3.1.1.

Electron beam lithography

EBL, a pivotal nanofabrication technique, precisely sculpts the electron beam resist at the nanoscale based on a computer-generated pattern. Acknowledged for its remarkable properties—high resolution, extensive design flexibility, and reliable repeatability—EBL has emerged as the predominant technique for dielectric fabrication in recent decades^[149]. Commercial EBL systems, typically operating within the 30 to 100 KeV energy range, are well-suited for fabricating sub-wavelength nanoantennas. The transfer techniques following exposure and development vary based on the specific dielectric material design and application. They are categorized into the bottom-up or top-down approach, with corresponding etching or lift-off processes. These approaches are grounded in film deposition methods, encompassing electron beam (E-beam) evaporation, sputtering deposition, chemical vapor deposition, and atomic layer deposition (ALD). The fundamental EBL process commences with the spin coating of a radiation-sensitive resist onto a designated substrate. The pattern is then delineated into the resist through high-energy electron radiation. In positive-type resist, the exposed area becomes soluble during development, while in negative-type resist, a cross-linking reaction in the exposed region renders it insoluble, and the background dissolves away. In the bottom-up approach, film deposition precedes lift-off to construct desired structures. Conversely, the top-down approach initiates with film deposition to achieve the dielectric layer at the desired thickness on the substrate before engaging in the EBL process, culminating the entire procedure with etching.

In the standard EBL fabrication procedure depicted in Fig. 4(a), monocrystalline silicon metasurfaces are crafted using the EBL technique, followed by a metal lift-off and dielectric etching process. Among these two approaches, the lift-off method utilizes a hard mask constructed with Cr, serving as a protector for subsequent etching. Yang et al. conducted experiments on commercially available silicon on the sapphire wafer, fabricating nanodisks with varying parameters of diameter/period arrays, showcasing high-performance structural color^[28]. This approach exhibits advantages in terms of a broad gamut, high saturation, brightness, and resolution. The high-resolution scanning electron microscope (SEM) images in Fig. 4(b) reveal a roughness smaller than 10 nm. Due to its precise control over geometric precision, EBL stands out as the most widely used process for defining functional nanostructures with unique shapes. As depicted in Fig. 4(c), Gao et al. proposed C-shaped Si nanoantennas, demonstrating high-efficiency cyan and blue third-harmonic generation (THG) holograms^[22]. This efficiency is achieved by introducing abrupt phase changes from 0 to $2\pi$ to the C-elements, precisely controlled by the outer radius, inner radius, and opening angle. The mature and highly reproducible CMOS-compatible fabrication technology of silicon allows for the efficient production of nanostructures, both horizontally and vertically. Figure 4(d) illustrates the smooth sidewall of silicon rectangle pillars, with an angle of nearly 90° in the vertical direction after reactive ion etching (RIE)^[150]. This showcases the successful demonstration of a tunable full-color vectorial hologram with metamolecules.

Fig. 4

The utilization of EBL in the fabrication of all-dielectric metasurfaces. (a) The schematic depicts the fabrication process of silicon-based structural color utilizing EBL.^[28] Initially, a PMMA resist film is spin-coated onto the silicon-coated sapphire substrate. Subsequently, the resist is exposed to an electron beam and developed to form PMMA nanostructures. The sample is then transferred to an E-beam evaporator and coated directly with Cr films. After a lift-off process involving immersion in acetone, the PMMA is removed, and the nanostructures are effectively transferred to Cr. The silicon is later etched away using reactive ion etching with ${\mathrm{CHF}}_3$ and ${\mathrm{SF}}_6$ gases. Finally, Si metasurfaces are obtained by immersing the sample in chromium etchant. (b) The top-view SEM image of silicon nanodisks for color printing. The diameters and periods of these three colors are 170/320, 90/200, and 160/300 nm, respectively. The scale bar is 500 nm.^[28] (c) The top-view SEM image displays the C-shaped silicon nanoantenna from nonlinear holographic all-dielectric metasurfaces. The insets include zoomed-in images showing high-resolution side-view SEM images.^[22] (d) The SEM image with a tilted view of 45° presents the fabricated silicon rectangle pillars used for tunable full-color vectorial meta-holography.^[150] (e) Fabrication process of high-efficiency ${\mathrm{TiO}}_2$ metasurfaces^[67]: (I) Application of the resist on fused silica. (II) Recording of the reversed metasurface pattern into the resist using EBL. (III) Deposition of ${\mathrm{TiO}}_2$ through atomic layer deposition (ALD) into the exposed substrate. (IV) Formation of the final thick ${\mathrm{TiO}}_2$ film, exceeding half the width of the maximum feature size after ALD. (V) Removal of residual ${\mathrm{TiO}}_2$ and resist via reactive ion etching with a mixture of ${\mathrm{Cl}}_2$ and ${\mathrm{BCl}}_3$ ions. (VI) Completion of the final metasurface after eliminating remaining resist. (f) The top and side-view SEM image of fabricated ALD-based ${\mathrm{TiO}}_2$ metalenses at visible wavelengths (scale bar, 300 nm)^[15]. (g) The fabrication process (left) of the ${\mathrm{TiO}}_2$ metasurface with lift-off technology^[30]: Top: Patterning of the photoresist with EBL. Middle: Formation of ${\mathrm{TiO}}_2$ film through electron-beam evaporation technology. Bottom: Elimination of the photoresist through lift-off. Corresponding SEM image (right) of one metasurface (scale bar, 500 nm). (h) Tilt view SEM images of the etched ${\mathrm{TiO}}_2$ nanostructures, including circular pillars, crosses, rectangle pillars, crosses, and rectangle pillars.^[101] (i) Fabrication flow of the GaN-based structures for a broadband achromatic metalens in the visible spectra.^[12] (j) SEM image of the on-axis focusing GaN-based metalens, applied in CMOS image sensor.^[151]

