This PDF file contains the front matter associated with SPIE Proceedings Volume 11803, including the Title Page, Copyright information, and Table of Contents.

Plenary talk: Engineering new nonlinear optical nanomaterials and nanophotonic structures

Charge-transfer based surface-enhanced Raman scattering (SERS) is a promising tool for chiral-label-free discrimination of small molecules with the superiority of chirality signal amplification, synchronous distinction and identification, and high sensitivity. Such chiral-label-free SERS strategy need neither any chiral auxiliary nor circularly polarized light, which opens up a new way for chiral discrimination.

Electrochemical tip-enhanced Raman spectroscopy (EC-TERS), which provides molecular fingerprint information with nanometer-scale spatial resolution, is a promising technique to study the structure-activity relationships of the electrochemical interface. In this work, we developed the electrochemical tip-enhanced Raman spectroscopy (EC-TERS) that possesses high sensitivity and nanoscale spatial resolution, as well as methods to fabricate TERS tips with a high enhancement. Based on the developed systems, we in-situ monitor the plasmon driven decarboxylation reaction. The spatial distribution of the effective hot carriers was visualized by TERS imaging of the nanoscale reaction region, which provides mechanistic insights into plasmon driven reactions.

The reactivity of electrochemically-active molecular architectures immobilized on electrode surfaces was investigated by electrochemical-TERS, at relatively high potential sweep rate and on broad potential ranges. A complex electrochemical mechanism, involving reaction intermediates and multiple reaction paths, could be resolved on electroactive architectures based on nitrobenzene derivatives. Further EC-TERS investigations on these derivatives assembled as mono- or multilayers on the electrode surface emphasized the influence of the structure of the molecular assemblies on their reactivity. Under specific conditions, azo bonds formation between nitrobenzene derivatives observed by TERS can result from the electrochemical polarization/reaction, and not from photochemical processes.

In this presentation, the molecular sensitivity of scattering-type near-field scanning optical microscopy (s-SNOM) will be demonstrated by imaging an organic thin film with thickness gradient that continuously vary from zero to over 200 nm on different substrates. We will then present recent s-SNOM experimental results that show phase separation and nanoscale pattern formation in thin films of blended polymers. The evolution of nanoscale domains and hierarchical patterns as a function of composition will be discussed. The results may help to understand the sensitivity of s-SNOM chemical imaging at the molecular “finger print” region of electromagnetic radiation and to realize the capability of the technique to resolve nanoscale domains and phase separation in multicomponent organic thin films.

The interface between bulk insulators SrTiO3 and LaAlO3 (LAO/STO) gives rise to a confined and highly conductive two-dimensional electron gas (2DEG) [1], which poses unique analytical challenges, due to its buried and sensitive nature. Scanning near-field optical microscopy (SNOM) was shown to detect the electronic properties of the LAO/STO 2DEG using highly confined optical near-fields. Here, we used our models to compare different vertical distributions of the interfacial free charge carriers and explore the resulting coupled plasmon-phonon polariton modes. Finally, we predict a transition from phonon-dominated to electron-dominated response at higher mobilities, which should be achievable in low-temperature SNOM measurements.

Experimental and theoretical studies of a metamaterial enhanced vibrational spectroscopy techniques are presented. We design a metamaterial consisting of vertically oriented metal insulator metal (MIM) structures with a 25 nm gap sandwiched between two metal films. By using vertical-oriented MIM structure we successfully detected 20 ppm concentration of carbon dioxide and butane molecules with negligible background noise. Metasmaterial structure was also applied for the vibrational CD spectroscopy by exploiting super-chiral field. We experimentally demonstrate high-sensitive mirror symmetric vibrational CD spectra of D- and L-alanine depending on the handedness of the metamaterial structures.

Abbe’s diffraction limit prevents straightforwardly resolving the micro/nanoscale heterogeneous materials with mid infrared light. In this presentation, I will describe our invention on peak force infrared (PFIR) microscopy . The PFIR microscopy utilizes temporal domain mechanical detection of the tip-enhanced infrared photothermal response of the sample with a nanoscopic atomic force microscope (AFM) tip. The PFIR works in both air phase and liquid phase. We have demonstrated the imaging capability of PFIR on a wide range of materials from block copolymer, amyloid fibril, cellular structures to secondary organic aerosols, oil shale source rock, and two-dimensional polaritonic materials. A spatial resolution of 6 nm is demonstrated across different types of samples. In addition, we have integrated the PFIR microscopy with other modalities of chemical measurement and imaging, including mechanical property measurement and surface potential mapping.

