Ion implantation has been widely used in various device fabrication applications, including that of optoelectronic components. Focused Ion Beam (FIB) is an especially versatile implantation method, since it can be used for well controlled doping with sub-micron spatial precision. Here, we report FIB induced gallium doping in micrometer-sized regions on shallow Silicon p-n junction devices and its effect on the device optoelectronic properties investigated through micro-spectroscopic measurements. The effect of varying dose levels has been quantified in terms of photo voltage, Raman spectroscopy, XPS and reflectance measurements to investigate the effect of radiation damage and surface amorphization. Based on these observations we report simultaneous occurrence of two scenarios, channeling of high-energy gallium ions beyond the junction depth, as well as formation of an amorphous silicon layer, which cumulatively degrade the optoelectronic properties of the diodes.

Manipulation of colloidal objects with light is important in diverse fields. While performance of traditional optical tweezers is restricted by the diffraction-limit, recent approaches based on plasmonic tweezers allow higher trapping efficiency at lower optical powers but suffer from the disadvantage that plasmonic nanostructures are fixed in space, which limits the speed and versatility of the trapping process. As we show here, plasmonic nanodisks fabricated over dielectric microrods provide a promising approach toward optical nanomanipulation: these hybrid structures can be maneuvered by conventional optical tweezers and simultaneously generate strongly confined optical near-fields in their vicinity, functioning as near-field traps themselves for colloids as small as 40 nm. The colloidal tweezers can be used to transport nanoscale cargo even in ionic solutions at optical intensities lower than the damage threshold of living micro-organisms, and in addition, allow parallel and independently controlled manipulation of different types of colloids, including fluorescent nanodiamonds and magnetic nanoparticles.

Noble metal dimers with sub‐nanometer separation support strong electromagnetic field enhancement which has practical applications in surface enhanced Raman scattering (SERS), photodetection, and photocatalysis. Monolayer graphene is an excellent spacer material to practically realize uniform separation between the dimers. Here, directed microwave‐assisted self‐assembly of Au nanoparticle dimers is reported, separated by graphene monolayer over 1 cm2 substrates. Detailed analytical models of Au particle formation kinetics explain the experimentally observed control of the density and selectivity of the dimer formation. SERS substrates with 7 × 106 cm−2 of Au–Graphene–Au dimers are obtained which yield a 35‐fold increase in the Raman spectral signal of graphene from a single dimer, and an enhancement factor in intensity per molecule of 107 allows ppb level detection of Rhodamine 6G. A system of such dimers can provide an efficient, reliable, and inexpensive solution for many nanophotonic applications that require ultrahigh field confinement, such as SERS and photodetection. 

Spatiotemporally controlled active manipulation of external micro‐/nanoprobes inside living cells can lead to development of innovative biomedical technologies and inspire fundamental studies of various biophysical phenomena. Examples include gene silencing applications, real‐time mechanical mapping of the intracellular environment, studying cellular response to local stress, and many more. Here, for the first time, cellular internalization and subsequent intracellular manipulation of a system of helical nanomotors driven by small rotating magnetic fields with no adverse effect on the cellular viability are demonstrated. This remote method of fuelling and guidance limits the effect of mechanical transduction to cells containing external probes, in contrast to ultrasonically or chemically powered techniques that perturb the entire experimental volume. The investigation comprises three cell types, containing both cancerous and noncancerous types, and is aimed toward analyzing and engineering the motion of helical propellers through the crowded intracellular space. The studies provide evidence for the strong anisotropy, heterogeneity, and spatiotemporal variability of the cellular interior, and confirm the suitability of helical magnetic nanoprobes as a promising tool for future cellular investigations and applications.