However, due to the persistent ohmic losses in silicon materials^[152–154], especially at shorter wavelengths, ${\mathrm{TiO}}_2$ has emerged as a promising alternative in recent years and holds the potential to replace silicon in the field of dielectric metasurfaces^[155–157]. ${\mathrm{TiO}}_2$ boasts a transparency window in the visible spectrum and a sufficiently high refractive index (2.87–2.31) for robust light–matter interactions. In the initial stages, Capasso’s group demonstrated high-efficiency ${\mathrm{TiO}}_2$ metasurfaces in the form of holograms for red, green, and blue wavelengths. The fabrication procedure, as illustrated in Fig. 4(e), involves using EBL to form reverse nanofins in resist, filling the exposed region with amorphous ${\mathrm{TiO}}_2$ through ALD, and then subjecting it to blanket RIE^[67]. The remaining electron beam resist is subsequently stripped, resulting in ${\mathrm{TiO}}_2$ nanofins with a roughness of less than 1 nm, as shown in the top image in Fig. 4(f). Additionally, they fabricated metalenses for the visible range [bottom image in Fig. 4(f)] with efficiencies as high as 86%^[15]. The geometrical parameters of the metalenses, defined by the resist, lead to the attainment of nearly 90° vertical sidewalls. However, current ${\mathrm{TiO}}_2$ metasurface fabrication techniques heavily rely on the ALD method, which is relatively slow and restricts thickness to values below 600 nm. In recent years, high-quality ${\mathrm{TiO}}_2$ films have been successfully formed using E-beam evaporation. In comparison to ALD, E-beam evaporation has proven to be more cost- and time-efficient^[158]. Xiao’s group showcased the application of ${\mathrm{TiO}}_2$ metasurfaces in dielectric full-color printing through EBL and a straightforward lift-off process, as illustrated in Fig. 4(g)^[30]. The nanostructures were constructed using E-beam evaporation to fill ${\mathrm{TiO}}_2$ into the reverse model. After a successful transfer of the reversed nanostructures onto the ${\mathrm{TiO}}_2$ film, ${\mathrm{TiO}}_2$ metasurfaces were ultimately formed through the lift-off process. The roughness of the ${\mathrm{TiO}}_2$ film measured at 0.66 nm signifies the achievement of an ultra-smooth surface. However, the lift-off method typically struggles to maintain perfect geometry due to the proximity effect during EBL. The cross-section of each subunit appears as a trapezoid rather than a rectangle in the vertical direction, as seen in Fig. 4(g). To address this issue, the researchers developed RIE technology for the ${\mathrm{TiO}}_2$ film fabricated by E-beam evaporation. Figure 4(h) demonstrates that the nanopillars exhibit nearly perfect vertical sidewalls, with a measured tilt angle of sidewalls around 89°–90°, effectively replacing the lift-off method^[101]. Apart from ${\mathrm{TiO}}_2$, GaN stands out as another promising dielectric material with a high refractive index and negligible loss in the visible range^[159]. It can be deposited using chemical vapor deposition. Tsai’s group has pioneered a top-down approach to creating a GaN metalens, as depicted in Fig. 4(i)^[12]. Employing integrated resonance unit elements within a metasurface framework, the successful demonstration of a flawless achromatic lens has been achieved^[12]. The fabrication process initiates with EBL, followed by an etching process to shape the nanostructures. Figure 4(j) provides a top-view SEM image of a GaN metalens after the etching process^[151]. While EBL possesses extraordinary advantages and has demonstrated numerous new concepts in metaphotonics, it is worth noting that it still grapples with disadvantages. These include limited area coverage, high expenses, and a complicated, time-consuming process. Additionally, there are costs associated with the purchase and maintenance of the EBL system.

3.1.2.

Focused ion beam lithography

FIB lithography stands as a potent and versatile instrument based on the intricate interactions between ions and solids. With capabilities extending beyond mere nanofabrication, FIB integrates functions for both fabrication and characterization. The ion beam, typically derived from a liquid metal tip such as gallium (Ga), replaces electrons to directly pattern features onto the target film without the requirement of a photomask^[160,161]. Through selective material removal or deposition, FIB enables the precise fabrication of planar micro/nano or three-dimensional (3D) structures, employing either the bottom-up or top-down approach^[162–164]. These effects emerge from the dynamic exchanges of energy and momentum between the incident ions and target atoms. FIB techniques are analogous to SEM, allowing for simultaneous milling and observation of the surface in the work area, a configuration known as the dual-beam system. It has become one of the most advanced fabrication platforms for dielectric metasurfaces.

Garg et al. utilized FIB for direct milling of nanoholes on a silicon substrate, showcasing a wide color palette achieved through variations in nanohole diameter. Due to the Gaussian profile of the Ga ion beam, the nanoholes exhibited a tapered morphology, as depicted in the inset and bottom SEM images in Fig. 5(a)^[165]. In comparison to alternative lithography techniques for color generation, the FIB process is notably simpler, requiring only a single step.

Fig. 5

The utilization of focused ion beam lithography in the fabrication of all-dielectric metasurfaces. (a) FIB gallium ion milling in a Gaussian profile for the direct fabrication of subwavelength nanoholes on silicon through scanning, enabling multicolor generation. The SEM image (right) displays the nanohole, with the corresponding enlarged cross-sectional view as an inset. SEM images (bottom) of color filter arrays, with the right corner in each image showing the cross-section SEM obtained by FIB cutting.^[165] (b) Chiral visible light metasurface patterned in monocrystalline silicon on sapphire using FIB. The SEM image on the left includes FIB paths and directions (curved arrows), and the inset is a tilted SEM image. The right image presents a 3D model of a unit cell.^[166] (c) All-dielectric phase-change reconfigurable metasurface consisting of a period grating fabricated by FIB milling in an amorphous ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST) film on silica. The SEM images depict tilt and cross-sectional views, shown on the left and bottom, respectively.^[167] (d) Schematic of the ZnO metasurface (top) for coherent vacuum ultraviolet light generation. Titled SEM image (bottom) of the FIB milled metasurface with scale bar of 300 nm.^[168] (e) The SEM image of a section of a silicon membrane nano-cantilever array from nanomechanically reconfigurable all-dielectric metasurfaces for sub-GHz optical modulation.^[169] (f) The process involves the stepwise fabrication of a dielectric slab waveguide photonic crystal, as illustrated on the left. On the right, SEM images provide both a top view and a cross-sectional view of dielectric structures fabricated using FIB milling lithography.^[170] (g) Side and top-view SEM images of self-rolled multilayer metasurfaces.^[171] (h) The 3D Archimedean spiral chiral metasurface milled by the gallium FIB and tilted SEM (right) images of the freestanding $\mathrm{Au}/{\mathrm{Si}}_3{\mathrm{N}}_4$ bilayer film.^[172]

In-depth exploration into the sophisticated 3D structures using FIB etching has been undertaken by Gorkunov et al. The researchers engineered a chiral, 4-fold rotationally symmetric monocrystalline silicon metasurface on sapphire through precise digital FIB patterning, as illustrated in Fig. 5(b)^[166]. This meticulously crafted metasurface demonstrates exceptional transparency within the visible spectrum while concurrently manifesting robust optical chirality. Moreover, FIB technology finds application in the fabrication of all-dielectric phase-change materials. Illustrated in Fig. 5(c), a nanograting array is precisely milled by FIB onto the phase-change medium, germanium antimony telluride (GST)^[167]. This method attains high-quality resonance and optical switchable all-dielectric metasurfaces in the NIR spectrum. Due to the FIB’s independence from material selectivity, it extends beyond the fabrication of dielectric materials from the visible to NIR region^[173,174], enabling the fabrication of materials in the UV region, such as ZnO, as demonstrated by FIB in Fig. 5(d) for the nonlinear optical generation of vacuum UV light at a wavelength of 197 nm^[168]. Moreover, the FIB not only demonstrates proficiency in handling single-layer dielectric films but also exhibits the capability to etch multilayer structures, as depicted in Fig. 5(f)^[170]. In this instance, an ${\mathrm{Nb}}_2{\mathrm{O}}_5$ layer is interposed between two ${\mathrm{SiO}}_2$ cladding layers to form a symmetric slab waveguide design. The creation of a high-quality structure is achieved through optimized FIB parameters, including the ion beam current, area dose, and a number of process loops, as illustrated in the SEM image on the right side of Fig. 5(f). However, deviations from optimal parameters result in increased surface roughness and deeper milling holes than desired, as evidenced in the inset of Fig. 5(f). It is noteworthy that FIB nanofabrication has emerged as a crucial and extensively employed technical tool in the production of 3D nanostructures and devices. Researchers have illustrated free-standing 3D bilayer structures fabricated through FIB, as depicted in the SEM images in Figs. 5(e), 5(g), and 5(h), respectively, showcasing the nano-cantilevers (polycrystalline silicon/indium tin oxide)^[169], self-rolled multilayer metasurfaces (${\mathrm{SiO}}_2/\mathrm{Au}$)^[171], and 3D Archimedean spiral chiral structures (${\mathrm{Si}}_3{\mathrm{N}}_4/\mathrm{Au}$)^[172], and they demonstrated the application of optical modulation. While FIB milling is the primary method for defining dielectric nanostructures, particularly the nanoholes in thick films, due to its maskless and resistless advantages, it comes with certain limitations. These include high cost, extremely low throughput, and poor geometric precision control caused by the redeposition effect during milling. Additionally, there are aspect ratio limitations and the potential for sample damage.

