Fabrication of functional micro–nano structures using phase holographically modulated femtosecond laser technology: a review

2.1 Iterative optimization algorithm

Iterative optimization algorithm, such as the Gerchberg–Saxton (GS) algorithm [71], weighted GS (GSW) algorithm [72], mixed-region-amplitude-freedom (MRAF) algorithm [73], optimal rotation angle (ORA) algorithm [74], adaptive-additive (AA) algorithm [75], simulated annealing method [76], and the Yang–Gu algorithm [77], rely on iterative feedback loops to optimize the phase distribution. These algorithms differ in their objective functions and weighting strategies, leading to different performances in terms of optical field uniformity, energy efficiency, and computational speed. They are commonly used to construct complex structures such as multifocal arrays, planar optical fields, and volumetric optical fields. However, these methods typically require a large number of iterations to converge and must be recalculated for different target optical fields, which limits their flexibility and real-time applicability.

2.2 Forward modeling algorithm

The forward modeling algorithm constructs target optical fields by superposing an analytical phase function. A representative example is the zonal modulation method [78], [79], which divides the incident optical field into regions and applies different phase profiles to generate two- or three-dimensional multifocal arrays. Other frequently employed models include Dammann gratings [80], Fresnel diffraction [81], and π-phase plates [78], [82]. These approaches offer high energy utilization and are well-suited for laser processing applications that require specific intensity or polarization distributions.

2.3 Deep learning–based algorithm

Deep learning–based algorithms have been introduced more recently for phase hologram generation, utilizing architectures such as convolutional neural networks (CNNs) [83] and diffractive neural networks (DNNs) [84]. These data-driven methods do not rely on physical models and can directly predict phase distributions, enabling rapid generation of highly complex optical fields. Additionally, the incorporation of phase gradient terms into the loss functions can reduce speckle noise. Although these methods may incur higher computational costs, they offer advantages for applications demanding high uniformity or real-time modulation.

In summary, each type of algorithm exhibits distinct advantages in terms of accuracy, efficiency, and applicability. The choice of phase algorithm should, therefore, be made based on the specific requirements of the intended application.

3.1 Dual-focus optical field–induced material phase transition and ablation

Wang et al. [86] used a 0-π phase modulation to transform a Gaussian femtosecond laser beam into a dual-focus optical field with zero intensity at the center. They modulated the phase transition and localized material transfer of a gold film by controlling the spatial distribution of free electrons, ultimately fabricating gold nanowires with a minimum linewidth of 56 nm between two ablation pits, as shown in Figure 2a. Furthermore, they fabricated integrated circuits with uniform linewidths and arbitrarily arranged patterns by switching phase holograms during beam scanning using the dynamic modulation capability of the SLM. Based on this approach, Xu et al. [87] increased the laser energy to induce Rayleigh instability in the nanowires, resulting in the rupture of the nanowires and the formation of nanoslits with a minimum spacing of 30 nm. They further demonstrated the application of these nanoslits in surface-enhanced Raman scattering (SERS) and terahertz filtering, as illustrated in Figure 2b. This method was also extended to fabricate nanoslit structures on silicon surfaces. Zhou et al. [88] used a dual-focus optical field to induce modifications on the surface of crystalline silicon, forming butterfly-shaped amorphous silicon areas. During subsequent KOH etching, stress concentration at the center of these butterfly-shaped mask structures led to their rupture, producing crack-like nanoslit structures as small as 9 nm in size, as shown in Figure 2c. Compared to strategies that use specifically modulated intensity distributions to control material transfer for nanostructure fabrication, directly reducing the focal spot size below the diffraction limit offers greater universality. Zhao et al. [89] used an SLM to generate a multifocal array beam, which was focused through a low numerical aperture (NA = 0.1) objective lens. Under the influence of near-field electromagnetic enhancement and incubation effects, this method enabled the fabrication of gold nanowires with a width of approximately λ/3, reaching 1/20 of the diffraction limit, as shown in Figure 2d. Due to the long depth-of-focus characteristic of the low-NA objective, this method enables large-area fabrication on nonplanar surfaces, such as flexible PET substrates, significantly enhancing parallel fabrication efficiency by a factor of eight. Moreover, this method eliminates the need for masks or complex postprocessing steps, providing an efficient and cost-effective solution for miniaturized electronic and optoelectronic devices.

