Characterization of semiconductor nanowires using optical tweezers

Toward Lossless Infrared Optical Trapping of Small Nanoparticles Using Nonradiative Anapole Modes

A challenge in plasmonic trapping of small nanoparticles is the heating due to the Joule effect of metallic components. This heating can be avoided with electromagnetic field confinement in high-refractive-index materials, but nanoparticle trapping is difficult because the electromagnetic fields are mostly confined inside the dielectric nanostructures. Herein, we present the design of an all-dielectric platform to capture small dielectric nanoparticles without heating the nanostructure. It consists of a Si nanodisk engineered to exhibit the second-order anapole mode at the infrared regime (λ=980  nm), where Si has negligible losses, with a slot at the center. A strong electromagnetic hot spot is created, thus allowing us to capture nanoparticles as small as 20 nm. The numerical calculations indicate that optical trapping in these all-dielectric nanostructures occurs without heating only in the infrared, since for visible wavelengths the heating levels are similar to those in plasmonic nanostructures.

Directional Assembly of ZnO Nanowires via Three-Dimensional Laser Direct Writing

The precise placement of semiconductor nanowires (NWs) into two- or three-dimensional (2D/3D) micro-/nanoarchitectures is a key for the construction of integrated functional devices. However, long-pending challenges still exist in high-resolution 3D assembly of semiconductor NWs. Here, we have achieved directional assembly of zinc oxide (ZnO) NWs into nearly arbitrary 3D architectures with high spatial resolution using two-photon polymerization. The NWs can regularly align in any desired direction along the laser scanning pathway. Through theoretical calculation and control experiments, we unveiled the laser-induced assembly mechanism and found that the nonoptical forces are the dominant factor leading to the directional assembly of ZnO NWs. A ZnO-NW-based polarization-resolved UV photodetector of excellent photoresponsivity was fabricated to demonstrate the potential application of the assembled ZnO NWs. This work is expected to promote the research on NW-based integrated devices such as photonic integrated circuits, sensors, and metamaterial with unprecedented controllability of the NW's placement in three dimensions.

Single GaAs nanowire based photodetector fabricated by dielectrophoresis

Mechanical manipulation of nanowires (NWs) for their integration in electronics is still problematic because of their reduced dimensions, risking to produce mechanical damage to the NW structure and electronic properties during the assembly process. In this regard, contactless NW manipulation based methods using non-uniform electric fields, like dielectrophoresis (DEP) are usually much softer than mechanical methods, offering a less destructive alternative for integrating nanostructures in electronic devices. Here, we report a feasible and reproducible dielectrophoretic method to assemble single GaAs NWs (with radius 35-50 nm, and lengths 3-5 μm) on conductive electrodes layout with assembly yields above 90% per site, and alignment yields of 95%. The electrical characteristics of the dielectrophoretic contact formed between a GaAs NW and conductive electrodes have been measured, observing Schottky barrier like contacts. Our results also show the fast fabrication of diodes with rectifying characteristics due to the formation of a low-resistance contact between the Ga catalytic droplet at the tip of the NW when using Al doped ZnO as electrode. The current-voltage characteristics of a single Ga-terminated GaAs NW measured in dark and under illumination exhibit a strong sensitivity to visible light under forward bias conditions (around two orders of magnitude), mainly produced by a change on the series resistance of the device.

Optically oriented attachment of nanoscale metal-semiconductor heterostructures in organic solvents via photonic nanosoldering

As devices approach the single-nanoparticle scale, the rational assembly of nanomaterial heterojunctions remains a persistent challenge. While optical traps can manipulate objects in three dimensions, to date, nanoscale materials have been trapped primarily in aqueous solvents or vacuum. Here, we demonstrate the use of optical traps to manipulate, align, and assemble metal-seeded nanowire building blocks in a range of organic solvents. Anisotropic radiation pressure generates an optical torque that orients each nanowire, and subsequent trapping of aligned nanowires enables deterministic fabrication of arbitrarily long heterostructures of periodically repeating bismuth-nanocrystal/germanium-nanowire junctions. Heat transport calculations, back-focal-plane interferometry, and optical images reveal that the bismuth nanocrystal melts during trapping, facilitating tip-to-tail “nanosoldering” of the germanium nanowires. These bismuth-semiconductor interfaces may be useful for quantum computing or thermoelectric applications. In addition, the ability to trap nanostructures in oxygen- and water-free organic media broadly expands the library of materials available for optical manipulation and single-particle spectroscopy.