3.1.3.

Laser-based nanomanufacturing

In comparison to EBL and FIB, laser-based nanomanufacturing distinguishes itself as a versatile and standard tool for mask-free fabrication techniques. It involves the utilization of lasers to selectively remove or modify material on a substrate, enabling the creation of intricate patterns or structures at the micro- or nanoscale. This process finds widespread application in the semiconductor industry for producing integrated circuits, microelectromechanical systems, and various nanodevices. Laser-based nanomanufacturing stands out for its high precision, offering an avenue for high-efficiency, cost-effective, and large-scale fabrication. It facilitates the development of extremely small and detailed 3D optical features. Different variations of laser-based lithography exist, including techniques like laser interference lithography (LIL)^[175,176] and laser direct writing (LDW)^{[138,177–182]}, each with its specific advantages and applications. In this section, we delve into the recent advancements in LIL and LDW for dielectric metasurface fabrication. We explore the capabilities of LDW technology, encompassing laser-induced techniques^{[137,183,184]}, laser melting or ablating^[185–188], and two-photon LDW^[189–197].

LIL is emerging as an innovative nano-patterning method without using a mask. Its prominence stems from its higher efficiency compared to EBL or FIB technology, coupled with a wide workspace and low cost. In LIL, two or more coherent light beams incident from different directions overlap on the photoresist layer, creating interference patterns. The control of these patterns is achieved through parameters such as the exposure dosage, development time, light wavelength, and angle of incident beams. Furthermore, laser interference beams with high-power pulses can directly process the material surface, relying on photothermal or photochemical mechanisms. In Fig. 6(a), the interference patterns of light intensity distribution with 2, 3, 4, and 6 overlapping beams are revealed^[198]. The simplest scenario involves the interference of two beams, where beams with the same phase and polarization direction are symmetrical, sharing the same angle ($\theta$) and azimuthal angles ($\phi$). In other instances, the overlap of 3 and 6 beams results in periodic structures arranged in a triangular lattice but with different periods. Particularly noteworthy is the interference pattern generated by the overlap of 4 beams, yielding a square array of intensity maxima. Leveraging this technology, researchers have devised various methods for constructing dielectric nanostructures. Chen et al. reported the fabrication of a 2D diffraction ${\mathrm{HfO}}_2$ grating designed for achieving polarization-independent high-efficiency properties. This structure was created through twice orthogonal exposures by LIL, where two coherent laser beams overlapped in a negative-type photoresist. Subsequent to this, ion-beam etching technology was employed, as depicted in the SEM image in Fig. 6(b)^[199]. Using the same procedures at positive photoresist, upon removal of the unexposed material, the pattern will change geometry from a nanodisk to a nanohole. Seo et al. introduced a large-area printed silicon hole array fabricated through LIL on a silicon-on-insulator (SOI) wafer designed for broadband reflectors. The corresponding SEM images are depicted in Fig. 6(c)^[200]. In addition to transforming the physical structure into the photoresist using the spatial intensity distribution of the interfering beams, Berzinš et al. demonstrated the application of single-pulse LIL with a four-beam picosecond laser, featuring a pulse duration of 300 ps and a wavelength of 532 nm [refer to the left image of Fig. 6(d)]. This technique was employed for the direct patterning of an amorphous silicon film into an array of Mie resonators, each a few hundred nanometers in diameter. As illustrated in the middle image of Fig. 6(d)^[93], the amorphous silicon film melted when exposed to sufficiently high temperatures, causing the continuous film to break into p-Si-based nanohole structures [Fig. 6(d), right top], referred to as the intermediate regime. If the laser energy density is further increased, it results in the formation of an array of Mie-resonant nanoparticles [Fig. 6(d), right bottom]. Moreover, LIL has faced challenges due to complex setups in multibeam lithography, leading to issues such as sensitivity and limited control over the fabricated structure. To address these concerns, researchers have introduced metasurface-mask-assisted LIL for 3D nanofabrication [left image of Fig. 6(e)]. This approach offers significant flexibility in patterning various periodic structures^[201]. The diamond metasurface mask [middle SEM image of Fig. 6(e)] generates different diffraction orders that interfere to achieve a specific desired intensity pattern, ultimately realizing 3D structures in the photoresist [right SEM image of Fig. 6(e)].

Fig. 6

The utilization of LIL in the fabrication of all-dielectric metasurfaces. (a) Schematic representation (top row) of the interference setup with two, three, four, and six overlapping beams, along with the corresponding light intensity patterns (bottom row)^[198]. (b) Top-view SEM of polarization-independent two-dimensional diffraction metal–dielectric grating.^[199] (c) SEM image (left) of the silicon nanohole array pattern on SOI substrate and the appearance (right) of large-area printed broadband membrane reflectors by LIL. The inset shows the enlarged SEM image.^[200] (d) Mie-resonant silicon-based metasurfaces via single-pulse LIL: Schematic (left) illustrating the four-beam interference setup and the resulting intensity distribution. Mechanism of silicon transformation (middle) utilizing the interference intensity pattern. Top view (right) SEM images of nanoholes (intermediate state, a-Si) and nanoparticles (Mie resonator state, p-Si) patterned from the Si film, respectively. Scale bar = 1 µm.^[93] (e) Metasurface-generated complex 3D optical fields for LIL: Concept (left) of a metamask generating specific diffraction orders to create a desired 3D pattern in the photoresist. Optical image (middle) of the fabricated optimized diamond metamask. Simulated and measured cross-sections (right) of the captured 3D intensity pattern under 514 nm laser illumination.^[201]

Indeed, the use of multiple beams in LIL enables the simultaneous exposure of a large area, making it a faster and more cost-effective method for maskless massively parallel patterning. However, it faces challenges in achieving wafer-scale pattern uniformity and the freedom of pattern geometry, making it less competitive than e-beam lithography in these aspects. The limitations of LIL may stem from the absence of suitable large-area mirrors or other interference optics. Additionally, LIL operates as a batch process with constraints on producing only periodic patterns^[202–204].

