1 Introduction
2 Design principles of metalenses
Fig.1 Schematic diagram of three phase modulation methods, i.e., resonance phase modulation ((a) and (b)), geometric phase modulation ((c) and (d)), and propagation phase modulation ((e) and (f)). (a) Simulated phase shift of the scattered light for V-antennas with various length h and angle between the rods Δ. (b) Schematic diagram of ordinary and anomalous reflection and refraction for y-polarized excitation. (c) Schematic diagram of a representative meta-atom array considered as model in the simulation. Bottom right: excitation of dipole moment when illuminating one meta-atom. (d) Schematic diagram of ordinary and anomalous refraction when illuminated with left- and right-circularly polarized light, respectively. (e) Side-view scanning electron microscope (SEM) image of a propagation phase-modulated metasurface. An array of nanopillars is indicated by red rectangle. (f) Schematic diagram of focusing a light pulse to the focal distance of a propagation phase-modulated metalens. The time-dependent electric field plots demonstrate that the light passing through the different parts of the metalens requires equal phase delay to arrive at the focal point without dispersion. (a) and (b) Reproduced with permission from Ref. [28]. (c) and (d) Reproduced with permission from Ref. [34]. (e) Reproduced with permission from Ref. [35]. (f) Reproduced with permission from Ref. [36] |
Fig.2 (a) Top: schematic diagram of meta-atoms. Bottom: finite difference time-domain (FDTD) simulations of the scattered electric field of the periodic array composed of individual antennas shown above. (b) Schematic diagram of the combination of propagation and geometric phases. (a) Reproduced with permission from Ref. [28]. (b) Reproduced with permission from Ref. [87] |
Fig.3 (a) SEM micrograph of the fabricated metalens. Wavelength= 532 nm. (c) Picture of the Nikon objective lens (100× CFI60, NA= 0.8). (b) and (d) Measured focal spot intensity profiles of the metalens in (a) and the commercial objective lens in (c) at the wavelength of 532 nm. (a), (b), and (d) Reproduced with permission from Ref. [68]. (c) From the Nikon website |
3 Innovation in full-color imaging
Fig.4 Multi-wavelength achromatic metalenses. (a) False colored SEM image of achromatic metalens. Inset: schematic side view of metalens designed to focus three different wavelengths into the same focal plane. (b) Schematic diagram of one integrated-resonant unit cell of the broadband achromatic metalens for the incident wavelengths varying from 1200 to 1680 nm. (c) Phase spectra for five different elements schematically shown on the right. The shaded region indicates the design bandwidth of 120 nm. (d) Calculated phase and dispersion for the meta-atom library schematically shown in the inset. (e) Calculated neff for all designed meta-atoms. The meta-atoms schematically shown in the inset are chosen to compose the broadband achromatic metalens by selecting the maximum Δneff. (a) Reproduced with permission from Ref. [102]. (b) Reproduced with permission from Ref. [88]. (c) Reproduced with permission from Ref. [89]. (d) Reproduced with permission from Ref. [104]. (e) Reproduced with permission from Ref. [105] |
Tab.1 Summary of broadband achromatic metalenses |
radius/mm | NA | wavelength range/nm | /Hz | minimum /p | focal length/mm | material | focusing efficiency | comment | Ref. |
---|---|---|---|---|---|---|---|---|---|
27.775 | 0.268 | 1200–1680 | 7.14 × 1013 | 0.29 | 100 | Au | 12% | reflection scheme | [88] |
220 | 0.02 | 470–670 | 1.91 × 1014 | 0.44 | 63 | TiO2 | 20% | [89] | |
750 | 0.075 | 475–700 | 2.03 × 1014 | N/A* | 9960 | TiO2 | 35% | refractive lens and metacorrector with air gap | [90] |
50 | 0.24 | 1300–1650 | 4.90 × 1013 | 0.32 | 200 | amorphous silicon | 20%–58% | [104] | |
50 | 0.24 | 1200–1650 | 6.82 × 1013 | 0.44 | 200 | ||||
100 | 0.13 | 1200–1650 | 6.82 × 1013 | 0.47 | 800 | ||||
50 | 0.88 | 1200–1400 | 3.57 × 1013 | 1.13 | 30 | N/A | |||
25 | 0.106 | 400–660 | 2.95 × 1014 | 0.42 | 235 | GaN | 40% | [106] | |
16.67 | 0.36 | 1310–1550 | 3.55 × 1013 | 0.12 | 38 | Si | 50.07%–55.53% | theoretical work | [109] |
13.2 | 0.2 | 460–700 | 2.24 × 1014 | 0.32 | 67 | TiO2 | 30% | [110] | |
7 | 0.086 | 430–780 | 3.13 × 1014 | 0.10 | 81.5 | SiN | 36%–55% | [105] | |
32 | 0.81 | 1470–1590 | 1.54 × 1013 | 0.27 | 22.95 | Si | 21%–27% | theoretical work | [112] |