The use of optical traps has been limited to materials dispersed in aqueous media, which restricts the materials and range of experiments. Here, the authors demonstrate the alignment and assembly of composite structures made of a bismuth nanocrystal and a germanium nanowire in organic solvents.

High-performance solar-blind SnO2 nanowire photodetectors assembled using optical tweezers

One-dimensional semiconducting SnO2 nanowires with wide bandgaps are promising candidates to build many important optoelectronic devices. Because building these devices involves the assembly of nanowires into complex structures, manipulation of the active materials needs to be done with high spatial precision. In this paper, an optical tweezer system, comprising a spatial light-modulator, a microscope, and optical elements, is used to individually trap, transfer, and assemble SnO2 nanowires into two-terminal photodetectors in a liquid environment. After the assembly using optical trapping, the two ends of the SnO2 nanowire photodetectors, which are connected with the electrodes, were further stabilized using a focused laser. During exposure to 275 nm deep-ultraviolet light, the as-assembled photodetectors show a high Iph/Idark ratio of 2.99 × 105, a large responsivity of 4.3 × 104 A W-1, an excellent external quantum efficiency of 1.94 × 105, and a high detectivity of 2.32 × 1013 Jones. The photoresponse-speed of the devices could be improved further using passivation with a polymer. The rise and decay times are about 60 ms and 100 ms, respectively. As a result of this study, we can confirm that non-contact optical trapping can enable the construction of nanowire architectures for optoelectronic, bioelectronic, and other devices.

Optical Trapping, Optical Binding, and Rotational Dynamics of Silicon Nanowires in Counter-Propagating Beams

Silicon nanowires are held and manipulated in controlled optical traps based on counter-propagating beams focused by low numerical aperture lenses. The double-beam configuration compensates light scattering forces enabling an in-depth investigation of the rich dynamics of trapped nanowires that are prone to both optical and hydrodynamic interactions. Several polarization configurations are used, allowing the observation of optical binding with different stable structure as well as the transfer of spin and orbital momentum of light to the trapped silicon nanowires. Accurate modeling based on Brownian dynamics simulations with appropriate optical and hydrodynamic coupling confirms that this rich scenario is crucially dependent on the non-spherical shape of the nanowires. Such an increased level of optical control of multiparticle structure and dynamics open perspectives for nanofluidics and multi-component light-driven nanomachines.

Gradient and scattering forces of anti-reflection-coated spheres in an aplanatic beam

Anti-reflection coatings (ARCs) enable one to trap high dielectric spheres that may not be trappable otherwise. Through rigorously calculating the gradient and scattering forces, we directly showed that the improved trapping performance is due to the reduction in scattering force, which originates from the suppression of backscattering by ARC. We further applied ray optics and wave scattering theories to thoroughly understand the underlying mechanism, from which, we inferred that ARC only works for spherical particles trapped near the focus of an aplanatic beam, and it works much better for large spheres. For this reason, in contradiction to our intuition, large ARC-coated spheres are sometimes more trappable than their smaller counter parts. Surprisingly, we discovered a scattering force free zone for a large ARC-coated sphere located near the focus of an aplanatic beam. Our work provides a quantitative study of ARC-coated spheres and bridges the gap between the existing experiments and current conceptual understandings.

Optical trapping and optical force positioning of two-dimensional materials

In recent years, considerable effort has been devoted to the synthesis and characterization of two-dimensional materials. Liquid phase exfoliation (LPE) represents a simple, large-scale method to exfoliate layered materials down to mono- and few-layer flakes. In this context, the contactless trapping, characterization, and manipulation of individual nanosheets hold perspectives for increased accuracy in flake metrology and the assembly of novel functional materials. Here, we use optical forces for high-resolution structural characterization and precise mechanical positioning of nanosheets of hexagonal boron nitride, molybdenum disulfide, and tungsten disulfide obtained by LPE. Weakly optically absorbing nanosheets of boron nitride are trapped in optical tweezers. The analysis of the thermal fluctuations allows a direct measurement of optical forces and the mean flake size in a liquid environment. Measured optical trapping constants are compared with T-matrix light scattering calculations to show a quadratic size scaling for small size, as expected for a bidimensional system. In contrast, strongly absorbing nanosheets of molybdenum disulfide and tungsten disulfide are not stably trapped due to the dominance of radiation pressure over the optical trapping force. Thus, optical forces are used to pattern a substrate by selectively depositing nanosheets in short times (minutes) and without any preparation of the surface. This study will be useful for improving ink-jet printing and for a better engineering of optoelectronic devices based on two-dimensional materials.

