We introduce the concept of a quasi-triply-degenerate state (QTDS) and demonstrate its relation to an effective zero refractive index (ZRI) in a two-dimensional (2D) square lattice photonic crystal (PC) of all dielectric pillars. A QTDS is characterized by a triple band structure (TBS), wherein two of the bands manifest a linear dispersion around the Γ-point, i.e. a Dirac-like cone, while the third is a flat zero refractive index (ZRI) band with a frequency that is degenerate with one of the other bands. Significantly, we find that while triple degeneracy of the bands is not observed, the three bands approach one another so close that the observable properties of PCs adapted to the QTDS frequency perform as expected of a ZRI material. We closely examine the ZRI band at the Γ-point and show that by varying the PC material and structure parameters, the ZRI band behavior extends over a wide range of dielectric refractive indices enabling materials made with polymeric constituents. Moreover, the ZRI characteristics are robust and tolerant over a range of frequencies. Furthermore, the computational screening we employ to identify QTDS parameters enables the rational design of low-loss 2D ZRI materials for a broad range of photonic applications, including distributing a common reference phase, cloaking and focusing light.

We present a bottom-up method for the fabrication of nanostructured photonic media and demonstrate proof-ofprinciple using a multiphysics modeling approach. The method involves magnetic field-directed template-assidted self-assembly of magnetic-plasmonic core-shell particles (e.g. Fe3O4@Au) into extended particle superstructures that have a desired photonic functionality. The superparamagnetic core enables adaptive magnetophoretic control of particle motion and provides interparticle coupling that drives the assembly process. The plasmonic shell provides unique optical and photothermal properties of assembled structures. Monte Carlo analysis and fullwave field theory are combined to investigate self-assembly and optical properties of particle superstructures. We demonstrate the method for two different Fe3O4@Au particle assemblies: heptamer particle structures and 1D particle chains. We demonstraet that the former supports Fano resonance behavior, while the latter exhibits extraordinary field enhancement and focused photothermal transduction. .

A study is presented of the self-assembly arid optical liehavior of one-dimensional (1D) chains of colloidal magnetic plasmonic core shell Fe3O4@Au nanoparticles. The superparamagnetic core enables adaptive magnetophoretic control of the particles and provides interparticle coupling that drives the assembly process. The plasmonic shell provides unique optical and photothermal properties of assembled structures. Monte Carlo analysis and full wave field theory are combined to investigate the self-assembly and Optical properties of 1D chains, respectfully. The Monte Carlo simulations demonstrate that the particle volume fraction and surface charge can be tailored to control the formation, aggregation, and spacing of chains. The optical analysis shows that as a chain forms its absorption spectrum red shifts and asymptotically converges to that of an infinite chain. In addition, strong field enhancement occurs in the gaps between neighboring particles at plasmon resonance, while photothermal transduction is focused and enhanced Within the center of the chain. The ability to adaptively Control the magnetic and optical behavior of colloidal plasmonic particles, using an external field opens up a host of opportunities for novel imaging, sensing, theranostic, and optofluidic applications. Moreover, while the theory is demonstrated for the analysis of ID particle chains, the numerical methods readily extend to 2D and 3D assemblies of multilayered core shell nanoparticles that have magnetic and plasmonic layers. As such, this approach is useful for the rational design of optically functional colloids and the bottom-up fabrication of nanostructured interfaces and media with novel magnetic and photonic functionality.

We study the field-directed self-assembly and photothermal behavior of one-dimensional (1D) chains of core-shell Fe3O4@Au magnetic-plasmonic nanoparticles. Monte Carlo analysis is used to predict the self-assembly of the nanoparticles when they are subjected to a uniform magnetic field and confined to a fluidic nanochannel. A coupled photonic and thermodynamic analysis is performed to analyze the optical and photothermal properties of the 1D chain structures. We show for the first time that the assembled chain structures exhibit a pronounced dip in their absorption spectrum at a wavelength that is strongly sensitive to changes in the refractive index of the surrounding medium. The plasmon enhanced features of these structures are well suited for a variety of theranostic modalities as we discuss.

