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Accurate single virus detection is critical for disease diagnosis and early prevention, especially in view of current pandemics. Numerous detection methods have been proposed with the single virus sensitivity, including the optical approaches and immunoassays. However, few of them hitherto have the capability of both trapping and detection of single viruses in the microchannel. Here, we report an optofluidic potential well array to trap nanoparticles stably in the flow stream. The nanoparticle is bound with single viruses and fluorescence quantum dots through an immunolabeling protocol. Single viruses can be swiftly captured in the microchannel by optical forces and imaged by a camera. The number of viruses in solution and on each particle can be quantified via image processing. Our method can trap and detect single viruses in the 1 mL serum or water in 2 h, paving an avenue for the advanced, fast, and accurate clinical diagnosis, as well as the study of virus infectivity, mutation, drug inhibition, etc. Ministry of Education (MOE) National Research Foundation (NRF) This work was supported by the Singapore National Research Foundation under the Competitive Research Program (NRFCRP13-2014-01), and the Singapore Ministry of Education (MOE) Tier 3 grant (MOE2017-T3-1-001).

We investigate gain-assisted optical forces on dye-enriched silver nanoshell in the quasi-static limit. We demonstrate the onset of nonlinear optical trapping of these resonant nanostructures in a ...

Metasurfaces become increasingly important for wavefront shaping and beam steering with high efficiency. We propose a wide-angle beam steering device based on all-dielectric metasurfaces consisting of two symmetrical blazed gratings for optical trapping of particles, which can convert the input Gaussian beam to a special beam with intensity gradient along the propagation direction. Two major types of metasurfaces (Pancharatnam-Berry phase and nanoposts) are compared, and dramatic difference is found between them, especially at oblique incidence. We show that the Pancharatnam-Berry phase-based metasurface is more tolerant to the incident angle and that significant optical gradient force can be generated over a large angle range of -50° to 50°, and the maximum attractive force is up to tens of pN/W, with a trapping range of 14 gym for particles of 2.5 μym in diameter. The proposed scheme exhibits great potential to trap a large fraction of particles floating in a microfluidic channel when the beam is dynamically steered.

We report the optical trapping of many particles with feasible laser powers by the nanostructured semiconductor-assisted(NASSCA) trapping technique. Furthermore, we have found that the random array of silicon needles with spacings substantially smaller than the nanoparticle sizes is advantageous not only for the trapping force-field enhancement but also for the realization of close-packed assembly of nanoparticles....

The ability to select, isolate, and manipulate micron-sized particles or small clusters has made optical tweezers one of the emergent tools for modern biotechnology. In conventional setups, the classification of the trapped specimen is usually achieved through the acquired image, the scattered signal, or additional information such as Raman spectroscopy. In this work, we propose a solution that uses the temporal data signal from the scattering process of the trapping laser, acquired with a quadrant photodetector. Our methodology rests on a pre-processing strategy that combines Fourier transform and principal component analysis to reduce the dimension of the data and perform relevant feature extraction. Testing a wide range of standard machine learning algorithms, it is shown that this methodology allows achieving accuracy performances around 90%, validating the concept of using the temporal dynamics of the scattering signal for the classification task. Achieved with 500 millisecond signals and leveraging on methods of low computational footprint, the results presented pave the way for the deployment of alternative and faster classification methodologies in optical trapping technologies.

This research was funded by the Ministerio de Ciencia e Innovacion de Espana (PID2019-106211RB-I00 and PID2019-105195RA-I00) and by Universidad Autonoma de Madrid and Comunidad Autonoma de Madrid (SI1/PJI/2019-00052). D.L. acknowledges a scholarship from the China Scholarship Council (No. 201808350097)., In recent years, optically trapped luminescent particles have emerged as a reliable probe for contactless thermal sensing because of the dependence of their luminescence on environmental conditions. Although the temperature effect in the optical trapping stability has not always been the object of study, the optical trapping of micro/nanoparticles above room temperature is hindered by disturbances caused by temperature increments of even a few degrees in the Brownian motion that may lead to the release of the particle from the trap. In this report, we summarize recent experimental results on thermal sensing experiments in which micro/nanoparticles are used as probes with the aim of providing the contemporary state of the art about temperature effects in the stability of potential trapping processes., Spanish Government PID2019-106211RB-I00 PID2019-105195RA-I00, Universidad Autonoma de Madrid and Comunidad Autonoma de Madrid SI1/PJI/2019-00052, China Scholarship Council 201808350097

ispartof: ACS PHOTONICS vol:8 issue:6 pages:1832-1839 status: published

Optical trapping of (sub)micron-sized particles is broadly employed in nanoscience and engineering. The materials commonly employed for these particles, however, have physical properties that limit the transfer of linear or angular momentum (or both). This reduces the magnitude of forces and torques, and the spatiotemporal resolution, achievable in linear and angular traps. Here, we overcome these limitations through the use of single-crystal rutile TiO 2 , which has an exceptionally large optical birefringence, a high index of refraction, good chemical stability, and is amenable to geometric control at the nanoscale. We show that rutile TiO 2 nanocylinders form powerful joint force and torque transducers in aqueous environments by using only moderate laser powers to apply nN·nm torques at kHz rotational frequencies to tightly trapped particles. In doing so, we demonstrate how rutile TiO 2 nanocylinders outperform other materials and offer unprecedented opportunities to expand the control of optical force and torque at the nanoscale.

