This PDF file contains the front matter associated with SPIE Proceedings Volume 11463, including the Title Page, Copyright information, and Table of Contents.

Many viruses contain DNA packed to such a high density that the mobility of the DNA inside the viral capsid is severely restricted, affecting the processes of DNA packaging and ejection. We study phage phi29, which uses an ATP-powered molecular motor to package DNA, by using optical tweezers to measure DNA ejection through the phi29 portal-motor channel after the removal of ATP during DNA packaging. We find that when initiated at low capsid filling levels, DNA exits faster than 10 kbp/s. When initiated at high filling levels, exit occurs with a dramatically reduced average velocity that decreases with increasing initial prohead filling. In individual exit measurements, complex dynamics and transient pausing are seen, which we attribute to the nonequilibrium DNA conformations thought to arise during DNA packaging. We also show that high concentration of Mg2+ slows exit dynamics, suggesting that the internal pressure of the confined DNA is the driving force for the ejection process.

Fundamental processes of life are carried out within cells by nanometer-scale molecular machines. Understanding how these tiny machines work reveals the basic physical underpinnings of life as well as provides opportunities for technological and medical advances. Single molecule biophysics, including optical tweezers, provides powerful experimental methods allowing us to directly observe the actions of individual molecules in real time. I will present new results from methods that combine two of the most powerful techniques: angstrom-resolution optical tweezers and single molecule fluorescence microscopy. I will describe some of the technical innovations involved in the research including tweezers stability and accuracy advances due to new acousto optic trap positioning device methods. I will then present very recent results where we have been able to perform high-resolution measurements of human telomerase protein machines extending DNA amid folding and unfolding DNA structures.

This presentation, originally published on 10 August 2020, was withdrawn per author request.

A plasmonic nanopore sensor enabling detection of bimodal optical and electrical molecular signatures was fabricated and tested for its ability to characterize low affinity ligand-receptor interactions. This plasmonic nanosensor uses a Self- Induced Back-Action (SIBA) mechanism for optical trapping to enable SIBA-Actuated Nanopore Electrophoresis (SANE) sensing through a nanopore located immediately below the optical trap volume. The ligand-receptor model consisted of a Natural Killer (NK) cell inhibitory receptor heterodimer molecule CD94/NKG2A that was synthesized to target a specific peptide-presenting Qa-1^b Qdm ligand. The latter interaction pair was used as a simplified model of lowaffinity interactions between immune cells and peptide-presenting cancer cells that occur during cancer immunotherapy. A cancer-irrelevant GroEL ligand was also targeted by the same receptor in control experiments to test for non-specific interactions. Although the analysis of different pairs of bimodal SANE sensor signatures enabled some level of discrimination between specific and non-specific interactions the separation was not complete, which suggested the need for multi-dimensional data analyses in future work. Nevertheless, the SANE sensor showed ability to quantify the fast dissociation rate (k_off) in this low-affinity model system that was previously shown to be challenging to quantify with commercial technologies. The k_off value of targeted peptide-presenting ligands is known to correlate with the subsequent activation of immune cells in vivo, suggesting the potential utility of the SANE sensor as a screening tool in cancer immunotherapy.

Single-molecule Förster resonance energy transfer (smFRET) is a powerful tool for probing nanoscale conformation and dynamics, but existing modalities have limitations. Solution-based confocal measurements have a short (~1ms), diffusion-limited observation time and extending the observation window by immobilization restricts the molecule’s translational and rotational degrees of freedom. We overcome these limitations by combining smFRET with the capability to isolate individual molecules in solution using an Anti-Brownian ELectrokinetic (ABEL) trap. Our platform, ABEL-FRET, enables photon-by-photon recording of smFRET over tens of seconds in solution and achieves near shot-noise limited resolution in FRET efficiency for short (10-30bp) DNA rulers. We further demonstrate that combining high-resolution smFRET spectroscopy with simultaneous inference of single-molecule diffusivity offers an expanded view of biomolecules and their complexes, filling a gap in the single-molecule toolkit.

The generation of localized temperature gradients is accompanied by new fundamental physics and also provides new tools for the control of molecules, particles or more complex matter in solution. We describe experiments, which use metal nano- and microstructures as optically pumped heat sources. Heat flowing from these structures along solid/liquid interfaces sets liquids into motion. With the help of such thermo-osmotic creep flows, we can trap particles and single molecules suspended in liquids without body forces but with forces balances. Also, the compression of macro-molecules becomes accessible. The inhomogeneous temperature, however, also modifies the Brownian dynamics. We report applications in the field polymer physics and protein aggregation, where such trapping techniques provide a unique new insight. We address the dynamics of heated colloids in optical tweezers with nanosecond time resolution and picometer spatial resolution to understand thermal non-equilibrium effects.

In standard optical tweezers optical forces arise from the interaction of a tightly focused laser beam with a microscopic particle. The particle is always outside the laser cavity and the incoming beam is not affected by the particle position. Here we describe an optical trapping scheme inside the cavity of a fiber laser where the laser operation is nonlinearly influenced by the displacement of trapped particle and there is a coupling between laser operation to the motion of the trapped particle and this can dramatically enhances optical tweezers action and gives rise to nonlinear feedback forces. This scheme operates using an aspheric lens at low numerical aperture (NA=0.125), NIR wavelength (λ = 1030 nm), and very low average power which results in about two orders of magnitude reduction in exposure to laser intensity compared to standard optical tweezers. Ultra-low intensity at our wavelength can grant a safe, temperature-controlled environment, away from surfaces for microfuidics manipulation of biosamples that are sensitive to light intensity. As the main advantage of our approach and highly relevant application, we observed that we can trap single yeast cells at a very low power, corresponding to an intensity of 0.036 mW μm^-2, that is more than a tenfold less intensity than standard techniques reported in the literature.