We study optical properties of near-infrared absorbing colloidal plasmonic nanostructures that are of interest for biomedical theranostic applications: SiO2@Au core-shell particles, Au nanocages and Au nanorods. Full-wave field analysis is used to compare the absorption spectra and field enhancement of these structures as a function of their dimensions and orientation with respect to the incident field polarization. Absorption cross-sections of structures with the same volume and LSPR wavelength are compared to quantify differential performance for imaging, sensing and photothermal applications. The analysis shows that while the LSPR of each structure can be tuned to the NIR, particles with a high degree of rotational symmetry, i.e. the SiO2@Au and nanocage particles, provide superior performance for photothermal applications because their absorption is less sensitive to their orientation, which is random in colloidal applications. The analysis also demonstrates that Au nanocages are advantaged with respect to other structures for imaging, sensing and drug delivery applications as they support abundant E field hot spots along their surface and within their open interior. The modeling approach presented here broadly applies to dilute colloidal plasmonic nanomaterials of arbitrary shapes, sizes and material constituents and is well suited for the rational design of novel plasmon-assisted theranostic applications.

We study the photothermal behavior of laser pulsed colloidal metallic nanoframe structures using three-dimensional (3D) photonic and thermofluidic computational models. The models predict the optical response of the nanoframe, photothermal transduction at plasmon resonance, heat transfer to the surrounding fluid, and the dynamics of nanobubble generation under conditions of superheating. We quantify for the first time the photothermal transduction of Au nanoframes as a function of their orientation with respect to the polarization of the incident field and, also, cooperative heating effects as a function of nanoframe spacing. We further demonstrate that laser illumination parameters and nanoframe properties can be tuned to control spatiotemporal heating and nanobubble dynamics.

3D computational analysis is performed to investigate and compare plasmonic and photothermal behavior of silica@Au core–shell nanoparticles and Au nanocages.

We proposed a bottom-up approach for the fabrication of magnetic-plasmonic nanostructures that exhibit tunable plasmon enhanced hots spots and Fano resonance behavior. The nanostructures are formed from the self-assembly of magnetic-plasmonic core-shell nanoparticles. The magnetic core enables magnetophoretic control of particles during assembly, while the plasmonic shell provides interesting and useful optical behavior. We demonstrate proof-of-concept using a combination of Monte Carlo analysis to predict self-assembly and full-wave computational analysis to study the optical properties of the assembled structures. Our analysis demonstrates that viable structures can be assembled using a magnetic template-assisted self-assembly protocol and that flexible tunability of the optical response can be achieved due to the strong sensitivity of nanogap hot spots and Fano resonance features on distinct geometric parameters and the surrounding medium. We demonstrate the self-assembly and Fano resonance response of a heptamer nanostructure formed from Fe3O4@Au nanoparticles and discuss its performance for biosensing. The ability to fabricate such nanostructures using bottom-up methods holds potential for numerous novel applications. Moreover, the photonic modeling approach demonstrated here broadly applies to arbitrary particle geometries, material properties, and assemblies and can be used for the rational design of such applications.

Potential solar energy applications of metamaterial absorbers require spectrally tunable resonance to ensure the overlap with intrinsic absorption profiles of active materials. Although those resonance peaks of metamaterial absorbers can be tuned precisely by lithography-fabricated nanopatterns with different lateral dimensions, they are too expensive for practical large-area applications. In this work, we will report another freedom to tune the spectral position of the super absorbing resonance, i.e. the spacer thickness. The structure was fabricated by evaporating an optically opaque metallic ground plate, a dielectric spacer layer, and a top metallic thin film followed by thermal annealing processes to form discrete nanoparticles. As the spacer thickness increases from 10-90 nm, two distinct shifts of the absorption peak can be observed [i.e. a blue-shift for thinner (10-30 nm) and a red-shift for thicker spacer layers (30-90 nm)]. To understand the physical mechanism, we characterized effective optical constants of top nanopattern layer and loaded them into numerical simulation models. A good agreement with experimental data was only observed in the thick spacer region (i.e. 30-90 nm). The optical behavior for thinner spacers cannot be explained by effective medium theory and interference mechanism. Therefore, a microscopic study has to be performed to reveal strongly coupled modes under metallic nanopatterns, which can be interpreted as separate antennas strongly coupled with the ground plate. Since the resonant position is sensitive to the spacer thickness, a tunable super absorbing metasurface is realizable by introducing spatial tunable materials like stretchable chemical/ biomolecules.