An important goal in nanotechnology is to control and manipulate submicrometer objects in fluidic environments, for which optical traps based on strongly localized electromagnetic fields around plasmonic nanostructures can provide a promising solution. Conventional plasmonics based trapping occurs at predefined spots on the surface of a nanopatterned substrate and is severely speed-limited by the diffusion of colloidal objects into the trapping volume. As we demonstrate, these limitations can be overcome by integrating plasmonic nanostructures with magnetically driven helical microrobots and maneuvering the resultant mobile nanotweezers (MNTs) under optical illumination. These nanotweezers can be remotely maneuvered within the bulk fluid and temporarily stamped onto the microfluidic chamber surface. The working range of these MNTs matches that of state-of-the-art plasmonic tweezers and allows selective pickup, transport, release, and positioning of submicrometer objects with great speed and accuracy. The MNTs can be used in standard microfluidic chambers to manipulate one or many nano-objects in three dimensions and are applicable to a variety of materials, including bacteria and fluorescent nanodiamonds. MNTs may allow previously unknown capabilities in optical nanomanipulation by combining the strengths of two recent advances in nanotechnology.

A wide variety of applications are envisioned and demonstrated with artificial micro‐ and nanomachines, ranging from targeted drug or gene delivery, microsurgery, environmental sensing, and many more. Here, it is demonstrated how helical nanomachines can be used to measure and map the local mechanical properties of a complex heterogeneous environment. The positions of the nanomachines are precisely controlled using externally applied magnetic fields, while their instantaneous orientations provide estimation of the viscosity of the surrounding medium with high spatial and temporal accuracy. The measurement technique can be applied to both Newtonian as well as shear thinning media, and all experimental results are in good agreement with the theoretical analysis. It is believed that this novel application of helical nanomachines can be particularly relevant to biophysical studies and microfluidic technologies.

Magnetic nanomotors with integrated theranostic capabilities can revolutionize biomedicine of the future. Typically, these nanomotors contain ferromagnetic materials, such that small magnetic fields can be used to maneuver and localize them in fluidic or gel-like environments. Motors with large permanent magnetic moments tend to agglomerate, which limits the scalability of this otherwise promising technology. Here, we demonstrate the application of a microwave-synthesized ferrite layer to reduce the agglomeration of helical ferromagnetic nanomotors by an order of magnitude, which allows them to be stored in a colloidal suspension for longer than six months and subsequently be manoeuvred with undiminished performance. The ferrite layer also rendered the nanomotors suitable as magnetic hyperthermia agents, as demonstrated by their cytotoxic effects on cancer cells. The two functionalities were inter-related since higher hyperthermia efficiency required a denser suspension, both of which were achieved in a single microwave-synthesized ferrite coating.

Chiral metamaterials are obtained by assembling plasmonic elements in geometries with broken mirror symmetry, which can have promising applications pertaining to generation, manipulation and detection of optical polarisation. The materials used to fabricate this promising nanosystem, especially in the visible–NIR regime, are limited to noble metals such as Au and Ag. However, they are not stable at elevated temperatures and in addition, incompatible with CMOS technologies. We demonstrate that it is possible to develop a chiro-plasmonic system based on a refractory material such as titanium nitride (TiN) which does not have these disadvantages. The building block of our metamaterial is a novel core–shell helix, obtained by coating TiN over silica nanohelices. These were arranged in a regular two-dimensional array over cm-scale areas, made possible by the use of scalable fabrication techniques such as laser interference lithography, glancing angle deposition and DC magnetron sputtering. The measured chiro-optical response was extremely broadband (<500 nm to >1400 nm), and had contributions from individual, as well as collective plasmon modes of the interacting nanohelices, whose spectral characteristics could be easily controlled by varying the direction of the incident radiation.

In 1977, Volodin et al. observed that the electrohydrodynamic instability of charged helium surfaces can lead to the loss of the electrons from the surface in the form of bubbles. These are Multielectron Bubbles (MEBs) which contain electrons pinned on their inner surfaces. The MEBs form a model system for studying electrons on curved surfaces, which are predicted to have many interesting properties. Recent experiments showed that above the Lambda point, MEBs could be trapped using a Paul-trap and their sizes are mainly determined by the amount of vapour present inside. Here, we report the experimental observation of the coalescence of two MEBs which were moving upward in bulk liquid helium-4. The charge and radii of the MEBs were determined before and after the coalescence. The merging of two similar charged bubbles was possible because of the presence of vapour inside the merging MEBs.

