A digital photon forging method of high-repetition-rate ultrafast laser

Abstract

The application discloses a kind of digital photon forging methods of high repetition rate ultrafast laser, it is related to laser material processing technical field, by digital programming to three-stage frequency coordination control of laser pulse sequence, to solve the problem that traditional laser shock strengthening is difficult to consider heat damage and strengthening depth simultaneously.Three-stage control includes: primary frequency (1 kHz-10 MHz) control processing efficiency, adapt to high-speed scanning;Secondary frequency is introduced heat relaxation window between two envelopes by setting sub-pulse envelope and its duty cycle, inhibit heat accumulation;Third frequency (1-100 GHz) matches material ultrafast relaxation process, so that sub-pulse interval is shorter than shock wave decay time, realize stress wave nonlinear superposition.Pulse sequence after modulation acts on material surface, induces to generate local plasma and shock wave (photon forging effect), so as to form modified layer with compression strain, high hardness, low roughness and modulated optical properties on material surface and subsurface.

Description

Technical Field

[0001] This invention relates to the field of laser material processing technology, specifically to a surface impact hardening method that utilizes high-repetition-rate ultrafast laser pulse sequences to perform non-thermal strain-controlled surface hardening of materials. This application is particularly applicable to surface modification of heat-sensitive high-performance materials (such as amorphous, polycrystalline, or single-crystal thin films) to synergistically improve their mechanical properties, surface integrity, and optical functions. Background Technology

[0002] Laser Shock Peening (LSP) is an advanced surface engineering technique. Its basic principle involves using a high-power, short-pulse laser beam to irradiate the material, inducing instantaneous vaporization and ionization of the surface material to form a high-pressure plasma. This plasma expands violently under the constraint of a layer (such as a transparent medium or water film), generating a high-amplitude shock wave that propagates into the material's interior. During propagation, the shock wave induces plastic deformation in the material, introducing residual compressive stress, refining grains, and increasing dislocation density, ultimately significantly improving the material's surface hardness, fatigue resistance, and wear resistance. Due to its non-contact nature, high controllability, and minimal heat-affected zone, this technology has significant application value in aerospace, precision manufacturing, and nuclear energy fields.

[0003] However, existing laser shock peening technologies are mainly divided into two categories based on the different laser pulse widths used, and each has inherent defects that are difficult to overcome: Limitations of Femtosecond Laser Shock Enhancement (Insufficient Depth of Action). Femtosecond lasers are characterized by extremely short pulse widths (typically less than or equal to 1 picosecond) and extremely high peak power. Their energy deposition time is much shorter than the material's thermal diffusion time, theoretically enabling "cold processing" without thermal effects. However, existing femtosecond laser shock enhancement technology is limited by two key factors: first, femtosecond lasers typically operate at low repetition frequencies (less than or equal to 1 kHz), resulting in low processing efficiency; second, the energy deposition time of a single femtosecond pulse is too short. Although the induced shock wave has a high peak pressure, its extremely short duration makes it difficult for the shock wave energy to penetrate deep into the material, limiting the depth of plastic deformation. This results in a shallow reinforced layer, failing to provide effective fatigue life gains for engineering components subjected to heavy loads.

[0004] Limitations of Nanosecond Laser Shock Enhancement (Significant Thermal Damage). Nanosecond laser pulses have relatively long widths (typically 10-30 nanoseconds), which facilitates shock wave propagation and the generation of deep plastic deformation, resulting in a deeper residual compressive stress layer. However, the long energy deposition time leads to significant thermal effects. During laser irradiation, the material surface melts and vaporizes, forming a wide heat-affected zone, which may induce undesirable phase transitions, thermal stress cracks, and surface ablation. This is generally unacceptable for heat-sensitive materials (such as amorphous alloys, high-temperature alloys, and precision-tempered components) or precision devices with extremely high surface quality requirements.

[0005] To address the challenges of cold and deep anchor sections, some existing technologies have explored strategies such as using composite laser sources or increasing the repetition frequency. The nanosecond-femtosecond composite laser shock peening method, while aiming to combine the advantages of both, is essentially a simple superposition of two independent laser systems (nanosecond and femtosecond), resulting in a complex system structure and high cost. More importantly, the synergistic mechanism between the two pulses in time and space is complex, with a narrow process window and poor controllability, failing to fundamentally solve the dynamic matching problem between energy deposition, thermal diffusion, and shock wave propagation.

[0006] High-repetition-rate nanosecond laser shock peening attempts to increase energy input per unit time by increasing the repetition frequency of the nanosecond laser to improve efficiency. However, this not only fails to solve the inherent heat accumulation problem of nanosecond lasers, but also exacerbates the thermal effect due to high-frequency energy injection, leading to a higher risk of thermal damage to the material surface and limiting its application range.

[0007] In summary, the core technical challenge of existing laser shock peening technologies is the inability to achieve efficient and deep deposition of shock wave energy within materials without introducing significant thermal damage. In other words, it is impossible to simultaneously obtain the "cold processing" characteristics of femtosecond lasers and the "deep strengthening" effect of nanosecond lasers. Therefore, developing a novel laser shock peening method that can effectively decouple the thermal and mechanical effects during laser energy injection, achieving non-thermally dominated and deeply controllable shock wave stress field modulation, is a pressing technical challenge in this field. Summary of the Invention

[0008] The purpose of this invention is to address the aforementioned problems by providing a digital photonic forging method using high-repetition-rate ultrafast lasers. Without introducing significant thermal effects, this method achieves efficient and deep transmission and accumulation of shock wave energy within the material through precise design and control of laser pulses in the time domain, thereby obtaining a reinforced layer with excellent surface quality and deep compressive strain.