At the nanoscale, simple liquids are expected to exhibit complex fluid-mechanical behavior, including viscoelasticity and violation of the no-slip boundary condition. We have observed these phenomena experimentally by optically exciting and probing the vibrations of metal nanoparticles in liquids.

We report optical tweezers based microrheology measurements of gelatin supported deep eutectic solvent-based gels. These ionically conducting gels are intended for application in the design of flexible biosensors. Gels with 10wt% gelatin from porcine skin in a liquid mixture of choline chloride, 1,2-propanediol, and water in a 1:2:1 molar ratio showed viscosity of the order of 1.1 Pa.sec and shear modulus of greater than 100Pa. Methods included oscillating bead phase and amplitude response measurements, as well the use of particle tracking to monitor Brownian motion. The design of a temperature controlled microscope sample cell is also presented.

Time-division multiplexing presents an attractive opportunity to probe multi-colloidal interactions in optical traps at short time-scales. In this paper, we demonstrate a stroboscopic system capable of arbitrary control of multiple trapped colloids with sensing at kHz rates and validate it using several simple multi-colloidal experiments. We expect this methodology will be of benefit in the study of group colloidal hydrodynamics and systems of active colloids, particularly where a temporal sensitivity beyond that of camera-based position sensing is required. In addition, our multiplexing enables in situ calibration that is robust to environmental anomalies, shape distortions of colloids and scattering interference from other particles.

Through innovative management of light, heat and electric field in opto-thermoplasmonic fluidics, we have developed a micro/nanorobot platform for versatile manipulation of variable synthetic micro/nanoparticles and biological cells. Five manipulation modes have been achieved and can be switched on-demand. High-throughput self-navigation of micro/nanorobots has been realized with feedback control. The multimodal and nanoscale manipulation enables in situ single-cell characterizations to achieve high-resolution 3D cellular imaging and membrane protein profiling with simple and low-power optics. With the superior functionalities and user-friendliness, our micro/nanorobot technique will become a powerful tool in colloid science, life sciences, and nanotechnology.

Computational approaches to microscopy allow high frame volumetric imaging by numerical reconstructions from 2D holographic patterns that encode the full 3D structure of the scene. Furthermore, highly efficient iterative algorithms ensure quick hologram computation for the live SLM refresh in the optical trapping system. The combination of holographic optical tweezers and a 3-axis implementation of holographic microscopy in the same setup leads to simultaneous dynamical manipulation and real-time volume reconstruction of microscopic samples. We present a novel interface to holographic optical tweezers that allows for an immersive and interactive experience of micromanipulation of colloids and living cells.

We monitor the rotation of a flagellated bacterium in a single laser beam optical trap. Bacillus subtilis, a rod shaped bacterium shows both run and tumble sequences as a free swimmer, the swimming aided by flagellar rotations. We detect and characterize the changes in flagellar and cell body rotations of the bacterium through detection of the forward scattered signal using a quadrant photo-detector(QPD). Simultaneously, the rotations in trap are visualized in time via video microscopy and the rotational frequency is measured through power spectral analysis . While the body rotation results in the appearance of a peak at lower frequencies (approx. 3 Hz) against the background Lorentzian spectrum, flagella rotations result in a broad higher frequency peak (approx. 88 Hz) in the power spectrum. The resultant peaks are modeled by a solution to the Langevin equation that takes into account the drag forces acting on the system. By monitoring the flagellar rotation speed’s variation with time, through continuous frequency measurement, we are able to determine the extent of photodamage to the cell and thereby, the time it takes to completely stop the rotations. We observe periodic fluctuations in the rotation frequency of the flagella that varies between a relatively higher and a lower value, with each of them gradually decreasing with time, until no further rotations are seen. In further, measurements, look for effects on the rotating bacterium in a fluid environment with altered pH. We observe a significant increase in time before the rotation of the flagella completely stops when the pH of the suspension media is lowered. Thus, direct monitoring of the optically trapped bacterium and onset of photodamage, is enabled through sequential power spectrum recording and this can clearly reveal the response of the bacterium to environmental changes. The experimental setup offers a simple and convenient way to confining and studying a single bacterium’s response to different fluid environments in real time.

Optical Tweezers (OTs) is not only a novelty in laser physics but also an indispensable tool in cell biology studies, especially in the precise manipulation of single living cells and detection of ultra-low intercellular bio-forces. The reversible aggregation of red blood cells (RBCs) strongly influences the blood rheological properties and is critical for blood microcirculation monitoring and therapy. However, the mechanism behind and the factors influencing the aggregation dynamics are still not clear. In this study, the peculiarity of red blood cell interaction was investigated and the potential factors influencing the aggregation process were clarified by optical tweezers. The intercellular interaction force in cell pairs, the role of cell deformation and adhesion time in interaction dynamics, as well as the effects of low-level laser irradiation, were evaluated and discussed.

The ability to use a wide range of wavelengths for deep penetration is important in order to target or avoid absorption bands of the biological media. By utilizing the nonlinear optical effect in the scattering bio-soft-matter, we demonstrate the self-trapping and guiding of light in sheep red blood cell suspensions and bacterium suspensions for a range of different wavelengths. By master/slave-type coupling, biological waveguides formed at one wavelength can effectively guide a wide spectrum of light at low power. Finally, we investigate propagation and guiding of optical vortex beams in biological suspensions.

The methods used to measure force fields have not changed in the last 30 years; their disadvantages have limited the possibility of measuring nanoscopic forces in many potential applications, such as experiments with non-conservative force fields and out-of-equilibrium conditions. We propose a new powerful, simpler, robust, and faster algorithm to measure force fields, Force Reconstruction via Maximum-likelihood-estimator Analysis(FORMA). FORMA has allowed us to retrieve the conservative and non-conservative components of a force field acting on a Brownian particle from the analysis of its displacements, proving to have essential advantages over established techniques.