Figure 2:

Dual-focus optical field–induced material phase transition and ablation for FMNS fabrication. (a) Gold nanowires [86], (b) gold nanoslit structures [87], and (c) crack-like silicon-based nanoslit structures [88] fabricated using a 0-π phase-modulated dual-focus femtosecond laser optical field. (d) Nanowires exceeding the optical diffraction limit were fabricated using near-field electromagnetic enhancement and incubation effects [89]. (a) Is reprinted from Ref. [86], with permission. Copyright 2015 John Wiley and Sons; (b) is reprinted from Ref. [87], with permission. Copyright 2019 American Chemical Society; (c) is reprinted with permission from Ref [88] © Optica Publishing Group; (d) is reprinted from Ref. [89], with permission. Copyright 2022 John Wiley and Sons.

At present, this type of technology is primarily applied to the fabrication of metals and metal film materials because of their moderate ablation thresholds. When applied to materials with lower damage thresholds, such as paraffin wax or hydrogels, even the minimal energy at the periphery of the laser spot can induce ablation or modification. This leads to poor fabricating resolution and prevents the achievement of subdiffraction-limit precision. Conversely, when applied to hard and brittle materials such as glass, sapphire, or ceramics, the wide bandgap characteristics of these materials require significantly higher laser pulse energies to induce damage. However, such high pulse energies may exceed the maximum energy threshold that the SLM can withstand. As a result, this technology is currently unsuitable for fabricating wide-bandgap, hard, and brittle materials.

3.2 Multimodal optical fields for FMNS fabrication

The multimodal optical field refers to an optical field capable of switching between multiple patterns. By utilizing specific effects associated with multimodal optical fields, it is possible to fabricate nanostructures with lateral dimensions below the optical diffraction limit. Qiu et al. [90] achieved simultaneous and precise control of both micron- and nanostructures in FMNS fabrication on the SiC surface by using an SLM to manipulate the optical field shape and polarization direction of the femtosecond laser, combined with micro-jet generation assisted by liquid. As shown in the SEM image in Figure 3a, the cross-sectional morphology (such as V-shaped grooves and blazed gratings) was determined by the geometric shape of the patterned laser optical field, with the characteristic width of the stripes inside the microgrooves reaching as small as 120 nm. Huang et al. [91] proposed a subwavelength-patterned pulsed laser lithography (PPLL) technology, generating quasi-binary phase masks using SLM, combined with gradient grayscale boundaries and circularly polarized light, which effectively suppressed diffraction effects and polarization asymmetry. This enabled the fabrication of high-uniformity structures, such as antennas and catenary-shaped subwavelength units on metal films and phase-change materials, with feature sizes as small as 303 nm (approximately 76 % of the optical diffraction limit), as shown in Figure 3b. Using PPLL technology, they further modulated the circular optical field into a linear form, successfully fabricating a grating with a linewidth of 54 nm. Hasegawa et al. [92] used SLM to modulate a single femtosecond laser focal point into an array of five focal points and adjusted the phase difference between the center and surrounding focal points to approximately π using algorithms. Interferometric cancellation between adjacent focal points in the array reduced the lateral size of the central focal point. The central focal spot size was reduced to 60 % of the diffraction limit by optimizing the spacing and energy ratio between the central and surrounding focal points. They used this method to fabricate microholes with diameters as small as 39 % of the optical diffraction limit on fused silica, as shown in Figure 3c. Wang et al. [93] used modulated femtosecond lasers to fabricate various sizes of micro-pit structures on polyolefin (PO) films with thermally shrinkable shape memory properties. When the PO film was heated to its glass transition temperature (∼140 °C), thermal shrinkage occurred, reducing the minimum micro-pit size to approximately 500 nm. A four-level amplitude holographic modulation was achieved using the difference in micro-pit transmittance before and after PO film shrinkage. As displayed in Figure 3d, the switching of the hologram from an “angry face” to a “smiling face” upon heating, as well as the decryption of “888”: into “790,” was experimentally verified, providing a new method for anticounterfeiting and information encryption.