We present a complete description of “topological darkness” in a four-dimensional space regarding optical constants (i.e. n and k) of effective media, wavelengths and incident angles, which is essential for enhanced light-matter interaction in thin-films.

A nanoplasmonic absorber consisting of a metal-dielectric-metal stack with a top layer of densely packed nanostars is designed and numerically investigated. The multimodal absorber can achieve 91% average absorption efficiency in the wavelength range of 450-700 nm. Remarkably, super absorption exhibits desirable insensitivity to both incident angles and polarization states. Electric field intensity was significantly enhanced in a nanogap region between adjacent nanostars, corresponding to an electric field enhancement factor of up to 300. Broadband absorption is attributed to the excitation of multiple resonance modes. Underlying mechanisms of individual modes were well revealed by a thorough physical analysis. The proposed absorber shows great potentials to enhance performances of various optical or optoelectronic devices.

We differentiate the spacer-dependent peak shift in coupled and decoupled super absorbing metasurfaces based on magnetic resonance and interference mechanism, respectively, which was experimentally validated by low-cost structures fabricated by lithography-free processes.

We developed an ultra-broadband super-absorbing metasurface substrate for SERS sensing. In contrast to conventional substrates working for limited excitation wavelengths, this structure can work for almost “all” available laser lines from 450-nm to 1000-nm.

Most reported surface-enhanced Raman spectroscopy (SERS) substrates can work for individual excitation wavelengths only. Therefore, different substrates have to be used for different excitation wavelengths, which consumes more biological/chemical materials, substrates, and measurement time. Here, an ultrabroadband super absorbing metasurface that can work as a universal substrate for low cost and high performance SERS sensing is reported. Due to broadband light trapping and localized field enhancement, this structure can work for almost “all” available laser lines from 450 to 1100 nm. This predicted feature is validated by SERS experiment using five different excitation laser lines, obtaining a high enhancement factor of 5.3 × 10 7 and very good uniformity over large areas.

Although intensive research efforts have been performed to realize compact/portable metamaterial absorbers, the super absorbing metasurface for ultraviolet (UV) wavelengths has not been reported. Here, we propose an aluminum-based metal-dielectric-metal patterned structure to realize super absorption and strong field localization for UV wavelengths. Its feasibility to function as a high performance substrate for UV surface-enhanced Raman spectroscopy will also be discussed.

We developed an ultra-broadband super-absorbing metasurface substrate for SERS sensing. In contrast to conventional substrates working for limited excitation wavelengths, this structure can work for almost “all” available laser lines from 450-nm to 1000-nm.

We demonstrate a strong enhancement of second harmonic generation based on a three-layered super absorbing metasurface consisting of an ultrathin spacer layer sandwiched by an array of random metallic nanoparticles and a metal ground plate.

Phase is an inherent and important feature for coherent processes, which, unfortunately, has not been completely understood for surface plasmon polariton (SPP) and matter interactions. Here we propose a practical approach to extract the phase change dispersion during the interaction between free-space light, SPPs and nanogroove/slit based on far-field information only. Numerical simulation and experimental validation were both presented using nanoslit-groove plasmonic interferometers, agreeing well with theoretical near-field analysis. This approach is generally feasible to extract the intrinsic phase dispersion of other plasmonic nanostructures and can reveal more fundamental features of SPP-matter interactions.

The phase change dispersion during the surface plasmon wave coupling process was extracted experimentally using a slit-groove interferometer and validated through numerical simulation, enriching the fundamental understanding of plamsonic subwavelength optics on a chip.