1 Introduction

1.1 Refractive, Reflective, and Diffractive Optics

Light can be manipulated via refractive, reflective, and diffractive optics. Refractive optics rely on the bending of light as it passes through materials with different refractive indices, such as lenses, to converge or diverge light rays. Reflective optics, on the other hand, employ mirrors to redirect light, maintaining its original wavelength and phase. Diffractive optics utilize the interference and diffraction of light waves, often through periodic structures, to shape and manipulate light, enabling devices like diffraction gratings and holograms [1,2,3,4]. Each of these optical approaches offers unique advantages and limitations, making them valuable tools in optical design, imaging, and various technological applications, depending on the specific requirements of a given system.

Diffractive optics exhibit several distinct advantages over refractive and reflective optics. One notable strength lies in their ability to achieve substantial reductions in thickness and weight. Diffractive elements can replace thick refractive lenses or bulky reflective mirrors with ultra-thin, lightweight surfaces that accomplish similar optical functions [2, 5, 6]. This characteristic is particularly advantageous in applications where space and weight constraints are critical, such as in aerospace and telecommunications systems. Furthermore, diffractive optics offer exceptional versatility in terms of optical design, enabling precise control of phase and amplitude, which is challenging to achieve with refractive or reflective components. This versatility makes diffractive optics well-suited for applications like beam shaping, spectral dispersion, and wavefront manipulation [7,8,9]. However, it's important to note that diffractive optics also have their limitations, including sensitivity to wavelength and limited efficiency, which must be carefully considered in their design and implementation.

Meanwhile, metasurfaces, which have recently attracted strong attention, also provide additional advantages over traditional optics, in terms of thickness, weight, and versatility. Unlike conventional optics, which rely on bulky and thick lenses or mirrors, metasurfaces are ultrathin, planar structures that manipulate light at the subwavelength scale. This property enables a dramatic reduction in thickness and weight, making metasurfaces highly desirable for applications where size and weight constraints are critical, such as in miniaturized optical systems and wearable devices [10,11,12]. Metasurfaces also provide precise control over the phase, amplitude, and polarization of light, allowing for the creation of complex optical functionalities in a single, compact layer. This versatility makes metasurfaces invaluable in areas such as beam steering, holography, and flat optics, offering a transformative approach to optical design and enabling the development of more compact, lightweight, and efficient optical systems [13,14,15]. However, it's important to note that metasurfaces are still subject to challenges, including limitations in the operating wavelength range and fabrication complexities that must be addressed for widespread adoption in practical applications.

1.2 Green Manufacturing Process for ESG

In the realm of semiconductor manufacturing, planar optical elements, and flat diffractive optical elements (DOEs) undergo fabrication through a series of semiconductor processes or traditional cutting methods. The production of thin, flat DOEs within the semiconductor industry involves a multi-step process that significantly impacts Environmental, Social, and Governance (ESG) considerations and green manufacturing practices.

Typically, in the case of DOEs, intricate patterns ranging from hundreds to several nanometers in size are meticulously created through a repetitive lithography process. Materials such as silicon and gallium arsenide, commonly used in semiconductor processes, are employed, with a substantial amount of energy consumed during the production of these ingots. The conventional Czochralski growth process historically consumed up to 100 kW/h per kilogram of silicon ingot produced [16]. However, advancements in modern furnace designs, continuous silicon feedstock supply methods, and improvements in gas flow dynamics have significantly reduced this energy consumption to less than 40 kW/h [16, 17]. Despite these improvements, addressing ESG policies remains crucial, particularly as the energy used in silicon ingot production often relies on fossil fuels. Continuous technological developments are essential to further minimize energy consumption and align with sustainable practices.

The semiconductor manufacturing process demands meticulous and sophisticated waste treatment and management practices to mitigate the ecological footprint associated with compounds such as photoresist, polishing slurry, and various etching materials. A prominent South Korean chip manufacturing company engaged in mass production utilizes over 150 chemical components across more than 450 chemical products, resulting in an annual consumption of over 45,000 tons of chemicals [18, 19].

An alternative method for fabricating flat optical elements is through traditional subtractive manufacturing. However, this approach generates various gases, chemical additives, coolants, lubricants, fuel energy, and cutting residues during processing. For instance, chlorine-based, fluorine-based, and boric acid-based vapors emitted during glass manufacturing, along with substances like sodium nitrate, potassium nitrate, and sulfate used as oxidizing and reducing agents to control glass properties, are considered hazardous substances or carcinogens [20].

This traditional manufacturing practice contributes to significant environmental damage through the release of hazardous waste and emissions. Coolants and lubricants introduce pollutants that can contaminate water sources, while factory emissions contribute to air pollution. The high power and energy demand also lead to increased carbon emissions, exacerbating climate change. Failing to address these challenges in traditional manufacturing hinders the industry's environmental stewardship [21].

Green manufacturing practices, involving the reduction of hazardous substances, optimization of energy usage, and the adoption of sustainable materials and recycling, become essential to align these technologies with ESG goals. By addressing the environmental impact associated with these processes, industries can demonstrate their commitment to responsible ESG practices and contribute to a more environmentally conscious and sustainable future. In the context of ESG considerations, the practices in traditional semiconductor and machining processes often conflict with environmental responsibility. To align with ESG principles and embrace green manufacturing, a transition to sustainable machining technologies and processes is imperative. This shift minimizes resource consumption, reduces waste generation, and prioritizes energy efficiency in the manufacturing of ultra-thin smart optics. Through such efforts, industries can mitigate their environmental footprint, adhere to ESG commitments, and contribute to a more environmentally sustainable manufacturing landscape.