In contrast to the LIL technique, LDW employs a focused laser beam to directly pattern a substrate, making it well-suited for creating intricate, customized 2D or 3D functional structures. LDW, also known as direct printing and digital writing, operates as a serial process where the laser beam traverses the substrate, selectively exposing or modifying the material. Over the past decade, research efforts have intensified, leading to the development of new applications for dielectric metasurfaces using LDW. This technique utilizes laser melting or ablating, laser-induced methods, and two-photon LDW, providing a single-step, lithography-free, and cost-efficient approach for large-scale fabrication of both ordered and disordered structures. Notably, LDW eliminates the need for a clean room and vacuum chambers. Figure 7(a) illustrates the utilization of patterned pulse LDW lithography for the fabrication of metasurfaces comprising sub-wavelength structural units^[205]. Employing the pulse LDW system depicted on the left im age of Fig. 7(a), the film undergoes ablation and recasting, forming fine H-shaped units [top right SEM image of Fig. 7(a)] as a result of the scanning of the patterned femtosecond laser (520 nm) achieved by the phase mask on the spatial light modulator (SLM). Subsequently, H-shaped arrays for dielectric metasurfaces are produced, with the etching process utilizing the nanopatterned films as hard masks [bottom image of Fig. 7(a)]. Furthermore, Bochek et al. experimentally demonstrated the direct fabrication of the tunable metasurfaces from ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ films by exploiting the LDW technique^[206]. Figure 7(b) displays the SEM images of the metasurface fabricated by laser (780 nm) inscription on GST films deposited on glass and sapphire substrates. Another one-step LDW fabrication method is based on the laser-induced technique. Figure 7(c) illustrates the application of the laser-induced graphene technique for processing a complex amplitude-modulating metasurface device^[207]. A $15\mathrm{mm}\times 15\mathrm{mm}$ metasurface composed of C-shaped graphene antennas can be fabricated by one-step laser writing in 34 s [visible in the photograph and SEM image in Fig. 7(c) on the right]. More complex 3D structures can also be fabricated using the laser-induced method. Lu et al. introduced a polarization-directed chiral growth of dielectric materials (${\mathrm{PdO}}_2-x$, CdS) induced by a laser with the vector beam^[208]. As shown in the right image of Fig. 7(d), the 3D chiral unit of metasurfaces is induced in an inorganic precursor solution under continuous-wave laser irradiation with different vectors.

Fig. 7

The utilization of LDW in the fabrication of all-dielectric metasurfaces. (a) The patterned pulse LDW approach for creating sub-wavelength features. The schematic illustration (top) shows the pulse LDW system, and the SEM image of fabricated H-shaped arrays with sub-wavelength feature size is visible. Scale $\mathrm{bars}=5\mathrm{\mu m}$. Fabrication flow (bottom) of structures by etching after pulse LDW (Cr used as masks on fused silica substrate), along with SEM images of the H shapes.^[205] (b) The fabrication process of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ metasurfaces by LDW. The schematic on the left illustrates the laser casting process of a GST film through LDW. The AFM image in the middle showcases the fabricated metasurface with a square array, while the SEM images on the right depict the metasurface on glass and sapphire substrates, respectively.^[206] (c) THz optical pattern recognition enabled by a complex amplitude modulating metasurface through LDW: Schematic (left) of the fabrication of laser-induced graphene C-shaped antennas by LDW with different orientations and angles. Optical and SEM image (right) of the fabricated metasurface.^[207] (d) Polarization-directed growth of spiral nanostructures by LDW with vector beams: Illustration of optical setup (left). Simulated beam profile with different vectors and SEM images of the chiral patterns formed with different vector beams (right).^[208]

Another advanced LDW technology two-photon lithography, also referred to as two-photon polymerization, represents an advanced LDW technology for the fabrication of true 3D micro/nanostructures. This method exploits nonlinear photon absorption by photopolymers, enabling the construction of well-defined structures with sub-100-nm resolution. While standard photolithography is commonly employed for 2D structure fabrication, relying on one-photon absorption in a photosensitive medium, the two-photon process involves tightly focusing the beam of an ultra-fast laser inside the volume of a transparent material. This leads to the absorption of two photons, inducing local polymerization. In Fig. 8(a), the true 2D photonic structures or dielectric metasurfaces, along with their inverted counterparts featuring square C4v and hexagonal C6v lattice symmetry, are showcased. These structures are fabricated using two-photon LDW with femtosecond pulses at 780 nm. Experimental results demonstrate a transition from the multi-order diffraction regime characteristic of photonic crystals to the regime where only zero-order diffraction is observed, serving as a distinctive feature of metasurfaces^[209]. Zhan et al. have introduced a computational inverse design method and implemented the fabrication of arrays of spherical Mie scatterers with the aim of generating 3D optical field patterns through two-photon lithography. The SEM image in Fig. 8(b) illustrates the outcome of this process, showcasing the arrays of scatterers. Notably, these scatterers exhibit a deviation from perfect spherical shapes^[210]. Following this, a more intricate inverse-designed structure intended for a free-form NIR polarization beam splitter is produced through 3D printing using two-photon lithography. The SEM image of the device is depicted in Fig. 8(c)^[211]. The device was printed layer by layer vertically on top of the fused silica substrate. Furthermore, two-photon lithography offers an exceptional opportunity for the direct printing of optical functional metasurfaces on a fiber tip, as compared to techniques such as EBL^[212] and FIB^[213]. Plidschun et al. employed two-photon lithography to create nanopatterns on the end faces of modified single-mode optical fibers for multi-focus holography [Fig. 8(d)]^[214]. Additionally, Hadibrata et al. achieved 3D printing of a metasurface consisting of a circular grating-like structure on an optical fiber tip for metalens applications [left SEM image of Fig. 8(f)]^[215]. Simultaneously, they employed a computational inverse-design method based on an objective-first algorithm to design this structure. They showcased the application of the fabricated metalens with a fiber tip using a homemade two-photon polymerization process, as depicted in the right image of Fig. 8(f), where the pattern “NU” is produced using the LDW setup. In Fig. 8(e), perfect absorber metasurfaces with a unique 3D helix-based dielectric architecture for the IR spectral range are presented using the LDW^[216]. The fabricated samples showcase resonant absorption bands with peak absorbance exceeding 80% in the wavelength range of 6–11 µm. LDW is typically a slower process, involving the scanning of a focused laser beam point by point, especially in the context of 3D printing by two-photon lithography, making it unsuitable for wafer-scale fabrication. Nevertheless, despite these limitations, LDW continues to be a valuable technique in applications where high precision and resolution are paramount, such as in microelectronics, photonics, and biomedical devices.

Fig. 8

The utilization of two-photon lithography in the fabrication of all-dielectric metasurfaces. (a) Dielectric metasurfaces in the optical diffraction with a fine structure: SEM images of fabricated dielectric particle/inverted counterpart structures and corresponding optical diffraction patterns.^[209] (b) SEM images of the device for controlling 3D optical fields via inverse Mie scattering.^[210] (c) SEM images of an inverse-designed near-infrared polarization beam splitter fabricated by two-photon LDW.^[211] (d) SEM image of a two-photon LDW-printed hologram on the single-mode fiber facet (scale bar is 20 µm).^[214] (e) Realization of a helix-based perfect absorber for IR spectral range using two-photon LDW: Fabrication process (left) of dielectric templates. Zoom-in SEM image (right) of a single-turn metallic helix.^[216] (f) Inverse design and 3D printing of a metalens on an optical fiber tip for two-photon LDW: SEM image (left) of the fabricated metalens on top of the fiber. Schematic of the homebuilt two-photon polymerization setup (right). The inset shows the SEM image of the pattern “NU” produced by the metalens using the homemade setup.^[215]

3.2.1.

