This PDF file contains the front matter associated with SPIE Proceedings Volume 11462, including the Title Page, Copyright information, and Table of Contents.

Welcome to the Plasmonics: Design, Materials, Fabrication, Characterization, and Applications XVIII conference

This talk will describe how strongly coupled plasmonic nanoparticle arrays can support high-quality optical modes at high-symmetry points in their photonic band structure. These collective lattice excitations, often called surface lattice resonances (SLRs), show flat-banded modes whose mechanistic origins depend on the nature of the localized plasmon resonance of the constituent nanoparticles. Access to these high-symmetry modes is now possible because of advances in scalable nanofabrication processes.

We demonstrate the direct printing of silver nano- and microstructures via two-photon absorption. The samples show very low porosity, high conductance and optical quality surfaces. These structures might find their application in plasmonics and antenna applications.

We investigated effective organization state of plasmonic metal nanoparticles on a substrate theoretically and experimentally. In particular, we revealed effective organization state of silver nanoplates (AgPLs) which were electrostatically immobilized with various densities on to a glass substrate on refractive index sensitivities. It was clearly observed that the substrate with higher density exhibit the higher sensitivity.

Digital and optical image processing techniques have become ubiquitous and algorithms and optical methods for noise reduction, edge detection, image enhancement and object identification are widely used. Digital approaches, however, cannot access the phase of a field and require energy and time which scale with the amount of data acquired, but conventional optical methods are bulky, prohibiting their use in compact devices and platforms such as smartphones and nanosatellites. This presentation discusses theoretical aspects and experimental results demonstrating the use of metasurfaces and other compact nanophotonic devices for direct spatial frequency filtering of images.

Obtaining nanoscale spatial information without real-space imaging, but from diffraction patterns, is already a valuable tool in metrology We investigate the potential of metasurfaces for nano-optical sensing and metrology with subwavelength resolution. We aim to exploit complex light scattering from a metasurface, programmable illumination, and retrieval of spatial information of a sample from far-field scattering images. We demonstrate an inversion technique based on singular value decomposition whereby we can retrieve the spatial position of a pointlike light source in a plasmon antenna with lambda/50 resolution just on basis of measured far field radiation patterns. Also, we argue that wavefront-shaping applied to plasmon oligomers enables selective generation of sub-diffractive field patterns that could form an optimal and complete basis for spatially-resolved sensing at the nanoscale. Our endeavours use stochastic optimization to choose wavefronts and plasmonic structures.

Single-wall carbon nanotubes (SWCNTs) provide a unique 1D environment in which to examine the physics of interplay between multivalley Dirac band structure and strong Coulomb interactions. Although the 1D nature of individual SWCNTs has stimulated much interest, its macroscopic manifestation has been difficult to observe. We have recently developed a controlled vacuum filtration technique to fabricate wafer-scale films of highly aligned and densely packed SWCNTs. Here, we summarize our accomplishments using these unique samples. We made the first observation of intersubband plasmons – quantum plasmons whose excitation energy is comparable to the quantum confinement energy. We further built an exciton-polariton architecture, which displayed a continuous transition from the ultrastrong-coupling regime to the weak-coupling regime through facile polarization control. The vacuum Rabi splitting exhibited cooperative enhancement when the number of excitons was increased.

Van der Waals (vdW) materials have recently attracted a lot of attentions, because they possess a wide range of optoelectronic properties and can be applied over a wide spectral regions. For example, black phosphorus and graphene can exhibit direct and narrow band gaps, making them appeal to the mid-infrared or even far-infrared devices applications. The single layer transition metal dichalcogenides materials (TMDCs, i.e., MX2; M=Mo, W; X=S, Se, Te), have been known as direct gap semiconductors, useful in the infrared and visible spectral regimes. Additionally, due to the vdW nature, these materials can be assembled vertically to form the complex vdW heterostructures, and more importantly, can be easily transferred onto different device substrates without stringent constraints on lattice matching at the interfaces. Notably, such features can potentially lead to the novel hybrid device platforms that simultaneously take advantage of the state-of-the-art semiconductor manufacturing technologies and exotic characteristics of vdW materials In this talk, we will demonstrate novel dielectric metalenses by exploiting the vdW molybdenum disulfide (MoS2) and hexagonal boron nitride as optical materials. By using the incomplete phase-based design approach (i.e. the maximum phase shift being less than 2π), we show the thicknesses of our created vdW metalenses can be far below the operating wavelength (∼0.1λ to 0.5λ). This circumvents the current fabrication challenges of making dielectric metalenses that require the high-aspect-ratio nanoscale scattering elements. More importantly, we demonstrate that the developed MoS2 (hexagonal boron nitride) metalenses not only can focus the near-infrared (visible) light into the diffraction limited spots, but also can be useful for creating the optical images. This could enable further downscaling of optoelectronics systems, and will significantly benefit the modern imaging, motion detection and spectroscopic applications. Furthermore, by exploiting the nature of vdW interactions, we show our proposed metalenses can be readily peeled off and then transferred onto the flexible and transparent polydimethylsiloxane (PDMS) substrate. When the mechanical strain was applied on the PDMS substrate, the axial focus length of transferred vdW metalens can be widely tuned in the range from 250 to 400 µm. This highlight the possibilities of realizing integrable and tunable dielectric metalenses based on our developed vdW nanophotonics. In addition to vdW nanophotonics, we will demonstrate that the vdW-based light emitters and photodetectors can be coupled with diverse photonic structures such as the waveguides and plasmonic resonators, and show the performed hybrid device platforms can hold great promise for the photonic circuits applications.

Recently, light absorption in optical nanoantennas has attracted growing interest. In particular, hot carrier generation in metallic (plasmonic) nanostructures and thermo-optical effects in dielectric Si and Ge nanoresonators offer novel opportunities for light-harnessing and energy conversion devices. Here, we report the construction, optoelectronic and photoelectrochemical characterization of plasmon-driven photodiodes and photocathodes based on a metal/p-type gallium nitride (p-GaN) heterostructure that operate within the visible regime via hot-hole injection. Next, we discuss how self-induced optical heating in Si and Ge nanoresonators affects their optical response and how it could be employed in optical devices and metasurfaces.

The effects of loss on the plasmon resonance peak in dispersive absorbing media are investigated. We find that dispersive loss can lead to narrower plasmon resonances. We also measure the plasmon peak for gold nanospheres embedded in P3HT, poly(3-hexylthiophene), using a dark field microscope.

