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Ultrasound in bulk liquid and superfluid helium has been used for multiple studies, including generation of quantum turbulence, and inducing homogenous or heterogenous cavitation. Both hemispherical and planar ultrasonic transducers have been used in the past, which optimize the focusing volume and the degree of focusing. In this paper, we demonstrate the application of a cylindrical piezoelectric transducer to achieve a linear focusing configuration. We have developed a new method to calibrate the pressure generated by the ultrasonic transducer, where we measured the threshold ultrasound drive at which mist was observed. The results were compared with those from a hemispherical geometry. The linear focusing configuration was further demonstrated to observe the cavitation of single electron bubbles arranged in a linear array. Our experiments are relevant to studies in liquid helium that require large pressure oscillations in controlled volumes.
A scallop-like swimmer going back-and-forth (reciprocal motion) does not produce any net motility. We discuss a similar artificial microswimmer that is powered by magnetic fields. In the presence of thermal noise, the helical swimmer exhibits enhanced diffusivity during reciprocal actuation. The external magnetic drive can be further modified to break the reciprocity. Equipped with only the information on swimmer trajectories and orientations, we discuss quantitative methods to estimate the degree of reciprocity and non-reciprocity in such scenarios. This work proposes a quantitative measure and validates the same with numerical simulations, further supported by experiments.
Recent interests in layered transition-metal dichalcogenides (TMDCs), such as WSe2, MoS2, etc, arise due to their attractive electrical, optical, and mechanical properties with potential applications in energy storage, generation, and many more. Embedding these 2D materials in plasmonic cavities can further enhance light–matter interactions and alter their properties, resulting in diverse and efficient optoelectronic applications. The strain due to the geometry and charge transfer due to the plasmonic materials can further modify the TMDCs’ optical response for sensing applications and as single photon emitters in on-chip optoelectronic applications. This work discusses one such 2D-plasmonic hybrid configuration of a silver sphere on a gold disc with WSe2 sandwiched in between. We perform non-invasive Raman and PL studies of this system to estimate the field enhancement and discuss strain and doping induced in the TMDC.
Magnetic nanobots are of great technological importance due to their benign actuation method, biocompatibility and medium-independent behaviour. They can be driven to propel in a particular direction using external magnetic fields. They can also be self-propelled under certain conditions where orientations of the nanobots remain independent of the energy input, implying individual entities behave independently of each other. The latter requires a precise magnetic field design, which, as we show in this work, is non-trivial experimentally. Specifically, the induced or stray magnetic fields in the plane of the nanobot motion can hinder autonomous motion altogether. We address the technique used to eliminate these unnecessary magnetic fields in two ways and especially highlight a method based on analyzing the trajectories of the nanobots.
This paper describes the construction and performance of a spacious and easily portable three-coil magnetic manipulation system. A uniform rotating magnetic field with a maximum field strength of 15 mT in a volume of 10 mm × 10 mm × 10 mm working space has been achieved.
The three-coil system has been primarily designed for maneuvering helical magnetic nanorobots inside complex biological fluids where a greater magnetic field is desirable to induce maneuverability.
The coil design can aid us in our goal of maneuvering magnetic nanorobots inside living animals in the near future while simultaneously imaging them through an optical microscope.
Magnetic nanobots are of great technological importance due to their benign actuation method, biocompatibility and medium-independent behaviour. They can be driven to propel in a particular direction using external magnetic fields. They can also be self-propelled under certain conditions where orientations of the nanobots remain independent of the energy input, implying individual entities behave independently of each other. The latter requires a precise magnetic field design, which, as we show in this work, is non-trivial experimentally. Specifically, the induced or stray magnetic fields in the plane of the nanobot motion can hinder autonomous motion altogether. We address the technique used to eliminate these unnecessary magnetic fields in two ways and especially highlight a method based on analyzing the trajectories of the nanobots. (A) The schematic shows the presence of a small induced oscillating field in XY plane along with an applied field in the Z direction. (B) The schematic of the feedback system where the nanobots along with the sensor are used to nullify the induced field. (C) The trajectory due to the induced field (blue), the one after correcting for the field using the sensor (red) and the trajectory after using the nanobot to rectify the field (green).
