Optical Tweezers with Metasurfaces

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Zhe Shen	--	3457	2023-08-16 07:04:18	\|
2	references update	Fanny Huang	-3 word(s)	3454	2023-08-17 08:23:58	\|

This entry is adapted from the peer-reviewed paper 10.3390/photonics10060623

Optical tweezers (OTs) have made significant progress, realizing the non-contact optical manipulation of target objects through the interaction between light and matter. In addition to trapping particles with the intensity gradient of the beam, a series of complex optical elements are required to properly modulate the beams to expand the operation of optical manipulation. The development of metasurfaces alleviates this problem. Due to the merits of miniaturization, planarization, multi-function, and integration of metasurfaces, these kinds of novel devices have been applied in OT systems.

metasurfaces optical tweezers

1. Optical Tweezers with Metasurfaces as Objective Lenses

A frequent idea has been to use metasurface devices to replace traditional objective lenses. In 2019, Z. Shen et al. made a numerical study of metasurfaces composed of cells of Si cuboid nanofins on a SiO₂ substrate ^[1]. The metasurface cells yield Pancharatnam–Berry (PB) phases with incident circularly polarized beams. Each cell acts as a half-wave plate. The phase of circularly polarized beams can be modulated according to the rotation angle about the z-axis of each cell. By arranging the nanofins on the substrate at appropriate rotation angles, the metasurface can generate the required phase response to obtain the required light field when the circularly polarized beam is incident. Instead of using a traditional objective lens, the metasurface concentrates the beam and forms the focus. Through theoretical calculation it was shown that, in the water environment, the optical forces on the Si Rayleigh particles pointed to the focal point of the outgoing beam, and stable trapping of target particles was achieved.

Further experimental work has been carried out on metasurfaces in OTs. Suwannasopon et al. also designed a metasurface on the principle of the PB phase. The metasurface was composed of periodically arranged Au nanofins ^[2]. In the experiment, a 30 mW laser with a wavelength of 1064 nm was used to irradiate the metalens, and a focus point was obtained at 800 μm. The devices and the set of the metasurface used in the experiment.

In 2021, Kunhon Shen et al. trapped nanoparticles in a vacuum environment with a metasurface device for the first time ^[3]. Instead of PB metasurfaces, they designed a metasurface insensitive to the polarization of incident beams. This metasurface modulates the beam’s phase through the accumulated phase changes when the beam passes through the cells of the metasurface. A cell provides the same phase response for two incident beams with orthogonal polarization directions (for example, two linearly polarized beams with mutually perpendicular polarization directions), and these can be called “polarization-insensitive metasurfaces”. The beam focused by the metasurface forms a stable three-dimensional (3D) potential well where SiO₂ nanoparticles with a diameter of 170 nm can be trapped. The transfer of particles was obtained between the potential wells formed by the traditional objective lens and the metasurface. In 2020, Chantakit et al. trapped nanoparticles with the help of the PB metasurface devices and arranged the particles in the pattern of letters “META” ^[4]. The operation demonstrated that OTs with metasurfaces could manipulate particles flexibly.

The works above demonstrated that metasurfaces could effectively replace traditional objective lenses and be used in OTs. The entire size of a metasurface can be reduced to tens or even a few microns. Compared with conventional lenses, these metasurfaces are remarkably compact. Based on miniaturization, OTs with metasurfaces can also trap target particles stably. Miniaturization also provides convenience for flexible manipulation of the OTs’ trapping position. The successful shifts of particles between the potential wells generated with traditional lenses and metasurfaces also prove that OTs with metasurfaces can be effectively applied with current OT systems. It should be noted that although dielectric cells have good transmissivity, the metasurface may have some disadvantages in efficiency compared with traditional objective lenses. However, the merits of metasurfaces outweigh such flaws. A typical process to manufacture a metasurface is to apply atomic layer deposition (ALD) ^[5]. This process is compatible with the manufacturing process of chips, which makes OTs with metasurfaces conducive for application in that context. Based on the works mentioned above, other kinds of metasurfaces, such as wideband achromatic metasurfaces ^[6], are also expected to be used in OTs, further enriching the development of OTs.

2. Optical Tweezers with Metasurfaces Integrating Multi-Devices

On the basis of using metasurfaces as substitutes for traditional objective lenses, researchers have further explored the application of metasurfaces in OTs. Since a metasurface is composed of many independent cells, the local phase distribution can be flexibly modulated, and multiple devices can be integrated into a single metasurface. For example, the phase distribution of a spiral phase plate and an objective lens were added onto a single metasurface ^[7]. In 2020, Chantakit et al. experimentally generated an optical vortex (OV) with metasurfaces ^[4]. Their metasurface can be regarded as combining a convex lens and a spiral phase plate. The OT can also rotate the particle along the halo in addition to trapping particles. Simulation of the generation of a focused OV with a PB metasurface was performed by Z. Shen et al. in 2021 ^[7]. The diagram of the metasurface and the cell. They generated OVs with a single metasurface through numerical calculation and comprehensively discussed the force exerted on the particles near the focal plane.