[0009] The technical solution of the present invention is as follows: A digital photonic forging method using high-repetition-rate ultrafast lasers includes the following steps: An ultrafast laser system is used to generate ultrafast laser pulse sequences, with the pulse width of the ultrafast laser being less than 10 ps; Each main pulse is digitally time-domain modulated to include at least three repetition frequency components, wherein: The first-level repetition frequency is used to control processing efficiency and adapt to the needs of high-speed scanning processing. The second-level repetition frequency is used to form a thermal relaxation window between the envelopes of adjacent sub-pulses to suppress heat accumulation and avoid undesirable thermally induced phase transitions or material damage. The third repetition frequency is used to make the sub-pulse interval shorter than the decay time of the laser-induced shock wave in the material, so as to achieve nonlinear superposition and enhancement of stress waves; The modulated pulse sequence is focused onto the material surface to induce localized plasma and excite shock waves. A modified layer is formed by the nonlinear superposition of stress waves from sub-pulses, and the modified layer is located on the material surface and subsurface.

[0010] The above methods address the issue of shallow penetration depth in femtosecond lasers by nonlinearly superimposing stress waves from sub-pulse sequences, effectively extending the duration of high-amplitude mechanical stress and penetrating the compressive strain layer deep into the material's subsurface, thus overcoming the bottleneck of insufficient penetration depth from a single ultrashort pulse. Regarding the thermal damage issue of nanosecond lasers, the pulse width is less than 10 ps, ​​the energy deposition time is much shorter than the thermal diffusion time, and the second-order repetition rate introduces a thermal relaxation window between adjacent envelopes, effectively suppressing heat accumulation and preventing material melting, ablation, and the formation of heat-affected zones. Addressing the complexity and difficulty in controlling composite schemes, a single laser system and digital programming can achieve synergistic effects across multiple effects, eliminating the need for two independent laser sources, resulting in a simple structure and high controllability.

[0011] Furthermore, the third-level repetition frequency is set to 1 GHz to 100 GHz, and its sub-pulse interval is configured to be shorter than the characteristic decay time of the shock wave in the material, thereby ensuring that the energy of the subsequent sub-pulse can be injected before the transient high-pressure environment created by the preceding pulse is fully relaxed, driving the shock wave pressure to achieve effective nonlinear superposition and enhancement.

[0012] The above method ensures that subsequent sub-pulses are injected before the transient high-pressure environment induced by the preceding pulse has fully relaxed, driving the shock wave pressure to achieve effective nonlinear superposition and enhancement. The 1-100 GHz frequency band matches the ultrafast relaxation processes (picosecond to nanosecond range) of most solid materials (metals, semiconductors, ceramics), exhibiting broad material applicability. Compared to low repetition rate (kHz-MHz) schemes, this frequency band allows the pulse interval to enter the sub-nanosecond to picosecond range, a key threshold condition for triggering the stress wave accumulation effect.

[0013] Furthermore, each main pulse cycle corresponding to the second-level repetition frequency contains a sub-pulse envelope consisting of multiple sub-pulses. The sub-pulse envelope has a duty cycle, and the thermal relaxation window between adjacent sub-pulse envelopes is configured to suppress thermal accumulation, provide the material with the necessary thermal diffusion time, and effectively suppress the cumulative thermal effect that may be caused by high-frequency energy injection.

[0014] By actively introducing a thermal relaxation window through the above method, the necessary thermal diffusion time is provided to the material, effectively suppressing the cumulative thermal effects that may be caused by high-frequency energy injection, and avoiding undesirable thermally induced phase transitions (such as crystallization of amorphous materials) or surface damage. By adjusting the duty cycle, the balance between mechanical strengthening effect and thermal management can be independently optimized, achieving impact hardening dominated by "cold working". This design makes this method applicable to thermally sensitive material systems (such as amorphous semiconductors, high-temperature alloys, and tempered precision parts), solving the problem that traditional nanosecond lasers are difficult to apply.

[0015] Furthermore, the number of sub-pulses within each main pulse is adjustable, maintaining a constant output power. The impact hardening effect can be adjusted by controlling the number of sub-pulses, and there is an optimization pulse number to achieve the best balance between efficiency and effect.

[0016] Experiments have shown that, under a fixed single-pulse energy, there exists an optimal number of sub-pulses that yields the best hardening effect (lowest roughness, most significant densification, and greatest improvement in photoelectric properties). When the number of sub-pulses is too high, the material enters the work-hardening saturation stage, the energy threshold required for further compression increases, and some indicators no longer improve or even decline. This discovery reveals the competitive mechanism between "optical pressure saturation" and "material dynamic response relaxation," providing a quantitative basis for optimizing process parameters. By controlling the number of sub-pulses, an optimal balance between efficiency and effect can be achieved, avoiding ineffective energy injection and reducing energy consumption and thermal risks.