Piconewton scale forces can be measured with optical tweezers by monitoring the deflection of the trapping laser transmitted through a particle. This deflection is caused by a momentum transfer from the beam to the particle, measured as a change in voltage at a correctly placed position sensitive detector. To monitor these forces a conversion constant needs to be determined,which provides a mapping between units of voltage and Newtons. We propose, and experimentally verify, a new technique of detector calibration which outperforms previously developed methods. We also provide a quantitative comparison of different calibration techniques and the circumstances in which they are applicable.

Photonic Force Microscopy (PFM) uses optically trapped nanoparticles to measure forces in the sub-piconewton range. This makes it a very soft probing technique that is perfectly suited to investigate surface interactions with biological samples. In addition, PFMs can be used in scan mode to create surface-profiles with a resolution better than the optical diffraction limit. A common problem for probing techniques that operate in contact mode is sticking of the probe to the sample. To overcome this problem, we present an intermittent contact mode PFM to improve the technique’s robustness by reducing contact times and binding between sample and probe.

Light is a powerful tool to manipulate matter, but existing approaches for manipulation, such as optical traps and tweezers, usually necessitate tightly focused light to realize the stabilizing optical potential. This in turn imposes fundamental limitations on the properties of the manipulated object and the type of dynamics that can be achieved. Here, we discuss how incorporation of engineered sub-wavelength elements can remove the need for a focused optical potential. By tailoring the anisotropy of light scattering along the object’s surface, the stabilizing potential can be self-created by the object, and not by the laser beam. We outline a general formalism for the motion of an object with sub-wavelength structure in a light field, and discuss several kinds of photonic platforms that can realize such “self-stabilizing” dynamics.

For many micro- and nano-photonic applications, current 3D prototyping approaches are unable to provide the necessary resolution or material integration. Optical tweezers (OT) are a potentially attractive solution due to their ability to manipulate various small objects with high precision. Here we show a custom-built automated OT 3D assembly platform that operates with manipulation speeds up to 0.22 mm/s and positioning accuracy better than 50 nm. Furthermore, to the best of our knowledge, we assemble the largest 3D structure to date using an OT platform, consisting of several hundred objects of multiple compositions.

Current display approaches, such as VR, allow us to get a glimpse of multimodal 3D experiences, but users need to wear headsets as well as other devices in order to trick our brains into believing that the content we are seeing, hearing or feeling is real. Light-field, holographic or volumetric displays avoid the use of headsets, but they constraint the user’s ability to interact with them (e.g. content is not reachable to user’s hands, user’s constrained to specific locations) and, most importantly, still cannot simultaneously deliver sound and touch. In this talk, we will present the Multimodal Acoustic Trapping Display (MATD): a mid-air volumetric display that can simultaneously deliver visual, tactile and audio content, using phased arrays of ultrasound transducers. The MATD makes use of ultrasound to trap, quickly move and colour a small particle in mid-air, to create coloured volumetric shapes visible to our naked eyes. Making use of the pressure delivered by the ultrasound waves, the MATD can also create points of high pressure that our bare hands can feel and induce air vibrations that create audible sound. The system demonstrates particle speeds of up to 8.75 m/s and 3.75 m/s in the vertical and horizontal directions, respectively. In addition, our technique offers opportunities for non-contact, highspeed manipulation of matter, with applications in computational fabrication and biomedicine.

We combine acoustic and optical trapping in a versatile, low-cost micro-fluidic chip for contact-free manipulation and imaging of sub-millimeter sized live biological samples in liquids. Our fully reconfigurable hybrid ‘sono-optical’ device opens up for 3D patterning where ultrasound in three orthogonal directions provide confinement and alignment of the sample suspended in the resonator, and tunable holographic optical tweezers enable us to modify and refine the acoustic trapping landscape on a finer spatial scale. We can induce sustained rotations of samples, as spheroids, embryos etc., providing access to the image data required for volumetric reconstruction of the sample by diffuse optical tomography. Our approach paves the way for long-term biological studies of micro-organism, developing embryos or larvae, or of cancer spheroids and organoids, in terms of local or global mechanical probing or in terms of non-invasive 3D visual inspection.

The force exerted by electromagnetic fields is of fundamental importance in physics. Intense debates on the conventionally accepted Lorentz formulation and the recently suggested Einstein–Laub formulation still continue due to lack of experimental evidences. To distinguish these two formulations, we experimentally investigated the topological charge of optical force in a solid dielectric, and found that the force exerted by a Gaussian beam has components with topological charge of both 2 and 0, which agrees with neither the Lorentz nor Einstein–Laub formulation. Instead, we found a modified Helmholtz theory could explain our experimental results. This work not only contributes to the ultimate determination of the correct force formulation in classical electrodynamics, but also has broad and far-reaching impact on many subjects involving electromagnetic forces.

Extremely large forces and torques have been reported when light is scattered by 2-dimensional metamaterial surfaces. Potential applications include force actuators in optically driven micro-machines, and diffractive solar sails. Here we consider refractive-index patterning of a dielectric substrate achieved photo-lithographically and demonstrate, using a computational electromagnetic approach, how a periodic structure may be optimised to maximise the optical forces and torques generated. We will show that enhancements of two orders of magnitude in the optical force are available in specific cases.

The presented work will provide an overview of 3D microfabrication capabilities enabled by concurrent use of holographic optical tweezers and two-photon lithography in a single system. The hybrid nature of this technique allows for fabrication, manipulation, and assembly of microparticles in 3D. Exploiting these capabilities has enabled fabrication of some unprecedented metamaterials such as engineered microgranular crystals and lattices with embedded strain energy. We also demonstrated in situ characterization of 2PL structures by the means of optical manipulation and image processing.

We derive expressions for the energy, linear momentum, and angular momentum content of circularly polarized Laguerre-Gaussian wave-packets propagating in free space. The vectorial nature of the electromagnetic field is taken into account, and the various consequences of paraxial approximation, which is typically invoked in theoretical treatments of the Laguerre-Gaussian beams, are examined.