Figure 3:

Multimodal optical fields for FMNS fabrication. (a) Schematic and SEM images of FMNS that were fabricated on the SiC surface using femtosecond laser optical field modulation combined with a liquid-assisted method [90]. (b) Annular structures and grating structures were fabricated using PPLL technology [91]. (c) Modulated focal points were achieved with a lateral size below the optical diffraction limit through interferometric cancellation between adjacent focal points [92]. (d) Micro-pit structures of various sizes fabricated on thermally shrinkable shape memory films using a modulated femtosecond laser, and the switching of the hologram from an “angry face” to a “smiling face” upon heating, as well as the decryption of “888” into “790” [93]. (a) Is reprinted from Ref. [90], with permission. Copyright 2024 John Wiley and Sons; (b) is reprinted from Ref. [91], under the terms of the Open Access Publishing Agreement; (c) is reprinted from Ref. [92], under the terms of the Open Access Publishing Agreement; (d) is reprinted from Ref. [93], with permission. Copyright 2024 American Chemical Society.

Table 1 provides a summary of the fabrication resolution of FMNS achieved by different phase holographically modulated femtosecond laser technologies.

4.1 Spherical aberration compensation

When femtosecond laser pulses are focused inside a material, the refractive index mismatch between the objective’s working medium and the material causes beam refraction at their interfaces, leading to spherical aberration at the focal point. Spherical aberration elongates the focal region axially and reduces the peak intensity, thereby influencing the fabrication quality and precision inside the material. Based on the phase modulation capability of an SLM, spherical aberration can be compensated by adjusting the wavefront of the incident beam before it enters the objective lens, thereby correcting the beam refraction at the interface and reducing focal distortion.

Cumming et al. [95] fabricated chiral helical photonic crystals inside arsenic trisulfide glass using an SLM-based spherical aberration phase compensation method. As displayed in Figure 4a, photonic crystals fabricated with spherical aberration compensation demonstrated superior morphological uniformity and agreement between experimental and simulated circular dichroism spectra compared to those without compensation. Kontenis et al. [96] further developed a phase precompensation method that dynamically adjusts the compensation phase according to fabrication depth. The microstructures fabricated by this method depth. The microstructures fabricated by this method have nearly identical axial and lateral dimensions, as shown in Figure 4b. Roth et al. [97] combined a slit mask with spherical aberration compensation phase modulation to maintain a consistently circular focal spot at different depths inside polymethyl methacrylate (PMMA), ensuring high uniformity of fabricated microchannels at various depths, as displayed in Figure 4c. Besides planar samples, spherical aberration phase compensation can also correct beam refraction when incident on curved surfaces. Salter et al. [98] analyzed the geometric characteristics of circular fiber cross sections and derived a phase compensation model for cylindrical surfaces. This model was applied to fabricate Bragg gratings inside optical fibers, as shown in Figure 4d. Compared to the uncompensated case, spherical aberration compensation significantly improved the fabrication quality of Bragg gratings, and the measured reflection spectrum showed better agreement with theoretical calculations.

Figure 4:

Improvement of FMNS quality through spherical aberration correction. (a) Improvement in fabrication uniformity in chiral helical photonic crystals using spherical aberration phase compensation method [95]. (b) Microstructures fabricated using the phase precompensation method [96]. (c) High uniformity microchannel fabricated in PMMA using spherical aberration phase compensation method combined with slit mask [97]. (d) Improvement in the fabrication quality of Bragg gratings inside optical fibers using spherical aberration phase compensation designed for cylindrical surfaces [98]. (a) Is reprinted from Ref. [95], under the terms of the Open Access Publishing Agreement; (b) is reprinted from Ref. [96], under the terms of the Open Access Publishing Agreement; (c) is reprinted from Ref. [97], under the terms of the Open Access Publishing Agreement; (d) is reprinted from Ref. [98], with permission (CC BY 4.0).

