potentials is applied first to decouple the Biot’s wave equations into four scalar Helmholtz equations, and then, general solutions to those equations are obtained. After that, the propagator matrix method and the vector surface harmonics are employed to derive the solutions for a multilayered poroelastic half-space subjected to surface tractions. It is known that the original propagator algorithm has the loss-of-precision problem when the waves become evanescent. At present, an orthogonalization procedure is inserted into the matrix propagation loop to avoid the numerical difficulty of the original propagator algorithm. Finally, a high-order adaptive integration method with continued fraction expansions for accelerating the convergence of the truncated integral is adopted to numerically evaluate the integral solutions expressed in terms of semi-infinite Hankel-type integrals with respect to horizontal wavenumber. Furthermore, to validate the present approach, the response of a uniform poroelastic half-space is examined using the formulation proposed in this article. It is shown that the numerical results computed with this approach agree well with those computed with the analytical solution of a uniform half-space.

Ultrahigh NA, high aspect ratio interference lithography with resonant dielectric underlayers

High aspect ratio imaging for immersion interference lithography in the ultrahigh numerical aperture regime, where evanescent fields are responsible for the exposure, is demonstrated experimentally using a resonant dielectric underlayer system consisting of HfO stacked upon SiO2. Improvements in producing these high aspect ratio grating structures compared with previous work [P. Mehrotra, C. A. Mack, and R. J. Blaikie, Opt. Express 21, 13710 (2013)] are shown, which has allowed subsequent lift-off pattern transfer for ∼55 nm half-pitch gratings patterned using a 405 nm exposure wavelength, corresponding to better than λ/7 resolution. Resist structures with aspect ratios close to 3:1 (height to half-pitch) demonstrate the limitations of our lithography system and highlight necessary improvements for higher aspect ratio resist structures to be achieved. Model and preliminary experimental results are presented for a scheme to mitigate resist collapse with very high aspect ratio structures.

Sampling error profile analysis (SEPA) for model optimization and model evaluation in multivariate calibration

A novel method called sampling error profile analysis (SEPA) based on Monte Carlo sampling and error profile analysis is proposed for outlier detection, cross validation, pretreatment method and wavelength selection, and model evaluation in multivariate calibration. With the Monte Carlo sampling in SEPA, a number of submodels are prepared and the subsequent error profile analysis yields a median and a standard deviation of the root‐mean‐square error (RMSE) for the submodels. The median coupled with the standard deviation is an estimation of the RMSE that is more predictive and robust because it uses representative submodels produced by Monte Carlo sampling, unlike the normal method, which uses only 1 model. The error profile analysis also calculates skewness and kurtosis for an auxiliary judgment of the estimated RMSE, which is useful for model optimization and model evaluation. The proposed method is evaluated with 3 near‐infrared datasets for wheat, corn, and tobacco. The results show that SEPA can diagnose outliers with more parameters, select more reasonable pretreatment method and wavelength points, and evaluate the model more accurately and precisely. Compared with the results reported in published papers, a better model could be obtained with SEPA concerning RMSECV, RMSEC, and RMSEP estimated with an independent prediction set.

Illumination Dependent Optical Properties of Plasmonic Nanorods Coupled to Thin-Film Cavities

The scattering spectra and intensity of gold nanorods placed at varied distances above gold films have been simulated and measured under various conditions, demonstrating that scattering characteristics of the nanorod-film system are highly dependent on illumination conditions. Studying the surrounding electric fields of nanorods reveals that the illumination-dependent properties of the system are induced by the interference in the nanorod-film system. Both simulations and experiments show that optimising the nanorod-film distance can greatly enhance scattering magnitudes up to ~20 times for certain illumination conditions. We propose an application of the studied system in facilitating photo-thermal conversion.

Body Force and Fluid Source Equivalents for Dynamic Dislocations in Fluid-Saturated Porous Media

In this study, an explicit expression is derived for body forces and fluid sources equivalent to a general dynamic dislocation in a fluid-saturated porous medium. The components of the source discontinuity vector across the source plane to represent dynamic point dislocations are found by using the surface vector harmonics. Furthermore, we demonstrate that point forces, couples, simple fluid sources, and fluid dipoles can be conversely replaced by point dislocations acting across the horizontal plane. It is shown that the discontinuity in the components of the generalized stress-displacement vector for representing these point sources agrees with those found in our previous research, and thus, the validity of our results for equivalent body forces and fluid sources is examined. With these equivalent body forces and fluid sources, the wave fields in stratified fluid-saturated porous solids can be evaluated by using the reflection and transmission matrix method.

Resonant absorption in dielectric thin films for humidity sensing

We have designed and fabricated novel humidity sensors based on thin-film systems capable of sustaining optical resonances in subwavelength thick absorbing dielectric coatings on reflecting surfaces, as a result of coupling between molecular absorption and Fabry-Perot resonances. Specifically, our experiments demonstrate that a strong resonance can be observed in the reflection spectrum of a 105 nm thick Rhodamine 6G-doped polyvinyl alcohol (PVA) coating on a silver surface under certain illumination conditions; this resonance shows an excellent spectral and temporal sensitivity to the change of environmental humidity. In addition, we also demonstrate the humidity sensing potential of a 47 nm thick titanium dioxide coating on an aluminium substrate as a more robust system for practical implementation.