Dynamic adjustable optical tweezers based on polarization-sensitive transverse bifocal superlens

Abstract

The invention relates to dynamic adjustable optical tweezers based on a polarization-sensitive transverse bifocal superlens, and belongs to the field of optical tweezers technology and micro-nano optics. According to the super-lens, dynamic regulation and control of a focus are realized by using sensitive response of the silicon nano-column array to polarization through a composite propagation phase and a geometric phase. Under linear polarization or elliptically polarized light, the system can generate transversely symmetrical double focuses; under the circularly polarized light, a single-side focus is formed. By adjusting the polarization state of incident light, a double-particle capture mode, a single-particle capture mode and a particle transverse transport mode can be flexibly switched. According to the invention, the physical structure does not need to be changed, the dynamic control can be realized only by external optical regulation and control, the structure is compact, the integration degree is high, the control is flexible and the like, and a new technical approach is provided for an on-chip particle control system.

Description

Technical Field

[0001] This invention relates to the field of micro-nano optics and light field manipulation technology, specifically to a dynamically tunable optical tweezer based on a polarization-sensitive transverse double-focal superlens, which utilizes the composite phase modulation characteristics of the metasurface to achieve multi-mode capture and dynamic transport of nanoparticles. Background Technology

[0002] Since its inception in the 1980s (Reference 1: Ashkin A, et al. Observation of a single-beam gradient force optical trap for dielectric particles. Opt Lett 1986, 11: 288–290.), optical tweezers have become a powerful tool for manipulating microscopic particles, demonstrating great potential in fields such as biomedicine, microfluidics, and materials science. Traditional optical tweezers typically rely on high numerical aperture (NA) microscope objectives, resulting in large, complex systems that are difficult to integrate and still have limitations in achieving multifocal dynamic manipulation.

[0003] In recent years, metalens composed of subwavelength nanostructure arrays have opened up new paths for the miniaturization and functionalization of optical systems. By precisely designing the geometric parameters and spatial arrangement of the unit structure, metalens can achieve flexible manipulation of the light wavefront at the subwavelength scale. Applying metalens technology to the field of optical tweezers is expected to overcome the shortcomings of traditional optical tweezers. Existing research has demonstrated axial particle manipulation based on metalens (Reference 2: Markovich H, et al. Optical manipulation along an optical axis with a polarization sensitive meta-lens. Nano letters, 2018, 18(8): 5024-5029.) and two-dimensional particle trapping (Reference 3: Chantakit T, et al. All-dielectric silicon metalens for two-dimensional particle manipulation in optical tweezers. Photonics Research, 2020, 8(9): 1435-1440.).

[0004] However, most existing research focuses on manipulating the optical axis or achieving static capture on a plane. Systematic research on how to achieve dynamic, controllable particle transport in the transverse dimension, and how to flexibly switch between different capture modes (such as single-point capture, dual-point capture, and intensity-adjustable capture), remains limited. Existing solutions are often single-function and lack sufficient flexibility and tunability to meet the demands of complex particle manipulation applications.

[0005] Therefore, this invention combines two control methods into the same superlens structure. Utilizing the sensitive response of the composite phase to polarization, it enables flexible switching between various manipulation modes, such as two-particle capture, single-particle capture, capture force control, and lateral particle transport, simply by adjusting the polarization state of the incident light. This invention requires no modification to any physical structure and achieves dynamic particle manipulation solely through external optical control, significantly improving the flexibility and versatility of manipulation. Summary of the Invention

[0006] This invention proposes a dynamically adjustable optical tweezers based on a polarization-sensitive transverse bifocal superlens. This optical tweezers utilizes a composite modulation mechanism of propagation phase and geometric phase, enabling a single superlens device to produce spatially separated focusing responses to incident light with different polarization states. Thus, without altering the physical structure, multiple particle trapping modes can be flexibly switched simply by adjusting the polarization state of the incident light.

[0007] 1. Specific content of the present invention

[0008] (1) The optical tweezers structure proposed in this invention is as follows: Figure 1 As shown, the core component is an all-dielectric superlens, built on a silicon dioxide (SiO2) substrate. Its surface is composed of micro / nano-structure units made up of rectangular silicon (Si) nanopillars arranged in a periodic array. All nanopillars have a uniform height (H = 750 nm), but their length, width, and in-plane rotation angles must be specifically set according to their spatial coordinates. To achieve independent decoupling and control of left-handed circularly polarized light (LCP) and right-handed circularly polarized light (RCP), the target focal length f and the lateral focal offset Δx of the superlens are first determined. Based on the principle of compound phase modulation, a phase distribution function is constructed on the surface of the superlens, enabling the device to focus the LCP and RCP components at symmetrical lateral positions on either side of the optical axis.