4.2 Noise suppression for improving structural uniformity

The Gerchberg–Saxton (GS) iterative algorithm is a widely used phase compensation method [71] due to its advantages of simple algorithm flow, fast convergence speed, and high computational efficiency [99], [100]. Compared to noniterative algorithms, iterative algorithms such as the GS algorithm were able to generate structured optical fields with arbitrary shapes and intensity distributions through phase modulation of a single laser beam, offering greater versatility. However, since most iterative algorithms modulate only the amplitude distribution of the structured optical field in the focal plane, the phase distribution of the modulated optical field often exhibits randomness. This irregular phase distribution may lead to interference effects – either destructive or constructive – between adjacent focal points due to phase mismatches, thereby introducing speckle noise, degrading the uniformity of the structured optical field, and ultimately affecting the quality of FMNS fabrication.

To improve the uniformity of structured optical fields, Sun et al. [101] used a phase holography–based optical field modulation technology to generate a single-row multifocal optical field. Simultaneously, they used simultaneous spatial and temporal focusing (SSTF) technology to stagger the arrival times of each focal point at the sample, effectively eliminating interference between them. A continuous and uniform line–shaped optical field was achieved by further reducing the spacing between focal points, as shown in Figure 5a. Taking a different approach, Li et al. [102] divided the modulated optical field into a signal area and a noise area. In the signal area, phase modulation was employed to eliminate speckle noise caused by phase interference, while a weighted amplitude constraint algorithm was used to improve convergence. In the noise area, energy efficiency–weighted modulation was introduced to redistribute energy exceeding the threshold energy into nonprocessing areas. This method achieved 95 % optical field uniformity and 73 % energy efficiency in an 80 × 80 pixel pattern, with a speckle contrast as low as 1.5 %. They successfully fabricated complex microstructures with feature sizes ranging from 10–50 μm using TPP, achieving an edge precision of 1.26 μm, as demonstrated in Figure 5b. Zhang et al. [103] applied the aforementioned noise suppression approach to the TPP process. By optimizing the number of overlaid modulating phase masks, they enhanced the uniformity of the structured optical field to nearly 100 % on the focal plane, thereby significantly improving the fabrication quality of micropatterned structures. This method was further applied to fabricating Dammann gratings with various shapes, as displayed in Figure 5c. In addition to improving the fabrication quality of FMNS in static projection fabrication, the multifield noise homogenization method can also be applied to dynamic scanning fabrication to improve the uniformity of the fabricated structure, as displayed in Figure 5d [94].

Figure 5:

Improvement of FMNS uniformity through noise suppression. (a) Generation of a uniform line–shaped optical field and fabrication process by staggering the arrival times of each focal point at the sample via SSTF technology, effectively eliminating interference between focal points [101]. (b) Schematic and fabrication results of microstructures fabricated using a spatially modulated femtosecond laser method based on an improved algorithm [102]. (c) Improvement of structured optical field uniformity through optimization of the iterative algorithm [103]. (d) High-uniformity FMNS fabricated via dynamic scanning using the multifield noise homogenization method [94]. (a) Is reprinted from Ref. [101], with permission (CC BY 4.0); (b) is reprinted from Ref. [102], with permission, Copyright 2024 John Wiley and Sons; (c) is reprinted from Ref. [103], with permission (CC BY 4.0); (d) is reprinted with permission from Ref [94] © Optica Publishing Group.

Table 2 provides a summary of the advantageous effects of FMNS achieved by different phase holographically modulated femtosecond laser technologies.

5.1 Parallel fabrication with multifocal optical fields

The initial application of phase holography spatial optical field modulation in femtosecond laser fabrication was to transform a single femtosecond laser focal point into an array of equally intense focal spots, enabling parallel fabrication of FMNS structures. In 2005, Hayasaki et al. [104] from Tokushima University, Japan, first realized multifocal points femtosecond laser parallel fabrication based on phase holography modulating using an SLM. By loading a phase hologram computed via the GS algorithm onto the SLM, they successfully fabricated a 1 × 9 array of micro-pits on a glass surface in a single projection. Subsequently, multifocal points optical fields have been widely applied across various fabrication technologies. Zhang et al. [105] proposed a high-throughput 3D printing technology integrating an SLM with a galvanometer scanning system, achieving high-speed scanning of holographic multifocal point arrays. In this method, the target optical field was positioned at the diffraction center of the SLM, and a holographic correction beam was introduced to suppress zero-order diffraction, achieving parallel fabrication with over 400 uniform focal points, as displayed in Figure 6a. Xu et al. [106] used a laterally distributed multifocal points array to induce TPP for the rapid fabrication of filter sieve structures inside microchannels, as displayed in Figure 6b. This technology also allowed precise control of the pore size and density of the sieve by adjusting the number and spacing of focal points, which can be used to sort and collect cells with different sizes. Carstensen et al. [107] used a femtosecond laser to generate a multifocal points array arranged in a Fresnel zone plate and Bessel zone plate pattern. They selectively transformed silver films deposited on silica nanopillar arrays into nanoparticles with distinct resonance spectra by using the femtosecond laser–induced dewetting effect of metal thin films. Consequently, the Fresnel zone plate and Bessel zone plate were successfully fabricated using a single-shot projection, as displayed in Figure 6c.