Micro-robotics is exploding in popularity, driven by the need to control the position of individual cells and other micronsized particles. There are many examples of optical tweezer-based micro-robots; here we introduce the first microrobotic system that relies on the related technology of optoelectronic tweezers (OET). The optoelectronic micro-robots described here are straightforward to manufacture and can be programmed to carry out sophisticated, multi-axis operations. One particularly useful program is a serial combination of “load,” “transport,” and “deliver,” which can be applied to manipulate a wide range of micron-dimension payloads. Importantly, micro-robots programmed in this manner are much gentler on fragile mammalian cells than conventional OET techniques. The micro-robotic system described here was demonstrated to be useful for single-cell isolation, clonal expansion and RNA sequencing, applications that are becoming increasingly important in the post-CRISPR life-science research landscape. We propose that the OET micro-robotic system, which can be implemented using a microscope and consumer-grade optical projector, will be useful for a wide range of applications in the life sciences and beyond.

We use optical tweezers to study the mechanic and hydrodynamic properties of micro-fabricated helices suspended in a fluid. In rigid helices we track Brownian fluctuations around mean values with a high precision and over a long observation time. Through the statistical analysis of fluctuations in translational and rotational coordinates we recover the full mobility matrix of the micro-helix including the off diagonal terms related with roto-translational coupling. Exploiting the high degree of spatial control provided by optical trapping, we can systematically study the effect of a nearby wall on the roto-translational coupling, and conclude that a rotating helical propeller moves faster near a no-slip boundary. We also study the relaxation dynamics of deformable micro-helices stretched by optical traps. We find that hydrodynamic drag only weakly depends on elongation resulting in an exponential relaxation to equilibrium.

Electrodynamic interactions among nano-particle constituents of optical matter (OM) systems (in solution) formed in optical traps can exhibit dynamics arising from asymmetric center-of-mass forces and torques due to broken symmetry or through many body interactions. This talk will describe the origins of negative torque in 3-or more nano-particle OM arrays and the spin-to-orbital angular momentum conversion that drives the mechanical motion. The scattered fields can be exploited to create self-assembling optical matter machines. Understanding these systems and their dynamics requires close interaction of specialized electrodynamics-Langevin dynamics simulations with field strength and spatial phase that match the experimental conditions.

Optically bound matter offers a route to study mesoscale electrodynamics interactions among nanoparticles. Here we report our recent work on light-driven self-organization of plasmonic nanoparticles. We observed new phenomena, such as phase transition, self-stabilization and negative torque, in optical matter systems ranging from 2 to 100 optically bound silver or gold nanoparticles. In particular, optical torque reversal can happen in stable dimers of two optically bound nanoparticles. These phenomena can be understood by electrodynamic simulations. Our results demonstrate the rich dynamics in optically bound matter.

We apply a novel optical technique – optothermally-gated photon nudging – to construct all-dielectric chiral metamaterials with silicon nanoparticles (SiNPs) and silicon nanowires (SiNWs). Through coordinating optical heating and radiation-pressure forces, a SiNP is manipulated and delivered to the vicinity of a SiNW to form chiral nanostructures. By dynamically moving the SiNP along the nanowire from one side to the other end, the chirality of the nanostructure can be tailored on-demand. Owing to simultaneous electric and magnetic resonances in dielectric nanostructures, the assembled chiral metamaterials support strong enhancement of optical near-field chirality for high-performance enantiodiscrimination of chiral molecules.

A beam of light may possess both spin and orbital angular momentum. In non-paraxial conditions part of the spin converts into orbital angular momentum through the spin-orbit angular momentum conversion phenomenon. This effect has important consequences at the nanoscale, particularly in nano-manipulation and nano-photonics. In this work, we thoroughly analyze the rotation of microscopic beads subjected to a tightly focused Laguerre-Gaussian beam. Particularly, we observe the rotation of particles along circular trajectories that will depend strongly on the combination of topological charges and the state of polarization. Based on Richard and Wolf theory for non-paraxial beam focusing, we found a very good agreement between the experimental results and the theoretical model based on calculation of the optical forces using the generalized Lorenz-Mie theory.

Transfer of spin angular momentum to birefringent materials is widely used in optical tweezers because of the vast array of applications and the ease with which it is generated. With circularly or elliptically polarized light, spin angular momentum is imparted to internally birefringent materials and objects with shape birefringence. In this work, we use polarimetry to spatially map the change in angular momentum of light traveling through birefringent objects. By directly measuring the change in polarization of light passing through materials, we can infer the transferred torque. Our objects are trapped with a linearly polarized beam at 660 nm and polarimetry is performed using a counter-propagating low-power probe beam at 633 nm. We measure six output polarizations each for a range of different input polarizations of the probe to form a polarization map. Using this technique we perform polarimetry on rhombohedral calcite crystals trapped in two distinct orientations, one face up with one side normal to the probe beam, and one corner up with the optic axis running parallel to the beam axis. The polarization changes significantly where the probe beam travels through an edge or corner of the crystal and is uniform across crystal faces. We show the differences in the polarimetry measurements between these orientations to fully understand the generated torque.

A novel approach is introduced to determine the time evolution of optical forces and torques on arbitrary shape nanostructures by combining Maxwell's stress tensor with the surface integral equation method (SIE). Conventional time averaging of Maxwell’s stress tensor allows obtaining an elegant form in terms of surface currents for the force exerted on nanostructures. Unfortunately, the information about the time dependence of the force – which can be very important in ultrafast photonics experiments and in nano-manipulation applications – is lost in such an approach. To overcome this, we have developed a time-domain method based on the inverse Fourier transform of the frequency-domain SIE. The calculations in the frequency domain allow accurately taking into account the dispersion of the permittivity function of the system and the use of surface currents enables the rigorous treatment of intricate geometries for the scatterer. Furthermore, the integration of Maxwell’s stress tensor directly on the scatterer’s boundary significantly reduces the required computation time and increases the accuracy of the method. We show quite unusual sum frequency-like terms in the dynamics of the force appearing in Maxwell’s stress tensor, which normally vanish for the time-averaged force. To illustrate this effect, we study how the pulse duration influences the dynamics of optical force in the case of a rectangular shape and Gaussian pulses illuminating thin film at normal incidence. In the framework of the developed numerical method, we study the influence of the sum-frequency-like terms on the dynamics of optical forces in the case of a spherical scatterer.