Grayscale lithography

Grayscale lithography stands out among advanced lithography methods, offering a novel approach to designing and fabricating intricate nanostructures. In contrast to nanolithography techniques like EBL, FIB, and laser lithography, which primarily focus on the fabrication of nanostructures with the same height, grayscale lithography introduces an additional level of control by enabling variable light intensity during the exposure process. This innovation addresses the limitation of these nanofabrication methods that often neglect the ability to vary the height of nanostructures on a single metasurface. In grayscale lithography, the intensity of light can be finely tuned, providing a continuum of exposure levels rather than a binary on-off state that achieves arbitrary structure control in the Z direction and X–Y direction simultaneously. This fine control empowers the creation of nanostructures with varying heights or depths, significantly enhancing the spatial regulation of nanostructures. This capability opens the door to the development of more sophisticated photonic devices with tunable height, including applications in color printing, multi/hyperspectral imaging, and metalenses^[228–232]. EBL stands out as a compelling choice for grayscale nanopatterning. Geng et al. innovated a grayscale nanofabrication method using EBL and ALD, as illustrated in the schematic in Fig. 9(a)^[233]. The process involves employing EBL to inscribe designed nanopatterns with varying irradiation doses, resulting in resist nanoholes with different depths that serve as a grayscale template. Subsequently, ALD is utilized to fill these nanoholes with ${\mathrm{TiO}}_2$. Finally, ${\mathrm{TiO}}_2$ nanostructures with diverse heights are constructed after removing the resist and the underlying ${\mathrm{TiO}}_2$ film through etching. The SEM images depict a simultaneous structural change in three dimensions of the nanopillars along the X–Y–Z directions. Additionally, they proposed achieving structural color display through height modulation [as seen in the bottom image of Fig. 9(a)]. An alternative to EBL for grayscale nanolithography is FIB milling^[234,235]. Hentschel et al. reported the fabrication of resonant Mie voids through FIB milling into silicon wafers. The top SEM image in Fig. 9(b) displays voids with varying diameters and depths. Furthermore, they explored applications in color printing using voids of constant diameter and varying depth, as depicted in the middle and bottom images of Fig. 9(b)^[234].

Fig. 9

The utilization of grayscale lithography in the fabrication of all-dielectric metasurfaces. (a) Grayscale lithography enables the creation of height-gradient-tunable nanostructure arrays, as illustrated in the schematic (top). This method utilizes grayscale EBL, ALD, and a transfer process for assembling nano-configurations with varying heights. The bottom row displays typical SEM images of ${\mathrm{TiO}}_2$ nanopillars with height variations, showcasing the structural color metasurface after implementing height regulation.^[233] (b) FIB grayscale lithography facilitates color printing using Mie voids in a silicon substrate. The top part illustrates conically shaped voids with varying diameters and depths in a bulk silicon wafer. In the middle, optical microscope and SEM images showcase selected sizes and depths of Mie voids. The bottom section presents an optical microscope image of the color-printed image.^[234] (c) Two-photon grayscale LDW is employed for 3D-printed low-index nanopillars, as outlined in the schematic of the fabrication process (left). The right side features a tilted view SEM of the 3D printed colorful painting, revealing pillars with varying heights, diameters, and periodicities.^[236] (d) Custom multispectral filter arrays are fabricated using EBL and laser grayscale lithography. The left section demonstrates a dose-modulated pixel array by EBL, displaying the dose-modulated pattern, optical micrograph, and corresponding AFM data. The middle part outlines the fabrication flow of photolithography-based multispectral filter arrays using a binary photomask. The right side presents an optical micrograph (transmission) of a different region of the wafer, with a labeled equivalent exposure pattern (inset).^[237]

Two-photon LDW presents a novel and alternative approach for grayscale nanolithography. Wang et al. explored color generation from nanopillars with varying heights, periodicities, and diameters in dielectric material using two-photon grayscale lithography [as depicted in the left image of Fig. 9(c)]^[236]. The SEM images of fabricated nanopillars with different heights are shown in the right image of Fig. 9(c). This method allows for the creation of arbitrary colorful and grayscale images by precisely controlling the parameters. In addition to these techniques, UV lithography can also be employed in grayscale lithography. As illustrated in Fig. 9(d), to showcase multispectral filter arrays in a single lithographic step across the visible to NIR spectrum, researchers combined grayscale lithography with metal–insulator–metal Fabry–Perot cavities, where the exposure dose controls cavity thickness. Initially, low-volume EBL grayscale lithography was employed, followed by the demonstration of large-volume UV mask-based photolithography using the binary mask approach^[237].

3.2.2.

Multistep lithography

Multistep lithography involves a sequence of lithographic processes aimed at creating intricate nanostructures. This strategy incorporates multiple patterning steps, each contributing to the final design. Multistep lithography is chosen when a single lithographic step proves insufficient for achieving the desired complexity. In Fig. 10(a), Lu et al. demonstrated the two-step lithography fabrication of a versatile silicon-based metasurface with superwettability for self-cleaning and dynamic color response^[238]. This involved utilizing a Cr mask for the optical metasurface through EBL, followed by fabricating the resist mask for wettability-supporting structures using laser beam lithography. The process continued with plasma etching and mask removal, concluding with a treatment to achieve the desired wettability state. In another instance, researchers employed a combination of EBL and photolithography to fabricate a synthetic aperture metalens, as outlined in Fig. 10(b)^[221]. This fabrication process included patterning a-Si nanopillar-based metalens and an aluminum (Al) mask to block unwanted light transmission outside the metalens aperture. The patterning steps were carried out through EBL and photolithography, respectively. Hybrid patterning techniques, combining various lithography methods, have been developed to realize metasurfaces with more complex nanostructures. Additionally, these techniques enable the creation of hybrid metasurfaces comprising both metal and dielectric materials with different geometries. In the left image of Fig. 10(c), Guo et al. investigated multipolar coupling in metal–dielectric metasurfaces, involving arrays of gold nanorods coupled to silicon nanodisks^[222]. The fabrication process involved a two-step EBL process combined with a precision alignment step. The first step defined the silicon nanodisks through EBL and etching on an SOI wafer. In the second step, cross-shaped gold alignment marks were lithographically defined at the corners of the nanodisk arrays for fabricating plasmonic gold nanorods through EBL and lift-off. The resulting nanoantennas are depicted in the SEM image on the right side of Fig. 10(c).

Fig. 10

The utilization of multistep lithography in the fabrication of all-dielectric metasurfaces. (a) A versatile metasurface with self-cleaning and dynamic color response, fabricated through EBL and laser lithography: Two super wettability states switchable with hydrophobic treatment or ${\mathrm{O}}_2$ hydrophilic treatment (left).^[238] Illustration of the multistep fabrication process for the sample (right). (b) Schematic of the fabrication process for the synthetic aperture metalens using EBL and photolithography.^[221] (c) Hybrid metal–dielectric metasurfaces: Stepwise fabrication process of stacked hybrid nanoantenna metasurfaces (left). SEM images of the fabricated metasurfaces (right).^[222]

3.2.3.

Scanning probe lithography

SPL is another direct-write nanolithography technique that has emerged as an area of research in the field of atomic force microscopy. SPL employs a probe or tip in close proximity to a sample to directly pattern nanometer-scale features on the sample surface. This method offers high precision and flexibility in the creation of nanostructures. The interaction between the probe and surface is pivotal in the patterning process, and SPL can be categorized into various modes based on this interaction, including thermal, electrical, mechanical, and diffusive processes during the patterning operation. Figure 11(a) illustrates the SPL operation, depicting the intricate interactions involved in the process^[239].