Helical magnetic nanomotors can be actuated using an external magnetic field and have potential applications in drug delivery, colloidal manipulation, and bio-microrheology. Recently, they have been maneuvered in biological environments such as vitreous humour, dentinal tubules, peritoneal fluid, stromal matrix, and blood, which are promising developments for clinical applications. However, their biocompatibility and biodistribution are vital parameters that must be assessed before further use. An extensive quantitative evaluation has been performed for these parameters for the first time through in vitro and in vivo experiments. Investigations of cell death, proliferation, and DNA damage ascertain that the motors are non-toxic. Also, an unbiased transcriptomic analysis affirms that the motors are not genotoxic till 20 motors/ cell. Toxicity studies in mice reveal that the motors show no signs of toxicity up to a dose of 55 mg/ kg body weight. Further, the biodistribution studies show that they remain in the blood circulation after injection and at later stages possibly adhere to the walls of the blood vessel because of adsorption. However, perfusion with physiological saline decreases this adsorption/adhesion. Overall, we demonstrate the biocompatibility of nanomotors in live cellular and organismal systems, and a systemic biodistribution analysis reveals organ-specific retention of motors.
The idea of “fantastic voyagers” carrying out medical tasks within the human body has existed as part of popular culture for many decades. The concept revolved around a miniaturized robot that can travel inside the human body and perform complicated functions such as surgery, navigation of otherwise inaccessible biological environments, and delivery of therapeutics. Since the last decade, significant developments have occurred in this arena that are yet to enter mainstream biomedical practices. Here, we define the challenges to make this fiction into reality. We begin by chalking the journey from pills, nanoparticles, and then to micro-nanomotors. The review describes the principles, physicochemical contexts, and advantages that micro-nanomotors provide. The article then describes micro-nanomotors’ obstacles such as maneuverability, in vivo imaging, toxicity, and biodistribution.
Artificial micro/nanomachines have been envisioned and demonstrated as potential candidates for targeted drug or gene delivery, cell manipulation, environmental and biological sensing and in lab on chip applications. Here, we have used helical nanomachines to measure the local rheological properties of a viscoelastic media. The position of the helical nanomachine/nanopropeller was controlled precisely using magnetic fields with simultaneous measurements of the mechanical properties of a complex and heterogeneous fluidic environment. We demonstrated that the motion of the helical nanopropeller is extremely sensitive to fluid elasticity and the speed of propulsion of the nanopropeller can be used as a measure of the local elastic relaxation time. Taken together, we report a promising new technique of mapping the rheological properties by helical nanopropellers with very high spatial and temporal resolutions, with performance superior to existing techniques of passive or active microrheology.
Light–matter interaction in graphene can be engineered and substantially enhanced through plasmonic sensitization, which has led to numerous applications in photodetection, sensing, photocatalysis and spectroscopy. The majority of these designs have relied on conventional plasmonic materials such as gold, silver and aluminum. This limits the implementation of such devices to the ultraviolet and visible regimes of the electromagnetic spectrum. However, for many practical applications, including those relevant to security and defense, the development of new strategies and materials for sensing and detection of infra red (IR) light is crucial. Here we use surface enhanced Raman spectroscopy (SERS), for direct visualization and estimation of enhanced light–matter interaction in graphene in the mid-IR regime, through sensitization by an unconventional plasmonic material. Specifically, we fabricate a hybrid device consisting of a single layer graphene and a two-dimensional array of nanodiscs of aluminum doped zinc oxide (AZO), which is a highly doped semiconductor, exhibiting plasmonic resonance in the mid-IR. We find that the enhancement in the SERS signal of graphene is of similar magnitude to what has been achieved previously in the visible using conventional plasmonic materials. Our results establish the potential of such hybrid systems for graphene-based optical and optoelectronic applications in the mid-IR.
Surface plasmon polaritons, in spite of being lossy, are known to preserve their coherence over long distances. In this paper, we investigate the coherence of surface plasmons propagating on a silver thin film surface using free-space optical Mach–Zehnder interferometry, where they are found to maintain their temporal coherence up to 80 µm, thus setting, for the first time to the best of our knowledge, a new limit. This can be important in applications integrating plasmonics with quantum information processing, where decoherence is a major challenge.
The role of quantum fluctuations in the self-assembly of soft materials is relatively unexplored, which could be important in the development of next-generation quantum materials. Here, we report two species of nanometer-sized bubbles in liquid helium-4 that contain six and eight electrons, forming a versatile, platform to study self-assembly at the intersection of classical and quantum worlds. These objects are formed through subtle interplay of the short-range electron-helium repulsion and easy deformability of the bulk liquid. We identify these nanometric bubbles in superfluid helium using cavitation threshold spectroscopy, visualize their decoration of quantized vortex lines, and study their creation through multiple methods. The objects were found to be stable for at least 15 milliseconds at 1.5 kelvin and can therefore allow fundamental studies of few-body quantum interactions under soft confinements.