[0009] (2) When a near-infrared beam with a wavelength of 980 nm is incident perpendicularly from the substrate side, the electric field distribution characteristics on the focal plane are significantly different depending on the polarization state of the incident light: when the incident light is linearly polarized or elliptically polarized, the superlens produces two high-intensity focused spots at the lateral symmetrical position (x = ±400 nm) on the focal plane, and the relative intensity of the two spots depends on the ellipticity of the incident light; when the incident light is adjusted to the left-hand circularly polarized state (LCP), the light field energy on the focal plane is completely concentrated on a single focus on one side of the optical axis (x = -400 nm), and the spot on the other side disappears; when the incident light is adjusted to the right-hand circularly polarized state (RCP), the light field energy is completely transferred to a single focus on the other side of the optical axis (x = +400 nm).

[0010] (3) Based on the above-mentioned light field distribution characteristics, the optical tweezers proposed in this invention can realize the following particle manipulation modes: Dual-well balance adjustment mode: Under linearly polarized or elliptically polarized light irradiation, two optical potential wells are formed in the lateral direction, which can capture two particles simultaneously; adjusting the ellipticity can continuously change the depth of the two potential wells (i.e., the capture stiffness); Dynamic transport switching mode: Utilizing the difference in the focal positions corresponding to LCP and RCP, by rapidly switching the rotation direction of the incident light, the optical potential wells are forced to undergo instantaneous jumps between two preset lateral positions, and the generated optical gradient force drives the captured particles to be transported rapidly between the two focal points in a non-mechanical manner.

[0011] 2. The advantages of this invention are as follows:

[0012] (1) This invention uses a superlens to replace the complex optical path of traditional optical tweezers, and does not require any mechanical moving parts. The lateral movement of the focal point and intensity adjustment can be achieved simply by changing the polarization state of the incident light, which greatly improves the system's integration and response speed.

[0013] (2) This invention integrates multiple functions such as single-point capture, simultaneous dual-point capture, continuously adjustable capture force, and dynamic transport on the same structure. In particular, the dynamic transport mode, simulation shows, has high transfer efficiency and fast speed for particles with a radius of 100nm, providing a new solution for on-chip material transport.

[0014] (3) The optical tweezers operate under far-field conditions, avoiding mechanical contact damage to the sample. Furthermore, the superlens structure is spatially separated from the capture area, ensuring the durability and reusability of the device.

[0015] 3. The principle of this invention is as follows:

[0016] (1) Propagation Phase: The propagation phase originates from the optical path difference generated when electromagnetic waves propagate in media with different effective refractive indices. For the silicon nanopillars in this invention, their equivalent effective refractive index n effThe phase delay β depends on the lateral geometry of the nanopillar (e.g., length L and width W). Therefore, by changing the L and W of the nanopillar, the phase of the transmitted light can be tuned. Its phase delay β can be expressed as:

[0017]

[0018] Where λ is the incident wavelength and h is the height of the nanopillar. By scanning different combinations of L and W, the propagation phase covering the range of 0-2π can be obtained.

[0019] (2) Geometric phase arises from the interaction between circularly polarized light and the anisotropic unit structure. When an anisotropic unit (such as a rectangular nanopillar) is rotated by an angle θ in the plane, the cross-polarized component in the transmitted light, which has the opposite chirality to the incident light, acquires an additional phase delay. This phase delay is only related to the rotation angle, and its value is:

[0020]

[0021] The "+" and "-" signs correspond to left-hand circularly polarized (LCP) and right-hand circularly polarized (RCP) incident light, respectively.

[0022] (3) This invention combines the above two phases—transmission phase and geometric phase—into a single high-refractive-index medium structure, allowing orthogonal polarization states to generate two independent optical responses. Specifically, to focus LCP light at a lateral position of -Δx and RCP light at +Δx, the required target phase distribution function is derived from the standard focusing lens formula:

[0023]

[0024] Where f is the focal length. Substituting these two target phases, which contain lateral offset information, into the derivation results of the Jones matrix theory, the required physical parameters for each nanopillar can be determined:

[0025]