[0017] Furthermore, the first-stage repetition frequency is 1 kHz to 10 MHz and is matched with the scanning speed, so that the impact area of ​​adjacent main pulses can be continuously covered, meeting different efficiency requirements from precision local processing to high-speed large-area processing.

[0018] Using the above method, a master pulse repetition frequency of 1 kHz-10 MHz supports different efficiency requirements from precision local processing to high-speed large-area machining, significantly improving processing throughput. Matching the scanning speed ensures continuous coverage of the impact area of ​​adjacent master pulses, achieving a uniform hardening effect and avoiding striped unevenness or missed processing. Compared with traditional femtosecond lasers (≤1 kHz), the processing efficiency can be improved by more than three orders of magnitude, showing significant potential for industrial application.

[0019] Furthermore, the digital time-domain modulation also includes independently programmed control of at least one of the following parameters: pulse energy, sub-pulse envelope width, and duty cycle of the second-level repetition frequency; the parameters are matched and adjusted according to the dynamic mechanical properties of the target material to achieve customized processing.

[0020] The above method achieves fully digital and predictable precision control of the laser shock peening process, transforming core physical parameters into programmable laser time-domain parameters. Optimal parameter combinations can be derived in reverse engineering based on the intrinsic properties of the target material, such as density, sound velocity, and shock wave attenuation constant, enabling customized processing tailored to specific needs and significantly improving the process window width and consistency of results. Compared to traditional trial-and-error methods, this method drastically shortens the process development cycle and reduces R&D costs.

[0021] Furthermore, by controlling the beam scanning path or wavefront shaping, point-like, line-like, or area-like illumination patterns can be achieved to selectively or regionally harden the material surface, wherein: Point illumination is used for precision enhancement of micro-areas; Linear lighting is used for weld or edge reinforcement; Area lighting is used for uniform treatment of large areas.

[0022] Using the methods described above, point lighting is suitable for precision reinforcement of micro-areas (such as localized hardening of microelectromechanical systems and micro / nano devices). Line lighting is suitable for strengthening welds, edges, or curved contours (such as leading edges of aerospace blades and mold edges). Area lighting is suitable for large-area uniform processing (such as semiconductor wafers and optical windows). A single system can adapt to complex geometries and different application scenarios without changing equipment or switching processes, greatly improving process flexibility and equipment utilization.

[0023] Furthermore, the modified layer has at least one of the following characteristics: Atomic-level smooth surfaces, where roughness corresponds to material smoothness; The compressive strain layer has high hardness and high residual compressive stress; Optical bandgap modulation corresponds to the optimization of the optical bandgap; An increase in refractive index corresponds to a densification of the material. The amorphous structure was maintained, and no transformation to the crystalline phase occurred.

[0024] With the above settings, a surface roughness of <0.5 nm is achieved, resulting in an atomically smooth surface, superior to traditional laser shock stabilization (typically submicron to micron level), allowing direct application in optical or precision interface applications without subsequent polishing. A compressive strain layer introduces high-amplitude residual compressive stress, significantly improving the material's fatigue resistance, wear resistance, and stress corrosion resistance. A blue shift in the optical bandgap and an increase in refractive index indicate material densification and a reduction in defect states, allowing direct modulation of optical properties (such as antireflection and waveguide applications), achieving a synergistic improvement in both mechanical and optical properties. Amorphous structure preservation demonstrates the "cold" nature of this method, preventing unwanted crystallization, which is crucial for maintaining the isotropy and superior properties of amorphous materials.

[0025] This application also includes a surface-modified material having a densified modified layer on its surface prepared by a digital photonic forging method using a high repetition rate ultrafast laser. The modified layer maintains an amorphous structure, and the densification is manifested in a decrease in the physical thickness of the material while an increase in its optical refractive index.

[0026] A novel surface-modified material is provided, whose modified layer combines high hardness, high residual compressive stress, atomically smooth surface, and tunable optical properties, making it suitable for optoelectronic devices, semiconductor devices, and high-performance mechanical components. The preservation of the amorphous structure ensures the material's isotropy and grain boundary-free characteristics, avoiding performance degradation caused by crystallization (such as increased leakage current and increased brittleness).

[0027] This application also includes a laser processing system for implementing a digital photonic forging method using high-repetition-rate ultrafast lasers, comprising: Ultrafast laser source, used to output laser pulses with a pulse width of less than 10 ps; The programmable time-domain pulse shaping module is used to digitally modulate each main pulse with a three-level repetition frequency to generate a composite pulse train with the number of sub-pulses, the envelope width of the sub-pulses, and the duty cycle. The beam focusing and scanning module is used to focus the modulated pulse sequence onto the material surface and control the scanning path to achieve various illumination methods such as point, line and surface; The digital controller is configured to: reverse-set the repetition frequency of the first, second, and third stages, the duty cycle of the sub-pulse envelope, and the number of sub-pulses based on the dynamic mechanical properties of the target material, and coordinate the control of the ultrafast laser source, the time-domain pulse shaping module, and the beam focusing and scanning module.

[0028] A complete and industrially scalable hardware system solution is provided, capable of implementing the aforementioned digital photonic forging method. The digital controller enables coordinated control of the laser source, pulse shaping, and scanning, avoiding the problems of multi-source asynchrony and spatiotemporal matching difficulties in existing composite laser systems. The reverse parameter setting function gives the system intelligent and adaptive capabilities, automatically generating optimal process parameters based on the input material type and desired modification effect, lowering the operational threshold and making it suitable for industrial mass production.

