Open Access
Add to BibSonomy
Abstract
Single-crystal silicon (c-Si) is a vital component of photonic devices and has obvious advantages. Moreover, femtosecond-pulsed laser interactions with matter have been widely applied in micro/nanoscale processing. In this paper, we report the modification mechanisms of c-Si induced by a femtosecond laser (350 fs, 520 nm) at different pulse fluences, along with the mechanism of this technique to trim the phase error of c-Si-based devices. In this study, several distinct types of final micro/nanostructures, such as amorphization and ablation, were analyzed. The near-surface morphology was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The main physical modification processes were further analyzed using a two-temperature model. By employing Raman spectroscopy, we demonstrated that a higher laser fluence significantly contributes to the formation of more amorphous silicon components. The thickness of the amorphous layer was almost uniform (approximately 30 nm) at different induced fluences, as determined using transmission electron microscopy. From the ellipsometry measurements, we demonstrated that the refractive index increases for amorphization while the ablation decreases. In addition, we investigated the ability of the femtosecond laser to modify the effective index of c-Si microring waveguides by either amorphization or ablation. Both blue and red shifts of resonance spectra were achieved in the microring devices, resulting in double-direction trimming. Our results provide further insight into the femtosecond laser modification mechanism of c-Si and may be a practical method for dealing with the fabrication errors of c-Si-based photonic devices.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Silicon (Si) is one of the most widely used semiconductor materials in photonic devices because of its high refractive index, transparency in the near-infrared band, and compatibility with complementary metal oxide semiconductor (CMOS) fabrication processes. So far, silicon-on-insulator (SOI) has been an attractive Si-based platform for achieving large-scale integration of photonic integrated circuits (PICs) [1–9]. Although the SOI fabrication technology has advanced significantly, unexpected fabrication errors are presently a major obstruction to the mass production of Si-based photonic devices [10–12]. In particular, passive and phase-sensitive devices such as Mach–Zehnder interferometers and microring resonators (MRRs) exhibit a clear response to fabrication errors that cannot be adjusted in operations once fixed in the processes. Therefore, it is necessary to employ a post-fabrication trimming technique to correct fabrication-induced variations.
Femtosecond lasers have been widely applied for the modification of semiconductor materials owing to their attractive features such as high instantaneous intensity, ultrashort pulse duration, contactless processing, and micro/nanoscale fabrication without any postprocessing [13–18]. The heat-affected zone of femtosecond pulses is minimal (i.e., nonthermal) compared to that in picosecond and nanosecond laser processing [19–22]. Single-crystal silicon (c-Si) is one of the best-studied semiconductors with regard to interactions with femtosecond laser irradiation [23–31], where micromachining or modification may exhibit specific phenomena such as ablation, melt-quenched amorphous phase, oxidation, and recrystallization. Izawa et al. [29,30] indicated that the thickness of an amorphized layer does not depend on the number and fluence of pulses, whereas the pulse width does, and further confirmed that the thickness of the amorphization layer is related to the effective optical penetration depth. Moreover, Bai et al. [24] indicated that the amorphization efficiency of c-Si is highly dependent on the femtosecond laser polarization and that the amorphization efficiency of a horizontally polarized laser beam is greater than that of a circularly polarized one. Zhang et al. [31] analyzed the amorphous phase of Si originating from a non-thermal melting-to-recrystallization transformation sequence using simulations and experimental observations. Bonse et al. [25] investigated the surface modification of c-Si, and the single-pulse threshold fluences for melting, ablation, and polycrystalline recrystallization were determined quantitatively. Furthermore, Garcia-Lechuga et al. [28] analyzed the direct laser writing of amorphous lines in c-Si, and obtained a maximum amorphous layer thickness of 128 nm upon multipulse irradiation at 3 µm wavelength. To that end, some of these modifying phenomena have been widely explored and already used in applications, for example, in post-fabrication trimming of c-Si-based devices. Several research groups [11,12,32–35] have focused on the surface modification of c-Si waveguides to demonstrate femtosecond laser irradiation as a permanent phase-error correction technique for c-Si-based PICs. Both amorphization and ablation processes can be used to tune the phase error by altering the effective index of the c-Si waveguides. Compared to other permanent tuning methods, the femtosecond laser technique is not only contactless, fast, and robust, but also does not require any changes to the fabrication process and can be readily adapted to wafer-scale applications. Nevertheless, in phase error correction based on the femtosecond laser trimming technique, additional optical losses are introduced in the c-Si waveguide. Hence, to efficiently correct fabrication errors and reduce the introduction loss in c-Si-based devices, it is worth further investigating the interaction mechanism between the ultrafast laser and c-Si.
