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Article

Controlled Growth of Multifilament Structures with Deep Subwavelength Features in SiC via Ultrafast Laser Processing

by
Xiaoyu Sun
1,2,3,
Haojie Zheng
2,3,
Qiannan Jia
1,2,3,
Limin Qi
1,2,3,
Zhiqi Zhang
2,3,
Lijing Zhong
4,*,
Wei Yan
2,3,*,
Jianrong Qiu
5,* and
Min Qiu
2,3,*
1
College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China
2
Zhejiang Key Laboratory of 3D Micro/Nano Fabrication and Characterization, Department of Electronic and Information Engineering, School of Engineering, Westlake University, Hangzhou 310030, China
3
Westlake Institute for Optoelectronics, Fuyang, Hangzhou 311421, China
4
Institute of Light+X Science and Technology, College of Information Science and Engineering, Ningbo University, Ningbo 315211, China
5
College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China
*
Authors to whom correspondence should be addressed.
Photonics 2025, 12(10), 973; https://doi.org/10.3390/photonics12100973
Submission received: 2 September 2025 / Revised: 26 September 2025 / Accepted: 29 September 2025 / Published: 30 September 2025
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Abstract

Silicon carbide (SiC) is a promising semiconductor material for electronics and photonics. Ultrafast laser processing of SiC enables three-dimensional nanostructuring, enriching and expanding the functionalities of SiC devices. However, challenges arise in delivering uniform, high-aspect-ratio (length-to-width) nanostructures due to difficulties in confining light energy at the nanoscale while simultaneously regulating intense photo modifications. In this study, we report the controllable growth of long-distance, high-straightness, and high-parallelism multifilament structures in SiC using ultrafast laser processing. The mechanism is the formation of femtosecond multifilaments through the nonlinear effects of clamping equilibrium, which allow highly confined light to propagate without diffraction in parallel channels, further inducing high-aspect-ratio nanostripe-like photomodifications. By employing an elliptical Gaussian beam—rather than a circular one—and optimizing pulse durations to stabilize multifilaments with regular positional distributions, the induced multifilament structures can reach a length of approximately 90 μm with a minimum linewidth of only 28 nm, resulting in an aspect ratio of over 3200:1. Raman tests indicate that the photomodified regions consist of amorphous SiC, amorphous silicon, and amorphous carbon, and photoluminescence tests reveal that silicon vacancy color centers could be induced in areas with lower light power density. By leveraging femtosecond multifilaments for diffraction-less light confinement, this work proposes an effective method for manufacturing deep-subwavelength, high-aspect-ratio nanostructures in SiC.

1. Introduction

Silicon carbide (SiC), an indirect wide bandgap semiconductor material characterized by a high refractive index, high hardness, and brittleness, shows promising applications in electronic devices, bio-diagnostics, and energy conversion [1,2]. To enhance and expand the functionalities of SiC devices, the incorporation of micro–nano structures is essential. Ultrafast laser processing technology—renowned for its high spatial resolution (0.1~1 μm), versatility in three-dimensional structuring, and high fabrication throughput—is well-suited for this purpose. Researchers have successfully fabricated a variety of micro-/nanostructures in SiC using ultrafast lasers, including atomic vacancies [3,4], nanovoids [5], periodic strain layers [6], refractive-index-modified regions [7], and microcracks [8]. These structures have been utilized as color centers for light emission, optical waveguides for signal transmission, and in wafer slicing. To further broaden the application scope, it is crucial to prepare uniform, high-aspect-ratio nanostructures, such as nanostripes. However, this demand presents a well-known challenge. SiC is a crystalline material with high hardness and brittleness; during laser processing, the growth and evolution of photomodified regions in SiC exhibit significant instability, leading to the torsion and bending of induced nanostripes [9]. Therefore, effectively inducing long-distance and uniform nanostripes in SiC has become a key problem to address.
Laser filamentation is a unique nonlinear optical phenomenon that can produce long-distance, highly confined optical fields and plasma channels through the nonlinear effects of clamping equilibrium [10]. This phenomenon has been extensively investigated in gaseous and liquid environments. However, much remains unknown about the filament formation processes in solid materials. On one hand, the plasma density of filaments produced by femtosecond pulses in solid media typically ranges from 1018 to 1020 cm−3, which is at least 3 orders of magnitude higher than that in air [11]. The higher plasma density results in greater pulse energy deposition, leading to permanent physical modifications and even significant structural, material changes. This presents an exciting research opportunity for exploiting laser filaments to prepare fine structures in materials. On the other hand, the higher atomic density in solid media results in greater energy loss during filament transmission compared to that in gas and liquid media. Moreover, various physical effects—such as increased group velocity dispersion [12], complex thermal processes [13], and inhomogeneous refractive index disturbances—further complicate the evolution laws and internal mechanisms of filamentations. Although there are reports of successfully inducing high-aspect-ratio structures with single filaments in solids [14], the creation of multifilaments is often associated with a disordered positional distribution [15,16,17] due to unpredictable random noises that are amplified by nonlinear effects. To produce controllable multifilaments, one approach is to use multibeam interference [18]. However, in this scheme, the spacing between adjacent filaments is constrained by the diffraction limit, which prevents the achievement of subwavelength features, while the duty cycle of the resulting multifilament structures remains relatively low. An alternative strategy involves modifying a circular beam into an elliptical one while utilizing a single beam [19,20,21,22]. This method efficiently suppresses random multifilaments, ensuring that their regular positions adhere to the mirror symmetry of the ellipse with respect to the major and minor axes [22,23].
In this paper, we investigate ultrafast laser processing of long-distance, uniform, high-aspect-ratio nanostructures in SiC through regularized multifilamentation and demonstrate the effectiveness of this processing method. The key to achieving long-distance multifilaments with regular positional distributions is the utilization of an elliptical Gaussian beam, along with further optimization of pulse durations. Under optimal conditions, we show that the induced multifilaments can reach lengths of approximately 90 μm with a minimum linewidth of only 28 nm, resulting in an aspect ratio of over 3200:1. Raman measurements indicate that the photomodified SiC regions transform from single-crystalline SiC (c-SiC) to amorphous SiC (α-SiC), which further decomposes into amorphous silicon (α-Si) and amorphous carbon (α-C); photoluminescence (PL) tests reveal that silicon vacancy (Vsi) color centers are produced along the growth traces of the multifilaments in the low power density region.

