All-dielectric silicon metalens for two-dimensional particle manipulation in optical tweezers Download: 627次
1. INTRODUCTION
Nowadays, optical tweezers—as cutting-edge technology—pave the way to new intriguing application opportunities in the fields of biophotonics and biomedical research, such as studies of cell interaction, embryology, cancer research, or molecular motor characterization [1–
In recent years, all-dielectric metasurfaces made of low-loss and high-refractive-index materials have been introduced [16–
Here we present a versatile optical tweezers setup based on transmission-type all-dielectric silicon metasurface lenses that can not only optically trap microbeads at a fixed position but also optically manipulate them without using traditional optical elements. The shaping of the intensity profile of the trapping beam by adding a spatially variant phase modulation to the incident beam is based on the Pancharatnam–Berry (PB) phase concept [22]. The abrupt phase change follows for circularly polarized light that is converted to its opposite helicity. This concept enables our device to work either as a convex or concave lens based on the used input circular polarization state [14,23,24]. We demonstrate polarization-sensitive two-dimensional (2D) drag and drop manipulation of polystyrene microbeads suspended in water. Furthermore, we expanded the concept to realize a dielectric vortex metalens, which was used to create a donut-shaped intensity distribution in the focal region without the need for an additional phase mask (q plate). Theoretical concepts for the orbital angular momentum (OAM) transfer with dielectric vortex metalenses already exist but have not yet been experimentally demonstrated [24]. In this work, we show that optically trapped particles can indeed rotate in a circular motion based on the topological charge of the helical phase front. With our approach, we demonstrate metasurface-enhanced optical tweezers, which show a high transmission efficiency with simultaneous flexibility in beam shaping that can be used for a broad range of applications in miniaturized “lab-on-a-chip-ready” systems.
2. METHODS
2.1 A. Schematic Concept, Metasurface Design, and Nanofabrication
The concept of the metalens optical tweezers is shown schematically in Fig.
Fig. 1. Schematic concept and optical characterization of the all-dielectric metalens. (a) The conceptual image illustrates the trapping of a polystyrene microbead with the help of an all-dielectric metalens, which converts an RCP incident Gaussian beam to a focused LCP beam. (b) SEM image of the fabricated metalens that consists of amorphous silicon nanofin array. The red inset shows a region at the edge of the metalens. (c) and (d) Cross sections of the intensity distribution of the focused beam along the optical axis for incident beams with different circular polarizations, drawn on a logarithmic scale. For LCP (RCP) illumination, the metalens acts as a concave (convex) lens, which results in a virtual (real) focal point at ( ). The metalens is located at . (e) and (g) Transverse intensity distributions at the virtual (real) focal point position, drawn on a linear scale. (f) and (h) Red dots: 1D intensity cross sections along the white dashed line shown in panels (e) and (g), respectively. Black dashed line presents Gaussian fits to the measured data points, which provide an FWHM of 0.9 μm in both considered cases.
To implement the metalens, we designed and fabricated a 2D circular nanofin array made of amorphous silicon. The radially changing rotation angle of the nanofins is determined by the desired PB phase modulation , such that , where stands for left or right circular polarization (LCP or RCP), is the free-space wave vector, is the distance of the nanofin from the center of the lens, and is the focal length of the metalens [13,14].
By using rigorous coupled-wave analysis (RCWA) with periodic boundary conditions, we found the optimal structure dimensions for a single nanofin with maximum efficiency of polarization conversion from one circular state of polarization to the other. Details of the design method can be found in earlier works [26,27]. Accordingly, the nanofin geometries are defined by the length of 200 nm, the width of 120 nm, and the center-to-center spacing of 360 nm [Fig.
We used three different kinds of all-dielectric silicon metasurfaces for our experimental study. A metalens, a linear phase gradient metasurface, and a vortex metalens were fabricated on a 1.1 mm thick glass substrate using silicon deposition, electron beam patterning, and reactive ion etching [28]. At first, a 600 nm thick amorphous silicon (a-Si) film was prepared through plasma-enhanced chemical vapor deposition (PECVD). Then a poly(methyl methacrylate) (PMMA) resist layer was spin-coated onto the a-Si film and baked on a hot plate at 170°C for 2 min. Next, the nanofin structures were patterned by using standard electron beam lithography (EBL). The sample was then developed in 1∶3 methyl isobutyl ketone (MIBK): isopropyl alcohol (IPA) solution and washed with IPA before being coated with a 20 nm thick chromium layer using electron beam evaporation. Thereafter, a liftoff process in acetone was executed to remove the remaining PMMA from the surface. We used inductively coupled plasma reactive ion etching (ICP-RIE) to transfer the structures from the chromium mask to silicon. After dry etching the silicon, a thin layer of chromium mask was left on top of the silicon nanofins, and we used a wet etching process to completely remove the residual chromium mask.
