In metal optics gold assumes a special status because of its practical importance in opto-electronic and nano-optical devices, and its role as a model system for the study of the elementary electronic excitations that underlie the interaction of electromagnetic fields with metals. However, largely inconsistent values for the frequency dependence of the dielectric function describing the optical response of gold are found in the literature [1][2][3]. We performed precise spectroscopic ellipsometry measurements on evaporated gold, template-stripped gold, and single-crystal gold to determine the optical dielectric function across a broad spectral range of 300 nm -25 µm (0.05 -4.14 eV) with high spectral resolution. We fit the data to the Drude free-electron model, with an electron relaxation time τD = 14 ± 3 fs and plasma energy ωp = 8.48 eV. We find that the variation in dielectric functions for the different types of samples is small compared to the range of values reported in the literature. Our values, however, are comparable to the aggregate mean of the collection of previous measurements from over the past six decades. This suggests that although some variation can be attributed to surface morphology, the past measurements using different approaches seem to have been plagued more by systematic errors than previously assumed.

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Inexpensive, reproducible and high-throughput fabrication of nanometric apertures in metallic films can benefit many applications in plasmonics, sensing, spectroscopy, lithography and imaging. Here we use template stripping to pattern periodic nanohole arrays in optically thick, smooth Ag films with a silicon template made via nanoimprint lithography. Ag is a low-cost material with good optical properties, but it suffers from poor chemical stability and biocompatibility. However, a thin silica shell encapsulating our template-stripped Ag nanoholes facilitates biosensing applications by protecting the Ag from oxidation as well as providing a robust surface that can be readily modified with a variety of biomolecules using well-established silane chemistry. The thickness of the conformal silica shell can be precisely tuned by atomic layer deposition, and a 15-nm-thick silica shell can effectively prevent fluorophore quenching. The Ag nanohole arrays with silica shells can also be bonded to polydimethylsiloxane (PDMS) microfluidic channels for fluorescence imaging, formation of supported lipid bilayers, and real-time, label-free SPR sensing. Additionally, the smooth surfaces of the template-stripped Ag films enhance refractive index sensitivity compared with as-deposited, rough Ag films. Because nearly centimeter-sized nanohole arrays can be produced inexpensively without using any additional lithography, etching or lift-off, this method can facilitate widespread applications of metallic nanohole arrays for plasmonics and biosensing.

With a template-stripping fabrication technique, we demonstrate the mass fabrication of high-quality, uniform, ultrasharp (10 nm) metallic probes suitable for single-molecule fluorescence imaging, tip-enhanced Raman spectroscopy (TERS), and other near-field imaging techniques. We achieve reproducible single-molecule imaging with sub-20-nm spatial resolution and an enhancement in the detected fluorescence signal of up to 200. Similar results are obtained for TERS imaging of carbon nanotubes. We show that the large apex angle (70.5°) of our pyramidal tip is well suited to scatter the near-field optical signal into the far-field, leading to larger emission enhancement and hence to a larger quantum yield. Each gold or silver pyramidal probe is used on-demand, one at a time, and the unused tips can be stored for extended times without degradation or contamination. The high yield (>95%), reproducibility, durability, and massively parallel fabrication (1.5 million identical probes over a wafer) of the probes hold promise for reliable optical sensing and detection and for cementing near-field optical imaging and spectroscopy as a routine characterization technique.

Self-assembled plasmonic nanoring cavity arrays are formed alongside the curvature of highly packed metallic nanosphere gratings. The sub-10-nm gap size is precisely tuned via atomic layer deposition and highly ordered arrays are produced over a cm-sized area. The resulting hybrid nanostructure boosts coupling efficiency of light into plasmons, and shows an improved SERS detection limit. These substrates are used for SERS detection of the biological analyte, adenine, followed by concurrent localized surface plasmon resonance sensing.

We experimentally demonstrate dielectrophoretic concentration of biological analytes on the surface of a gold nanohole array, which concurrently acts as a nanoplasmonic sensor and gradient force generator. The combination of nanohole-enhanced dielectrophoresis, electroosmosis, and extraordinary optical transmission through the periodic gold nanohole array enables real-time label-free detection of analyte molecules in a 5 µL droplet using concentrations as low as 1 pM within a few minutes, which is more than 1000 times faster than purely diffusion-based binding. The nanohole-based optofluidic platform demonstrated here is straightforward to construct, applicable to both charged and neutral molecules, and performs a novel function that cannot be accomplished using conventional surface plasmon resonance sensors.

We demonstrate the integration of a black phosphorus photodetector in a hybrid, three-dimensional architecture of silicon photonics and metallic nanoplasmonics structures. This integration approach combines the advantages of the low propagation loss of silicon waveguides, high-field confinement of a plasmonic nanogap, and the narrow bandgap of black phosphorus to achieve high responsivity for detection of telecom-band, near-infrared light. Benefiting from an ultrashort channel (∼60 nm) and near-field enhancement enabled by the nanogap structure, the photodetector shows an intrinsic responsivity as high as 10 A/W afforded by internal gain mechanisms, and a 3 dB roll-off frequency of 150 MHz. This device demonstrates a promising approach for on-chip integration of three distinctive photonic systems, which, as a generic platform, may lead to future nanophotonic applications for biosensing, nonlinear optics, and optical signal processing.

We evaluate and compare the sensitivity of gold nanodisks on silica substrates and nanoholes made in silica-supported gold films, two of the most common sensor structures used in plasmonic biosensing. An alumina overcoat was applied by atomic layer deposition (ALD) to precisely control the interfacial refractive index and determine the evanescent plasmonic field decay length. The results are in good agreement with analytical models and biomolecular binding experiments for the two substrates. We found that nanodisks outperform nanoholes for thin dielectric coatings (<∼20 nm), while the opposite holds true for thicker coatings (>∼20 nm). The optimum nanoplasmonic transducer element for a given biorecognition reaction can be chosen based on experimentally determined bulk sensitivities/noise levels and theoretically estimated evanescent field decay lengths.