Nanoscale Light-matter Interaction Research Articles

Enhanced light-matter interactions at photonic magic-angle topological transitions

In analogy to Lifshitz transitions in electronic systems, topological transitions have recently attracted widespread attention in photonic metamaterials, metasurfaces, and two-dimensional materials, enabling exotic regimes for light-matter interactions. Here, we discuss and study enhanced photonic local density of states in twisted hyperbolic bilayers, enabled by topological transitions emerging at specific twist angles. Our results enhance the understanding of nanoscale light-matter interactions in stacked optical materials as they are rotated with respect to each other in the context of twistronics and suggest emerging applications of these concepts for photonics, including for the manipulation of radiative heat transfer and the control and harvesting of light at the nanoscale.

Ultrafast Transient Holographic Microscopy.

Nanotechnology is increasingly being applied in many emerging technologies, ranging from metamaterials to next-generation nanodrugs. A key ingredient for its success is the ability to specifically tailor ultrafast nanoscale light–matter interactions over very large areas. Unfortunately, dynamic imaging by ultrafast nanoscopy so far remains limited to very small 2D areas. This shortcoming prevents connecting single-particle observations with large-scale functionality. Here, we address this experimental challenge by combining concepts of ultrafast spectroscopy, wide-field nanoscopy, and digital holography. We introduce an ultrafast holographic transient microscope for wide-field transient nanoscale imaging with high frequency all-optical signal demodulation. We simultaneously record ultrafast transient dynamics of many individual nano-objects and demonstrate time-resolved spectroscopy of gold nanoparticles over a large volume irrespective of their x–y–z position. Our results pave the way to single-shot 3D microscopy of 2D and 3D materials on arbitrary time scales from femtosecond carrier dynamics in optoelectronic materials to millisecond dynamics in complex tissues.

High order discontinuous Galerkin methods for time-domain and frequency-domain nanophotonics

The discontinuous Galerkin (DG) method is a general numerical modeling approach that has been extensively studied in the last 20 years for the solution of many systems of partial differential equations in physics. Its development for the numerical treatment of the system of Maxwell equations was initiated by the applied mathematics community in the early 2000s. It is now a very popular method for time-domain electromagnetics, which is increasingly used and further developed by the applied physics and electrical engineering communities. More recently, a specific variant of DG, referred to as hybridized DG (or HDG), has been proposed for frequency-domain electromagnetics. DG methods possess nice features that make them particularly attractive for dealing with heterogeneous media and irregularly shaped or curved geometries, and more generally with multiscale problems. Not surprisingly, the method has also been adopted by researchers in the nano-optics field. In this paper, we report on our recent efforts for extending the capabilities of this family of methods for the numerical modeling of nanoscale light-matter interactions.

LPSE: A 3-D wave-based model of cross-beam energy transfer in laser-irradiated plasmas

The new component of the “laser–plasma simulation environment” (LPSE) described here is a practical numerical model that solves the coupled vector equations for the propagation of nearly monochromatic, coherent, electromagnetic waves in inhomogeneous unmagnetized plasmas. It operates efficiently in the numerically challenging semiclassical regime where characteristic plasma scale lengths are many times greater than the wavelength of the light and solutions are highly oscillatory. Solutions can be obtained in one, two, or three spatial dimensions and time. The model includes the effects of nonlinear coupling of electromagnetic waves to the low-frequency plasma perturbations (i.e., ion-acoustic response) that are responsible for stimulated Brillouin scattering. Induced plasma perturbations are assumed to be imposed on a prescribed large-scale inhomogeneous background that includes spatially varying plasma density and flow. Our code is directly relevant to the problem of cross-beam energy transfer in laser-driven inertial confinement fusion. It may also be applicable in other areas where eikonal solutions of multicomponent wave equations (or coupled wave equations) are insufficient, such as optical scattering from ultrasound, electron dynamics in quantum devices or in nanoscale light–matter interactions.

A Single-Crystalline Silver Plasmonic Circuit for Visible Quantum Emitters.