We take advantage of the strong temperature modulation of the graphene conductivity to propose an all-optical technique of excitation and manipulation of plasmons in graphene and thin metallic films. Through spatial patterning of the temperature of electrons in a graphene film, the graphene conductivity acquires a periodic profile, enabling plasmons to be excited directly by diffraction of a probe beam in the imprinted thermal grating. We show that, when graphene is placed in the vicinity of a thin metallic film, this technique can be used to excite and manipulate the plasmons supported in this hybrid structure. We further demonstrate the ability of graphene, thin metals films, and graphene-metal hybrid systems to undergo photothermal optical modulation with depth as large as > 70% over a wide spectral range extending from the visible to the terahertz spectral domains.

Physical and side channel attacks on Internet of Things (IoT) devices employing cryptographic software are an increasing threat to the security of the Internet. Preventing these attacks may require new, hardware-based approaches to encryption. Here, we present a possible solution, consisting of a multimodal metal-insulator-metal (MIM) plasmonic ring resonator-based Physically Unclonable Function (PUF). Device-specific field distribution patterns with strong sub-wavelength confinement act as a device-specific cryptographic identifier to ensure private communications. This is possible because our MIM structures are ultra-responsive to fabrication variations, such as sidewall roughness, metal/insulator thicknesses, coupling lengths, ring diameters, and material impurities.

Metal-based nanostructures made from low-loss plasmonic materials allow a targeted and strong enhancement of light-matter interaction in a broad wavelength range. As a result, the far-field single-photon emission rates from solid-state quantum defects can overcome both the rate of dipole dephasing and that of plasmon absorption in metals. This approach promises the advent of single-photon sources featuring bitrates up to the THz range and operating at cryogen-free temperatures. We establish simple and intuitive fundamental enhancement limits for plasmonic systems coupled to quantum emitters and present practical methods for achieving these advantageous regimes.

Quantum strong coupling between emitters and cavities generates hybrid modes which provide a platform for quantum devices. The atom based systems require precise control over the position of atoms within the cavity and are difficult to be integrated on a chip. The quantum dots-photonic crystal system is limited to the cryogenic temperature. On the contrary, the molecule-plasmonic cavity is a good candidate for chip scale, room temperature operating strong coupling units due to the extremely small mode volume of plasmonic nanocavities. However, to precisely position a single or a few molecules into a plasmonic nanocavity is challenging. In this work, a few molecules are integrated into the nanocavity through oligonucleotides. The clear Rabi splitting is observed and the anti-crossing curve shows a clear verification of coupling. The number of fluorophore integrated into the nanocavity is estimated to be one. The deterministic strong coupling may be realized based on this configuration.

We use a quantum mechanical model to study electron energy-loss spectroscopy (EELS) from crystalline noble metal films, revealing intrinsic features associated with their crystallographic orientation. Within the random-phase approximation, we employ a 1-D potential across the film that captures the main features of the electronic band structure of such different surfaces, including the electron-spill out and bulk atomic-plane corrugation. Additionally, we examine the role of vertical transitions among quantum well states, severely affected by their in-plane effective mass.

Currently, there are no cheap and compact sources in the mid-infrared range that can be modulated at high frequencies. While hot membranes are common IR sources, their thermal inertia limit the modulation rates to a few tens of Hertz. Moreover, available thermal sources are unpolarized, isotropic, broadband and have a low efficiency. However, there is no fundamental limit that imposes these properties. It turns out that they can be strongly modified by using appropriate nanostructures. In this presentation, we report the design, fabrication and characterization of infrared incandescent sources, modulated faster than 10 MHz with a controlled spectrum and polarization.

Plasmonic nanoparticles can be used to engineer radiation decay of a dipole in close proximity to the surface of the particle. We present a theoretical analysis of the quantum yield of an electric dipole near a silver or gold nanoparticle of several different sizes. Specifically, we detail the calculation and simulation of the normalized quantum yield of an electric dipole coupled with a plasmonic nanoparticle. We find that the local electric field near the electric dipole is enhanced and has its characteristics altered.

Photoluminescence enhancement of MoS2 monolayers on TiN thin films grown on sapphire by molecular-beam-epitaxy (MBE) is observed. The PL spectra of MoS2 flakes on MBE-grown 58-nm-thick TiN crystalline film and on reference sapphire substrate are obtained at room temperature using a confocal laser scanning microscope with 405, 445, 488 and 561 nm excitation wavelengths. The maximum PL enhancement for B-exciton (6-fold) and A- trion (15-fold) is obtained at the excitation wavelength 488 nm that matches most closely to the epsilon-near-zero wavelength, 473 nm, of TiN film. A good agreement is observed between measured and calculated enhancements. The enhancement is attributed to increased light absorption when excitation wavelength matches the epsilon-near-zero wavelength of TiN film.

Plasmonic nanoparticles can be used to engineer radiation decay of a dipole in close proximity to the surface of the particle. In this talk I will review our recent theoretical and experimental results on the enhancement and quenching of radiation from a dipole in close proximity to a silver or gold nanoparticle of a few different sizes. We apply the theory of radiation decay engineering to explain our recent experimental results of enhancement of exciton emission in a composite semiconductor thin film decorated with gold nanoparticles.

Plasmons can be excited by inelastic tunneling of electrons [1,2]. This provides a broadband source of plasmons that can be integrated into plasmonic nanocircuitry. The emission wavelength and bandwidth can be controlled via plasmonic resonances which enhance the inelastic tunneling process. Here we present an electrically-driven multi-element Yagi-Uda antenna that emits light into one specific direction [3]. Furthermore, we discuss mode-specific electric excitation of plasmons in a two-wire transmission line and its application towards polarization-controlled nano-light sources.

We present an electrically tunable metasurface and demonstrate an ultrafast beam steering and distance-ranging. A unit cell of the proposed device consists of plasmonic antennas and an ITO film as an active, tunable layer. By individually applying electrical biases to the top and bottom of the unit cell, we achieve in the near-infrared range a phase change up to 360 degrees while keeping the amplitude constant. An adjustable gradient phase profile allows for all solid-state-electronic beam steering. Using the Time-of-Flight principle, we demonstrate for the first time metaphotonic-light detection and ranging (Meta-LiDAR).