1.3 Light-Weight Planar Optics: Key Requirements

Planar optics for photovoltaics, smart windows, and smart lighting are highly required, as described in Fig. 1. Light-weight deployable planar lenses represent a promising technological innovation for space satellite applications. These planar lenses, which can be compactly stowed during launch and then unfurled or deployed in space, offer several advantages. They enable the miniaturization of satellite payloads, reducing launch costs and providing increased mission flexibility. Deployable planar lenses can be used for a variety of purposes, such as enhancing imaging capabilities, enabling compact radar systems, or improving communication systems by focusing and steering electromagnetic waves. Their planar and lightweight design makes them well-suited for integration into small satellites or CubeSats. The deployment mechanism, materials, and precise control systems are key considerations in designing these lenses for space deployment. As technology advances and deployment mechanisms become more reliable, deployable planar lenses are likely to find increasing utility in future space missions, contributing to more cost-effective and versatile satellite systems. Light-weight mobile camera lenses are of paramount importance in modern high-performance mobile phone cameras, offering portability and versatility in capturing high-quality images and videos. Their manufacturing methods often involve the use of advanced materials like plastic and composite elements to reduce weight while maintaining optical performance. The advantages of lightweight mobile camera lenses include improved ease of use, reduced strain on device components, and enhanced image stabilization. These lenses also contribute to the overall reduction in device weight and size, making them indispensable for mobile devices where portability and user experience are critical. Research efforts are continually focused on developing novel materials, optical designs, and fabrication techniques to further enhance lightweight lens performance, ensuring that mobile cameras continue to meet the demands of today's tech-savvy consumers. Thin endoscopic probes hold importance for minimally invasive procedures and diagnostics in medical applications. Their manufacturing typically involves the use of advanced materials and precision machining techniques to create compact, lightweight, and flexible optical systems that can navigate the narrow and tortuous pathways of the human body. The advantages of thin endoscopic probe optics include reduced patient discomfort, shorter recovery times, and fewer complications compared to traditional open surgeries. Moreover, their small size and flexibility allow access to previously inaccessible regions within the body, facilitating precise imaging, diagnostics, and therapeutic interventions. Ongoing research focuses on further miniaturization, enhanced imaging capabilities, and integration with other diagnostic and therapeutic tools, ensuring that thin endoscopic probe optics continue to advance medical practices and improve patient outcomes.

Fig. 1
figure 1

A) Tunability requirements of tunable DOE including beam steering, focal length control, and transmittance control (b) Advantages of laser direct writing (c-h) Applications of tunable DOE. (c) Space optics (Adopted from [22] Figure 1) (d) Smart window (Adopted from [23], Figure 1) (e) Smart photovoltaics (Adopted from [24], Figure 1) (f) Laser machining (Adopted from [25], Figure 3) (g) optical metrology (Adopted from [26], Figure 3) (h) Bio application (Adopted from [27], Figure 12)

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1.4 Tunable Planar Optics: Key Requirements

The smart tunability of ultra-thin optics is crucial for their adaptability and versatility in a variety of optical applications stated above. Key requirements for tunability include the ability to adjust focal length, enabling precise focusing and imaging; the control of aberrations, minimizing optical distortions for high-quality imaging; transparency modulation, allowing for dynamic light transmission adjustments; and the capability to manipulate beam propagation angles, facilitating beam steering and redirection [28,29,30,31,32]. Achieving these tunability requirements typically involves the integration of advanced materials, such as liquid crystal or electroactive polymers, and the incorporation of responsive mechanisms, like piezoelectric actuators or voltage-driven devices [33,34,35,36]. Meeting these requirements empowers ultra-thin optics to address a broad range of optical design challenges and adapt to dynamic operating conditions, making them valuable tools in contemporary optical systems.

2 Design and Fabrication of Diffractive Optical Elements

2.1 Design of Diffractive Optical Elements

The design of DOEs conventionally has been pursued through an analytic solution-based approach derived from the Rayleigh-Sommerfeld diffraction integral [37,38,39,40]. While this theory offers intuitive solutions for straightforward imaging function of the diffractive optics, recent demands for various functionalities in DOEs, and the requirements for a rapid design process have led to the adoption of numerical optimization-based approaches to overcome the limitations of the conventional analytic solution-based approaches [41,42,43]. The numerical design approach already has been successfully applied to various DOEs implementations, such as achromatic imaging DOE, multiple elements DOEs, large-scale optics, hybrid optics, and photon sieves [44,45,46,47,48,49,50,51,52].

