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High-efficiency broadband achromatic metalens for near-IR biological imaging window - Nature Communications

Results . Working principle . In our work, we control the metalens characteristics by carefully designing the group delay, since the material’s optical absorption is intrinsic and cannot be modified. The group delay range can be increased through simple geometry change, i.e., by making structures with larger aspect ratio/height. Here, we increase the pillar height to h ?=?1500?nm, which is 2.5 times larger compared to the previous reports. Similar to ref. 20 , four types of nanopillars with circular-, ring-, square- and bipolar concentric ring-shaped cross-sections were employed as the metasurfaces building blocks (see Insets in Fig.? 2a ) to eliminate the polarization dependence. A parameter sweep of the nanopillar size and shape was done to build a library with the values of phase, group delay, and transmittance at the designed operational wavelength of 760?nm (see Supplementary Fig.? 2 ). For an achromatic metalens with the diameter D ?=?30??m and NA?=?0.24, a particle swam optimization method was applied to optimize the nanostructures, minimizing the required group delay range and maximizing the smallest feature size 37 . Figure? 2a shows the layout of a quarter of the developed metalens. It consists of four sets of TiO 2 nanopillars with different cross-sections and a minimal feature size of 40?nm. The required group delay (black curves) of the achromatic metalens was matched to the propagating group delay in nanopillars (dots in Fig.? 2b ). As a result, broadband achromatic performance was expected. Figure? 2c shows the numerically calculated focal lengths. The incident light with the wavelength 650 to 1000?nm, respectively, is focused by the TiO 2 metalens to the designed position with a <7% variation, covering the first biological imaging window. The corresponding focusing efficiency is plotted as dots in Fig.? 2d . The minimal and maximal focusing efficiencies are 72% at 650?nm and 93% at 850?nm, respectively. The averaged value in the entire operating spectrum is 83%. Note that the average efficiency can be up to 93% for the small NA of 0.1 and diameter of 25??m (squares in Fig.? 2d and details can be seen in Supplementary Fig.? 3 ), surpassing all the current achromatic metalens demonstrations. In addition, as the nanopillars have at least fourfold rotational symmetry, the achromatic metalens is expected to be polarization insensitive, which is critical for biological applications 38 . Therefore, the developed TiO 2 metalens with h ?=?1500?nm can potentially fill the so-called low-efficiency gap in available achromatic metalens designs. Fig. 2: Design of the near-IR achromatic metalens. a The layout of the quarter of the designed metalens. Four fundamental building blocks (unit cells) are enlarged in the insets. b The required group delay for broadband achromatism (solid line) and the values provided by the TiO 2 nanostructures. c and d are the numerically calculated focal lengths and efficiencies of TiO 2 metalenses with NA?=?0.24 (dots) and NA? =?0.1 (open squares) at different wavelengths. The diameters of two metalens are 30 and 25??m, respectively. Full size image Experiments . Experimental realization of TiO 2 metalenses is a challenging task since the state of art pillar height is only 990?nm 6 , 30 . To overcome this challenge, we developed a top-down etching-based technology for fabricating TiO 2 metalenses. First, 1500-nm-thick TiO 2 membrane was deposited by the electron beam evaporation and coated with PMMA A2 resist, which was patterned with electron beam lithography. A lift-off process was applied to transfer the nanopatterns to Cr mask. Then, the membrane was etched with the reactive ion etching process (see details in Fig.? 3a and “Methods”). After removing the Cr mask, the TiO 2 nanostructures were created. Figure? 