Based on device integration, further integration of multi-functions in OTs has also become possible with the assistance of metasurfaces. Splicing together two monolithic metasurfaces is the most straightforward idea. Ma et al. designed a corresponding metasurface in such a way ^[8]. The metasurface can be divided into two areas; each region independently modulated the light to form a focus or an OV and trapped or rotated particles. Tianyue Li and Xingyi Li proposed further ideas ^[9]^[10]. Researchers come first to Tianyue Li’s work. PB metasurfaces can provide two categories of independent phase modulation. One category is the accumulated propagation phase when the beam transmits through the cells of the metasurfaces. The other is the PB phase determined by the nanofins’ rotation angles. These two types of phases do not interfere with each other, so Li et al. utilized a single piece of metasurface to produce two non-interfering focuses ^[9]. A focus and an OV were generated simultaneously at different focal lengths, and the combination of OT and OS was achieved simultaneously. Xingyi Li’s team designed a metasurface that generated two different light fields with varying incident beams ^[10]. By employing the concepts of propagation phase and PB phase, when the left-handed circularly polarized beam (LCP) was incident, the light spot was observed on the focal plane and acted as an OT. A halo was observed when the right-handed circularly polarized beam (RCP) was incident, and the function of OS was achieved.

The integration advantage of metasurfaces allows their use in OTs to realize the integration of devices and even the integration of multiple functions. Such integration enables combining more devices and functions on a single metasurface, providing new possibilities for dynamic manipulation. Furthermore, it reduces the size of the already compact OTs compared with OTs using traditional objective lenses, enabling the OTs with metasurfaces to meet more requirements of the lab-on-chip. Such a feature also reduces the difficulty of optical path assembly and the requirements for optical axis alignment, and facilitates the work of the whole system.

3. Optical Tweezers with Structured Beams Generated with Metasurfaces

Thanks to the flexible discrete adjustment of the local phase distribution, metasurfaces have unique advantages in generating structured beams that require compact phase modulation. OTs with structured beams generated using metasurfaces can be viewed as a further development of the advantage of integration. Wang et al. developed a novel plasmonic OT based on a metasurface ^[11], which was able to trap the target particle precisely. The plasmonic OT traps and manipulates particles with the surface plasmon polariton (SPP). In Wang’s work, left and right-handed polarized beams are simultaneously incident on the metasurface, and the polarization-sensitive metasurface allows them to focus at different distances. Then, the silver film is irradiated, and the SPPs are produced. Finally, the light field consists of a halo and a focal point can be formed. The light spot plays a role in trapping the target particles. The surrounding halo can isolate the rest of the particles, aiming to accurately trapping the target particles.

Using the spin-decoupled phase control method, Zhu et al. designed metasurfaces from which different responses can be generated for left-handed and right-handed polarized beams ^[12]. The metasurfaces they designed can generate two focal points generated by pure radially or angular polarized beams, respectively. However, the authors only briefly discussed the possibility of trapping particles with the metasurfaces.

Kuo et al. generated more complex Airy beam OTs with a metasurface device in 2021 ^[13]. An Airy beam has the characteristic of transverse self-acceleration, allowing the beam to form a bending track. The unique bending track has outstanding merits in OTs. The nanoparticles can be trapped on the main and side lobes of the Airy beam and travel along the beam. Such a feature can be utilized for the directional transportation of particles.

Structured beams applied in OTs further demonstrate the integration capacity of metasurfaces. Compared with the conventional devices currently used, the metasurface has more freedom in generating beams by its discrete phase adjustment, polarization, and even transmittance. Meanwhile, the metasurface provides a beam modulation ability comparable to that of a traditional spatial light modulator (SLM), with micron size. Considering that an SLM is limited by the power threshold of the incident beam, metasurfaces have higher potential for application in OTs. Concerning the enrichment and development of the manipulation form and prospects for OTs brought by structured beams, such as Airy beams, it could be foreseen that these new structured beams, which have not yet been applied in this context, have great potential for expanding the functions and application space of OTs. The advantages of metasurfaces for generating complex structured beams mean that they represent a powerful tool for achieving these ideas.