In this study, we investigated the interactions of a femtosecond-pulsed laser with c-Si and then used this technique to trim the phase error of the c-Si-based MRRs. Using a femtosecond-pulsed top-hat beam instead of a Gaussian beam profile, ablation and amorphization were induced on c-Si. The near-surface morphology was characterized by optical microscopy (OM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The final microstructures of the c-Si before and after laser irradiation were analyzed using transmission electron microscopy (TEM), Raman spectroscopy, and ellipsometry. In addition, we analyzed the modification of c-Si-based waveguides induced by a femtosecond laser, and observed the tuning of the MRRs phase error using this technique. The topics discussed in this work provide additional insights into femtosecond-laser-modified c-Si and promote its use in correcting the fabrication errors of c-Si-based PICs.
2. Materials and methods
2.1. Experimental samples
In the experiments, commercial c-Si (99.999% purity) in the form of p-type (100) Si wafers was selected as the target material. For the experiment, polished Si wafers (thickness, 0.6 mm) were sliced mechanically into 1 × 1 cm small sheet. Before the experiment, the samples were surface-cleaned to remove impurities and organics.
In this study, c-Si-based MRRs were fabricated by electron-beam lithography and dry etching. The MRRs device was based on a SOI substrate with a top Si layer thickness of 220 nm and a width of 450 nm, including an oxide layer buried of 2 µm. In addition, the radius of the microring was approximately 25 µm, which supported only the TE mode.
2.2. Experimental setup
The samples (c-Si and c-Si-based MRRs) were modified via a femtosecond laser (Spirit-1040, Spectra-Physics) that delivered a 350-fs pulse train at a repetition rate of 100 kHz with a frequency-doubled wavelength of 520 nm. The experimental setup was based on an online charge-coupled device camera system and automatic control system to achieve laser precision processing, as shown in Fig. 1. The laser beam was coupled to a 10× objective lens (Olympus, 0.30NA), which the beam waist was about 2.8 µm at 520 nm wavelength, and focused vertically on the sample surface. The focal spot exhibited a uniform intensity distribution when a top-hat beam was used. The sample was placed on a 3-dimension (3D) processing platform with a resolution of 50 nm in the x- and y-directions. The scanning intervals were at the micron level, and the moving rate was varied in the range of 10–1000 mm/s to achieve irradiation spots in the array on the c-Si substrate. The samples were placed in a super-clean room and non-contact environment, which ensured the uniformity and repeatability of the irradiated spots. More precise laser beam control, alignment, and focusing were required for the c-Si waveguide structure of the MRRs, owing to their small sizes.
2.3. Characterization methods
The structural changes within the laser-irradiated areas were measured using a Raman spectrometer (HORIBA, Kyoto, Japan). The laser power of the 532-nm-wavelength excitation laser that reached the c-Si surface was maintained below the energy that would thermally induce modification, and it was equipped with 100× (Olympus, 0.9 NA) microscopic objectives. Ellipsometry (HORIBA France SAS, France) was performed in the spectral region of 200–2000 nm to characterize the optical properties. The surface characteristics of the samples were inspected using OM (Olympus, 0.9NA) and SEM (ZEISS, Auriga). The surface morphology was observed using AFM (NT-MDT, Russia). An ultraviolet–visible light–near-infrared spectrophotometer (Shimadzu, 3600-plus) was used to measure the reflection and absorption of the samples. In addition, the compositional changes in the c-Si substrate before and after laser irradiation were analyzed using TEM (Talos F200X, Thermo Fisher).
3. Results and discussion
3.1 Interactions of the femtosecond-pulsed laser with the c-Si
3.1.1 Analysis for laser modification of the c-Si
To determine the different modification threshold conditions for c-Si, laser-irradiated areas on the c-Si substrate were implemented using a femtosecond laser at various laser energies, and they were characterized using OM, SEM, and AFM techniques. Figure 2 shows several selected laser-irradiated arrays on a c-Si substrate generated at fluences of 0.23, 0.35, 0.51, 0.59, and 0.86 J/cm2, respectively. Here, the irradiation areas look tilted due to the spatial elliptical distribution of the beam. The upper row of the image displays OM micrographs of the laser-irradiated arrays, and the corresponding SEM micrographs are shown in the second row. In addition, the two lower rows display the AFM topography and cross-sectional views in the AFM image (corresponding to the white dotted line in Fig. 2(c)). The size and brightness of the laser-irradiated areas changed significantly depending on the laser fluence, as shown in Fig. 3(a) and 3(b). The laser threshold fluences for different modifications of the c-Si substrate were determined using the following equation [30,36]:
where D (µm) is the diameter of the femtosecond laser-irradiated areas on the c-Si substrate, 2w is the spatial laser beam diameter, F0 is the laser peak fluence, and Fth is the laser modification threshold fluence. The evaporation threshold (Tev) and melting temperature (Tm) were approximately 3510 K and 1685 K, respectively [27,37].