The numerical design process for DOEs consists of three representative steps, including phase optimization, pattern generation, and verification, which can be further divided into detailed procedures, as shown in Fig. 2(a). The phase optimization step involves optimizing phase parameters to minimize or maximize merit functions, aiming to achieve the optical specifications of the target DOE. As shown in Fig. 2(a) (step 1), a continuous phase map across the entrance pupil can be generated by using polynomials with the function of spatial variables in commercial ray-tracing tools. For the modeling of symmetric or asymmetric surface phase, Zemax® provides the surface types of "binary1" and "binary2", respectively [53]. These surface types comprise various orders of polynomials with coefficients, which can be optimized to minimize merit functions. Subsequently, the optimized phase map is applied to a modulo operation concerning 2π, resulting in a residual map that corresponds to the kinoform profile, as shown in Fig. 2(a) (step 2). The extracted phase map of the kinoform is then quantized, as shown in Fig. 2(a) (step 3), during the DOE pattern generation process. Depending on the efficiency requirements and fabrication feasibility, threshold values can be set to create either single-level or multiple-level patterns. Increasing the number of levels can enhance efficiency from 10 to 99% [35]. This pattern generation and quantization process allows rapid calculations by importing the optimized phase map data from the ray-tracing software into computation software such as MATLAB® or Python®. The performance of the generated DOE pattern can be verified using physical optics propagation (POP) within ZEMAX®, which is based on scalar diffraction theory. The POP function allows the evaluation of primary optical specifications, such as focal length, f-number, spot size, and depth of focus. Figure 2(b) presents the results of imported a 1-inch diameter DOE pattern with focal length 80 mm, estimating beam profiles depending on propagation distance. In the near-field, the DOE pattern transfers directly to the beam profile, however as the beam propagates, the beam size gradually shrinks, focusing a spot accurately at the 80 mm focal length. Beam cross-sections are plotted against propagation distance, allowing depth of focus evaluation at the focal length. Additionally, it clearly reveals side-effects such as focusing due to high-order diffraction in the near-field. For cross-evaluation purposes, Finite-Difference Time-Domain (FDTD) software also enables the verification of the DOE pattern, as shown in Fig. 2(a) (step 5) [54, 55]. Additionally, evaluation in the FDTD with a more versatile configuration is available, such as DOE illumination with oblique incident angles or sources with divergence angles, which is not fully supported in the ray-tracing software [53]. Once the design and verification processes are completed, the pattern is prepared for fabrication through drawing work. The resolution of drawing files, such as gds or dwg, is adjusted with accounting for fabrication precision, and efficiency of the drawing work flow.

Fig. 2
figure 2

A Design process of DOEs. The comprehensive DOE design process consists of 5 steps, incorporating optimization of the surface phase, conversion to Kinoform/binary phase, and verification of the pattern. Each step utilizes ray-tracing, computational, or finite element analysis software as appropriate for precise modeling and optimization. (b) Verification of a diffractive pattern in the design process. The designed pattern can be imported for the verification of beam propagation characteristics, including amplitude, phase, and wavefront, in both near-field and far-field regions by using ray-tracing or finite analysis tools. (c) Analytical design and efficiency of refractive optics and diffractive optics (adopted from [35] Table 1)

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Diffractive optics employ micro- or nanostructures etched onto a flat surface to control the phase and amplitude of incident light waves. This precise control over light wavefronts enables ultrathin, planar optical elements capable of achieving complex optical functions. Diffractive optics offer several advantages over traditional refractive optics, including reduced thickness and weight, as well as the ability to correct chromatic aberrations and implement multifunctional optical elements. These characteristics make diffractive optics particularly valuable in applications where space and weight constraints are critical, such as in compact imaging systems and wearable devices, offering a versatile and lightweight alternative to traditional refractive optics. Ultrathin diffractive optics design encompasses various techniques, including the use of Fresnel zone plates, binary Fresnel phase plates, and kinoforms, which differ significantly from traditional optics design. Fresnel zone plates use concentric rings with varying widths to focus light, while binary Fresnel phase plates employ discrete phase levels to manipulate wavefronts, enabling lightweight and compact optical elements. Kinoforms, on the other hand, optimize the phase distribution to achieve specific optical functions, minimizing energy loss. These ultrathin designs offer advantages like reduced thickness, weight, and chromatic aberrations compared to traditional bulky lenses or mirrors. Thus, they are well-suited for applications where size and weight constraints are paramount, such as in portable imaging devices, augmented reality displays, and miniaturized optical systems, representing a transformative approach to optical design and enabling more compact and lightweight optical solutions.

2.2 Traditional Manufacturing Methods for DOE

The manufacturing process of thin, flat diffractive optical elements (DOEs) in the semiconductor industry involves numerous steps that impact ESG considerations and green manufacturing practices. Typically, in the case of DOEs, aligned patterns ranging from hundreds to several nanometers in size are created through a repetitive lithography process. The materials used in DOEs, such as silicon or gallium arsenide, are common in semiconductor processes, and the production of these ingots consumes vast amounts of energy. Additionally, the vacuum environment in which general photolithography processes occur increases energy consumption in the overall process [56,57,58,59].

A variety of compounds are employed in actual semiconductor manufacturing. Due to security considerations in the semiconductor industry, determining the exact composition of materials is often challenging [60,61,62,63]. Nevertheless, a range of substances, including resin, photoresist (PR), polymer compounds, surfactants, organic acids, organic compounds, pigments, and more, are commonly utilized. The manufacturing process incorporates a minimum of five substances, including sulfuric acid, chromic acid, ethylene oxide, silica, potassium dichromate, etc., all of which fall under Group 1 carcinogens as per the classification by the International Agency for Research on Cancer (IARC) [64,65,66,67,68]. Additionally, the process may involve the use of Group 2B carcinogens, including diborane, carbon black, 1,4-dioxane, pyrocatechol, antimony trioxide, methyl isobutyl ketone, ethylbenzene, diethanolamine, titanium dioxide, lead, dichloromethane, naphthalene, alpha-methyl, styrene, 1,2-benzenediol, nitrilotriacetic acid, cumene, among others [18, 69, 70].

Furthermore, the semiconductor process utilizes substances such as ethylene oxide and hydroquinone, as well as phenol and N-butyl glycidyl ether, all categorized as Group 2 germ cell mutagens and falling under Group 1B. Reproductive toxicants in the semiconductor process are categorized into different groups. Group 1A includes carbon monoxide and lead, whereas Group 1B encompasses 2-ethoxyethanol, 2-methoxyethanol, ethyl cellosolve acetate, N, N-dimethylacetamide, and sodium tetraborate. Additionally, Group 2 reproductive toxicants consist of toluene, cyclohexylamine, and hexane [18].