Plasmonic waveguides are key elements in nanophotonic devices, serving as optical interconnects between nanoscale light sources and detectors. Multimode operation in plasmonic two-wire transmission lines promises important degrees of freedom for near-field manipulation and information encoding. However, highly confined plasmon propagation along gold nanostructures is typically limited to the near-infrared region due to ohmic losses, excluding all visible quantum emitters from plasmonic circuitry. We report on the top-down fabrication of complex plasmonic nanostructures in single-crystalline silver plates. We demonstrate the controlled remote excitation of a small ensemble of fluorophores by a set of waveguide modes and the emission of the visible luminescence into the waveguide with high efficiency. This approach opens up the study of a nanoscale light-matter interaction between complex plasmonic waveguides and a large variety of quantum emitters available in the visible spectral range.

Single-crystalline Cu2O thin films of optical quality as obtained by the oxidation of single-crystal Cu thin films at low temperature

High-quality, single-crystal-like Cu2O thin films of various thicknesses (10 nm–45 nm) were prepared at a low temperature (150 °C) by controlling layer-by-layer oxidation of wafer-scale Cu thin films sputtered along the (111) direction using a pure single-crystal Cu target. The cross-sectional images of the thin films reveal high crystallinity of Cu2O layers except for 60° twinning in the sequential stacking order as evidenced by high-resolution transmission electron microscopy, which is consistent with the absence of the photoluminescence (PL) signals arising from atomic-scale vacancies. The optical properties of our Cu2O films were investigated using temperature-dependent PL and Raman spectroscopy. All of the Cu2O thin films exhibit characteristic band-to-band transitions together with the series of yellow excitonic transitions slightly below the fundamental bandgap. The spectral locations for the PL are approximately consistent with those for the bulk counterpart. The excellent optical quality of our Cu2O was further demonstrated by significantly reduced quasi-direct transition that occurs at symmetry-breaking crystal imperfection, which relaxes the stringent momentum conservation rule. We identified the three main Raman scattering modes of the Cu2O thin films, where the two forbidden modes of Γ15(1) and Γ12−+Γ25− are resonantly allowed by the proximity of the incident photon energy to the green bandgap. We believe that our synthesis technique can be utilized for the preparation of single-crystal-like metal oxide thin films at low production temperatures with precise thickness control for the development of novel optoelectronic devices and for the exploration of the nanoscale light-matter interaction as well.

Simulating Maxwell–Schrödinger Equations by High-Order Symplectic FDTD Algorithm

A novel symplectic algorithm is proposed to solve the Maxwell-Schr\"odinger (M-S) system for investigating light-matter interaction. Using the fourth-order symplectic integration and fourth-order collocated differences, Maxwell-Schr\"odinger equations are discretized in temporal and spatial domains, respectively. The symplectic finite-difference time-domain (SFDTD) algorithm is developed for accurate and efficient study of coherent interaction between electromagnetic fields and artificial atoms. Particularly, the Dirichlet boundary condition is adopted for modeling the Rabi oscillation problems under the semi-classical framework. To implement the Dirichlet boundary condition, image theory is introduced, tailored to the high-order collocated differences. For validating the proposed SFDTD algorithm, three-dimensional numerical studies of the population inversion in the Rabi oscillation are presented. Numerical results show that the proposed high-order SFDTD(4,4) algorithm exhibits better numerical performance than the conventional FDTD(2,2) approach at the aspects of accuracy and efficiency for the long-term simulation. The proposed algorithm opens up a promising way towards a high-accurate energy-conservation modeling and simulation of complex dynamics in nanoscale light-matter interaction.

Plasmonic nanoantenna-dielectric nanocavity hybrids for ultrahigh local electric field enhancement.

A dielectric nanostructure with a high refractive index can exhibit strong optical resonances with considerable electric field enhancement around the entire structure volume. Here we show theoretically that a dielectric structure with this feature can boost the local electric field of a small plasmonic nanoantenna placed nearby. We construct a hybrid system of a plasmonic nanoantenna and a dielectric nanocavity, where the nanocavity is a concentric disk-ring structure with a lossless material n = 3.3 and the nanoantenna is a gold nanorod dimer. The resonant electric field enhancement at the gap center of the antenna in the hybrid structure reaches more than one order of magnitude higher than that of the individual antenna. The dielectric structure plays two roles in the hybrid system, namely the amplified excitation field and an environment causing the redshift of the antenna resonance. The hybrid configuration is applicable to the cases with various geometries and different materials of the hybrid system. Our results can find applications in enhanced nanoscale light-matter interactions such as surface-enhanced Raman scattering, nonlinear optics, and plasmon-exciton couplings.