We explore a computational, finite element approach to the study of flexible particles in optical tweezers, illustrating the method with optical deformation of membranes and nanowires and an optically driven microscopic swimmer.

We study the normal fluctuations of an MCF-7 cell membrane and calibrate the optical trap to detect pitch motion to get information about the rocking motion of a birefringent particle. We could show both theoretically and experimentally that the Z power spectrum has a power-law behavior of (frequency)^−5/3, and We find that the power spectrum of slope fluctuations is proportional to (frequency)⁻¹. We could extract parameters like bending rigidity directly from the power spectrum fitting parameters in 5 Hz to 1 kHz range. Our method was powerful enough to identify pitch rotation for a spherical birefringent particle to a high resolution using optical tweezers.

In this contribution we discuss the influence of relative humidity on photoacoustic measurements from both an experimental and theoretical perspective. We present a refined model of the photoacoustic (PA) signal that accounts for elevated particle temperatures and different levels of relative humidity. We use this new model together with the photoacoustic data collected with our photothermal single-particle spectrometer (PSPS) to retrieve the mass accommodation coefficients of water on organic aerosol particles. The single-particle nature of our experiments is achieved by employing counter-propagating tweezers. Furthermore, we investigate the influence of relative humidity on the eigenfrequency of the PA cell.

In this work is presented a study of the photophoresis phenomenon induced in a group of silver microparticles suspended in distilled water. To generate this phenomenon, the light of a laser source with a wavelength of 445 nm is guided through a multimode optical fiber which is inserted inside a thin cell, where the microparticles in water are contained. Due to the high absorption by silver microparticles, an uneven uniform heat distribution is generated inside the particles and, consequently, the solvent is heated too. In such manner, the particles will experience a movement directed towards the light source. As time passes, a greater number of silver micro particles are agglomerated at the tip of the optical fiber. To observe the phenomenon in situ, a 20x microscope objective was used together with a CMOS microscope camera.

We present both the 3D trapping and manipulation of microbubbles by temperature gradients, induced by low power CW laser in absorbing liquid (ethanol). Two optical fibers were used: a multimode one for bubble generation and a single-mode one for both trapping and manipulating. One distal end of the multimode fiber was coupled to a Qswitched pulsed laser (λ=532 nm and pulse width τ_p=5 ns). The light propagates in the fiber and gets absorbed at silver nanoparticles, previously photodeposed at the other distal end, heating up the surrounding liquid and generating the microbubbles. On the other hand, a CW laser (λ = 1550 nm) was coupled to one distal end of the single-mode fiber, the other distal end was immersed in ethanol, inducing thermocapillary force, also called Marangoni force, that is the cornerstone in the trapping and manipulating of bubbles. The bubble generated on the multimode fiber travels towards the single-mode fiber by a careful switching of the temperature gradients. In addition to the Marangoni force, the microbubble immersed in ethanol suffers both drag force and buoyancy force. So, the equilibrium among these forces drives the 3D trapping and manipulation of the microbubble. To our best knowledge, this is the first time that 3D trapping and manipulation using low CW power es presented.

Thermophoresis is a phenomenon of drift motion of colloidal particles in a temperature gradient. There has been a growing interest in exploiting its potential applications in sorting, concentrating, and separation of particles and macromolecules. However, the success of such applications has been uneven because the phenomenon is difficult hard to predict quantitatively. Thermophoresis is known to be attributed by many competing for thermally driven effects, including the electroosmosis in the electric double layer, a concentration gradient of ionic species in the solvent, dispersion forces, depletion forces, and hydrogen bonding. Because the different attributes have different temporal responses, measuring their time dependence could be used to distinguish these completion attributes. This research sets out to identify different attributes by investigating the frequency dependence of the particle's drift motion in a harmonically driven temperature gradient. Measured by phoretic force spectroscopy, the results of the frequency-dependent magnitude and phase delay of the drift motion of an optically trapped particle in the harmonic temperature gradient are will be reported in this paper.

We present opto-thermoelectric speckle tweezers (OTEST) for large-scale and high-throughput trapping of particles. OTEST combine optical speckle with plasmonic substrate to generate a thermal speckle field that consists of many random thermal hotspots to trap a large number of particles using thermoelectric forces. We demonstrate trapping of dielectric and metallic particles with sizes as small as 100 nm in the speckle field. Finally, we integrate OTEST with microfluidic systems to demonstrate filtration of the smaller-sized particles from a mixed solution of 200 nm and 1 µm particles.

An original approach of highly efficient fiber-based optical tweezers using 3D printed diffractive optical elements at an optical fiber facets is presented. As an example Fresnel lens structures, with focal lengths in the range of 50 to 200 µm, are fabricated by femtosecond two photo lithography. Compared to conventional fiber tip tweezers based on chemically wet etched fiber tips, significant trapping efficiency enhancement by a factor of up to 50 is observed in both axial and transverse direction. An outlook on further concepts of enhanced optical fiber tweezers based on 3D diffractive structures will complete the presentation.

Optical trapping has become a powerful tool in numerous fields such as biology, physics, chemistry, etc. In conventional optical trapping systems, trapping and imaging share the same objective lens, confining the region of observation to the focal plane. we developed an optical tweezers system that allows for simultaneous optical trapping and imaging technique in the axial plane. The versatility and usefulness of the system in axial-plane trapping and imaging are demonstrated by investigating its trapping performance with various optical fields, including Bessel, Airy, and snake-like beams. The potential applications of the reported technique are suggested to several research fields, including optical pulling, longitudinal optical binding, tomographic phase microscopy, and super-resolution microscopy.