Addressing ESG concerns and advancing green manufacturing in this context involves exploring alternative materials and more environmentally friendly processes [71, 72]. This includes investigating sustainable substrates and coatings, as well as developing energy-efficient etching techniques, such as plasma etching [73]. Additionally, waste reduction and recycling efforts can be implemented, and the adoption of green energy sources for manufacturing facilities can minimize the carbon footprint associated with the energy-intensive aspects of the process [57]. Furthermore, research into the integration of cleaner and more sustainable fabrication methods, such as nanoimprint lithography, holds potential for reducing the environmental impact of thin, flat DOE production. By taking these steps and embracing green manufacturing principles, the semiconductor industry can work towards more eco-friendly and sustainable production of DOEs, aligning with global efforts to reduce the environmental impact of technology manufacturing.

2.3 Direct Laser Writing

Direct Laser Writing (DLW) has emerged as a forefront manufacturing technique, presenting several key advantages over precedent methods [74,75,76,77,78]. One of its most noteworthy attributes is its green manufacturing profile due to the facile process and wide selection of materials [79,80,81,82]. DLW significantly reduces the environmental impact compared to methods like photolithography, primarily due to its elimination of hazardous chemicals [83,84,85]. Unlike photolithography, which relies on photoresists, developers, and etching chemicals, DLW is a direct-write process that operates without the need for such environmentally harmful materials. It offers a wide range of substrate options including glass, textiles, and biomass materials such as wood, leaves, and charcoal [5, 86, 87]. By virtue of its chemical-free approach, DLW not only reduces waste generation but also minimizes the risk of environmental contamination [88,89,90,91].

Additionally, DLW systems can be more energy-efficient, working with simple manufacturing processes without the need for specialized treatments. The facile processing steps and the direct utilization of energy from the laser source contribute to its energy efficiency [92,93,94,95,96,97]. Moreover, DLW systems can often be operated with lower power requirements, further reducing their environmental footprint [98, 99]. In terms of future developments, ongoing research in DLW technology aims to enhance its green manufacturing techniques. This includes exploring new materials that are more environmentally friendly, optimizing process parameters to minimize energy consumption, and integrating DLW with other sustainable manufacturing processes. Collaborative efforts between researchers and industry partners are driving innovations in DLW technology, guiding the way for even greener and more eco-conscious manufacturing processes in the future.

2.4 Nanoimprinting

Nanoimprinting is a nanofabrication technique based on the principle of replicating nanoscale patterns from a master template onto a substrate. It involves pressing the template, containing desired nanostructures, into a soft, moldable material that solidifies upon cooling or exposure to UV light, resulting in precise replication of nanostructures on the substrate's surface [100,101,102,103,104,105]. Nanoimprinting offers a greener and more eco-friendly alternative to traditional semiconductor lithography [106,107,108,109,110]. Unlike conventional lithography, which often uses hazardous chemicals and energy-intensive processes, nanoimprinting generally requires fewer chemicals, emits fewer volatile organic compounds, and consumes less energy [111,112,113].

Additionally, the process can utilize biodegradable or recyclable materials, further reducing its environmental impact [102]. This enhanced eco-friendliness and reduced power consumption make nanoimprinting a promising and sustainable choice for nanofabrication, aligning with green manufacturing practices and contributing to a more energy-efficient and environmentally conscious technological landscape [114,115,116,117]. Looking ahead, advancements in nanoimprinting technology hold promise for even greater sustainability gains. Continued research efforts are focused on improving process efficiency, exploring novel materials with enhanced eco-friendliness, and addressing any remaining challenges in scalability and cost-effectiveness [118]. By addressing these aspects, nanoimprinting can further solidify its position as a key enabler of green manufacturing practices, contributing to a more sustainable technological landscape.

3 Electrically-Tunable Planar Optics

Figures 3, 4, 5, 6, 7, 8, 9 illustrates the principle of tunable DOE with optical tuning effect, medium, and the principle of operation. Optical tuning can be realized by active modulation of refractive index with liquid crystals, active modulation of geometry of fluid and elastomer, and active modulation of the bandgap of 2D materials. We have discussed these cases one-by-one in detail in this forthcoming section.

Fig. 3
figure 3

Principle of tunable DOE: (a) Optical tuning effect, lens medium, and the principle of tunable DOE (a-1) Principle of liquid crystal (adopted from [119] Figure 1) (a-2) Principle of modulation of shape (adopted from [36] Figure 4) (a-3) Principle of modulation of bandgap (adopted from [120] Figure 1) (b) Tunable DOE enabled by liquid crystal (adopted from [121] Figure 2) (c) Tunable Fresnel zone plate enabled by modulation of shape (adopted from [122] Figure 2) (d) Modulation of bandgap of terahertz beam (adopted from [120] Figure 1)

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Fig. 4
figure 4

(a) Corning’s electrowetting-based liquid lens in a soft vacuum setting and Optotune’s pressure-driven liquid lens in a soft vacuum setting (Adopted from [123], Figure 4 and 6) (b) 91 element hexagonal piston-tip-tilt mirror array with a ~ 4 mm active aperture packaged on controller board. Zernike aberrations were created with an array of 61 piston-tip-tilt micromirrors with 27-μm stroke. ZEMAX® and experimental layouts of the 4X active optical zoom (Adopted from [124], Figure 1, 2, and 10) (c) 0.676 m SiC lightweight mirror assembly showing the support frame, mirror body, passive support, and active support. For clarity, part of the model has been cut off along the wave lines. Integrated system on its test platform showing the experimental apparatus of heat and active optics. Influence functions of mirror surface in which the piston and tilt have been removed, measured with a Fizeau interferometer. (Adopted from [22], Figure 1, 6, and 9) (d) Four different configurations of shadow-casters and detectors were leveraged in the payload. CONFIG-3 and CONFIG-4 contained Fresnel zone plates as shadow-casters (Adopted from [125], Figure 2)