Fig. 2. Morphology characteristics of laser-irradiated c-Si at different laser fluences characterized by (a) OM, (b) SEM, and (c) AFM images; (d) height distribution that resulted from cross-sectional views of the white dotted line in AFM topography.
Here, laser-irradiated areas are defined as those with local destruction of the native oxide layer, melt-quenched amorphous (M-amorphous), and ablated areas. Likewise, the OM and SEM images clearly identify the correlation of the modification threshold with the morphology distribution of the laser-irradiated areas. We further classify this process into two ablation regimes based on how ablation occurs. The first regime is complete ablation and the second regime, which corresponds to the lowest laser evaporation fluence, is partial ablation. When the laser fluence reached 0.86 J/cm2, the first ablation regime occurred, which was darker in the OM image owing to the comparatively lower reflectivity of the ablation areas than that of the c-Si surface. Briefly, ablation can result from a phase explosion with a steep increase in the heating rate, and this process has been verified in numerous studies [21,27,37]. In addition, the modification mechanism was suspected to change, and it was mainly manifested as the thickness thinning of c-Si with a maximum removal depth of approximately 800 nm. As the laser fluence was gradually decreased to 0.51 and 0.59 J/cm2, the second ablation regime appeared. The laser-irradiated areas were brighter but also accompanied by local ablation, as evidenced by the large amounts of evaporated Si particles shown in Fig. 2(b) and 2(c). This phenomenon possibly occurs because the unstable laser energy output becomes closer to the phase-explosion threshold. In addition, ablation can induce amorphization, and recrystallization is possible at higher laser fluences irradiation, as discussed below. In general, as the laser fluence gradually decreased to 0.35 J/cm2, the areas became brighter and the surface roughness became smoother. It was inferred that the laser modification energy was closer to the melting threshold than to the evaporation threshold. Importantly, with a further decrease in laser fluence to 0.23 J/cm2, more uniform areas appeared, with a surface roughness of approximately several nanometers. This was not caused by the local destruction of the native oxide layer but was associated with physical processes (i.e., amorphization). Finally, as the laser fluence decreased below 0.23 J/cm2, which was slightly above the point of visual changes modified by the laser irradiation, the areas almost disappeared (not shown in the figure).
3.1.2 Analysis for reflectance measurement
Figure 3 shows the variation tendencies of the reflectance spectra in the laser-irradiated areas induced by different modification threshold conditions. In brief, different laser modifications for c-Si can lead to differences in the optical properties, such as the complex refractive index. To a certain extent, the variation in the reflectance intensity decreased as the laser fluence gradually increased. When the laser energy became greater than the evaporation threshold, approximately 0.51 J/cm2, ablative removal of c-Si from the surface was observed, as indicated by the darker color in the optical image. Similarly, the intensity of the reflectance spectra also decreased compared with that of the pristine c-Si substrate. With further reduction of the laser energy, for example, to 0.23 J/cm2, it is possible that a direct modification of optical properties occurs rather than a change in morphology. This indicates an increase in absorption owing to laser-irradiated c-Si resulting in its microstructural evolution or nanomilling evaporation.
3.1.3 Analysis for two-temperature model (TTM) simulation
To further examine the laser-Si interactions, it is necessary to know the main physical processes for laser energy absorption, diffusion, and transport in its microstructure. The TTM is suitable for studying the interactions between ultrafast lasers and semiconductor materials. Thermal effects involve a rapid energy exchange from the femtosecond laser to electrons with a femtosecond-level exchange time. However, the energy relaxation time from the free electrons to the lattice is relatively long, within tens of picoseconds. In this study, we used the TTM to establish a theoretical model for the interactions between a femtosecond laser and c-Si. Considering thermal diffusion and energy absorption under a high-energy laser, the thermal equilibrium equation of the energy transfer between the electrons and the lattice is evaluated as follows [14,27,31,37]:
Considering the ultrafast energy exchange for the interactions between the femtosecond laser and the c-Si samples, the effect of thermal equilibrium for the top-hat beam is similar to that of the Gaussian beam, and the laser source term could be given by the following formula [14,38]:
Table 1. Thermophysical parameters used in this model
It is worth noting that high laser energy irradiation of c-Si not only induced a solid-to-liquid phase transition but also changed its properties because the molten material exhibited metallic behavior. To determine a reasonable approximate modification process for the completely ablated areas, owing to the lack of physical parameters for molten Si, we referred to the theoretical analysis in [30] to alter parameters such as Ke, Kl, and Cl. Specifically, some parameters were related to the laser wavelength, which was approximated using the 532 nm wavelength parameters. The finite difference method (FDM) was used to solve the partial differential equations of the TTM.