2. Materials and Methods

The SiC samples used for ultrafast laser processing were commercially produced, undoped 4H-SiC wafers with a thickness of 500 μm and a bandgap of 3.2 eV. This wafer was cut into pieces measuring 10 mm in length and 5 mm in width. An amplitude femtosecond laser—with a wavelength of 1030 nm (resulting in a 3-photon absorption process during laser fabrication of SiC), a pulse repetition rate of 100 kHz, a tunable pulse duration ranging from 130 fs to 10 ps, and a tunable single pulse energy up to 200 μJ—was utilized. The laser output beam features an elliptical Gaussian intensity distribution, as shown in Figure 1a. The beam waist diameter of the major axis (1/e2 diameter) is ω y = 2.63 mm, while the minor axis beam waist diameter is ω x = 2.07 mm, yielding an ellipticity of e = ω y / ω x = 1.27. The use of the elliptical beam regularizes the positions of multifilaments formed in SiC, adhering to the mirror symmetry of the ellipse with respect to the major and minor axes [23]. We used a pair of cylindrical lenses with focal lengths of 100 mm and 125 mm to adjust the light spot to an approximately circular distribution with an ellipticity of 1.07. Then, we used another pair of cylindrical lenses with focal lengths of 50 mm and 100 mm to adjust the ellipticity of the light spot for processing SiC. The top and side views of the induced structure under different ellipticity conditions of the laser beam are shown in Appendix A, Figure A1. When the ellipticity is too low, multifilament structures cannot be formed. When the ellipticity is too high (e.g., ε = 3.31), the growth direction of the filaments may converge towards the optical axis due to the strong converging effect of self-focusing, making it difficult to form parallelly arranged multifilaments. Therefore, in subsequent experiments, the beam with a moderate ellipticity of 1.27 was used.
The processing system is sketched in Figure 1b. A power attenuation component, consisting of a half-wave plate and a Brewster window, was incorporated into the optical path to allow for a continuous adjustment of the laser power. The laser beam was focused inside the SiC sample using a 20× microscope objective with a numerical aperture (NA) of 0.5. The focal spot diameter was about 2.5 μm, and the focal depth was 100 μm below the sample surface. The sample was positioned on a 3D displacement stage. Two processing methods were implemented: static irradiation and dynamic scanning; during scanning, the moving speed was controlled at 200 μm/s. Because the transient overdense plasma channels grow and expand parallel to the electric field polarization direction under the laser irradiation [24], the induced multifilaments extend along the same direction. When the polarization direction rotates by 90° (along the Y-direction), although multifilament structures can be generated under the effect of elliptical light spots, they cannot be further expanded because the polarization direction is not parallel to the minor axis of the elliptical beam; thus, uniform multifilaments cannot grow long, as shown in Appendix A, Figure A3. Therefore, both the scanning direction and the polarization direction were aligned parallel to the minor axis of the elliptical beam—specifically chosen to be the [11 2 ¯ 0] direction of SiC in experiment—to enable the continuous growth of the multifilaments along the scanning direction, as shown in Figure 1c.
After laser processing, the surface of the sample was polished to reveal filiform structures in photomodified regions. Then, the morphology of the induced structures was first observed by an optical microscope. Figure 1d shows the top view and side view (perpendicular to the polarization direction) of typical nanostructures fabricated under the static irradiation processing scheme. From the top view, it can be observed that the multifilament structure expands along the direction parallel to the polarization of the electric field. This phenomenon is closely related to the generation of overdense plasma in laser-induced SiC and the metal-like characteristics it presents [24]. The side-view images clearly reveal long-distance, uniform parallel multifilament structures, which will be investigated in detail later. Specifically, Raman and PL spectra were collected to assess composition changes of the photomodified regions after the laser processing. Furthermore, to analyze the nanostructural characteristics, the samples were etched by ICP using O2 and SF6 gas [25]. Finally, the etched samples were characterized using scanning electron microscopy (SEM).