[0029] Compared with existing technologies, the advantages of this invention are: 1. Breaking through the traditional contradiction between "cold processing" and "deep strengthening": This application significantly extends the equivalent action time of high-amplitude mechanical stress within the material by nonlinear superposition of stress waves induced by high-repetition-rate subpulse sequences, without relying on thermal mechanisms (pulse width <10 ps, ​​energy deposition time much shorter than thermal diffusion time). This allows for the compression strain layer to penetrate deep into the subsurface of the material while completely avoiding or greatly suppressing material melting, crystallization, and the formation of heat-affected zones, achieving a deep strengthening effect that is difficult to achieve with traditional femtosecond lasers, and solving the fundamental bottleneck of insufficient depth of single ultrashort pulse action; 2. Achieved "fully digital" and "predictable" precise control of the laser shock peening process: The core innovation of this application lies in transforming the core physical parameters of shock peening (shock wave pressure, action time, energy deposition rhythm) into programmable laser time-domain parameters (three-stage pulse repetition frequency, sub-pulse envelope, energy). By establishing a mapping relationship between material properties (density, sound velocity, shock wave attenuation constant) and these digital parameters, the strengthening effect can be designed forward and controlled backward, realizing a leap from "experience-based trial and error" to "model prediction and digital setting," significantly improving the width of the process window and the consistency and repeatability of the processing results. 3. It creates multi-dimensional, customizable material property modification capabilities: Thanks to the aforementioned digital control capabilities, this application can "program" the material surface's performance according to end-application requirements. It can not only achieve conventional hardness enhancement and residual compressive stress introduction, but also simultaneously and independently control other key properties. (1) Surface integrity: By optimizing the uniformity of the shock wave, an atomically smooth surface (roughness <0.5nm) can be obtained, and surface defect states can be effectively passivated; (2) Optical function: Through densification and defect engineering, the refractive index, extinction coefficient and optical band gap of the material can be actively modulated, providing surface modification means for integrated photonic devices; (3) Structural state: Metastable phases or nanocrystalline structures can be induced under non-equilibrium conditions, realizing synergistic design of structure and properties. This "one-stop" multifunctional modification capability greatly expands the application scope of laser shock technology; 4. Excellent process flexibility and versatility: The method is compatible with various processing modes such as point, line, and surface, enabling both localized ultra-precision strengthening of key parts of micro and nano devices and efficient and uniform processing of large areas or complex curved surfaces. Furthermore, its non-thermal dominant mechanism makes it suitable for thermally sensitive material systems, such as amorphous semiconductors, high-temperature alloys, and tempered precision parts, solving the problem that traditional heat-assisted or high-energy nanosecond lasers are difficult to apply to these materials. 5. Improved processing efficiency and economic benefits: The high main pulse repetition frequency (up to the MHz level) ensures processing efficiency. More importantly, through the nonlinear cumulative effect of the sub-pulse sequence, the required enhancement effect can be achieved at a lower single main pulse energy. This reduces the requirements for laser peak power and energy, which helps to reduce system cost, energy consumption and maintenance complexity, while reducing the scrap rate caused by excessive energy, and has significant potential for industrial application.

[0030] In summary, this application fundamentally reconciles the opposing requirements of "cold" and "deep" in laser shock peening through an innovative laser temporal energy arrangement strategy, providing a novel, effective, and highly controllable solution for surface modification of high-performance materials and precision components. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the principle of three-level pulse repetition frequency modulation in GHz mode.

[0032] Figure 2 This is a comparison of the simulated time-domain distribution of impact pressure in a GHz mode with the time-domain distribution in a kHz laser mode.

[0033] Figure 3 This is an XRD spectrum of a sample processed with different numbers of sub-pulses in a GHz mode.

[0034] Figure 4 This is a graph showing the surface roughness variation trend of a sample processed with different numbers of sub-pulses in a GHz mode.

[0035] Figure 5 This is a graph showing the thickness variation trend of a sample processed with different numbers of sub-pulses in a GHz mode.

[0036] Figure 6 This is a graph showing the trend of refractive index change of a sample processed with different numbers of sub-pulses in a GHz mode.

[0037] Figure 7 This is a high-resolution XPS spectrum of the Ga 3d level of a sample processed with different numbers of sub-pulses in a GHz mode.

[0038] Figure 8This is the UV-Vis absorption spectrum of the Ga 3d level of a sample processed with different numbers of sub-pulses in a GHz mode.

[0039] Figure 9 The image shows the It curves of the Ga 3d energy levels of samples processed with different numbers of sub-pulses in a GHz mode under 254 nm illumination. Detailed Implementation

[0040] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0041] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0042] To address the inherent contradiction in existing laser shock peening technologies—the difficulty in simultaneously achieving "cold processing" capabilities and "deep strengthening" effects—specifically, the insufficient shock wave energy deposition due to the short action time and low repetition rate of femtosecond lasers, and the surface damage easily caused by the significant thermal effects of nanosecond lasers, please refer to [link to relevant documentation]. Figure 1-9 A digital photonic forging method using high-repetition-rate ultrafast lasers aims to achieve efficient and deep-level transfer and accumulation of shock wave energy within the material through precise temporal design and control of laser pulses without introducing significant thermal effects. This results in a reinforced layer with excellent surface quality and deep compressive strain. The specific steps include: An ultrafast laser system is used to output pulse sequences, each of which consists of multiple sub-pulses; the output laser pulse width of the ultrafast laser system is less than 10 ps.

