An interacting dipole model to explore broadband transverse optical binding

Observing and taming unusual effects arising from non-trivial light-matter interaction has always triggered scientists to better understand Nature and develop technological tools towards implementing novel applications. Recently, several unusual optomechanical effects have been unveiled when subtle spin-orbit interactions come at play to build up optical forces and torques that are hardly seen in everyday life, such as negative optical radiation pressure, transverse optical forces, or left-handed optical torques. To date, there are only a few experimental proposals to reveal these effects despite tremendous conceptual advances. In particular, spin-dependent lateral forces and their angular analog are done either at the expense of direct observations or at the cost of specific instrumental complexity. Here we report on naked-eye identification of light-induced spin-dependent lateral displacements of centimeter-sized objects endowed with structured birefringence. Its angular counterpart is also discussed and the observation of left-handed macroscopic rotational motion is reported. The unveiled effects are ultimately driven by lateral optical force fields that are five orders of magnitude larger than those reported so far. These results allow structured light-matter interaction to move from a scientific curiosity to a new asset for the existing multidisciplinary optical manipulation toolbox across length scales. In addition, this highlights the spin-orbit optomechanics of anisotropic and inhomogeneous media, which is just beginning to be explored.

The impact of optical forces in the physical and biological sciences now enables the manipulation of objects ranging in size from a cell down to a single atom. The mechanical effects of optical fields have profound and far-reaching consequences, and attention is increasingly focused upon the opportunities for the non-contact assembly of particles into specific geometries. The present overview focuses on the two aspects of multi-particle trapping and optical binding. These can broadly be grouped as methods based on light-mediated inter-particle interactions, offering potential for the organization of large numbers of micro- or nano-particles using optical forces alone.

High-Tc superconductivity is surpassed by few, if any, other unsolved problems in contemporary physics in terms of its richness, complexity, impact on other fields, and potential technological importance in energy applications. Recent discoveries reveal that the highest transition temperatures emerge under extraordinary experimental conditions such as the strong perturbation of a single atomic layer material by the underlying substrate, application of ~200 GPa pressure or ~MV/cm static electric fields, or irradiation by intense femtosecond optical fields. Research in superconductivity under extreme experimental conditions is challenging and requires the development of novel experimental methods and new theoretical tools suitable to tackle the problem. Our team has approached the challenges of transient superconductivity under intense optical fields by rethinking the types of experimental observables that are best suited to elucidate the physics of light-induced effects. This exercise has identified the need for: i) ultra-fast spatio-temporal probes enabling access to plasmonic properties and also nano-scale inhomogeneities that are ubiquitous in unconventional superconductors; ii) meso-scale structures and hybrid meta-surfaces imperative for strong enhancement of optical fields and iii) theoretical approaches suitable to explain and predict the response of superconductors under ultra-fast photo-excitation. Co-PIs have already made major advances with developing capabilities i)-iii). Therefore, our team is poised to make significant progress in transient superconductivity by exploring novel spatio-temporal effects that currently remain unattainable. Co-investigators will search for light-induced superconductivity in the high-Tc cuprates to build upon spectacular recent results still lacking independent confirmations. Novel data acquisition methods combined with nano-plasmonic imaging will allow our team to test the hypothesis of photo-induced superconducting pairing. Spatio-temporal experiments will provide insights into the interplay between superconductivity and competing orders in the cuprates. Co-PIs will also investigate strong light-matter interaction and superconductivity in FeSe monolayers. Finally, we will study spatio-temporal electrodynamics of planar Josephson junctions. This latter direction will allow our team to thoroughly characterize one of the most fundamental examples of inhomogeneity in all of unconventional superconductivity. The proposed experiments in combination with theoretical analysis will provide information that is difficult to obtain using alternative methods. Basov and Averitt will carry out pump-probe spectroscopy and nano-imaging experiments. Averitt and Hone will design and fabricate state-of-the-art meta-surfaces. Theoretical and computational studies of ultrafast transient phenomena will be carried out by Millis and Fogler. Modeling of time-resolved nano-optical effects will be done by Fogler. The bulk of the requested budget will be used to support graduate students at Columbia & UCSD that will be co-supervised by co-PIs. The proposed program is transformative, since it will enable an entire suite of experiments previously either impossible or technically implausible. It will deliver critically important insights not only into mechanisms of unconventional superconductivity but also to many other correlated quantum materials.

We have developed a NSOM technique that can map both the near optical field and the optical force using an atomic force microscope. This technique could be very useful for characterizing MEMs/NEMs devices, plasmonic nanoantennas, nano-photonic devices and biologically active substrates. Unlike conventional NSOM techniques that rely on an aperture fabricated on the end of an AFM tip to collect the optical signal this apertureless technique uses a lockin amplifier locked to the AFM tip vibrational frequency, to correlate the amplitude modulation of the back reflected optical signal to the strength of the optical field. And since we are not limited by the fabrication of an aperture the spatial resolution of the map is limited only by the size of a sharp AFM tip which for metallic coated tips can have a radius of curvature of 10 to 20 nm. For optical force mapping the incident laser is modulated and the lock-in amplifier is used to correlate the amplitude modulation of the vibrating AFM tip to strength of the optical gradient force. And in this way one can get a very accurate mapping of both the optical force and the optical field for any substrate of interest as long as it can be back illuminated. Lastly with an electrically monolithic substrate it is possible to correlate the amplitude modulation of the tunneling current to the optical field and obtain a spatial mapping that has a resolution of an STM, about 1 nm or maybe less.