We report a theoretical modeling on the excitation of chiro-optical activity in chiral assemblies comprising of dielectric nanoparticles using a semianalytical method based on coupled dipole approximation. In stark contrast to plasmonic counterparts where electric dipolar coupling dominates chirooptical response, our analysis reveals a magnetic-dominated response, which originates from the dynamic electromagnetic coupling between the magnetic dipole moments of the individual high-index dielectric nanoparticles. The calculated circular dichroism response practically remains insensitive to interparticle spacing but shows strong dependence on the size of nanoparticle and refractive index, in close agreement with numerical simulations. Such a chiral assembly represents a novel material platform where individual achiral dielectric nanoparticles with strong magnetic resonances can be exploited as building blocks for engineering new type of chiral light−matter interaction, and being low in light absorption loss, this could be promising for applications in nanophotonics.

Photonic manipulation with plasmonic materials is typically associated with high ohmic losses, which has triggered interest in alternative strategies based on low loss dielectric materials. Here we describe a novel dielectric nanomaterial capable of supporting strong Mie resonances from the visible to IR regimes. The fundamental block of this metamaterial is based on nanopillars in a core–shell configuration, with a large refractive index (RI) contrast between the (low RI) core and the (high RI) shell. The material showed strongly tunable optical resonances that varied from visible to near and mid IR as a function of shell thickness, core diameter and inter-pillar spacing. The numerical simulations, which are in good agreement with the experimental results, suggest the optical response to be dominated by magnetic dipole resonances. This versatile material platform is CMOS compatible, can be fabricated in a scalable manner as thin films, can act as strong scatterers in colloidal suspensions and thereby can provide several promising technological opportunities in nanophotonics.

Micro- and nanomotors are nonliving micro- and nanoparticles that are rendered motile by supplying energy from external sources, for example, through asymmetric chemical reactions or the application of electric, magnetic, optical, or acoustic fields. Their study is interesting for two reasons. First, nanomotors can impact future biomedical practices, where one envisions intelligent multifunctional nanomachines swarming toward a diseased site and delivering therapeutics with high accuracy. The second motivation stems from the prevalence of self-powered systems in nature, ranging from intracellular transport to human migration, which are nonequilibrium phenomena yet to be completely understood. Nanomotors provide a promising route toward the study of complex active matter phenomena with a welldefined and possibly reduced set of variables. Among different ways of powering nanomotors, magnetic field deserves a special mention because of its inherent biocompatibility, minimal dependence on properties of the surrounding medium, and remote powering mechanism. In particular, magnetically actuated propellers (MAPs), which are helical structures driven by rotating fields in fluids and gels, have been demonstrated to be highly suitable for various microfluidic and biotechnology applications. Unfortunately, this method of actuation requires direct application of mechanical torque by the applied field, implying that the system is driven and therefore cannot be considered self-propelled. To overcome this fundamental limitation, we discuss an alternate magnetic drive where the MAPs are powered by oscillating (not rotating) magnetic fields. This technique induces motility in the form of back-and-forth motion but allows the directionality to be unspecified, and therefore, it represents a zero-force, zero-torque active matter where the nanomotors behave effectively as selfpropelled entities. The MAPs show enhanced diffusivity compared with their passive counterparts, and their motility can be tuned by altering the external magnetic drive, which establishes the suitability of the MAPs as model active particles. Enhancement of the diffusivity depends on the thermal noise as well as the inherent asymmetries of the individual motors, which could be well-understood through numerical simulations. In the presence of small direct-current fields and interactions with the surface, the swimmers can be maneuvered and subsequently positioned in an independent manner. Next, we discuss experimental results pertaining to the collective dynamics of these helical magnetic nanoswimmers. We have studied nonmagnetic tracer beads suspended in a medium containing many swimmers and found the diffusivity of the beads to increase under magnetic actuation, akin to measurements performed in dense bacterial suspensions. In summary, we envision that rendering the system of MAPs active will not only provide a new model system to investigate fundamental nonequilibrium phenomena but also play a vital role in the development of intelligent theranostic probes for futuristic biomedical applications.