Enabled by multiple optical traps, holographic optical tweezers can manipulate multiple particles in parallel flexibly. Spatial light modulators are widely used in holographic optical tweezers, in which Gaussian point (GP) trap arrays or special mode optical trap arrays including optical vortex (OV) arrays, perfect vortex (PV) arrays, and Airy beam arrays, etc., can be generated by addressing various phase holograms. However, the optical traps in these arrays are almost all of the same type. Here, we propose a new method for generating a hybrid optical trap array (HOTA), where optical traps such as GPs, OVs, PVs, and Airy beams in the focal plane are combined arbitrarily. Also, the axial position and peak intensity of each them can be adjusted independently. The energy efficiency of this method is theoretically studied, while different micro-manipulations on multiple particles have been realized with the support of HOTA experimentally. The proposed method expands holographic optical tweezers’ capabilities and provides a new possibility of multi-functional optical micro-manipulation.

Here we numerically and experimentally studied the optical trapping on a microsphere from an axicon lensed fiber (ALF). The optical force from the fiber with different tapered lengths and by incident light at different wavelengths is calculated. Numerically, the microsphere can be trapped by the fiber with tapered outline y=±x/0.5 and y=±x at a short incident wavelength of 900 nm. While for the fiber with tapered outline y=±x/2, the microsphere can be trapped by the light with longer wavelength of 1100 nm, 1300 nm, or 1500 nm. The optical trapping to a polystyrene microsphere is experimentally demonstrated in a microfluidic channel and the corresponding optical force is derived according to the fluid flow speed. This study can provide a guidance for future tapered fibre design for optical trapping to microspheres.

Thermal control of liquids with high (micrometric) spatial resolution is required for advanced research such as single molecule/cell studies (where temperature is a key factor) or for the development of advanced microfluidic devices (based on the creation of thermal gradients at the microscale). Local and remote heating of liquids is easily achieved by focusing a laser beam with wavelength adjusted to absorption bands of the liquid medium or of the embedded colloidal absorbers. The opposite effect, that is highly localized cooling, is much more difficult to achieve. It requires the use of a refrigerating micro-/nanoparticle which should overcome the intrinsic liquid heating. Remote monitoring of such localized cooling, typically of a few degrees, is even more challenging. In this work, a solution to both problems is provided. Remote cooling in D2O is achieved via anti-Stokes emission by using an optically driven ytterbium-doped NaYF4 microparticle. Simultaneously, the magnitude of cooling is determined by mechanical thermometry based on the analysis of the spinning dynamics of the same NaYF4 microparticle. The angular deceleration of the NaYF4 particle, caused by the cooling-induced increase of medium viscosity, reveals liquid refrigeration by over −6 K below ambient conditions, This work was supported by the Ministerio de Ciencia e Innovación de España (PID2019-106211RB-I00 and PID2019-105195RA-I00) and by Universidad Autónoma de Madrid and Comunidad Autónoma de Madrid (SI1/PJI/2019-00052). E.O.R gratefully acknowledges the financial support provided by the Spanish Ministerio de Universidades, through the FPU program (FPU19/04803). K.P. acknowledges financial support from NCN, Poland, grant number 2018/31/D/ST5/01328

Hypertrophic cardiomyopathies (HCMs) are the leading cause of acute cardiac failure in young individuals. Over 300 mutations throughout β-cardiac myosin, including in the motor domain, are associated with HCM. A β-cardiac myosin motor mutation (R712L) leads to a severe form of HCM. Actin-gliding motility of R712L-myosin is inhibited, despite near-normal ATPase kinetics. By optical trapping, the working stroke of R712L-myosin was decreased 4-fold, but actin-attachment durations were normal. A prevalent hypothesis that HCM mutants are hypercontractile is thus not universal. R712 is adjacent to the binding site of the heart failure drug omecamtiv mecarbil (OM). OM suppresses the working stroke of normal β-cardiac myosin, but remarkably, OM rescues the R712L-myosin working stroke. Using a flow chamber to interrogate a single molecule during buffer exchange, we found OM rescue to be reversible. Thus, the R712L mutation uncouples lever arm rotation from ATPase activity and this inhibition is rescued by OM.

As water is normally used as the immersion medium in optically trapped microsphere microscopy, the high-refractive-index barium titanate glass (BTG) microsphere shows a better imaging performance than the low-index polystyrene (PS) or melamine formaldehyde (MF) microsphere, but it is difficult to be trapped by single-beam optical trapping due to its overly high refractive index. In this study, coated BTG microspheres with a PS coating have been computationally explored for the combination of optical trapping with microsphere-assisted microscopy. The PS coating thickness affects both the optical trapping efficiency and photonic nanojet (PNJ) property of the coated BTG sphere. Compared to the uncoated BTG sphere, the coated BTG sphere with a proper PS coating thickness has a highly improved trapping efficiency which enables single-beam optical trapping, and a better PNJ with a higher optical intensity Imax and a narrower full width at half maximum (FWHM) corresponding to better imaging performance. These coated BTG spheres also have an advantage in trapping efficiency and imaging performance over conventional PS and MF spheres. The coated BTG microsphere is highly desirable for optically trapped microsphere super-resolution microscopy and potentially beneficial to other research areas, such as nanoparticle detection.

