Wenqi Zhu

Quantum mechanical effects in plasmonic structures with subnanometre gaps

Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale. Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. This review outlines the findings of many groups into quantum mechanical effects in nanogap plasmons, and discusses outstanding challenges and future directions.

Quantum mechanical limit to plasmonic enhancement as observed by surface-enhanced Raman scattering.

Plasmonic nanostructures enable light to be concentrated into nanoscale hotspots, wherein the intensity of light can be enhanced by orders of magnitude. This plasmonic enhancement significantly boosts the efficiency of nanoscale light-matter interactions, enabling unique linear and nonlinear optical applications. Large enhancements are often observed within narrow gaps or at sharp tips, as predicted by the classical electromagnetic theory. Only recently has it become appreciated that quantum mechanical effects could emerge as the feature size approaches atomic length-scale. Here we experimentally demonstrate, through observations of surface-enhanced Raman scattering, that the emergence of electron tunnelling at optical frequencies limits the maximum achievable plasmonic enhancement. Such quantum mechanical effects are revealed for metallic nanostructures with gap-widths in the single-digit angstrom range by correlating each structure with its optical properties. This work furthers our understanding of quantum mechanical effects in plasmonic systems and could enable future applications of quantum plasmonics.

Lithographically Fabricated Optical Antennas with Gaps Well Below 10 nm

Metal nanostructures that effi ciently capture or radiate electromagnetic waves at optical frequencies offer a means to concentrate electromagnetic energy into deep subwavelength regions. Wessel noted that these structures can therefore be considered antennas. [ 1 ] Recent work has focused on more effi cient designs, termed ‘optical antennas’, which employ small gaps or very sharp tips. [ 2‐4 ] Optical antennas present opportunities for ultrasensitive spectroscopy, near-fi eld scanning optical microscopy, and compact subwavelength light sources. [ 5‐7 ] However, the achievable feature sizes are usually determined by fabrication, being approximately given by the gap size or tip sharpness. Here, we report a top-down fabrication procedure to fabricate pairs of nanoparticles separated by a controllable gap size that can be as small as 3 nm. As an application, we show that the enhancement factors of surfaceenhanced Raman scattering (SERS) increase signifi cantly for smaller gap sizes, indicating greatly enhanced electromagnetic fi elds within the gaps. We anticipate that the fabrication method we introduce here for nanoparticle pairs with nanoscale gaps would be useful not only for SERS, where it could potentially enable single-molecule sensitivity, but also for other applications in plasmonics. Raman spectroscopy is a powerful analytical method, enabling molecules to be identifi ed through their characteristic vibrational spectra. Through SERS, the Raman cross-section of molecules adsorbed to nanostructures can be increased by orders of magnitude. SERS has attracted renewed attention since the fi rst demonstrations of singlemolecule sensitivity. [ 8 , 9 ] Increasing the adoption of the SERS technique further, however, requires fabrication methods capable of routinely delivering reproducible substrates with high enhancement factors. Dimer structures, consisting of two metallic nanoparticles closely placed together, are the simplest optical antenna structures that are confi rmed to be single-molecule SERS active. [ 10 , 11 ] It is believed that substantial electromagnetic fi elds generated in dimer gaps are one of the main enhancement mechanisms in single-molecule SERS. [ 12 , 13 ] This motivates the development of a reproducible

Surface-Enhanced Raman Scattering with Ag Nanoparticles Optically Trapped by a Photonic Crystal Cavity

We demonstrate a reusable and reconfigurable surface enhanced Raman scattering (SERS) platform by optically trapping Ag nanoparticles with a photonic crystal cavity integrated with a microfluidic chip. High-performance SERS is performed in a very reproducible manner, owing to the fact that Ag aggregates are produced by optical trapping in a controllable process that is monitored in real-time by the cavity resonance shift that occurs with the trapping of each additional nanoparticle.

Directional Raman scattering from single molecules in the feed gaps of optical antennas.