Three dimensional chiral plasmonic nanostructures exhibit large circular dichroism which can be altered by changing the geometry, size of nanostructures and the plasmonic material. Here, we have explored the possibility of using chiral nanostructured films as reusable substrates for sensing the presence of analytes. These wafer-scale fabricated plasmonic substrates show a strong sensitivity to change in the refractive index of the surrounding medium. Crucially, the chiral plasmonic films retain their circular dichroism once the analyte is completely washed off and dried. This methodology of sensing offers a simple solution to the difficulties associated with expensive and time-consuming fabrication steps and device-to-device variability issues often associated with nanoplasmonic sensing devices.

Multielectron bubbles (MEBs) are cavities in liquid helium which contain a layer of electrons trapped within few nanometres from their inner surfaces. These bubbles are promising candidates to probe a system of interacting electrons in curved geometries, but have been subjected to limited experimental investigation. Here, we report on the observation of fission of MEBs under strong electric fields, which arises due to fast rearrangement of electrons inside the bubbles, leading to their deformation and eventually instability. We measured the electrons to be distributed unequally between the daughter bubbles which could be used to control the charge density inside MEBs.

Multielectron bubbles (MEBs) are charged cavities in liquid helium which provide an interesting platform for the study of electrons on curved surfaces. Very recently, we have reported an experiment to trap these objects in a two-dimensional Paul trap, where they could be observed from ten to hundreds of milliseconds. During this time, the vapor inside the bubble condensed which resulted in a steady reduction in their size such that beyond a certain time the MEBs could no longer be detected. In this paper, we present experimental data on the lifetime of the bubbles as a function of their initial radius and compare the results with a theoretical model.

In a recent experiment, we have used a linear Paul trap to store and study multielectron bubbles (MEBs) in liquid helium. MEBs have a charge-to-mass ratio (between 10−4−4 and 10−2−2C/kg) which is several orders of magnitude smaller than ions (between 1066 and 1088 C/kg) studied in traditional ion traps. In addition, MEBs experience significant drag force while moving through the liquid. As a result, the experimental parameters for stable trapping of MEBs, such as magnitude and frequency of the applied electric fields, are very different from those used in typical ion trap experiments. The purpose of this paper is to model the motion of MEBs inside a linear Paul trap in liquid helium, determine the range of working parameters of the trap, and compare the results with experiments.

An electron bubble in liquid helium-4 under the saturated vapor pressure becomes unstable and explodes if the pressure becomes more negative than −1.9 bars. In this paper, we use focused ultrasound to explode electron bubbles. We then image at 30,000 frames per second the growth and subsequent collapse of the bubbles. We find that bubbles can grow to as large as 1 mm in diameter within 2 ms after the cavitation event. We examine the relation between the maximum size of the bubble and the lifetime and find good agreement with the experimental results.

We report the observation of a circular differential two-photon photoluminescence (TPPL) response from a three-dimensional chiral metamaterial, comprising a system of achiral (spherical) metal nanoparticles arranged on a chiral (helical) dielectric template. The enhanced dipolar response of the individual particles arising from their strong electromagnetic coupling resulted in strong photoluminescence under peak illumination intensities as low as 2 × 10³ W/cm². The TPPL signal was of approximately equal magnitude but of opposite sign, which depended on both the circular polarization state of the incident beam and the handedness of the helical geometry. The strong chiro-optical effect observed in these experiments may be relevant to technologies related to nonlinear plasmonics, in particular imaging applications where control over the polarization state of the imaged photons may be desirable.

Three dimensional chiral plasmonic nanostructures exhibit large circular dichroism which can be altered by changing the geometry, size of nanostructures and the plasmonic material. Here, we have explored the possibility of using chiral nanostructured films as reusable substrates for sensing the presence of analytes. These wafer-scale fabricated plasmonic substrates show a strong sensitivity to change in the refractive index of the surrounding medium. Crucially, the chiral plasmonic films retain their circular dichroism once the analyte is completely washed off and dried. This methodology of sensing offers a simple solution to the difficulties associated with expensive and time-consuming fabrication steps and device-to-device variability issues often associated with nanoplasmonic sensing devices.