[0043] The laser pulse sequence is digitally time-domain structure modulated to generate a composite pulse train containing at least two levels, wherein the at least two levels include: a) First-level repetition frequency, used to control processing efficiency; b) At least one higher-level repetition frequency, which is higher than the first-level repetition frequency, for injecting energy in the form of multiple sub-pulses within the period of the first-level repetition frequency. The modulated composite pulse train is focused onto the material surface, and a modified layer is induced to form on the material surface and subsurface through the photonic forging effect. The photonic forging effect refers to the laser-induced local plasma, which generates a shock wave through restricted expansion, and the shock wave exerts a mechanical effect on the material.

[0044] Among these, at least one of the higher-level repetition frequencies is set to between 1 GHz and 100 GHz, and its pulse interval is configured to be shorter than the relaxation time of the key excited state induced by laser in the material, so as to achieve nonlinear accumulation of physical effects.

[0045] Digital time-domain structure modulation is a three-level repetition rate control architecture, such as Figure 1 and Figure 2 As shown, it specifically includes: First-level repetition frequency: As the first-level repetition frequency, ranging from 1 kHz to 10 MHz, it is used to match the scanning speed and achieve efficient processing over a large area; Secondary repetition frequency: As the first higher-level repetition frequency, it is defined as the repetition frequency of the sub-pulse envelope, which contains multiple sub-pulses within its period. The duty cycle of the secondary repetition frequency is configured to form a thermal relaxation window between adjacent sub-pulse envelopes to suppress heat accumulation. Third-order repetition frequency: As a second, higher-order repetition frequency, it is defined as the repetition frequency of a single subpulse within the subpulse envelope, ranging from 1 GHz to 100 GHz, and is used to match the ultrafast relaxation process of materials and drive the nonlinear superposition of shock wave stress.

[0046] Digital temporal structure modulation also includes independent programmable control of the following parameters, which are matched and adjusted according to the dynamic mechanical properties of the material; the parameters include at least one of pulse energy, pulse width, sub-pulse repetition frequency, and sub-pulse envelope width.

[0047] This method uses beam scanning or shaping to achieve point, line, or surface illumination, and selectively or regionally hardens the material surface.

[0048] The effects of shock waves on materials include: removing surface dangling bonds, achieving material densification, inducing controllable phase transitions, and modulating at least one of the following optical properties: refractive index, extinction coefficient, absorption spectrum, transmittance, and optical band gap. The hardening process is carried out in a confined layer or a specific atmosphere.

[0049] This application also includes a surface modification material, the surface modification layer of which is prepared by a digital photonic forging method using a high repetition rate ultrafast laser; the device prepared by the surface modification material is an optoelectronic device, a semiconductor device, or a mechanical component.

[0050] This application also includes a laser processing system for implementing a digital photonic forging method using a high-repetition-rate ultrafast laser, comprising: an ultrafast laser source, a time-domain pulse shaping module, a beam focusing and scanning module, and a digital controller for coordinating the control of the above modules.

[0051] The core concept of this application lies in breaking through the limitations of the traditional single-pulse mode and creatively elevating the interaction process between laser and material from an "energy pulse" to a "digitally programmable mechanical waveform." Specifically, by digitally segmenting and recombining each main pulse in the time domain, a sub-pulse sequence with independently and precisely controllable parameters is constructed. The design principle of this sequence is to ensure that the arrival time of subsequent sub-pulses on the material surface precisely matches the dynamic phase of the shock wave induced by the preceding sub-pulse propagating and attenuating within the material. When the time interval between sub-pulses is set to be shorter than the attenuation time constant of the shock wave in the material, nonlinear coherent superposition of shock wave stress can be achieved within the material. This mechanism essentially constructs a "photonic forging" process: it retains the instantaneous high pressure and non-thermal interaction characteristics brought by the high peak power of ultrafast lasers (typical pulse width less than 100 ps), effectively avoiding thermal accumulation damage; and through the stress accumulation of the sub-pulse sequence, it effectively extends the action time of high-amplitude pressure waves, thereby efficiently coupling energy to deeper layers of the material and solving the fundamental bottleneck of shallow single-pulse interaction depth.

[0052] To achieve the above concept, the technical solution of this application deeply integrates the effects of "digital programming" and "photon forging," and its core is embodied in a three-level repetition rate collaborative control architecture: Digital multi-level pulse sequence modulation: An ultrafast laser system with high-precision time-domain shaping capability is employed, and the output pulses are digitally reconstructed at the following levels: Level 1 Repetition Rate Control (Efficiency Control Level): Sets the main pulse repetition frequency of the system, typically ranging from 1 kHz to 10 MHz. This level parameter is directly related to the scanning processing speed and is designed to meet different efficiency requirements, from precision local processing to high-speed large-area processing.

[0053] Second-level frequency repetition rate control (thermal management level): Within each main pulse cycle, a pulse envelope consisting of multiple sub-pulses is generated through programming, and the repetition frequency of this envelope is defined. The key innovation lies in the ability to actively introduce a thermal relaxation window between adjacent envelopes by controlling the duty cycle of this envelope. This window provides the material with the necessary thermal diffusion time, thereby effectively suppressing the cumulative thermal effects that may be caused by high-frequency energy injection and avoiding undesirable thermally induced phase transitions or material damage.