Figure 6:

Improvement of fabrication efficiency through projection of multifocal optical fields. (a) Schematic and results of microstructure fabrication using high-speed scanning and high-throughput 3D printing technology based on holographic multifocal points arrays [105]. (b) Schematic and results of microchannel filter sieves fabricated via parallel multifocal points optical field–induced TPP [106]. (c) Fresnel zone plates and Bessel zone plates fabricated via a single-shot projection [107]. (d) Schematic and results of concave lens array fabrication based on axial length differences among focal points within a multifocal points array [108]. (e) Microlens arrays fabricated via multifocal points optical fields with different focal lengths [110]. (a) Is reprinted from Ref. [105], with permission. Copyright 2024 American Chemical Society; (b) is reprinted from Ref. [106], with permission (CC BY 4.0); (c) is reprinted from Ref. [107], with permission. Copyright 2018 American Chemical Society; (d) is reprinted from Ref. [108], with permission (CC BY 4.0); (e) is reprinted from Ref. [110], under the terms of the Open Access Publishing Agreement.

The use of an SLM not only enables the transformation of a single focal point into an array of focal spots with uniform energy but also allows for precise control over the axial length and energy distribution of individual focal points within the multifocal array. This capability facilitates customized multifocal parallel fabrication, enabling more refined manipulation of the physicochemical properties and structural morphology of the fabricated materials. Liu et al. [108], [109] used variations in axial length among focal points within a multifocal points array to quantitatively control the modified volume inside fused silica. This method can induce various modified structures in a single-shot projection, which were subsequently fabricated using wet etching to form microlens arrays with multiple numerical apertures (NA), as displayed in Figure 6d. Similarly, Chen et al. [110] fabricated a micro-pit array on curved fused silica substrates using a multifocal optical field with hexagonally packed focal spots of varying focal lengths. These micro-pits were subsequently enlarged into microlens arrays using wet etching, as displayed in Figure 6e.

These studies effectively solved the problem of balancing high spatial resolution with an extended depth of field in microlens applications. Furthermore, Silvennoinen et al. [111] used grayscale multifocal points optical field projection to fabricate grayscale patterns composed of micro-pits of varying sizes on the surface of silicon. Compared to binary micro-pit structures, these grayscale-patterned structures provided more intricate pattern details. Based on the principle that reduced graphene oxide (rGO) with different reduction degrees modulates incident laser differently of incident laser, Li et al. [112] fabricated high-order phase holograms on rGO by controlling the focal point’s position and relative energy ratio between focal points in the multifocus points optical field and using a grayscale multifocus points femtosecond laser.

