Accessible high-performance double nanohole tweezers.

Nanohole optical tweezers have been used by several groups to trap and analyze proteins. In this work, we demonstrate that it is possible to create high-performance double nanohole (DNH) substrates for trapping proteins without the need for any top-down approaches (such as electron microscopy or focused-ion beam milling). Using polarization analysis, we identify DNHs as well as determine their orientation and then use them for trapping. We are also able to identify other hole configurations, such as single, trimers and other clusters. We explore changing the substrate from glass to polyvinyl chloride to enhance trapping ability, showing 7 times lower minimum trapping power, which we believe is due to reduced surface repulsion. Finally, we present tape exfoliation as a means to expose DNHs without damaging sonication or chemical methods. Overall, these approaches make high quality optical trapping using DNH structures accessible to a broad scientific community.

Isolating and enhancing single-photon emitters for 1550 nm quantum light sources using double nanohole optical tweezers.

Single-photon sources are required for quantum technologies and can be created from individual atoms and atom-like defects. Erbium ions produce single photons at low-loss fiber optic wavelengths, but they have low emission rates, making them challenging to isolate reliably. Here, we tune the size of gold double nanoholes (DNHs) to enhance the emission of single erbium emitters, achieving 50× enhancement over rectangular apertures previously demonstrated. This produces enough enhancement to show emission from single nanocrystals at wavelengths not seen in our previous work, i.e., 400 and 1550 nm. We observe discrete levels of emission for nanocrystals with low numbers of emitters and demonstrate isolating single emitters. We describe how the trapping time is proportional to the enhancement factor for a given DNH structure, giving us an independent way to measure the enhancement. This shows a promising path to achieving single emitter sources at 1550 nm.

Bright upconverted emission from light-induced inelastic tunneling.

Upconverted light from nanostructured metal surfaces can be produced by harmonic generation and multi-photon luminescence; however, these are very weak processes and require extremely high field intensities to produce a measurable signal. Here we report on bright emission, 5 orders of magnitude greater than harmonic generation, that can be seen from metal tunnel junctions that we believe is due to light-induced inelastic tunneling emission. Like inelastic tunneling light emission, which was recently reported to have 2% conversion efficiency per tunneling event, the emission wavelength recorded varies with the local electric field applied; however, here the field is from a 1560 nm femtosecond pulsed laser source. Finite-difference time-domain simulations of the experimental conditions show the local field is sufficient to generate tunneling-based inelastic light emission in the visible regime. This phenomenon is promising for producing ultrafast upconverted light emission with higher efficiency than conventional nonlinear processes.

Switchable Metal-Insulator Phase Transition Metamaterials.

We investigate the switching of a gap plasmon tunnel junction between conducting and insulating states. Hysteresis is observed in the second and the third harmonic generation power dependence, which arises by thermally induced disorder ("melting") of a two-carbon self-assembled monolayer between an ultraflat gold surface and metal nanoparticles. The hysteresis is observed for a variety of nanoparticle sizes, but not for larger tunnel junctions where there is no appreciable tunneling. By combining quantum corrected finite-difference time-domain simulations with nonlinear scattering theory, we calculate the changes in the harmonic generation between the tunneling and the insulating states, and good agreement is found with the experiments. This paves the way to a new class of metal-insulator phase transition switchable metamaterials, which may provide next-generation information processing technologies.

Probing the acoustic vibrations of complex-shaped metal nanoparticles with four-wave mixing.

We probe the acoustic vibrations of silver nanoprisms and gold nano-octahedrons in aqueous solution with four-wave mixing. The nonlinear optical response shows two acoustic vibrational modes: an in-plane mode of nanoprisms with vertexial expansion and contraction; an extensional mode of nano-octahedrons with longitudinal expansion and transverse contraction. The particles were also analyzed with electron microscopy and the acoustic resonance frequencies were then calculated by the finite element analysis, showing good agreement with experimental observations. The experimental mode frequencies also fit with theoretical approximations, which show an inverse dependence of the mode frequency on the edge length, for both nanoprisms and nano-octahedrons. This technique is promising for in situ monitoring of colloidal growth.

Nanorod Surface Plasmon Enhancement of Laser-Induced Ultrafast Demagnetization.

Ultrafast laser-induced magnetization dynamics in ferromagnetic thin films were measured using a femtosecond Ti:sapphire laser in a pump-probe magneto-optic Kerr effect setup. The effect of plasmon resonance on the transient magnetization was investigated by drop-coating the ferromagnetic films with dimensionally-tuned gold nanorods supporting longitudinal surface plasmon resonance near the central wavelength of the pump laser. With ~4% nanorod areal coverage, we observe a >50% increase in demagnetization signal in nanorod-coated samples at pump fluences on the order of 0.1 mJ/cm(2) due to surface plasmon-mediated localized electric-field enhancement, an effect which becomes more significant at higher laser fluences. We were able to qualitatively reproduce the experimental observations using finite-difference time-domain simulations and mean-field theory. This dramatic enhancement of ultrafast laser-induced demagnetization points to possible applications of nanorod-coated thin films in heat-assisted magnetic recording.