[0026] (4) In summary, the target phase distribution can be directly mapped to the rotation angle θ and propagation phase φ of the nanopillar at each coordinate point on the surface of the superlens using equations (5) and (6). pro By searching for the corresponding nanopillar size in a pre-established database of geometric parameters, the specific physical parameters of each unit on the lens surface can be uniquely determined. This principle ensures that a single device can produce spatially separated independent responses to LCP and RCP inputs, thus forming the physical basis for realizing polarization-switched lateral dynamic optical manipulation. Attached Figure Description

[0027] Figure 1 (a) is a schematic diagram of the working principle of the dynamic optical tweezers system based on the polarization-sensitive superlens of the present invention, showing the effect of the beam being focused at different lateral positions (F1 and F2) on both sides of the optical axis under the incident light of left-hand circularly polarized light (LCP) and right-hand circularly polarized light (RCP); (b) is a schematic diagram of a single silicon nanopillar unit structure constituting the superlens. The unit is located on a silicon dioxide (SiO2) substrate, with a height of H, a lattice period of S, and a length and width of L and W, respectively; (c) is a top view of the overall structure of the superlens, showing the periodic arrangement of the nanopillar array on the substrate surface.

[0028] Figure 2 (a) is a simulation of the electric field intensity distribution of the superlens on the xz section under linearly polarized light incidence, showing that two symmetrical focal points F1 and F2 are formed simultaneously on both sides of the optical axis; (b) is a two-dimensional electric field intensity distribution on the focal plane (z = 8 μm) under linearly polarized light incidence, clearly demonstrating the lateral separation characteristics of the two focal points; (c) is a one-dimensional normalized light intensity distribution curve along the x-axis of the focal plane, with the two peaks corresponding to the positions of the two focal points; (d) is the optical force distribution curve of a 100 nm radius polystyrene particle along the x-axis of the focal plane, with alternating positive and negative force distributions indicating the existence of two stable equilibrium points; (e) is the corresponding... Figure 2 The optical potential well depth distribution curve (d) shows that two deep potential wells are formed at the two focal locations.

[0029] Figure 3 (a) is the electric field intensity distribution of the xz section when the incident light ellipticity γ = -0.5, with the intensity at the focal point on the left being higher than that on the right; (b) is the electric field intensity distribution of the xz section when the incident light ellipticity γ = 0.5, with the intensity at the focal point on the right being higher than that on the left; (c) is the electric field intensity distribution of the xz section when the incident light is left-handed circularly polarized (LCP, γ = -1), with the light field focused only at a single point on the left; (d) is the electric field intensity distribution of the xz section when the incident light is right-handed circularly polarized (RCP, γ = 1), with the light field focused only at a single point on the right.

[0030] Figure 4 The curves show the dependence of the potential depth of the two optical traps (left potential trap L_U and right potential trap R_U) on the ellipticity of the incident light, demonstrating the ability to continuously control the depth of the two potential traps by adjusting the polarization state.

[0031] Figure 5 This is a schematic diagram of the principle of transverse switching of optical potential wells and the distribution of single-particle potential wells. The two curves represent the left deep potential well generated by left-handed circularly polarized light and the right deep potential well generated by right-handed circularly polarized light, respectively. The arrows indicate the process of transporting particles from the left potential well to the right potential well through polarization switching.

[0032] Figure 6This is a histogram of the spatiotemporal evolution of the particle dynamic transport process. The figure shows the positional distribution of 1000 particles with a radius of 100 nm along the x-axis and the corresponding optical potential trap distribution at different times from t = 0.0 ms to t = 0.4 ms after the incident light instantly switches from LCP to RCP, verifying the efficient lateral transfer of particles within 0.4 ms. Detailed Implementation

[0033] To make the technical problem to be solved, the technical solution, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. This embodiment constructs a polarization-sensitive transversely bifocal superlens optical tweezers operating in the near-infrared band (λ = 980 nm), and its specific implementation is as follows:

[0034] First, the basic physical parameters and simulation conditions of the superlens were determined. The lattice constant of the superlens unit cell was set to S = 520 nm, satisfying the Nyquist sampling theorem. The functional structure was a silicon (Si) nanopillar with a height of H = 750 nm, and the substrate was silicon dioxide. To establish a phase modulation library, the length L and width W of the nanopillar were scanned using the finite-difference time-domain method, with a scanning range of 50 nm to 450 nm. The selection criteria were: (1) high unit transmittance; (2) the propagation phase difference along the long axis and short axis satisfies the Nyquist sampling theorem. To ensure high polarization conversion efficiency, this embodiment selects eight preferred nanopillar size combinations (Table 1) and different rotation angles to achieve full coverage of phases from 0 to 2π.