Optical trapping and laser interferometry enable the non-invasive manipulation of colloids, which can be used to investigate the microscopic mechanics of surrounding media or bound macromolecules. For efficient trapping and precise tracking, the sample media must ideally be homogeneous and quiescent whereas such conditions are usually not satisfied in vivo in living cells. In order to investigate mechanics of the living-cell interior, we introduced (1) the in-situ calibration of optical trapping and laser interferometry, and (2) 3-D feedback control of a sample stage to stably track a colloidal particle. Investigating systematic errors that appear owing to sample heterogeneity and focal offsets of a trapping laser relative to the colloidal probe, we provide several important caveats for conducting precise optical micromanipulation in living cells. On the basis of this study, we further improved the performance of the techniques to be used in cells, by optimizing the position sensitivity of laser interferometry and the stability of the feedback simultaneously.

Microsphere-assisted microscopy serves as an effective super-resolution technique in biological observations and nanostructure detections, and optical trapping is widely used for the manipulation of small particles like microspheres. In this study, we focus on the selection of microsphere types for the combination of the optical trapping and the super-resolution microsphere-assisted microscopy, by considering the optical trapping performances and the super-resolution imaging ability of index-different microspheres in water simultaneously. Finally, the polystyrene (PS) sphere and the melamine formaldehyde (MF) sphere have been selected from four typical index-different microspheres normally used in microsphere-assisted microscopy. In experiments, the optically trapped PS/MF microsphere in water has been used to achieve super-resolution imaging of a 139 nm line-width silicon nanostructure grating under white light illumination. The image quality and the magnification factor are affected by the refractive index contrast between the microspheres and the immersion medium, and the difference of image quality is partly explained by the photonic nanojet. This work guides us in selecting proper microspheres, and also provides a label-free super-resolution imaging technique in many research fields.

The ability to apply force and torque directly to micro-and nanoscale particles without contact in optical traps has generated wide range of applications in scientific research and engineering. However, most of the particles for such optical manipulations are comprised of a single material that is chosen from the library of naturally available materials. Hence, the design and performance of the particles have been severely constrained by the narrow range of the given set of physical and chemical properties. Here, we overcome these limitations by implementing the concept of dielectric multilayer metamaterial based on the effective medium theory. It allows to obtain a designed combination of refractive index and birefringence suitable for each specific need, by choosing different consisting materials and changing the relative composition ratio of them. Aiming for being highly birefringent yet easily trap-pable and chemically stable, we have designed and fabricated highly uniform Nb 2 O 5 /SiO 2-multilayered and square cuboid-shaped nanoparticles. The resulting maximum birefringence is similar to that of calcite CaCO 3 crystal while the refractive index can be as low as that of other common dielectric probes such as silica and polystyrene. These desired properties are successfully demonstrated by tight 3D-trapping and generation of ∼16 nN·nm torque and ∼5 kHz rotation frequency at a ∼100 mW single-beam optical trap in water. This extension to the library of optically trappable materials beyond the existing ones paves the way for further improvements and opportunities in advanced optical manipulation systems.

Date of Conference: 24 August-4 September 2020 Conference Name: SPIE NanoScience Engineering, 2020: Optical Trapping and Optical Micromanipulation XVII 2020 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.

We realized integrated photonics multi-waveguide devices for optical trapping and Raman spectroscopy of particles in a fluid. In these devices, multiple beams directed towards the device center lead to a local field enhancement around this center and thus counteract the effect of light concentration near the facets, which is a disadvantage of dual-waveguide traps. Thus, a trapping region is created around the center, where a single particle of a size in a wide range can be trapped and studied spectroscopically, free from the influence of surfaces. We report the design (including simulations), fabrication and performance demonstration for multi-waveguide devices, using our Si3N4 waveguiding platform as the basis. The designed ridge waveguides, optimized for trapping and Raman spectroscopy, emit narrow beams. Multiple waveguides arranged around the central microbath result from fanning out of a single input waveguide using Y-splitters. A second waveguiding layer is implemented for detection of light scattered by the trapped particle. For reliable filling of the device with sample fluid, microfluidic considerations lead to side channels of the microbath, to exploit capillary forces. The interference of the multiple beams produces an array of hot spots around the bath center, each forming a local trap. This property is clearly confirmed in the experiments and is registered in videos. We demonstrate the performance of a 2-waveguide and a 16-waveguide device, using 1 and 3 μm polystyrene beads. Study of the confined Brownian motion of the trapped beads yields experimental values of the normalized trap stiffness for the in-plane directions. The stiffness values for the 16-waveguide device are comparable to those of tightly focused Gaussian beam traps and are confirmed by our own simulations. The Raman spectra of the beads (in this work measured via an objective) show clear peaks that are characteristic of polystyrene. In the low-wavenumber range, the spectra have a background that most likely originates from the Si3N4 waveguides.

Based on the gradient force of evanescent waves in silica waveguides and add-drop micro-ring resonators, the optical trapping and manipulation of micro size particles is demonstrated in a self-locked scheme that maintains the on-resonance system even if there is a change in the ambient temperature or environment. The proposed configuration allows the trapping of particles in the high Q resonator without the need for a precise wavelength adjustment of the input signal. On the one hand, a silicon dioxide waveguide having a lower refractive index and relatively larger dimensions facilitates the coupling of the laser with a single-mode fiber. Furthermore, the experimental design of the self-locked scheme reduces the sensitivity of the ring to the environment. This combination can trap the micro size particles with a high stability while manipulating them with high accuracy.

