Optical Modification of 2D Materials: Methods and Applications

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost‐ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.


Introduction
Within the past few decades, 2D layered materials [1] have garnered vast interest due to their remarkable physical and chemical properties. Besides graphene, there is an extensive range of materials that can be layered down to the monolayer limit, such as transition metal dichalcogenides [2] (TMDs), hexagonal boron nitride [3] (hBN), and phosphorene. [4] Some of the intriguing properties of these materials include high thermal and electrical conductance of graphene, [5] thickness-dependent bandgap (TMDs, black phosphorus (BP)), and high chemical stability of hBN. [4] Recently, stacking different 2D materials on top of another one to form van der Waals heterostructures has also widened the scope of the known 2D materials for devices with unprecedented performance and unique functionalities. [6] One of the encouraging prospects of 2D materials is the possibility to build almost all the necessary components for devices

Light-Matter Interactions
The light-matter interactions in materials describe not only their typical optical properties (e.g., absorption and photoluminescence) but also the unique physical and chemical ways they can affect the material properties upon illumination. As we humans can experience during summer under the sun, light can be powerful. It can quickly induce diverse physical changes in materials, depending on how the energy of the light interacts with the material. The main physical effects include thermal, mechanical, acoustic, and electric effects. Thermal effects (e.g., ablation) are caused by absorption and subsequent conversion of light into heat. Thermal energy is typically spread in the material by vibrations. Still, it can also create a photoacoustic effect, where a pulse of light energy is converted into acoustic waves via pressure mainly caused by thermoelastic expansion. Mechanical effects of light can be slightly harder to picture. Photons have momentum and can therefore physically affect things via a process called radiation pressure. While not often relevant, radiation pressure does play a role in many critical microsized applications. Finally, light can create versatile electric effects. These are usually a consequence of light-excited electrons in the material to higher energy states. These excitations can relax by emitting electrons through the photoelectric effect or by diffusion through the photovoltaic effect. Both of these phenomena can lead to the creation of photocurrent, which in turn is the basis for many critical optoelectronic applications.
Many chemical reactions are often also caused by thermal energy as heat can cause reactive agents to move more quickly, making the interaction between them more likely to occur. Heat can also speed up chemical processes, and it can be required to kick-start some chemical reactions. However, heat is not the only way, light can also affect chemical reactions. The energy that photon carry can be enough to break bonds, once again excite electrons, or it can be absorbed and be used to overcome the activation energy of the reaction. A great example of this can be found in nature, as the photosynthesis of plants requires light to happen. Chemical reactions caused by light are called photochemical reactions. Typical light-activated reactions on materials in ambient conditions are surface reactions, such as oxidation and reduction. By controlling the environment, different functionalization processes are also possible.
The methods presented in this review are mainly based on either thermal or nonthermal laser-induced structural modifications of 2D materials. Thermal modification is caused by longer irradiation processes where light's photon energy is transferred into the material's phonon energy upon absorption, breaking atomic bonds once an energy threshold is overpassed. This process is typical for continuous wave (CW) lasers. Nonthermal modification, on the other hand, uses pulsed lasers that deliver a large amount of optical energy to the material in a very short time. This creates hot carriers that can couple with phonons much faster, allowing shorter reaction timescales. Thus, energy from pulsed lasers causes minimal thermal effects but can cause rapid structural modifications via ionization or ejection. [197] Laser parameters, such as power and wavelength, are also extremely important factors when considering the induced light-matter interactions. While low power lasers (≈nW) are safe to use for measurements without structural damage, midrange CW laser powers (>µW) can induce irreversible changes in the neutral-to-charged exciton ratios of 2D materials [79] via optical doping, and furthermore, high power lasers almost always induce some type of structural modification. The laser wavelength can also produce changes in physical and structural properties of materials [198] but it also limits the minimum spot size of CW lasers. However, pulsed lasers are typically not limited by the wavelength but rather by their input intensity profile. [199] This and the structural damage caused by pulsed lasers make them a prime candidate for micromachining processes such as ablation, while CW irradiation is optimal for processes requiring constant thermal energy input, for instance, CVD.
Other considerable factors for structural modification of 2D materials are beam profiles and substrates. Certain beam profiles, such as the Gaussian profile, can be suboptimal for heating-related processes as they are not uniform. [16,51] Using different beam shapes, such as a donut-like shape, the location and distribution of light-induced effects could be easily controlled. [99] Additionally, varying substrates can have a significant effect on the light-induced interactions in 2D materials. Substrate materials with high thermal conductivity could diminish the heating effect of the 2D material, and differences in surface roughness can impact charge trapping processes. [79] Laser parameters that affect optical modification processes are summarized in Figure 2.