5.2 Patterned optical field projection fabrication

When the spacing between focal points in a multifocal points optical field is reduced to the size of a single focal spot, the modulated optical field can be regarded as a patterned optical field. Patterned optical fields are widely used in the projection-based fabrication of micropatterned FMNS. Since a single optical field can directly fabricate the desired patterned structure, this approach not only significantly enhances fabrication efficiency but also effectively avoids issues associated with single-focal direct writing. These issues include increased recast layer thickness and an expanded heat-affected zone at the pattern edges, which result from excessive pulse accumulation due to the frequent start-stop motion of the translation stage. Hasegawa et al. [113] used an SLM loaded with a cylindrical lens phase to modulate the focal field of a femtosecond laser into a line-shaped beam, which was then projected onto a glass surface to fabricate periodic microgroove structures. Since each microgroove was fabricated with a single pulse, this method not only improved fabrication efficiency but also eliminated the formation of laser-induced periodic surface structures (LIPSS) at the groove bottom, which typically occurs due to the multipulse incubation effect in single-focal direct writing. Consequently, the surface smoothness at the groove bottom was significantly improved, as displayed in Figure 7a. Except for line-shaped optical fields, annular optical fields of various forms can be generated using vortex phases [114], Bessel phases [115], or a combination of both [116]. These optical fields can be applied in different fabrication scenarios. For instance, in conjunction with TPP, annular optical fields with varying diameters along the optical axis have been used to fabricate microtubules for cellular or drug transport, as displayed in Figure 7b [116]. Additionally, the closed-loop nature of annular optical fields enables rapid drilling of metallic thin films, as demonstrated in Figure 7c [115]. Beyond simple line-shaped optical fields, Liu et al. [117] designed more complex patterned optical fields using a noniterative phase design method. This method involves defining a pattern area at the center of the phase map, overlaying a grating phase onto the nonpattern area to spatially separate the reflected optical fields from the two areas, and finally filtering out the reflected light from the nonpattern area to obtain the desired patterned optical field, as displayed in Figure 7d. Li et al. [118] used the principle of two-beam interference by integrating an SLM as one of the mirrors in a Michelson interferometer. A patterned optical field was generated by modulating a pulse with a pattern-designed phase hologram loaded onto the SLM, followed by interference with an unmodulated pulse reflected from another mirror, as displayed in Figure 7e. Furthermore, this method allows arbitrary modulation of the energy distribution within the patterned optical field, thereby improving energy utilization efficiency.

Figure 7:

Improvement of fabrication efficiency through projection of patterned optical fields. (a) Improvement of microgroove fabrication efficiency using a line-shaped optical field [113]. (b) Fabrication of microtubules with different cross-sectional shapes using an annular optical field [116]. (c) Rapid drilling of aluminum thin films enabled by an annular optical field [115]. (d) Generation of a patterned optical field by spatially separating the reflected light from the patterned and nonpatterned areas using a grating phase [117]. (e) Modulation of patterned optical fields with arbitrary shapes and intensity distributions based on two-beam interference [118]. (a) Is reprinted from Ref. [113], under the terms of the Open Access Publishing Agreement; (b) is reprinted from Ref. [116], with permission. Copyright 2017 John Wiley and Sons; (c) is reprinted from Ref. [115]. Published by IOP Publishing Ltd. CC BY 4.0; (d) is reprinted from Ref. [117], with permission. Copyright 2018 Elsevier; (e) is reprinted from Ref. [118], with permission, Copyright 2018 John Wiley and Sons.

5.3 3D optical field volume projection fabrication

Compared to 2D-shaped optical fields, such as multifocal points array, line-shaped, and planar optical fields, a single 3D optical field enables in situ volume projection fabrication of 3D structures within a transparent material, providing a more significant improvement in fabrication efficiency. The simplest form of a 3D optical field is an axially elongated optical field, represented by Bessel beams and their variations (i.e., Bessel-like beams). Bessel beams are primarily used for fabricating high-aspect-ratio microhole structures. Compared to varying the parameters of axicon lenses, generating Bessel beams using an SLM provides greater flexibility, as the intensity distribution of the modulated optical field can be controlled by adjusting the phase hologram.

Yao et al. [119] continuously tuned the nondiffractive region length of Bessel beams by overlaying focusing or diffusing phases onto a zero-order Bessel phase, as displayed in Figure 8a. This method enables the fabrication of microholes with different aspect ratios without moving the sample, simply by modifying the computational parameters of the phase hologram [120]. Furthermore, in additive manufacturing technologies such as femtosecond laser–induced TPP or multiphoton polymerization (MPP), Bessel beams and other axially elongated optical fields allowed for in situ projection fabrication of high-aspect-ratio micropillar structures, as displayed in Figure 8b [121]. Wu et al. [81] utilized the interference of multiple parallel vortex beams to generate 3D chiral microstructures, leveraging capillary forces for subunit assembly of these structures, as demonstrated in Figure 8c. The fabrication efficiency was enhanced by more than 100 times.