4. Dynamic Manipulation with Metasurfaces in Optical Tweezers

Compared with traditional OTs, the advantage of the multi-function of the metasurface brings new potential for dynamical manipulation in OTs. The change of incident beams may control the generated light fields due to the anisotropy of the metasurface to the incident beam. The intuitive idea is to achieve focal scanning by titling the incident beams. For a traditional object lens, the outgoing beam will also tilt accordingly when the incident beam is tilted. A similar phenomenon will also occur on metasurface devices. M. X. He led the team to design the metasurface, which converted the transverse intensity distribution of the Gaussian beam into the intensity distribution along the propagation direction, forming a light field distribution sufficient to trap target particles ^[14]. The oblique scanning of the incident beam can also drive the scanning of the trapped potential well-formed, which further has the potential to manipulate particles dynamically.

The further idea is to control the light field distribution with the polarization-sensitive metasurface to achieve dynamic manipulation. Markovich et al. designed the metasurface based on the critical operation of creating polarization-sensitive Fresnel zones whose properties predefine the focal spot positions ^[15]. The metasurface generates the focus points of different focal lengths at the optical axis with the incident linearly polarized beams of different polarization angles. By rotating the polarization direction of the incident beam, the focus position can be changed to realize the dynamic manipulation of particles along the propagation direction. Xu et al. used a similar idea but used a series of metasurface groups to obtain the traverse directional transport of particles ^[16]. Their metasurface is composed of silica bands in gold film. The silica bands are divided into four groups, and the bands in each group are parallel and form a hexagon. The stripes between different groups form a certain angle to each other and rotate 0, 60, 120, and 180 degrees counterclockwise about the z-axis from left to right, respectively. When the polarization direction is parallel to the silica bands of the metasurfaces, the focus light intensity formed by transmission is the strongest. When the polarization direction of the incident light rotates, a directional moving “belt” of hot point can be obtained, and this “hot spot” plays the role of a “conveyor belt” for transporting particles.

Yin et al. utilized the method of changing the illumination configuration to obtain directional manipulation of the particles ^[17]. The metasurface was composed of a series of Au nanopillars, covered with a polydimethylsiloxane (PDMS) layer, and placed beneath the microfluidic channel. Three configurations of illumination were applied. The first two configurations use two coherent, counter-propagating beams with controlled relative phase and strength to illuminate the metasurface. These are the E-antinode (electric field antinode) and the B-antinode (magnetic field antinode), where the magnetic or electric field is zero. The third lighting configuration is a single-beam incident from the PDMS direction. The microfluidic channel has two outlets. Under three illumination configurations, the metasurface can generate focused light fields pointing to different outlets. These light fields act like pipes, guiding particles to the corresponding outlets. The illumination configurations can be artificially selected to guide particles to the desired exit. Such directional and adjustable particle transport implies the potential of the metasurface OTs for active optical sorting.

The anisotropy of metasurfaces for incident beams can produce different modulation effects when incident beams change, giving metasurfaces the advantage of multi-function. This merit is revealed by OTs that use a single metasurface to achieve dynamic manipulation perfectly. The metasurface applied in OTs can expand the degree of the freedom of beam control, and such a feature enables expansion of the freedom of dynamic manipulation. This characteristic greatly expands OTs’ simplicity and application prospects in dynamic manipulation. Dynamic manipulation based on metasurfaces can also be applied to active optical sorting. A simple change of the incident beam can realize the transfer of hot spots in the light field, allowing free control of target particles. Such features mean that OTs with metasurfaces have great potential for precise dynamic manipulation.

5. Particle Sorting in Optical Tweezers with Metasurfaces

Passive sorting through the different motion states of different particles is a simpler and more feasible approach than performing artificially active sorting through the dynamic manipulation of OTs with metasurfaces. In dynamic manipulation, it is natural to find that particles’ size and other parameters may influence their movement states. Z. Shen et al. developed a particle sorting method through a numerical study ^[18]. The structure of the metasurface is illustrated. The metasurface worked as a cylindrical lens with a phase gradient. Through simulation and theoretical calculation, they concluded that particles could be trapped at the striped focal spot. The force caused by the phase gradient drove the particles to move directionally. Particles of different radii moved at different velocities. Based on such a feature, particles can be sorted according to their sizes after traveling for some distance.

Zhang et al. adopted the methods of the “optical conveyor belt” to transmit particles ^[19], similar to the idea of Xu’s work in Section 4. The SiO₂ bands embedded in the gold film were replaced by oval Au nano blocks arranged in stripes on the SiO₂ substrate.