Novel physical chemistry approaches in biophysical researches with advanced application of lasers: Detection and manipulation

Novel methodologies utilizing pulsed or intense CW irradiation obtained from lasers have a major impact on biological sciences. In this article, recent development in biophysical researches fully utilizing the laser irradiation is described for three topics, time-resolved fluorescence spectroscopy, time-resolved thermodynamics, and manipulation of the biological assemblies by intense laser irradiation. First, experimental techniques for time-resolved fluorescence spectroscopy are concisely explained in Section 2. As an example of the recent application of time-resolved fluorescence spectroscopy to biological systems, evaluation of the viscosity of lipid bilayer membranes is described. The results of the spectroscopic experiments strongly suggest the presence of heterogeneous membrane structure with two different viscosity values in liposomes formed by a single phospholipid. Section 3 covers the time-resolved thermodynamics. Thermodynamical properties are important to characterize biomolecules. However, measurement of these quantities for short-lived intermediate species has been impossible by traditional thermodynamical techniques. Recently, development of a spectroscopic method based on the transient grating method enables us to measure these quantities and also to elucidate reaction kinetics which cannot be detected by other spectroscopic methods. The principle of the measurements and applications to some protein reactions are reviewed. Manipulation and fabrication of supramolecues, amino acids, proteins, and living cells by intense laser irradiation are described in Section 4. Unconventional assembly, crystallization and growth, amyloid fibril formation, and living cell manipulation are achieved by CW laser trapping and femtosecond laser-induced cavitation bubbling. Their spatio-temporal controllability is opening a new avenue in the relevant molecular and bioscience research fields. This article is part of a Special Issue entitled "Biophysical Exploration of Dynamical Ordering of Biomolecular Systems" edited by Dr. Koichi Kato.

Using back focal plane interferometry to probe the influence of Zernike aberrations in optical tweezers

We experimentally investigate the influence of geometric aberrations in optical tweezers using back focal plane interferometry. We found that the introduction of coma aberrations causes significant modification to the Brownian motion of the trapped particle, producing an apparent cross-coupling between the in-plane aberrated axis and the weaker propagation axis. This coupling is evidenced by the emergence of a second dominant low frequency Lorentzian feature in the position power spectral density. The effect on Brownian motion was confirmed using a secondary unaberrated probe beam, ruling out the possibility of systematic optical effects related to the detection system.

Optical Binding of Nanowires

Multiple scattering of light induces structured interactions, or optical binding forces, between collections of small particles. This has been extensively studied in the case of microspheres. However, binding forces are strongly shape dependent: here, we turn our attention to dielectric nanowires. Using a novel numerical model we uncover rich behavior. The extreme geometry of the nanowires produces a sequence of stationary and dynamic states. In linearly polarized light, thermally stable ladder-like structures emerge. Lower symmetry, sagittate arrangements can also arise, whose configurational asymmetry unbalances the optical forces leading to nonconservative, translational motion. Finally, the addition of circular polarization drives a variety of coordinated rotational states whose dynamics expose fundamental properties of optical spin. These results suggest that optical binding can provide an increased level of control over the positions and motions of nanoparticles, opening new possibilities for driven self-organization and heralding a new field of self-assembling optically driven micromachines.

Real-time monitoring and visualization of the multi-dimensional motion of an anisotropic nanoparticle

As interest in anisotropic particles has increased in various research fields, methods of tracking such particles have become increasingly desirable. Here, we present a new and intuitive method to monitor the Brownian motion of a nanowire, which can construct and visualize multi-dimensional motion of a nanowire confined in an optical trap, using a dual particle tracking system. We measured the isolated angular fluctuations and translational motion of the nanowire in the optical trap, and determined its physical properties, such as stiffness and torque constants, depending on laser power and polarization direction. This has wide implications in nanoscience and nanotechnology with levitated anisotropic nanoparticles.