Controlling light from single emitters is an overarching theme of nano-optics. Antennas are routinely used to modify the angular emission patterns of radio wave sources. Optical antennas translate these principles to visible and infrared wavelengths and have been recently used to modify fluorescence from single quantum dots and single molecules. Understanding the properties of single molecules, however, would be advanced were one able to observe their vibrational spectra through Raman scattering in a very reproducible manner but it is a hugely challenging task, as Raman scattering cross sections are very weak. Here we measure for the first time the highly directional emission patterns of Raman scattering from single molecules in the feed gaps of optical antennas fabricated on a chip. More than a thousand single molecule events are observed, revealing that an unprecedented near-unity fraction of optical antennas have single molecule sensitivity.

Double resonance surface enhanced Raman scattering substrates: an intuitive coupled oscillator model

The strong coupling between localized surface plasmons and surface plasmon polaritons in a double resonance surface enhanced Raman scattering (SERS) substrate is described by a classical coupled oscillator model. The effects of the particle density, the particle size and the SiO2 spacer thickness on the coupling strength are experimentally investigated. We demonstrate that by tuning the geometrical parameters of the double resonance substrate, we can readily control the resonance frequencies and tailor the SERS enhancement spectrum.

Wafer-scale metasurface for total power absorption, local field enhancement and single molecule Raman spectroscopy

The ability to detect molecules at low concentrations is highly desired for applications that range from basic science to healthcare. Considerable interest also exists for ultrathin materials with high optical absorption, e.g. for microbolometers and thermal emitters. Metal nanostructures present opportunities to achieve both purposes. Metal nanoparticles can generate gigantic field enhancements, sufficient for the Raman spectroscopy of single molecules. Thin layers containing metal nanostructures (“metasurfaces”) can achieve near-total power absorption at visible and near-infrared wavelengths. Thus far, however, both aims (i.e. single molecule Raman and total power absorption) have only been achieved using metal nanostructures produced by techniques (high resolution lithography or colloidal synthesis) that are complex and/or difficult to implement over large areas. Here, we demonstrate a metasurface that achieves the near-perfect absorption of visible-wavelength light and enables the Raman spectroscopy of single molecules. Our metasurface is fabricated using thin film depositions, and is of unprecedented (wafer-scale) extent.

High Directivity Optical Antenna Substrates for Surface Enhanced Raman Scattering

A two-dimensional array of gold optical antennas integrated with a one-dimensional array of gold strips and mirrors is introduced and fabricated. The experimental results show that this design achieves average surface-enhanced Raman scattering (SERS) enhancement factors as high as 1.2 × 10(10) , which is more than two orders of magnitude larger than optical antennas without the gold strips and gold mirror.

Si Microwire Solar Cells: Improved Efficiency with a Conformal SiO2 Layer

Silicon microwire arrays have attracted considerable attention recently due to the opportunity they present as highly efficient and cost-effective solar cells. In this study, we report on efficient Si microwire array solar cells with areas of 1 cm(2) and Air Mass 1.5 Global conversion efficiencies of up to 10.6%. These solar cells show an open-circuit voltage of 0.56 V, a short-circuit current density of 25.2 mA/cm(2), and a fill factor of 75.2%, with a silicon absorption region that is only 25 μm thick. In particular, the maximum overall efficiency of the champion device is improved from 8.71% to 10.6% by conformally coating the wires with a 200 nm thick SiO2 layer. Optical measurements reveal that the layer reduces reflection significantly over the entire visible range.

Efficient polarization beam splitter pixels based on a dielectric metasurface

The polarization dependence of the reflection, refraction, and diffraction of electromagnetic waves from materials is measured in applications that extend from small (e.g., ellipsometry of semiconductor chips) to large scales (e.g., remote sensing for planetary science and weather radar). Such applications employ polarimeters that are in turn based on devices with polarization-selective absorption or reflection/refraction properties (e.g., prisms). The latter devices are generally bulky, thereby limiting their integration into compact systems. The former devices are inherently lossy, as they function by absorbing the unwanted polarization. Here, we experimentally demonstrate a conceptually novel method for pixel-level polarimetry. Each pixel contains amorphous-silicon nanoridges and deflects incident light in a polarization-dependent manner. As photons are sorted by polarization rather than filtered, the approach permits high efficiency. A high transmission efficiency of 90% and a high extinction ratio of 15 times are demonstrated.