Within the past years, structured light has paved the way to advanced optical manipulation. Besides well-established amplitude and phase structuring, we will present the pioneering topical role of polarization modification for innovative trapping landscapes. Beyond, by combining scalar and vectorial modulation, fully-structured light becomes available including sophisticated spin and orbital energy flow topologies – thus, trapping gets full structure. Additionally, we present tight focusing as a tool for 4D light field customization, demonstrate its application as well as identification. These tailored focal fields of 3D polarization will revolutionize trapping of polarization sensitive objects, allowing for the formation of advanced functional matter.

This year joint sessions are planned between, our conference, OTOM, and a related conference on "Emerging Topics in Artificial Intelligence" (“ETAI”). It is clear that such methods must become a part of our shared conversation, as we discuss tools for designing both systems and analysis for the future. Training is required, not only Machine Learning and Deep Neural Networks, but for the students in our own research groups and beyond: if we commit to training a next generation in programmable optics, they will design hybrid approaches that will help to address many key problems. This talk aims to outline a path forward, focused on leveraging the power of our shared community, to make our own lives more productive, to reduce barriers to a broad set of students, and to empower us, together, to make a difference.

Deep learning is emerging as a powerful technique in many areas of sciences, from evolutionary biology to quantum physics. They may use artificial neural networks (ANNs) to automatically learn to identify and extract the relevant features present in an input dataset. In this paper we will describe the use of such approaches in a number of applications. We show how to recover high-resolution images from sub-sampled information in Airy beam light sheet microscopy as well as analyse and understand measurement using laser speckle. Due consideration will be given to the role of noise in datasets, limitations of these approaches and future directions

Making sense of the brain network and functions from individual neuronal activity is a challenging task. We developed a data analysis pipeline, combining unsupervised learning and supervised learning methods to reveal some of the intricacies of brain function in zebrafish. We were interested in particular to apply it to the senses of hearing, and balance. Using Optical Tweezers, we manipulated optically and individually each of the four ear stones which reside in the inner ear. Consequently, we simulated sound and acceleration in an alive zebrafish with laser beams. I will present the study of behaviour and neural responses to these stimulations.

Deep learning has emerged as a class of optimization algorithms proven to be effective for a variety of inference and decision tasks. Similar algorithms, with appropriate modifications, have also been widely adopted for computational imaging. Here, we review the basic tenets of deep learning and computational imaging, and overview recent progress in two applications: super resolution and phase retrieval.

Optical levitation of nanoscale particles promises a completely new experiment in force sensing and the foundations of quantum physics and thermodynamics. However, most of these experiments have hardly made use of the extraordinary versatility of optical micromanipulation technology. We present a novel optical holographic trapping platform that levitates a nanosphere in vacuum in a fully controllable double-well potential. We show the power and versatility of our platform by demonstrating a generalised version of Landauer’s principle, where a memory is first encoded in an out-of-equilibrium classically-squeezed state. We infer produced work and heat over a large number of repetitions of the protocols, and we observe that the energy cost to erase a memory is greatly reduced and can in principle be made negative. Our results pave the way to fully customizable vacuum optical trapping in arbitrary potentials, and opens up to the study of non-linearities in ground-state cooled particles.

Levitated nano-oscillators in vacuum are among the highest quality mechanical oscillators, and thus hold great promise for testing fundamental quantum physics, precision measurements and studies of nano thermodynamics. The aim of this work is twofold: to cool all translational and rotational degrees of freedom of levitated particle with anisotropic susceptibility and to investigate quantum physics with submicron particles. To this end, a silicon nanorod is trapped by optical tweezers in ultra-high vacuum. Due to the anisotropy of the susceptibility tensor, the nanorod has an enhanced interaction with the light field as compared to a spherical particle of the same volume. By controlling the polarization of the trapping light field, feedback will be employed to cool the librational motion of the particle. We aim to explore high mass quantum physics by looking for quantization and superposition of the angular momentum of the nanorod.

The confinement of quantum electromagnetic fluctuations between two, isotropic macroscopic objects results in a force, i.e. the Casimir force. This force depends on both the geometry and the optical properties of the materials involved. An additional effect has been predicted for optically anisotropic materials, which can cause a rotation, i.e. a Casimir torque. Here we present our recent measurements of both of these phenomena. First, I will describe our results pertaining to the Casimir force between two spheres – a geometry that has previously eluded measurement due to experimental difficulties. Second, I will discuss additional geometries including pillars and holes that are now possible with this measurement technique and why they are interesting. Finally, I will conclude with a discussion of our recent measurement of the Casimir torque.

The universal law of gravitation has undergone stringent tests for many decades over a significant range of length scales, from atomic to planetary. Of particular interest is the short distance regime, where modifications to Newtonian gravity may arise from axion-like particles or extra dimensions. We have constructed an ultra-sensitive force sensor based on optically-levitated microspheres with a force sensitivity of $10^{-17}$ N/$sqrt{rm Hz}$ for the purpose of investigating non-Newtonian forces in the 1-100 $mu$m range. Microspheres interact with a variable-density attractor mass made by alternating silicon and gold segments with periodicity of 50 $mu$m. The attractor can be located as close as 10 $mu$m from a microsphere. I describe the characterization of this system, its sensitivity, and some preliminary results. Further technological developments to reduce background are expected to provide orders of magnitude improvement in the sensitivity, probing beyond current constraints on non-Newtonian interactions.

