Optical Manipulation along an Optical Axis with a Polarization

Jun 27, 2018 - (35). Here, the problem of optomechanical manipulation along the optical ... Figure 2. (a) Schematics of optical trapping setup with fl...
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Optical Manipulation along Optical Axis with Polarization Sensitive Meta-lens Hen Markovich, Ivan Shishkin, Netta Hendler, and Pavel Ginzburg Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01844 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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Optical Manipulation along Optical Axis with Polarization Sensitive Meta-lens Hen Markovich1,2, Ivan I. Shishkin1,2 , Netta Hendler1, and Pavel Ginzburg1,2,* 1 2

School of Electrical Engineering, Tel Aviv University, Tel Aviv, 69978, Israel Light-Matter Interaction Centre, Tel Aviv University, Tel Aviv, 69978, Israel

Abstract: The ability to manipulate small objects with focused laser beams opens a broad spectrum of opportunities in fundamental and applied studies, where precise control over mechanical path and stability is required. While conventional optical tweezers are based on refractive optics, developing compact trapping devices which could be integrated within fluid cell is of high demand. Here, plasmonic polarization sensitive metasurface-based lens, embedded within a fluid, is demonstrated to provide several stable trapping centers along the optical axis. The position of a particle is controlled with the polarization of the incident light, interacting with plasmonic nanoscale patch antennas, organized within overlapping Fresnel zones of the lens. While standard diffractive optical elements face challenges to trap objects in axial direction outside the depth of focus, bi-focal Fresnel meta-lens demonstrates the capability to manipulate a bead along 4 micrometers line. Additional fluorescent module, incorporated within the optical trapping setup, was implemented and enabled accurate mapping of optical potential via particle tracking algorithm. Auxiliary micro- and nano- structures, integrated within fluidic devices, provide numerous opportunities to achieve flexible optomechanical manipulation, including transport, trapping and sorting, which are highly demanded in labon-a-chip applications and many others. *[email protected]

Keywords: optomechanical manipulation, plasmonics, metasurfaces, optical tweezers Optomechanical manipulation tools, known as optical tweezes, remain the subject to both fundamental and applied studies since late 1960s 1. In the past decades they became one of the most frequently used instruments in biophysical studies, where control over mechanical motion and force

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measurements with sub-picoNewton resolution is required (e.g. 2, 3). Single-trap optical tweezers use the laser beam tightly focused by a microscope objective to a diffraction-limited spot in order to achieve trapping of microscopic objects. In the case where independent manipulation of multiple individual particles is demanded, the holographic optical tweezers (HOT) are commonly used 4. However, since HOT configurations rely on high-numerical aperture optics for traps formation, they are limited to relatively short working distances and can be used with only relatively thin substrates (not thicker than 250 micrometers), which puts limitations on their further integration directly within microfluidics. This limitation could be overcome by using Bessel beams generated by axicons, however the stable position of the trapped particle along propagation axis is defined by the balance of gravity and scattering force 5, which limits applicability of such tools for microfluidics. Approaches towards replacing bulky optical elements, employed in traditional optical tweezers designs, are under extensive continuous development. For example, diffractive and refractive optical elements obey the classical diffraction limit, which prevents creation of multiple focal spots, positioned in a close subwavelength proximity one with respect to another. As the result, optical trapping of small particles with nanoscale resolution is not possible. As a partial solution to this challenge, plasmonic tweezers, which utilize localized resonant phenomena in small noble metal particles

6

and thin films, were demonstrated to provide deep subwavelength localization with

superior trapping capabilities 7,8, including realization with nearfield probes 9, plasmon-assisted trapping using radially polarized beams 10, gold nanopillars 11, bowtie antennas 12, structured surfaces (e.g.

13 14 15 16

, , , ), metamaterials

17 18

, , and many others However, majority of existing plasmonic

tweezers realizations are static and do not allow control over the position of the particle dynamically. Another limitation (apart from being diffraction limited) of conventional optical tweezers is related to the need of high numerical aperture (NA) focusing optics, which imposes certain limitations on optical manipulation of trapped particle along the optical (axial) axis of the system. Achieving significant optical forces outside the depth of focus of high-NA trapping objective (typically in the range of 400 nm) (few techniques for depth of focus extensions have been reported, e.g. 19) becomes a challenging task and thus particles are only trapped within vicinity of the focal spot. The position of the laser focus with respect to image plane could be altered by the change of the divergence angle of the laser beam, which could be done by adding an additional lens in the laser path 20. However, such scheme does not allow fast motion of the trapped particle along the optical axis. A possible solution, which can allow overcoming the bulkiness of the optical tweezers setup, would be to use diffractive optical elements instead of refractive optics (lenses and objectives). Due to their compact size they can be easily incorporated in lab-on-chip platforms