Hybridizing Strong Quadrupole Gap Plasmons Using Optimized Nanoantennas with Bilayer MoS2 for Excellent Photo‐Electrochemical Hydrogen Evolution

AbstractPlasmonic nanoantennas with efficient scattering and strong electromagnetic field confinement are integrated into atomically thin transition metal dichalcogenides to gain insight into nanoscale light–matter interactions. Herein, unusual quadrupole gap plasmons (QGPs) are introduced in the tailored nanoantennas. The field enhancement of such QGPs is further maximized by optimizing the structure of the nanoantennas using a statistical method. In addition, the optimized nanoantennas are employed on a chemical vapor reaction‐grown bilayer molybdenum disulfide (MoS2) to tune and intensify the optical response of MoS2. The optimized QGP structure performance was enhanced by a factor of 27.87 and a beneficial continuous bilayer MoS2 can be applied in the hydrogen evolution reaction (HER) with superior outcomes. MoS2@QGP acts as an excellent photocatalyst for HER activity because of its accelerated electron injection process, which is faster than the electron–hole pair recombination, supported by the good light‐trapping capacity of the QGP structure and plentiful accepting sites of the continuous bilayer MoS2 film. The experimental results demonstrate that the QGP enhances the intrinsic catalytic properties of bilayer MoS2 for photocatalytic reactions.

Quadrupole Gap Surface Plasmon Integrated Bilayer MoS2 for Excellent Photoelectrochemical Hydrogen Evolution

Plasmonic nanoantennas with efficient scattering and strong electromagnetic field confinement were integrated to atomically thin transition metal dichalcogenides (TMDC) to shed light on the nanoscale light-matter interactions. Herein, we optimized the quadrupole gap surface plasmonic (QGSP) structure by Taguchi design to maximize field enhancement and integrated to CVD grown bilayer molybdenum disulfide (MoS2), beneficially tuning the optical properties of bilayer MoS2. The optimized QGSP structure with 27.87 factor enhanced performance and advantageous 1cm2 continuous bilayer MoS2 finds potential application for hydrogen evolution reaction (HER) with superior outcome. MoS2@QGSP act as an excellent photocatalyst for HER activity on account of accelerated electron injection process in comparison to electron-hole pair recombination supported from skilled light trapping capacity of QGSP structure and plentiful accepting sites of large surface continuous bilayer MoS2 film. This report demonstrates the gap surface plasmon enhanced intrinsic catalytic properties of bilayer MoS2 for photocatalytic reactions.

Ultrafast time-resolved measurement of energy transport at the metal-liquid interface

The nanoscale light-matter interaction at metallic interfaces has many important applications, especially when it is crucial to enhance the surface-to-volume ratio and to achieve high spatial energy confinement. Here, we report an ultrafast time-resolved measurement to study photo-excited transport at the metal-liquid interfaces of colloidal gold nanoparticles (AuNPs). By using the transient absorption spectroscopy method together with the stimulated emission depletion of fluorescence molecules, we simultaneously measured the perturbations of energy states on both sides of the interfaces within a nanoscale distance. Our measurement results showed the evidence of ultrafast coupling between AuNPs and their surrounding solvent molecules at the picosecond time scale. This method can be extended to study the energy transfer mechanisms at the various interfaces for biology, chemistry, or optoelectronics.

Plasmonic metasurface cavity for simultaneous enhancement of optical electric and magnetic fields in deep subwavelength volume.

