All-Optical, Air-Coupled Ultrasonic Detection of Low-Pressure Gas Leaks and Observation of Jet Tones in the MHz Range.

We used an ultrasensitive, broadband optomechanical ultrasound sensor to study the acoustic signals produced by pressurized nitrogen escaping from a variety of small syringes. Harmonically related jet tones extending into the MHz region were observed for a certain range of flow (i.e., Reynolds number), which is in qualitative agreement with historical studies on gas jets emitted from pipes and orifices of much larger dimensions. For higher turbulent flow rates, we observed broadband ultrasonic emission in the ~0-5 MHz range, which was likely limited on the upper end due to attenuation in air. These observations are made possible by the broadband, ultrasensitive response (for air-coupled ultrasound) of our optomechanical devices. Aside from being of theoretical interest, our results could have practical implications for the non-contact monitoring and detection of early-stage leaks in pressured fluid systems.

Complex 10-nm resolution nanogap and nanowire geometries for plasmonic metasurface miniaturization.

Emerging electromagnetic inverse design methods have pushed nanofabrication methods to their limits to extract maximum performance from plasmonic aperture-based metasurfaces. Using plasmonic metamaterial-lined apertures as an example, we demonstrate the importance of fine nanowire and nanogap features for achieving strong miniaturization of plasmonic nanoapertures. Metamaterial-lined nanoapertures are miniaturized over bowtie nanoapertures with identical minimum feature sizes by a factor of 25% without loss of field enhancement. We show that features as small as 10 nm can be reliably patterned over the wide areas required of metasurfaces using the helium focused ion beam microscope. Under imperfect fabrication conditions, we achieve 11-nm-wide nanogaps and 12-nm-wide nanowires over an area of 13 µm2, and successfully validate our results with optical characterization and comparable full-wave simulations.

Investigation of the Electrical Properties of Microtubule Ensembles under Cell-Like Conditions.

Microtubules are hollow cylindrical polymers composed of the highly negatively-charged (~23e), high dipole moment (1750 D) protein α, ß- tubulin. While the roles of microtubules in chromosomal segregation, macromolecular transport, and cell migration are relatively well-understood, studies on the electrical properties of microtubules have only recently gained strong interest. Here, we show that while microtubules at physiological concentrations increase solution capacitance, free tubulin has no appreciable effect. Further, we observed a decrease in electrical resistance of solution, with charge transport peaking between 20-60 Hz in the presence of microtubules, consistent with recent findings that microtubules exhibit electric oscillations at such low frequencies. We were able to quantify the capacitance and resistance of the microtubules (MT) network at physiological tubulin concentrations to be 1.27 × 10-5 F and 9.74 × 104 Ω. Our results show that in addition to macromolecular transport, microtubules also act as charge storage devices through counterionic condensation across a broad frequency spectrum. We conclude with a hypothesis of an electrically tunable cytoskeleton where the dielectric properties of tubulin are polymerisation-state dependent.