3b, c depicts the top-view scanning electron microscope (SEM) images of the developed TiO 2 metalens. It consists of 4725 nanopillars with four types of different cross-sections. Note that the nanopillars locations, shapes and feature sizes closely follow the numerical design. The tilt-view SEM images of TiO 2 metalenses in Fig.? 3d , the inset in Fig.? 1 , and the Supplementary Fig.? 5 show that the nanopillars have nearly perfect vertical sidewalls with the measured tilt angle of sidewalls of around 89°?90°. Considering the smallest feature in the metalens, the achieved aspect ratio is around 37.5 or higher (Supplementary Fig.? 5 ). The developed procedure for obtaining high-quality TiO 2 nanostructures enabled the experimental realization of the metalens with the designed phase, group delay, and the corresponding high-efficiency broadband achromatism. Fig. 3: The experimentally fabricated TiO 2 metalens with NA?=?0.24. a The schematic of the fabrication process. A highly directional etching process was employed to fabricate TiO 2 nanostructures. b and c are the top-view SEM images of achromatic metalens with different resolutions. Four types of nanostructures can be clearly identified. d The corresponding tilt-view SEM image of the metalens. e and f are the intensity profiles of focal spots in x-z plane and x-y planes at different wavelengths. The bottom panels depict the intensity distributions along the diameter (solid lines) and the fitted curves (dashed lines). g The images of element-6, group-7 of the 1951 Unites States Air Force resolution target recorded by the achromatic metalens. Full size image The optical properties of the demonstrated TiO 2 achromatic metalens were characterized using optical setup in Supplementary Fig.? 6 . The focal lengths at different wavelengths were obtained by measuring the cross-sectional intensity profiles along the propagation direction (Fig.? 3e ). In the broad wavelength range from 650 to 1000?nm, the incident light was focused to a bright spot ~60??m away from the metalens with a small variation. The achieved values of both the focal length and the NA?=?0.24 match the numerical design very well. To demonstrate the realization of a broadband achromatic metalens for the first biological imaging window, Fig.? 3f shows the light intensity profiles in the x-y plane at different wavelengths. The focal spot is a circular point at each wavelength. By fitting the intensity distributions along the diameter, all of the calculated Strehl ratios are larger than 0.81 and the full widths at half maximum (FWHM) deviate <9% from the theoretical values, clearly demonstrating that the focal spots are diffraction limited. The 1951 United State Air Force (USAF) resolution test chart was used as a target to test the imaging capability of the developed metalens. For the incident light with the wavelength of 650?nm, the metalens is able to resolve element-4 of group 8, giving a resolution above 1.38??m. While the resolution limit is dependent on the wavelength, the image of element-6 of group 7 was clearly recorded at all wavelengths without tuning the metalens and the target (see Fig.? 3g ), confirming the broadband achromatism of our metalens very well. Compared with the achromatism, the focusing efficiency is a more critical criterion for filling the near-IR gap. Similar to refs. 2 , 8 , 9 , we measured the efficiency by directly comparing the optical power of the focused light with the power of the incident light. The dots in Fig.? 4a shows the experimentally recorded efficiencies of the TiO 2 metalens with NA?=?0.24. The lowest efficiency is ~65% at 650?nm, whereas the highest value is even ~85% at 800?nm. The averaged efficiency for the entire spectral range is ~77.1%, only a few percent lower than the numerical design. This is consistent with the high-quality fabrication of nanopillars since the efficiency reduces rapidly if the sidewall deviates from 90° in the vertical direction. Fig. 4: Focusing efficiencies of the developed TiO 2 metalenses. a The experimentally recorded focusing efficiency of the TiO 2 metalens with NA?=?0.24 (dots) and NA?=?0.1 (open squares) as the function of the incident wavelength. Here, the incident light is un-polarized. b The dependence of the focusing efficiencies at 800?nm on the polarization. The insets in ( a ) and ( b ) are images of the focal spots of the metalens with NA?