Based on this model, the changes in the peak lattice temperature and carrier-lattice thermal equilibrium with laser irradiation intensity were calculated, as shown in Fig. 4(a) and 4(b), respectively. The rough evaporation threshold (Tev) and melting point (Tm) were marked by red dashed lines in Fig. 4(a). The simulation results were compared with experimentally observed surface morphologies. For the laser energy at 0.23 J/cm2, the peak lattice temperature was near the melting temperature of c-Si, which indicated the presence of melting and that surface morphology modification occurred. Likewise, we calculated the change in the electron and lattice temperatures, establishing a thermal electron–lattice equilibrium after 3ps, as shown in Fig. 4(b). When the laser energy was increased to 0.35 J/cm2, the peak lattice temperature was much higher than the melting temperature, resulting in amorphization but no ablation of c-Si. As the laser fluence further increased to 0.51 J/cm2, the peak lattice temperature approached the evaporation temperature of c-Si. In other words, before heating to the evaporation point with a higher laser fluence, ablation occurred earlier, a phenomenon similar to that reported in [31]. For 0.59 J/cm2, the electron and lattice temperatures reached thermal equilibrium at approximately 3460 K, which was close to the evaporation point where lattice structure breaks occurred. This simulation is in agreement with the observations shown in Fig. 2, indicating that evaporation and partial ablation occurred under this laser fluence. When the laser fluence reached 0.86 J/cm2, the peak lattice temperature was much higher than the evaporation point, resulting in complete ablation with boiling and evaporation of materials.
Fig. 4. (a) Variation in the peak lattice temperatures after laser irradiation with different laser fluences; (b) thermal equilibrium of electron and lattice temperatures after laser irradiation at laser fluences of 0.23 and 0.59 J/cm2.
3.1.4 Analysis for Raman measurements
Raman measurements were performed to study the microstructural evolution of the femtosecond-laser-irradiated areas on the c-Si substrate, as shown in Fig. 5. In our experiments, the intensity distribution of the reshaped beam was uniform, and micro-Raman spectroscopy was used to analyze the centers of the irradiated areas. The pristine c-Si substrate had a dominant peak centered at ∼520 cm-1, which was related to the transverse optical (TO) mode of Si–Si vibrations in the crystalline phase [24,25,40]. For laser pulses with a fluence of 0.23 J/cm2, a weak envelope peak could be observed at 490 cm-1, indicating that amorphous silicon (a-Si) had formed in irradiation areas. Most notably, this broad peak at approximately 490 cm-1 is attributed to the stretching vibration mode of the Si–Si bonds owing to the TO mode of the a-Si network [41,42]. Moreover, with a further increase of the laser-annealed temperature, the intensity of Raman peak at ∼520 cm-1 gradually decreased while the intensity at ∼490 cm-1 increased. This means that a higher laser energy may lead to the formation of more a-Si components in the laser-irradiated areas. However, laser pulses with a fluence of 0.86 J/cm2 led to the ablation of the c-Si substrate, and a broad peak around 490 cm-1, attributed to the contributions of the amorphous phase, was still observed. This implies that even for laser energies slightly above the ablation threshold, the removal of c-Si materials is accompanied by the formation of a very thin melt layer, which can be resolidified as a-Si components.
Fig. 5. Raman spectra of the laser-irradiated areas on c-Si substrate induced at various laser fluences.