21

. One of the frequently used approaches to

achieve lensing with flat diffractive elements is Fresnel Zone Plate (FZP). Fresnel lenses have been first demonstrated as a viable trapping platform for microfluidics in 22. The metallic FZP on the facet of optical fiber has been demonstrated as a tool for a manipulation of cells inside liquid 23. Another

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demonstration of the metallic zone plate used the Fractal Zone Plate for trapping of multiple particles in few diffractive focal orders of the lens 24. The applications of zone plates in microfluidics include individual particle and cell trapping 25 and parallel fluorescence detection from individual dye droplets 26

. It is worth noting that the zone plates can be designed to be index-matched to the used liquid,

which reduces aberrations 27. The chromatic dispersion of metallic FZP has been used to demonstrate the possibility to shift the focal point by tuning the wavelength of light

28

. Introducing additional

functionalities to diffractive flat lenses promise to enlarge their capabilities in optomechanical applications. This may be done by utilizing the concept of metasurfaces, which provides a set of design rules for engineering optical interactions on flat landscapes, e.g. capabilities, e.g.

29 30 31

, ,

(also with tuning

32

). Few of them were used to achieve polarization multiplexing in order to

reconfigure the imaging properties of patterned arrays of elliptic nanowires 33, beam shaping 34, and chromatic aberration control 35. Here the problem of optomechanical manipulation along the optical axis is solved by implementing a thin polarization sensitive meta-lens directly inside the fluid cell (Fig. 1). The lens is based on two overlapping polarization-sensitive Fresnel zone plates, implemented with the help of aperiodic arrays of carefully designed nanoantennas. The focal point position for the lens along the optical axis is controlled by the polarization of the incident laser beam. As the result, flexible manipulation between two focal points is achieved (Fig. 1).

Figure 1. An artist’s view of the proposed concept – controlling the position of optically trapped particle along the optical axis with the help of polarization of the incident laser beam, interacting with metasurface-based micro lens.

The manuscript is organized as follows – the design and characterization of polarization-sensitive meta-lens is followed by the demonstration and analysis of optical trapping capabilities, which come before the Summary section.

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Design and characterization of the meta-lens In order to achieve a flexible optomechanical manipulation along optical axis, several focal points with tunable field intensity hot spots should be created. Polarization state of the incident light provides the required degree of freedom for control over the position of a trapped object. Here, the polarization-sensitive Fresnel lens for achieving bi-focal characteristics (36 for demonstration at the GHz spectral range) is designed and implemented. The key operation concept is to create polarizationsensitive Fresnel zones, whose properties pre-define the focal spot positions, which are different for linear orthogonal polarizations of the incident illumination. For the optical realization of the concept, an array of nanoantennas was fabricated on transparent glass substrate (Fig. 2(b)). Polarization sensitivity of the whole structure is obtained by composing the corresponding Fresnel zones from elongated patches, where polarization of the incident field is collinear with the elongation axis of the nanoantennas. The radii of the zones for each polarization were chosen according to Fresnel zone equation for the zone plates with foci at 3 um and 7 um respectively. A total of 15 and 20 zones considered in the resulting structure respectively. Since radii of Fresnel zones solely define prosperities of the meta-lens, vertical and horizontal zones should overlap in a certain region in order to provide well separated focal points for orthogonal polarizations (36 for detailed discussion). Cross-shaped patches (inset to Fig. 2(b)) correspond to regions, where mutually orthogonal Fresnel zones overlap. It should be noted, that such patch-based design results in pronounced back reflection (approximately 35-40% of laser power is back reflected under a plane wave illumination), while about 5% of incident power is guided to main focal spot of the lens. In order to create the model of the structure, a square node grid with period of 500 nm was superimposed with corresponding Fresnel zones for two orthogonal polarizations. In case the coordinate of the node was falling within limits of one zone, a single patch was placed. When the node belonged to two zones for different polarizations simultaneously, the cross was placed in the node. The dimensions of individual rectangular patches after a set of numerical optimizations were chosen to be 400 nm by 100 nm with the distance between the centers of 500 nm. The numerical simulations were performed in CST Microwave Studio. The meta-lens was fabricated using electron beam lithography. A 70nm gold layer was sputtered on a patterned PMMA-coated diced fused silica wafer (1mm thick, 5nm ITO adhesion layer) followed by lift-off process. SEM image of the fabricated structure appears in Fig. 2(b). The inset demonstrates the enlarged zone, where vertical, horizontal and overlapping (cross-antennas) Fresnel zones can be clearly visible.