Figure 8:

Improvement of fabrication efficiency through 3D optical fields volume projection fabrication. (a) Fabrication of microhole structures with varying depth-to-diameter ratios using Bessel-like beams with adjustable nondiffractive regions [119]. (b) High-aspect-ratio micropillar structures fabricated using an axially elongated optical field [121]. (c) 3D chiral microstructures fabricated based on multiple parallel vortex optical field [81]. (a) Is reprinted from Ref. [119], under the terms of the Open Access Publishing Agreement; (b) is reprinted with permission from Ref [121] © Optica Publishing Group; (c) is reprinted from Ref. [81], with permission. Copyright 2022 John Wiley and Sons.

For 3D optical fields with more complex intensity distributions, such as 3D multifocal points optical fields or arbitrarily shaped volumetric optical fields, phase hologram iterative algorithms suitable for 3D optical field modulation are required, as their phase holograms lack fixed analytical functions. Jesacher et al. [122] used a 3D GS algorithm to generate a phase hologram for a 3D multifocal points optical field with a like-crystalline distribution. They successfully fabricated uniform 3D cavity structure arrays inside lithium niobate and fused silica by introducing spherical aberration compensation. Ren et al. [123] improved the uniformity of focal intensities among different focal point arrays along the optical axis by improving the 2D Fourier iterative algorithm with a 3D Fourier transform based on vector Debye diffraction. This method facilitated the fabrication of uniformly sized patterned lattice structures at different depths within the sample, as displayed in Figure 9a. Zhang et al. [124] applied the Yang–Gu algorithm to generate a 3D annular multifocal optical field, allowing the fabrication of Type-III optical waveguide structures and waveguide coupling devices within lithium niobate through a single scan, as displayed in Figure 9b. Furthermore, based on the electro-optic refractive index of lithium niobate crystal, signal coupling between different optical waveguides and the adjustment of spectral coefficients were achieved by applying a voltage to change the refractive index of the material.

Figure 9:

Improvement of fabrication efficiency through 3D multifocal optical fields and arbitrarily shaped volumetric optical fields projection fabrication. (a) Schematic of a uniformly patterned lattice structure fabricated at different depths using a 3D Fourier transform iterative algorithm based on vector Debye diffraction [123]. (b) Type-III optical waveguide structures and waveguide coupling devices fabricated in a single scan using a 3D annular multifocal points optical field [124]. (c) 3D structures fabricated via TPP using either volumetric projection or serial stitching of a 3D optical field [125]. (d) Metallic helical chiral structures fabricated using a dual-helical beam based on the MPP effect [126]. (a) Is reprinted with permission from Ref [123] © Optica Publishing Group; (b) is reprinted from Ref. [124], with permission from Photonics Research; (c) is reprinted from Ref. [125], with permission. Copyright 2019 John Wiley and Sons; (d) is reprinted from Ref. [126], with permission (CC BY 4.0).

Furthermore, 3D optical fields with complex intensity distributions can also be applied in additive manufacturing technology, including femtosecond laser-induced TPP, MPP, and metal ion reduction. Yang et al. [125] used the nonlinear effects of TPP to fabricate high-quality 3D structures by improving iterative algorithms and optimizing the energy distribution of the 3D optical field, as displayed in Figure 9c. For small-scale structures, a single modulated optical field was sufficient for projection fabrication. However, for large and complex structures, a 3D object could be longitudinally sliced to generate a series of 2D patterns and corresponding phase holograms, which were subsequently stitched together to fabricate intricate 3D structures. Liu et al. [126] first modulated a femtosecond laser beam into a dual-helical structured optical field and subsequently employed the MPP effect to reduce silver ions in a silver nitrate solution into silver nanoparticles. These nanoparticles were then self-assembled into a metallic dual-helical structure following the distribution of the structured optical field, as displayed in Figure 9d. The fabricated dual-helical structures exhibited low surface roughness and high reproducibility because the heat accumulation caused by pulse overlap was reduced, making this method suitable for the fabrication of chiral optical devices in the mid-infrared spectral range. Although 3D-shaped optical fields significantly improve the fabrication efficiency of femtosecond lasers, their current applications are primarily focused on structural formation. Research on the application of 3D optical field modulation in fabricating micro–nano optical devices is still in its early stages.

Table 3 provides a summary of the fabrication efficiency of FMNS achieved by different phase holographically modulated femtosecond laser technologies.