Fig. 11

The utilization of scanning probe lithography in the fabrication of all-dielectric metasurfaces. (a) SPL for imaging and patterning applications.^[239] (b) Nanopatterning achieved through combined thermal SPL and dry etching, along with corresponding SEM images of silicon fabrication using a ${\mathrm{SiO}}_2$ hard mask.^[240] (c) Fabrication of a tunable metasurface based on ${\mathrm{VO}}_2$ using electric-field SPL with precise depth control: Topography image of square patterns modulated by different tip bias voltages and followed by sonication. The scale bar is 2 µm (left). Schematic representation of the main steps in fabricating the designed metasurface (right).^[241]

In thermal SPL, localized material modifications, such as crystallization, evaporation, and melting, are induced using thermal energy from a heated tip^[242]. Lisunova et al. showcased high-aspect-ratio nanopatterning in silicon through combined thermal SPL and dry etching, as illustrated in Fig. 11(b)^[240]. This method relies on the self-amplified depolymerization of a poly(phthalaldehyde) (PPA) thin film in contact with a heated probe. The SPL tool was utilized to inscribe patterns in the PPA thin film, which were then transferred into the ${\mathrm{SiO}}_2$ hard mask through etching. In the final etching step, the ${\mathrm{SiO}}_2$ pattern was further transferred into Si using deep RIE. The SEM images portraying the transferred patterns and the silicon sidewall are presented in the inset of Fig. 11(b), showcasing remarkable accuracy and uniformity, with a sidewall angle profile measuring approximately $87°\pm 2°$. SPL, grounded in a tip-induced electric field, emerges as a promising avenue for prototyping. Researchers have demonstrated a precise etching strategy for ${\mathrm{VO}}_2$ by directly writing on a ${\mathrm{VO}}_2$ film with a negative tip bias, followed by sonication removal of the written area. As depicted in the left image of Fig. 11(c), the results indicate that ${\mathrm{VO}}_2$ can be etched layer by layer through alternately repeating tip modulation and sonication, allowing for the creation of arbitrary patterns^[241]. Leveraging this approach, a metasurface was designed by arranging ${\mathrm{VO}}_2$–gold nanoblocks with varying sizes and heights, enabling spectrally selective tunable reflectivity in the near- and mid-IR, as shown in the right image of Fig. 11(c).

These innovative lithography techniques, encompassing grayscale lithography, multistep lithography, and probe scanning lithography, broaden the horizons of nanostructure design and fabrication. Researchers and engineers can select the most fitting method according to the particular demands of their applications, providing enhanced flexibility in crafting intricate nanostructures. Nonetheless, it is essential to note that all of these methods are characterized by low throughput and high costs. Substantial enhancements are imperative to enable large-area patterning for widespread application in large-scale manufacturing processes.

3.3.1.

UV lithography

Wafer-scale UV lithography employs UV light sources with specific wavelengths to pattern semiconductor wafers during the fabrication of integrated circuits. In semiconductor lithography, two types of photolithography tools are utilized in transferring patterns onto silicon wafers during the manufacturing process: the “stepper system” and the “scanner system”^[266]. The stepper employs a step-and-repeat process, where the reticle (mask) and wafer remain stationary for each exposure, exposing one small part of the wafer at a time. The pattern is replicated across the wafer by repeatedly exposing and incrementally stepping the wafer position. On the other hand, the scanner utilizes a scanning system where the reticle and wafer move synchronously during the exposure process, exposing the entire wafer simultaneously.

Light is systematically scanned across the reticle, and the resulting exposed pattern is projected onto the entire wafer. Utilizing these systems, three UV lithography techniques are currently employed in metasurface fabrication: i-line ($\lambda =365\mathrm{nm}$) stepper lithography, argon fluoride (ArF) DUV ($\lambda =193\mathrm{nm}$) immersion scanner lithography, and krypton fluoride (KrF) stepper DUV ($\lambda =248\mathrm{nm}$) lithography. The inherent planarity of metasurfaces aligns with fabrication processes directly compatible with the mature integrated circuit industry, providing opportunities for mass production. Employing i-line stepper lithography, as depicted in Fig. 12(a), researchers successfully fabricated a single 2-cm-diameter [refer to photograph in Fig. 12(a)] silicon-based NIR metalens, showcasing high-quality imaging and diffraction-limited focusing^[267]. However, the large wavelength of i-line stepper lithography prevents the creation of nanostructures with smaller feature sizes suitable for working at visible wavelengths. Current state-of-the-art photolithography tools employ DUV light with wavelengths of 248 nm (KrF) and 193 nm (ArF). KrF DUV lithography has found applications in semiconductor manufacturing, striking a balance between resolution and cost-effectiveness for specific uses. As illustrated in Fig. 12(c), Park et al. presented centimeter-scale nanopillar-based metalenses made of glass fabricated using KrF DUV lithography, demonstrating the capability of focusing and imaging at visible wavelengths^[268]. Furthermore, Leitis et al. employed KrF DUV lithography to introduce a versatile method for wafer-scale and cost-effective fabrication of Ge metasurfaces. This approach facilitates mid-IR photonics and biosensing in a high-throughput manner on silicon wafers, supporting optically transparent free-standing ${\mathrm{Al}}_2{\mathrm{O}}_3$ membranes [indicated by the blue color label in the SEM image of Fig. 12(b)]^[269]. ArF DUV lithography stands as a more sophisticated technology compared to i-line and KrF lithography, proving instrumental in the manufacturing of dielectric metasurfaces with reduced feature sizes. Xu et al. showcased the mass production of a Si nanopyramid-based metasurface polarizing bandpass filter on a 12 in. wafer. This achievement was realized through the utilization of 193 nm ArF DUV immersion lithography and direct inductively coupled plasma etching, strategically employed to manipulate polarization in the shortwave IR wavelength range^[270]. Moreover, metasurface-based subtractive color filters have been manufactured on 12 in. glass wafer substrates utilizing 193 nm ArF DUV lithography. Through variations in pillar height and pitch, metasurfaces displaying different colors are attained, as illustrated in Fig. 12(e). It is noteworthy that the patterned a-Si layer is transferred from the Si wafer to a glass wafer via a bonding glue-assisted layer transfer process, as depicted in the bottom right of Fig. 12(e)^[100,271]. They also showcased an a-Si metalens designed for fingerprint imaging operating at 940 nm. This metalens was fabricated on a 12 in. glass wafer using the 193 nm ArF DUV lithography. The enlarged optical image of the central die on the wafer is depicted in Fig. 12(f), highlighting the metalens, which is labeled by a red circle^[272]. Most recently, Park et al. showcased an all-glass metalens with a diameter of 100 mm designed for operation in the visible spectrum, employing KrF DUV projection lithography as illustrated in the top panel of Fig. 12(g)^[136]. A photograph and a tilted SEM image of the 100-mm-diameter metalens are presented in the bottom left of Fig. 12(g), respectively. Furthermore, they successfully demonstrated direct astronomical imaging of the Moon at visible wavelengths, as depicted in the bottom right of Fig. 12(g).