Light carries momentum which can influence the matter through optical forces. Nowadays, studying of optical forces became a promising field due to widespread usage of single and organized molecular and atomic structures in chemical and biological research, nanophoton-ics, different miniaturized electronic devices. Optical binding is an optical force appearing in the ensemble of particles that controls their motion and can lead to self-organization of structural elements. If an array of nanoparticles is illuminated by light, each element of the array scatters incident wave and a set of potential wells is created which defines stable positions of particles.

The principle of optical trapping is conventionally based on the interaction of optical fields with linear-induced polarizations. However, the optical force originating from the nonlinear polarization becomes significant when nonlinear optical nanoparticles are trapped by femtosecond laser pulses. Herein we develop the time-averaged optical forces on a nonlinear optical nanoparticle using high-repetition-rate femtosecond laser pulses, based on the linear and nonlinear polarization effects. We investigate the characteristics of transverse and longitudinal optical forces for particles exhibiting self-focusing and defocusing effects. It is shown that the self-focusing effect increases the trapping force strength and improves the confinement of particles, whereas the self-defocusing effect leads to the splitting of potential well at the focal plane and destabilizes the optical trap, resulting in ejections of trapped particles along the direction of the beam’s propagation. The optical forces exerted on the nonlinear optical particles are experimentally related to the trapping stiffness. It is expected that the self-focusing (or self-defocusing) effect increases (or decreases) the trapping efficiency and stiffness. Our results successfully explain the reported experimental observations and provide theoretical support for capturing nonlinear nanoparticles with femtosecond laser trapping.

In the field of optical micromanipulations the optical landscapes are also used for sorting of micro (nano) particles. This sorting processes are actuated by intense laser fields devised in an arrangement to a particular sorting problem. The primary forces acting on the particles are given by gradient and scattering forces resulting from the design of the optical landscape. Anyway the field scattered by particles gives rise to additional optical forces. This effect is called optical binding and the particles due to their mutual forces induced by light may create an optically bound structure. We study these scattering and forces numerically by the Coupled dipole method (CDM). With this method we can model groups of arbitrary shaped particles of different composition even inhomogeneous. We are interested in asymmetric configurations because the net force acting on the particles is nonzero despite of the group of particles remains optically bound. By these means we offer a new sorting mechanism based on asymmetric scattering of optically bound particles.

In the phenomenon known as optical binding, optical fields induce significant forces between microparticles of dielectric matter. Most experimental studies have centered on particles of spherical morphology, assumed to be isotropic and able to tumble freely in a fluid. However, when birefringent micro-crystals and anisotropic nanoparticles such as carbon nanotubes are held in an optical trap, it is essential to account for their orientation. These particles are susceptible not only to optical forces but also torques, and there is considerable interest in their response to light that conveys angular momentum - especially optical vortices. Before the full effects of such interactions can be fully understood, however, it is necessary to cultivate a thorough understanding of the rotational effects that operate in optical binding with conventional laser radiation. Here, the orienting effect of the radiation on each individual particle, as well as the orienting influences they exert on each other, need robust theory to account for partial alignment with the throughput radiation. The aim of this paper is to develop, from results based on quantum electrodynamics and perturbation theory, analytical expressions for the observables associated with pair-wise optical binding in anisotropic, non-polar particles. The intricacies of weighted rotational averaging and tensor analysis are tackled, deploying newly devised methods to resolve results into forms amenable to experimental application. Analyzing the resulting equations allows the identification of terms corresponding to specific properties of the considered particles, including terms reflecting the degree of anisotropy. It is then straightforward to recognize criteria for the validity of commonly held approximations.

Optical fields can exert forces of chiral nature on molecules and nanoparticles, which would prove extremely valuable in the separation of enantiomers with pharmaceutical applications, yet it is inherently complex, and the varied frameworks used in the literature further complicate the theoretical understanding. This paper unifies existing approaches used to describe dipolar optical forces and introduces a symmetry-based “force basis” consisting of twelve vector fields, each weighted by particle-specific coefficients, for a streamlined description of force patterns. The approach is rigorously applied to evanescent waves and dielectric nanofibers, yielding concise analytical expressions for optical forces. Through this, we identify optimal strategies for enantiomer separation, offering invaluable guidance for future experiments. Published by the American Physical Society 2024

Optical gradient forces between monolayer infinite-width graphene sheets as well as single-mode graphene nanoribbon pairs of graphene surface plasmons (GSPs) at mid-infrared frequencies were theoretically investigated. Although owing to the strongly enhanced optical field, the normalized optical force, fn, can reach 50 nN/μm/mW, which is the largest fn as we know, the propagation loss is also large. But we found that by changing the chemical potential of graphene, fn and the optical propagation loss can be balanced. The total optical force acted on the nanoribbon waveguides can thus enhance more than 1 order of magnitude than that in metallic surface plasmons (MSPs) waveguides with the same length and the loss can be lower. Owing to the enhanced optical force and the significant neff tuning by varying the chemical potential of graphene, we also propose an ultra-compact phase shifter.