The possibility to use one-dimensional magnetoplasmonic crystals as localized and sensitive sensors of the DC magnetic field is shown. The achievable sensitivity of such sensors is estimated to be up to 10-5 Oe at a local area of 1 mm2. The performance of demonstrated sensors strongly depends on the geometry-driven magnetic properties and can be tuned by the change of materials and thicknesses of functional layers. Demonstrated results in details explain the mechanisms of tuning the properties of MPlCs and give a discussion on possible applications.

The hexagonal silver-nanoparticles (Ag-NPs) array grown in self-assembled anodic aluminum oxide (AAO) nanochannels have been successfully demonstrated to show reliable enhanced Raman-scattering signal. In this work, we systematically explored the roles of multipolar resonances for Ag-NP/AAO substrates in affecting the signal amplification, the background noise, as well as the signal-to-noise ratio at three typical wavelengths for Raman spectroscopy (532 nm, 633nm, and 785 nm). In addition, a duel-resonant system consisting of Ag-NPs embedded in a sinusoidal-shaped AAO/Al substrate was also investigated. The grating structure enables the excitation of surface plasmon polaritons (SPPs) to further couple with the generated hots spots between adjacent Ag-NPs and meanwhile provides a diffraction dispersion which is beneficial for spectral detection.

A theory is developed for calculating the electromagnetic (EM) eigenstates of Maxwell’s equations for a two-constituent composite where the magnetic permeability as well as the electric permittivity have different uniform values in the two constituents. The physical electric field E(r) produced in the system by a given source current density is expanded in this set of bi- orthogonal eigenstates for any position r. Once all these eigenstates are known for a given host and a given microstructure then calculation of E(r) only involves performing three-dimensional integrals of known functions and solving sets of linear algebraic equations.

A uniformly-charged spherical shell of radius R, mass m, and total electrical charge q, having an oscillatory angular velocity Ω(t) around a fixed axis, is a model for a magnetic dipole that radiates an electromagnetic field into its surrounding free space at a fixed oscillation frequency ω. An exact solution of the Maxwell-Lorentz equations of classical electrodynamics yields the self-torque of radiation resistance acting on the spherical shell as a function of R, q, and ω. Invoking the Newtonian equation of motion for the shell, we relate its angular velocity Ω(t) to an externally applied torque, and proceed to examine the response of the magnetic dipole to an impulsive torque applied at a given instant of time, say, t = 0. The impulse response of the dipole is found to be causal down to extremely small values of R (i.e., as R → 0) so long as the exact expression of the self-torque is used in the dynamical equation of motion of the spherical shell.

Nonradiating anapoles are superposition of internal modes that can act as an energy reservoir by reducing the far-field scattering. We report experimental excitation of the electrodynamic anapole mode in isotropic silicon nanospheres at the optical frequencies using radially polarized beam illumination. The superposition of equal and out-of-phase amplitudes of the Cartesian electric and toroidal dipoles produces by a pronounced dip in the scattering spectra with the scattering intensity almost reaching zero – a signature of anapole excitation. The total scattering intensity associated with the anapole excitation is found to be more than 10 times weaker, and the internal energy is found to be 6 times greater for illumination with radially vs. linearly polarized beams. Our approach provides a simple, straightforward alternative path to realize electrodynamic anapole mode at the optical frequencies.

As powerful wave-front shapers, meta-surfaces can be used as planar lenses, polarizers, vortex generators, and other components. A general design approach is the finite difference time domain (FDTD) technique, which is robust but computationally costly. The discrete dipole approximation (DDA) is a rigorous and fast alternative, but has not been widely used in nanophotonic design because of computational complexity resulting from dipole-substrate interactions. Here we present a substrate-compatible DDA formulation using a one-dimensional Green’s function in cylindrical coordinates that accurately handles singularities and high-frequency oscillations. It is significantly faster with similar accuracy compared to several other methods, including FDTD.

Recently, meta-optics based on innovative nanostructures has attracted significant attention in scientific community because of their superior properties in local-field enhancement and ultrasmall mode volume. For practical applications such as all-optical switch, optical nonlinearity of metamaterials is indispensable. However, the intrinsic nonlinear responses of most nanomaterials are not adequate. In this invited talk, I will review our recent findings on ultra-large optical nonlinearity in metallic and dielectric nanostructures. With ~100 nm particles, significant nonlinear response on scattering was found, based on resonant heating of the nanostructure as well as strong photothermal effect. The excitation intensity is typically less than 10 mW/m2, which is much smaller than most nonlinear optics requirements, and thus leading to effective nonlinear coefficient n2 that is several orders larger than the bulk counterparts. I will present the applications of such a huge nonlinearity in a single nanostructure toward ultrasmall all-optical switch and label-free super-resolution microscopy.

In this talk, I will discuss how light can be sculpted with engineered nanostructures to enhance chiral light-matter interactions. With these nanostructures, we have developed optical force nanoscopes to visualize and quantify molecular chirality with high sensitivity and resolution. Specifically, we have designed and developed a cavity-enhanced atomic force microscope to image chiral optical forces with nanometer spatial resolution and piconewton force sensitivity. We use this technique to measure the chirality of DNA molecules, on the order of few tens of molecules. These studies provide a foundation for new sensing and imaging techniques at the single molecular to cellular level in-situ and in real time.

Three kinds of β-Sn-based plasmonic materials, including β-Sn nanoparticles, β-Sn/graphene hybrid structures and β-Sn@Ag core@shell nanoparticles, have been prepared to explore their near-field enhancement performance. The results indicate that such materials could act as traditional noble metallic nanostructures to support tunable resonance frequency. In addition, surface enhanced Raman scattering (SERS) determinations and corresponding calculations confirm that such novel plasmonic materials not only could serve as a SERS platform, but also could be an alternate efficient plasmonic material for the Si-based optoelectronic and/or photovoltaic devices by taking advantage of the near-field enhancement effect.

We have recently demonstrated extremely high second-order and third-order susceptibilities at near-infrared frequencies in ultrathin-metal/dielectric heterostructures, both of which are a few orders of magnitude higher than those of classic plasmonic materials. In this talk, we will present our recent efforts on quantum engineering of these plasmonic heterostructures for achieving extreme optical nonlinearities.

Dielectric nanostructures with a high refractive index and a low optical loss have recently attracted considerable attention as an alternative to plasmonic nanostructures. The electromagnetic multipoles excited in the high-index dielectric nanostructures enable the manipulation of light beyond the diffraction limit and offer high electromagnetic field enhancement comparable to that exhibited by the plasmonic nanostructures. Here, we demonstrate all-dielectric field enhanced spectroscopy using high-index dielectric nanoantenna and metasurface, enabling us to perform Raman and Infrared spectroscopies with single molecule detection sensitivity.