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Figure 2. (a) Schematics of optical trapping setup with fluorescent module. Main functional elements are indicated on the scheme. (b) SEM image of the bi-focal Fresnel meta-lens. The performance of meta-lens was evaluated as follows: the light from infrared laser (980nm) is focused on bi-focal lens from the bottom using 200mm achromatic lens. Since the laser is focused to a spot with ~ 100 µm diameter, which is roughly 2 times larger than the diameter of meta-lens (50 µm), the incident light could be considered as a plane wave in order to simplify subsequent numerical evaluation. Fig. 2(a) summarizes the layout of experimental setup. In order to assess the focusing capabilities, a 60X water immersion objective (NA = 1.3), was used to acquire the intensity distributions at different heights in order to reconstruct the field profile created by the meta-lens. The glass slide with the meta-lens was mounted on a piezo-stage and a set of images with different positions of the lens with respect to immobile collection optics (60X objective) was collected using CMOS camera (the travel range of the stage with mounted sample was -10 µm to +10 µm with respect to the collecting objective focus, with the 50nm steps between adjutant images). The image acquisition and piezo motion were controlled using custom LabView software. Real time scanning camera images appear in the online Supplementary Information. The obtained images of intensity distribution along the optical axis were stitched together and appear in Fig. 3 (a,b,e,f). A pair of well-defined focal points for orthogonal polarizations of the incident light can be clearly identified and fully correspond to the numerical full wave simulation (Fig. 3(c,d,g,h)), performed for the fabricated structure. It is worth noting that the design of the meta-lens was obtained after a set of numerical optimizations, where the quality of focal spots and their separation were used as constraints.

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Figure 3. Meta-lens performance – experiment (a,b,e,f) and numerical simulation (c,d,g,h) for both polarizations, indicated on the panels. Experimental (a,b) and numerical (d,h) reconstruction of the beam profile. Experimental (b,f) and numerical (d,h) field intensity distribution at the focal plane, parallel to the substrate. All plots are normalized to their maximum and plotted in the linear scale. Field cross-sections (b,d) and (f,h) are taken at heights of 3 and 7 µm from the meta-lens surface respectively. All planes have been cut to size of 8µm by 8µm in order to get a detailed picture of the focal spots. The optical properties of the meta-lens were assessed basing on the obtained field reconstruction. In order to calculate numerical aperture, the size of the Airy disc was extracted from the reconstructed three-dimensional intensity distribution giving values of 0.64 µm (NA = 0.7) and 0.8 µm (NA = 0.56) for 3 µm and 7 µm foci respectively. Photo-damage of the structure was observed under approximately 1 kW/cm2 illumination at the wavelengths 450-470 nm, which match with high absorption region of gold. Optical trapping performances In order to test the possibility of optomechanical manipulation dye-doped latex spheres and silica beads with 2 µm diameter were used (fluorescence of doping dye will be used hereafter for implementing particle tracking algorithm). It should be noted that in case of single beam optical trapping the gravity compensates the radiation pressure and, as the result, more stable trapping can be achieved. In order to prevent bead sticking to the lens and substrate surface, passivation using a thin layer of Bovine Serum Albumin (BSA)37 have been done before each measurement and followed by rinse with water in the end.

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Fig. 4 demonstrates the camera snapshots of the meta-lens with the particle trapped at lower (3µm, panel (a)) and upper (7µm, panel (b)) focal spots. The distances are measured from the glass slide with the meta-lens pattern. Real time camera videos are presented in online Supplementary Information. Comparison between the contrast of the trapped bead images allows to qualitatively identify positions – the collection objective is focused on the substrate and, as the result, the bead at the upper trap is more blurred. In order to confirm the possibility to manipulate particle along optical axis the trapped bead was continuously transferred between two focal points of meta-lens. The polarization of the incident light was controlled by rotation of half-wave plate. The videos of bead transfer between the traps are provided in Supplementary Information. Typical trapping experiments are performed in solutions, where multiple particles are suspended. As the result, several beads might appear in the field of view of the objective (several of them can be seen in Fig. 4). Their appearance makes quantitative optical trap stiffness measurement with the help of consecutive image processing challenging 38. A more common approach to measure trap stiffness is to use quadrant photo diode (QPD), which monitors the displacement of a particle from the trap center by measuring the shift of interference pattern of the light scattered on a particle 39. In the case of metalens (with extra-feature of polarization sensitivity), additional diffraction orders from this auxiliary structure interfere with the scattered field from the particle, making the QPD-based analysis to be less reliable. In order to surpass the beforehand mentioned challenges and achieve accurate quantitative calibration of optical trap stiffness, additional fluorescence excitation module was introduced on top of the tweezers setup (Fig. 2(a)). A broadband supercontinuum laser (SC-Pro, YSL Photonics) was filtered using tunable band-pass filter (VLF, YSL Photonics) down to ~20nm wide band (450nm-470nm). The excitation light was coupled to the beam path using a dichroic mirror (Semrock). A 200mm lens inserted in beam path of fluorescence excitation was used to create epi-illumination of the visualized area. Additional shortpass filter was used to clean up the spectrum of the excitation laser. Fluorescent images (false black and white colors) appear as insets in Fig. 4. Bright signal without significant noise background can be clearly identified. Red circles around the image indicate the physical edge of the particle, estimated with the particle tracking algorithm (to be elaborated hereafter).

