Liquid microlens applied to super-resolution imaging

Abstract

This invention relates to a liquid microlens for super-resolution imaging, which is formed by confining a high-refractive-index liquid through a micrometer-sized cavity structure in a transparent thin film. The specific features and usage method are as follows: Step 1, a photoresist cylinder is prepared on a quartz substrate using photolithography; Step 2, a plano-convex lens is fabricated by thermal reflow treatment of the photoresist cylinder structure; Step 3, polydimethylsiloxane (PDMS) is spin-coated onto the upper surface of the plano-convex lens, and after heat curing, the PDMS film is peeled off to form a film with a spherical cavity structure; Step 4, a high-refractive-index liquid is dropped into the cavity position in the PDMS film and covers the surface of the sample to be tested. The liquid microlens formed by the PDMS cavity film and the high-refractive-index liquid acquires high-frequency structural information of the sample surface and transmits it to an optical microscope, thereby achieving super-resolution imaging of the optical microscope.

Description

Technical Field

[0001] This invention relates to a thin film, specifically to a liquid microlens for super-resolution imaging, belonging to the field of optical microscopy imaging technology. Background Technology

[0002] Traditional optical microscopes fall under the category of far-field imaging, and their resolution is limited by the diffraction limit, making it difficult to resolve details in samples smaller than half the working wavelength. The diffraction limit makes traditional optical microscopy insufficient for observing microscopic structures in life sciences and electronics. Therefore, overcoming the diffraction limit and achieving super-resolution imaging has always been a hot research topic in photonics. Electron microscopy and scanning probe microscopy, developed in the last century, have nanoscale resolution capabilities, but these techniques cannot observe living samples. Recent advancements in fluorescence super-resolution microscopy and Fourier transform microscopy have had a significant impact on bio-optical imaging techniques, with fluorescence super-resolution microscopy earning the 2014 Nobel Prize. Fluorescence super-resolution microscopy achieves super-resolution imaging by specially labeling samples with fluorescence and using a special excitation source. Fourier transform microscopy acquires corresponding low-resolution images by changing the sample illumination direction and then performs phase retrieval and aperture synthesis in the frequency domain to achieve high spatial resolution imaging.

[0003] However, the aforementioned techniques still have certain limitations and shortcomings. For example, fluorescent labeling affects the sample, image reconstruction is subject to noise interference, and the imaging speed is slow, making direct sample observation impossible. Therefore, researchers have been exploring and developing novel label-free, wide-field illumination optical super-resolution microscopy techniques. In recent years, researchers have discovered that micron-scale lenses possess unique optical properties, with photon nanojet streams formed by focusing electromagnetic waves exhibiting light intensity and subwavelength full width at half maximum (FWHM) far exceeding those of the irradiated wave. By imaging the sample (real or virtual image) using spherical and non-spherical microlenses placed on the sample surface, and then performing secondary imaging on the image formed by the microlenses using an optical microscope, super-resolution imaging of the sample can be achieved. When using low-refractive-index microspheres for imaging, the microspheres are randomly dropped onto the sample surface and partially immersed in liquid to enhance their super-resolution imaging capability. However, liquid evaporation causes dynamic changes in the microsphere resolution and magnification. Completely immersing high-refractive-index microspheres in liquid avoids this limitation, but this method cannot accurately position the microspheres in the region of interest of the sample. To address this problem, some researchers have assembled microspheres and probes and then moved the microspheres to perform imaging. However, the overall imaging system is quite complex and inconvenient to operate.

[0004] In addition, researchers have fabricated microlenses using various methods and processes, such as hot embossing, photoresist hot reflow, and crystal growth. While solid immersion lenses fabricated on samples can accurately position the microlens within a single area of ​​the sample requiring observation, these lenses cannot move on the sample surface and may damage the sample. Fabricating movable microlens films allows for movement on the sample surface and precise positioning in any area requiring observation; however, current microlens films rarely achieve close contact with the sample (the gap between the sample and the film is less than one illumination wavelength), hindering high-resolution imaging. Furthermore, embedding barium titanate microspheres in colloidal films can also achieve movable and super-resolution imaging, but the aberrations of spherical lenses severely affect their imaging performance, and the field of view of spherical lenses is much smaller than that of plano-convex microlenses. Therefore, a new approach is urgently needed to address these technical problems. Summary of the Invention

[0005] This invention addresses the problems existing in the prior art by providing a liquid microlens for super-resolution imaging. This technical solution involves preparing a low-refractive-index thin film with a spherical cavity structure, filling the film with a high-refractive-index liquid and covering the sample, thereby achieving accurate positioning, convenient movement, and super-resolution imaging without damaging the sample.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: a liquid micron lens for super-resolution imaging, characterized by comprising the following steps:

[0007] Step 1: Prepare photoresist cylinders on a quartz substrate using photolithography.