3. Results and Discussion

3.1. Optimization of Pulse Duration

The generation of laser filaments is essentially a dynamic balance between the Kerr self-focusing effect and the plasma defocusing effect induced by electron transitions following photon absorption in materials [10]. Therefore, the occurrence of a filament signifies the formation of a plasma channel along the proapagation direction of light. When the instantaneous laser power density is low, the electron plasma density within the material is likewise relatively low. At this stage, once the laser irradiation ceases, the electrons and holes will recombine immediately, preventing permanent damage to the material. However, once the laser power density reaches a sufficiently high level, strong nonlinear interactions lead to the production of a large number of free electrons. If the density of these free electrons is high enough, it can result in permanent structural damage [26].
An interesting topic worth discussing is that the polarization state plays a very important role in the processes of self-focusing formation, multiphoton ionization triggering and multifilament structure expansion. Firstly, for media with anisotropic nonlinearity, due to the reduced nonlinearity associated with averaged polarization rotation, the self-focusing critical power of circular polarization is approximately 1.5 times that of linear polarization [27]. This suggests that processing with a linear polarization state should be more efficient in self-focusing. Secondly, when it comes to material modification, ionization is generally more crucial than self-focusing. In materials like quartz and sapphire—where high-order nonlinear effects dominate and sixth-order nonlinearity is present—linear polarization typically induces a higher ionization rate than circular polarization [28]. However, in this experiment, photoionization in SiC is estimated to be a third-order process. Therefore, the ionization behavior is expected to correspond with that observed in the lower-order nonlinear ionization process, wherein circular polarization typically leads to a higher ionization rate compared to linear polarization [29,30], making it potentially more effective at inducing modifications in SiC. Last but not least, our experiments confirm that polarization state plays a decisive role in the growing process of multifilament structures. As shown in Appendix A Figure A4, when a circularly polarized beam is used for processing, the lateral expansion of the laser-modified structures shows no preferred direction due to the continuous changes in polarization direction over time, and the parallel multifilament structure features no longer form. However, when a linearly polarized beam is used for processing, the multifilament structure can expand parallel to the polarization direction, thereby forming a multifilament region with high parallelism and high linearity. Therefore, linear polarization conditions were employed for laser processing in this paper.
In our experiment, an elliptical Gaussian beam was used; the threshold power for the Kerr self-focusing induced by this beam in SiC is calculated using the following formula [19]:
P cr   =     1 + e 2 e λ 2 4 π n 0 n 2 .
Here, e =   ω x / ω y = 1.27 is the eccentricity of the elliptical beam, λ = 1030 nm is the laser wavelength, n0 = 2.6 and n2 ≃ 1.88 × 10−15 cm2/W [31] are the linear and nonlinear refractive indices of SiC, respectively. The threshold power Pcr is calculated to be 3.6 × 105 W. Considering the situation that the single pulse energy is 400 nJ, when the pulse duration is 130 fs, the instantaneous peak power is 3.08 × 106 W, which far exceeds Pcr. Under the focusing condition of a 20× objective lens with a NA value of 0.5, the instantaneous power density at the focus is about 1.93 × 1014 W/cm2. At this power density, the tunnel ionization rate significantly surpasses the multiphoton ionization rate (Appendix A, Figure A5), leading to an intense ionization process and the generation of high-density localized plasma, which causes severe damage with extremely unstable morphological structures. Keeping the single pulse energy constant, as the pulse duration increases to 1 ps, the instantaneous peak power decreases to 4 × 105 W, slightly above Pcr. The instantaneous power density at the focus becomes 1.06 × 1013 W/cm2, and, accordingly, the Keldysh parameter [32] is approximately 1. At this stage, the rates of multiphoton ionization and tunnel ionization are similar, resulting in relatively uniform and stable modification of the material morphology. However, a small amount of irregular structural damage still occurs during the long-distance propagation of the elliptical beam. Further increasing the pulse duration to 10 ps, the instantaneous peak power decreases to 4 × 104 W, far below Pcr, approximately one ninth of it, but the quality of multifilament structure is the best. Calculations indicate that the power density at the focal point is about 1.06 × 1012 W/cm2, which is sufficient to induce multiphoton ionization. Moreover, a longer pulse duration provides adequate time to facilitate subsequent intense collisional ionization, leading to the generation of a high-density plasma that causes material modifications [33,34]. Meanwhile, the generation of avalanche ionization at long pulse duration can lead to a reduction in the threshold for material modification [35]. Under varying pulse durations, the dominant ionization mechanisms differ, resulting in distinct energy deposition processes and material modification effects. Consequently, the total length of the modified region decreases from 225 μm (at a 130 fs pulse duration) to 140 μm (at a 10 ps pulse duration), representing approximately a 40% reduction. However, this also significantly enhances the straightness and uniformity of the multifilament region within it (see Appendix A Figure A2).