Fuel-Free Synthetic Micro-/Nanomachines

Inspired by the swimming of natural microorganisms, synthetic micro-/nanomachines, which convert energy into movement, are able to mimic the function of these amazing natural systems and help humanity by completing environmental and biological tasks. While offering autonomous propulsion, conventional micro-/nanomachines usually rely on the decomposition of external chemical fuels (e.g., H₂ O₂ ), which greatly hinders their applications in biologically relevant media. Recent developments have resulted in various micro-/nanomotors that can be powered by biocompatible fuels. Fuel-free synthetic micro-/nanomotors, which can move without external chemical fuels, represent another attractive solution for practical applications owing to their biocompatibility and sustainability. Here, recent developments on fuel-free micro-/nanomotors (powered by various external stimuli such as light, magnetic, electric, or ultrasonic fields) are summarized, ranging from fabrication to propulsion mechanisms. The applications of these fuel-free micro-/nanomotors are also discussed, including nanopatterning, targeted drug/gene delivery, cell manipulation, and precision nanosurgery. With continuous innovation, future autonomous, intelligent and multifunctional fuel-free micro-/nanomachines are expected to have a profound impact upon diverse biomedical applications, providing unlimited opportunities beyond one's imagination.

Stable optical trapping and sensitive characterization of nanostructures using standing-wave Raman tweezers

Optical manipulation and label-free characterization of nanoscale structures open up new possibilities for assembly and control of nanodevices and biomolecules. Optical tweezers integrated with Raman spectroscopy allows analyzing a single trapped particle, but is generally less effective for individual nanoparticles. The main challenge is the weak gradient force on nanoparticles that is insufficient to overcome the destabilizing effect of scattering force and Brownian motion. Here, we present standing-wave Raman tweezers for stable trapping and sensitive characterization of single isolated nanostructures with a low laser power by combining a standing-wave optical trap with confocal Raman spectroscopy. This scheme has stronger intensity gradients and balanced scattering forces, and thus can be used to analyze many nanoparticles that cannot be measured with single-beam Raman tweezers, including individual single-walled carbon nanotubes (SWCNT), graphene flakes, biological particles, SERS-active metal nanoparticles, and high-refractive semiconductor nanoparticles. This would enable sorting and characterization of specific SWCNTs and other nanoparticles based on their increased Raman fingerprints.

Photocontrolled Supramolecular Assembling of Azobenzene-Based Biscalix[4]arenes upon Starting and Stopping Laser Trapping

Laser trapping in chemistry covers various studies ranging from single molecules, nanoparticles, and quantum dots to crystallization and liquid-liquid phase separation of amino acids. In this work, a supramolecular assembly of azobenzene-based biscalix[4]arene is generated in ethyl acetate using laser trapping; its nucleation and growth are elucidated. No trapping behavior was observed when a 1064 nm laser beam was focused inside of the solution; however, interesting assembling phenomena were induced when it was shined at the air/solution interface. A single disk having two layers was first prepared at the focal point of ∼1 μm and then expanded to the size of a few tens of micrometers, although no optical force was exerted outside of the focal volume. Upon switching the trapping laser off, needles were generated at the outer layer of the assembly, giving a stable sea urchin-like morphology to the generated assembly. At a 30-50% dilution of the initial solution in ethyl acetate, a mushroom-like morphology was also observed. Laser trapping-induced assembly of azobenzene-based biscalix[4]arene was quite different from the sharp-ellipsoidal aggregates obtained by the spontaneous evaporation of the solution. These trapping phenomena were specifically observed for biscalix[4]arene in the trans conformation of azo-benzene moiety but not for the cis-form, suggesting that the laser trapping of this azobenzene-based biscalix[4]arene is photocontrollable. Dynamics and mechanism of the supramolecular assembling are considered, referring to laser trapping-induced nucleation and liquid-liquid phase separation of amino acids.

Concentric Circular Grating Generated by the Patterning Trapping of Nanoparticles in an Optofluidic Chip

Due to the field enhancement effect of the hollow-core metal-cladded optical waveguide chip, massive nanoparticles in a solvent are effectively trapped via exciting ultrahigh order modes. A concentric ring structure of the trapped nanoparticles is obtained since the excited modes are omnidirectional at small incident angle. During the process of solvent evaporation, the nanoparticles remain well trapped since the excitation condition of the optical modes is still valid, and a concentric circular grating consisting of deposited nanoparticles can be produced by this approach. Experiments via scanning electron microscopy, atomic force microscopy and diffraction of a probe laser confirmed the above hypothesis. This technique provides an alternative strategy to enable effective trapping of dielectric particles with low-intensity nonfocused illumination, and a better understanding of the correlation between the guided modes in an optical waveguide and the nanoparticles in a solvent.