[0008] Step 2: Perform thermal reflow treatment on the photoresist cylindrical structure to fabricate a plano-convex lens;

[0009] Step 3: Coat the upper surface of the plano-convex lens with PDMS uniformly, peel off the PDMS film to form a film with a spherical cavity structure;

[0010] Step 4: Drop a high refractive index liquid into the cavity structure obtained in step (3) and cover it on the sample to be tested.

[0011] Step 1 is as follows: Positive AZ4903 photoresist is uniformly coated onto a quartz substrate. The photoresist thickness is controlled to 10µm using a spin coater, and the substrate is baked at 80℃ for 5 minutes. Finally, the photoresist is exposed using a mask, and after development, cylindrical structures with different aspect ratios are obtained.

[0012] Step 2 is as follows: The quartz substrate with photoresist cylinders prepared by photolithography in step 1 is transferred to a hot plate and subjected to photoresist thermal reflow treatment to form a plano-convex lens.

[0013] Step 3 is as follows: Mix the PDMS pre-cured material and curing agent at a mass ratio of 10:1, let it stand to remove air bubbles, and then pour it onto the plano-convex lens obtained in step 2. Spin-coat the PDMS and transfer it to a hot plate to cure the PDMS. Peel off the PDMS film to obtain a film with a spherical cavity structure with different aspect ratios.

[0014] Step 4 is as follows: A certain volume of high refractive index liquid is dropped onto the PDMS film. The high refractive index liquid fills the concave cavity structure to form a plano-convex lens structure. The film is then placed on the sample and imaged under an optical microscope.

[0015] As an improvement of the present invention, in step 4, a high refractive index liquid is filled into the cavity structure of the thin film and covered on the sample, with the sample and the high refractive index liquid in direct contact, and the thin film covering the high refractive index liquid.

[0016] As an improvement of the present invention, the refractive index of the high-refractive-index liquid is in the range of 1.7-1.8, and the material used for the thin film with the spherical cavity structure is PDMS. The refractive index of the high-refractive-index liquid filling the spherical cavity structure is higher than that of the PDMS thin film, which can form a plano-convex lens with a relative refractive index of 1.20-1.28 and an aspect ratio of 0.25-0.5.

[0017] Compared to existing technologies, this invention offers the following advantages: It obtains a plano-convex lens structure through thermal reflow of photoresist, and further fabricates an imaging thin film with a high-precision spherical cavity structure. A high-refractive-index liquid is filled between the imaging thin film and the sample to form a plano-convex lens, ensuring close contact between the sample and the imaging thin film, thereby achieving super-resolution imaging with a large field of view. Simultaneously, the refractive index of the PDMS thin film is lower than that of the filling high-refractive-index liquid, further improving the imaging effect. Furthermore, the transparent thin film proposed in this invention for super-resolution imaging possesses the ability to be accurately positioned, easily moved, and without damaging the sample. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the fabrication process of the transparent thin film for super-resolution imaging according to the present invention;

[0019] Figure 2 This is a schematic diagram of the imaging system of the transparent thin film applied to super-resolution imaging according to the present invention;

[0020] Figure 3 This is a schematic diagram of Embodiment 2 of the transparent film of the present invention applied to super-resolution imaging;

[0021] Figure 4 This is a schematic diagram of Embodiment 3 of the transparent film of the present invention applied to super-resolution imaging.

[0022] In the figure: 1. Objective lens, 2. PDMS film, 3. High refractive index liquid, 4. Sample. Detailed Implementation

[0023] To enhance understanding of the present invention, the embodiments will be described in detail below with reference to the accompanying drawings.

[0024] Example 1: See Figure 1 A liquid microlens for super-resolution imaging, specifically comprising the following steps:

[0025] Step 1 is as follows: The quartz substrate is ultrasonically cleaned in acetone solution, dried with a nitrogen gun, and then uniformly coated with positive AZ4903 photoresist. The photoresist thickness is controlled to 10µm by the spin coater speed. Then, the quartz wafer with 10µm photoresist is transferred to a hot plate and baked at 80°C for 5 minutes. Finally, the photoresist is photolithographically masked and developed for 5 minutes to prepare photoresist cylinders with different aspect ratios on the quartz wafer.