3.2. Evolution of Photomodified Regions and Formation of Multifilaments

A pulse duration of 10 ps was selected. Under the static irradiation processing scheme, the SiC sample was processed by incrementally increasing the number of laser pulses from 1 to 106. Throughout this process, successive nanostructures were generated, illustrating the evolution of the photomodifications resulting from strong light–matter interactions. Their morphologies at varying pulse numbers are sketched in Figure 2b, and the exemplified optical microscope images are provided in Figure 2c. Initially, when the injected pulse number reached 10, only the black structural damage point near the focal point was observed. This phenomenon is attributed to that the density of the excited free electrons is highest at the focal point due to the highest light power density, thus causing the threshold power for damage to the lattice structure to be reached first. As the pulse number increases further, this excessively damaged region, labeled as region I, continues to grow away from the focal point in the direction opposite to that of the beam propagation.
As the pulse number increases to about 1000, region I reaches its maximum extent. The distance from the focal plane—approximately the starting point of region I—to its furthest extent is about 77 μm, see Figure 2c. Above region I, the power density decreases to an appropriate level conducive to the generation of the laser multifilaments, resulting in the formation of photomodified long-distance nanostripes. This multifilament region is labeled as region II. The efficient growth of region II occurs at the pulse number ranging from several hundred to 10,000, with an extent length of about 85 μm.
When the pulse number exceeds 10,000, in addition to its continuous growth, the photomodified region above region II displays a distinct brown coloration under the optical microscope. Subsequent PL test indicates the presence of Vsi centers in this new region (see Appendix A, Figure A6, and later discussions associated with Figure 3c for more details), which is labeled as region III.
To further elucidate the growth process of the photomodified regions, a simulation of geometrically focused light-field-intensity distribution inside SiC using MATLAB R2019b is shown in Figure 2a, supporting the growth processes shown in Figure 2b,c. When an elliptical Gaussian beam is focused into SiC, the combined effects of wavefront focusing and nonlinear effects [20,36] lead to the formation of multiple optical field channels in front of the focal point, exciting overdense electron plasma. These multifilament channels converge and merge as the beam gradually focuses toward the focal position. Consequently, the closer the beam gets to the focal position, the higher the power density due to focusing, resulting in a more rapid increase in excited free electron density. The highest free electron density first reaches the lattice damage threshold, leading to material damage in region I [26]. The initial modified region near the focal area induced by the multifilament effect can act as an effective scattering center. Subsequent pulses are not only affected by nonlinear self-focusing but also modulated by the near-field enhancement effect of the previously modified subwavelength-sized structures [37,38,39], thereby depositing energy upstream (i.e., towards the laser incident side) and achieving material writing. As the number of pulses increases, this process repeats continuously, ultimately leading to the observed progressive reverse growth. Subsequently, as the pulse number increases, the multifilament region (region II) and the color center region (region III) develop in succession above the focal point. These observations are consistent with the fact that the growth rate of the photomodified regions increases with the power density—which rises as the beam approaches the focal point—along with the idea that photomodification variations among the different regions also depend on power density.
Based on experimental observation and analysis, we found that the alignment direction of the multifilament structures arises from the combined effects of two mechanisms, namely (1) multifilamentation and (2) polarization-related modulation, analogous to the formation mechanism of nanogratings in fused silica [40] and diamond [41]. Specifically, multifilamentation causes the formed filaments to align parallel to the short axis of the elliptical laser beam, while polarization-related modulation drives the filaments to align parallel to the laser polarization direction. When the short axis of the elliptical laser beam aligns with the polarization direction, the two effects constructively interfere with each other, resulting in well-defined, highly parallel multifilament structures that align along the shared direction of the beam’s short axis and the laser polarization, as observed in our experiment.
We also examined the scanning processing scheme and obtained similar observations of the three photomodified regions as those observed in the static irradiation processing scheme; see Figure 3a. Furthermore, we performed Raman and PL tests to assess material changes qualitatively. First, it is known that single-crystalline (c) SiC can transform to amorphous (α) SiC under ultrafast laser processing, which then decomposes into α-Si and α-C [42]. Additionally, α-C can further convert into graphite phase [6,43]. To confirm these material changes, we conducted mapping tests on the sample at the Raman peaks of c-SiC 780 cm−1, α-SiC 880 cm−1, α-Si 480 cm−1, and the D-band of α-C at 1350 cm−1, and the results are shown in Figure 3b. At 780 cm−1 (the c-SiC peak), the Raman signal in the photomodified region is significantly weaker than the unmodified region, indicating that the crystal phase of SiC has been diminished following ultrafast laser processing. It is worth noting that the Raman signal of the c-SiC in the upper area of the photomodified region is slightly stronger than that in the lower region, indicating less crystal phase damage in the upper area with lower light power density. The enhanced Raman signal strength at 880 cm−1, 480 cm−1, and 1350 cm−1 in the photomodified region indicates that the crystal phase has indeed transformed into α-SiC, α-Si, and α-C. Moreover, a Raman peak at 520 cm−1 was observed during the test (see Appendix A, Figure A7), indicating the generation of a small amount of single-crystalline Si in the region of extremely high electron temperature and high electron plasma density. Furthermore, PL mappings at a wavelength of 917 nm—emission wavelength of silicon vacancy color centers—were conducted [44], and the results are shown in Figure 3c. The PL intensity in the excessively damaged area I and the multifilament region II is significantly weaker than that in the unmodified region, indicating a structural transformation of silicon vacancies in the extremely high temperature environment. Nevertheless, the PL signal in region III is noticeably enhanced, suggesting that in this region, the breaking of Si-C bonds in the SiC lattice and the displacement of silicon atoms can induce the formation of new Si vacancies.