[0054] Third-level frequency repetition rate modulation (nonlinear superposition driving stage): Within the envelope of the second-level sub-pulse, a third-level sub-pulse sequence with a higher frequency is further set, with a typical repetition frequency range of 1 GHz to 100 GHz. The core function of this frequency is to match the ultrafast electronic or lattice relaxation processes in the material. By setting the interval of this level of sub-pulse to be shorter than the relaxation time of the key excited states in the material (such as shock waves, plasma), it is ensured that the energy of the subsequent sub-pulse can be injected before the transient high-pressure environment created by the preceding pulse is fully relaxed, thereby driving the shock wave pressure to achieve effective nonlinear superposition and enhancement.

[0055] This cascaded control mode of "main pulse repetition rate - sub-pulse envelope - sub-pulse within the envelope" enables precise programming of the spatiotemporal dynamics of laser-material interaction, providing a digital tool for the dynamic mechanical response of matching materials.

[0056] Photonic forging and the nonlinear accumulation mechanism of stress waves: The modulated pulse sequence is focused onto the material surface, and its high peak power density ionizes the surface material in a very short time (pulse width typically less than 10 ps), forming a localized high-pressure plasma. This plasma expands violently in a confined environment (such as air or a confinement layer), emitting high-intensity shock waves into the material. The key physical mechanism of this invention lies in: through digital programming, the time interval between the second and third stage sub-pulses is made shorter than the characteristic decay time of the shock wave in the material. Before the stress wave generated by the preceding sub-pulse has completely dissipated, the subsequent sub-pulse has arrived and injected new energy, thereby achieving nonlinear superposition and accumulation of stress waves within the material. This effect is equivalent to significantly extending the duration of high-amplitude mechanical stress, overcoming the limitations of shallow depth of single-pulse or low-repetition-rate pulses, and driving deep plastic deformation of the material. This method has high process adaptability and flexibility. Parameter matching: Based on the intrinsic properties of the target material (such as density, sound velocity, and thermal conductivity) and the desired modification targets (hardening layer depth, surface roughness, and optical properties), the optimal combination of parameters such as repetition frequency, pulse energy, and envelope shape at each level can be derived in reverse and set digitally to achieve true "tailor-made" customized processing.

[0057] Flexible processing: By controlling the beam scanning strategy or wavefront shaping, it can easily achieve various processing modes such as point (micro-area precision strengthening), line (contour or weld strengthening) or surface (uniform processing of the whole area), adapting to complex geometries and different application scenarios.

[0058] Multi-dimensional process adaptation and customized processing: The digital characteristics of this application give it a high degree of process adaptability: Parameter matching: Based on the physical properties of the target material (such as density, sound velocity, hardness) and the desired modification effect (such as hardened layer depth, residual stress amplitude), the optimal pulse energy, width, and parameters of each sub-pulse can be reverse-engineered and programmed to achieve customized processing.

[0059] Flexible approach: By controlling the scanning path and shaping of the beam, various lighting and processing methods can be easily achieved, such as point (precise local strengthening), line (weld or edge strengthening), or surface (large area uniform processing), to meet the needs of different structures and application scenarios.

[0060] Overall performance improvement effect: Through the aforementioned digital photonic forging process, a stable compressive strain layer can be formed on the surface and subsurface of the material, simultaneously triggering a synergistic improvement in multiple properties: Surface quality optimization: The uniform plastic compression effect of shock waves can significantly reduce surface roughness and effectively eliminate or passivate surface dangling bonds.

[0061] Structural densification and phase transition control: High-amplitude shock waves promote atomic rearrangement, increase material density, and can induce beneficial phase transitions under non-thermal or controlled thermal conditions.

[0062] Optical performance modulation: Densification and defect reduction can directly change the optical constants of materials, enabling active control over refractive index, extinction coefficient, absorption spectrum, transmittance, and optical band gap.

[0063] Mechanical property enhancement: The final formation of a reinforced layer with high hardness and high residual compressive stress significantly improves the material's wear resistance, fatigue resistance and other mechanical properties.

[0064] In another specific embodiment, a surface impact hardening method based on high repetition rate ultrafast laser is disclosed, taking amorphous gallium oxide (a-Ga2O3) thin film as an example, including the following steps: 1. Sample preparation: Amorphous gallium oxide thin films were prepared on silicon substrates using magnetron sputtering. The deposited thin film samples were then fixed on a three-dimensional precision moving platform.

[0065] 2. Laser Parameter Settings: The high-repetition-rate laser system and its control unit were turned on, and the laser parameters were set as follows: center wavelength 1030 nm, main pulse repetition frequency 200 kHz, and single pulse width ~250 fs. Through control system programming, each main pulse was modulated into a sequence containing a specific number of sub-pulses. In this embodiment, the number of sub-pulses was set to 15, 30, and 90 for comparative experiments, maintaining a constant output power. The repetition frequency of the sub-pulses was set in the GHz range to ensure that the pulse interval was shorter than the decay time of the shock wave in the material.