Optical Synthesis of 2D Materials
LCVD is an innovative CVD method based on laser-driven heatenabled chemical reactions to synthesize graphene (Figure 3a). This local heating at room temperature environment provides the means for high-quality thin films and broadens the scope of substrates [47][48][49][50][51][52][53] and precursors, some of which are unsuitable for conventional CVD. [17,46,[49][50][51][52][53][54] Local heating in LCVD enables many advantages over traditional CVD, highlighting the usefulness of laser-based modification. In addition, using lasers in the CVD methods opens many avenues for tuning the chemical reactions; moreover, the locality and controllability can add new functions to the synthesis method. Laser parameters can be used to control the material deposition rate and thickness. [47,49,53] Using a microscopic laser beam enables the patterning of ribbons, [47,53] mazes, and spirals [52] on graphene with potential developments for on-flake microcircuits. Additionally, controlling the laser intensity profile might improve   Reprinted with permission. [47] Copyright 2011, AIP Publishing. b) Laser-based direct synthesis of TMDs from laser decomposed precursor materials. Adapted under the terms of the CC-BY 4.0 license. [55] Copyright 2021, Springer Nature. c) Laser-induced photochemical synthesis of graphene on polyimide precursor. Reproduced with permission. [66] Copyright 2020, American Chemical Society. d) Laser-assisted liquid phase exfoliation from highly ordered pyrolytic graphite. Reproduced with permission. [65] Copyright 2011, AIP Publishing. e) Atomic force microscopy image of laser-etched multilayer graphene. The Raman spectra indicate that the laser-thinned area exhibits monolayer graphene Raman signals. Reproduced with permission. [25] Copyright 2011, American Chemical Society.
the flake uniformity, which is complicated with a Gaussian beam profile. [51] In conclusion, LCVD offers optimum control of graphene synthesis and patterning for novel electronic and optoelectronic devices.
Recently, another pioneering bottom-up synthesis method, that utilizes thermal energy from lasers, has been reported. [55] In the method (as seen in Figure 3b), a laser is used to decompose spin-coated precursor materials, typically ammonium transition metal tetrachalcogenide compounds, into TMDs in ambient conditions, setting it apart from LCVD. The technique is scalable, spatially selective, produces high-quality thin films, and can be used for LDW. Furthermore, the deposition thickness and process speed could be adjusted with laser fluence and irradiation time. This optical synthesis method offers an in situ modifiable route to fabricate and pattern TMDs at an industrial scale.

Laser-Induced Photochemical Synthesis
Laser-induced photochemical synthesis is a fabrication method for the production of porous graphene via light-based reduction of aromatic compounds (see Figure 3c). [58] This is a simple process that can be conducted in ambient conditions and expanded to a variety of carbon containing precursors. [58] These precursors can be common compounds, such as polyimide plastic, [58,66] lignin, and Kevlar, [59] or unusual materials, for instance, bread and wood. [58] Although, laser-induced photochemical synthesis is mainly utilized for graphene synthesis, it could potentially be extended to other single element 2D materials, offering additional versatility. Furthermore, the laser-induced photochemical synthesis is promising for green fabrication methods with potential applications in the recycling industry.

Laser-Assisted LPE
Laser-assisted liquid phase exfoliation typically utilizes the thermal energy of light to exfoliate 2D materials [56,57,65] (Figure 3d). Light with sufficient energy can be used to break the CC bonds of the material via ablation, and the fluence of the light can be used to tune the fragment size. [65] The biggest advantages of laser-assisted liquid phase exfoliation are its simplicity and fast processing speed, which is 3 times faster than the conventional liquid phase exfoliation. [56] Moreover, tunable laser parameters provide control over flake size [56,57] and morphology. [65] Additionally, using lasers with extremely short pulse duration circumvents the disruptive thermal effects, for example, reducing defects that are typically created with continuous wave lasers. [67] In conclusion, laser-assisted liquid phase exfoliation presents notable advantages for high-speed industrial scale production of few-layered 2D materials and their applications. [68]

Structuring via Optical Ablation
Optical thinning is a light-based top-down synthesis method that uses high-power lasers to ablate and evaporate thick materials down to thin layers. [24][25][26][27][28][60][61][62][63][64] This method typically utilizes photothermal effects for laser-driven sublimation to break down the material layer-by-layer with extremely high precision. The substrate plays a vital role in interactions between laserinduced heat, for example, the substrate can tune the ablation rate, allowing thinning the bulk material down to produce highquality monolayer (see Figure 3e). By modifying laser parameters, ablation speeds up to 10 mm 2 min −1 can be achieved, [26] showing promise for industry-scale applications.

Defect Introduction
Optical defect introduction uses the energy of light to modify 2D material surface structure locally. Optically introduced 2D material defects typically include chalcogen vacancies in TMDs, [16,32,76] hBN color centers [78] and spin defects, [99] and graphene point defects. [33,39,193,194] The concentration of point defects and vacancies can be tuned with laser power, wavelength, [15,31,32,37,38,76] irradiation time, [76] and by the choice of substrate. [37,38] Precise design and placement of these defects is the basis for many modification and synthesis methods and can be used to improve the device performance, add new device functionalities, and aid in deterministic growing processes. [100] For instance, a recent demonstration of using laser irradiation to create nucleation centers for heterostructure CVD [100] has illustrated that defects can be used for extremely controllable synthesis regardless of lattice differences. In conclusion, optical defect engineering is a versatile and tunable local modification method for advanced 2D material devices.