It is well known that the miscibility or immiscibility of dissimilar liquids can be a sensitive function of external physical conditions, and fractional composition. In regions of thermodynamic instability, phase separation can often be instigated by local nucleation, in regions of the fluid where fluctuations in concentration rise above a threshold value and compositional instabilities arise. Such fluctuations, normally stochastic, are strongly influenced by a dynamic interplay of local intermolecular forces. In fluid regions illuminated by off-resonance laser light, the forces between neighbouring molecules can be significantly modified by an ‘optical binding’ effect that depends on the polarizabilities of the components. In liquid mixtures, this enables the forces between molecules of the same, or different, chemical composition to be differentially modified. In consequence, a suitably guided laser beam can locally control nucleation events, and so precipitate phase separation.

Single photon sources are desired for quantum computing and communication applications. Ideally, these sources would provide single photons on demand at the low-loss fiber optical communication wavelength of 1550 nm. Single Erbium ions provide a good candidate for such sources because they emit at the right wavelength and they are very stable, however, the challenges remain of isolating a single emitter, coupling its emission to optical fiber and enhancing its emission rate. Nanoaperture optical tweezers provide a pathway to solving these issues by trapping and identifying single emitters, enhancing their emission rate and providing efficient coupling to an optical fiber. Recently, we have made progress in each of these areas (trapping, enhancing and coupling), which will be reviewed in this talk.

When particles are held in optical tweezers, we assume that they are in thermal equilibrium. Here we show that this condition only holds for high symmetry cases, e.g. perfectly isotropic particles in unaberrated, linearly polarized Gaussian traps. We show both experimentally and theoretically that when a birefringent microsphere is held in a linearly polarised Gaussian optical trap in vacuum, spontaneous oscillations emerge that grow rapidly in amplitude and become increasingly coherent as the air pressure is reduced. Furthermore, when parametrically driven, these self-sustained oscillators exhibit an ultrahigh quality factor exceeding 200 million, which can be highly sensitive to external perturbations.

Since the development of optical tweezers in 1970, it has evolved into a sophisticated tool for the measurement of molecular-scale forces for single molecules and molecular assemblies. More recently, researchers have explored expanding the capability of these tools through the addition of a third beam capable of exciting fluorescence, performing Raman scattering, and other optical probes. The combination of force and optical spectroscopies allows for uniquely powerful insight into the structure, dynamics, and fundamental mechanisms driving molecular scale chemistry and functionality. Here, we will discuss the design of a three-beam optical tweezers instrument configured to enable excitation of plasmonic resonances in hybrid constructs comprising molecular and inorganic components, with an emphasis on enabling both force and optical actuation of conformational dynamics.

Tailored light represents a fundamental tool for complex optical tweezing and micromanipulation. Despite many successful applications, there is still unexploited potential in fully structuring amplitude, phase and polarization. This is especially true if multiple spatial degrees of freedom are combined as e.g. the transverse plane as well as longitudinal extent, leading to three-dimensional (3d) volumes. This gives easy access to versatile features as spatio-temporal singularities or singularity networks in polarization landscapes. Within this contribution, we employ two fully-structured counter-propagating light fields to enable 3d beam shaping in amplitude, phase and polarization with a sub-wavelength resolution and a dynamic exchange between phase and polarization singularities upon propagation.

Molecular self-assembly has been recognized as an important technique for organizing building blocks into ordered structures with promising applications in various fields. Spontaneous self-assembly of diphenylalanine(FF) pep- tide derivatives which have remarkable sequence similarity with Alzheimer’s Aβ peptide, give rise to micro-rods and microtubes. But, the reported structures suffer from a lack of control. Herein, we report a new technique for laser-assisted self-assembly of diphenylanaline where we obtain stable annular ring microstructures using thermo-optical tweezers. In this method, a dense aqueous dispersion of the material which has high absorption at the laser wavelength is taken in a sample holder so that some material is adsorbed on the top surface. A hot spot is created on the top surface when the adsorbed material absorbs the high intensity at the focus of the laser beam, giving rise to a water vapor bubble. Due to Gibbs Marangoni convection, self-assembly of FF peptides occurs around the bubble in the form of rings. This method of fabricating rings is fast with complete control over the spatial location, size, and thickness of the rings. Interestingly, these self-assembled ring struc- tures display wave-guiding and spectrally asymmetric Fano resonances. We demonstrate the potential of such micro-structures in sensing by changes in their Fano spectral line and wave-guiding response on binding with Congo red dye, which is commonly used to stain amyloid proteins. Such intriguing waveguiding systems may have promising applications in biological and chemical sensing, precision diagnostics of various neurodegenerative diseases and in the fabrication of multifarious micro-optical devices.

Traumatic brain injury (TBI) occurs when an external shock causes injury to the brain. The mechanism of the disease is not completely understood yet. Studies have shown that astrocytes play various roles following brain injury. However, the exact functional role of them after TBI is still a matter of debate. Laser-induced shock waves (LIS) can create a precise controllable mechanical force that is capable of injuring or lysing cells to simulate the brain injury at the cellular level. Here, we propose a system that enables us to induce injuries in CNS cells with LIS and observe the whole process under a Quantitative phase microscope (QPM). Our system is also capable of adding another laser for optically trapping the cells to keep them at a certain distance from the center of the shockwave, as this distance is one of the important factors which determines the level of injury.

Manipulation and trapping of particles have taken a huge relevance in recent years thanks to many applications with revolutionary contributions to diverse fields. Several experiments have demonstrated that thermal effects can improve the current micromanipulation techniques such as DNA manipulation or assembly of colloidal crystals. In this work, we present the effect of laser-induced thermal effects, such as convection currents and thermophoresis, on the trap stiffness (spring constant) constant of an optical trap of 3-micrometer particles suspended in water. These effects are a consequence of light absorption in a thin layer of hydrogenated amorphous silicon (a-Si:H) deposited at the bottom of the chamber which generates a thermal gradient. Since these effects (and its correspondent forces) are symmetric around the beam focus, trapped particles, experience an increment in the trapping force. Around the beam focus, the drag force associated with convective currents is directed upwards and are compensated by optical scattering force. Depending on the laser power, the trap stiffness increases significantly, so a trapped particle can be dragged along the cell (by displacing the sample and leaving the beam fixed) at velocities around 90 μm/s without escaping the trap, whereas in the absence of the a-Si:H film, the escape velocity of the particle in the trap drops to velocities around 30 μm/s. This presents a simple, yet effective, option for optical manipulation at low powers (<5 mW) and its possible applications in the manipulation of a variety of biological micro samples.