Post-print (lokagerð höfundar), We propose an experimentally friendly scheme for trapping quasi-relativistic electrons in graphene by an electromagnetic beam with circular polarization and a spatially inhomogeneous profile with an intensity dip. The trapping is achieved due to the effect of band-gap opening outside the trapping region. The proposed mechanism allows for noninvasive electron confinement in graphene without any need for the chemical patterning of the sample or the application of metallic gates., This work was supported by the megagrant 14.Y26.31.0015 and Goszadanie no. 3.2614.2017/4.6, and GosZadanie No 3.1365.2017/4.6 and RFBR project 16-32-6012 of the Ministry of Education and Science of Russian Federation, Icelandic Research Fund, Grant No. 163082-051, and Horizon2020 project CoExAN. K.D. thanks National University of Belarus for hospitality during the work on the project.

Because inorganic nanosheets, such as clay minerals, are anisotropic, the manipulation of nanosheet orientation is an important challenge in order to realize future functional materials. In the present study, a novel methodology for nanosheet manipulation using laser radiation pressure is proposed. When a linearly polarized laser beam was used to irradiate a niobate (Nb6O 17 4- ) nanosheet colloid, the nanosheet was trapped at the focal point so that the in-plane direction of the nanosheet was oriented parallel to the propagation direction of the incident laser beam so as to minimize the scattering force. In addition, the trapped nanosheet was aligned along the polarization direction of the linearly polarized laser beam.

Background: Sickle cell disease (SCD) is prevalent in Basrah city and affects the red blood cell (RBC) deformability and thereby causes disease symptoms. Hydroxyurea (HU) is effective in reducing morbidity and mortality in SCD patients by different mechanisms. Objectives: The aim of the study was to investigate the effect of HU on RBC deformability among SCD patients by direct laser optical trapping (OT) technique. Materials and Methods: Blood samples from SCD patients and control groups were prepared in the medical laboratory of Basrah Center for Hereditary Blood Diseases and transferred into physics laboratory wherein the laser system was presented and built-in. RBCs from each sample were exposed to three different powers of laser 5, 15, and 20 mW for 15 s and then were released and followed for 2 min. The images for each trapped RBC were obtained and at relaxation sequential times. The percentage changes in the diameters of trapped RBCs were measured for control and patient groups. Results: SCD patients were divided into two groups depending on whether they were receiving HU (39 patients) or not (43 patients). They were matched with 50 healthy individuals (control) regarding age and gender. We found that all the trapped RBCs were affected during the trapping time and then returned toward near normal with some differences between the groups and according to the power used. The deformability of HU group was better and closure to the control. Conclusion: The presented laser system and OT technique with optimal power are effective to study the RBC characteristics and deformability. HU is effective in improving RBC deformability among SCD patients.

The optical trapping, sensing, and imaging of nanostructures and biological samples are research hotspots in the fields of biomedicine and nanophotonics. However, because of the diffraction limit of light, traditional optical tweezers and microscopy are difficult to use to trap and observe objects smaller than 200 nm. Near-field scanning probes, metamaterial superlenses, and photonic crystals have been designed to overcome the diffraction limit, and thus are used for nanoscale optical trapping, sensing, and imaging. Additionally, photonic nanojets that are simply generated by dielectric microspheres can break the diffraction limit and enhance optical forces, detection signals, and imaging resolution. In this review, we summarize the current types of microsphere lenses, as well as their principles and applications in nano-optical trapping, signal enhancement, and super-resolution imaging, with particular attention paid to research progress in photonic nanojets for the trapping, sensing, and imaging of biological cells and tissues.

The focusing properties of elegant third-order Hermite–Gaussian beams (TH3GBs) and the radiation forces exerted on dielectric spherical particles produced by such beams in the Rayleigh scattering regime have been theoretically studied. Numerical results indicate that the elegant TH3GBs can be used to simultaneously trap and manipulate nanosized dielectric spheres with refractive indexes lower than the surrounding medium at the focus and those with refractive indexes larger than the surrounding medium in the focal vicinity. Furthermore, by changing the radius of the beam waist, the transverse trapping range and stiffness at the focal plane can be changed.

Highly evanescent waveguides with a subwavelength core thickness present a promising lab-on-chip solution for generating nanovolume trapping sites using overlapping evanescent fields. In this work, we experimentally studied Si3N4 waveguides whose sub-wavelength cross-sections and high aspect ratios support fundamental and higher order modes at a single excitation wavelength. Due to differing modal effective indices, these co-propagating modes interfere and generate beating patterns with significant evanescent field intensity. Using near-field scanning optical microscopy (NSOM), we map the structure of these beating modes in three dimensions. Our results demonstrate the potential of NSOM to optimize waveguide design for complex field trapping devices. By reducing the in-plane width, the population of competing modes decreases, resulting in a simplified spectrum of beating modes, such that waveguides with a width of 650 nm support three modes with two observed beats. Our results demonstrate the potential of NSOM to optimize waveguide design for complex field trapping devices.

