Kenneth J. Chau

All-angle negative refraction and active flat lensing of ultraviolet light

Decades ago, Veselago predicted that a material with simultaneously negative electric and magnetic polarization responses would yield a ‘left-handed’ medium in which light propagates with opposite phase and energy velocities—a condition described by a negative refractive index. He proposed that a flat slab of left-handed material possessing an isotropic refractive index of −1 could act like an imaging lens in free space. Left-handed materials do not occur naturally, and it has only recently become possible to achieve a left-handed response using metamaterials, that is, electromagnetic structures engineered on subwavelength scales to elicit tailored polarization responses. So far, left-handed responses have typically been implemented using resonant metamaterials composed of periodic arrays of unit cells containing inductive–capacitive resonators and conductive wires. Negative refractive indices that are isotropic in two or three dimensions at microwave frequencies have been achieved in resonant metamaterials with centimetre-scale features. Scaling the left-handed response to higher frequencies, such as infrared or visible, has been done by shrinking critical dimensions to submicrometre scales by means of top-down nanofabrication. This miniaturization has, however, so far been achieved at the cost of reduced unit-cell symmetry, yielding a refractive index that is negative along only one axis. Moreover, lithographic scaling limits have so far precluded the fabrication of resonant metamaterials with left-handed responses at frequencies beyond the visible. Here we report the experimental implementation of a bulk metamaterial with a left-handed response to ultraviolet light. The structure, based on stacked plasmonic waveguides, yields an omnidirectional left-handed response for transverse magnetic polarization characterized by a negative refractive index. By engineering the structure to have a refractive index close to −1 over a broad angular range, we achieve Veselago flat lensing, in free space, of arbitrarily shaped, two-dimensional objects beyond the near field. We further demonstrate active, all-optical modulation of the image transferred by the flat lens.

Enhancing the efficiency of slit-coupling to surface-plasmon-polaritons via dispersion engineering

We describe a simple method for enhancing the efficiency of coupling from a free-space transverse-magnetic (TM) plane-wave mode into a surface-plasmon-polariton (SPP) mode. The coupling structure consists a metal film with a dielectric-filled slit and a planar, dielectric layer on the slit-exit side of the metal film. By varying the dielectric layer thickness, the wavevector of the SPP mode on the metal surface can be tuned to match the wavevector magnitude of the modes emanating from the slit exit, enabling high-efficiency radiation coupling into the SPP mode at the slit exit. An optimal dielectric layer thickness of approximately 100 nm yields a visible-frequency SPP coupling efficiency approximately 4 times greater than the SPP coupling efficiency without the dielectric layer. Commensurate coupling enhancement is observed spanning the free-space wavelength range 400 nm < or = lambda(0) < or = 700 nm. We map the dependence of the SPP coupling efficiency on the slit width, the dielectric-layer thickness, and the incident wavelength to fully characterize this SPP coupling methodology.

A gigahertz surface magneto-plasmon optical modulator

We propose a novel high-speed magnetooptic (MO) modulator based on the optical excitation of surface magneto-plasmon (SMP) waves in bismuth-substituted yttrium iron garnet (Bi-YIG). A model describing magnetization dynamics of the Bi-YIG film is used in conjunction with an SMP reflectivity model to evaluate the performance of the device. In developing the reflectivity model, the dispersion relation for the SMP waves at a Bi-YIG-metal interface is derived. The optical response of the device is modeled for various driving pulses, and the performance of the device is discussed in terms of response time, efficiency, and bandwidth. Using practical material parameters, it is found that the modulator has an improved efficiency over MO modulation devices relying on bulk propagation for comparable driving current pulse durations, as well as the added benefits of a miniature design, multigigahertz operation, and tunable bandwidth. The analysis presented here provides a useful framework for the design and development of magneto-plasmon photonic devices.

Simulations of radiation pressure experiments narrow down the energy and momentum of light in matter.

Consensus on a single electrodynamic theory has yet to be reached. Discord was seeded over a century ago when Abraham and Minkowski proposed different forms of electromagnetic momentum density and has since expanded in scope with the gradual introduction of other forms of momentum and force densities. Although degenerate sets of electrodynamic postulates can be fashioned to comply with global energy and momentum conservation, hope remains to isolate a single theory based on detailed comparison between force density predictions and radiation pressure experiments. This comparison is two-fold challenging because there are just a handful of quantitative radiation pressure measurements over the past century and the solutions developed from different postulates, which consist of approximate expressions and inferential deductions, are scattered throughout the literature. For these reasons, it is appropriate to conduct a consolidated and comprehensive re-analysis of past experiments under the assumption that the momentum and energy of light in matter are degenerate. We create a combined electrodynamic/fluid dynamic simulation testbed that uses five historically significant sets of electrodynamic postulates, including those by Abraham and Minkowski, to model radiation pressure under diverse configurations with minimal assumptions. This leads to new interpretations of landmark investigations of light momentum, including the Balazs thought experiment, the Jones-Richards and Jones-Leslie measurements of radiation pressure on submerged mirrors, observations of laser-deformed fluid surfaces, and experiments on optical trapping and tractor beaming of dielectric particles. We discuss the merits and demerits of each set of postulates when compared to available experimental evidence and fundamental conservation laws. Of the five sets of postulates, the Abraham and Einstein-Laub postulates provide the greatest consistency with observations and the most physically plausible descriptions of electrodynamic interactions. Force density predictions made by these two postulates are unique under many conditions and their experimental isolation is potentially within reach.