Several studies have been proposed to control particle trajectory in liquid solutions using optically induced thermal gradient. Upon introducing different solutes such as salts and surfactants along with microparticles in these solutions, an additional optically induced thermoelectric trapping force is generated due to the differential motion of ions in the solution under thermal field. As the complexity of the solution increases, it becomes increasing difficult to understand particle response towards laser irradiance. More importantly, the existing models to study the thermoelectric behavior of the particle assumes a constant temperature gradient across the particles, which becomes obsolete in the micro-regime due to discontinuity of thermal conductivity at the particle-solution interface. For a better understanding of trapping and manipulation behavior of particles under light induced thermoelectric field, the temperature gradient distortion must be considered. In this work, full-scale finiteelement solver model has been proposed to determine the temperature variation around a microparticle under laser heating. The resultant temperature distribution is utilized to numerically evaluate the thermoelectric field and the trapping potential of the laser induced opto-thermoelectric trap. To experimentally validate this methodology, polystyrene micro-particles are trapped opto-thermoelectric-ally in CTAC solution and compared the experimental trapping stiffness to theoretical estimates obtained from the model. It is observed that trapping stiffness saturates as surfactant concentration increases which can be optimized by choosing the lowest CTAC concentration at the onset of saturation. The model implemented here can be easily extended to arbitrarily shaped particles, particles with non-uniform surface morphology, different combinations of core-shell particles and electrolyte solutions, which can be implemented to study different phenomenon such as optical pulling, rotation and translation.

It is well known that thermo-optical tweezers leads to deposition of continuous patterns on substrates mediated by Marangoni convection currents around micro-bubbles. While performing the deposition, we also find that there is an accompanying emission. On first look, one would expect this to be thermal broadband emission. However, the spectrum of emission seems to start from green and extend all the way to near infra red. Such peak wavelengths would correspond to 3000 K or even higher. Generation of such high temperatures at the local hot spot would melt the glass substrate. However, such melting facets have never been seen. Thus we speculate that the emission is actually two photon fluorescence from the incident light on the deposited pattern. Soft Oxometallate material is known to exhibit photoluminescence in the green-red region of spectrum. This kind of emission is also observed in carbon nanotubes when incident with a focused 1064 nm light, the origin of which appears to be similar multiphoton fluorescence processes.

The present work shows the oscillation of a microbubble using temperature gradients. This gradient is caused by the absorption of laser light by silver nanoparticles (AgNPs) immobilized on the tip of a single-mode optical fiber FO (9/125 μm). The immobilization of these nanoparticles was performed using the technique known as photodeposition. Subsequently, the tip with the nanoparticles was immersed in ethanol. We used a infrared (λ=1550 nm) laser with fiber optic output which was controlled (modulate) with a waveform generator. When the laser pulse is at its high level, a radial temperature gradient is generated and the liquid near the tip of the optical fiber evaporates creating a microbubble. This microbubble remains attached to the face of the optical fiber due to the Marangoni force (FM) that brings it to the point of highest temperature. When the laser pulse changes to its low level, the temperature gradient disappears and the Marangoni force becomes zero. This causes the buoyancy force (FB) to become predominant driving the microbubble to the surface. However, for a new laser pulse the cycle repeats itself, keeping the microbubble oscillating within a region. As the laser modulation frequency increases the oscillation distance of the microbubble decreases.

We have developed a versatile optothermal microrobot platform that enables low-power optical manipulations of variable synthetic particles and biological cells. An Internet-based interface has been developed to allow user(s) to manipulate the microrobots from their smartphones, laptops and desktops from anywhere at any time, enabling connected workspaces for anywhere productivity. Five manipulation modes (i.e., rotating, rolling, pushing, pulling and braking) have been achieved, which can be switched on-demand for the variable tasks. The multimodal and nanoscale manipulation of the robots enables in situ single-cell characterizations to achieve three-dimensional cellular imaging and membrane protein profiling.

We show a new instability in sessile water droplets when a particle is trapped close to the edge interface of air and water by optical tweezers when the light beam heats up the glass substrate and generate thermophoretic forces that direct the particle outward from the tweezers trap. There is competition between the optical trapping and the thermophoretic forces which direct the particle away from the trap to generate this instability.

An optical system based on a photonic nanojet created by an optically trapped microsphere is proposed for enhanced trapping and remote manipulation of upconverting nanoparticles. The nanojet-induced enhancement in the optical forces acting on a 8 nm single upconverting nanoparticle are compared to those achievable by the traditional approach based on the use of high numerical aperture optics. The results presented in this work prove that this is a simple and effective method to remotely trap, manipulate, and detect single upconverting nanoparticles. This is an innovative approach to overcome the current challenges in the remote manipulation of probes in biosensing or in single particle studies.

Recently, the effect of optical nonlinearity in laser trapping has been investigated under pulsed excitation, and it was observed that the inclusion of nonlinearity significantly modulates trapping potential for metallic nanoparticles using dipole approximation. In this paper, we present theoretical studies on nonlinear laser trapping for silver nanoparticles using generalized Lorenz-Mie theory. We observe a reversal in the direction of asymmetry of potential well and splitting of the potential well due to nonlinear effects which is further modulated with an increase in laser power.

Recent theoretical and experimental results have shown how the trapping force/potential can be dramatically modulated due to optical and thermal nonlinearity. Compared with dielectrics, metals show even more interesting behavior (for example, trap-splitting, enhanced forward scattering, etc.) owing to higher-order optical nonlinearities. Hence, we present a comparison study for dielectric and metallic nanoparticles using generalized Lorenz-Mie theory.