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Fig. 5
figure 5

Smart photovoltaic applications (a) enhanced light-harvesting in photovoltaic devices enabled by gratings (Adopted from [126], Figure 1) (b) light management to optimize the photovoltaics (Adopted from [127], Figure 2)

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Fig. 6
figure 6

Smart window and light (a) Solar radiation transmittance control for effective heating (Adopted from [128], Figure 1) (b) Effective light management for harvesting enabled by transmittance and diffusion control (Adopted from [129], Figure 5 and 6)

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Fig. 7
figure 7

DOE applications for laser machining (a) Beam splitting enabled by DOE for effective nanofabrication of Ag film (Adopted from [25], Figure 3) (b) Beam shaping accomplished by DOE for effective laser welding of aluminum alloy (Adopted from [130], Figure 1)

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Fig. 8
figure 8

DOEs for effective optical metrology (a) Dynamic beam-steering by a pair of rotating diffractive elements to control viewing direction on camera (Adopted from [26], Figure 1 and 3) (b) DOE exploited for achromatic extended-depth-of-field imaging (Adopted from [131], Figure 1, 3, and 5)

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Fig. 9
figure 9

a, b, c) Anterior segment OCT images for different electrical tunable lens (ETL) current values (focus position indicated by yellow triangle) and (d) composite image generated by stacking cross-sections with 50 different foci (Adopted from [132], figure 5) Image of 100 nm fluorescent beads acquired by confocal microscopy (e) before and (f) after correction of sample-induced aberration using a spatial light modulator (SLM). Stimulated emission depletion (STED) microscopy images with (g) correction to the excitation beam only and (h) correction to both excitation and depletion beam paths. (i) Axial line profiles of dashed boxes in (g) and (h) in the horizontal direction (Adopted from [133], figure 4) (j) Schematic illustration and (k) picture of the endoscopic OCT probe with a microfluidic lens (Adopted from [27], figure 12)

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3.1 Active Modulation of Refractive Index

The modulation of refractive index by liquid crystal (LC) materials is a pivotal technique for achieving electrically tunable Diffractive Optical Elements (DOEs) with versatile optical functionalities. LCs often exhibit anisotropic optical properties due to their orientation-dependent refractive indices. The principle lies in the ability of LC molecules to align themselves under the influence of an external electric field, thereby changing the refractive index experienced by incident light [134,135,136, 137]. By patterning the alignment of LC molecules on substrates and applying an electric field, precise control over the phase and amplitude of light passing through the LC layer can be achieved. This dynamic modulation enables the creation of electrically tunable DOEs [138,139,140,141]. The applications of LC-based DOEs are diverse and impactful. One prominent application is in adaptive optics, where LC-based DOEs are used to correct atmospheric distortions in astronomical observations, enabling the capture of sharper images of celestial objects. In free-space optical communication, these DOEs facilitate beam steering, allowing for rapid and precise redirection of laser beams for point-to-point communication and data transfer [142,143,144,145,146]. Additionally, LC-based DOEs find use in variable-focus lenses for imaging devices, such as smartphones and digital cameras, providing dynamic and rapid focusing capabilities. Moreover, LC-based DOEs are essential in emerging technologies like augmented reality (AR) and virtual reality (VR) headsets, where they enable adjustable optical elements to achieve immersive and comfortable user experiences. LC-based beam steering is valuable in lidar systems for autonomous vehicles, enhancing 3D mapping and obstacle detection [147,148,149,150]. Furthermore, LC-based DOEs have applications in tunable spectral filters, spatial light modulators, and holography, contributing to advancements in spectroscopy, optical signal processing, and 3D display technologies [21].

3.2 Active Modulation of Geometrical Shape

The modulation of geometrical shapes in tunable Diffractive Optical Elements (DOEs) represents a versatile approach for dynamic control over optical wavefronts and beam manipulation. This modulation is achieved through two primary methods: adjusting the gap between the DOE zones or dynamically changing the surface shape. The principle of gap modulation involves altering the spacing between adjacent DOE elements, effectively changing the phase delay experienced by incident light. Alternatively, dynamic surface shape modulation encompasses physically deforming the DOE surface to induce desired phase shifts, enabling real-time optical adjustments. For the medium employed in these tunable DOEs, elastomers and fluids are preferred alternatives due to their ease of actuation and deformation capabilities. Elastomers exhibit mechanical flexibility, enabling precise shape changes with minimal energy input. Fluids, on the other hand, provide adaptability to external forces and can be readily manipulated for shape modulation. Various actuation mechanisms can be employed to drive the tunable DOEs. These include Dielectric Elastomer Actuators (DEAs), piezoelectric elements, external fluid pumps, electromagnetic actuation, electrostatic actuation, and electrothermal effects. DEAs offer rapid and reversible deformations when subjected to an electric field, making them suitable for real-time optical adjustments. Piezoelectric elements provide precise and predictable shape changes in response to voltage inputs, ensuring accurate optical control. External fluid pumps introduce controlled pressure changes to deform the DOE surface. Electromagnetic, electrostatic, and electrothermal actuation methods leverage electromagnetic fields, electric charges, and thermal effects, respectively, to induce surface deformations, offering diverse options for achieving tunable DOEs. Applications of these tunable DOEs facilitate multiple domains, including adaptive optics for astronomical observations, beam shaping and steering in laser processing, variable-focus lenses in imaging devices, and reconfigurable optical systems for telecommunications.