3.3. Pulse-Energy-Dependent Regulation of Multiple Filaments

Under the conditions of a fixed laser processing pulse width of 10 ps and a focal point 100 μm below the sample surface, by adjusting the pulse energy, multifilament structures with distinct energy-dependent characteristics were successfully formed inside the SiC crystal. Subsequently, through sample polishing and ICP etching treatment, and by observing the etched structures with SEM, as shown in Figure 4, the above-mentioned energy-dependent effects were further revealed. The experimental results show that when the pulse energy was lower than 280 nJ, no obvious modification features were observed inside the SiC, indicating that the energy threshold for multifilament formation had not been reached. As the pulse energy increased to 280 nJ, three slender filament structures appeared in the etched area, but with poor uniformity and accompanied by additional damage features (Figure 4a). When the energy was raised to the range of 320–400 nJ, the number of filaments increased to 4, and the uniformity gradually improved, with reduced additional damage features (Figure 4b–d). Notably, during this process, the growth position of the multifilament gradually moved upward against the laser propagation direction (z-axis), a phenomenon clearly presented in the optical micrographs (see Appendix A, Figure A8 and Figure A9). When the pulse energy reached 600 nJ, the number of multifilaments increased to 5, and the uniformity further improved (Figure 4e), at which point the multifilament region had extended to the sample’s upper surface (see Appendix A, Figure A8 and Figure A9). With the continuous increase in energy to 800 nJ, the number of multifilaments further increased to 7 (Figure 4f). This series of experimental phenomena indicates that the formation and development of multifilament structures exhibit significant energy dependence: as the pulse energy increases, the range of areas meeting the multifilament growth threshold gradually moves upward, leading to an increase in the growth length of the multifilaments; at the same time, higher energy input enhances the plasma self-organization effect, not only increasing the number of multifilament but also significantly improving the uniformity of the structure. These findings provide important experimental evidence for the precise regulation of multifilament structures inside SiC through the adjustment of pulse energy.

3.4. High-Aspect-Ratio Nanostripes

To further characterize the photomodified region, we selected the most uniform multifilament area processed at 600 nJ pulse energy. Figure 5a shows the side optical microscopic image of this area. Particularly, we highlight the multifilament region (region II)—marked by the white dashed boxes in Figure 5a, where long-distance, uniform nanostripes are generated, following the traces of the laser multifilaments. A number of SEM images were captured to show the morphology of the photomodified region. Due to the magnification, the beginning, central, and end portions are displayed separately, as shown in Figure 5c. It reveals the presence of five parallel nanostripes, denoted as Li (I = 1, 2, … 5), with vertical spacing denoted by Si. Obviously, an appropriate measure of the uniformity of the nanostripes is to plot Si as a function of stripe length. The total length of the nanostripes after etching is about 97 μm, and we measured Si every 4.85 μm. The evolutions of Si are plotted in Figure 5b. The data show that the spacing S1 between the nanostripes L1 and L2 starts at 38.8 μm from the leftmost, maintaining a constant value of 1092 nm in the middle, and gradually reducing to 1075 nm at the rightmost end; the variations are gentle. The spacing S2 varies from 938 nm to 1133 nm at the leftmost side. Interestingly, S2 remains nearly constant over a length of about 30 μm (from 28 um to 58 um, cf. Figure 5b); that is, the nanostripes L2 and L3 maintain extremely high straightness and parallelism here. In comparison to self-assembled nanogratings [9,45], the confined light in the multifilament channels demonstrates strong directivity, significantly reducing the scattering effects of defects encountered during the photomodification processes. Therefore, the multifilament-induced nanostripes can exhibit very stable and uniform growth characteristics, particularly in the center area of the multifilament region (Figure 5c). Additionally, the width of L3 was about 29 nm, while the width of L2 was about 28 nm. Considering that the total length was approximately over 90 μm, the aspect ratio of the nanostripes L2 and L3 reached approximately over 3200:1.
Furthermore, the analysis of the spacing at the rightmost side reveals that Si experienced notable reductions at around 73 μm. This is attributed to the effect of geometric focusing (cf. Figure 2b). Further closer to the focal point at around 90 μm, S2 even abruptly decreased from 957 nm to 351 nm, while S3 decreased from 879 nm to 371 nm, both falling below 400 nm. In short, at positions far from the focal point, the nonlinear clamping effect during the multifilament formation process enables the optical field to maintain good collimation, thereby resulting in nanostripe spacing remaining relatively unchanged. However, near the focal point, the filament plasma is unable to balance the geometric focusing effect, leading to a gradual decrease in the nanostripe spacing. These results suggest that the multifilament processing scheme can offer an alternative approach for adjusting the nanostripe spacing by engineering the focused optical field, rather than relying on traditional methods of changing the laser wavelength.