Experimental and numerical investigation of terahertz transmission through strongly scattering sub-wavelength size spheres

We report on experimental and numerical studies of terahertz propagation in strongly scattering random media. The experimental variations of the terahertz pulse group delay and scattering-induced effects such as temporal pulse distortion, spectral decay, and power attenuation as a function of sample thickness are in excellent agreement with those predicted from a Monte Carlo photon migration model. The transmitted pulses are analyzed with a classical effective medium approximation. Due to the subwavelength size of the random scatterers, it is found that the effective medium approximation underestimates the accumulated pulse phase acquired by the high frequencies during pulse propagation.

A plasmonic random composite with atypical refractive index

We present a material composite consisting of randomly oriented elements governed by non-resonant interactions. By exploiting near-field plasmonic interaction in a dense ensemble of subwavelength-sized dielectric and metallic particles, we reveal that the group refractive index of the composite can be increased to be larger than the effective refractive indices of constituent metallic and dielectric parent composites. These findings introduce a new class of engineered photonic materials having customizable and atypical optical constants.

Revisiting the Balazs thought experiment in the case of a left-handed material: electromagnetic-pulse-induced displacement of a dispersive, dissipative negative-index slab.

We propose a set of postulates to describe the mechanical interaction between a plane-wave electromagnetic pulse and a dispersive, dissipative slab having a refractive index of arbitrary sign. The postulates include the Abraham electromagnetic momentum density, a generalized Lorentz force law, and a model for absorption-driven mass transfer from the pulse to the medium. These opto-mechanical mechanisms are incorporated into a one-dimensional finite-difference time-domain algorithm that solves Maxwells equations and calculates the instantaneous force densities exerted by the pulse onto the slab, the momentum-per-unit-area of the pulse and slab, and the trajectories of the slab and system center-of-mass. We show that the postulates are consistent with conservation of global energy, momentum, and center-of-mass velocity at all times, even for cases in which the refractive index of the slab is negative or zero. Consistency between the set of postulates and well-established conservation laws reinforces the Abraham momentum density as the one true electromagnetic momentum density and enables, for the first time, identification of the correct form of the electromagnetic mass density distribution and development of an explicit model for mass transfer due to absorption, for the most general case of a ponderable medium that is both dispersive and dissipative.

High-throughput diffraction-assisted surface-plasmon-polariton coupling by a super-wavelength slit

We propose a novel SPP coupling scheme capable of high SPP throughput and high SPP coupling efficiency based on a slit of width greater than the wavelength, immersed in a uniform dielectric. The dispersive properties of the slit are engineered such that the slit sustains a low-loss higher-order waveguide mode just above cutoff, which is shown to be amenable to wavevector matching to the SPP mode at the slit exit. The SPP throughput and SPP coupling efficiency are quantified by numerical simulations of visible light propagation through the slit for varying width and dielectric refractive index. An optimal SPP coupling configuration satisfying wavevector matching is shown to yield an order-of-magnitude greater SPP throughput than a comparable slit of sub-wavelength width and a peak SPP coupling efficiency ≃ 68%. To our knowledge, this is the first investigation of coupling between higher-order waveguide modes in slits of super-wavelength width and SPP modes.

Subsurface probing of terahertz particle plasmons

Here, the authors exploit the potential barrier at the interface between dissimilar metals to probe frequency dependent subsurface charge induction on metallic microparticles excited with terahertz radiation. The authors’ experimental data and model show that terahertz electromagnetic charge induction on the microparticles occurs over a distance comparable to the skin depth. This work provides a technique to probe subsurface terahertz charge induction in subwavelength metallic structures and may open research avenues of low-frequency plasmonic behavior.

Terahertz Time-Domain Investigation of Axial Optical Activity from a Sub-wavelength Helix.

Chiral media are characterized by preferential interaction with either left- or right- circularly polarized radiation, whereupon an optically active medium and its enantiomorph possess rotary powers of opposing sign due to mirror handedness of their micro- or nano-structures. Here, we report on the first time-resolved investigations of few-cycle pulse propagation along the axis of a sub-wavelength size helix. Time-resolved measurements of the electric field pulse scattered from the helix enable temporal discriminations of transient scattering mechanisms within the helix. Our main finding is that polarization circularization associated with axial propagation through the helix is non-instantaneous, and requires several picoseconds to develop before reaching steady state values. Using a 3D FDTD model, we describe the field and Poynting vector dynamics within the helix leading to steady state polarization circularization. Our conclusions not only support the established picture that optical activity arises from multiple scattering within the helical structure, but also show that this operative mechanism requires a finite time to induce steady state polarization circularization.