Localized surface plasmon resonance (LSPR) in different metallic nanostructures with sharp field gradient and extremely strong field intensity below the diffraction limit are the keys of near-field optical nano-tweezers for efficiently manipulating tiny bio-object. To further enhance the field gradient and intensity of LSPR and improve accessibility for the target object in nano-tweezers, we propose and realize a plasmonic bowtie notch design with assisted curved grooves for coupling more surface plasmon polariton waves into the LSPR. In experiments, our presented design shows the improved low power consumption for stably trapping bio-sized particles, which is promising for realizing an efficient nano-tweezers.

In this talk, I will present our recent development on the “Meta”-optical fiber integrated with optical metasurfaces and epsilon-near-zero (ENZ) materials. I will present the development of ultrathin optical metalens cascaded on the facet of a photonic crystal optical fiber that enables light focusing in the telecommunication regime. I will also discuss the integration of ENZ materials with optical fiber for advanced light manipulation and optical modulation. The integration of an metalens/ENZ thin film and optical fiber will open the path to revolutionary ultracompact in-fiber optical devices for optical imaging, sensing, and fiber communication and laser applications.

Metasurfaces provide a great flexibility in light management through interaction with nanostructured surface. Although successful in demonstrating fantastic optical functionalities, metasurfaces still suffer from high optical losses which highly constraint on enhancing their optical performance. In this talk, I will present a design method to realize a plasmonic metasurface with a circular polarization conversion efficiency higher than 50% in transmission at near-infrared wavelengths. We further demonstrate several metasurface-based components such as beam deflector and flat focusing lens with record operating efficiency based on the proposed metasurface.

In this article we review some machine learning methods for the design of nanomaterials. The first part will discuss how to use neural network to build a predictive model of the optical properties of a certain material or structure. The second part is dedicated to the optimization and reverse engineering of an optical material using generative networks.

With modern nanofabrication technology, researchers and companies can reliably produce 3-dimensional patterns with feature sizes much smaller than the wavelength of visible light. The ability to do this in a scalable fashion brings nanophotonic research into the realm of commercial technology. For example, metasurfaces achieve high optical performance in fractions of the thickness of traditional bulky optical components and can be designed for unique, custom functionalities. By expanding the design space beyond the metasurface regime and allowing for photonic designs in full three dimensions, we can further increase the degrees of freedom at our disposal. This new design space is complex and inherently involves multiply scattering structures. In order to efficiently search for good solutions, we use an inverse design procedure based on the adjoint variable method. Taking advantage of this large design space, we can computationally optimize multi-functional meta-optical devices that achieve novel functionalities in minimal footprints. We demonstrate wavelength splitting photonic filters with application to color filter arrays on modern-day image sensors. These filters are designed to replace absorbing filters and instead re-route colors to specific sensor locations, thus recovering previously lost transmission. We show that these devices work with a variety of realistic fabrication restrictions and demonstrate their abilities experimentally in the microwave regime where we can realize layered devices via simple techniques like 3D printing. Finally, we comment on potential future applications and avenues where inverse design can help solve inherently difficult engineering challenges in nanophotonics.

We address some fundamental limits in the coupling of radiation to 2D polaritons using free-propagating plane-waves. We study the scattering properties of 0D and 1D scatterers over a 2D or finite-thickness layer and quantify the coupling of light-to-polaritons cross-section for this process as a function of the scatterer effective polarizability. We formulate our results for both 0D and 1D scatterers, including material edges, and present them in simple closed-form expressions. We finally propose coupling light to plasmons using a graphene edge as a configuration which maximizes the coupling efficiency. Furthermore, we show that the adequate shaping of the impinging radiation can be used as a strategy to overcome the limitations of using plane-waves, and we demonstrate how to optimize this shaping for several different purposes.

Maintaining thermal gradient inside a thermoelectric device is a key for sustainable energy conversion from heat to electricity. Here, we propose a metamaterial perfect absorber which induce a sustainable thermal gradient inside a thermoelectric device by emitting local heat via absorption of thermal radiation released from a heat source. The metamaterial perfect absorber can absorb the thermal radiation emitted from a heat source via a magnetic resonance mode and emit the plasmonic local heat as a consequence of plasmon loss. Metamaterial perfect absorber attached on a top of a thermoelectric device absorbs radiation heat and generate plasmonic local heat, which propagate to the underneath thermoelectric device, leading to a thermal gradient inside the device. Power generation efficiency of the thermoelectric device with metamaterial perfect absorber is estimated by numerical and experimentally.

Tamm plasma-polariton (TPP) resonance is a confined state at the interface between photonic crystals and the metal. In this presentation, we have demonstrated the concept of bound states in the continuum with TPP, which allows the ultra-sharp resonance and large Q-factor. By combining liquid crystals and TPP devices, the quality factor of resonance would be tunable. Several developments and applications, like narrow-band filters, thermal emitters, sensors, photodetectors would be discussed.

The subwavelength nature of the plasmonic resonances, observed in patterned thin metallic films, makes it an attractive choice for use in application such as security features, product branding and data storage and imaging. However, these have largely been limited by the practical issues of cost and robustness arising from the use of gold, silver or aluminum. Here we pattern TiN thin films, a well explored alternative plasmonic material, with sub wavelength apertures, arranged with hexagonal periodicity, which exhibit extraordinary transmission in the visible and near IR spectrum. These TiN structures are shown to withstand different levels of mechanical stresses, while gold doesn’t, making TiN an attractive platform for use as security features.

Random light fields demonstrate Rayleigh intensity statistics and only possess short-range correlations. We recently demonstrate a method of simultaneously customizing the intensity statistics of speckle patterns while introducing long-range spatial correlations among the speckle grains. The tailored speckle patterns exhibit radically different topologies and varying degrees of spatial order. The various families of speckles are created by encoding high-order correlations into the phase front of a monochromatic laser beam with a spatial light modulator. This work provides a versatile framework for creating complex light fields and controlling their statistical properties for varied applications in microscopy, imaging, and optical manipulation.

