Publications

Super-Planckian near-field radiative heat transfer allows effective heat transfer between a hot and a cold body to increase beyond the limits long known for black bodies. Until present, experimental techniques to measure the radiative heat flow relied on steady-state systems. Here, we present a dynamic measurement approach based on the transient plane source technique, which extracts thermal properties from a temperature transient caused by a step input power function. Using this versatile method, that requires only single sided contact, we measure enhanced radiative conduction up to 16 times higher than the blackbody limit on centimeter sized glass samples without any specialized sample preparation or nanofabrication.

Alberta has one of the largest oil reserves in the world. Large-scale commercial oil production from oil sands in Alberta for the past 40 years has led to accumulation of tailings water in tailings ponds covering areas ranging over 150 km². Less than 1% of this area has been certified as reclaimed leading to both economic and environmental consequences. Research is underway to reduce tailings ponds reclamation time from decades to weeks by developing new polymer flocculants, better tailings treatment methods and recovering bitumen from tailings. Information about impact of residual bitumen on the shear strength, trafficability, densification, hydraulic conductivity, consolidation, post-reclamation settlement for oil sands tailings is insufficient. Outstanding challenges exist in understanding bitumen and clay interaction in tailings to help with the development of techniques which accelerate clay sedimentation and enhance bitumen recovery. To shed light on the bitumen-clay interactions, here we develop advanced three-dimensional optical tomography approaches approaching sub-micron resolution. In this paper, we report, the first ever Total Internal Reflection Fluorescence (TIRF) microscope tomography for Mature Fine Tailings (MFT) samples to reveal bitumen distribution on clay in MFT. We employ a unique evanescent wave illumination approach as opposed to conventional fluorescence microscopy with enhanced axial resolution and high signal-to-noise ratio. The resolution of TIRF is further improved by using an Axial Super-Resolution Evanescent-wave Tomography (AxSET) technique. The information obtained from this study not only gives evidence of the presence of hydrophilic and oleophilic clays but with aid of 3D reconstruction using advance image processing also validates that bitumen is partially coating some of clay surfaces, thus verifying the presence of biwettable clays in oil sands MFT. The advances from our imaging work can aid the development of bitumen recovery techniques for environmental and economic impact.

Strong nanoscale light–matter interaction is often accompanied by ultraconfined photonic modes and large momentum polaritons existing far beyond the light cone. A direct probe of such phenomena is difficult due to the momentum mismatch of these modes with free space light, however, fast electron probes can reveal the fundamental quantum and spatially dispersive behavior of these excitations. Here, we use momentum-resolved electron energy loss spectroscopy (q-EELS) in a transmission electron microscope to explore the optical response of plasmonic thin films including momentum transfer up to wavevectors (q) significantly exceeding the light line wave vector. We show close agreement between experimental q-EELS maps, theoretical simulations of fast electrons passing through thin films and the momentum-resolved photonic density of states (q-PDOS) dispersion. Although a direct link between q-EELS and the q-PDOS exists for an infinite medium, here we show fundamental differences between q-EELS measurements and the q-PDOS that must be taken into consideration for realistic finite structures with no translational invariance along the direction of electron motion. Our work paves the way for using q-EELS as the preeminent tool for mapping the q-PDOS of exotic phenomena with large momenta (high-q) such as hyperbolic polaritons and spatially dispersive plasmons.

The key feature of a thermophotovoltaic (TPV) emitter is the enhancement of thermal emission corresponding to energies just above the bandgap of the absorbing photovoltaic cell and simultaneous suppression of thermal emission below the bandgap. We show here that a single layer plasmonic coating can perform this task with high efficiency. Our key design principle involves tuning the epsilon-near-zero frequency (plasma frequency) of the metal acting as a thermal emitter to the electronic bandgap of the semiconducting cell. This approach utilizes the change in the reflectivity of a metal near its plasma frequency (epsilon-near-zero frequency) to lead to spectrally selective thermal emission, and can be adapted to large area coatings using high temperature plasmonic materials. We provide a detailed analysis of the spectral and angular performance of high temperature plasmonic coatings as TPV emitters. We show the potential of such high temperature plasmonic thermal emitter coatings (p-TECs) for narrowband near-field thermal emission. We also show the enhancement of near-surface energy density in graphene-multilayer thermal metamaterials due to a topological transition at an effective epsilon-near-zero frequency. This opens up spectrally selective thermal emission from graphene multilayers in the infrared frequency regime. Our design paves the way for the development of single layer p-TECs and graphene multilayers for spectrally selective radiative heat transfer applications.