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. Figure 4. Camera snapshot image of the trapped silica bead in the (a) lower focal point, (b) upper focal point. Insets – fluorescent images of trapped dye-doped latex beads. Red circles – particle’s edges, as they were estimated with the particle tracking algorithm. The trap stiffness was measured using fluorescence pattern detection. The fluorescence was collected from latex dye-doped microspheres (2 µm, PolyRed from PolySciences) excited by filtered supercontinuum laser as described above. It is worth noting that particle position could be tracked by means of holographic microscopy as well 40. The video sequence of fluorescence from trapped bead was collected and analyzed using custom software. The fluorescence intensity from a single particle in optical trap follows Gaussian distribution, which could be precisely identified after background subtraction. The center of the distribution is assumed to coincide with the center of the bead. The processing of the video sequence of trapped particle motion resulted in the particle trajectories. The probability density of the particle’s position in the trap appears in Fig. 5(b,d) for lower and upper trapping centers respectively. The projections of the probability profiles along x- and y- axis are demonstrated in panels (a,c). The probability density demonstrates isotropic behavior within a good approximation. Furthermore, the distribution profile resembles Gaussian shape, which is fully consistent with the law of large numbers.

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Figure 5. (a) Position distribution for X and Y coordinates of particle in trap at 3 µm height (a) and 7µm (c). Probability distribution for 3 um trap is given in (b), for 7 um trap in (d).

The obtained data on the position of the trapped particle was statistically analyzed using equipartition theorem. The extracted values of trap stiffness are kx = 0.18pN/µm and ky = 0.19 pN/µm for 3µm trap and kx = 0.21 pN/µm and ky = 0.32 pN/µm for 7µm trap respectively for highest measured power of trapping laser (27.6mW). A linear fit of stiffness-power curves yields power-normalized stiffnesses of 0.007 pN/µm·mW and 0.008 pN/µm·mW along x-axis and 0.009 pN/µm·mW and 0.013 pN/µm·mW along y-axis for 3 µm and 7 µm traps respectively. The obtained values here are 3-4 times smaller compared to data, reported in plates designs

36

22

. However, taking into account typical efficiencies of Fresnel zone

(2.6% and 3.4% power conversion efficiencies were shown for the zone plates with 4

and 7 zones respectively), the reported values, normalized to the conversion efficiency, are relatively high. Furthermore, they can be further improved with advanced optical designs, including antireflection coatings and others. It also worth mentioning that the additional functionality of polarization-controlled trapping along the optical axis, implemented via polarization sensitive Fresnel zone multiplexing, did not show significant efficiency degradation of trap stiffness, validating the accuracy of the design. Recent developments in all-dielectric metasurfaces (e.g. 41, 42) and, especially, demonstrations of flat extremely high NA lenses, based on all-dielectric metasurfaces

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valuable directions for additional improvements of trapping capabilities and pave ways for new flexible opto-fluidic platforms.

Outlook and Conclusion Flexible optomechanical manipulation along optical axis was demonstrated experimentally. Polarization-sensitive meta-lens, designed according to metasurfaces concept, was implemented on the inner facet of a fluid cell. A pair of stable focal points for mutually orthogonal laser polarizations were demonstrated and utilized towards microparticle tweezing. Dye-doped latex 2µm bead was shown to be continuously transferable along the optical axis over the distance of 4 microns, prevailing the depth of focus of high NA objectives by more than order of magnitude. The demonstrated solution shows pathways to achieve flexible optomechanical manipulation within a fluid cell without involving bulky optical elements, e.g. objectives, galvanoscanners and spatial light modulators. Incorporation of additional fluorescent module was implemented and allowed to perform accurate tracking of a single particle in the optical trap, enabling quantitative measurement of optical trap stiffness in cluttered surrounding environment. The concept of multifunctional fluid cell, capable to control mechanical motion of particles, molecules, cells and other objects within it via tailoring field distributions of externally injected laser radiation paves new ways towards realization of compact multifunctional opto-fluidic devices without a demand for large refractive optical elements.

Acknowledgments The authors declare no competing financial interest. The study was supported by PAZY Foundation.

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