International audience

[EN] Laser tweezers afford quantum dot (QD) manipulation for use as localized emitters. Here, we demonstrate fluorescence by radiative energy transfer from optically trapped colloidal QDs (donors) to fluorescent dyes (acceptors). To this end, we synthesized silica-coated QDs of different compositions and triggered their luminescence by simultaneous trapping and two-photon excitation in a microfluidic chamber filled with dyes. This strategy produces a near-field light source with great spatial maneuverability, which can be exploited to scan nanostructures. In this regard, we demonstrate induced photoluminescence of dye-labeled cells via optically trapped silica-coated colloidal QDs placed at their vicinity. Allocating nanoscale donors at controlled distances from a cell is an attractive concept in fluorescence microscopy because it dramatically reduces the number of excited dyes, which improves resolution by preventing interferences from the whole sample, while prolonging dye luminescence lifetime due to the lower power absorbed from the QDs., H.R.-R. is supported by an FPI-UAM 2015 fellowship (BES-2009-027909). Authors acknowledge funding from the Spanish Ministry of Economy and Competitiveness through MAT2017-85617-R and MAT2015-71806-R. B.H.J. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the Maria de Maeztu (IFIMAC) and Severo Ochoa (IMDEA Nanoscience) Programmes for Units of Excellence in R&D.

We demonstrate a silicon-based wavelength-division multiplexing (WDM) tree splitter for optofluidic switching of nanoparticles in a lab-on-a-chip or nanofluidic system. The gradient force and scattering force induced by the evanescent field can, respectively, lead to trapping and transportation of colloidal polystyrene (PS) spheres directly above the waveguide. Guiding of PS into any designated branch within a cascaded tree splitter is achieved by switching of the excitation wavelength. As compared to that based on microrings, an optimized design of the reported tree splitter approach offers a number of advantages in terms of device compactness, wavelength tolerance, response speed, and trap stability, while maintaining the inherent low-loss and low-power performance features of WDM splitters. A network of such splitters can readily lead to a platform for high-throughput and large-scale particle manipulation in nanofluidic systems.

We propose an optical tweezers setup based on an annular-shaped laser beam that is efficient to trap 2.8 μ m-diameter superparamagnetic particles. The optical trapping of such particles was fully characterized, and a direct absolute comparison with a geometrical optics model was performed. With this comparison, we were able to show that light absorption by the superparamagnetic particles is negligible for our annular beam tweezers, differing from the case of conventional Gaussian beam tweezers, in which laser absorption by the beads makes stable trapping difficult. In addition, the trap stiffness of the annular beam tweezers increases with the laser power and with the bead distance from the coverslip surface. While this first result is expected and similar to that achieved for conventional Gaussian tweezers, which use ordinary dielectric beads, the second result is quite surprising and different from the ordinary case, suggesting that spherical aberration is much less important in our annular beam geometry. The results obtained here provide new insights into the development of hybrid optomagnetic tweezers, which can apply simultaneously optical and magnetic forces on the same particles.

Optical fiber tweezers have been gaining prominence in several applications in Biology and Medicine. Due to their outstanding focusing abilities, they are able to trap and manipulate microparticles, including cells, needing any physical contact and with a low degree of invasiveness to the trapped cell. Recently, we proposed a fiber tweezer configuration based on a polymeric micro-lens on the top of a single mode fiber, obtained by a self-guided photopolymerization process. This configuration is able to both trap and identify the target through the analysis of short-term portions of the back-scattered signal. In this paper, we propose a variant of this fabrication method, capable of producing more robust fiber tips, which produce stronger trapping effects on targets by as much as two to ten fold. These novel lenses maintain the capability of distinguish the different classes of trapped particles based on the back-scattered signal. This novel fabrication method consists in the introduction of a multi mode fiber section on the tip of a single mode (SM) fiber. A detailed description of how relevant fabrication parameters such as the length of the multi mode section and the photopolymerization laser power can be tuned for different purposes (e.g., microparticles trapping only, simultaneous trapping and sensing) is also provided, based on both experimental and theoretical evidences.

Manipulation of micro- and nano-sized objects with optical tweezers is a well-established, albeit still evolving technique. While many objects can be trapped directly with focused laser beam(s), for some applications indirect manipulation with tweezers-operated tools is preferred. We introduce a simple, versatile micro-tool operated with holographic optical tweezers. The 40 &micro, m long dumbbell-shaped tool, fabricated with two-photon laser 3D photolithography has two beads for efficient optical trapping and a probing spike on one end. We demonstrate fluids viscosity measurements and vibration detection as examples of possible applications.

International audience; Focal shift of radially laguerre polarized Bessel Gaussian beams induced by cosine phase masks are investigated theoretically by vector diffraction theory. Results show that when the radially polarized laguerre Bessel Gaussian beam with cosine phase plate is focused, the focal pattern differs considerably with frequency parameter (C) in the cosine function term. Increasing the value of frequency parameter in the cosine part of the phase mask, focal shift may occur, simultaneously, the focal shift direction may change. Moreover, by altering frequency parameter of the phase mask will change the energy distributions of maximum intensity peak and other small intensity peaks.

Optical Trapping and Optical Micromanipulation XV (Proceedings), ISBN:978-1-5106-2017-9, ISBN:978-1-5106-2018-6

Ponència presentada a: Unconventional Optical Imaging (Strasbourg, France: 22-26 April 2018) This paper is devoted to investigate the application of different dynamic light structures generated by a self-calibrated Liquid Crystal on Silicon (LCoS) display for microparticle manipulation. Two major studies based on implementing different DOEs, to thoroughly characterize the LCoS display and to achieve optical-inspired particle manipulation, are proposed, respectively. On the one hand, we dynamically introduced the billet-lens configuration and the micro-lens array pattern on the LCoS display, from which the self-calibration of the studied device is implemented. In this case, both the phase-voltage relation and the surface profile were determined and optimized to the optimal performance for microparticle manipulation. On the other hand, we performed the optical manipulation of microparticles by addressing configurable three-dimensional light structures obtained from different phase driven split-lens configurations initiated by the same but optimized LCoS display. Experimental results demonstrated that, by addressing certain phase distributions on the LCoS display, the microparticle can be trapped in the light cones and manipulated by providing certain continuous split-lens configurations.