Doping
Optical doping is a versatile tool to modify 2D material properties, such as electrical and optical conductivities [86] and photoluminescence (PL). [82,83] This method is especially advantageous for 2D materials [13,16,[79][80][81][82][83][84][85][86]101] as the method introduces doping via the surface modification [13] instead of the subsurface effects created in the ion implantation process. Optical doping also works via the introduction of optical defects and their subsequent filling with dopants from a precursor material [16,81,101,195] or modulation doping induced by incandescent light. [102] Figure 4a shows a schematic description of the defect introduction pathway. Laser irradiation induces chalcogen vacancies that are subsequently replaced by phosphine (PH 3 ) which acts as a p-type dopant for MoS 2 . This process enhances the PL intensity from MoS 2 , as shown in Figure 4b.
As defect introduction can be controlled with laser power, [16,79] irradiation time, [16] and substrate type, [79] so can defect-based optical doping. However, there is a clear difference between doping with CW and pulsed lasers: pulsed laser typically removes neutral exciton contribution causing PL to be dominated by trion emission, while CW light normally modifies the trions to neutral exciton ratio depending on charge doping type (p or n). [16,79] The exact cause of this phenomenon is unknown, but it is speculated to be related to charge transfer from non-native atoms. [79] Conversely, the modulation doping is easily understood as light-induced charge carrier separation and while not permanent, it is erasable and repeatable. [102] In conclusion, optical doping can retain or improve the optoelectrical performance of 2D material devices while introducing interesting physical phenomena.

Strain
TMDs Sublimation and spallation [98,196] fs [98] 800 [98] 1.6 [98] Graphene Bulging due to defect introduction, [33,38] local strain rearrangement according to substrate [97] CW, [97] fs [33,38] 515, [33,38] 532, [97] 560 [33] 5-25 × 10 −3 [38]  The low intensity red area in this figure is the PL from nontreated MoS 2 , while the intense red area at the center corresponds to an enhanced PL intensity from the laser-treated MoS 2 demonstrating high site selectivity. Reproduced with permission. [16] Copyright 2015, John Wiley and Sons. c) Schematic of optically controlled reduction process on GO. Reproduced with permission. [23] Copyright 2010, Elsevier. d) Optical image of a thin MoTe 2 flake with two areas of different reflection contrast. The left-side area has low contrast as a result of laser irradiation, while the right-side area remains pristine. Raman scattering area scans of the in-plane vibrations of the 2H phase (top-right) and of the 1T′ phase(bottom-right) clearly indicate the phase change. Clear change in reflectivity and thickness in an optically phase-transitioned TMD material during the thinning process. Reproduced with permission. [30] Copyright 2009, Royal Society of Chemistry. e) Working principle and laser fluence dependence of laser-assisted phase transition and thinning of MoTe 2 . Low fluence is optimum for nonlinear measurements (left), while medium fluence enables structural modifications such as layer-by-layer thinning (middle), and foremost, high fluence enables crystalline phase modifications (right). Reproduced with permission. [63] Copyright 2020, American Chemical Society. f) Strain-induced LDW pattern on graphene. The different topography features were achieved by controlling the laser pulse energy and irradiation time at different spots on the graphene flake. These spots were separated by 0.5 µm in a total area of 12 × 12 µm 2 . Reproduced with permission. [40] Copyright 2021, Royal Society of Chemistry. g) Optical straining process. Adapted with permission. [38] Copyright 2018, American Chemical Society.
of laser phase changed MoTe 2 in Figure 4d). Raman scattering measurements reveal that the laser-irradiated areas exhibit Raman modes that correspond to the 1T′-phase of MoTe 2 , while the nonirradiated area remains in the pristine 2H phase (Figure 4d). Although the process is typically irreversible, [29,30] some degree of control can be achieved by tuning the laser wavelength, [30,32,93,95,96] power, [30] and irradiation time. [30] Additionally, optical crystalline phase change can also be combined with laser-induced thinning [63] and LCVD. [31] As an example, Figure 4e shows a schematic description of three ultrafast laserassisted applications on MoTe 2 controlled by the laser fluence.
Optically controlled phase engineering can be an accurate areaselective and multifunctional tool for fabrication and improvement of 2D materials for optoelectronic applications.

Strain and Topographical Modification
Local introduction of strain in 2D materials exhibits reliable modifications of material properties with applications in energy [110] and quantum devices. [111] Optically induced strain can be used to engineer structures both during and after material fabrication, [33,[38][39][40]97,98,112] providing an additional advantage over conventional methods, such as nanopillar array fabrication. Furthermore, optically induced strain can be used as a LDW method for shaping 3D patterns, including symbols and pyramids. [33,38,40] Figure 4f shows the topography of optically forged graphene obtained by atomic force microscopy. The optical forging of graphene was obtained using a femtosecond laser while graphene was kept in an inert atmosphere. This method has also been utilized to induce luminescence from graphene [39] and to induce quenching in the luminescence from MoS 2 . [112] Optically induced strain is highly depending on the material preexisting strain condition, also depending on the substrate, laser power, wavelength, [33,[38][39][40]97,98] and irradiation time. The method has been demonstrated via defect-induced lattice expansion, [33,[38][39][40]77] local strain rearrangement according to the substrate topology, [97] and strain introduction by interaction of sublimation and spallation. [98] Figure 4g shows a schematic description of optical forging of graphene where the graphene layer adopts a Gaussian shape (right) over the flat substrate (left). This Gaussian shape is consistent with the Gaussian profile of the spatial intensity distribution of the laser. In conclusion, optically induced strains and their topographical modification are a versatile method to modify 2D materials for applications in quantum devices (e.g., strain-introduced quantum emitters) and sensing.