Photonic Torque Microscopy of the Nonconservative Force Field for Optically Trapped Silicon Nanowires

We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

Characterization of Individual Magnetic Nanoparticles in Solution by Double Nanohole Optical Tweezers

We study individual superparamagnetic Fe3O4 (magnetite) nanoparticles in solution using a double nanohole optical tweezer with magnetic force setup. By analysis of the trapping optical transmission signal (step size, autocorrelation, the root-mean-square signal, and the distribution with applied magnetic field), we are able to measure the refractive index, magnetic susceptibility, remanence and size of each trapped nanoparticle. The size distribution is found to agree well with scanning electron microscopy measurements, and the permeability, magnetic susceptibility and remanence values are all in agreement with published results. Our approach demonstrates the versatility of the optical tweezer with magnetic field setup to characterize nanoparticles in fluidic mixtures with potential for isolation of desired particles and pick-and-place functionality.

Nonconservative dynamics of optically trapped high-aspect-ratio nanowires

We investigate the dynamics of high-aspect-ratio nanowires trapped axially in a single gradient force optical tweezers. A power spectrum analysis of the dynamics reveals a broad spectral resonance of the order of kHz with peak properties that are strongly dependent on the input trapping power. A dynamical model incorporating linear restoring optical forces, a nonconservative asymmetric coupling between translational and rotational degrees of freedom, viscous drag, and white noise provides an excellent fit to experimental observations. A persistent low-frequency cyclical motion around the equilibrium trapping position, with a frequency distinct from the spectral resonance, is observed from the time series data.

Determining the 3D orientation of optically trapped upconverting nanorods by in situ single-particle polarized spectroscopy

An approach to unequivocally determine the three-dimensional orientation of optically manipulated NaYF4:Er(3+),Yb(3+) upconverting nanorods (UCNRs) is demonstrated. Long-term immobilization of individual UCNRs inside single and multiple resonant optical traps allow for stable single UCNR spectroscopy studies. Based on the strong polarization dependent upconverted luminescence of UCNRs it is possible to unequivocally determine, in real time, their three-dimensional orientation when optically trapped. In single-beam traps, polarized single particle spectroscopy has concluded that UCNRs orientate parallel to the propagation axis of the trapping beam. On the other hand, when multiple-beam optical tweezers are used, single particle polarization spectroscopy demonstrated how full spatial control over UCNR orientation can be achieved by changing the trap-to-trap distance as well as the relative orientation between optical traps. All these results show the possibility of real time three-dimensional manipulation and tracking of anisotropic nanoparticles with wide potential application in modern nanobiophotonics.

Spatially Resolved Doping Concentration and Nonradiative Lifetime Profiles in Single Si-Doped InP Nanowires Using Photoluminescence Mapping

We report an analysis method that combines microphotoluminescence mapping and lifetime mapping data of single semiconductor nanowires to extract the doping concentration, nonradiative lifetime, and internal quantum efficiency along the length of the nanowires. Using this method, the doping concentration of single Si-doped wurtzite InP nanowires are mapped out and confirmed by the electrical measurements of single nanowire devices. Our method has important implication for single nanowire detectors and LEDs and nanowire solar cells applications.

Fano resonance-induced negative optical scattering force on plasmonic nanoparticles

We demonstrate theoretically that Fano resonance can induce a negative optical scattering force acting on plasmonic nanoparticles in the visible light spectrum when an appropriate manipulating laser beam is adopted. Under the illumination of a zeroth-order Bessel beam, the plasmonic nanoparticle at its Fano resonance exhibits a much stronger forward scattering than backward scattering and consequently leads to a net longitudinal backward optical scattering force, termed Fano resonance-induced negative optical scattering force. The extinction spectra obtained based on the Mie theory show that the Fano resonance arises from the interference of simultaneously excited multipoles, which can be either a broad electric dipole mode and a narrow electric quadrupole mode, or a quadrupole and an octupole mode mediated by the broad electric dipole. Such Fano resonance-induced negative optical scattering force is demonstrated to occur for core-shell, homogeneous, and hollow metallic particles and can therefore be expected to be universal for many other nanostructures exhibiting Fano resonance, adding considerably to the flexibility of optical micromanipulation on the plasmonic nanoparticles. More interestingly, the flexible tunability of the Fano resonance by particle morphology opens up the possibility of tailoring the optical scattering force accordingly, offering an additional degree of freedom to optical selection and sorting of plasmonic nanoparticles.

Label-free free-solution nanoaperture optical tweezers for single molecule protein studies

Recent advances in nanoaperture optical tweezers have enabled studies of single nanoparticles like proteins in label-free, free-solution environments.