Plasmon nanofocusing is a phenomenon that creates a localized strong light field at the apex of a tapered metallic structure by propagating plasmons on the metallic taper toward the apex with compressing light energy. One of interesting properties of plasmon nanofocusing is that it can be excited over a broad wavelength range. In this talk, focusing on broadband property of plasmon nanofocusing, we introduce our recent works from fundamental studies to advanced applications. We also discuss potentials of the broadband nanolight source created by plasmon nanofocusing for future applications.

Inspired by the proposal that single molecules will be functional elements of future nanoelectronic devices, there exists considerable interest in understanding charge transport in individual molecules. To study charge transport in single-molecule junctions, we exploit the STM microscope’s Blinking approach. It is a “current vs. time” molecular capturing procedure, able to electrically detect spontaneous individual molecular junctions under a constant sub-nm precise interelectrode distance. Here, we will present a novel plasmon-supported methodology (PBJ), based on Blinking to increase the timescale of the junctions. The (stabilising) force of the nearfield gradient is exploited to provide additional endurance to junctions, increasing the detected lifetime from hundreds of milliseconds to the order of seconds. Also, we will present our advances exploiting PBJ under electrochemical control, trapping redox metalloproteins resonant to the localized surface plasmon excitation wavelength.

On and near the noble metal nanodimer, even single molecule can be detected by surface-enhanced Raman scattering and fluorescence (SERS and SEF), respectively. The positions of the SERS and SEF-active single molecule were observed beyond the diffraction limit by super-resolution imaging. The spatial fluctuation becomes narrower by more intense excitation laser light. The laser intensity dependence of the spatial fluctuation was observed not in the large aggregate but in the nanodimer. It indicates the single molecular optical trapping via plasmon resonance. Moreover, the intensities of single pulse signals in the blinking SERS and SEF were barely fluctuated under the intense excitation light. The power spectral density of the fluctuated positions in the optically trapping shows a line. It represents not harmonic but random movement of the optically trapped single molecule, which is consistent with the power law analysis of the blinking SERS.

We demonstrate a dynamic nano-mechanical strain-engineering of naturally-formed wrinkles in a WSe2 monolayer, with real-time investigation of nano-spectroscopic properties using hyperspectral adaptive tip-enhanced PL (a-TEPL) spectroscopy. First, we characterize nanoscale wrinkles through hyperspectral a-TEPL nano-imaging with <15 nm spatial resolution which reveals the modified nano-excitonic properties by the induced tensile strain at the wrinkle apex, e.g., an increase in the quantum yield due to the exciton funneling, decrease in PL energy up to ~10 meV, and a symmetry change in the TEPL spectra caused by the reconfigured electronic bandstructure. We then dynamically engineer the local strain by pressing and releasing the wrinkle apex through an atomic force tip control. This nano-mechanical strain-engineering allows us to tune the exciton dynamics and emission properties at the nanoscale in a reversible fashion.

Monolayer transition metal dichalcogenides, coupled to metal plasmonic nanocavities, have recently emerged as new platforms for strong light–matter interactions. These systems are expected to have nonlinear-optical properties that will enable them to be used as entangled photon sources, compact wave-mixing devices, and other elements for classical and quantum photonic technologies. Here, we report the first experimental investigation of the nonlinear properties of these strongly coupled systems, by observing second harmonic generation from a WSe2 monolayer strongly coupled to a single gold nanorod. The pump-frequency dependence of the second-harmonic signal displays a pronounced splitting that can be explained by a coupled-oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a nonperturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. Our study thus lays the groundwork for understanding the nonlinear properties of strongly coupled nanoscale systems.

Understanding intermolecular electronic energy transfer mechanism in a donor-acceptor system is important for engineering light harvesting in photosynthesis and photovoltaics. Extensive efforts have been made to investigate the intermolecular electronic energy transfer from the energy, and time domains. However, it is still unknown how different types of electronic energy transfer are manifested in real space, due to the diffraction limitation in conventional far-field optics. The scanning tunneling microscope induced luminescence (STML) technique can do nano-imaging beyond diffraction limit and allows to optically access each individual constituent of a donor–acceptor molecular system. In this talk, I shall present the real-space investigation on the electronic energy transfer in donor-acceptor systems with sub-nanometer resolved STML imaging technique.