Fig. 12

The utilization of UV lithography in the fabrication of all-dielectric metasurfaces. (a) The schematic diagram illustrates the production of large-area metalenses utilizing photolithographic stepper technology. An accompanying photograph of the fabricated metalens (upper right) showcases a 2 cm diameter. A SEM image of the metalens center (center right) reveals the nanocylinder constituting the metalens. Scale bar: 2 µm.^[267] (b) The fabrication process of wafer-scale membrane-based metasurfaces for mid-infrared photonics and biosensing. In the top right photograph, a fully processed 4 in. silicon wafer is depicted, showcasing large-area metasurfaces. The bottom right provides a high-magnification SEM image of the metasurface unit cell, revealing the thin ${\mathrm{Al}}_2{\mathrm{O}}_3$ membrane in blue and the Ge resonators in orange.^[269] (c) The left image illustrates the fabrication process of an all-glass, large metalens at visible wavelengths utilizing DUV projection lithography. On the right, zoomed-in SEM images showcase the nanopillars of the metalens, while a photograph displays the fabricated 1 cm metalenses on a 4 in. ${\mathrm{SiO}}_2$ wafer.^[268] (d) Photograph of the 12 in. silicon metasurface wafer, meticulously fabricated through the Multiple-Projects Wafer (MPW) line at Microelectronics IME for the production of polarizing bandpass filters. A detailed examination of the wafer center is presented in the zoomed-in view (left). Additionally, a tilted SEM image and a cross-sectional TEM image showcase a single silicon pyramid with intricate details (right).^[270] (e) Image of a metasurface-based subtractive color filter crafted on a 12 in. glass wafer through a CMOS platform. In the top-right section, there are photo images of the central die subsequent to wafer dicing. The bottom-right segment illustrates the schematic of the fabrication process for layer transfer onto the glass wafer.^{[100, 271]} (f) The left column depicts CMOS-compatible a-Si metalenses on a 12 in. glass wafer designed for fingerprint imaging, with the central circled region on the wafer highlighting the metalens. The right column showcases SEM images captured at both the central and outer zones of the metalens.^[272] (g) The fabrication process of an all-glass 100-mm-diameter visible metalens for imaging the cosmos is depicted in the top panel. The bottom-left section features a photograph of the metalens, providing a size comparison with a table tennis racket, along with a SEM image showcasing the fused silica nanopillars constituting the metalens. In the bottom-right section, a photograph of the astro-imager is presented, consisting solely of a 100-mm-diameter metalens, an exchangeable optical filter, and a cooled CMOS monochromatic sensor. Additionally, an acquired image of the Moon is displayed.^[136]

Each of these lithography techniques signifies a milestone in the evolution of semiconductor manufacturing, with continuous endeavors to extend the boundaries of resolution and enhance the overall efficiency of the metasurface fabrication process.

3.3.2.

Nanoimprint lithography

Nanoimprint lithography (NIL) has emerged as a pivotal tool in nanofabrication, providing a cost-effective, high-throughput, and large-scale production alternative to traditional photolithography techniques. In contrast to optical lithography, NIL is a nanofabrication technique that involves replicating patterns from a mold onto a resist-coated substrate, typically using a polymer material through physical deformation or curing. There are two main types of NIL based on the curing mechanism: thermal nanoimprint lithography (T-NIL) and ultraviolet nanoimprint lithography (UV-NIL)^[273–276]. Thermal NIL relies on heating and cooling, wherein the resist material is softened or made molten by heating it to a temperature above its glass transition temperature. The mold is then pressed into the softened resist material, creating a replica of the mold pattern when its temperature is brought below the glass transition temperature. UV-NIL eliminates the need for heating and cooling processes using UV light to cure a UV-sensitive resist material. In comparison to T-NIL, UV-NIL is widely employed in metasurface fabrication due to its higher resolution and faster processing. Moreover, soft NIL, involving flexible molds made of polymeric materials like polydimethylsiloxane (PDMS), has been explored in the NIL process. This method, termed as soft NIL, enables the fabrication of metasurfaces on arbitrary surfaces^[277,278]. The resist imprints the reverse structure from the original mold, not only serving as the hard mask but also directly functioning as the optical device. Yao et al. fabricated heterogeneous all-dielectric metasurfaces consisting of a-Si, ${\mathrm{Si}}_3{\mathrm{N}}_4$, and ${\mathrm{SiO}}_2$, acting as an efficient ultra-broadband reflector through the NIL patterning process and etching. In the fabrication process illustrated in Fig. 13(a), the NIL-patterned resist is employed to create a Cr mask. Once the Cr mask is formed, RIE recipes are developed to etch the dielectric materials, resulting in a well-defined etching profile^[279]. Moreover, the replication and transfer of nanostructured photoresist patterns onto the substrate can function as an etching mask for the final structure. As illustrated in Fig. 13(b), the primary stamp precisely transferred the master’s pattern into the SU-8 photoresist. Subsequently, through optimized etching, well-controlled structures on the silicon can be obtained^[280]. The conventional UV-NIL process typically employs the resist as the sole hard mask. However, to enhance the functional capabilities of the NIL resist, Yoon et al. pioneered the development of a ${\mathrm{TiO}}_2$ nanoparticle-embedded UV-curable resin, illustrated in Fig. 13(c)^[281]. Consequently, a dielectric metalens can be directly fabricated with a single step of UV-NIL, eliminating the need for secondary processes such as deposition or etching. In a subsequent investigation, Kim et al. introduced a metahologram crafted from a ${\mathrm{TiO}}_2$ nanoparticle-embedded resist (PER), achieving a remarkable efficiency record of 90.6%. This metahologram was manufactured using a one-step NIL process, as depicted in Fig. 13(d)^[126]. Furthermore, this one-step nanoimprinting facilitates transfer onto various substrates, ranging from flexible films to diverse hard surfaces like PC, PP, and curved glasses. To address issues such as warpage and breakage of nanostructures during mold detachment, Choi et al. innovatively replaced the detaching process with wet etching of a replica mold crafted from a water-soluble polymer, polyvinyl alcohol (PVA). The fabrication process for wet etching NIL is illustrated in Fig. 13(e)^[282]. ${\mathrm{TiO}}_2$ PER is applied to the PVA replica mold through spin coating. Following the curing of PER under sufficient pressure and UV illumination, the substrate adheres to the PVA mold. As PVA is a water-soluble polymer, the PVA mold covering the substrate dissolves, leaving only the replicated ${\mathrm{TiO}}_2$ nanostructures on the substrate. This method, unlike conventional NIL, avoids introducing shear stress in cured nanostructures, enabling the replication of various patterns without faults. Moreover, through the utilization of multilayer NIL and solution-phase epitaxy (SPE), high-aspect-ratio ZnO nanopillar-based metalenses are successfully demonstrated^[283]. The main procedure, as illustrated in Fig. 13(f), involves initially replicating an OrmoStamp mold from the silicon mold. Subsequently, a nanoimprint resist is spin-coated on top of the multilayer structure, patterned by a thermal nanoimprint with the OrmoStamp mold, and followed by dry etching of the ${\mathrm{SiO}}_2$ layer and the PMMA layer. The PMMA layer is fully etched to expose the ZnO seed layer. Finally, ZnO nanopillars are grown in a precursor solution, and the residual ${\mathrm{SiO}}_2$ and resist layers are removed by dry etching, resulting in the acquisition of the ZnO metalens.