Ultrafast polarization response of an optically trapped single ferroelectric nanowire

One-dimensional potassium niobate nanowires are of interest as building blocks in integrated piezoelectric devices, exhibiting large nonlinear optical and piezoelectric responses. Here we present femtosecond measurements of light-induced polarization dynamics within an optically trapped ferroelectric nanowire, using the second-order nonlinear susceptibility as a real-time structural probe. Large amplitude, reversible modulations of the nonlinear susceptibility are observed within single nanowires at megahertz repetition rates, developing on few-picosecond time-scales, associated with anomalous coupling of light into the nanowire.

Optical trapping of individual human immunodeficiency viruses in culture fluid reveals heterogeneity with single-molecule resolution

Optical tweezers use the momentum of photons to trap and manipulate microscopic objects, contact-free, in three dimensions. Although this technique has been widely used in biology and nanotechnology to study molecular motors, biopolymers and nanostructures, its application to study viruses has been very limited, largely due to their small size. Here, using optical tweezers that can simultaneously resolve two-photon fluorescence at the single-molecule level, we show that individual HIV-1 viruses can be optically trapped and manipulated, allowing multi-parameter analysis of single virions in culture fluid under native conditions. We show that individual HIV-1 differs in the numbers of envelope glycoproteins by more than one order of magnitude, which implies substantial heterogeneity of these virions in transmission and infection at the single-particle level. Analogous to flow cytometry for cells, this fluid-based technique may allow ultrasensitive detection, multi-parameter analysis and sorting of viruses and other nanoparticles in biological fluid with single-molecule resolution.

5D-Tracking of a nanorod in a focused laser beam--a theoretical concept

Back-focal plane (BFP) interferometry is a very fast and precise method to track the 3D position of a sphere within a focused laser beam using a simple quadrant photo diode (QPD). Here we present a concept of how to track and recover the 5D state of a cylindrical nanorod (3D position and 2 tilt angles) in a laser focus by analyzing the interference of unscattered light and light scattered at the cylinder. The analytical theoretical approach is based on Rayleigh-Gans scattering together with a local field approximation for an infinitely thin cylinder. The approximated BFP intensities compare well with those from a more rigorous numerical approach. It turns out that a displacement of the cylinder results in a modulation of the BFP intensity pattern, whereas a tilt of the cylinder results in a shift of this pattern. We therefore propose the concept of a local QPD in the BFP of a detection lens, where the QPD center is shifted by the angular coordinates of the cylinder tilt.

Ultrasensitive size-selection of plasmonic nanoparticles by Fano interference optical force

In this paper, we propose a solution for the ultrasensitive optical selection of plasmonic nanoparticles using Fano interference-induced scattering forces. Under a Gaussian beam excitation, the scattering of a plasmonic nanoparticle at its Fano resonance becomes strongly asymmetric in the lateral direction and consequently results in a net transverse scattering force, that is, Fano interference-induced force. The magnitude of this transverse scattering force is comparable with the gradient force in conventional optical manipulation experiments. More interestingly, the Fano scattering force is ultrasensitive to the particle size and excitation frequency due to the phase sensitivity of the interference between adjacent plasmon modes in the particle. Utilizing this distinct feature, we show the possibility of size-selective sorting of silver and gold nanoparticles with an accuracy of about ±10 nm and silica-gold core-shell nanoparticles with shell thickness down to several nanometers. These results would add to the toolbox of optical manipulation and fabrication.

Optical trapping and manipulation of nanostructures

Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate - nanoscale - range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.

Using molecular tweezers to move and image nanoparticles

The ability to manipulate nanoparticles is significant in nanoscale science and technology. As sizes of the objects scale down to the sub-10 nm regime, it imposes a great challenge for the conventional optical tweezers. There has been much effort to explore alternative manipulation methods including using nanostructures, electron beams, scanning probes, etc. In this paper, an overview of the latest advances in trapping and manipulation of nanoparticles with a focus on the emergent electron tweezers is provided.

Coherence and Raman sideband cooling of a single atom in an optical tweezer

We investigate quantum control of a single atom in a tightly focused optical tweezer trap. We show that inevitable spatially varying polarization gives rise to significant internal-state decoherence but that this effect can be mitigated by an appropriately chosen magnetic bias field. This enables Raman sideband cooling of a single atom close to its three-dimensional ground state (vibrational quantum numbers n(x)=n(y)=0.01, n(z)=8) even for a trap beam waist as small as w=900  nm. The small atomic wave packet with δx=δy=24  nm and δz=270  nm represents a promising starting point for future hybrid quantum systems where atoms are placed in close proximity to surfaces.

