Xiang-Tian Kong

Important efforts are currently under way in order to develop further the nascent field of plasmonic photocatalysis, striving for improved efficiencies and selectivities. A significant fraction of such efforts has been focused on distinguishing, understanding, and enhancing specific energy-transfer mechanisms from plasmonic nanostructures to their environment. Herein, we report a synthetic strategy that combines two of the main physical mechanisms driving plasmonic photocatalysis into an engineered system by rationally combining the photochemical features of energetic charge carriers and the electromagnetic field enhancement inherent to the plasmonic excitation. We do so by creating hybrid photocatalysts that integrate multiple plasmonic resonators in a single entity, controlling their joint contribution through spectral separation and differential surface functionalization. This strategy allows us to create complex hybrids with improved photosensitization capabilities, thanks to the synergistic combination of two photosensitization mechanisms. Our results show that the hot electron injection can be combined with an energy-transfer process mediated by the near-field interaction, leading to a significant increase in the final photocatalytic response of the material and moving the field of plasmonic photocatalysis closer to energy-efficient applications. Furthermore, our multimodal hybrids offer a test system to probe the properties of the two targeted mechanisms in energy-related applications such as the photocatalytic generation of hydrogen and open the door to wavelength-selective photocatalysis and novel tandem reactions.

Although the concept of chirality has its origin in chemistry, the field of photonics is actively exploring and utilizing it. A new family of 2D chiral lattices constructed from fully achiral unit cells is introduced here. This type of chiral structure differs fundamentally from previously reported arrays made of chiral unit cells. In this system, circular dichroism (CD) appears due to the electromagnetic interaction between unit cells and the formation of lattice plasmons (LP). Importantly, the CD is strongly enhanced for anisotropic rectangular lattices, whereas the chiral signal in square lattices is found to be relatively weak. The results cover two configurations. The first has an index‐matched environment, and the second includes an asymmetric arrangement of refractive indices. The index‐matched model is preferable and supports ultrasharp LP resonances. Overall, excellent control of the spectral position and broadening of the CD peaks is also demonstrated by tuning the geometry and matrix refractive indices. These results can be useful for engineering polarization filters and chiral biosensors.

Inverted glancing angle deposition produces closely-spaced, out-of-plane structures around apertures. These nanofingernails exhibit chiral hybrid multi-polar modes, tilted Poynting vector power flows, and hotspots that depend on illuminating circular-polarization handedness.

Significance: Traditional optical microscopy methods are limited by diffraction. This makes it hard to image and study nanostructures in the midinfrared, a relatively unexplored spectral region of chemical significance. Here we use infrared (IR) photothermal heterodyne imaging (IR-PHI), a far-field superresolution optical technique, to study the Fabry–Pérot resonances of single gold nanowires in the 2,800 to 4,000-cm ⁻¹ region. Because the spatial resolution of IR-PHI is on the order of a few hundred nanometers, we are able to directly resolve the structure of the Fabry–Pérot resonances and assign the mode order. This cannot be done with traditional optical measurements. IR absorption spectra recorded using IR-PHI show narrow linewidths, in several cases below the limit imposed by Au’s Drude relaxation parameter.

Despite successful modeling of graphene as a 0.34-nm-thick optical film synthesized by exfoliation or chemical vapor deposition (CVD), graphene-induced shift of surface-plasmon resonance (SPR) of gold films has remained controversial. Here we report the resolution of this controversy by developing a clean CVD graphene transfer method and extending Maxwell-Garnett effective-medium theory (EMT) to two-dimensional (2D) materials. A SPR shift of 0.24° is obtained and it agrees well with 2D EMT in which wrinkled graphene is treated as a 3-nm graphene/air layered composite, in agreement with the average roughness measured by atomic force microscopy. Because the anisotropic built-in boundary condition of 2D EMT is compatible with graphene's optical anisotropy, graphene can be modeled as a film thicker than 0.34 nm without changing its optical property; however, its actual roughness, i.e., effective thickness, will significantly alter its response to strong out-of-plane fields, leading to a larger SPR shift.

The ability to control and manipulate temperature at nanoscale dimensions has the potential to impact applications including heat-assisted magnetic recording, photothermal therapies, and temperature-driven reactivity. One challenge with controlling temperature at nanometer dimensions is the need to mitigate heat diffusion, such that the temperature only changes in well-defined nanoscopic regions of the sample. Here we demonstrate the ability to use far-field laser excitation to actively shape the thermal near-field in individual gold nanorod heterodimers by resonantly pumping either the in-phase or out-of-phase hybridized dipole plasmon modes. Using single-particle photothermal heterodyne imaging, we demonstrate localization bias in the photothermal intensity due to preferential heating of one of the nanorods within the pair. Theoretical modeling and numerical simulation make explicit how the resulting photothermal images encode wavelength-dependent temperature biases between each nanorod within a heterodimer, demonstrating the ability to actively manage the thermal near-field by simply tuning the color of incident light.

Nature is chiral, thus chirality is a key concept required to understand a multitude of systems in physics, chemistry, and biology. The field of optics offers valuable tools to probe the chirality of nanosystems, including the measurement of circular dichroism, the differential interaction strength between matter and circularly polarized light with opposite helicity. Simultaneously, the use of plasmonic systems with giant light‐interaction cross‐sections opens new paths to investigate and manipulate systems on the nanoscale. Consequently, the interest in chiral plasmonic and hybrid systems has continually grown in recent years, due to their potential applications in biosensing, polarization‐encoded optical communication, polarization‐selective chemical reactions, and materials with polarization‐dependent light–matter interaction. Experimentally, chiral properties of nanostructures can be either created artificially using modern fabrication techniques involving inorganic materials, or borrowed from nature using bioassembly or biomolecular templating. Herein, the recent progress in the field of plasmonic chirality is summarized, with a focus on both the theoretical background and the experimental advances in the study of chirality in various systems, including molecular‐plasmonic assemblies, chiral plasmonic nanostructures, chiral assemblies of interacting plasmonic nanoparticles, and chiral metal metasurfaces and metamaterials. The growth prospects of this field are also discussed.