Applications of Optically Modified 2D Materials
As discussed in Sections 2 and 3, optical modification of 2D materials provides direct control over material properties, such as phase, [29][30][31][32]78,[93][94][95][96] electronic bandgap, [28] and photoluminescence [16,79] of 2D materials via photoinduced physical and chemical processes. Optical modification methods provide a fast, cost-effective, and simultaneous control over multiple properties, which has attracted significant attention for applications ranging from electronics and photonics to biological and environmental applications.

Contact Engineering
The performance of 2D material electronic and optoelectronic devices is strongly affected by the electrical contact resistance at the material-contact interface. Site impurities normally increase the Schottky barrier height at the material-contact interface, resulting in a non-Ohmic resistance. Reducing the Schottky barrier height is of great importance for improving device performance. Optical modification at the material-contact interface for improving device performance has been demonstrated in TMDs via local phase transition and oxidation, resulting in twofold [88] and 50-fold [32] conductivity enhancement, respectively. Figure 5a shows an illustration of a MoTe 2based field-effect transistor where the crystalline phase of MoTe 2 was locally modified by laser irradiation to improve contact resistance. In addition, optical modification can be used in the developments of transparent and flexible electronics. For instance, laser-reduced GO electrodes -which offer better electrical performance than chemically reduced GO -were recently reported for organic semiconductor diodes. [123,124] Optical modification method enables the customized design of 2D material contacts with controlled sizes and shapes and with improved interfaces in normal ambient conditions for enhanced electronic and optoelectronic performance.

Tuning Electrical Performance
Modifying the electrical properties of active 2D materials is a direct way to improve or tune the performance of electronic and optoelectronic devices that utilize these materials. Optically modified 2D material devices have been demonstrated with laserdoping-, [13,16,[114][115][116][117][118] reduction-, [19,104] oxidation-, [64] and ablation- [125] induced bandgap engineering. These laser-initiated changes can reduce threshold voltage, increase drive current, [126] modify the conductance, and widen the operation spectral range of optoelectronic devices. As an example, optical doping has been used to create p-n junctions [114,116,118] and transistors [16,[116][117][118] with significant property changes, such as carrier density change [114] of 7 × 10 12 cm −2 in graphene and respective DC and AC voltage gains [115] up to 28 in MoTe 2 . The optical modification method provides versatile ways to modify the properties of 2D material electrical devices for improved operation.

Memory Devices
The increasing amount of digital information drives forward developments in memory devices. Novel nonvolatile memories with more memory and shorter data-access time are becoming an industry standard for high-capacity storage. The unique material properties of 2D materials can further enhance the operation of memory devices, however, their current long and expensive fabrication times are hindering the progress. [127] Optical modification methods provide a faster and cost-effective alternative, while enabling multilevel memory access when used in combination with electrostatic gate control. Novel laser-modified 2D-material-based memory devices have been engineered via optical ablation [119] and LDW. [120,121] For example, laser-ablated flexible graphene write-once-read-many memory card [119] was realized with 10 ns write and erase time, and its data density reaches 500 000 bits cm −2 . These properties are superior to other novel 2D material memory devices. [122] Figure 5b shows an illustration of the process flow in the fabrication of a graphene-based memory device. This device can be built either on glass or on a flexible substrate of polyimide. Graphene is grown on the substrate (steps i and ii), and gold structures are built on it (step iii). Next, the exposed graphene layer in between the gold structures is etched by laser irradiation (step iv), and the etching process produces two distinct geometries for the graphene layer (step v), that represent the binary elements of the memory device. Optical modification has proven to unlock the true potential of 2D material memory devices.

Sensors
2D materials are promising sensor materials due to their large surface to volume ratio, [138] which can be further expanded Adv. Mater. 2022, 34, 2110152 Figure 5. Typical 2D material electrical, sensing, and energy devices created with optical fabrication methods. a) Schematic of phase transitioned 2H and 1T′ heterojunction for contact engineering. Reproduced with permission. [32] Copyright 2015, American Association for the Advancement of Science. b) Fabrication process of the graphene write-once-read-many memory device. Reproduced with permission. [119] Copyright 2010, American Chemical Society. c) Schematic of laser-oxidized TMD-TM-oxide-TMD humidity sensor. Reproduced with permission. [22] Copyright 2020, Springer Nature. d) Typical fabrication process of a laser fabricated micro-supercapacitor. Reproduced with permission. [137] Copyright 2013, John Wiley and Sons. e) 30 000 micro-supercapacitors optically produced on a flexible substrate. Reproduced with permission. [43] Copyright 2020, Springer Nature. by patterning via optical methods. [22,107,128,129] Furthermore, the chemical composition, and thus electrical properties, of the laser-treated areas can be adjusted with the laser power to increase sensitivity (up to 1 ppb [128] ) and tune the sensing reaction time. [107,128] As an example, optical oxidation of TMDs has been shown to modify transition-metal-oxide surface conduction for ultrasensitive temperature, moisture, and gas sensing with a sensitivity of 1 ppm. [22] Figure 5c shows a schematic illustration of a NbS 2 ,Nb 2 O 5 -based sensor. The Nb 2 O 5 channel was directly patterned on pristine NbS 2 by laser writing. Various other optical modification methods, such as optical ablation, could also prospectively be used to replicate conventionally fabricated ultrasensitive sensors, [139] guaranteeing avenues for further research.