Fig. 13

The utilization of nanoimprint lithography in the fabrication of all-dielectric metasurfaces. (a) The fabrication process of an all-dielectric heterogeneous metasurface, designed as an efficient ultra-broadband reflector, is depicted on the left. The cross-sectional SEM image on the right provides a detailed view of the fabricated metasurface.^[279] (b) Schematic representation of the fabrication process for silicon nanostructures with adjustable geometries through soft nanoimprint lithography and reactive ion etching (left). The initial imprinted SU8 nanostructure is presented in the first column before etching. The second column displays the results after well-controlled etching times, showcasing the tunable meta-atom geometries (right).^[280] (c) Single-step manufacturing of hierarchical dielectric metalenses in the visible. SEM images depict the master mold, soft mold, and final plum pudding metalens, respectively. Scale bars: 1 µm. Scale bars of inset optical micrographs: 100 µm.^[281] (d) The ${\mathrm{TiO}}_2$ nano-PER metahologram is fabricated through a one-step process of nanoparticle-embedded-resin printing. The initial row showcases the SEM image of the master mold meticulously crafted using EBL on a silicon substrate. The subsequent image displays the intricate final structure of the printed ${\mathrm{TiO}}_2$ nano-PER. In the second row, the successful replication of the ${\mathrm{TiO}}_2$ nano-PER metahologram is exhibited on diverse substrates, including flexible polycarbonate (PC) and ultrathin polypropylene (PP). Scale bars: 1 µm.^[126] (e) Illustration depicting the achievement of high-aspect-ratio metalenses through nanoimprint lithography utilizing water-soluble stamps (left). SEM images showcasing the replicated metalens (right).^[282] (f) Schematic illustration depicting the fabrication of a metalens through multilayer nanoimprint lithography and solution phase epitaxy. The inset images on the right showcase SEM images of the ZnO metalens.^[283]

NIL offers the advantages of high resolution, large-area fabrication, and cost-effectiveness. The reusability of stamps further contributes to a substantial reduction in fabrication costs and environmental impact. However, it is important to note that this method still necessitates the fabrication of a mold using high-resolution equipment.

3.3.3.

Self-assembly of nanostructures

Self-assembly (SA) is a process wherein smaller components spontaneously unite to create larger, well-defined, and stable aggregates^[284]. Recently, it has emerged as a promising alternative to lithographic techniques for crafting features and structures at the nanometer scale. This approach provides a cost-effective and scalable method to achieve intricate structures, followed by subsequent etching or deposition processes. Self-assembled monolayers (SAMs) consist of single layers of molecules that autonomously organize into ordered lattices on the surface of a substrate. Various SAMs, formed using organic molecules, have found extensive application in the fabrication of dielectric metasurfaces. Zhang et al. presented a flexible, all-dielectric metasurface fabricated through nanosphere self-assembly, showcasing its potential applications in sensing^[285]. The principal fabrication steps are illustrated in Fig. 14(a): initially, a thin layer of Si is deposited onto a polyethylene terephthalate (PET) substrate. Subsequently, a monolayer of polystyrene (PS) spheres self-assembles and is transferred onto the Si surface [top right SEM image in Fig. 14(a)]. Following this, plasma etching is employed to reduce the size of the PS spheres, serving as a mask for a second RIE procedure. This preserves the Si cylinder-based metasurface directly beneath the PS spheres. The SEM image of the Si cylinders is depicted in the bottom right of Fig. 14(a). Moreover, Valentine et al. elucidated the design and manufacturing of large-scale perfect reflectors using Si-based cylinder resonators on SOI substrate. The patterning was achieved through nanosphere lithography and RIE as illustrated in Fig. 14(b)^[98]. The SEM images, coupled with the optical image, unequivocally showcase the capability of nanosphere lithography to produce high-quality dielectric resonators across a sizable area. In a subsequent investigation, a pioneering manufacturing technique was introduced, leveraging grayscale nanosphere lithography. This method enables the cost-effective and scalable fabrication of metasurfaces featuring arbitrary, nonperiodic phase profiles. To attain nonperiodic phase control, nanosphere lithography is integrated with illumination from an SLM, facilitating spatial control of the dose and exposure size, as illustrated in Fig. 14(c)^[232]. To assess the efficacy of the technique, they produced large-scale metalenses operating at a wavelength of 1.7 µm. The fabricated metalenses showcased diffraction-limited focusing and achieved an impressive relative efficiency exceeding 83%. Moreover, Gupta et al. proposed a straightforward self-assembly fabrication method combined with nanoimprinting approaches, eliminating the need for nanospheres. They showcased the controlled dewetting of optical glass thin films to achieve all-dielectric metasurfaces over a large area. The self-assembly of various optical nanostructures with feature sizes down to $\sim 100\mathrm{nm}$ and interparticle distances as small as 10 nm is experimentally demonstrated. The fabrication approach is illustrated in Fig. 14(d)^[286]. Soft lithography is initially employed to imprint a PDMS mask onto a thermoplastic or sol–gel layer through nanoimprinting [Fig. 14(d), top]. Subsequently, a thin layer of chalcogenide glass is deposited through evaporation. The annealing process induces dewetting of the glass layer, leading to its breakup into an array of well-dispersed and self-ordered nanoparticles, as depicted in the SEM images in Fig. 14(d). This approach is both simple and scalable, as underscored in the bottom of Fig. 14(d), where an EPFL logo has been created by embossing a $20\times 11{\mathrm{cm}}^2$ polycarbonate sheet with nanostructured assembled PDMS molds. In addition to the aforementioned large-scale fabrication techniques, researchers have also demonstrated a novel method utilizing an anodized aluminum oxide (AAO) template combined with a ${\mathrm{TiO}}_2$ metasurface^[287]. Furthermore, they have explored the direct construction of larger-scale nanostructures on resist^[288–290].

Fig. 14

The utilization of self-assembly in the fabrication of all-dielectric metasurfaces. (a) Illustration of the primary fabrication steps for the flexible, all-dielectric metasurface utilizing nanosphere lithography. The accompanying images on the right showcase SEM images of self-assembled polystyrene (PS) spheres (top) and Si cylinders (bottom) formed after RIE. Scale bars: 1 µm.^[285] (b) Illustration of the self-assembly-based nanosphere lithography technique for the large-scale fabrication of all-dielectric metasurface perfect reflectors, accompanied by a camera image depicting the pattern of PS spheres on an SOI substrate (left column). SEM images showcase PS spheres with a diameter initially at 820 nm and subsequently downscaled to 560 nm after etching (middle column). The top and tilted views exhibit the final metamaterial, comprising an array of Si cylinders. Scale bar: 2 µm.^[98] (c) Image of large-scale metasurfaces created through grayscale nanosphere lithography. A grayscale pattern is produced by the DMD system utilizing a 365 nm I-line UV light source, transmitted through a projection system incorporating an objective lens. The schematic depicts the air–water interface for nanosphere self-assembly, while the SEM image showcases the close-packed nanosphere monolayer after self-assembly, with a scale bar of 5 µm. Additionally, an optical image of the fabricated metalens is presented. Scale bar: 200 µm.^[232] (d) The left panel illustrates the schematic, while the right panel displays the SEM image of the self-assembly process of nanostructured glass metasurfaces through templated fluid instabilities. The fabrication initiates with thermal or ultraviolet nanoimprinting of the requisite pattern on a substrate, as shown in the top section (scale bar, 400 nm). Subsequently, a thin-film deposition of high-index optical glass is performed in the middle section (scale bar, 1 µm). The final step involves annealing to induce the dewetting process, depicted in the bottom section (scale bar, 1 µm). A schematic of the dewetting process is also provided. An optical photograph showcases a large area ($20\mathrm{cm}\times 11\mathrm{cm}$) EPFL logo-shaped metasurface on a polymer substrate, with an inset SEM image revealing the corresponding nanostructure (scale bar, 1 µm).^[286]