We report continuous-wave (CW) lasing at 535 nm from a single lead halide perovskite (CsPbBr3) quantum dot (PQD) in a gap plasmon nanocavity with an ultralow threshold (lower than 90 mWcm-2) under 4 K. The ultrasmall mode volume dramatically enhances the light-matter interaction to achieve CW lasing. By raising the temperature, a clear lasing threshold can be observed at temperatures above 80 K. The demonstration of single QD lasing, which operates in the strong coupling regime, provides a new approach for realizing electrically driven lasing and integration into ultracompact optoelectronic devices. The detail mechanism of plasmonic lasing will be discussed.

Monolayer semiconductors with spin-dependent contrasting phenomena at K and K’ valleys feature addressable valley degree of freedom for valleytronic applications. Herein, we demonstrate that chiral Purcell effects can versatilely control the relaxation of targeted valley excitons at monolayer semiconductors, allowing the actively tunable modulation of valley dynamics at room temperature. We achieve the tunable valley modulation by embedding a monolayer WSe2 in our model system, known as a moiré chiral metamaterial, with actively controllable chiral plasmonic modes. Our work provides advanced understanding on mechanisms to distinguish the effects of spin-dependent excitation and near-field-controlled relaxation on valley emission in hybrid systems of valley excitons and plasmonic cavities. We have also shown that large room-temperature valley modulation can be achieved outside of strong-coupling regime with active tunability.

Plasmonic nanostructures have been explored for a wide variety of applications due to their ability to control light at the nanoscale. However, plasmonic nanoantennas based on conventional metals are difficult to tune after fabrication due to fixed material properties. By contrast, the properties of organic conducting polymers can be tuned via their redox state, and they can be optically metallic in their oxidized state [1]. Here, I will present our recent work demonstrating that nanodisks of a highly conducting polymer can sustain plasmonic resonances in the near-infrared and act as switchable optical nanoantennas [2]. 1 S. Chen, P. Kühne, V. Stanishev, S. Knight, R. Brooke, I. Petsagkourakis, X. Crispin, M. Schubert, V. Darakchieva, and M.P. Jonsson, J. Mater. Chem. C 7, 4350 (2019). 2 S. Chen, E.S.H. Kang, M.S. Chaharsoughi, V. Stanishev, P. Kühne, H. Sun, C. Wang, M. Fahlman, S. Fabiano, V. Darakchieva, and M.P. Jonsson, Nature Nanotech. 15, 35 (2020).

Plasmonic sensors show great potential in healthcare with nanohole arrays being most suited to highly sensitive, multiplexed label-free biosensing in point-of-care diagnostics. Extraordinary optical transmission (EOT) occurs when incident light is coupled to plasmonic modes in a nanohole array. When biomolecules bind to the surface of the nanohole array the refractive index surrounding it changes. We use optical microspectrometry and imaging to measure the EOT resonance wavelength shift due to the presence of these biomolecules. We demonstrate the biosensing capabilities of our plasmonic sensor with the detection of molecular and protein binding events, and cancer biomarkers.

We experimentally investigate the simultaneous microfluidic control and visualization of a heated region using gold nanoisland/VO₂ thin films. Gold nanoisland/VO₂ thin films are self-assembled using sputtering and glancing angle deposition techniques. The film shows a metal-insulator transition at approximately 70 °C. Therefore, the film is beneficial for visualizing the heated region inside the microfluidic chamber under a simple optical microscope. Furthermore, the photothermal property of the thin film enables a microbubble and a rapid Marangoni flow to be generated in water. The region heated above 70 °C around the bubble is visualized by the metal- insulator transition of the film. The visualization shows that the Marangoni flow is generated when a portion of the bubble is heated above 70 °C.

Various scatterers such as rough surfaces or nanostructures are typically used to enhance the low efficiency of Raman spectroscopy (surface-enhanced Raman scattering). In this work, we find fundamental upper bounds on the Raman enhancement for arbitrary-shaped scatterers, depending only on its material constants and the separation distance from the molecule. According to our metric, silver is optimal in visible wavelengths while aluminum is better in the near-UV region. Our general analytical bound scales as the volume of the scatterer and the inverse sixth power of the distance to the active molecule. For periodic scatterers, a second bound with surface-area scaling is presented. Simple geometries such as spheres and bowties are shown to fall short of the bounds. However, using topology optimization based inverse design, we obtain surprising structures maximizing the Raman enhancement. These optimization results shed light to what extent our bounds are achievable.

We demonstrated gold nanodimer arrays could improve the signal-to-noise ratio (SNR) of fluorescence correlation spectroscopy (FCS). In this research, we explore the feasibility of plasmon-enhanced FCS for biomolecular study using a nanodimer array whose gap size was 18 nm. Fluorescence nanobead with a diameter of 40 nm was first examined to verify if gold nanodimer arrays can enhance SNR of fluorescence and scattering intensities. We emphasize that plasmon-enhanced FCS can improve the precision for analyzing the dynamics of the particle by combining scattering characteristics of nanodimer arrays to surface plasmon resonance imaging technique. We have also observed the fluorescence enhancement and plasmon scattering in the movement of lysosome in HEK293 cells. It was found that we could measure diffusion properties such as diffusion coefficients and anomalous exponents with a low standard deviation.

Investigation of new plasmonic material platforms with large optical nonlinearity is crucial for the continued development of nonlinear optics and its applications. Here we report an enhanced second order nonlinear effect in metallic quantum wells (QWs) where the intersubband transition plays a dominant role. Centrosymmetry in these metallic QWs is broken by forming multilayers with chemically and structurally distinct barrier oxides above and below a metal nanofilm. For Au-based QWs, we show that a large χ(2), measured by second harmonic generation (SHG), around 229.6 pm/V at the near infrared (NIR) wavelength of 940 nm was achieved in an asymmetric metallic QW of SiO2|Au|HfO2 on a fused silica substrate.

A large amount of experimental and theoretical works deals with the second harmonic generation from different plasmonic geometries. Since they often consider relatively long optical pulses, many of these studies are focused on the investigation of a quasi-monochromatic response of the system and can be understood through the excitation of one, possibly two, optical modes. On the other hand, when the excitation pulse duration is short (say, below several tens of fs), the excitation spectrum becomes broader and a very interesting dynamics emerges from the interplay between several optical modes. In this work, the dynamics of modes at the second harmonic frequency for two silver spheres of different diameters and a nanorod is investigated numerically and shown to be quite different. For the pulsed illumination with length close to the modes lifetime, apart from different relative contributions of dipolar and quadrupolar multipoles in the far-field, we have been able to observe and explain non constant phase difference between multipoles, which is not accessible in continuous wave regime. Short pulse durations also allow us to observe only one mode, while another one has already decayed. For the case of the nanorod we also perform an eigenmode analysis, which allows to understand the modes interplay that explains the observed spectra. In the paper, we also show a method allowing a significant reduction of required computational steps to find the response of a plasmonic nanostructure to a pulsed illumination with arbitrary frequency-domain method.