4. Conclusions

In conclusion, by utilizing an elliptical Gaussian beam, we achieved the formation of long-range ordered multifilament structures induced by a single beam in solid SiC media, with the spacing regulation range spanning from submicron to nanoscale. This enabled the creation of ultrahigh-aspect-ratio (>3200:1) nanostripes with unprecedented uniformity and parallelism. Such control over subwavelength features in a crystalline semiconductor surpasses previous diffraction-limited approaches. We infer that different ionization mechanisms under varying pulse durations [10] may have a crucial impact on the morphology of the final formed multifilament structure in SiC. Under short pulse duration of less than 1 ps, plasma generation is mainly dominated by multiphoton ionization and tunneling ionization. The intense photoionization process and high free electron density can lead to damage to the multifilament structure. However, under longer pulse durations, the avalanche ionization mechanism plays a particularly prominent role. The reduction in the multifilament generation threshold under this ionization mechanism and the near-field enhancement effect under pulse accumulation may play an important role in improving the uniformity and quality of the multifilament structure. The growth process of multifilament structures was also studied by increasing the number of pulses. The multifilament region located in front of the focal point, extending approximately tens of micrometers, grows in the direction opposite to the beam propagation as the pulse number increases. Raman measurements confirmed that SiC is transformed from a single-crystalline phase to an amorphous phase, which further decomposes into amorphous silicon and amorphous carbon. Moreover, the enhanced PL signal of Vsi color centers was observed at the top of the multifilament region, where the laser power density was relatively low.
The quality of the multifilament-induced nanostripes still has considerable room for improvement. For instance, by further optimizing the laser beam—such as enhancing the symmetry of the spot, optimizing the ellipticity of the light spot [46], implementing complex vector-field induced modulation [47], and employing spatio-temporal control [48]—we anticipate achieving the ability to fabricate more uniform high-aspect-ratio nanostructures in SiC. We believe that an in-depth study of this technology and its underlying mechanisms will significantly enhance the processing efficiency and quality of nanostructures in solid materials, further broadening their functionalities and application scenarios [49].

Author Contributions

Conceptualization, X.S. and L.Z.; methodology, X.S., H.Z., Q.J., L.Q., and Z.Z.; software, X.S. and Q.J.; validation, X.S. and W.Y.; formal analysis, X.S., L.Z., and W.Y.; investigation, X.S., H.Z. and Q.J.; resources, J.Q. and M.Q.; data curation, X.S., W.Y., and M.Q.; writing—original draft preparation, X.S.; writing—review and editing, L.Z., W.Y., and M.Q.; visualization, X.S.; supervision, W.Y. and M.Q.; project administration, X.S., W.Y., and M.Q.; funding acquisition, M.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. U21A20494).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors thank Westlake Center for Micro/Nano Fabrication and Instrumentation and Service Center for Molecular Sciences for the facility support and technical assistance.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
SiCSilicon carbide
α-SiCAmorphous silicon carbide
c-SiCCrystalline silicon carbide
α-SiAmorphous silicon
α-CAmorphous carbon
VsiSilicon vacancy
PLPhotoluminescence
NANumerical aperture
SEMScanning electron microscopy