Controlled maneuvering of artificial nanomotors in various biological environments can have impactful biomedical applications, such as drug delivery, microsurgery etc. In spite of the various strategies available for the fabrication of nanomotors, practical issues such as toxic fuel requirement, corrosion of the nanomotor and the liquid viscosity has limited their usage to simple bio fluids such as serum, diluted urine etc. By incorporating conformal ferrite coatings to our system of magnetic nanopropellers, we have been able to maneuver artificial nanomotors in undiluted human blood. The motion of the nanopropellers in human blood showed an interesting stick and slip motion, originating from the colloidal jamming of the blood cells. The amount of jamming was found to be related to the concentration of the blood cells, and therefore establishes the suitability of the propellers in probing mechanical properties of blood and other interesting suspensions.

Chiral metamaterials have recently gained attention due to their applicability in developing polarization devices and in the detection of chiral molecules. A common approach towards fabricating plasmonic chiral nanostructures has been decorating metallic nanoparticles on dielectric chiral scaffolds, such as a helix. This resulted in the generation of a large chiro-optical response over a wide range of the electromagnetic spectrum. It has been shown previously that the optical tunability of these chiral metamaterials depends on the geometrical aspects of the overall structure, as well as the nature of the plasmonic constituents. In this study, we have investigated the role of the underlying dielectric scaffold with numerical simulations, and experimentally demonstrated that it is possible to enhance and engineer their chiro-plasmonic response significantly by choosing dielectric scaffolds of appropriate materials.

There is considerable interest in powering and maneuvering nanostructures remotely in fluidic media using noninvasive fuel-free methods, for which small homogeneous magnetic fields are ideally suited. Current strategies include helical propulsion of chiral nanostructures, cilia-like motion of flexible filaments, and surface assisted translation of asymmetric colloidal doublets and magnetic nanorods, in all of which the individual structures are moved in a particular direction that is completely tied to the characteristics of the driving fields. As we show in this paper, when we use appropriate magnetic field configurations and actuation time scales, it is possible to maneuver geometrically identical nanostructures in different directions, and subsequently position them at arbitrary locations with respect to each other. The method reported here requires proximity of the nanomotors to a solid surface, and could be useful in applications that require remote and independent control over individual components in microfluidic environments.

Combining oblique angle deposition with standard graphene transfer protocols, two planar arrays of metal nanoparticles are fabricated that are vertically separated by atomic dimensions, corresponding precisely to the thickness of a single layer of graphene, i.e., 0.34 nm. Upon illumination of light at an appropriate wavelength, the local electromagnetic field at the junction of the dimers can be increased dramatically, thereby resulting in the most sensitive graphene–plasmonic hybrid photodetector reported to date.

Investigations of two-dimensional electron systems (2DES) have been achieved with two model experimental systems, covering two distinct, non-overlapping regimes of the 2DES phase diagram, namely the quantum liquid phase in semiconducting heterostructures and the classical phases observed in electrons confined above the surface of liquid helium. Multielectron bubbles in liquid helium offer an exciting possibility to bridge this gap in the phase diagram, as well as to study the properties of electrons on curved flexible surfaces. However, this approach has been limited because all experimental studies have so far been transient in nature. Here we demonstrate that it is possible to trap and manipulate multielectron bubbles in a conventional Paul trap for several hundreds of milliseconds, enabling reliable measurements of their physical properties and thereby gaining valuable insight to various aspects of curved 2DES that were previously unexplored.