2.2 B. Optical Characterization of the Metalens and Vortex Metalens
Figures
The metalens diffraction efficiency is crucial for the application in optical tweezers. Note that any polarization-unconverted light (same polarization as the incident polarization state) does not carry the metalens phase information and therefore, it does not contribute to the focusing. It only increases the radiation pressure on the particle and decreases the trap efficiency. We measured the metasurface diffraction efficiency by using a metasurface diffraction grating, which is fabricated with silicon nanofin parameters identical to those of metasurface lenses on the same substrate. A conceptual schematic of the measurement is illustrated in Fig.
Fig. 2. Measurement of the metasurface diffraction efficiency and the optical characterization of the vortex metalens. (a) Schematic image of the diffraction efficiency measurement: the metasurface diffraction grating with LCP incident light deflects the RCP beam to the st order of diffraction, while the unconverted LCP part remains at the zeroth diffraction order. (b) The cross-section intensity distribution of the 1st, 0th, and st orders of diffraction for the incident LCP and RCP beams, further divided into the respective co- and cross-polarization states. (c) and (d) The cross-section intensity distributions of the beams converted by the vortex metalens along the optical axis on a logarithmic scale for better visibility. For LCP to RCP (RCP to LCP) conversion, the metalens acts as a concave (convex) vortex lens, which results in a virtual (real) focal point at ( ). The helical phase factor results in zero intensity on the optical axis in the focal region. (e) and (g) Transverse intensity distributions of the donut-shaped beam profiles at the focal point positions indicated by white arrows in panels (c), (d), drawn on a linear scale. (f) and (h) Red dots: 1D intensity cross sections along the white dashed lines shown in panels (e) and (g), respectively.
Fig. 3. Schematic illustration of the measurement setup. A nonpolarizing beam splitter (BS) is used to insert white light (WL) for sample illumination at the front side of the metasurface (MS, either metalens, vortex metalens, or metasurface diffraction grating) and to measure the incident laser power with a power meter (PM). Three different configurations can be used (blue dashed boxes). (a) Metalens-based optical tweezers system. Laser light with the desired polarization state is focused by the metalens onto the microbeads sample (S) while the focal plane of the metalens is imaged on the camera (CAM) through a microscope objective (MO) and a tube lens (TL). (b) Optical propagation measurement setup. The polarization states were separated by a polarization analyzer consisting of a quarter-wave plate (Q) and a linear polarizer (P). (c) Setup for efficiency measurement with the metasurface diffraction grating. (d) Arrangement of the MS and S in the metalens-based optical tweezers system. H, half-wave plate; CL, collimating lens; F, filter; f1, f2, and f3, lenses.
As a next step, we characterized the optical properties of the vortex metalens in the same way as for the regular metalens [Figs.
2.3 C. Experimental Setup
Next, we characterized the performance of the different fabricated metalenses for optical trapping of microbeads. The metalens optical tweezers setup is shown in Fig.
3. EXPERIMENTAL RESULTS AND DISCUSSION
For demonstrating the optical trapping, we dispersed polystyrene microbeads (Polysciences Polybeads) in purified water and loaded the resulting suspension into a concavity glass slide (cavity depth 1.2–1.5 mm), which is sealed with a 140 μm thick cover glass. In our experimental setup, the metalens sample and this cover glass were faced towards each other [Fig.