Higher-order dynamic polarizability tensors are formulated using the irreducible Cartesian basis, which makes polarizability tensors and Green’s tensors to be symmetric. Basis transformation matrices are presented, which systematically transform Cartesian multipoles and local field quantities to spherical multipoles. These newly presented expressions allow systematic retrieval of higher-order dynamic polarizability tensors and formulation of coupled multipole method up to magnetic octupole. This multipole framework is (semi-)analytic and computationally efficient, allowing description of electromagnetically coupled meta-atoms. Meta-atoms in periodic or random lattices and those dispersed in three-dimensional random- and large-scale systems may be analyzed using this method.

We recently formulated the canonical boundary-value problem of propagation of surface plasmon-polariton (SPP) waves along the direction of periodicity of a one-dimensional photonic crystal. Here we present the general formulation of that canonical problem supporting the oblique propagation of SPP waves in the interface plane. The general dispersion equation has been obtained using the rigorous coupled-wave approach (RCWA) for the oblique propagation and numerically solved using the Muller’s method. The RCWA formulation is developed for the general eigenvalue problem solved here. A periodicity in the wavenumbers of the SPP waves was observed. Furthermore, the regions of high losses for the SPP waves, dubbed as plasmonic bandgaps, were observed in the photonic band diagram of the SPP waves. These plasmonic badngaps can be used to construct optical filters for the SPP waves.

In this work we present the synthesis of Au@Ag bimetallic nanoparticles via seed-mediated method and their nonlinear optical properties. For the synthesis of nanoparticles, 18 nm gold seeds were synthesized by Turkevich method followed by the silver ion reduction with ascorbic acid. The nanoparticles dispersion was analyzed by UV-Vis spectroscopy and the spectra suggest the formation of spherical core@shell bimetallic nanoparticles. SEM images were obtained in order to corroborate this. The nanoparticles dispersions present negative and positive nonlinear absorption coefficient and negative refraction index, according to z-scan measurements. The nonlinear optical properties can be tailor modifying the thickness of shell.

Stimulated laser beam scattering on a plasma layer with slowly varying plasma thickness and density is investigated. The scattering process was described by shortened equations for scattered electromagnetic, sound and pump waves. Spacetime perturbations evolution from local and distributed over entire volume initial sources is considered. Conditions of occurrence and shape of the perturbation propagating along the plasma layer for different initial conditions is determined. Scattered wave parameters were used to calculate wave energy for different scattering angles and incident angles at a given pump wave power. The prospects of using the results for plasma layer diagnostics are considered.

Nanoscale electron pulses are increasingly in demand, including as probes of nanoscale ultrafast dynamics and for emerging light source and lithography applications. Using electromagnetic simulations, we show that gold plasmonic lenses as multiphoton photoemitters provide unique advantages, including emission from an atomically at surface, nanoscale pulse diameter regardless of laser spot size, and femtosecond-scale response time. We then present fabrication of prototypes with sub-nm roughness via e-beam lithography, as well as electro-optical characterization using cathodoluminescence spectromicroscopy. Finally, we introduce a DC photogun at LBNL built for testing ultrafast photoemitters. We discuss measurement considerations for ultrafast nanoemitters and predict that we can extract tens of pA photocurrent from a single plasmonic lens using a Ti:Sa oscillator. Altogether, this lays the groundwork to develop and test a broad class of plasmon-enhanced ultrafast nanoemitters.

We study optical response of a plasmonic crystal based on multi-gated 2D structure with periodic modulation of the electron density in the device channel. In such a structure, the plasma wave velocity is periodically modulated as well. We consider the simplest model of periodically alternating stripes of the electron density and plasma wave velocity: active regions with high plasma wave velocity and passive regions with low plasma wave velocity. Terahertz radiation applied to such a structure excites plasmonic resonances both in the active and passive stripes. The width of the resonances is determined by the momentum relaxation rate. For sufficiently large relaxation rates, the resonances in the passive regions strongly overlap and only “active resonances” survive. In this regime, the plasmonic oscillations in the active regions exponentially decay into the passive regions, so that different active regions are disconnected at plasmonic frequencies but connected at zero dc frequency. We assume that dc current is applied to this plasmonic crystal and calculate radiation-induced correction to the dissipation in the channel. We demonstrate that with increasing the dc current this correction changes sign, which results in amplification of the optical signal.

A simple scheme for T-matrix retrieval with higher efficiency and accuracy by taking particle symmetries into account is presented. T-matrix is calculated using matrix inversion, where the matrix elements are numerically calculated. This proposed scheme additionally generates extra matrix elements using the particle symmetries and reciprocity. This process consumes little effort in terms of computations, while the retrieved T-matrix shows improved accuracy for geometric symmetries and reciprocity. We also study required symmetry operations depending on the symmetries of meta-atoms (point group), and the shapes of their T-matrices. This scheme allows efficient and accurate calculation of T-matrix of meta-atoms for further calculations and group-theoretical analysis.

In this study, we will demonstrate the fabrication of a free-standing Au membrane with designed nano-patterns using Nanospherical-Lens Lithography. First, we will fabricate the Au membrane with designed nano-holes on top of the photoresist thin film. Then, the Au membrane is released in solvent and recovered on top of another thick metal membrane with larger holes. The surface plasmon resonance of the designed nano-holes is in the spectral range of mid infrared, which should be able to demonstrate surface-enhanced infrared absorption spectrum. We are currently measuring several IR-active molecules to demonstrate surface enhanced infrared absorption.

We have developed microbubble-assisted rapid concentration and ultrahigh-sensitive detection of multiple chiral metabolites. Through the strong Marangoni convection, the drag force can efficiently drag and print the biomolecules with hundreds of molar masses on the plasmonic substrates for chiral analysis, breaking the limit of conventional electrophoresis and thermophoresis in the manipulation of molecules. We show that we can detect and differentiate 100 pM D/L pure glucose solution, which shows 107 times greater sensitivity than the state-of-the-art chiral sensing techniques. Finally, we successfully detect the enantiomer excess in purified urine and therefore push the label-free lab-on-a-chip detection of chiral biomarkers for diabetes mellitus detection.