[0066] 3. Processing Procedure: The sample area to be processed is moved to the laser focusing spot using the control system. The scanning path is set to S-shape, the scanning speed is 600 mm / s, and the scan line spacing is 30 μm. With ambient air as a constraint layer, the laser scanning program is initiated to perform regional impact treatment on the film surface.

[0067] 4. Characterization and Results Analysis: Structural analysis: X-ray diffraction (XRD) spectra showed that no crystallization peaks were observed in any of the laser-treated samples (see [link]). Figure 3 This indicates that, under the parameters set in this invention, the interaction time between the laser and the material is extremely short, and the energy is injected in the form of shock wave mechanical energy rather than being converted into lattice thermal energy, thereby effectively suppressing the transformation from amorphous to crystalline phase and maintaining the amorphous structural characteristics of the material.

[0068] Surface morphology and densification: Atomic force microscopy (AFM) measurements showed that the surface roughness of the sample was significantly reduced after treatment with optimized parameters (e.g., sub-pulse number 30) (see [link]). Figure 4 This is because the superimposed shock waves induced by the high-repetition-rate subpulse sequence exert a uniform and continuous plastic compression on the material surface, smoothing out the surface micro-undulations. Spectroscopic ellipsometry measurements show that the physical thickness of the treated film decreases, while the optical refractive index increases (see...). Figure 5 , Figure 6 These factors together constitute direct evidence of material densification. The reduction in thickness is a manifestation of material compression in the normal direction, while the increase in refractive index, according to the Lorentz-Lorenz relation, is directly related to the increase in the material's bulk density or atomic packing density, which is the result of shock wave-induced atomic-scale rearrangement and bond compression.

[0069] Evolution of chemical bonding states: X-ray photoelectron spectroscopy (XPS) analysis showed a positive shift of approximately 0.12 eV in the binding energy of the Ga 3d orbitals in the treated sample (see [link to article]). Figure 7 The shift of binding energy towards higher energies indicates that under the high-pressure transient environment generated by the shock wave, the Ga-O bond length may be compressed, the bond strength increases, and the covalent component is enhanced, thereby changing the effective potential of the Ga atom.

[0070] Optical bandgap modulation: UV-Vis absorption spectroscopy shows a blue shift in the absorption edge of the processed sample (see...). Figure 8This corresponds to the broadening of the optical band gap. This phenomenon is directly related to the aforementioned densification and increased bond strength. The band gap in amorphous semiconductors is closely related to the short-range order and defect state density of the atomic network. The densification and bond strengthening caused by shock hardening effectively reduce the band tail states caused by dangling bonds and coordination defects in the amorphous network, resulting in a widening of the band gap between the valence band top and the conduction band bottom, which macroscopically manifests as a blue shift of the absorption edge.

[0071] Mechanism for improving photoelectric performance: The treated thin film was fabricated into a solar-blind ultraviolet detector device and tested under 254 nm illumination. Key performance indicators such as photoresponse current and responsivity were significantly improved compared to the untreated sample (see [link to product description]). Figure 9 The performance improvement stems from multiple optimizations in the aforementioned microstructure: 1) Improved carrier transport: Densification and surface smoothing reduce scattering centers caused by grain boundaries (for polycrystalline materials) or structural undulations, potentially increasing carrier mobility. 2) Reduced defect states: Suppression of band-tailed states lowers the density of nonradiative recombination centers, extending the lifetime of photogenerated carriers and improving collection efficiency. 3) Optimized interface contacts: Smoother, denser surfaces facilitate the formation of superior metal-semiconductor ohmic or Schottky contacts, reducing contact resistance. These synergistic effects induced by shock hardening collectively lead to enhanced device photoelectric response.

[0072] Nonlinear accumulation and saturation effect: This embodiment reveals the nonlinear relationship between the impact hardening effect and the pulse sequence parameters by setting different numbers of sub-pulses (15, 30, 90). Experimental results show that more sub-pulses do not necessarily lead to better results. Under the condition of 90 sub-pulses, the improvement in surface finish, densification degree, and final device performance is lower than or close to that under the optimized condition of 30 sub-pulses, and some indicators even show a downward trend.

[0073] This phenomenon stems from a competitive mechanism between "optical pressure saturation" and "material dynamic response relaxation." When a sub-pulse sequence acts on a material, the transient high-pressure shock wave generated by each pulse drives the atomic network to compress and rearrange. In the early stages of the sequence (e.g., when the number of sub-pulses increases from 15 to 30), the shock waves from subsequent pulses can arrive before the lattice strain (or amorphous network distortion) induced by the previous pulse has fully relaxed, thus achieving effective stress superposition, producing cumulative plastic deformation, and the effect continuously intensifies.

[0074] However, when the number of sub-pulses becomes excessive (e.g., increasing to 90), the material gradually approaches a "dynamic equilibrium state" under continuous, high-frequency impact loading. On one hand, the energy injected initially begins to dissipate, producing a weak but not negligible local temperature rise. Although far from reaching the crystallization threshold, this may slightly reduce the material's yield strength, decreasing its plastic response to subsequent impacts. On the other hand, after undergoing a certain degree of compression densification, the material's atomic packing density and bonding strength have significantly increased, and the energy threshold required for further compression also rises accordingly. At this point, under a fixed single-pulse energy, the energy injected by subsequent pulses is insufficient to drive further plastic deformation of the same magnitude, thus entering the work-hardening saturation stage. Furthermore, excessive pulses may also introduce a very slight fatigue effect on the surface or induce nanoscale re-fluctuations. Therefore, although the 90-pulse condition still maintains the amorphous state and is superior to the untreated sample, its overall modification effect no longer increases linearly with the number of pulses, and may even be slightly diminished due to the aforementioned saturation and weak thermal effects.

