Xiang Meng

Graphene Plasmon Enhanced Vibrational Sensing of Surface-Adsorbed Layers

We characterize the influence of graphene nanoribbon plasmon excitation on the vibrational spectra of surface-absorbed polymers. As the detuning between the graphene plasmon frequency and a vibrational frequency of the polymer decreases, the vibrational peak intensity first increases and is then transformed into a region of narrow optical transparency as the frequencies overlap. Examples of this are provided by the carbonyl vibration in thin films of poly(methyl methacrylate) and polyvinylpyrrolidone. The signal depth of the plasmon-induced transparency is found to be 5 times larger than that of light attenuated by the carbonyl vibration alone. The plasmon-vibrational mode coupling and the resulting fields are analyzed using both a phenomenological model of electromagnetically coupled oscillators and finite-difference time-domain simulations. It is shown that this coupling and the resulting absorption enhancement can be understood in terms of near-field electromagnetic interactions.

Engineering metal-nanoantennae/dye complexes for maximum fluorescence enhancement

We theoretically investigate the fluorescence enhancement of a molecule placed in a variable (4 - 20 nm) gap of a plasmonic dimer, with different dye molecules as well as different nanoparticle geometries, using a fully vectorial three-dimensional finite-difference time-domain (3D FDTD) method. This work extends previous studies on molecular fluorescence in the vicinity of metal interfaces and single nanoparticles and shows how the radiative emission of a molecule can be further enhanced by engineering the geometry of a plasmonic structure. Through the use of rigorous 3D FDTD calculations, in conjunction with analytic guidance based on temporal coupled-mode (TCM) theory, we develop a design procedure for antennae assemblies that is useful both for general understanding of molecule-metal structure interaction and experimental efforts in plasmon-enhanced molecular spectroscopy.

Rigorous theoretical analysis of a surface-plasmon nanolaser with monolayer MoS 2 gain medium.

Lasers based on monolayer (ML) transition-metal dichalcogenide semiconductor crystals have the potential for low threshold operation and a small device footprint; however, nanophotonic engineering is required to maximize the interaction between the optical fields and the three-atom-thick gain medium. Here, we develop a theoretical model to design a direct bandgap optically pumped nanophotonic integrated laser. Our device utilizes a gap-surface-plasmon optical mode to achieve subwavelength optical confinement and consists of a high-index GaP nanowire atop an ML MoS2 film on an Ag substrate. The optical field and material medium are analyzed using a three dimensional finite-difference time-domain method and a first-principles calculation based on the density functional theory, respectively. The nanolaser is designed to have a threshold of ∼0.6  μW under quasi-continuous wave operation on an excitonic transition at room temperature.

Two-color field enhancement at an STM junction for spatiotemporally resolved photoemission

We report measurements and numerical simulations of ultrafast laser-excited carrier flow across a scanning tunneling microscope (STM) junction. The current from a nanoscopic tungsten tip across a ∼1  nm vacuum gap to a silver surface is driven by a two-color excitation scheme that uses an optical delay-modulation technique to extract the two-color signal from background contributions. The role of optical field enhancements in driving the current is investigated using density functional theory and full three-dimensional finite-difference time-domain computations. We find that simulated field-enhanced two-photon photoemission (2PPE) currents are in excellent agreement with the observed exponential decay of the two-color photoexcited current with increasing tip-surface separation, as well as its optical-delay dependence. The results suggest an approach to 2PPE with simultaneous subpicosecond temporal and nanometer spatial resolution.

New Advances in Nanophotonic Device Physics

Advanced silicon and plasmonic nanophotonics is undergoing rapid progress due to its manifold applications in high data communication links and other applications in imaging and sensing. Our group has been at the forefront of new devices and device physics. In this talk we will first review progress in our group in a wide variety of fundamental technologies and physics needed to extend the advances in nanophotonics. We will then illustrate these ideas with several new devices types that we have recently demonstrated at Columbia based on new simulation modalities. Our approach then to modeling and simulation is to use fully accurate methods and techniques and to achieve new capabilities based on massively parallel and high-performance computation. Much of our advances are based on new hardware strengths and testing with distributed and parallel systems.

Coupling of strongly localized graphene plasmons to molecular vibrations

In this chapter, we first present a determination of the out-of-plane confinement of the plasmons in graphene nanoribbons. Using light with a free-space wavelength of ∼6 μm, we excite plasmons in graphene nanoribbons that are ∼100 nm wide. A red-shift in the plasmon frequency is induced by a thin layer of Poly (methyl methacrylate) (PMMA) adsorbed onto the nanoribbons surface due to dielectric screening effect. With increasing thickness of the PMMA layer, we observe a saturation of the frequency shift, from which we deduce an out-of-plane field plasmon field decay length of ∼10 nm. The strongly confined plasmons in graphene produce significant enhancement of the field intensity. We then show that this enhancement strengthens the coupling of graphene plasmon to vibrations in the PMMA molecules. The enhanced interaction is manifested through induced transparency in the graphene plasmon optical response when the plasmon and the vibrational frequencies are matched. We also show that this coupling is of an electromagnetic nature by comparing the evolution of the line shape as a function of the detuning of the two frequencies to simulations using the finite-difference time-domain method. The content of most of this chapter has appeared in [1].

Emerging plasmonic applications explored with cluster parallel computing

We numerically investigate the optical properties of a variety of nanostructures using the full 3D finite-difference time-domain (FDTD) method. Rigorous computational design of nanostructures is shown to be an important tool for optimized devices in emerging plasmonic applications.

Two-color assisted field enhancement for vacuum tunneling of a metallic-tip/substrate systems

We investigate the optical field enhancement for two-color ultrafast laser pulses incident on a nanoscopic tungsten probe tip suspended over a silver substrate. Experimental measurements of vacuum tip tunneling are paired with computations using full 3D finite-difference time-domain (FDTD) calculations. The measured spatial response of the tip current matches the calculated dependence.

Designer metal-nanoantennae/dye complexes for maximum fluorescence enhancement

We theoretically investigate the fluorescence enhancement of a representative set of dye-molecules excited by three classes of nanoantennae, using a fully vectorial three-dimensional finite-difference time-domain (3D FDTD) method. Through these 3D FDTD calculations, in conjunction with analytic guidance using temporal coupled-mode (TCM) theory, we develop a design procedure for antennae assemblies that allow achieving fluorescence enhancements of 200-900 over the emission intensity in the bare dye molecule. The enhancement from these commercially available fluorochrome conjugates, namely, CFTM568, CFTM660R and CFTM790 are fully investigated using spherical-dimer, elliptical-dimer, and bowtie nanoantennae. These results demonstrate a method for rationally designing arbitrary metallic nanoparticle/emitter assemblies prior to their synthesis and assembly to achieve optimum fluorescence enhancement.

Metal-Semiconductor-Metal Monolithic Silicon-Waveguide Photodiode Design and Analysis

We present a monolithic Si-nanowire-waveguide based Metal-Semiconductor-Metal photodetector design and compare to similar p-i-n structures. Key characteristics are simplicity of design, dark current of 1nA at 15V, and calculated frequency response of ≈14 GHz.