=?0.1 at different wavelengths and different polarization, respectively. Full size image Compared with the previous reports, the observed averaged efficiency is record-high. More interesting, we find that the averaged focusing efficiency of the developed TiO 2 metalens can be furthered improved at smaller NAs. When the NA is 0.1, the minimal and maximal focus efficiencies of the metalens at 650?nm and 700?nm are 85% and 90.2%, respectively (see open squares in Fig.? 4a and details in Supplementary Fig.? 3 ). The averaged efficiency from 650 to 1000?nm is as high as 88.5%. Since the Fresnel loss at the bottom and etched interfaces are not fully corrected, the averaged high-efficiency can reach above 90% that is superior to the commercial micro-lenses. According to the prediction in refs. 4 , 5 , such high-efficiency broadband achromatic TiO 2 metalens can become a game changer in practical applications of flat photonics. Polarization insensitivity is another important characteristic for applications in the near-IR window. Figure? 4b shows the focusing efficiencies of the demonstrated TiO 2 metalenses at 800?nm at different polarization states. With the change of polarization, the efficiency of metalenses with NA?=?0.24 and NA?=?0.1 are almost flat at 85 and 89% for different linear polarization and circular polarization, consistent with the results of the non-polarized case in Fig.? 4a . Similar polarization-insensitive characteristics hold true for all other wavelengths (see Supplementary Fig.? 8 ). After we have demonstrated that the developed metalenses can successfully address the low-efficiency gap, we further explored the potential of TiO 2 metalenses in upconversion imaging. In this experiment, the lanthanide-doped nanocrystals (NCs), which are widely used in bioimaging and labeling, were spread and aggregated on a glass substrate 39 . An additional layer of polystyrene (PS) spheres was deposited on top of the NC clusters. Under a conventional microscope, the NCs clusters were buried in PS spheres and cannot be identified (see Fig.? 5a ). Then, the continuous wave (CW) laser at 980?nm was focused by the TiO 2 metalens onto the lanthanide-doped NCs clusters. The upconversion fluorescence centered at 655?nm with a linewidth of 20?nm was collected by the same metalens and recorded by a CCD camera (see Supplementary Fig.? 9 ). By scanning the sample with a three-dimensional translation stage, the image can be reconstructed by combing the collected signals for each excitation point. The results are plotted in Fig.? 5b . In contrast to conventional microscope image, the sharp edges of the NCs based microplate can be clearly seen in upconversion imaging. The resolution limit is around 1.46??m, which is determined by the point spread function and the diffraction limit at the emission wavelength. For a direct comparison, the upconversion microscope image has been captured with a commercial achromatic objective lens with NA?=?0.26 (Fig.? 5c ). Both the metalens and the objective lens can capture the structural information of microplate with two-photon excitation with no obvious difference in resolution and intensity distribution (see Supplementary Figs.? 10 and 11 ). Fig. 5: Upconversion imaging with the metalens. a The microscope image of nanocrystals (NCs) on a microplate covered with polystyrene spheres under the white light illumination. The sample is marked with the dashed lines. b The upconversion fluorescent image recorded by the achromatic metalens with NA?=?0.24. c The upconversion fluorescent image recorded by a commercial achromatic objective lens with N ?=?0.26 (MY10X-823, Mitutoyo). d The microscope image of HeLa cells with upconversion NCs. e and f and the upconversion fluorescent images recorded by the metalens and a commercial objective lens. Full size image Potential application of the developed TiO 2 achromatic metalens for biological imaging was also evaluated. For that, the HeLa cells containing lanthanide-doped NCs were prepared (Fig.? 5d ) and optically excited. Figure? 5e depicts the fluorescent image of HeLa cells under two-photon excitation. Compared with the bright field microscope image in Fig.? 5d , more detailed internal structures of the cell were captured. The image quality and the resolution were close to the ones recorded by the commercial objective lens with the similar NA (Fig.? 5f ). Thus, the demonstrated achromatic TiO 2 metalens can perform on par with the commercial products for optical imaging. Considering its compactness, flatform, high-efficiency, polarization insensitivity, and diffraction-limit resolution, the proposed achromatic TiO 2 metalenses could trigger a revolution in on-board biomedical diagnosis. .

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