3.2 Double-direction trimming of c-Si-based MRRs
3.2.1 TEM observation of the laser-modified amorphization
To further investigate the formation mechanism of the final structures, particularly the amorphous layer, TEM was performed to observe the pristine and modified areas. Figure 6 presents the typical cross-sectional TEM images on the irradiated areas at the laser energy of 0.23 and 0.51 J/cm2, respectively. For comparison, low-energy laser-irradiated M-amorphous areas are shown in the upper image, whereas partially ablated areas are displayed in the lower rows. From the TEM results, we could not observe a regular lattice structure compared to that in the pristine c-Si sample, indicating that amorphization occurred. A fine boundary surface was formed between the modified a-Si and pristine c-Si areas. For laser pulses with a fluence of 0.23 J/cm2, the amorphized section was almost uniform, and a thickness of approximately 30 nm was measured. The thickness of the a-Si layer is related to the effective optical penetration depth; thus, a thickness of approximately 30 nm was induced at a laser wavelength of 520 nm, which is similar to the results reported in [29]. More importantly, amorphization induced upon ablation of c-Si was also observed, as shown in the lower row of Fig. 6(c). This also verifies the assumption of the formation of a very thin a-Si layer resulting from resolidification of the melt layer. Furthermore, amorphization in laser-irradiated areas at laser fluences of 0.35, 0.59, and 0.86 J/cm2 was also observed (not shown here). Nevertheless, we observed that the crystallographic orientation of Si around the amorphous layer was same as the unirradiated areas. This indicated that the areas irradiated by higher laser pulse energies resulted in recrystallization rather than amorphization due to the release of latent heat maintaining the melt layer [27,31].
Fig. 6. TEM images of femtosecond-laser-irradiated areas on c-Si substrate at laser energy of 0.23 and 0.51 J/cm2, respectively: (a) and (c) overview, (b) and (d) high-resolution image.
3.2.2 Optical property of the femtosecond-laser-irradiated areas
Femtosecond laser processing as a micro-nanoscale fabrication method can optimize c-Si and c-Si-based devices performance based on light–matter interactions; nevertheless, the loss introduced also deserves further attention. Figure 7(a) shows the variations in the absorption spectra of the laser-irradiated areas. The spectral absorbance of the pristine c-Si substrate was lower than that of the laser-irradiated areas, whereas the M-amorphous was lower than that of the ablation. The optical constants of the pristine c-Si substrate and laser-irradiated areas were measured and fitted using ellipsometry, as shown in Fig. 7(b). The pristine c-Si substrate was transparent in the optical communication band, but the extinction coefficient k increased upon laser irradiation. It has been reported that the refractive index is sensitive to the coordination chemistry and composition around the Si lattice [43]; thus, the formation of amorphization or larger structural defects resulting from laser irradiation significantly influences the real part of the optical constant (n value). For 0.23 J /cm2, the refractive index at 1550 nm wavelength was significantly higher than that of the pristine c-Si substrate (n = 3.45 at λ = l550 nm). This is approximate with the refractive index values at a wavelength of 1550 nm for laser-induced amorphization reported in [32,44,45]. In contrast, for 0.51 J/cm2, the refractive index of laser-irradiated samples decreased in the optical communication band. This implies that some of the surface c-Si removal results in structural defects; thus, the refractive index is lower.
Fig. 7. Variation of (a) absorption spectra and (b) optical constants for femtosecond-laser-irradiated areas on c-Si substrate.
3.2.3 Double-direction tuning of c-Si-based MRRs
We examined the femtosecond-laser-irradiated c-Si waveguides of the MRRs to investigate their trimming effects on the resonant wavelength shift, as shown in Fig. 8. The structural property of the MRRs is sensitive to model confinement; thus, the effective index change can shift its resonant wavelength, which can be computed by [33]
Fig. 8. (a) Diagrams of the femtosecond laser trimming position for the MRRs and its waveguide cross section, (b) optical image and (c) SEM image of the MRRs. The location of femtosecond laser irradiation areas on MRR is outlined in red dots line.
Positions 1 and 2 were modified by the top-hat beam with laser fluence at 0.23 J/cm2 (the M-amorphous) and 0.51 J/cm2 (the ablation), respectively, as shown in Fig. 8(a). The location of irradiation areas on MRR waveguides was outlined, as depicted by the dashed circles in Fig. 8(b). A theoretical model for post-fabrication trimming of the MRRs was established by the finite-difference time-domain (FDTD) method, and the optical parameters of this model were obtained from the experimental results. Figure 9 shows a plot of the resonance wavelength shift for the MRRs devices at different laser-irradiated positions. In general, the theoretical analysis was consistent with the experimental results, and the variation trends of the resonant wavelength shift were similar. In our experiments, the free spectral range (FSR) of the MRRs was approximately 3.7 nm with a tunable laser source scanning resolution of 1 pm. For 0.23 J/cm2, only redshifts, near 0.5 nm, were observed in the resonances. The most likely physical mechanism for this change is the conversion of the thin layer of c-Si at the top waveguide surface to a-Si; similar phenomena have been previously reported [32,33]. Moreover, the microstructure changes resulting from laser-Si interactions are quite complex, which may include recrystallization of Si, as will be discussed further. In addition, an increase in the optical loss of the MRRs was observed, as shown by the decrease in intensity of the red spectra compared with that of the black spectra. On the contrary, with laser-irradiated fluence at approximately 0.51 J/cm2, blueshifts of approximately 1.5 nm were observed. Similarly, the working mechanism is suspected to change, probably introducing significant extra losses and limiting the tuning range.