It has been hard to achieve simultaneous plasmonic enhancement of nanoscale light-matter interactions in terms of both electric and magnetic manners with easily reproducible fabrication method and systematic theoretical design rule. In this paper, a novel concept of a flat nanofocusing device is proposed for simultaneously squeezing both electric and magnetic fields in deep-subwavelength volume (~λ3/538) in a large area. Based on the funneled unit cell structures and surface plasmon-assisted coherent interactions between them, the array of rectangular nanocavity connected to a tapered nanoantenna, plasmonic metasurface cavity, is constructed by periodic arrangement of the unit cell. The average enhancement factors of electric and magnetic field intensities reach about 60 and 22 in nanocavities, respectively. The proposed outstanding performance of the device is verified numerically and experimentally. We expect that this work would expand methodologies involving optical near-field manipulations in large areas and related potential applications including nanophotonic sensors, nonlinear responses, and quantum interactions.

Trapping and Deposition of Dye-Molecule Nanoparticles in the Nanogap of a Plasmonic Antenna.

Plasmonic nanostructures, which allow light focusing at the deep subwavelength scale, and colloidal nanoparticles with unique optoelectronic properties are nowadays fabricated with nanometer precision. However, to fully control and exploit nanoscale light–matter interactions in hybrid plasmonic–nanophotonic devices, both materials must be assembled in heterostructures with similar precision. Near-field optical forces have recently attracted much attention, as they can precisely trap and position nanoparticles at plasmonic hotspots. However, long-range attraction and the surface bonding of nanoparticles usually require other specific techniques, such as electrothermal heating and surface chemical treatments. This Letter reports on the optical trapping and deposition of dye–molecule nanoparticles in the nanogap of a gold antenna. The nanoparticles are captured by focusing a near-infrared laser beam on a targeted plasmonic antenna. This single-step deposition process requires only a few seconds under 1.4–1.8 MW·cm–2 continuous-wave illumination and shows a polarization dependence smaller than expected. Fluorescence and electronic microscopy observations suggest that nanoparticle deposition arises from a trade-off between optical and thermal effects.

Coupling of Epsilon-Near-Zero Mode to Gap Plasmon Mode for Flat-Top Wideband Perfect Light Absorption

Epsilon-near-zero (ENZ) materials, when probed at or near wavelengths corresponding to their zero permittivity crossing points, have unique and interesting properties that can be exploited for enhancing nanoscale light–matter interactions. Here, we experimentally show that over an order of magnitude increase in the absorption band of a periodically patterned metal–dielectric–metal structure can be obtained by integrating an indium tin oxide (ITO) subwavelength nanolayer into the insulating dielectric gap region. Through incorporation of a 12 nm thick ITO layer between the patterned gold nanodisks and the SiO2 dielectric layer, a 240 nm wide, flat-top perfect (>98%) absorption centered at 1550 nm wavelength is enabled. The demonstrated wideband, perfect absorption resonance is shown to be due to coupling between the gap plasmon mode of the metasurface and the ENZ mode in the nanoscale ITO film.

Surface-enhanced FAST CARS: en route to quantum nano-biophotonics

AbstractQuantum nano-biophotonics as the science of nanoscale light-matter interactions in biological systems requires developing new spectroscopic tools for addressing the challenges of detecting and disentangling weak congested optical signals. Nanoscale bio-imaging addresses the challenge of the detection of weak resonant signals from a few target biomolecules in the presence of the nonresonant background from many undesired molecules. In addition, the imaging must be performed rapidly to capture the dynamics of biological processes in living cells and tissues. Label-free non-invasive spectroscopic techniques are required to minimize the external perturbation effects on biological systems. Various approaches were developed to satisfy these requirements by increasing the selectivity and sensitivity of biomolecular detection. Coherent anti-Stokes Raman scattering (CARS) and surface-enhanced Raman scattering (SERS) spectroscopies provide many orders of magnitude enhancement of chemically specific Raman signals. Femtosecond adaptive spectroscopic techniques for CARS (FAST CARS) were developed to suppress the nonresonant background and optimize the efficiency of the coherent optical signals. This perspective focuses on the application of these techniques to nanoscale bio-imaging, discussing their advantages and limitations as well as the promising opportunities and challenges of the combined coherence and surface enhancements in surface-enhanced coherent anti-Stokes Raman scattering (SECARS) and tip-enhanced coherent anti-Stokes Raman scattering (TECARS) and the corresponding surface-enhanced FAST CARS techniques. Laser pulse shaping of near-field excitations plays an important role in achieving these goals and increasing the signal enhancement.