Energy Storage Devices
Research of energy storage devices, particularly supercapacitors and batteries, is steadily increasing as the demand for miniaturized, portable, and self-sustaining devices grows. Optical modification of 2D materials is particularly promising for micro-energy-storage-device fabrication due to their ability to create flexible on-chip devices rapidly and economically. Multiple optical methods [43,[130][131][132][133][134][135][136][137] have been reported as suitable methods for micro-supercapacitor fabrication at industrial production. [43] Figure 5d shows a schematic illustration of MoS 2based micro-supercapacitor fabrication that can be implemented in an industrial scale. A paint containing MoS 2 nanosheets is sprayed on a silicon wafer with prepatterned gold contacts, and later the MoS 2 -based coating is patterned by laser irradiation. In other recent advances, high-speed industrial scale production of graphene-based micro-supercapacitors has reached a laser-assisted patterning of 30 000 capacitors in 10 min. [42] Figure 5e shows an image of a flexible substrate with microsupercapacitors built on it. The inset figure shows a microscope image of the structures, the scale bar is 150 µm long. Notably, the energy densities of the fabricated devices range [130][131][132] from 1 × 10 −3 to 0.23 Wh cm −3 , competing with the state-ofthe-art micro-supercapacitors with typical energy densities in the mWh cm −3 . [140] Further, a LDW method to fabricate sulfurand nitrogen-doped graphene electrodes with different nanoparticles has been realized for long-term cycles with almost no fading in charge capacities. [141] Given the current growing demand for electrical vehicles and portable devices, the availability and economic advantages of laser-assisted fabrication methods will be further investigated for efficient, durable, lightweight, and miniaturized energy storage devices.

Photonic and Optoelectronic Applications
The unique physical properties of 2D materials enable numerous photonic and optoelectronic applications, such as saturable absorbers, [142] photodetectors, [88,118,[143][144][145] ultrathin lenses, [44,[146][147][148][149][150] photodetectors, [88,118,143] and quantum photonic devices. [78] Reduced dimensionality of on-chip devices is under particularly heavy research due to the increased demand for miniature devices. Optical modification methods of 2D materials offer an auspicious solution as they are a swift and economical way to achieve versatile on-chip photonic and optoelectronic devices.

Photodetectors
2D materials and their heterostructures offer prospective avenues to keep developing novel photodetectors with wide spectral range. Optical modification methods [88,118,143] can be used to create heterostructures without the need for multiple layer transfer. For instance, local laser oxidation of In 2 Se 3 was used to produce lateral In 2 Se 3 -In 2 O 3 heterostructure photodetectors with optical detectivity [88] of 3.8 × 10 14 Jones, which outperforms most novel 2D material photodetectors by several orders of magnitude [144] and it is up to par to the state-of-the-art siliconbased photodetectors. [151] This high detectivity is induced by the intrinsic band offset of the heterojunction and a created local charge depletion layer. [88] To further highlight the promise of optically fabricated photodetectors, plasmonic terahertz sensors, previously conventionally demonstrated on graphene nanoribbons, [145] could be prospectively fabricated optically. Optically modified 2D material photodetectors outperform their conventionally fabricated counterparts and offer avenues for further research for applications in security and medicine industries. [152]

Polarizers and Absorbers
Ultrathin polarizers with conventional materials can be found in only a few demonstrations to date due to the challenging engineering behind their fabrication, which results in a low extinction ratio and suboptimal efficiency. [153] However, 2D materials offer great advantages to overcome these factors. Additionally, optical modification [154] provides high-precision methods suitable for isolated components as well as larger ensembles. Indeed, C-shaped polarizers were laser-patterned on GO and they showcased an extinction ratio of over 3000, which is over a magnitude larger than other guided resonance polarizers. [153] The same method has also been demonstrated for the fabrication of saturable [142] and complete absorbers [155] for laser applications. Figure 6a shows a schematic of the experimental setup of a mode-locked laser that uses GO or reduced GO (rGO) as saturable absorber. Laser modification of 2D materials can overcome the material limitations of thin polarizers and absorbers, providing versatile local modification methods for integrated photonic circuits.

Ultrathin Lenses
Ultrathin lenses are light, compact, and can potentially be aberration-free, which make them extremely viable for miniature broadband applications in aerospace, [147] cellphones, [156] and microscopy. [157] Standard ultrathin lenses, however, exhibit performance limitations due to their chemical and thermal instability, [148] and their reduction to monolayer thickness leads to a reduction in modulation efficiency. [44] As a solution to these challenges, 2D materials in combination with optical modification methods offer a stable lens medium. Laser reduction of GO [146][147][148] can be used to create scattering patterns that overcome the modulation inefficiency. Figure 6b shows a schematic description of circular patterns of rGO created by laser-irradiating GO, which was used for ultrathin lens applications. Optically patterned [44,[146][147][148][149][150] ultrathin lenses exhibit a broad operation bandwidth in the range [146] of 400-1500 nm, resilience [147] to a broad pH range, and multifunctionality (such as simultaneous phase and beam modulation [44,146] ). Figure 6c shows a schematic of a supercritical ultrathin lens of MoS 2 for subdiffraction-limited focusing. This ultrathin lens operates for a wide range of wavelengths, and the subdiffraction-limited focal distance exhibits a linear relationship with the wavelength. Optically modified 2D material ultrathin lenses advance almost every aspect of photonic systems while enabling more complex applications, such as particle nanotracking. [158]