Controlled motion of artificial nanomotors in biological environments, such as blood, can lead to fascinating biomedical applications, ranging from targeted drug delivery to microsurgery and many more. In spite of the various strategies used in fabricating and actuating nanomotors, practical issues related to fuel requirement, corrosion, and liquid viscosity have limited the motion of nanomotors to model systems such as water, serum, or biofluids diluted with toxic chemical fuels, such as hydrogen peroxide. As we demonstrate here, integrating conformal ferrite coatings with magnetic nanohelices offer a promising combination of functionalities for having controlled motion in practical biological fluids, such as chemical stability, cytocompatibility, and the generated thrust. These coatings were found to be stable in various biofluids, including human blood, even after overnight incubation, and did not have significant influence on the propulsion efficiency of the magnetically driven nanohelices, thereby facilitating the first successful “voyage” of artificial nanomotors in human blood. The motion of the “nanovoyager” was found to show interesting stick–slip dynamics, an effect originating in the colloidal jamming of blood cells in the plasma. The system of magnetic “nanovoyagers” was found to be cytocompatible with C2C12 mouse myoblast cells, as confirmed using MTT assay and fluorescence microscopy observations of cell morphology. Taken together, the results presented in this work establish the suitability of the “nanovoyager” with conformal ferrite coatings toward biomedical applications.

Plasmonic nanostructures in chiral geometries are suitable candidates for various device applications pertaining to optical polarization. These devices can show large chiro-optical effects, implying a strongly differential response to right and left circularly polarized light. In general, three-dimensional plasmonic structures show larger optical activity, but are typically not suitable for wafer-scale fabrication. As an alternate strategy, we have considered the optical response of stacked planar chiral geometries, which were found to exhibit very large chiro-optical response. Further, the plasmonic chirality of such stacked metamaterials can be tuned in the visible, by simply varying the thickness of the stack. This novel design of layered achiral metamaterials will be easier to fabricate than standard three dimensional geometries, and is suitable for various photonic device applications requiring polarization control.

200 nm thick films of gallium nitride were grown on sapphire substrate using molecular beam epitaxy. Gold nanoparticles were fabricated on the grown films by thermal evaporation followed by annealing. Aluminium nanostructures were fabricated on another set of films using nanosphere lithography. Interdigited electrodes were fabricated using standard lithography to form metal-semiconductor-metal photodetectors. The performance of bare gallium nitride films were compared with the samples that had Au nanoparticles and Al nanostructures. An enhancement of the photocurrent with negligible change in dark current was observed in both cases.

Helical magnetic nanopropellers have been a subject of active research in the last few years. In this work we present the details of the numerical calculation to model their motion in the presence of thermal fluctuations. Also pertaining to their possible use in microfluidic devices, we have included the effect of adjacent walls. The results of our numerical calculations show non-Gaussian features in the power spectrum of the propulsion velocity, in close resemblance with experimental observations.

We have studied the effects of geometrical variability on the chiro-optical response of helically arranged metal nanoparticles. A semi-analytical approach based on coupled dipole approximation model was used to study the effects of variation in shape, size, position, spacing and orientation of the metal nanoparticles. Within the extent of geometrical variability studied in our model, we found the chiro-optical response did not depend strongly on the size, position and spacing for either spherical or non spherical (ellipsoid) metal nanoparticles. On the other hand, the variability in the orientation of the ellipsoids can have significant effects on the chiro-optical response. These variability issues need to be taken into consideration in designing novel photonic polarization devices, as well as the optical system relevant for their investigation.

We present experimental results on the generation and collapse of multielectron bubbles in liquid helium. By applying voltage pulses to a tungsten tip above the surface of the liquid, millimetre sized deformations were formed. Using high speed photography, we have imaged the disintegration of these deformations into bubbles of sizes ranging from ten to few hundred microns. At temperatures less than 2 K, the bubbles split into smaller bubbles and then disappeared in a time scale of few milliseconds. Smaller bubbles were formed at temperatures around 3 K, but were visible for more than hundreds of milliseconds. Although we have not been able to measure their charge directly, some of these bubbles responded to electric fields, implying these were indeed multielectron bubbles. With the existing theoretical picture, it is not possible to understand the strong dependence of the lifetime of multielectron bubbles on temperature.

The dependence on an applied electric field of the ionization current produced by an energetic electron stopped in liquid helium can be used to determine the spatial distribution of secondary electrons with respect to their geminate partners. An analytic expression relating the current and distribution is derived. The distribution is found to be non-Gaussian with a long tail at larger distances.