Fig. 9. Resonance spectra of (a) simulation and (b) experiment for an MRRs before and after irradiation with a femtosecond laser fluence of 0.23 and 0.51 J/cm2.
The quality factor (Q) of the MRRs was also monitored after each laser irradiation to ensure that the device response varied during trimming. As shown in Fig. 9, the Q of the MRRs decreased after laser irradiation. It is inferred that the decrease in Q values after laser trimming can be attributed to the increased absorption and roughness compared to the pristine smooth c-Si waveguides surface. In principle, it was observed that the femtosecond laser trimming technique is an effective and permanent method for post-fabrication double-direction tuning of MRRs. However, the main problem is the fact that the laser-trimming waveguides suffer from extra loss. In future post-fabrication trimming PICs, such problems can be further studied with the cladding oxide or by altering the laser focusing position, irradiation power, and trimming speed.
4. Conclusions
We investigated femtosecond-laser-induced surface modification of c-Si and used this technique to achieve double-direction tuning of c-Si-based MRRs. Through OM, SEM, and AFM, it was demonstrated that the surface roughness was highly dependent on the laser-irradiated fluences. From the Raman scattering spectra, it was inferred that a higher laser-annealing fluence led to the formation of more a-Si components, resulting in a stronger Raman peak at ∼490 cm-1. The a-Si section was almost uniform, and its thickness of approximately 30 nm was quantitatively determined using TEM. Moreover, it was found that the refractive index of the formation of amorphization induced by a laser energy at 0.23 J/cm2 was significantly higher than that of the pristine c-Si substrate in the optical communication band, while the refractive index decreased due to removal of some of the surface Si at higher ablation energy. Finally, we investigated the application of femtosecond laser pulses to directly modify c-Si-based waveguides for post-fabrication trimming by altering the effective index resulting from either amorphization or ablation. The results confirmed that the resonance spectra of the MRRs could be double-direction tuned. Our findings not only serve to understand laser-Si interactions, but also advance this technique for use in the post-fabrication trimming of PICs on a Si-based platform.
Funding
National Natural Science Foundation of China (62375274); Shanghai Technology Innovation Project (XTCX-KJ-2023-01).
Acknowledgments
We would like to thank Jun Wang and Zixin Wang for their assistance with the Raman spectrometer measurements for this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. P. Abbamonte, G. Blumberg, A. Rusydi, et al., “Crystallization of charge holes in the spin ladder of Sr14Cu24O41,” Nature 431(7012), 1078–1081 (2004). [CrossRef]
2. A. H. Atabaki, Sajjad Moazeni, F. Pavanello, et al., “Integrating photonics with silicon nanoelectronics for the next generation of systems on a chip,” Nature 556(7701), 349–354 (2018). [CrossRef]
3. G. Kim, H. Park, J. Joo, et al., “Single-chip photonic transceiver based on bulk-silicon, as a chip-level photonic I/O platform for optical interconnects,” Sci. Rep. 5(1), 11329 (2015). [CrossRef]
4. D. Perez, I. Gasulla, L. Crudgington, et al., “Multipurpose silicon photonics signal processor core,” Nat. Commun. 8(1), 636 (2017). [CrossRef]
5. S. Y. Siew, B. Li, F. Gao, et al., “Review of silicon photonics technology and platform development,” J. Lightwave Technol. 39(13), 4374–4389 (2021). [CrossRef]
6. J. Wang and Y. Long, “On-chip silicon photonic signaling and processing: a review,” Sci. Bull. 63(19), 1267–1310 (2018). [CrossRef]