3.3 Active Modulation of the Material Bandgap

Graphene, known for its exceptional electrical, optical, and mechanical properties, serves as an electrically controlled THz modulator through bandgap modulation. Its optical absorption behavior includes dominant interband transitions in the optical and near-infrared spectrum, along with intraband transitions in the far-infrared and terahertz range. This versatility allows graphene to find applications in various terahertz technologies, enabling modulation, detection, and ultrafast carrier recombination. Notably, the material's optical response can be dynamically tailored by adjusting its Fermi level through electrical, optical, or chemical means. By leveraging the field-effect configuration in semiconductors, graphene enables precise control of electron and hole concentrations, with the choice of gate dielectric significantly impacting the operational speed of the device.

These electrically controlled modulators offer a means to observe and manipulate changes across the THz spectrum. In one such instance, Sensale-Rodriguez et al. demonstrated a tunable terahertz modulator using monolayer graphene based on intraband absorption [120]. The device achieved a spectrally-flat intensity modulation of approximately 15% in the frequency range of 570–630 GHz with an insertion loss of about 0.2 dB and a modulation frequency of around 20 kHz, surpassing the performance of AlGaAs/GaAs-based devices. Modifications using a reflection geometry further enhanced the modulation depth to approximately 64% with an insertion loss of roughly 2 dB. The back-gate electrode's role as a reflecting layer enabled enhanced modulation depth at room temperature. In another study, Ju et al. showcased the potential of graphene in terahertz modulation through tunable plasmons in patterned graphene nanoribbons [151]. Their work emphasized the material's capability to couple terahertz waves into subwavelength charge-density oscillations with polarization-sensitive behavior. Additionally, Lee et al. introduced a graphene-metamaterial integrated device capable of both amplitude and phase modulation, achieving a maximum amplitude modulation depth of 47% at resonance [152]. The use of multilayer graphene (MLG) film further increased the modulation depth, showcasing the practical application of these electrically controlled modulators in observing and manipulating changes across the THz spectrum.

With advancements in amplitude and phase modulation, and the establishment of multidimensional arrays, terahertz spatial modulators can now effectively adjust focal length, enable beam steering, and facilitate wavefront shaping. This breakthrough is underpinned by the utilization of graphene for the electric control of the bandgap, thus enabling spatial modulation. Various techniques, including chemical doping and external bias voltage, have been instrumental in achieving substantial modulation depths. For instance, one method involved the segmentation of graphene into a 4 × 4-pixel array utilizing oxygen plasma, resulting in a notable 50% modulation depth with voltage variations [153]. Furthermore, the application of an ionic liquid electrolyte gating technique led to the development of a transmissive graphene spatial light modulator (SLM) capable of achieving an impressive modulation depth of 80% at 1 THz, maintaining 10% at 1 kHz [154]. Similarly, a graphene metasurface demonstrated the potential for dynamic phase modulation alongside variations in carrier concentration, enabling dynamic beam scanning at 0.98 THz through precise bias distribution [155]. Despite challenges associated with the production of large-area and uniform graphene films, the prospects for utilizing graphene as a spatial modulator remain exceptionally promising.

4 Representative Applications of Tunable Planar Optics

In this chapter, the six potential applications of tunable planar optics have been discussed, particularly space optics, smart photovoltaics, smart window, laser machining, optical metrology, and biomedical imaging.

4.1 Space optics

Application in space typically requires more endeavors than on the ground. The same has been applied to the tunable planar optics. The harsh space environment has hindered its active implementations despite its potential to benefit various space technologies. The optical elements must endure adversarial space environments, such as vacuum, radiation, and dynamic temperature cycles. However, as inter-satellite optical communication has drawn attention and small satellites are prevalent more than ever, tunable optics have been researched for application in space. Compared to conventional optical communication payloads, which require volumetric mechanical actuators for beam steering, tunable optics can usher into optical communication in small satellites. The authors of [123] demonstrated that the liquid lens could endure the vacuum environment. Two types of commercial-off-the-shelf liquid lenses were confirmed to operate normally in a soft vacuum environment. Specifically, an electrowetting-type liquid lens demonstrated almost identical performance under the harsh environment as in ambient conditions. While a pressure-driven liquid lens initially experienced air bubbles, it ultimately performed similarly to ambient conditions after several weeks in the vacuum. The experiment results allude to its possibility of being adopted in space technologies. Since the properties of diffractive planar optics could become advantages in the space development field, diffractive optics have been applied to imaging systems of satellites or surveyors. For common refractive optics, it is essential to increase the diameter of the lens for capturing high-resolution images. However, increased mass and volume of optics require higher launch costs. To reduce the weight and the volume of optics and achieve a speedy system, an alternative method that replaces moving optics by conventional gimbals with light-weight carbon fiber reinforced polymer (CFRP) variable radius-of-curvature mirrors (VRMs) and MEMS deformable mirrors (DMs) was proposed [124]. Just by operating the actuation of the mirrors, the focal lengths and the magnification of the entire system can be adjusted without huge movement of conventional mechanical parts. With an experimental setup that consists of micromachined deformable membrane mirrors and static spherical mirrors, the entire system achieved 4X magnification with an increase of resolution of 2X by adjusting the actuation of mirrors. Since lightweight thin planar optics lack structural stability and rigidity, they are easily affected and corroded by hazard space environments. Active optics with segment mirrors can instead be used to compensate for deformation induced by gravity and to correct low-order aberrations because of temperature change and gravity gradient [22]. To realize active optics, many kinds of actuation are being developed, one of the examples is using a whiffletree-supported mirror. A prototype of 0.676 m diameter is supported by 9 axial and 3 lateral supports, and they serve as hard points. Also, the locations of 9 actuators are selected by solving least squares considering the deformation of mirrors measured by the Fizeau interferometer and bonding interface between mirror and supports. Actuators act along the optical axis of mirrors as push–pull style actuators do. Several tests for correcting gravity-gradient and thermal change are conducted and some Zernike terms (which represent deformation on the unit disk) are corrected by actuators. Follow-up research is progressing to correct more Zernike terms. The results from the performance test promise the ability of such hybrid support to compensate for unexpected deformation and validation of space-active optics. Diffractive Optical Elements (DOEs) are utilized for observing astrophysical phenomena, including but not limited to solar flare observations. For example, aboard the CORONAS-PHOTON satellite, the RT-2/CZT payload was launched into Low Earth Orbit (LEO) in 2009 [125]. Four configurations constituted the payload, two of which used Fresnel Zone Plates (FZPs). Along with a Cadmium Zinc Telluride (CZT) and a Complementary Metal Oxide Semiconductor (CMOS) as detectors, the FZPs were leveraged as shadow-casters for indirect imaging that enables imaging of solar flares in hard X-rays. The simulation demonstrated that the configuration of dual FZPs and a CMOS detector could accomplish an angular resolution of 54 arcseconds with a field-of-view of 4.29 degrees. The performance was sufficient to embrace the full Sun with the finest angular resolution among the four configurations.