Artificially designed self-propelled objects can allow studying active matter phenomena with great detail that is not possible in natural, e.g. biological systems. Here, we show experimental results on helical shaped, magnetically actuated, reciprocal swimmers, where the degree of randomness in the reciprocal sequence plays an important role in determining their effective motility. Here, for the first time we show the results at high activity levels where the degree of randomness is further affected by the presence of the surface, which in turn results in a non-monotonic increase of motility as a function of magnetic drive. It will be interesting to extend these studies to denser systems where the swimmers can interact with each other through hydrodynamic forces.
Multielectron bubbles (MEBs) are cavities in liquid helium that contain electrons and possibly vapour. They provide a versatile platform for exploring properties of interacting electrons in two dimensions in a regime of densities that has not been previously available. Since the electrons are pushed against the deformable curved inner surface of the bubble, MEBs also provide an environment to study the roles of curvatures. In this review, we describe the current understanding of MEBs, with a focus on their stability issues, and highlight recent experimental studies of this interesting system.
Controlled manipulation of nanoscale objects in fluids is relevant to both fundamental studies and technological advances in nanotechnology. While standard techniques of nanomanipulation, such as optical and plasmonic tweezers have limitations in simultaneous trapping and transport of nanoscale cargo, magnetically driven plasmonic nanorobots under optical illumination provide a promising solution. These so called mobile nanotweezers (MNT) use strongly localized electromagnetic field near plasmonic nanostructures to trap objects with high efficiency and can simultaneously be driven by magnetic fields to selectively trap, transport and release colloidal cargo. Upon illumination, apart from strong optical gradient forces due to local electric field enhancement, additional fluidic forces arise due to the heat generated by absorption of light. Here, we present a method to understand and engineer thermally induced fluidic forces in mobile nanotweezers. The temperature enhancement and associated thermofluidic forces are studied as a function of MNT geometry. We also discuss illumination at wavelengths slightly detuned from plasmon resonance frequency, which produces sufficient field enhancement with negligible generation of heat, and therefore much reduced thermophoretic and convective forces. This allowed us to engineer thermoplasmonic forces in MNTs for enhanced trapping performance and diverse applications.
A rheological probe that can measure mechanical properties of biological milieu at well-defined locations with high spatial resolution, on a time scale faster than most biological processes, can further improve our understanding of how living systems operate and behave. Here, we demonstrate nanorobots actively driven in realistic ex vivo biological systems for fast mechanical measurements with high spatial accuracy. In the various demonstrations of magnetic nanobots as mechanical probes, we report the first direct observation of the internalization of probes by a living cell, the accurate measurement of the ‘fluid phase’ cytoplasmic viscosity of ~200 cP for a HeLa cell, demonstration of intracellular measurements in cells derived from human patients; all of which establish the strength of this novel technique for measurements in both intra- and extracellular environments.
Acoustic cavitation is a powerful technique to probe electron bubbles inside the liquid helium. The critical pressure to explode a bubble depends on the number and quantum state of electrons inside the bubble and if the bubble is trapped on a vortex. Here, we report cavitation events that occur at pressure magnitudes approximately 70% lower compared to single electron bubbles. We have considered various possibilities, e.g., single electron bubbles trapped on vortex lines or primary electrons depositing the energy at the acoustic focus and compared the results of our experiments with past measurements reported in the literature. We consider the possibility these new species of bubbles are multielectron bubbles with a small (< 20) number of electrons and discuss future experiments to confirm the same.
There have been several reports of plasmonically enhanced graphene photodetectors in the visible and the near infrared regime but rarely in the ultraviolet. In a previous work, we have reported that a graphene-silver hybrid structure shows a high photoresponsivity of 13 A/W at 270 nm. Here, we consider the likely mechanisms that underlie this strong photoresponse. We investigate the role of the plasmonic layer and examine the response using silver and gold nanoparticles of similar dimensions and spatial arrangement. The effect on local doping, strain, and absorption properties of the hybrid is also probed by photocurrent measurements and Raman and UV-visible spectroscopy. We find that the local doping from the silver nanoparticles is stronger than that from gold and correlates with a measured photosensitivity that is larger in devices with a higher contact area between the plasmonic nanomaterials and the graphene layer.
Multielectron bubbles (MEBs) are cavities in liquid helium containing a layer of electrons pinned to the inner surface of the bubbles. Previous experimental work carried out with MEBs in bulk helium-4 above the lambda point showed MEBs can contain vapor, which condenses in a time approximately proportional to the volume of the bubble, and this observation was further confirmed by numerical simulations. In the present work, we describe experiments where the MEBs are held against a solid substrate. We found the rate of vapor condensation and therefore the speed of collapse of the bubble to be orders of magnitude faster compared to MEBs in bulk. We discuss a numerical model and the associated difficulties to explain this difference.