Chiral metamaterials can have diverse technological applications, such as engineering strongly twisted local electromagnetic fields for sensitive detection of chiral molecules, negative indices of refraction, broadband circular polarization devices, and many more. These are commonly achieved by arranging a group of noble-metal nanoparticles in a chiral geometry, which, for example, can be a helix, whose chiroptical response originates in the dynamic electromagnetic interactions between the localized plasmon modes of the individual nanoparticles. A key question relevant to the chiroptical response of such materials is the role of plasmon interactions as the constituent particles are brought closer, which is investigated in this paper through theoretical and experimental studies. The results of our theoretical analysis, when the particles are brought in close proximity are dramatic, showing a large red shift and enhancement of the spectral width and a near-exponential rise in the strength of the chiroptical response. These predictions were further confirmed with experimental studies of gold and silver nanoparticles arranged on a helical template, where the role of particle separation could be investigated in a systematic manner. The “optical chirality” of the electromagnetic fields in the vicinity of the nanoparticles was estimated to be orders of magnitude larger than what could be achieved in all other nanoplasmonic geometries considered so far, implying the suitability of the experimental system for sensitive detection of chiral molecules.

Helical propulsion is at the heart of locomotion strategies utilized by various natural and artificial swimmers. We used experimental observations and a numerical model to study the various fluctuation mechanisms that determine the performance of an externally driven helical propeller as the size of the helix is reduced. From causality analysis, an overwhelming effect of orientational noise at low length scales is observed, which strongly affects the average velocity and direction of motion of a propeller. For length scales smaller than a few micrometers in aqueous media, the operational frequency for the propulsion system would have to increase as the inverse cube of the size, which can be the limiting factor for a helical propeller to achieve locomotion in the desired direction.

We report on the development of a system of micron-sized reciprocal swimmers that can be powered with small homogeneous magnetic fields, and whose motion resembles that of a helical flagellum moving back and forth. We have measured the diffusivities of the swimmers to be higher compared to nonactuated objects of identical dimensions at long time scales, in accordance with the theoretical predictions made by Lauga [Phys. Rev. Lett. 106, 178101 (2011)]. Randomness in the reciprocity of the actuation strokes was found to have a strong influence on the enhancement of the diffusivity, which has been investigated with numerical calculations.

We report on a wafer scale fabrication method of a three-dimensional plasmonic metamaterial with strong chiroptical response in the visible region of the electromagnetic spectrum. The system was comprised of metallic nanoparticles arranged in a helical fashion, with high degree of flexibility over the choice of the underlying material, as well as their geometrical parameters. This resulted in exquisite control over the chiroptical properties, most importantly the spectral signature of the circular dichroism. In spite of the large variability in the arrangement, as well as the size and shape of the constituent nanoparticles, the average chiro-optical response of the material remained uniform across the wafer, thus confirming the suitability of this system as a large area chiral metamaterial. By simply heating the substrate for a few minutes, the geometrical properties of the nanoparticles could be altered, thus providing an additional handle towards tailoring the spectral response of this novel material.

We consider the rotational motion of an elongated nanoscale object in a fluid under an external torque. The experimentally observed dynamics could be understood from analytical solutions of the Stokes equation, with explicit formulae derived for the dynamical states as a function of the object dimensions and the parameters defining the external torque. Under certain conditions, multiple analytical solutions to the Stokes equations exist, which have been investigated through numerical analysis of their stability against small perturbations and their sensitivity towards initial conditions. These experimental results and analytical formulae are general enough to be applicable to the rotational motion of any isolated elongated object at low Reynolds numbers, and could be useful in the design of non-spherical nanostructures for diverse applications pertaining to microfluidics and nanoscale propulsion technologies.

The authors studied the formation of a wafer-scale network of connected colloidal beads by reactive ion etching. The dimensions of the connections have been studied as a function of etching time for colloidal beads of different sizes, and could be well controlled. The authors have found that the nano-network forms and disappears for the same time of etching independent of the diameter of the polystyrene beads. With recent interest of connected colloidal networks in various optical sensing applications, such as photonic crystals, as surface-enhanced Raman scattering substrates, the studies have potential uses in the development of wafer-scale nanophotonic sensors.