We present an optical sensor for solute concentration in solutions by using aluminum-doped zinc oxide (AZO) coatings on silica single-mode/coreless/single-mode and single-mode/multimode/single-mode fiber structures. The fiber sensor is based on the detection of the second lossy-mode resonance (LMR) order of the AZO coated fiber structures which covers the near-infrared C and L wavelength bands. AZO thin films with Zn:Al atomic concentration proportions of 92%:8% were deposited on the fiber structures by radio frequency magnetron sputtering technique. Wavelength displacement of the second order LMR notch as a function of concentration variation on isopropyl-alcohol/glycerin liquid mixtures where measured by a simple optical transmission setup, detected in the 1.55 μm wavelength range. In glycerin concentration percentages (c%) variations from 5% to 30%, maximum sensitivity of 1.42 nm/c% was obtained for coreless-fiber-based structures.

This study explores optical characteristics in quantum dots. CdSe quantum dots samples have been prepared and an optical photoluminescence experimental setup has been created to measure the light emission from the quantum dots as a function of time and laser intensity. Initial baseline measurements and photoluminescence spectrum has been measured. This preliminary work sets up future studies of quantum dot photobrightening, which is when the emitted light from a CdSe quantum dots gradually increases with time while under constant laser illumination. Future work will investigate photobrightening as a function of laser intensity and with the presence of plasmonic nanoparticles to give insight into plasmonic enhancement and light interaction between plasmonic particles, quantum dots, and photobrightening effects. Results of this study can add value to future quantum dot technologies.

We theoretically calculated a metamaterial perfect absorber that exhibits broadband perfect absorption in the visible region using TiN nanostructures as resonator. We designed a 200 nm diameter TiN nanodisks array covered with a 5 nm thick TiO2 film. We used the FDTD method to obtain numerical values for reflectance, transmittance, and absorptance. By optimizing the structure design, we found that the average absorption with the TiN nanoresonators is 97% in the wavelength range from 350 nm to 750 nm. The calculated electric field distribution indicates a strong localized optical field around the TiN nanodisk which enhanced the light absorption.

We propose a strategy for designing infrared absorbers with predefined spectral response using aluminum gratings as building blocks. We begin by defining 3 target spectra with resonances in the 7 – 15 micron wavelength range. Using FDTD simulations and interpolation, we create a reference library of aluminum gratings to investigate the relationship between their structural parameters and spectral properties. Next, we develop a search algorithm to find gratings from this library corresponding to resonances in the target spectra. Finally, we present an approach for designing hybrid structures from these gratings to generate each of the 3 target spectra.

One of the major challenges in integrated silicon photonics is the light coupling into and out of photonic circuits. Prism coupling, among waveguide coupling methods in the silicon photonics is a rather complex and costly one mostly due to the requirement of a prism with index of refraction higher than that of the effective index of the propagating mode in the waveguide. Surface plasmon polariton (SPP) allows the deployment of a fused silica prism instead of the high index prism, as indicated by our simulation using Lumerical FDTD software and Otto configuration of the SPP excitation scheme. Using the dispersion diagram the surface plasmon angle is determined to be 44◦ for the TM-mode of the incident beam. This is followed by examining the relevant parameters affecting the light coupling efficiency to a silicon waveguide of input dimensions of 220 750 nm. In Lumerical FDTD solver, two-dimensional simulation of plane waves of p-polarization enters the optical system at surface plasmon angle. This model reveals a coupling efficiency of 54% which at the very least is better than that of traditional prism coupling methods. For fabrication purposes, we could impose geometrical constraints on airgap height, making it a function of the waveguide and metal (silver in this case) strip thickness. Furthermore, we will show and discuss the effect of choice of the thickness for the Si waveguide, airgap, and silver layer on coupling efficiency from our mathematical model and Lumerical FDTD simulations.

This work aims to improve the performance of a magnetic field sensor based on a magnetoplasmonic crystal. To achieve that, a detailed characterization of the magnetic properties of the sensor was done using the switching field distribution and first-order reversal curve diagram methods. The information obtained by these methods revealed the existence of several interacting magnetic phases corresponding to the magnetization processes of material deposited onto different parts of the diffraction grating. The obtained data are in agreement with modeling and calculations.

The work is devoted to the development of magnetic field sensor based on magnetoplasmonic crystal and demonstrates the results of studying spectral and field dependences of reflectivity and transverse magneto-optical Kerr effect (TMOKE) geometry. It is shown, that magneto-optical properties of studied samples non monotonously change due to the different contributions of surface plasmon-polaritons excitation, magnetic and optical properties into the TMOKE enhancement process. Presented samples allow one to achieve the sensitivity of tens μOe to the DC magnetic field magnitude in an area of 1 mm2 and perform a mapping of external uniaxial magnetic field without moving the MPlC.

Phase-changing materials are promising due to their sharp temperature dependent characteristics and have high potential of being integrated in optical switching and sensing techniques. Among such materials, vanadium dioxide (VO₂) is the most utilitarian because of its transition temperature being close to the room-temperature. VO2-based bolometers utilize the material’s large temperature coefficient of resistivity to detect infrared radiation. However, to achieve large sensitivity, the active radiation absorption area needs to be large enough that allows sufficient temperature buildup from incident radiation absorbed by VO₂, thus requiring large pixel dimen- sion and degrading the spatial resolution of bolometric sensing. Moreover, the absorption by the VO₂ material is not optimized for a specific frequency band in most of the applications. On the other hand, plasmonic nanos- tructures can be tuned and designed to selectively and efficiently absorb a specific band of the incident radiation for local heating and thermal imaging. In this work, we propose to incorporate plasmonic nanostructures with VO₂ nanowires that amplifies the slope of impedance change due to the thermal variations to achieve a higher sensitivity. We present the numerical analysis of the mid-infrared electromagnetic radiation absorption by the proposed detector showing near-perfect absorption by the plasmonic absorbers. Besides, the thermal buildup and the nanowire resistance change is predicted for different substrate, as the substrate plays a big role in heat distribution. We show high sensitivity and ultra-low noise equivalent temperature difference (NEDT) by our novel bolometric detector. We also discuss the fabrication of the VO₂.