[0075] This observation highlights the core advantage and necessity of the "digital programming control" of the present invention: by precisely controlling the parameter of the "time dimension" of the number of sub-pulses, the optimal energy injection window for a specific material can be actively sought and locked, maximizing the nonlinear cumulative benefit of the shock wave while avoiding saturation and potential thermal negative effects, thereby achieving the optimal balance between efficiency and effectiveness.

[0076] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed and specific, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

Claims

1. A digital photonic forging method using high repetition rate ultrafast laser, characterized in that, Includes the following steps: An ultrafast laser system is used to generate ultrafast laser pulse sequences, with the pulse width of the ultrafast laser being less than 10 ps; Each main pulse is digitally time-domain modulated to include at least three repetition frequency components, wherein: The first-level repetition frequency is used to control processing efficiency and adapt to the needs of high-speed scanning processing. The second-level repetition frequency is used to form a thermal relaxation window between the envelopes of adjacent sub-pulses to suppress heat accumulation and avoid undesirable thermally induced phase transitions or material damage. The third repetition frequency is used to make the sub-pulse interval shorter than the decay time of the laser-induced shock wave in the material, so as to achieve nonlinear superposition and enhancement of stress waves; The modulated pulse sequence is focused onto the material surface to induce localized plasma and excite shock waves. A modified layer is formed by the nonlinear superposition of stress waves from sub-pulses, and the modified layer is located on the material surface and subsurface.

2. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, The third-level repetition frequency is set to 1 GHz to 100 GHz, and its sub-pulse interval is configured to be shorter than the characteristic decay time of the shock wave in the material, thereby ensuring that the energy of the subsequent sub-pulse can be injected before the transient high-pressure environment created by the preceding pulse is fully relaxed, driving the shock wave pressure to achieve effective nonlinear superposition and enhancement.

3. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, Each main pulse cycle corresponding to the second-level repetition frequency contains a sub-pulse envelope consisting of multiple sub-pulses. The sub-pulse envelope has a duty cycle, and the thermal relaxation window between adjacent sub-pulse envelopes is configured to suppress thermal accumulation, provide the material with the necessary thermal diffusion time, and effectively suppress the cumulative thermal effect that may be caused by high-frequency energy injection.

4. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, The number of sub-pulses within each main pulse is adjustable, maintaining a constant output power. The impact hardening effect can be adjusted by controlling the number of sub-pulses, and there is an optimization pulse number to achieve the best balance between efficiency and effect.

5. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, The first-stage repetition frequency is 1 kHz to 10 MHz and is matched with the scanning speed, so that the impact area of ​​adjacent main pulses can be continuously covered, meeting different efficiency requirements from precision local processing to high-speed large-area processing.

6. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, The digital time-domain modulation also includes independently programmed control of at least one of the following parameters: pulse energy, sub-pulse envelope width, and duty cycle of the second-level repetition frequency; the parameters are matched and adjusted according to the dynamic mechanical properties of the target material to achieve customized processing.

7. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, By controlling the beam scanning path or wavefront shaping, point, line, or area illumination patterns can be achieved, enabling selective or regional hardening of material surfaces. Point illumination is used for precision enhancement of micro-areas; Linear lighting is used for weld or edge reinforcement; Area lighting is used for uniform treatment of large areas.

8. The digital photonic forging method using high repetition rate ultrafast laser according to claim 1, characterized in that, The modified layer has at least one of the following characteristics: Atomic-level smooth surfaces, where roughness corresponds to material smoothness; The compressive strain layer has high hardness and high residual compressive stress; Optical bandgap modulation corresponds to the optimization of the optical bandgap; An increase in refractive index corresponds to a densification of the material. The amorphous structure was maintained, and no transformation to the crystalline phase occurred.

9. A surface-modified material, characterized in that, Its surface has a densification modification layer prepared by a digital photonic forging method of high repetition rate ultrafast laser according to any one of claims 1-8, wherein the modification layer maintains an amorphous structure, and the densification is manifested by a decrease in the physical thickness of the material and an increase in the optical refractive index.

10. A laser processing system for implementing the digital photonic forging method using a high-repetition-rate ultrafast laser as described in any one of claims 1-8, characterized in that, include: Ultrafast laser source, used to output laser pulses with a pulse width of less than 10 ps; The programmable time-domain pulse shaping module is used to digitally modulate each main pulse with a three-level repetition frequency to generate a composite pulse train with the number of sub-pulses, the envelope width of the sub-pulses, and the duty cycle. The beam focusing and scanning module is used to focus the modulated pulse sequence onto the material surface and control the scanning path to achieve various illumination methods such as point, line and surface; The digital controller is configured to: reverse-set the repetition frequency of the first, second, and third stages, the duty cycle of the sub-pulse envelope, and the number of sub-pulses based on the dynamic mechanical properties of the target material, and coordinate the control of the ultrafast laser source, the time-domain pulse shaping module, and the beam focusing and scanning module.

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