This review presents an up-dated summary of the fundamentals and applications of optoelectronic photovoltaic tweezers for trapping and manipulation of nano-objects on the surface of lithium niobate crystals. It extends the contents of previous reviews to cover new topics and developments which have emerged in recent years and are marking the trends for future research. Regarding the theoretical description of photovoltaic tweezers, detailed simulations of the electrophoretic and dielectrophoretic forces acting on different crystal configurations are discussed in relation to the structure of the obtained trapping patterns. As for the experimental work, we will pay attention to the manipulation and patterning of micro-and nanoparticles that has experimented an outstanding progress and relevant applications have been reported. An additional focus is now laid on recent work about micro-droplets, which is a central topic in microfluidics and optofluidics. New developments in biology and biomedicine also constitute a relevant part of the review. Finally, some topics partially related with photovoltaic tweezers and a discussion on future prospects and challenges are included., This work was supported by MINECO, project number MAT2014-57704-C3-1-R. Alfonso Blázquez-Castro acknowledges funding under the Marie Skłodowska-Curie Action COFUND 2015 (EU project 713366–InterTalentum)

Here, we use a calibrated high gradient laser trap to directly measure the total time-averaged 3D force on a dielectric silica parti- cle in the regime of an ultrasonic standing wave. Acoustic radiation and acoustic streaming apply forces on an optically trapped particle within an acoustofluidic device. From measuring the induced displacements from the laser trap center in three dimen- sions the acoustic forces (0.2-50pN) can be calculated in dependence of the particle position and excitation frequency. Thus, the real pressure distributions within acoustofluidic devices can be mapped out. The three dimensional direct measurement, as pre- sented here, opens up the possibility to quantify so far inaccessible small scale phenomena such as the effects of: a.) local and global acoustic streaming, and b.) boundaries or close-by objects.

Ultrasonic standing waves are increasingly applied in the manipulation and sorting of micrometer-sized particles in microfluidic cells. To optimize the performance of such devices, it is essential to know the exact forces that the particles experience in the acoustic wave. Although much progress has been made via analytical and numerical modeling, the reliability of these methods relies strongly on the assumptions used, e.g. the boundary conditions. Here, we have combined an acoustic flow cell with an optical laser trap to directly measure the force on a single spherical particle in two dimensions. While performing ultrasonic frequency scans, we measured the time-averaged forces on single particles that were moved with the laser trap through the microfluidic cell. The cell including piezoelectric transducers was modeled with finite element methods. We found that the experimentally obtained forces and the derived pressure fields confirm the predictions from theory and modeling. This novel approach can now be readily expanded to other particle, chamber, and fluid regimes and opens up the possibility of studying the effects of the presence of boundaries, acoustic streaming, and non-linear fluids., Lab on a Chip, 15 (1), ISSN:1473-0197, ISSN:1473-0189

Optical trapping and manipulation using focused laser beams has emerged as a powerful tool in the biological and physical sciences. However, scaling this technique to metallic nanoparticles remains challenging due to the strong scattering force and optical heating effect. In this work, we propose a novel strategy to optically trap metallic nanoparticles even under the resonant condition using engineered optical field. The distribution of the optical forces can be tailored through optimizing the spatial distribution of a vectorial optical illumination to favour the stable trapping of a variety of metallic nanoparticles under various conditions. It is shown that this optical tweezers has the ability of generating negative scattering force and supporting stable three-dimensional trapping for gold nanoparticles at resonance while avoiding trap destabilization due to optical overheating. The technique presented in this work offers a versatile solution for trapping metallic nanoparticles and may open up new avenues for optical manipulation.

A novel photonic/plasmonic cavity based on a 1-D photonic crystal cavity vertically coupled to a plasmonic gold structure is reported. The design has been optimized to achieve an ultra-high Q/V ratio, therefore improving the light–matter interaction and making the device suitable for optical trapping applications. Accurate 3-D finite element method (FEM) simulations have been carried out to evaluate the device behavior and performance. The device shows ${\rm Q} = 2.8 \times 10^{3}$ and ${\rm V} = 4 \times 10^{-4} (\lambda/{\rm n})^{3}$ , which correspond to a ${\rm Q}/{\rm V} = 7 \times 10^{6} (\lambda/{\rm n})^{-3}$ with a resonance transmission around 50% at $\lambda_{\rm R} = 1589.62\ \hbox{nm}$ . A strong gradient of the optical energy has been observed in the metal structure at the resonance, inducing a strong optical force and allowing a single particle trapping with a diameter less than 100 nm. The device turns out very useful for novel biomedical applications, such as proteomics and oncology.

We present an optofluidic chip in polymethyl methacrylate (PMMA) that combines optical trapping of single particles with confocal Raman spectroscopy. We introduce the design of the optofluidic chip and the ray-tracing simulations combined with mathematical calculations used to determine the optical forces exerted on the particles and to model the excitation and collection of Raman scattering. The optical trapping is done using a single-beam gradient trap realized by a high numerical aperture free-form reflector, monolithically embedded in the optofluidic chip. The focused beam functions both as the excitation beam as well as the trapping beam. The embedded free-form reflector is also used to collect the Raman scattered light generated from the trapped particle. We discuss the fabrication process for the prototyping of the chip, which consists of an ultraprecision diamond turning step and a sealing step. Finally, we demonstrate the functionality of the optofluidic chip in a proof-of-concept experimental setup and trap polystyrene beads with diameters from 6 to 15 $\ \mu \text{m}$ . We characterize the maximal transverse optical trap strength in the sample flow direction using the drag force method, measuring average efficiencies that lie between 0.11 and 0.36, and perform confocal Raman measurements of these particles.