Resolving stable axial trapping points of nanowires in an optical tweezers using photoluminescence mapping

Axially resolved microphotoluminescence mapping of semiconductor nanowires held in an optical tweezers reveals important new experimental information regarding equilibrium trapping points and trapping stability of high aspect ratio nanostructures. In this study, holographic optical tweezers are used to scan trapped InP nanowires along the beam direction with respect to a fixed excitation source and the luminescent properties are recorded. It is observed that nanowires with lengths on the range of 3-15 μm are stably trapped near the tip of the wire with the long segment positioned below the focus in an inverted trapping configuration. Through the use of trap multiplexing we investigate the possibility of improving the axial stability of the trapped nanowires. Our results have important implication for applications of optically assisted nanowire assembly and optical tweezers based scanning probes microscopy.

Optical trapping of nanoparticles by ultrashort laser pulses

Optical trapping with continuous-wave lasers has been a fascinating field in the optical manipulation. It has become a powerful tool for manipulating micrometer-sized objects, and has been widely applied in physics, chemistry, biology, material, and colloidal science. Replacing the continuous-wave- with pulsed-mode laser in optical trapping has already revealed some novel phenomena, including the stable trap, modifiable trapping positions, and controllable directional optical ejections of particles in nanometer scales. Due to two distinctive features; impulsive peak powers and relaxation time between consecutive pulses, the optical trapping with the laser pulses has been demonstrated to have some advantages over conventional continuous-wave lasers, particularly when the particles are within Rayleigh approximation. This would open unprecedented opportunities in both fundamental science and application. This Review summarizes recent advances in the optical trapping with laser pulses and discusses the electromagnetic formulations and physical interpretations of the new phenomena. Its aim is rather to show how beautiful and promising this field will be, and to encourage the in-depth study of this field.

An optically actuated surface scanning probe

We demonstrate the use of an extended, optically trapped probe that is capable of imaging surface topography with nanometre precision, whilst applying ultra-low, femto-Newton sized forces. This degree of precision and sensitivity is acquired through three distinct strategies. First, the probe itself is shaped in such a way as to soften the trap along the sensing axis and stiffen it in transverse directions. Next, these characteristics are enhanced by selectively position clamping independent motions of the probe. Finally, force clamping is used to refine the surface contact response. Detailed analyses are presented for each of these mechanisms. To test our sensor, we scan it laterally over a calibration sample consisting of a series of graduated steps, and demonstrate a height resolution of ∼ 11 nm. Using equipartition theory, we estimate that an average force of only ∼ 140 fN is exerted on the sample during the scan, making this technique ideal for the investigation of delicate biological samples.

Flow-dependent double-nanohole optical trapping of 20 nm polystyrene nanospheres

We study the influence of fluid flow on the ability to trap optically a 20 nm polystyrene particle from a stationary microfluidic environment and then hold it against flow. Increased laser power is required to hold nanoparticles as the flow rate is increased, with an empirical linear dependence of 1 μl/(min×mW). This is promising for the delivery of additional nanoparticles to interact with a trapped nanoparticle; for example, to study protein-protein interactions, and for the ability to move the trapped particle in solution from one location to another.

Nanowire heating by optical electromagnetic irradiation

The dissipative absorption of electromagnetic energy by 1D nanoscale structures at optical frequencies is applicable to several important phenomena, including biomedical photothermal theranostics, nanoscale photovoltaic materials, atmospheric aerosols, and integrated photonic devices. Closed-form analytical calculations are presented for the temperature rise within infinite circular cylinders with nanometer-scale diameters (nanowires) that are irradiated at right angles by a continuous-wave laser source polarized along the nanowire's axis. Solutions for the heat source are compared to both numerical finite-difference time domain (FDTD) simulations and well-known Mie scattering cross sections for infinite cylinders. The analysis predicts that the maximum temperature increase is affected not only by the cylinder's composition and porosity but also by morphology-dependent resonances (MDRs) that lead to significant spikes in the local temperature at particular diameters. Furthermore, silicon nanowires with high thermal conductivities are observed to exhibit extremely uniform internal temperatures during electromagnetic heating to 1 part in 10(6), including cases where there are substantial fluctuations of the internal electric-field source term that generates the Joule heating. For a highly absorbing material such as carbon, much higher temperatures are predicted, the internal temperature distribution is nonuniform, and MDRs are not encountered.