6.25 | 0.1 | 450–700 | 2.38 × 1014 | 0.08 | 60 | TiO2 | 43%–78% | theoretical work | [113] |
0.9 | 0.99 | 3 | 13%–32% | ||||||
0.99 | 1.37 | 0.9 | 23%–36% |
Note: *This device is a hybrid lens consisting of a metacorrector and a spherical lens. Thus, the hyperbolic phase profile is not applicable here. |
Fig.5 Simulated maximum focusing efficiency of a metalens designed by the unit-cell approach for diffraction order Nd is equal to 1 (black), 3 (red), and 5 (blue), respectively. Inset: maximum efficiency as a function of unit-cell periods L divided by the wavelength l. Reproduced with permission from Ref. [113] |
4 Functional applications of metalenses
4.1 Flat high-resolution focusing and imaging
Fig.6 High-resolution metalenses. Vertical cuts of the measured focal spot intensity profile of the metalens designed at 532 nm with (a) NA= 0.80, (b) NA= 0.98, and (c) NA= 1.48 in immersion oil, respectively. (d) Diffraction-limited image (left) and super-oscillatory image with the application of metasurface filter (right). (e) Schematic illustration of the designed resolution-enhanced metalens described in Ref. [118]. (a) Reproduced with permission from Ref. [68]. (b) and (c) Reproduced with permission from Ref. [100]. (d) Reproduced with permission from Ref. [119]. (e) Reproduced with permission from Ref. [118] |
4.2 Multifocal metalenses
Fig.7 (a) Schematic diagram of the working principle of the bifocal lens proposed in Ref. [120]. (b) Schematic diagram of the working principle of the designed bifocal metalens [92]. Bottom right: top view of the designed metalens consisting of nano-fins with same height but different cross sizes and orientations. (c) Schematic diagram of the multispectral chiral imaging metalens where the left-circularly polarized (LCP) light and right-circularly polarized (RCP) light from the same object are focused into two spots. Spiral arrows indicate helicity of the incident light. Blue and green nano-fins (top view) impart the phase profile required to focus RCP light and LCP light, respectively. The upper half of the image is formed by focusing the LCP light reflected from the beetle, whereas the lower half of the image is formed by focusing the RCP light reflected from the beetle. (d) Schematic diagram of the twofold polarization-selective metalens indicated in Ref. [121]. (e) Left: under the illumination of linearly polarized THz waves, two longitudinally distributed polarization-rotated focal points are demonstrated. Right: under the illumination of x-polarized THz waves, two transversely distributed focal points are demonstrated. (f) Measured point spread function of the achromatic varifocal metalens by rotating the polarization of linearly polarized input light of visible wavelengths. (a) Reproduced with permission from Ref. [120]. (b) Reproduced with permission from Ref. [92]. (c) Reproduced with permission from Ref. [122]. (d) Reproduced with permission from Ref. [121]. (e) Reproduced with permission from Ref. [123]. (f) Reproduced with permission from Ref. [124] |
Fig.8 (a) Schematic diagram of the MEMS-assisted metasurface triplet operating as a compact focus tunable microscope. (b) Schematic diagram of the rotationally tunable multifocal Moiré metalens. (c) Schematic diagrams of the rotationally tunable metalens doublet showing negative focal length variation (left), no focus (middle), and positive focal length variation (right). (a) Reproduced with permission from Ref. [125]. (b) Reproduced with permission from Ref. [126]. (c) Reproduced with permission from Ref. [127] |
4.3 Other functional imaging
Fig.9 (a) Left-hand side: schematic diagram of the principle of QPGM. Right-hand side: phase gradient image (PGI) formed by combining three DIC images (I1, I2, and I3). (b) LCP incident on the metasurface results in an output beam with Gaussian intensity distribution and a bright-field image. RCP incident on the same metasurface results in an output beam with donut-shaped intensity distribution and a spiral phase contrast image. (c) Microscopic spectral tomography images of frog egg cells with aplanatic metalens at different wavelengths. (d) Schematic diagram of the metalens-array-based quantum source. (a) Reproduced with permission from Ref. [129]. (b) Reproduced with permission from Ref. [130]. (c) Reproduced with permission from Ref. [131]. (d) Reproduced with permission from Ref. [132] |