Appendix A

Photonics 12 00973 g0a1
Figure A1. Top view and cross-sectional morphology of SiC after laser processing with different ellipticities: (a) e = 1.07, (b) e = 1.27, and (c) e = 3.31. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 400 nJ, and a total pulse number of 105.
Figure A1. Top view and cross-sectional morphology of SiC after laser processing with different ellipticities: (a) e = 1.07, (b) e = 1.27, and (c) e = 3.31. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 400 nJ, and a total pulse number of 105.
Photonics 12 00973 g0a1
Photonics 12 00973 g0a2
Figure A2. Side view of photomodified regions processed by different pulse durations. The scanning processing scheme was used. The pulse energy is 400 nJ, and the scanning speed is 200 um/s.
Figure A2. Side view of photomodified regions processed by different pulse durations. The scanning processing scheme was used. The pulse energy is 400 nJ, and the scanning speed is 200 um/s.
Photonics 12 00973 g0a2
Photonics 12 00973 g0a3
Figure A3. (a,b) Top and side views of laser-modified structures using linear polarization along the Y-direction. (c,d) Top and side views of laser-modified structures using linear polarization along the X-direction. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 400 nJ, and a total pulse number of 105.
Figure A3. (a,b) Top and side views of laser-modified structures using linear polarization along the Y-direction. (c,d) Top and side views of laser-modified structures using linear polarization along the X-direction. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 400 nJ, and a total pulse number of 105.
Photonics 12 00973 g0a3
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Figure A4. (a,b) Top and side views of laser-modified structures using a circular polarized beam. (c,d) Top and side views of laser-modified structures using a linear polarized beam. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 800 nJ, and a total pulse number of 105.
Figure A4. (a,b) Top and side views of laser-modified structures using a circular polarized beam. (c,d) Top and side views of laser-modified structures using a linear polarized beam. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 800 nJ, and a total pulse number of 105.
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Figure A5. The effect of laser peak intensity on multiphoton ionization rate and tunneling ionization rate induced by ultrafast laser in SiC.
Figure A5. The effect of laser peak intensity on multiphoton ionization rate and tunneling ionization rate induced by ultrafast laser in SiC.
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Figure A6. Optical (upper) and PL (lower) mapping images of a photomodified region processed with a pulse number of 106. The static irradiation processing scheme was used. The pulse energy is 400 nJ, and the pulse duration is 10 ps.
Figure A6. Optical (upper) and PL (lower) mapping images of a photomodified region processed with a pulse number of 106. The static irradiation processing scheme was used. The pulse energy is 400 nJ, and the pulse duration is 10 ps.
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Figure A7. Raman spectra of SiC processed with different pulse durations. The scanning processing scheme was used. The pulse energy is 400 nJ, and the scanning speed is 200 um/s.
Figure A7. Raman spectra of SiC processed with different pulse durations. The scanning processing scheme was used. The pulse energy is 400 nJ, and the scanning speed is 200 um/s.
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Figure A8. Optical image of multifilament structures processed with various pulse energies from 280 to 800 nJ. The areas marked with red brackets are regions of multifilament structure.
Figure A8. Optical image of multifilament structures processed with various pulse energies from 280 to 800 nJ. The areas marked with red brackets are regions of multifilament structure.
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Figure A9. Enlarged optical image of the corresponding multifilament areas within the red brackets in Figure A8. Multifilament structures were processed with various pulse energies of (a) 280 nJ, (b) 320 nJ, (c) 400 nJ, (d) 480 nJ, (e) 600 nJ, and (f) 800 nJ.
Figure A9. Enlarged optical image of the corresponding multifilament areas within the red brackets in Figure A8. Multifilament structures were processed with various pulse energies of (a) 280 nJ, (b) 320 nJ, (c) 400 nJ, (d) 480 nJ, (e) 600 nJ, and (f) 800 nJ.
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Figure A10. Side view of photomodified regions processed at different repetition rates. The pulse energy is 400 nJ, and the total pulse number for different pulse repetition rates is kept at 105. The repetition rate shows no significant effect on the formation of multifilament structures inside SiC.
Figure A10. Side view of photomodified regions processed at different repetition rates. The pulse energy is 400 nJ, and the total pulse number for different pulse repetition rates is kept at 105. The repetition rate shows no significant effect on the formation of multifilament structures inside SiC.
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Figure 1. Ultrafast laser processing of SiC: (a) Measured intensity distribution of the elliptical Gaussian beam. (b) Sketch of the ultrafast laser processing system. (c) Schematic illustration of laser irradiation on SiC samples. Two processing methods—static irradiation and dynamic scanning—were employed. The beam propagation direction k, the polarization direction of the electric fields E, and the scanning direction v are indicated. (d) Typical top view (left) and side view (perpendicular to the polarization direction) of multifilament structures were captured using an optical microscope. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 400 nJ, and a total pulse number of 104.
Figure 1. Ultrafast laser processing of SiC: (a) Measured intensity distribution of the elliptical Gaussian beam. (b) Sketch of the ultrafast laser processing system. (c) Schematic illustration of laser irradiation on SiC samples. Two processing methods—static irradiation and dynamic scanning—were employed. The beam propagation direction k, the polarization direction of the electric fields E, and the scanning direction v are indicated. (d) Typical top view (left) and side view (perpendicular to the polarization direction) of multifilament structures were captured using an optical microscope. The static irradiation processing scheme was employed, with a pulse duration of 10 ps, a pulse energy of 400 nJ, and a total pulse number of 104.
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Figure 2. Growth processes of photomodified regions as the pulse number increases: (a) Simulated focused light field intensity distribution within SiC. (b) Schematic diagram of the growth process at varying pulse numbers. (c) Optical microscope images of laser-induced photomodifications in the Y-Z cross-section (perpendicular to the electric field polarization direction) at varying pulse numbers. Three distinct regions are observed: region I—excessively damaged region; region II—multifilament region; region III—color center region. The pulse energy is 400 nJ, the pulse duration is 10 ps, and the laser wavelength is 1030 nm. (d–f) Enlarged figures of excessively damaged region I (d), multifilament region II (e), and color center region III (f), produced by various pulse numbers.
Figure 2. Growth processes of photomodified regions as the pulse number increases: (a) Simulated focused light field intensity distribution within SiC. (b) Schematic diagram of the growth process at varying pulse numbers. (c) Optical microscope images of laser-induced photomodifications in the Y-Z cross-section (perpendicular to the electric field polarization direction) at varying pulse numbers. Three distinct regions are observed: region I—excessively damaged region; region II—multifilament region; region III—color center region. The pulse energy is 400 nJ, the pulse duration is 10 ps, and the laser wavelength is 1030 nm. (d–f) Enlarged figures of excessively damaged region I (d), multifilament region II (e), and color center region III (f), produced by various pulse numbers.
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Figure 3. Raman and PL mapping images: (a) Optical microscope image of the cross-section of SiC processed by the scanning processing scheme. The pulse energy is 400 nJ, the pulse duration is 10 ps, and the laser wavelength is 1030 nm. The moving speed of the SiC sample was controlled at 200 μm/s. (b) Raman mapping images at 780 cm−1 (c-SiC), 880 cm−1 (α-SiC), 480 cm−1 (α-Si), and 1350 cm−1 (α-C). (c) PL mapping image at 917 nm (emission wavelength of silicon vacancy color centers).
Figure 3. Raman and PL mapping images: (a) Optical microscope image of the cross-section of SiC processed by the scanning processing scheme. The pulse energy is 400 nJ, the pulse duration is 10 ps, and the laser wavelength is 1030 nm. The moving speed of the SiC sample was controlled at 200 μm/s. (b) Raman mapping images at 780 cm−1 (c-SiC), 880 cm−1 (α-SiC), 480 cm−1 (α-Si), and 1350 cm−1 (α-C). (c) PL mapping image at 917 nm (emission wavelength of silicon vacancy color centers).
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Figure 4. SEM images of etched multifilament structures processed with various pulse energies of (a) 280 nJ, (b) 320 nJ, (c) 400 nJ, (d) 480 nJ, (e) 600 nJ, and (f) 800 nJ.
Figure 4. SEM images of etched multifilament structures processed with various pulse energies of (a) 280 nJ, (b) 320 nJ, (c) 400 nJ, (d) 480 nJ, (e) 600 nJ, and (f) 800 nJ.
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Figure 5. Etched high-aspect-ratio nanostripes: (a) Optical microscope image of the photomodified region inside the processed SiC sample with a pulse energy of 600 nJ and a pulse duration of 10 ps using the scanning processing scheme. (b) Evolution of nanostripe spacing as a function of stripe length. (c) Enlarged SEM view of the center area of the multifilament region (dashed white boxes marked in (a)).
Figure 5. Etched high-aspect-ratio nanostripes: (a) Optical microscope image of the photomodified region inside the processed SiC sample with a pulse energy of 600 nJ and a pulse duration of 10 ps using the scanning processing scheme. (b) Evolution of nanostripe spacing as a function of stripe length. (c) Enlarged SEM view of the center area of the multifilament region (dashed white boxes marked in (a)).
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MDPI and ACS Style