We studied the development of surface instabilities leading to the generation of multielectron bubbles (MEBs) in superfluid helium upon the application of a pulsed electric field. We found the statistical distribution of the charge of individual instabilities to be strongly dependent on the duration of the electric field pulse. The rate and probability of generation of these instabilities in relation to the temporal characteristics of the applied field was also investigated.

When an electron is injected into liquid helium, it forces open a cavity that is free of helium atoms (an electron bubble). If the electron is in the ground 1S state, this bubble is spherical. By optical pumping it is possible to excite a significant fraction of the electron bubbles to the 1P state; the bubbles then lose spherical symmetry. We present calculations of the energies of photons that are needed to excite these 1P bubbles to higher energy states (1D and 2S) and the matrix elements for these transitions. Measurement of these transition energies would provide detailed information about the shape of the 1P bubbles.

Plasmonic interactions in a well-defined array of metallic nanoparticles can lead to interesting optical effects, such as local electric field enhancement and shifts in the extinction spectra, which are of interest in diverse technological applications, including those pertaining to biochemical sensing and photonic circuitry. Here, we report on a single-step wafer scale fabrication of a three-dimensional array of metallic nanoparticles whose sizes and separations can be easily controlled to be anywhere between fifty to a few hundred nanometers, allowing the optical response of the system to be tailored with great control in the visible region of the spectrum. The substrates, apart from having a large surface area, are inherently porous and therefore suitable for optical sensing applications, such as surface enhanced Raman scattering, containing a high density of spots with enhanced local electric fields arising from plasmonic couplings.

The angles at which a light beam gets diffracted by a grating depend strongly on the 
direction of incidence for diffraction angles close to a right angle. Accordingly, it is possible to amplify small beam deflections by placing a grating at an optimal orientation to the light path. We use this principle to amplify small beam deviations arising out of a light beam refracting at the interface of an optically active medium, and demonstrate a new technique of enhancing the limit of detection of chiro-optical measurements.

We study the motion of a ferromagnetic helical nanostructure under the action of a rotating magnetic field. A variety of dynamical configurations were observed that depended strongly on the direction of magnetization and the geometrical parameters, which were also confirmed by a theoretical model, based on the dynamics of a rigid body under Stokes flow. Although motion at low Reynolds numbers is typically deterministic, under certain experimental conditions the nanostructures showed a surprising bistable behavior, such that the dynamics switched randomly between two configurations, possibly induced by thermal fluctuations. The experimental observations and the theoretical results presented in this paper are general enough to be applicable to any system of ellipsoidal symmetry under external force or torque.

In this letter, we investigate the circular differential deflection of a light beam refracted at the interface of an optically active medium. We show that the difference between the angles of deviation of the two circularly polarized components of the transmitted beam is enhanced manyfold near total internal reflection, which suggests a simple way of increasing the limit of detection of chiro-optical measurements.

Significant progress has been made in the fabrication of micron and sub-micron structures whose motion can be controlled in liquids under ambient conditions. The aim of many of these engineering endeavors is to be able to build and propel an artificial micro-structure that rivals the versatility of biological swimmers of similar size, e.g. motile bacterial cells. Applications for such artificial “micro-bots” are envisioned to range from microrheology to targeted drug delivery and microsurgery, and require full motion-control under ambient conditions. In this Mini-Review we discuss the construction, actuation, and operation of several devices that have recently been reported, especially systems that can be controlled by and propelled with homogenous magnetic fields. We describe the fabrication and associated experimental challenges and discuss potential applications.

The development of SERS for the detection and identification of bacterial pathogens has recently attracted much interest motivated by both applications in clinical diagnostics and prevention, such as “super-bug” resistance, as well as concerns about bio-safety. The ability to provide unique vibrational signatures of bacteria at the single cell level illustrates the potential of SERS as a valuable analytical and structural spectroscopic tool.