7. J. Cheng, Z. He, Y. Guo, et al., “Self-calibrating microring synapse with dual-wavelength synchronization,” Photonics Res. 11(2), 347 (2023). [CrossRef]
8. D. Dai, D. Liang, P. Cheben, et al., “Next-generation silicon photonics: introduction,” Photonics Res. 10(10), NGSP1 (2022). [CrossRef]
9. G. Giamougiannis, A. Tsakyridis, M. Moralis-Pegios, et al., “Neuromorphic silicon photonics with 50 GHz tiled matrix multiplication for deep-learning applications,” Adv. Photonics 5(01), 1 (2023). [CrossRef]
10. W. A. Zortman, D. C. Trotter, M. R. Watts, et al., “Silicon photonics manufacturing,” Opt. Express 18(23), 23598–23607 (2010). [CrossRef]
11. V. Biryukova, G. J. Sharp, C. Klitis, et al., “Trimming of silicon-on-insulator ring-resonators via localized laser annealing,” Opt. Express 28(8), 11156–11164 (2020). [CrossRef]
12. H. Jayatilleka, H. Frish, R. Kumar, et al., “Post-fabrication trimming of silicon photonic ring resonators at wafer-scale,” J. Lightwave Technol. 39(15), 5083–5088 (2021). [CrossRef]
13. J. Huang, L. Jiang, X. Li, et al., “Controllable photonic structures on silicon-on-insulator devices fabricated using femtosecond laser lithography,” ACS Appl. Mater. Interfaces 13(36), 43622–43631 (2021). [CrossRef]
14. H. Ma, Y. Zhao, Y. Shao, et al., “Determining femtosecond laser fluence for surface engineering of transparent conductive thin films by single shot irradiation,” Opt. Express 29(23), 38591–38605 (2021). [CrossRef]
15. Y. Shen, Y. Jia, F. Chen, et al., “Femtosecond laser-induced optical waveguides in crystalline garnets: Fabrication and application,” Opt. Laser Technol. 164, 109528 (2023). [CrossRef]
16. D. Tan, Zhou Wang, X. Beibei, et al., “Photonic circuits written by femtosecond laser in glass: improved fabrication and recent progress in photonic devices,” Adv. Photonics 3(02), 1 (2021). [CrossRef]
17. K. X. Zhang, J.-D. Shao, G.-H. Hu, et al., “Fast fabrication of fishnet optical metamaterial based on femtosecond laser induced stress break technique,” Nanomaterials 11(3), 742 (2021). [CrossRef]
18. Z.-Z. Li, Z.-N. Tian, Z.-T. Li, et al., “Photon propagation control on laser-written photonic chips enabled by composite waveguides,” Photonics Res. 11(5), 829 (2023). [CrossRef]
19. J. Du, Z. Mu, L. Li, et al., “A Raman study on nanosecond-laser-induced multi-level switching of Ge2Sb2Te5 thin films,” Opt. Laser Technol. 144, 107393 (2021). [CrossRef]
20. J. Du, J. Zhou, L. Zhang, et al., “Investigation of the crystallization characteristics of intermediate states in Ge2Sb2Te5 thin films induced by nanosecond multi-pulsed laser irradiation,” Nanomaterials 12(3), 536 (2022). [CrossRef]
21. M. E. Shaheen, J.E. Gagnon, B.J. Fryer, et al., “Studies on laser ablation of silicon using near IR picosecond and deep UV nanosecond lasers,” Opt. Lasers Eng. 119, 18–25 (2019). [CrossRef]
22. Q. Fu, J. Qian, G. Wang, et al., “Heat accumulation effects in femtosecond laser-induced subwavelength periodic surface structures on silicon,” Chin. Opt. Lett. 21, 051402 (2023). [CrossRef]
23. E. Allahyari, J. Nivas, G. Avallone, et al., “Femtosecond laser surface irradiation of silicon in air: pulse repetition rate influence on crater features and surface texture,” Opt. Laser Technol. 126, 106073 (2020). [CrossRef]
24. F. Bai, H.-J. Li, Y.-Y. Huang, et al., “Polarization effects in femtosecond laser induced amorphization of monocrystalline silicon,” Chem. Phys. Lett. 662, 102–106 (2016). [CrossRef]
25. J. Bonse, K.-W Brzezinka, A.J Meixner, et al., “Modifying single-crystalline silicon by femtosecond laser pulses: an analysis by micro Raman spectroscopy, scanning laser microscopy and atomic force microscopy,” Appl. Surf. Sci. 221(1-4), 215–230 (2004). [CrossRef]
26. C. Chen, F. Zhang, Y. Zhang, et al., “Single-pulse femtosecond laser ablation of monocrystalline silicon: a modeling and experimental study,” Appl. Surf. Sci. 576, 151722 (2022). [CrossRef]