4.2 Smart Photovoltaics

Diffractive Optical Elements (DOEs) are widely utilized to revolutionize the field of smart photovoltaics, offering a dynamic solution for optimizing solar energy harvesting. These versatile optical elements enable several key functionalities, including dynamic light management within solar panels and concentrator photovoltaic systems [126, 156,157,158,159,160,161]. By precisely adjusting the optical properties of the DOE, such as focal length, phase profile, or diffraction pattern, incident sunlight can be expertly steered and concentrated onto photovoltaic cells. This enhances energy collection, particularly in scenarios where sunlight angles vary or sun tracking isn't practical [24, 127, 128, 162,163,164,165]. Additionally, tunable DOEs can control the spectral distribution of light, customize the photovoltaic response, improve optical efficiency, and selectively filter out unwanted components. They also play a pivotal role in adaptive solar tracking, ensuring optimal energy conversion. These capabilities make electrically tunable DOEs a promising technology for the advancement of smart photovoltaic systems and the efficient utilization of renewable solar energy.

4.3 Smart Window

Smart windows constitute a critical frontier in the field of building technologies, necessitating specific requirements, offering notable advantages, and inspiring evolving research trends. These windows must possess precise and rapid tunability over their optical properties, enabling dynamic control of light transmission, solar heat gain, and privacy levels. DOEs can offer a versatile and transformative solution for smart window applications. These DOEs can dynamically control the transmission and scattering of light, enabling a range of benefits. They allow for precise regulation of light and heat entering buildings, enhancing comfort and energy efficiency [166, 167]. Variable light transmission and glare reduction ensure occupants can tailor natural lighting while mitigating discomfort from excessive brightness. Privacy control is customizable, with the DOE altering its diffraction pattern to provide both privacy and diffused natural light [168, 169]. Moreover, energy-efficient heating, cooling, and lighting systems are optimized through real-time adjustments, contributing to reduced energy consumption and lower utility costs. Adaptive daylighting solutions and security features enhance overall building functionality and occupant well-being. Additionally, the integration of energy harvesting capabilities further enhances the energy efficiency of smart windows [170,171,172]. These combined advantages make electrically tunable DOEs a pivotal technology for smart building designs and energy-conscious environments.

Furthermore, energy-efficient smart lighting necessitates the integration of high-efficiency light sources, such as LEDs, and advanced directional light-control technologies to optimize energy consumption and lighting quality [173, 174]. Advantages encompass substantial energy savings, prolonged luminaire lifespan, improved visual comfort, and potential health and well-being benefits through tunable color temperature and intensity. Current research trends focus on enhancing the energy efficiency of smart lighting by developing novel materials, such as perovskite-based LEDs, and integrating IoT (Internet of Things) connectivity for real-time monitoring and control [129, 175,176,177]. Additionally, research is exploring human-centric lighting solutions that consider circadian rhythms and user preferences, driving the evolution of energy-efficient smart lighting toward more sustainable and user-centric illumination strategies in various applications, from homes to commercial spaces.