Probing the quantum tunneling limit of plasmonic enhancement by third harmonic generation.

Metal nanostructures provide extreme focusing of optical energy that is limited fundamentally by quantum tunneling. We directly probe the onset of the quantum tunneling regime observed by a sharp reduction in the local field intensity in subnanometer self-assembled monolayer gaps using third harmonic generation. Unlike past works that have inferred local limits from far-field spectra, this nonlinear measurement is sensitive to the near-field intensity as the third power. We calculate the local field intensity using a quantum corrected model and find good quantitative agreement with the measured third harmonic. The onset of the quantum regime occurs for double the gap size of past studies because of the reduced barrier height of the self-assembled monolayer, which will be critical for many applications of plasmonics, including nonlinear optics and surface enhanced Raman spectroscopy.

Quantification of an exogenous cancer biomarker in urinalysis by Raman spectroscopy.

We quantified an exogenous cancer biomarker, Acetyl amantadine (AcAm), directly from urine solution using surface enhanced Raman spectroscopy (SERS). SERS was used for the detection of AcAm using a commercial Raman substrate after beta-cyclodextrin encapsulation for capture of the analyte. We achieved a detection limit of 1 ng mL(-1) of AcAm in the mock urine in the absence of steroids without extraction or other pre-treatment methods required. With levels of corticosterone typical of urine, the limit of detection was 30 times higher. Since the approach works directly from samples containing the high concentrations of salts and organic co-solutes normal to urine, it has the potential to reduce cost and speed up processing with respect to methods that require pre-purification. Therefore, this is promising for clinical adoption for early cancer detection, particularly for lung cancer.

Effect of surface roughness on self-assembled monolayer plasmonic ruler in nonlocal regime.

Recently, self-assembled monolayers (SAMs) have been used for plasmonic rulers to measure the nonlocal influence on the Au nanoparticle - metal film resonance wavelength shift and probe the ultimate field enhancement. Here we examine the influence of surface roughness on this plasmonic ruler in the nonlocal regime by comparing plasmonic resonance shifts for as-deposited and for ultra-flat Au films. It is shown that the resonance shift is larger for ultra-flat films, suggesting that there is not the saturation from nonlocal effects previously reported for the spacer range from 0.7 nm to 1.6 nm. We attribute the previously reported saturation to the planarization of the as-deposited films by thinner SAMs, as measured here by atomic-force microscopy. This work is of interest both in probing the ultimate limits of plasmonic enhancement with SAMs for applications in Raman and nonlinear optics, but also in the study of SAMs planarization as a function surface roughness.

Trace cancer biomarker quantification using polystyrene-functionalized gold nanorods.

We demonstrate the application of polystyrene-functionalized gold nanorods (AuNRs) as a platform for surface enhanced Raman scattering (SERS) quantification of the exogenous cancer biomarker Acetyl Amantadine (AcAm). We utilize the hydrophobicity of the polystyrene attached to the AuNR surface to capture the hydrophobic AcAm from solution, followed by drying and detection using SERS. We achieve a detection limit of 16 ng/mL using this platform. This result shows clinical potential for low-cost early cancer detection.

Plasmon hybridization for enhanced nonlinear optical response.

We report the plasmon hybridization between silver nanoprisms and a thin gold film as a means to tune the plasmon resonance and to achieve enhanced optical second harmonic generation. The hybridization enhances the second harmonic counts by nearly three orders of magnitude when varying the spacer layer between the nanoprisms and the gold film. Finite-difference time-domain calculations agree within a factor of 2 with the experimental findings in terms of the predicted enhancement factor. This plasmon hybridization approach is promising for future applications, including multi-photon lithography and nonlinear sensing using metal nanoparticles.

One-step integration of metal nanoparticle in photonic crystal nanobeam cavity.

A single step process of integrating a resonantly tuned silver nanoparticle into photonic crystal nanobeam cavities fabricated by focused ion beam milling is presented. Even though the quality factor of the cavities is reduced by a factor of 20, the emission peak at the cavity resonance is enhanced by 5-fold with respect to the cavities without the metal nanoparticle. The fluorescence is also compared before and after etching away the nanoparticle. Experimental quality factors and wavelength shifts are found to agree reasonably well with simulation values. These results are promising for future single photon emission studies involving the incorporation of quantum dot or NV center emitters into hybrid plasmonic/photonic crystal cavities for enhanced emission rates while retaining reasonably high quality factors.