Stability analysis and thermal motion of optically trapped nanowires

We investigate the stability and thermal motion of optically trapped nanowires, with aspect ratios in the range 10-100. A simple analytical model is used to determine qualitative features of the system, assuming that the nanowire is weakly scattering and the incident beam is paraxial. As expected, the model predicts that the nanowire will align with the beam axis. In this configuration the translational stiffness coefficients of the trap approach their limiting values for long nanowires like O(L(-3)), where L is the nanowire length, the limit for the stiffness parallel to the beam axis being zero. The rotational stiffness coefficients vary more slowly, according to O(L(-1)). Also, it is predicted that defocusing decreases the translational stiffness perpendicular to the beam, while increasing rotational stiffness. These findings are reinforced by comparison with rigorous electromagnetic calculations which additionally reveal the effects of radiation pressure and finite scattering. A strong polarization effect is observed in the numerical simulations and coupled translational and rotational motions arise which influence the trap stability. The use of nanowire traps for force sensing is discusse.

Calibration of the optical torque wrench

The optical torque wrench is a laser trapping technique that expands the capability of standard optical tweezers to torque manipulation and measurement, using the laser linear polarization to orient tailored microscopic birefringent particles. The ability to measure torque of the order of kBT (∼4 pN nm) is especially important in the study of biophysical systems at the molecular and cellular level. Quantitative torque measurements rely on an accurate calibration of the instrument. Here we describe and implement a set of calibration approaches for the optical torque wrench, including methods that have direct analogs in linear optical tweezers as well as introducing others that are specifically developed for the angular variables. We compare the different methods, analyze their differences, and make recommendations regarding their implementations.

Optical trapping of a single protein

We experimentally demonstrate the optical trapping of a single bovine serum albumin (BSA) molecule that has a hydrodynamic radius of 3.4 nm, using a double-nanohole in an Au film. The strong optical force in the trap not only stably traps the protein molecule but also unfolds it. The unfolding of the BSA is confirmed by experiments with changing optical power and with changing solution pH. The detection of the trapping event has a signal-to-noise ratio of 33, which shows that the setup is extremely sensitive to detect the presence of a protein, even at the single molecule level.

Dark-field optical tweezers for nanometrology of metallic nanoparticles

Applications of metallic nanoparticles are based on their strongly size-dependent optical properties. We present a method for combining optical tweezers with dark field microscopy that allows measurement of localised surface plasmon resonance (LSPR) spectra on single isolated nanoparticles without compromising the strength of the optical trap. Using this spectroscopic information in combination with measurements of trap stiffness and hydrodynamic drag, allows us to determine the dimensions of the trapped nanoparticles. A relationship is found between the measured diameters of the particles and the peak wavelengths of their spectra. Using this method we may also resolve complex spectra of particle aggregation and interactions within the tweezers.

Plasmon-exciton interactions on single thermoresponsive platforms demonstrated by optical tweezers

Optical and hydrodynamic-size studies on single bare thermo-responsive microspheres, and microspheres covered either with Au nanoparticles, CdSe/CdS quantum dots, or a combination of both have been performed by optical tweezers. The photothermal heating of water in the focal region boosts the shrinkage of the microspheres, an effect that is intensified in the presence of Au nanoparticles. In contrast, bigger microspheres are measured when they are covered with quantum dots. Plasmon-exciton interactions are observable in the trap in the combined Au and quantum dots hybrid systems.

Size-scaling in optical trapping of silicon nanowires

We investigate size-scaling in optical trapping of ultrathin silicon nanowires showing how length regulates their Brownian dynamics, optical forces, and torques. Force and torque constants are measured on nanowires of different lengths through correlation function analysis of their tracking signals. Results are compared with a full electromagnetic theory of optical trapping developed in the transition matrix framework, finding good agreement.

Optical trapping of 12 nm dielectric spheres using double-nanoholes in a gold film

Optical tweezers have found many applications in biology, but for reasonable intensities, conventional traps are limited to particles >100 nm in size. We use a double-nanohole in a gold film to experimentally trap individual nanospheres, including 20 nm polystyrene spheres and 12 nm silica spheres, at a well-defined trapping point. We present statistical studies on the trapping time, showing an exponential dependence on the optical power. Trapping experiments are repeated for different particles and several nanoholes with different gap dimensions. Unusually, smaller particles can be more easily trapped than larger ones with the double-nanohole. The 12 nm silica sphere has a size and a refractive index comparable to the smallest virus particles and has a spherical shape which is the worst case scenario for trapping.