Holography
Novel holographic applications, based on plasmonic nonlinear holograms, are limited by an inefficient conversion of the pump beam into second-harmonic generation and absorption losses. 2D materials with broken inversion symmetry have high second-order nonlinear susceptibility, which can be used to overcome the low conversion efficiency, and they can be optically modified for optimum performance as a holograph medium. [159] Laser-patterned 2D material holographs with colorful 3D images (Figure 6d), wide angle observation, and restoration of polarization-dependent waves [106] have been realized with reduction of GO. [105,106,160] Optically created 2D material holographs enable versatile applications for displays [161] and data storage, [162] and further investigation on the optical reconstruction of milled TMD holograms [159] can be expected.

Quantum Applications
2D-material-based quantum light sources have been under intense research due to their strong light-matter interactions and easy integration into circuits. [163] The functionality of quantum light sources is based on trapped states that originate from point defects, [164,165] impurities, [166,167] or strain. [168][169][170] hBN-based quantum light sources have been demonstrated, and Adv. Mater. 2022, 34, 2110152   Figure 6. Optical-modification-based photonic and optoelectronic devices. a) Laser-reduced GO saturable absorber in a mode-locked laser setup. Reproduced with permission. [142] Copyright 2012, Optica Publishing Group. b) Schematic of optical ultrathin lens fabrication. Reproduced with permission. [146] Copyright 2015, Springer Nature. c) Broadband subdiffraction focusing working principle (top) and the focal spot radii of yellow, green, and blue light (bottom) shone through a TMD-based ultrathin lens. Reproduced with permission. [44] Copyright 2021, Springer Nature. d) Schematic of rGO holograph operation from laser patterning a digital image into rGO structures (left) to wide angle 3D image creation and full color object reconstruction (right). Reproduced with permission. [106] Copyright 2015, Springer Nature. their fabrication method includes laser-induced phase change [78] and laser tuning of photochemical reactions. [171] This quantum emitter exhibits room-temperature single photon emission [78] with a correlation factor g 2 (0) of 0.2-0.42, which is comparable to other novel 2D-material-based quantum light sources. [172] Other optical modification methods, such as strain, [33,[38][39][40]97,98] could also lead to novel developments on quantum devices, and to inducing other quantum effects, such as spin defects. [99] Overall, optically assisted methods for quantum applications are still in their infancy with extensive research opportunities due to the many possible ways to introduce trap states.

Bio-and Ecological Applications
The advancements in the fields of technology and medicine have increased the quality of life drastically, but they have also induced new societal challenges, such as antibiotic resistant microbes and environmental issues. Optical modification of 2D materials can be used to address some of these challenges by developing applications for oil-water separation, [173] fog collection, [174] and bactericidal surfaces. [175]

Moisture Collection
Organisms in dry climates maximize their water collection efficiency by developing droplet collecting surfaces. Similar surface structures have been reported for fog collection using a treatment that combines laser etching and ultrasonic vibrating of polydimethylsiloxane (PDMS)/graphene mixture on a copper mesh. [174] This treatment enables a combination of superhydrophobic areas with superwettable areas in the mesh, that increases fog collection efficiency from ≈3.7 g cm −2 h −1 (in a bare Cu mesh) to 5.4 g cm −2 h −1 (in the treated Cu mesh) at a fog flow of 45 cm s −1 . [174] Additionally, the graphene surface wettability is tunable via temperature, [176] enabling controllable water collection devices. The fog collection rating of the optically treated structures is comparable to other novel 2D material moisture collectors, but exhibiting a greater collection efficiency per fog flow. [177]

Oil-Water Separation
Technological advances for the separation of water from contaminants is crucial in facing environmental catastrophes, such as the harmful oil spills from accidents in the overseas shipping industry. Optically modified surfaces combined with 2D materials present great advances for these challenges. Identical to moisture collector, laser-etched graphene/PDMS mixtures with alternating or tunable wettability can be used to separate oil from water. [178] Figure 7a shows a schematic description of this process. A copper mesh is coated by a layer of graphene suspended in a solution of PDMS. Later, the coated mesh is irradiated with a high power, 200 ns pulsed laser producing Adv. Mater. 2022, 34, 2110152   Figure 7. Environmental and biological applications achievable via optical methods. a) Schematic of optical fabrication of superhydrophobic surfaces for fog collection and oil-water separation. Reproduced with permission. [178] Copyright 2019, American Chemical Society. b) Growth of bacterial cultures on PDMS, glass, and laser-induced graphene surfaces. Reproduced with permission. [175] Copyright 2019, John Wiley and Sons. c) Two-beam laser interference and Ag-nanoparticle-based SERS structure. Working principle of the laser interference and the interference pattern on the top, the resulting rGO structure and the Ag-coated SERS structure on the bottom. Reproduced with permission. [74] Copyright 2015, American Chemical Society. d) The process steps for growing oriented neural network on laser-etched graphene patterns involving laser etching procedure (left), neuron seed deposition (middle), and the ready structure (right). Reproduced with permission. [182] Copyright 2013, Springer Nature. a superhydrophobic and superoleophilic surface, upon water droplets remaining on top of the mesh, while oil (hexane, chloroform) was filtered through the mesh. Moreover, alternating laser etching and oxygen plasma treatment can be used to switch between the hydrophobic and hydrophilic states, creating a tunable water-oil filter. [178] The separation efficiency of the mesh is reported to be at least 97%, which is comparable to other novel oil-water separation methods. [179]