27. C. Florian, D. Fischer, K. Freiberg, et al., “Single femtosecond laser-pulse-induced superficial amorphization and re-crystallization of silicon,” Materials 14(7), 1651 (2021). [CrossRef]
28. M. Garcia-Lechuga, N. Casquero, A. Wang, et al., “Deep silicon amorphization induced by femtosecond laser pulses up to the mid-infrared,” Adv. Opt. Mater. 9, 2100400 (2021). [CrossRef]
29. Y. Izawa, Y. Izawa, Y. Setsuhara, et al., “Ultrathin amorphous Si layer formation by femtosecond laser pulse irradiation,” Appl. Phys. Lett. 90(4), 044107 (2007). [CrossRef]
30. Y. Izawa, Y. Setuhara, M. Hashida, et al., “Ablation and amorphization of crystalline Si by femtosecond and picosecond laser irradiation,” Jpn. J. Appl. Phys. 45(7R), 5791 (2006). [CrossRef]
31. X. Zhang, L. Zhang, S. Mironov, et al., “Effect of crystallographic orientation on structural response of silicon to femtosecond laser irradiation,” Appl. Phys. A 127(3), 196 (2021). [CrossRef]
32. D. Bachman, Z. Chen, A. M. Prabhu, et al., “Femtosecond laser tuning of silicon microring resonators,” Opt. Lett. 36(23), 4695–4697 (2011). [CrossRef]
33. D. Bachman, Z. Chen, R. Fedosejevs, et al., “Permanent fine tuning of silicon microring devices by femtosecond laser surface amorphization and ablation,” Opt. Express 21(9), 11048–11056 (2013). [CrossRef]
34. D. Bachman, Z. Chen, C. Wang, et al., “Postfabrication phase error correction of silicon photonic circuits by single femtosecond laser pulses,” J. Lightwave Technol. 35(4), 588–595 (2017). [CrossRef]
35. Y. Wu, H. Shang, X. Zheng, et al., “Post-processing trimming of silicon photonic devices using femtosecond laser,” Nanomaterials 13(6), 1031 (2023). [CrossRef]
36. S. Martin, A. Hertwig, M. Lenzner, et al., “Spot-size dependence of the ablation threshold in dielectrics for femtosecond laser pulses,” Appl. Phys. A 77, 883–884 (2003). [CrossRef]
37. G. D. Tsibidis, M. Barberoglou, P. A. Loukakos, et al., “Dynamics of ripple formation on silicon surfaces by ultrashort laser pulses in subablation conditions,” Phys. Rev. B 86(11), 115316 (2012). [CrossRef]
38. B. Chen, Y. Bian, Z. Li, et al., “Effect of laser beam profile on thermal transfer, fluid flow and solidification parameters during laser-based directed energy deposition of Inconel 718,” Materials 16(12), 4221 (2023). [CrossRef]
39. H. S. Sim, S. H. Lee, K. G. Kang, et al., “Femtosecond pulse laser interactions with thin silicon films and crater formation considering optical phonons and wave interference,” Microsyst. Technol. 14(9-11), 1439–1446 (2008). [CrossRef]
40. Q. Cheng, S. Xu, K. Ostrikov, et al., “Rapid, low-temperature synthesis of nc-Si in high-density, non-equilibrium plasmas: enabling nanocrystallinity at very low hydrogen dilution,” J. Mater. Chem. 19(29), 5134–5140 (2009). [CrossRef]
41. M. S. Amer, M. A. El-Ashry, L. R. Dosser, et al., “Femtosecond versus nanosecond laser machining: comparison of induced stresses and structural changes in silicon wafers,” Appl. Surf. Sci. 242(1-2), 162–167 (2005). [CrossRef]
42. D. Song, E.-C. Cho, G. Conibeer, et al., “Structural, electrical and photovoltaic characterization of Si nanocrystals embedded SiC matrix and Si nanocrystals/c-Si heterojunction devices,” Sol. Energy Mater. Sol. Cells 92(4), 474–481 (2008). [CrossRef]
43. K. U. M. Kumar and M. G. Krishna, “Chromium-induced nanocrystallization of a-Si thin films into the Wurtzite structure,” J. Nanomater. 2008, 1–6 (2008). [CrossRef]
44. M. J. A. de Dood, A. Polman, T. Zijlstra, et al., “Amorphous silicon waveguides for microphotonics,” J. Appl. Phys. 92(2), 649–653 (2002). [CrossRef]
45. J. Bonse, “All-optical characterization of single femtosecond laser-pulse-induced amorphization in silicon,” Appl. Phys. A 84(1-2), 63–66 (2006). [CrossRef]