3.2 A. Metalens Optical Tweezers for 2D Particle Manipulation
For the measurement, we adjusted the metalens real focal spot in an plane where polystyrene microbeads were attracted to the cover glass surface. Such surface adhesion forces like van der Waals and electrostatic interaction forces are known as DLVO forces (named after Derjaguin, Landau, Verwey, and Overbeek) [30]. In a horizontal beam path configuration, the transverse (lateral) gradient force generated by our metalens focus was strong enough to maintain a stable trap in 2D at a laser power of 30 mW. It also stabilized the particles against the force of gravity that tried to pull the particles out of the trap in the direction. By tuning the input circular polarization, particles were either be trapped and dragged in the medium (operation as a converging metalens) or they were attracted by the surface of the cover glass (operation as a concave metalens). Therefore, we can actively tune the 2D gradient force as well as the radiation pressure using the ellipticity of the polarization state. The convex metalens for RCP incident light generated a focal spot that was smaller than the particle diameter. Therefore, the radiation pressure on the particle increased while it was also partly counteracting the attraction between particle and cover glass. For the 2D lateral trapping, the particle was attracted by the trap center and could be dragged through the solution by moving the microbeads’ glass slide sample. To drop the particle at the intended location, we had to change the input circular state of polarization, so that the metalens would now work as a concave lens and the beam would diverge. Therefore, the radiation pressure on the particle vanished, and the particle stuck to the cover glass again. For demonstration purposes, we arranged different lateral patterns in the form of the letters “M,” “E,” “T,” and “A” with different particle diameters ranging from 2.0 to 4.5 μm [Figs.
Fig. 4. Metalens for 2D polarization-sensitive drag and drop manipulation of particles. Polystyrene particles with diameters of (a) 4.5 μm, (b) 3.0 μm, (c) 2.0 μm, and (d) 4.5 μm are dispersed in water and arranged by polarization-sensitive drag and drop using the metalens. (e) Radial stiffness versus power in trapping plane for polystyrene particles with a diameter of 4.5 μm. (f) GLMT simulation for the radial stiffness of optical traps with different particle refractive indices and various particle diameters. Dashed lines indicate the experimental values of the particle size and the relative refractive index.
We evaluated the lateral trapping stiffness by the standard calibration methods that are based on the particle motion in a stationary optical trap [Fig.
3.3 B. Vortex Metalens for OAM Transfer with a Single Tailored Beam
In a second metalens optical tweezers experiment, we studied the OAM transfer from our vortex metalens to a polystyrene microbead. We rotated the optical trapping part of the setup [marked with the blue dashed box in Fig.
Fig. 5. Vortex metalens for OAM transfer. (a) A vortex metalens with a numerical aperture of is used to generate a donut-shaped intensity distribution in the focal spot region. At a particle (4.5 μm diameter) is attracted by the lateral gradient force. The OAM is transferred onto the particle, resulting in a clockwise rotational movement. Simultaneously, the particle is slowly pushed out of the trap in the axial direction. (b) Trajectory plot of the particle’s rotational movement. (c) Radial stiffness of the vortex trap versus power in trapping plane .
Next, we observed that the polystyrene bead is undergoing a rotational movement along with the vortex beam profile at 19 mW laser power, consistent with the topological charge of the beam (
4. CONCLUSION
In summary, we demonstrated efficient all-dielectric transmission-type metasurfaces made of Si nanofins for optical micromanipulation in 2D optical tweezers. We utilized the geometric PB phase to enable a switchable metalens functionality—a convex and a concave lens based on the circular input polarization. With this concept, we could realize a polarization-sensitive drag and drop manipulation of polystyrene microparticles dispersed in water at a power-normalized radial stiffness of . Furthermore, we showed the OAM transfer onto particles with the help of a vortex metalens, realizing both vortex beam generation and focusing by one single metasurface element at a radial stiffness per unit power of . Hence, no additional phase masks for beam shaping were required. Our work paves the way for future devices based on metalens optical tweezers with possible integration of electronically addressable liquid crystals to switch the polarity of the metalens and that enable fully remotely controlled lab-on-a-chip optical tweezers.
5 Acknowledgment
Acknowledgment. T.C. acknowledges the support of the Science Achievement Scholarship of Thailand (SAST). The authors acknowledge the continuous support by Cedrik Meier by providing access to the electron beam lithography system. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 724306).
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Article Outline
Teanchai Chantakit, Christian Schlickriede, Basudeb Sain, Fabian Meyer, Thomas Weiss, Nattaporn Chattham, Thomas Zentgraf. All-dielectric silicon metalens for two-dimensional particle manipulation in optical tweezers[J]. Photonics Research, 2020, 8(9): 09001435.