[0026] Step 2 is as follows: The quartz wafer with photoresist cylinder prepared by photolithography in step 1 is transferred to a hot plate and baked at 200°C for 2 hours for heat reflow treatment. The photoresist melts at high temperature and forms a plano-convex lens under the action of surface tension.

[0027] Step 3 is as follows: Pour PDMS onto the plano-convex lens obtained in step 2, spin-coat PDMS to control its thickness to 35 μm, transfer it to a hot plate and bake at 90°C for 1 hour to cure PDMS, and peel off the PDMS film to obtain films with spherical cavity structures of different aspect ratios.

[0028] Step 4 is as follows: A certain volume of high refractive index liquid (composed of diiodomethane and sulfur, with a refractive index of 1.79 at a wavelength of 540 nm) is dropped onto the PDMS film. The high refractive index liquid fills the concave cavity structure, forming a plano-convex lens with a relative refractive index of 1.28. The film is then placed on the sample and imaged under an optical microscope.

[0029] Example 2:

[0030] The illumination wavelength λ was 540 nm, and the numerical aperture (NA) of the microscope objective was 0.9. The concave structure of the thin film had a bottom diameter of 20 μm and a depth of 10 μm, and the material was PDMS; the refractive index of the high-refractive-index liquid was 1.79; the sample to be tested was a Blu-ray disc with a linewidth of 200 nm and a groove of 100 nm.

[0031] The imaging effect diagram of the transparent film applied to super-resolution imaging is shown below. Figure 3 As shown. From Figure 3As can be seen, the thin film can clearly distinguish the periodic structure on the surface of the Blu-ray disc, and its resolution exceeds the optical diffraction limit. Under the same objective lens, the structure cannot be distinguished by an optical microscope alone.

[0032] Example 3:

[0033] The illumination wavelength λ was 540 nm, and the numerical aperture (NA) of the microscope objective was 0.9. The concave structure of the thin film had a bottom diameter of 20 μm and a depth of 10 μm, and the material was PDMS; the refractive index of the high-refractive-index liquid was 1.79; the sample under test was a triangular aluminum lattice with a detailed structure of 130 nm.

[0034] The imaging effect diagram of the transparent film applied to super-resolution imaging is shown below. Figure 4 As shown. From Figure 4 As can be seen, the micron-scale imaging film can clearly distinguish the detailed structure of two triangular aluminum lattices.

[0035] It should be noted that the above embodiments are not intended to limit the scope of protection of the present invention. Equivalent transformations or substitutions made based on the above technical solutions all fall within the scope of protection of the claims of the present invention.

Claims

1. A liquid microlens for use in super-resolution imaging, characterized in that, Specifically comprising the following steps: Step 1, preparing photoresist cylinder on quartz substrate by using photolithography process, the physical parameters of the photoresist cylinder are 10-40 μm in diameter and 10 μm in height; Step 2, making plano-convex lens by heat reflow treatment on the photoresist cylinder structure; Step 3, uniformly coating PDMS on the upper surface of the plano-convex lens, peeling off the PDMS film to form a film with spherical concave cavity structure, the spin coating thickness is 35 μm; the width-height ratio of the spherical concave cavity structure ranges from 0.25 to 0.5; Step 4, filling high refractive index liquid to form plano-convex lens, the refractive index of the high refractive index liquid is higher than that of the PDMS film, dropping the high refractive index liquid into the spherical concave cavity position in the PDMS film, and covering on the surface of the sample to be measured, and observing the surface of the sample by using optical microscope.

2. The liquid microlens for super-resolution imaging according to claim 1, wherein, The material of the photoresist cylinder in step 1 is AZ4903 series photoresist.

3. The liquid microlens for super-resolution imaging according to claim 1, wherein, The heat reflow parameters in step 2 are temperature 190℃ and time 1 hour.

4. The liquid microlens for super-resolution imaging according to claim 1, wherein, The PDMS film in step 3 is polydimethylsiloxane.

5. The liquid microlens for super-resolution imaging according to claim 1, wherein, The refractive index of the high refractive index liquid in step 4 ranges from 1.7 to 1.8; the magnification of the objective lens in the optical microscope can be selected from 10× to 100×, and the numerical aperture is 0.25 to 0.95.

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