Optical traps based on strongly confined electromagnetic fields at metal–dielectric interfaces are far more efficient than conventional optical tweezers. Specifically, these near-field nanotweezers allow the trapping of smaller particles at lower optical intensities, which can impact diverse research fields ranging from soft condensed matter physics to materials science and biology. A major thrust in the past decade has been focused on extending the capabilities of plasmonically enhanced nanotweezers beyond diffusion-limited trapping on surfaces such as to achieve dynamic control in the bulk of fluidic environments. Here, we review the recent efforts in optical nanotweezers, especially those involving hybrid forcing schemes, covering both surface and bulk-based techniques. We summarize the important capabilities demonstrated with this promising approach, with niche applications in reconfigurable nanopatterning and on-chip assembly as well as in sorting and separating colloidal nanoparticles.
We study the two-dimensional assemblies of interacting colloidal particles in a loosely focussed optical trap. As the optical confinement is increased, the system becomes ordered and we investigate how these crystallites maintain their order under externally imposed oscillation. For small amplitudes, the crystalline order remains intact and the system behaves like a rigid body as predicted by numerical simulations. However, the rigidity breaks at large amplitudes, which we infer to be caused by the anharmonic component of the confinement potential. These studies are general enough to be applied to other physical systems comprising ordered finite-sized assemblies under external dynamic perturbation.
Multielectron bubbles provide a unique platform to study electrons in two dimensions and on curved surfaces, at densities which cannot be accessed using electrons on bulk helium or in semiconductor interfaces. Usually, MEBs are created by applying a large electric field and thereby inducing electrohydrodynamical instability on a charged surface of liquid helium. In the present study, we describe a method to create instability of the charged surface using ultrasound, in the presence of small electric fields. The ultrasound was applied close to the charged liquid-vapor interface, resulting in the formation of a liquid column, which breaks into liquid droplets. The mechanical impact of the droplets falling back into the bulk liquid resulted in the formation of highly charged multielectron bubbles. We estimated the initial charge density of the bubbles above the lambda point to be close to 1013electrons/m2.
Richard Feynman’s 1959 vision of controlling devices at small scales and swallowing the surgeon has inspired the science-fiction Fantastic Voyage film and has played a crucial role in the rapid development of the microrobotics field. Sixty years later, we are currently witnessing a dramatic progress in this field, with artificial micro- and nanoscale robots moving within confined spaces, down to the cellular level, and performing a wide range of biomedical applications within the cellular interior while addressing the limitations of common passive nanosystems. In this review article, we discuss key recent advances in the field of micro/nanomotors toward important cellular applications. Specifically, we outline the distinct capabilities of nanoscale motors for such cellular applications and illustrate how the active movement of nanomotors leads to distinct advantages of rapid cell penetration, accelerated intracellular sensing, and effective intracellular delivery toward enhanced therapeutic efficiencies. We finalize by discussing the future prospects and key challenges that such micromotor technology face toward implementing practical intracellular applications. By increasing our knowledge of nanomotors’ cell entry and of their behaviour within the intracellular space, and by successfully addressing key challenges, we expect that next-generation nanomotors will lead to exciting advances toward cell-based diagnostics and therapy.
The invasion of cancer is brought about by continuous interaction of malignant cells with their surrounding tissue microenvironment. Investigating the remodelling of local extracellular matrix (ECM) by invading cells can thus provide fundamental insights into the dynamics of cancer progression. In this paper, we use an active untethered nanomechanical tool, realized as magnetically driven nanomotors, to locally probe a 3D tissue culture environment. We observed that nanomotors preferentially adhere to the cancer proximal ECM and magnitude of the adhesive force increased with cell lines of higher metastatic ability. We experimentally confirmed that sialic acid linkage specific to cancer-secreted ECM makes it differently charged, which causes this adhesion. In an assay consisting of both cancerous and non-cancerous epithelia, that mimics the in vivo histopathological milieu of a malignant breast tumor, we find that nanomotors preferentially decorate the region around the cancer cells.
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.
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.
For biomedical applications, such as targeted drug delivery and microsurgery, it is essential to develop a system of swimmers that can be propelled wirelessly in fluidic environments with good control. Here, we report the construction and operation of chiral colloidal propellers that can be navigated in water with micrometer-level precision using homogeneous magnetic fields. The propellers are made via nanostructured surfaces and can be produced in large numbers. The nanopropellers can carry chemicals, push loads, and act as local probes in rheological measurements.