We demonstrate nanometer precision manipulation of multiple nanoparticles at room temperature. This is achieved using the optical binding force, which has been assumed to be weak compared to the optical gradient and scattering forces. We show that trapping by the optical binding force can be over 20 times stronger than by the gradient force and leads to ultrastable, rigid configurations of multiple nanoparticles free in solution - a realization of "optical matter." In addition, we demonstrate a novel trapping scheme where even smaller nanoparticles are trapped between larger "anchor" particles. Optical binding opens the door for the observation of collective phenomena of nanoparticles and the design of new materials and devices made from optical matter.

Here, a method to eliminate the trade-off between quality factor (Q-factor) and sensitivity of a one dimensional slot mode photonic crystal nanobeam cavity biosensor is presented. Applied method utilizes an optomechanical feedback mechanism in order to generate transverse gradient optical forces inside the cavity. A pump mode is utilized in order to generate the optical force, triggered by intrusion of analyte into the background medium. The amount of generated force is controlled via an interference mechanism at the output realized by the feedback loop. By utilizing created optical force, slot width of the nanobeam cavity is dynamically tuned and the quality factor degradation due to the decrease in the refractive index contrast of the cavity is compensated by enhancing the field confinement inside the cavity. With the contribution of the slot width tuning to the resonant wavelength shift, sensitivity of the biosensor is increased without any degradation of the Q-factor. Numerical analyses regarding the cavity design and the elimination of trade-off are provided. Obtained results show that the both performance can be increased at the same time.

Two-fold variation over the aspect ratio of each disk and distance between disks gives rise to numerous events of avoided crossing of resonances of individual disks. For these events, the hybridized anti-bonding resonant modes can acquire a morphology close to the Mie resonant mode with the high orbital momentum of an equivalent sphere. The Q factor of such resonance can exceed the Q factor of an isolated disk by two orders of magnitude. We show that dual incoherent counterpropagating coaxial Bessel beams with power 1 m W / µ m 2 with frequency resonant to such anti-bonding Mie-like modes result in unprecedented optical binding forces up to tens of nano-Newtons for silicon micrometer-sized disks. We show also that the magnitude and sign of optical forces depend strongly on the longitudinal wave vector of the Bessel beams.

Known since Kepler's observation that a comet's tail is oriented away from the sun, radiation pressure stimulated remarkable discoveries in electromagnetism, quantum physics and relativity [1,2]. This phenomenon plays a crucial role in a variety of systems, from atomic [3-5] to astronomical [6] scales. The pressure of light is associated with the momentum of photons, and it is usually assumed that both the optical momentum and the radiation-pressure force are naturally aligned with the propagation of light, i.e., its wavevector. Here we report the direct observation of an extraordinary optical momentum and force directed perpendicular to the wavevector, and proportional to the optical spin (i.e., degree of circular polarization). Such optical force was recently predicted for evanescent waves [7] and other structured fields [8]. It can be associated with the enigmatic "spin-momentum" part of the Poynting vector, which was introduced by Belinfante in field theory 75 years ago [9-11]. We measure this unusual transverse momentum using a nano-cantilever capable of femto-Newton resolution, which is immersed in an evanescent optical field above the total-internal-reflecting glass surface. Furthermore, the transverse force we measure exhibits another polarization-dependent contribution determined by the imaginary part of the complex Poynting vector. By revealing new types of optical forces in structured fields, our experimental findings revisit fundamental momentum properties of light and bring a new twist to optomechanics.

Intense light traps and binds small particles, offering unique control to the microscopic world. With incoming illumination and radiative losses, optical forces are inherently nonconservative, thus non-Hermitian. Contrary to conventional systems, the operator governing time evolution is real and asymmetric (i.e., non-Hermitian), which inevitably yield complex eigenvalues when driven beyond the exceptional points, where light pumps in energy that eventually “melts” the light-bound structures. Surprisingly, unstable complex eigenvalues are prevalent for clusters with ~10 or more particles, and in the many-particle limit, their presence is inevitable. As such, optical forces alone fail to bind a large cluster. Our conclusion does not contradict with the observation of large optically-bound cluster in a fluid, where the ambient damping can take away the excess energy and restore the stability. The non-Hermitian theory overturns the understanding of optical trapping and binding, and unveils the critical role played by non-Hermiticity and exceptional points, paving the way for large-scale manipulation.