[0035] Table 1 Nanopillar numbering Length (nm) Width (nm) 1 70 380 2 400 160 3 140 250 4 150 310 5 380 70 6 160 400 7 250 140 8 320 150 Next, based on the formula in the invention principle, the target focal length f = 8000nm and the lateral focal offset Δx = 400nm are set. The required propagation phase and geometric phase rotation angle at each coordinate point (x, y) on the surface of the superlens are calculated. The calculation results are matched with the cell library established in step 1 to determine the specific size and rotation angle of the nanopillar at each location, thus completing the design of the superlens array.

[0036] Then, the constructed superlens model was imported into FDTD software for full-wave simulation. When linearly polarized light was incident, two symmetrical high-intensity focused spots were formed on both sides of the optical axis of the focal plane (x = -400nm and x = +400nm), verifying that linearly polarized light can simultaneously excite two designed focal points (such as...). Figure 1Adjusting the ellipticity γ of the incident light can continuously control the relative intensity of the two focal points: when γ = -0.5 (left-handed dominance), the intensity of the left focal point is significantly higher than that of the right; when γ = 0.5 (right-handed dominance), the intensity of the right focal point is enhanced. When the incident light is purely left-handed or purely right-handed circularly polarized light (γ = -1 or 1), the light field forms a single focal point only on the left or right side. At this time, the focusing efficiency and intensity of the single focal point reach their maximum values ​​(e.g., Figure 2 ).

[0037] Finally, polystyrene (PS) microspheres with a radius of 100 nm were selected as the manipulation targets for performance analysis. Calculations under linearly polarized light illumination showed that the potential well depths of the dual-well configuration were approximately 77.2 kJ / m². B T and 73.3k B T, much greater than the thermal energy of Brownian motion, is sufficient to achieve stable two-particle trapping; under circularly polarized light illumination, the single-trap depth can reach approximately 142k. B In terms of dynamic transport, the motion of particles in a fluid was simulated. Initially, the particles were captured at the left focal point by left-handed light. When the beam instantly switched to right-handed light, the optical potential trap jumped to the right. Simulation statistics showed that, driven by the optical gradient force, approximately 92.1% of the particles successfully transferred to the right target focal point within 0.4 ms after the polarization switch.

Claims

1. A dynamically adjustable optical tweezers based on a polarization-sensitive transverse bifocal superlens is proposed, characterized in that: This superlens is composed of an array of silicon (Si) nanopillar units with different geometric dimensions and rotation angles, placed on a silicon dioxide (SiO2) substrate. Its phase modulation is composed of a combination of propagation phase and geometric phase, which can focus the left-hand and right-hand circularly polarized (LCP / RCP) components of the incident light to opposite lateral positions on both sides of the optical axis at a working wavelength of 980nm.

2. According to claim 1, the surface array of the superlens is composed of square unit cells mainly composed of rectangular silicon nanopillars, characterized in that: The lattice constant of the square unit cell is 520 nm, and the height of the rectangular nanopillar is 750 nm. By changing the length (L) and width (W) of the nanopillar to control the propagation phase, and simultaneously rotating the nanopillar to control the geometric phase, independent wavefront shaping of light with different polarizations can be achieved.

3. The optical focusing characteristics of the superlens according to claims 1 and 2 are characterized by: When the incident light is linearly polarized or elliptically polarized, the superlens produces two symmetrical focal points laterally on the focal plane (focal length 8000 nm), with the focal points located approximately ±400 nm from the optical axis. The relative intensity of the two focal points can be continuously adjusted by regulating the ellipticity of the incident light. When the incident light is left-handed circularly polarized (LCP), a single focal point is formed only on one lateral side (x = -400 nm). When the incident light is right-handed circularly polarized (RCP), a single focal point is formed only on the other lateral side (x = +400 nm).

4. The dynamic particle manipulation function of the optical tweezers according to claims 1, 2, and 3, characterized in that... It enables flexible particle trapping and transport modes: when linearly polarized light is incident, stable trapping of two particles can be achieved simultaneously at two transverse focal points. For polystyrene (PS) particles with a radius of 100 nm, the potential well depths of the two optical traps are approximately 77.2 kJ / m². B T and 73.3k B T; By switching between left-handed and right-handed circularly polarized light, the controlled lateral transport of trapped particles between the two focal points can be achieved. For polystyrene particles with a radius of 100 nm, the particle transport efficiency can reach 92.1% within 0.4 ms after switching polarization states.

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