It is a well-acknowledged fact natural ferromagnetism is hard to find in optical spectrum. In this article, a metallic nanoantenna is reported which facilitates the achievement of artificial ferromagnetism at optical frequencies. The proposed nanoantenna consists of two rhombuses joined together by a square has been designed and analyzed for magnetic field enhancement using finite element method (FEM). The proposed design is aims at generating magnetic hot spot in the optical frequency range

We prepare metallic tungsten oxide(WO3-x) nanoparticles by RF sputtering without doping, and investigate optical properties of the nanoparticles. Optical absorption spectra show clear two independent absorption species in near-infrared region, which are originated from localized surface plasmon resonance and polaron. This concurrent activation property not only provides a stage for understanding the fundamental physics of plasmon and polaron of tungsten oxide nanoparticles, but also shows that the nanoparticles are interesting candidates for device application.

Resonant excitation and manipulation of high-index dielectric nanostructures (such as Silicon, Germanium) provide great opportunities for engineering novel optical phenomena and applications. Here, we report selective excitation and enhancement of multipolar resonances, and non-radiating optical anapoles in silicon nanospheres using cylindrical vector beams (CVBs). Our approach can be used as a spectroscopy tool to enhance and identify multipolar resonances as well as a straightforward alternate route to excite electrodynamic anapoles at the optical frequencies.

The surface plasmonic lithography (SPL) is low cost and simpler system configuration. Thus, for the nano-scale features, there have been many developments of the maskless SPL technologies. In this study, for below 10-nm patterning, SPL process based on the SP interference and metamaterial in bowtie and hexahedron structures is modeled and simulated by using the rigorous coupled-wave analysis (RCWA) method and the finite difference time domain (FDTD) method. SPL is not only capable to high resolution beyond the restriction of diffraction limit but also applicable to conventional light source. For 193-nm wavelength, the minimum FWHMs (the full width at half maximum) of the transverse magnetic (TM) intensity in xz plane and yz plane are 10-nm and 7-nm in a bowtie plasmonic structure, respectively. For hexahedron structures, the minimum 30-nm FWHM of TM intensity with 193-nm wavelength is improved to the minimum 16-nm FWHM by using metamaterial and SP interference.

We describe here the use of a metasurface geometry previously reported by the author, based upon geometric in- version of a set of conformal mapping contours, for application in the THz bandwidth as the basis for an uncooled microbolometer in downhole chemical spectroscopy. The resulting geometry forms a nearly continuous series of perfect absorption resonances in a broad bandwidth of the THz gap by an ultrathin (λ/1600) metasurface. The metasurface is derived from a geometric inversion of the Rhodonea, or more commonly called four-leaf roses, con- formal mapping contours and was found to exhibit a near zero index metamaterial behavior. The near zero index properties of the metasurface lead to an absorption phenomenom characterized by surface plasmon resonances that confine the absorption mechanism within the ultrathin metasurface plane and make the absorption prop- erties of the microbolometer design practically independent of the material properties of the remaining laminae. This unusual feature allows the metasurface to be integrated on a single dielectric support layer with a single V0₂ material thermometric layer which is now able to be operated at downhole elevated temperatures within its metal-insulator-transition region. Within this transition region the V0₂ layer is effectively a metallic electrical conductor and exhibits more than an order of magnitude enhancement in its thermometric propertie√s. This leads to a metasurface microbolometer design with predicted maximum detectivity D*= 2.2 × 10¹⁰cm√Hz/W and noise equivalent difference temperature NEDT of 1 mK at a modulation frequency of 50 Hz. These levels of THz detector performance conventionally would be achievable only with cryogenically cooled technologies and could represent a significant step in the effort towards deploying miniaturized uncooled THz sensor devices into oilfield exploration and production applications.

We investigate changes in the electron energy-loss spectra triggered by electron scattering by an optical mode depending on mode statistics (bosons vs fermions) and population (coherent, Fock states, and thermal).

Chiral metamaterials consisting of periodic asymmetric unit cells have a different complex refractive index depending on spin-states of impending light. These phenomena can be applied to chiral sensing applications, so enhanced chiral responses have been attracted to develop advanced spectroscopic devices. However, chiral devices, which made of metallic chiral metasurfaces, have weak circular dichroism due to ohmic loss of metal. Here, we proposed simulation results of chiral metasurfaces consist of helically located gold metallic nanodisc, called oligomers. The oligomers are located with C4 chirality, resulting in non-conversion efficiency for reducing noise when they are used for spectroscopy. Also, the oligomers have ultra-sharp circular dichroism that has been rarely reported in the near-infrared region. These results may have wide applications, including spectroscopy, thermal detectors, and biochemical distributors.

We present 2-D arrays of plasmonic nanostructures ─ gold nanopillars on a gold coated substrate ─ coated with a thin film of VO₂ (vanadium dioxide), as novel optical switches. The incident optical radiation is coupled into plasmonic modes due to the small gaps between the adjacent nanostructures, which leads to high electromagnetic fields in the gap regions. As VO₂ changes phase from the semiconducting to the metallic phase on heating or on application of voltage, there is a change in the overall plasmonic properties of the VO₂ coated 2-D plasmonic nanostructures, and a change in the reflectance spectra. We employ Rigorous Coupled Wave Analysis (RCWA) to calculate the switchability, i.e., the differential reflectance between the semiconducting and metallic state of the VO₂ coated nanostructures. An exhaustive analysis of the differential reflectance spectra is carried out, by optimizing the different geometrical parameters of the structure. Moreover, we demonstrate that these 2-D arrays of plasmonic nanostructures can be employed for switching with unpolarized light. These nanostructures can also be employed for multi-wavelength switching if a degree of asymmetry is introduced with different gaps or periodicities in the two dimensions. Thus, we propose 2-D VO₂ coated nanostructures which can be employed as plasmonic switches with unpolarized light.

In this study, we have used scanning probe microscopy (SPM) to validate temperature-dependent thermo-plasmonic calculation which is called iterative opto thermal analysis (IOTA). We have applied temperature-dependent Drude-Lorentz model to IOTA. To solve wave-coupled heat transfer equation, finite element method based multi-physics analysis tool has employed with wave optics module and heat transfer module under the proper boundary and initial conditions. For this study, various plasmonic structures were considered to acquire temperature and plasmonic field enhancement using IOTA and SPM experiments. As a result, we have improved plasmonic analysis with consideration of temperature-dependence.