Optical sorting with the OTs with metasurfaces can be divided into two aspects, active sorting, and passive sorting. The active method involves artificially changing the incident light to actively control the movement of particles with the help of the metasurface’s excellent dynamic optical modulation ability. The passive method uses particle motion speed differences during transmission to distinguish particles of different sizes. Compared with the active method, the passive approach is more convenient for sorting. It is necessary to distinguish the movement states of different particles in advance. The OTs with metasurface utilized for passive optical particle sorting mentioned in this section were based on the forms of dynamic optical manipulation mentioned in Section 4. Optical sorting could be considered as a further development of dynamic manipulation and further indicates the advantages of multi-functionality of metasurfaces in OTs. This type of optical particle sorting discards the process of coloring, resolving, and charging required in the current common sorting methods. Simply, the movement of particles can separate the particles according to size. In addition, particles with similar refractive indexes can be sorted with this method, which expands the application of optical sorting and improves the degree of freedom of sorting. Meanwhile, planarized metasurfaces can be easily applied to the chip to reduce the system’s volume. In addition, the planarization of the metasurface allows these optical tweezers to be easily combined with other devices on chip.

6. Near-Field Optical Tweezers with Metasurfaces

The OTs with metasurfaces mentioned above can be classified as far-field OTs. For a far-field OT, the beam converges to form the required light field distribution at a position with a distance from the surface of the metasurface and then manipulates the target particles. In this section, researchers introduce OTs with metasurfaces that trap particles on the surface of the metasurface. This kind of OT with metasurface can be called near-field OTs. A typical method utilized by near-field OTs with metasurfaces is to trap the particles at the hot spots between the metasurface cells. For example, Yang et al. developed OTs with quasi-bound states in the continuum (quasi-BICs) ^[20]. Quasi-BICs can achieve high quality factor (Q factor) resonances as well as high field enhancement. The metasurface comprised oval Si nano blocks on the silica substrates. With the help of a quasi-BICs system, the field enhancements were observed between cells.

Jiang et al. designed a plasmonic nano-ellipse metasurface composed of oval gold nano blocks on a silica substrate ^[21]. Nano blocks were arranged on rectangular grid points, with the long axes of any adjacent nano blocks perpendicular to each other. The linearly polarized beam was incident from the direction of the nano blocks. By rotating the polarization angle, it is possible to turn on/off the hot spots between adjacent nano blocks. The change of the hot spots then conveys the trapped particles on the nano blocks due to the rotation of the incident beam.

Another method adopted is to generate SPPs with the help of the metasurface and obtain optical manipulation with the SPPs. Rahim et al. ^[22] and Safkat et al. ^[23] followed similar ideas, both utilizing two groups of V-shaped metasurfaces. When the beam was incident, SPPs generated by the two metasurfaces were coherent, and the interference field was formed over the substrate. Hence, the central region of the substrate had a continuous distribution of electric field. The interference field can exert optical forces on particles on the substrate’s surface and directionally manipulate them. Rahim’s work successfully exerted intriguing optical pulling forces on particles regardless of their materials. According to their calculation, particles with sufficient radii were subject to the pulling force regardless of their material. Meanwhile, it is worth noting that Safkat drew interesting conclusions for smaller particles (with radii of around 200 nm) under similar configurations. For particles of different materials (dielectric, plasmonic, or chiral objects), the direction of the optical forces pointed to different paths through numerical study. These results indicate the system’s potential for being applied to optical sorting.

In addition to the coherence of SPPs from two sets of metasurfaces, the characteristic of metasurface cells being able to arrange independently at will provides a new possibility for near-field optical manipulation. For example, Yang et al. designed metasurfaces with cells arranged as a right-hand Archimedes’ spiral on a golden film ^[24]. Each cell was composed of two V-shaped nanocavities. When the linearly polarized beam was incident, the SPP formed as a halo, and the particles could be trapped at the hot halo. It should be noted that the specially arranged nanocavities introduced the OAM into the incident linearly polarized beam, enabling the system to rotate the trapped particles without the help of OVs. Particles were trapped at the halo, and the optical forces drove them to rotate along the halo.

Near-field OTs trap targets on their surface. With the help of delicate metasurface devices, the OT systems become smaller and more planar. This characteristic further expands the application scope of OTs with metasurfaces on chips. Moreover, metasurfaces in near-field OTs show a good field enhancement effect (typically with the help of SPP or quasi-BIC). This feature enhances the trapping depth of particles and tempers the requirements for light source power without weakening the trapping effect, making the near-field OTs with metasurfaces more widely used. By adjusting the metasurfaces or the incident beams, these near-field OTs with metasurfaces can accurately locate the potential well’s exact position and exhibit more flexible trapping and manipulation capabilities. The OTs mentioned above have not entirely overcome the shortcomings of the traditional evanescent wave near-field OTs; for example, the evanescent wave decays rapidly in a short distance and cannot manipulate the target particles at a longer distance. However, such innovations have provided a new degree of freedom for the optical field control of near-field OTs, providing near-field OTs with broad development prospects.

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