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1. Label-free alternating-current plasmonic nanopore sensing of nanoparticles

   https://doi.org/10.1117/12.2607884  

   Renkes, Scott; Kim, MinJun; Alexandrakis, George (March 2022, Label-free alternating-current plasmonic nanopore sensing of nanoparticles)

   Vo-Dinh, Tuan; Ho, Ho-Pui A.; Ray, Krishanu (Ed.)

   Alternating current (AC) modulation of command voltage applied across a Self-induced Back Action Actuated Nanopore Electrophoresis (SANE) sensor, a type of plasmonic nanopore sensor that we have developed previously, enables acquisition of new data types that could potentially enhance the characterization of nanoparticles (NPs) and single molecules. In particular, AC voltage frequency response provides insight into the charge and dielectric constant of analytes that are normally obfuscated using DC command voltages. We first analyzed Axopatch 200B data to map the frequency response of the empty SANE sensor in terms of phase shift and amplitude modulation, with and without plasmonic excitation. We then tested the frequency response of 20 nm diameter silica NPs and 20 nm gold NPs trapped optically, which made these particles hover over an underlying 25 nm nanopore at the center of the SANE sensor. By applying a DC command voltage with a superimposed AC frequency sweep while keeping the NPs optically trapped in the vicinity of the nanopores’s entrance, we have found that silica and gold NPs to have distinctly different electrical responses. This pilot work demonstrates the feasibility of performing AC measurements with a plasmonic nanopore, which encourages us to pursue more detailed characterization studies with NPs and single molecules in future work. 

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2. Tunable optical traps over nonreciprocal surfaces

   https://doi.org/10.1364/OE.476269  

   Paul, N_K; Gomez-Diaz, J_S (December 2022, Optics Express)

   We propose engineering optical traps over plasmonic surfaces and precisely controlling the trap position with an external bias by inducing in-plane nonreciprocity on the surface. The platform employs an incident Gaussian beam to polarize targeted nanoparticles, and exploits the interplay between nonreciprocal and spin-orbit lateral recoil forces to construct stable optical traps and manipulate their position within the surface. To model this process, we develop a theoretical framework based on the Lorentz force combined with nonreciprocal Green’s functions and apply it to calculate the trapping potential. Rooted on this formalism, we explore the exciting possibilities offered by graphene to engineer stable optical traps using low-power laser beams in the mid-IR and to manipulate the trap position in a continuous manner by applying a longitudinal drift bias. Nonreciprocal metasurfaces may open new possibilities to trap, assemble and manipulate nanoparticles and overcome many challenges faced by conventional optical tweezers while dealing with nanoscale objects. 

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3. Stand-off trapping and manipulation of sub-10 nm objects and biomolecules using opto-thermo-electrohydrodynamic tweezers

   https://doi.org/10.1038/s41565-020-0760-z  

   Hong, Chuchuan; Yang, Sen; Ndukaife, Justus C. (August 2020, Nature Nanotechnology)

   Optical tweezers have emerged as a powerful tool for the non-invasive trapping and manipulation of colloidal particles and biological cells1,2. However, the diffraction limit precludes the low-power trapping of nanometre-scale objects. Substantially increasing the laser power can provide enough trapping potential depth to trap nanoscale objects. Unfortunately, the substantial optical intensity required causes photo-toxicity and thermal stress in the trapped biological specimens3. Low-power near-field nano-optical tweezers comprising plasmonic nanoantennas and photonic crystal cavities have been explored for stable nanoscale object trapping4,5,6,7,8,9,10,11,12,13. However, the demonstrated approaches still require that the object is trapped at the high-light-intensity region. We report a new kind of optically controlled nanotweezers, called opto-thermo-electrohydrodynamic tweezers, that enable the trapping and dynamic manipulation of nanometre-scale objects at locations that are several micrometres away from the high-intensity laser focus. At the trapping locations, the nanoscale objects experience both negligible photothermal heating and light intensity. Opto-thermo-electrohydrodynamic tweezers employ a finite array of plasmonic nanoholes illuminated with light and an applied a.c. electric field to create the spatially varying electrohydrodynamic potential that can rapidly trap sub-10 nm biomolecules at femtomolar concentrations on demand. This non-invasive optical nanotweezing approach is expected to open new opportunities in nanoscience and life science by offering an unprecedented level of control of nano-sized objects, including photo-sensitive biological molecules. 

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4. Scanning force sensing at micrometer distances from a conductive surface with nanospheres in an optical lattice

   https://doi.org/10.1364/AO.457148  

   Montoya, Cris; Alejandro, Eduardo; Eom, William; Grass, Daniel; Clarisse, Nicolas; Witherspoon, Apryl; Geraci, Andrew_A (April 2022, Applied Optics)

   The center-of-mass motion of optically trapped dielectric nanoparticles in a vacuum is extremely well decoupled from its environment, making a powerful tool for measurements of feeble subattonewton forces. We demonstrate a method to trap and maneuver nanoparticles in an optical standing wave potential formed by retroreflecting a laser beam from a metallic mirror surface. We can reliably position a $\sim <\#comment/>170\mathrm{n}\mathrm{m}$diameter silica nanoparticle at distances of a few hundred nanometers to tens of micrometers from the surface of a gold-coated silicon mirror by transferring it from a single-beam tweezer trap into the standing wave potential. We can further measure forces experienced by the particle while scanning the two-dimensional space parallel to the mirror surface, and we find no significant excess force noise in the vicinity of the surface. This method may enable three-dimensional scanning force sensing near surfaces using optically trapped nanoparticles, promising for high-sensitivity scanning force microscopy, tests of the Casimir effect, and tests of the gravitational inverse square law at micrometer scales. 

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5. Optical dipole trapping of Holmium

   Yip, C.; Booth, D.; Zhou, H.; Collett, J.; Hostetter, J.; Saffman, M. (January 2018, 49th annual meeting of the APS division of Atomic, Molecular, & optical physics)

   Neutral Holmiums 128 ground hyperfi ne states, the most of any non-radioactive element, is a test bed for quantum con- trol of a very high dimensional Hilbert space, and offers a promising platform for quantum computing. Previously we have cooled Holmium atoms in a MOT on a 410.5 nm transition and characterized its Ry- dberg spectra. We report here on the first optical dipole trapping of Holmium with a 532 nm wavelength trap laser. The trap lifetime is close to 1 sec., limited by photon scattering from nearby transitions. The trapped atoms are used to measure the dynamic scalar and tensor polarizabilities which are compared with calculations based on measured oscillator strengths. We also report progress towards narrow line cooling and magnetic trapping of single atoms. 

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