Antibacterial Surfaces
Antibiotic resistant microbes are an ever-growing problem in the increasingly mobile world. Antimicrobial surfaces that are capable of preventing microbes from spreading in public places have been created by laser-induced photosynthesis of graphene and its composites. [175,180] Roughness and the subsequent hydrophobicity of the graphene surface synthesis by laser-induced photochemical method has been shown to inhibit microbial growth significantly. Figure 7b shows schematics (top) of Escherichia coli (a common bacteria that lives in human intestines), and optical images (bottom) on different substrates: PDMS (left), glass (middle), and graphene (right). The lasertreated graphene substrate produced colonies with 99.83% reduction compared to glass and a complete 100% reduction in biogrowth can be achieved by sun illumination. [175] Advances in the medical field are extremely important and optical modifications of 2D materials offer more advantageous applications as the research progresses.

Surface-Enhanced Raman Spectroscopy
Surface-enhanced Raman spectroscopy (SERS) uses surface structures and surface plasmons to enhance the optical excitation of phonon modes and incoherent vibrations in crystalline materials and molecules. [181] Optical reduction, straining, and ablation could offer a pathway to create suitable surface structures without the need for substrate engineering or additive compounds, such as nanoparticles. So far, two-beam laser interference reduction of GO, in combination with silver nanoparticle hybrid structures, has been applied for SERS mechanisms. Figure 7c shows a schematic description of the mechanism for the production of the SERS substrate from a GO film. The two 355 nm pulsed lasers interfere on the GO film producing rGO strips under the areas where the lasers interfere constructively. This treatment modifies the topography and the chemistry of the GO film, forming a superhydrophobic rGO/GO grating. Subsequently, the twobeam-laser-interference-treated surface was coated with a thin silver film and coated with rhodamine B (RhB) as test sample. The interplay between the superhydrophobic-surface-induced condensation effect and the silver plasmonic nanoparticles leads to a detection limit [74] of 10 −10 m for the RhB concentration, which is on the same range as other 2D material SERS devices. [181] Multipurpose optical modification routes could be used to simultaneously create superhydrophobic surfaces and strained or ablated surface structures, which could be used for SERS devices with enhanced detectivities.

Bioscaffolds
Nontoxic bioscaffolds for growing ordered neural networks present important advantages for the development of biotic prosthetics. The chemical stability and electrical sensitivity of graphene make it a prime candidate for this application. Optical fabrication of grid structures for growing linear neurons has been demonstrated via laser ablation in graphene [182] and laser interference reduction of GO. [183] Figure 7d shows a schematic description of this process. Single layer graphene (SLG) on glass is etched by a pulsed laser to create graphene strips (left). The sample was coated with poly-d-lysine (PDL) to improve the adhesion of hippocampal neuron (middle). After 7 days in vitro (DIV 7), the neural cells cultured on the laser-treated surface showed a clear preference to grow along the graphene strips avoiding the etched or nonreduced areas (right). This method for area-selective growth of neural cells is also promising for prospective biosensor applications.

Biocompatible Devices
Biocompatibility is paramount for the development of bioelectronics. While 2D materials are generally nontoxic, the body may still reject them as a foreign material. This can be overcome by coating the surface of the 2D materials with biocompatible molecules, such as proteins. Chemical activation of 2D material crystalline planes for protein adherence has been demonstrated via laser-induced oxidation in graphene. [184] The introduced proteins showed a clear aggregation preference toward these areas via noncovalent bonds. [184] Optical modification is promising for the development of structurally intricate biocompatible devices.

Perspectives
Novel 2D material applications require layer-by-layer modification of their physical and chemical properties such as crystalline phase, electronic structure, and thickness. Optical modification can offer fast, sustainable, and large-scale no-contact methods to structure and pattern and functionalize 2D materials. In addition, lasers offer highly tunable modification processes that do not require a harmful environment, nor cleanroom scale fabrication requirements. While optical methods have been demonstrated, a full integration in device manufacturing at an industrial scale is still under development. Many electrical and optical devices realized with nonoptical fabrication methods could also be created via all-optical processes presented in this review, showcasing multiple yet-to-be-explored avenues of research.