Paloma Rodríguez-Sevilla, Yuhai Zhang, Nuno de Sousa, Manuel I. Marqués, Francisco Sanz-Rodríguez, Daniel Jaque, Xiaogang Liu, Patricia Haro-González, "Microrheometric upconversion-based techniques for intracellular viscosity measurements", Optical Trapping and Optical Micromanipulation XIV, Proc. SPIE 10347, 103471S (25 August 2017); doi: 10.1117/12.2275944. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited, Proceedings Volume XIV Optical Trapping and Optical Micromanipulation Conference (San Diego, California, United States), Rheological parameters (viscosity, creep compliance and elasticity) play an important role in cell function and viability. For this reason different strategies have been developed for their study. In this work, two new microrheometric techniques are presented. Both methods take advantage of the analysis of the polarized emission of an upconverting particle to determine its orientation inside the optical trap. Upconverting particles are optical materials that are able to convert infrared radiation into visible light. Their usefulness has been further boosted by the recent demonstration of their three-dimensional control and tracking by single beam infrared optical traps. In this work it is demonstrated that optical torques are responsible of the stable orientation of the upconverting particle inside the trap. Moreover, numerical calculations and experimental data allowed to use the rotation dynamics of the optically trapped upconverting particle for environmental sensing. In particular, the cytoplasm viscosity could be measured by using the rotation time and thermal fluctuations of an intracellular optically trapped upconverting particle, by means of the two previously mentioned microrheometric techniques, This work was supported by the Spanish Ministerio de Educación y Ciencia (MAT2016-75362-C3-1-R) and by COST Action 1403. P.H.G. thanks the Spanish Ministerio de Economía y Competitividad (MINECO) for the Juan de la Cierva- Incorporación program. P.R.S. thanks MINECO and the Fondo Social Europeo (FSE) for the “Promoción del talento y su Empleabilidad en I+D+i” statal program (BES-2014-069410). M.I.M. acknowledges the financial support from MINECO Grant FIS2015-69295-C3-3-P and from the María de Maeztu Programme for Units of Excellence in R&D (Grant MDM-2014-0377). N.d.S. thanks MINECO for grants FIS2012-36113-C03, FIS2015-69295-C3-3-P, and MAT2014-58860-P and the Comunidad de Madrid (Contract No. S2013/MIT-2740). J. R. Procopio and M. T. Sevilla are acknowledged for material support. A. García-Martín is gratefully acknowledged for supporting computing facilities

The ability to manipulate and sense biological molecules is important in many life science domains, such as single-molecule biophysics, the development of new drugs and cancer detection. Although the manipulation of biological matter at the nanoscale continues to be a challenge, several types of nanotweezers based on different technologies have recently been demonstrated to address this challenge. In particular, photonic and plasmonic nanotweezers are attracting a strong research effort especially because they are efficient and stable, they offer fast response time, and avoid any direct physical contact with the target object to be trapped, thus preventing its disruption or damage. In this paper, we critically review photonic and plasmonic resonant technologies for biomolecule trapping, manipulation, and sensing at the nanoscale, with a special emphasis on hybrid photonic/plasmonic nanodevices allowing a very strong light–matter interaction. The state-of-the-art of competing technologies, e.g., electronic, magnetic, acoustic and carbon nanotube-based nanotweezers, and a description of their applications are also included.

Interest in microfluidics is rapidly expanding and the use of microchips as miniature chemical reactors is increasingly common. Microfluidic channels are now complex and combine several functions on a single chip. Fluid flow details are important but relatively few experimental methods are available to probe the flow in confined geometry. We use optical trapping of a small dielectric particle to probe the fluid flow. A highly focused laser beam attracts particles suspended in a liquid to its focal point. A particle can be trapped and then repositioned. From the displacement of the trapped particle away from its equilibrium position one estimates the external force acting on the particle. The stiffness (spring constant) of the optical trap is low thus making it a sensitive force measuring device. Rather than using the optical trap to position and release a particle for independent velocimetry measurement, we map the fluid flow by measuring the hydrodynamic force acting on a trapped particle. The flow rate of a dilute aqueous electrolyte flowing through a plastic microchannel (W × H × L = 5 mm × 0.4 mm × 50 mm) was mapped using a small silica particle (1 μm diameter). The fluid velocity profile obtained experimentally is in very good agreement with the theoretical prediction. Our flow mapping approach is time efficient, reliable and can be used in low-opacity suspensions flowing in microchannels of various geometries.