4.4 Laser Machining

Electrically tunable Diffractive Optical Elements (DOEs) have become an indispensable property in the realm of laser machining, significantly expanding the capabilities of laser-based manufacturing processes. These versatile optical elements offer a wide range of advantages that enhance precision, productivity, and energy efficiency across various applications. One of the key roles of tunable DOEs in laser machining is beam shaping. They can reshape the typically Gaussian beam profiles generated by lasers into desired intensity distributions, such as top-hat or donut shapes. This transformation ensures uniform energy delivery to the workpiece, resulting in consistent and high-quality machining outcomes. Whether it's cutting, engraving, or surface treatment, the ability to precisely tailor the laser beam profile is invaluable [130, 178,179,180,181,182]. Beam splitting is another crucial function facilitated by tunable DOEs. These optical elements allow a single laser beam to be split into multiple beams with precise control over their distribution. This capability is especially beneficial for parallel processing, where multiple workpieces or features can be machined simultaneously, significantly improving productivity and throughput [25, 183,184,185]. Furthermore, tunable DOEs offer the advantage of focal length control without the need for physical adjustments in the distance between the laser source and the workpiece. This dynamic focal length modification proves particularly useful in scenarios where different machining tasks require varying focus settings. It simplifies and streamlines the machining process while maintaining precision [119, 122, 186]. Real-time dynamic beam steering is another hallmark feature of tunable DOEs, allowing for the precise control of laser beam direction by adjusting phase shifts across the surface of the DOE. This dynamic beam steering capability is valuable in material ablation and cutting processes, especially in applications where rapid and precise beam positioning is essential for achieving intricate patterns or complex geometries [26, 187]. Moreover, these DOEs provide a high level of customization and flexibility, accommodating various laser machining tasks and seamlessly integrating into different laser systems, thereby meeting the diverse needs of contemporary laser-based manufacturing.

4.5 Optical Metrology

Electrically tunable Diffractive Optical Elements (DOEs) have established themselves as indispensable tools in the field of optical metrology, offering an extensive range of capabilities that enhance precision, versatility, and adaptability in a wide array of applications. Dynamic focusing is one of the primary functions that tunable DOEs excel at in optical metrology. They empower users with dynamic control over the focus and depth of field of laser beams, a crucial aspect for metrology applications that involve the measurement of 3D surfaces or complex geometries [188,189,190]. By carefully adjusting the DOE, the laser focus can be meticulously tailored to different surface profiles, ensuring that measurements are not only precise but also highly accurate, regardless of the intricacies of the target surface. Additionally, tunable DOEs facilitate high-precision laser interferometry, supporting distance, displacement, and vibration measurements. They play a key role in surface defect detection, projecting structured light for identifying flaws and irregularities [131, 191]. These DOEs contribute to accurate distance measurements in laser distance systems and enhance surface roughness assessment. In non-destructive testing, they assist in generating controlled laser-induced ultrasonic waves [26, 192, 193]. Their adaptability and real-time adjustments make them valuable in dynamic metrology tasks, promising continued advancements in the field.

4.6 Biomedical Imaging

Tunable optics, which allow for real-time control of optical parameters, have found diverse applications in bio-imaging. From optical coherence tomography (OCT) to super-resolution microscopy, their ability to manipulate focal planes and enhance imaging quality has greatly enhanced our understanding of biological structures and functions [132, 133, 194,195,196]. In particular, endoscopic OCT, with its potential for diagnosing luminal organs such as arteries and esophagus [197, 198], faces technical challenges due to its static and limited depth of field. To address these limitations, researchers have explored methods to dynamically control the focal length of optics [199, 200]. A notable example is the use of tunable microfluidic lenses [27, 201]. These lenses adjust their focal length dynamically by manipulating the shape of a liquid-filled chamber, providing a compact alternative to traditional, bulky lens systems. Nevertheless, introducing microfluidic lenses to endoscopic instruments presents challenges, including manufacturing complexity, durability, and precise control of lens properties. Additionally, there is growing interest in incorporating micro-electro-mechanical systems (MEMS) into endoscopic instruments [202,203,204]. While MEMS technology allows for miniaturized optical components with dynamic capabilities in line with the goals of tunable optics, it also encounters issues related to manufacturing complexity and cost-effectiveness. Tunable planar optics emerge as a promising alternative that might effectively address these obstacles. The flat and flexible structure of the tunable planar optics simplifies manufacturing processes and reduces production costs. Moreover, these optics can offer precise control of optical parameters, thus making them suitable for dynamic adjustments during endoscopic procedures. Consequently, the substitution of traditional bulky optics with thin and light-weight tunable planar counterparts can make endoscopic devices more agile, less invasive, and ultimately more patient-friendly.

5 Summary and Future Prospects

This review has investigated the design, green manufacturing, and application of electrically-tunable smart light-weight planar optics. Direct laser writing, with its distinct advantages, emerges as a key technique that minimizes the environmental impact compared to traditional methods, aligning with the growing emphasis on sustainability in manufacturing. DLW offers design flexibility, enabling precise control and various designs for tunable Diffractive Optical Elements (DOE). Its facile process eliminates the need for multiple steps and hazardous chemicals, significantly reducing environmental impact. DLW's chemical-free, energy-efficient operation aligns perfectly with green manufacturing principles, making it an ideal choice for sustainable fabrication of tunable DOE. Moreover, electrically-tunable planar optics can be realized by active modulation of refractive index, geometrical shape, and bandgap. The capability to tailor these optical elements’ focal length, aberration, transparency, and beam propagation angle is opening up new possibilities in space optics, photovoltaics, mobile phone lenses, and beyond.

In moving toward the future, the evolution of electrically-tunable smart lightweight planar optics hinges on three crucial avenues for exploration. Firstly, there is a need to broaden the spectrum of modulation materials, expanding beyond the current candidates such as graphene, MoS2, and MXene [205,206,207,208,209]. This exploration should consider materials not only for their tunability but also for compatibility with eco-friendly manufacturing processes. Secondly, the refinement of modulation techniques, encompassing advancements in precision, efficiency, and scalability, is essential for electrically-tunable planar optics. Lastly, the practical application of tunable diffractive optical elements (DOEs) across diverse fields, from space optics to biomedical imaging, requires research regarding real-world implementations. By addressing these requirements, this research field can be impelled forward, unlocking the full potential of tunable DOEs and facilitating a new era of versatile and sustainable optical solutions.