Micromagnets with collinear electric and magnetic dipole moments are embedded in a polymer to form a magnetic composite (see image). Each particle is a micromagnet that sits in a fluid‐filled cavity in which it is free to rotate. Under the application of a voltage, the multifunctional particles align with the applied electrostatic field to give rise to a net magnetization.
We show that magnetic-field-induced circular differential deflection of light can be observed in reflection or refraction at a single interface. The difference in the reflection or refraction angles between the two circular polarization components is a function of the magnetic-field strength and the Verdet constant, and permits the observation of the Faraday effect not via polarization rotation in transmission, but via changes in the propagation direction. Deflection measurements do not suffer from n−π ambiguities and are shown to be another means to map magnetic fields with high axial resolution, or to determine the sign and magnitude of magnetic-field pulses in a single measurement.
In an optically active liquid the diffraction angle depends on the circular polarization state of the incident light beam. We report the observation of circular differential diffraction in an isotropic chiral medium, and we demonstrate that double diffraction is an alternate means to determine the handedness (enantiomeric excess) of a solution.
A light beam changes direction as it enters a liquid at an angle from another medium, such as air. Should the liquid contain molecules that lack mirror symmetry, then it has been predicted by Fresnel that the light beam will not only change direction, but will actually split into two separate beams with a small difference in the respective angles of refraction. Here we report the observation of this phenomenon. We also demonstrate that the angle of reflection does not equal the angle of incidence in a chiral medium. Unlike conventional optical rotation, which depends on the path-length through the sample, the reported reflection and refraction phenomena arise within a few wavelengths at the interface and thereby suggest a new approach to polarimetry that can be used in microfluidic volumes.
In recent work, we have developed a new technique for the study of the properties of electron bubbles (negative ions) in liquid helium. We use ultrasound to measure the critical negative pressure Pc at which an electron bubble becomes unstable and explodes. The value of Pc is affected, for example, by the quantum state of the electron and is reduced if the bubble is attached to a quantized vortex. In the present experiments, we have discovered a new type of object that appears to be larger than the usual electron bubble. We will consider possible explanations of these observations.
In an experiment to investigate the possibility of using superfluid helium as a detection medium for low energy solar neutrinos, we have studied the currents produced by a radioactive source in a helium cell having a liquid/vacuum interface at 50 mK. A number of phenomena have been observed that appear not to have been described in the literature. These include the following: 1) The current at very low voltages in a cell having a free surface can be 100 times greater than in a filled cell. This additional current is associated with Penning ionization of metastable triplet dimers in surface states. 2) There is a large amplification of current in modest electric fields with a free surface present in the cell. This is the result of charges accelerated across the vacuum having sufficient energy to produce ionization and additional free charges upon hitting a liquid surface. The amplification becomes sufficiently large that breakdown occurs at potential differences across the vacuum of less than 1000 V. The dependence on 3He concentration of these phenomena has been studied.
We report on the observation of a new type of electron bubble in superfluid helium-4. This object appears to be larger than the normal electron bubble and is associated with the presence of quantized vortices in the liquid.
We report on the measurement of the lifetime of bubbles in superfluid helium that contain an electron in the 1 P state. The 1 P bubbles are produced by laser excitation of ground-state bubbles, and are detected by ultrasonic cavitation. Our measurements show that the lifetime of these excited bubbles is much less than the calculated lifetime for radiative decay and, hence, is determined by a nonradiative mechanism.
We have perforated a series of experiments to study cavitation in superfluid helium into which electrons are injected by field-emission from a sharp tip. The injected electrons force open small cavities in the liquid (“electron bubble”). These objects explode at a critical negative pressure Pc, and in previous experiments we have studied the cavitation that resugts from these explosions. In the present experiments we have detected cavitation events that occur before a negative pressure as large as Pc is reached. We suggest that these events may arise from a process in which two neutral helium dimers interact and an electron is injected into the liquid through Penning ionization.
We give a survey of recent experimental and theoretical work on the effect of light on electron bubbles in liquid helium. The light-induced change in the bubbles is measured using an ultrasonic technique. In helium at temperatures above about 1.7 K, we are able to produce and detect electron bubbles in the 1P quantum state. The properties of the electron bubbles are in agreement with theoretical expectations. However, the application of light to bubbles at low temperatures (T>1.5 K) resugts in. changes in the properties of the bubbles that are not yet understood.