Novel 2D Materials and Other Low-Dimensional Materials
Research on the optical modification of the common 2D materials is quite advanced, however, some of the methods described in this review, for instance, lack of doping and ablation demonstration even for well-known materials, such as hBN and phosphorene. Moreover, there is still a large group of novel 2D materials and many other low-dimensional materials, for example, perovskites, [185] 2D polymers, and different van der Waals heterostructures, with only few reported optical modification results. To illustrate, heterostructures could be created with defined twisting angles (e.g., for twistronics) and 3D structures via optical-modification-based strain and topographical engineering. Furthermore, light-based modification of hybrid 2D materials, such as flakes combined with 0D or 1D materials or biomolecules (e.g., metallic particles, nanowires, nanotubes, nanoislands), deserves further investigation due to their diverse combinations of light-matter interactions.
In addition to limited demonstrated materials, the majority of the current optical modification results focus on freestanding 2D materials or their bulks supported by silicon substrates. It is to be expected that innovative research into optical control of different forms of 2D materials, such as suspended or enveloped in complex environments, will emerge in the coming years. For example, combining multidirectional laser-based assembly and trapping enables manipulating and modifying 2D materials with flake-to-flake precision in a liquid environment. This can create a microscale production process, similar to the modern industrial assembly line. Overall, it is likely that research on lightbased modification of a large range of 2D materials and their mixed-dimensional heterostructures under different environments is going to increase in the following years.

Advanced Laser Tools
Currently, many of the studies on optical modification of 2D materials are performed with common laser sources (e.g., CW lasers used in Raman systems), while this is beneficial for simultaneous optical characterization and modification, it can hinder other potential advances in the research. Using advanced laser tools with optimized parameters, such as pulse duration, wavelength, and beam profile, will improve the optical modification performance in 2D materials. For example, the fast-developing laser technology enables the use of novel engineered laser beams, such as vortex beam, donut shaped beam, and control orbital angular momentum beams, for increasingly versatile light modification methods. Additionally, developments in available lasing wavelengths will introduce new available excitation and heating ranges and thus greater control on optical modification methods. To summarize, advances in laser technologies will also promote discoveries in light-based modification of 2D materials and beyond.

New Optical Modification Strategies
Thus far, optical modification of 2D materials mainly uses thermal, mechanical, and chemical effects. In the future, considering the large range of 2D materials, new optical modification strategies will likely appear. For example, direct optical writing of ferroelectric or magnetic domains could be possible, [186] expanding the scope of multiferroic 2D materials. Laser-induced acoustic wave or polariton generation could also be used to modify the physical properties of 2D materials. For example, laser-induced acoustic waves can be used to introduce time-(in)dependent modification for various applications, such as 2D material time crystals. [187] Additionally, light polariton propagation in 2D materials can be explored to produce selfinduced waveguide writing. [188] Further, combining optical modification with other photonic methods, such as near-field optics, micro-and nanoresonators, photonic crystal cavities, and plasmonics, can further boost the performance and tuning of 2D devices, potentially enabling even higher resolutions beyond the diffraction limit, and faster operating or patterning speeds. For instance, using near-field scanning spectroscopy techniques (e.g., scanningprobe-based [189] and scattering nanoparticles [190] ) might enable drawing subwavelength structures on 2D materials. This could be achieved through local introduction of structural modifications, which is similar to other demonstrated 2D material nanolithography methods but with the assistance of light. [190] All this will greatly expand the research scope of the optical modification, such as on-demand modification (e.g., bandgap engineering), forming artificial quantum dots/wires and moiré potentials by high-resolution optical modification.

Emerging Applications
Up to now, applications of optically modified 2D materials have mainly focused on providing an alternative to the existing nanofabrication methods. Consequently, the research and demonstration of devices with interesting parameters, such as magneticity, spin and surface states, are severely lacking. For instance, optical spin defect generation has already been demonstrated for 2D materials, [99] however, refining it for applications such as spintronics, quantum sensors and computing, and expanding the method to more complex structures could open new avenues in research and development. Additionally, many applications that use conventional fabrication methods, for instance, plasmonic enhancement of high-harmonic generation in nanoribbons, could also be emulated with optical methods providing new perspectives and results. Hybrid materials and advanced modification methods will also expand the scope of applications for light-based modification of 2D materials. For example, in bioelectronics, future research avenues could include growing and connecting tissues (e.g., neurons) directly onto 2D materials for applications in prosthetics and artificial retinas. Additionally, further refining biocompatibility via biomolecule/2D material hybrid structures could be realized for a range of bioapplications, such as targeted drug delivery and DNA sequencing. [191] A great recent example of this is the TMD biosensor for Covid-19 detection, [192] which could also be created using optical modification methods. Given the above, applications of optically modified 2D materials with new functions and operation fields are highly possible to be reported in the near future.

Conclusions
Research on optical modification of 2D materials has resulted in a cost-effective, controllable, and modifiable way to engineer structures that have previously been attainable only via harmful and time-consuming microfabrication methods. The ondemand optical modification has shown the great potential to not only fabricate and tune a wide range of 2D material devices, but also control their underlying fundamental properties as well, displaying wide variety and versatility. Finally, advancement in the research of lasers and more complex materials could enable optical modification to drastically change the perspectives of 2D material research.
Suvi-Tuuli Marianne Akkanen is a doctoral candidate in the Photonics Group at the Aalto University, Finland. She received her Master's degree in Nanochemistry with specialization in Optical Spectroscopy and Optical Modification from the University of Jyväskylä, Finland, and is currently working on photonics and optoelectronics of 2D materials. Her main research interests include optical modification of materials, graphene plasmonics, and magnetism in 2D materials.