Sun, X.; Zheng, H.; Jia, Q.; Qi, L.; Zhang, Z.; Zhong, L.; Yan, W.; Qiu, J.; Qiu, M. Controlled Growth of Multifilament Structures with Deep Subwavelength Features in SiC via Ultrafast Laser Processing. Photonics 2025, 12, 973. https://doi.org/10.3390/photonics12100973

AMA Style

Sun X, Zheng H, Jia Q, Qi L, Zhang Z, Zhong L, Yan W, Qiu J, Qiu M. Controlled Growth of Multifilament Structures with Deep Subwavelength Features in SiC via Ultrafast Laser Processing. Photonics. 2025; 12(10):973. https://doi.org/10.3390/photonics12100973

Chicago/Turabian Style

Sun, Xiaoyu, Haojie Zheng, Qiannan Jia, Limin Qi, Zhiqi Zhang, Lijing Zhong, Wei Yan, Jianrong Qiu, and Min Qiu. 2025. "Controlled Growth of Multifilament Structures with Deep Subwavelength Features in SiC via Ultrafast Laser Processing" Photonics 12, no. 10: 973. https://doi.org/10.3390/photonics12100973

APA Style

Sun, X., Zheng, H., Jia, Q., Qi, L., Zhang, Z., Zhong, L., Yan, W., Qiu, J., & Qiu, M. (2025). Controlled Growth of Multifilament Structures with Deep Subwavelength Features in SiC via Ultrafast Laser Processing. Photonics, 12(10), 973. https://doi.org/10.3390/photonics12100973

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