Vacuum consists of a bath of balanced and symmetric positive- and negative-frequency fluctuations. Media in relative motion or accelerated observers can break this symmetry and preferentially amplify negative-frequency modes as in quantum Cherenkov radiation and Unruh radiation. Here, we show the existence of a universal negative-frequency-momentum mirror symmetry in the relativistic Lorentzian transformation for electromagnetic waves. We show the connection of our discovered symmetry to parity-time (PT) symmetry in moving media and the resulting spectral singularity in vacuum fluctuation-related effects. We prove that this spectral singularity can occur in the case of two metallic plates in relative motion interacting through positive- and negative-frequency plasmonic fluctuations (negative-frequency resonance). Our work paves the way for understanding the role of PT-symmetric spectral singularities in amplifying fluctuations and motivates the search for PT symmetry in novel photonic systems.

Dipole–dipole interactions, which govern phenomena such as cooperative Lamb shifts, superradiant decay rates, Van der Waals forces and resonance energy transfer rates, are conventionally limited to the Coulombic near-field. Here we reveal a class of real-photon and virtual-photon long-range quantum electrodynamic interactions that have a singularity in media with hyperbolic dispersion. The singularity in the dipole–dipole coupling, referred to as a super-Coulombic interaction, is a result of an effective interaction distance that goes to zero in the ideal limit irrespective of the physical distance. We investigate the entire landscape of atom–atom interactions in hyperbolic media confirming the giant long-range enhancement. We also propose multiple experimental platforms to verify our predicted effect with phonon–polaritonic hexagonal boron nitride, plasmonic super-lattices and hyperbolic meta-surfaces as well. Our work paves the way for the control of cold atoms above hyperbolic meta-surfaces and the study of many-body physics with hyperbolic media.

Optical tomographic reconstruction of a three-dimensional (3D) nanoscale specimen is hindered by the axial diffraction limit, which is 2–3 times worse than the focal plane resolution. We propose and experimentally demonstrate an axial super-resolution evanescent wave tomography method that enables the use of regular evanescent wave microscopes like the total internal reflection fluorescence microscope beyond surface imaging and achieve a tomographic reconstruction with axial super-resolution. Our proposed method based on Fourier reconstruction achieves axial super-resolution by extracting information from multiple sets of 3D fluorescence images when the sample is illuminated by an evanescent wave. We propose a procedure to extract super-resolution features from the incremental penetration of an evanescent wave and support our theory by one-dimensional (along the optical axis) and 3D simulations. We validate our claims by experimentally demonstrating tomographic reconstruction of microtubules in HeLa cells with an axial resolution of ∼130 nm∼130 nm. Our method does not require any additional optical components or sample preparation. The proposed method can be combined with focal plane super-resolution techniques like stochastic optical reconstruction microscopy and can also be adapted for THz and microwave near-field tomography.

Optical metamaterials have redefined how we understand light in notable ways: from strong response to optical magnetic fields, negative refraction, fast and slow light propagation in zero index and trapping structures, to flat, thin and perfect lenses. Many rules of thumb regarding optics, such as μ = 1, now have an exception, and basic formulas, such as the Fresnel equations, have been expanded. The field of metamaterials has developed strongly over the past two decades. Leveraging structured materials systems to generate tailored response to a stimulus, it has grown to encompass research in optics, electromagnetics, acoustics and, increasingly, novel hybrid material responses. This roadmap is an effort to present emerging fronts in areas of optical metamaterials that could contribute and apply to other research communities. By anchoring each contribution in current work and prospectively discussing future potential and directions, the authors are translating the work of the field in selected areas to a wider community and offering an incentive for outside researchers to engage our community where solid links do not already exist.

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO₂ metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.

A major waste byproduct of oil sands in situ extraction is oil sands tailings, which are a mixture of water, clay, and residual bitumen. These tailings represent a huge ecological footprint in the form of tailings ponds, which not only render large land areas unusable but also prevent reuse of water. The slow dewatering of the tailings ponds poses a major challenge to the industry. The presence of complex inorganic–organic bitumen–clay mixtures in these tailings contributes to this problem. Hence, understanding the nature of the bitumen–clay association and the effect of bitumen on clay particle–particle interactions is important for the development of more effective chemicals or processes to accelerate particle aggregation and sedimentation during dewatering. Previous studies that investigate these interactions used techniques that are sensitive only toward inorganic clay but not sensitive towards organic bitumen. Here, we use a high-resolution total internal reflection fluorescence (TIRF) microscopy to help identify the accurate location and distribution of bitumen in mature fine tailings (MFT) samples. We report the first adaptation of TIRF beyond cell biology for visualization of bitumen and its interaction with clay. The high signal-to-noise ratio of TIRF microscopy and a high contrast between the clay and residual bitumen provide images that reveal a wealth of information about the bitumen coverage on clay as well as clay–clay aggregates and how the bitumen positions itself within these aggregates. These images confirm the presence of hydrophobic fine clay agglomerates along with the hydrophilic clay particles in MFT. It is also observed that bitumen coats clay particles, bridges clay agglomerates, and is mostly absent as free bitumen in the bulk of the MFT sample. Our work paves the way for the use of nanophotonic tools in oil sands imaging and provides strategic suggestions for the development of better methods for clay sedimentation and bitumen recovery.