Optical trapping and manipulation of objects down to the ˚Angstrom level has revolutionized research at the smallest scales in all natural sciences. The flexibility of optical trapping methods facilitates real-time monitoring of the dynamics of biological processes in model systems and even in living cells. Different optical trapping and manipulation approaches allow displacement of nanostructures with subnanometer precision and force measurements with femtonewton precision. Due to inherent constraints of optical methods, most optical trapping experiments are performed in water or simple aqueous solutions. However, in recent years, there is an ever-growing interest of shiftingfromsimpleaqueousmediatowardsmorebiologically-relevantmedia. Precise optical trapping and manipulation, combined with state-of-the-art microfabrication, will enable the development of microrobotic “surgeons” with tremendous potential for biomedical and microengineering applications. This review introduces the basics of optical trapping and discusses its applications for biological samples, with focus on trapping in biological media and strategies for overcoming the challenges of optical manipulation in complex environments as a stepping-stone for microrobotic “surgeons.”

8 pages; International audience; Radiation pressure forces from a moderately focused vertical laser beam are used to levitate transparent particles, a few micrometers in size. Having recalled basic results about levitation of spheres, and applications to long-working distance trapping, we turn to ellipsoid-shaped particles. Experiments are carried out with polystyrene particles, inside a glass chamber filled with water. The particles are lifted up to contact with the chamber top surface. We examine particle equilibrium in such conditions and show that the system ''bifurcates'' between static on-axis equilibrium with short ellipsoids, to sustained oscillations with longer ones. A similar Hopf bifurcation is found using a simple ray-optics model of the laser-ellipsoid interaction, providing a qualitative account of the observed oscillations.

We investigate the manipulation of microscopic and nanoscopic particles using the evanescent optical field surrounding an optical fiber that is tapered to a micron-scale diameter, and propose that this scheme could be used to discriminate between, and thereby sort, metallic nanoparticles. First we show experimentally the concept of the transport of micron-sized spheres along a tapered fiber and measure the particle velocity. Having demonstrated the principle we then consider theoretically the application to the optical trapping and guiding of metallic nanoparticles, where the presence of a plasmon resonance is used to enhance optical forces. We show that the dynamics of the nanoparticles trapped by the evanescent field can be controlled by the state of polarization of the fiber mode, and by using more than one wavelength differently detuned from the nanoparticle plasmon resonance. Such a scheme could potentially be used for selectively trapping and transporting nano- or microscopic material from a polydisperse suspension.

Optical microcavities that confine light in high-Q resonance promise all of the capabilities required for a successful next-generation microsystem biodetection technology. Label-free detection down to single molecules as well as operation in aqueous environments can be integrated cost-effectively on microchips, together with other photonic components, as well as electronic ones. We provide a comprehensive review of the sensing mechanisms utilized in this emerging field, their physics, engineering and material science aspects, and their application to nanoparticle analysis and biomolecular detection. We survey the most recent developments such as the use of mode splitting for self-referenced measurements, plasmonic nanoantennas for signal enhancements, the use of optical force for nanoparticle manipulation as well as the design of active devices for ultra-sensitive detection. Furthermore, we provide an outlook on the exciting capabilities of functionalized high-Q microcavities in the life sciences.

Optoelectronic tweezers (OET) are a promising technique for the realization of reconfigurable systems suitable to trap and manipulate microparticles. In particular, dielectrophoretic (DEP) forces produced by OET represent a valid alternative to micro-fabricated metal electrodes, as strong and spatially reconfigurable electrical fields can be induced in a photoconductive layer by means of light-driven phenomena. In this paper we report, and compare with the experimental data, the results obtained by analyzing the spatial configurations of the DEP-forces produced by a 532 nm laser beam, with Gaussian intensity distribution, impinging on a Fe-doped Lithium Niobate substrate. Furthermore, we also present a promising preliminary result for water-droplets trapping, which could open the way to the application of this technique to biological samples manipulation.

We investigate the use of a hybrid nanoresonator comprising a photonic crystal (PhC) cavity coupled to a plasmonic bowtie nanoantenna (BNA) for the optical trapping of nanoparticles in water. Using finite-difference time-domain simulations, we show that this structure can confine light to an extremely small volume of $${\sim } 30,000\, \hbox {nm} ^3 ({\sim } 30$$ zl) in the BNA gap whilst maintaining a high quality factor (5400–7700). The optical intensity inside the BNA gap is enhanced by a factor larger than 40 compared to when the BNA is not present above the PhC cavity. Such a device has potential applications in optical manipulation, creating high precision optical traps with an intensity gradient over a distance much smaller than the diffraction limit, potentially allowing objects to be confined to much smaller volumes and making it ideal for optical trapping of Rayleigh particles (particles much smaller than the wavelength of light).

An optical two-beam trap, composed from two counter propagating laser beams, is an interesting setup due to the ability of the system to trap, hold, and stretch soft biological objects like vesicles or single cells. Because of this functionality, the system was also named "the optical stretcher" by Jochen Guck, Josep Kas and co-workers some 15 years ago. In a favorable setup, the two opposing laser beams meet with equal intensities in the middle of a fluidic channel in which cells may flow past, be trapped, stretched, and allowed to move on, giving the promise of a high throughput device. Yet, single beam optical traps, aka optical tweezers, by far outnumber the existing optical stretchers in research labs throughout the world. The ability to easily construct an optical stretcher setup in a low-cost material would possibly imply more frequent use of the optical stretching technique. Here, we will outline the design, the production procedures, and results obtained in a fiber-based experimental setup built within an injection molded microfluidic polymer chip. The microfluidic chip is constructed with a three layer technology in which we ensure both horizontal and vertical focusing of the cells we wish to trap, thereby preventing too many cells to flow below the line of focus of the two counter propagating laser beams that are positioned perpendicular to the direction of flow of the cells. Results will be compared to that from other designs from previous work in the group.