Nanoantenna-Microcavity Hybrids with Highly Cooperative Plasmonic-Photonic Coupling.

Nanoantennas offer the ultimate spatial control over light by concentrating optical energy well below the diffraction limit, whereas their quality factor (Q) is constrained by large radiative and dissipative losses. Dielectric microcavities, on the other hand, are capable of generating a high Q-factor through an extended photon storage time but have a diffraction-limited optical mode volume. Here we bridge the two worlds, by studying an exemplary hybrid system integrating plasmonic gold nanorods acting as nanoantennas with an on-resonance dielectric photonic crystal (PC) slab acting as a low-loss microcavity and, more importantly, by synergistically combining their advantages to produce a much stronger local field enhancement than that of the separate entities. To achieve this synergy between the two polar opposite types of nanophotonic resonant elements, we show that it is crucial to coordinate both the dissipative loss of the nanoantenna and the Q-factor of the low-loss cavity. In comparison to the antenna-cavity coupling approach using a Fabry-Perot resonator, which has proved successful for resonant amplification of the antenna's local field intensity, we theoretically and experimentally show that coupling to a modest-Q PC guided resonance can produce a greater amplification by at least an order of magnitude. The synergistic nanoantenna-microcavity hybrid strategy opens new opportunities for further enhancing nanoscale light-matter interactions to benefit numerous areas such as nonlinear optics, nanolasers, plasmonic hot carrier technology, and surface-enhanced Raman and infrared absorption spectroscopies.

High-contrast wide-field evanescent wave illuminated subdiffraction imaging

In this Letter, we show how to obtain high-contrast wide-field evanescent wave illuminated subdiffraction imaging through controlling nanoscale light-matter interaction. The light coupling, propagation, and far-field imaging processes show strong polarization selectivity and film quality dependence, which is used to improve the image-contrast-to-noise ratio (CNR) and to enlarge the field of view (FOV). We demonstrate experimentally high CNR subdiffraction imaging with lateral resolution of 122nm and FOV of thousands of micrometers square.

Sculpting Fano Resonances To Control Photonic-Plasmonic Hybridization.

Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their respective components have either high-quality factors or low mode volumes. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode volumes to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune critical system parameters. In this Letter, we present a general and practical method to determine the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on experimentally measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters critical to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode volume and ultimately maximize the Purcell factor to be 104.

Reaction: New Insights into Molecular Electronics

Liang-Yan Hsu is an assistant research fellow at the Institute of Atomic and Molecular Science. He earned his PhD from Princeton University (2015) and was a postdoctoral fellow at Northwestern University (2017). His research focuses on theoretical chemical physics, nanoelectronics, quantum-transport methodology, and nanoscale light-matter interactions. Bih-Yaw Jin is a chemistry professor at National Taiwan University (NTU). His research interests include the electronic structure of conjugated organic materials, quantum molecular processes in condensed matter and molecular devices, and self-trapping phenomena in quantum systems. Chun-hsien Chen is a Distinguished Professor at NTU. His research interests involve electrochemistry, solid-liquid interfacial phenomena, and scanning-probe-microscopy-based I-V characterization of single molecules. Shie-Ming Peng is a chair professor at NTU, academician, and fellow of the World Academy of Sciences, Royal Society of Chemistry, Russian Academy of Engineering, and Japan Society for the Promotion of Science. He is a synthetic and structural coordination chemist with interests in metal-string complexes.

Reaction: Technological Aspects of Molecular Electronics

Dr. Emanuel Lortscher received his PhD with distinction (summa cum laude) in 2006 from the University of Basel in Switzerland and a master in physics in 2003 from the Swiss Federal Institute of Technology. He has been a research staff member at IBM Research since 2008. He has received various prizes, including the Swiss Physical Society Award for Applied Physics in 2007 and the Faculty Prize of the University of Basel in the same year. His current research focuses on fabrication, electronics, optics, and chemistry at the nanoscale for applications in novel computing applications, biology, medicine, and the Internet of Things.