Plasma-modulated quasi-phase-matched nonlinear frequency conversion method and system

Abstract

The application discloses a plasma modulation quasi-phase matching nonlinear frequency conversion method and system, and belongs to the technical field of nonlinear optics and plasma physics crossing. The application actively constructs a plasma grating with dynamic regulation of period, modulation depth and spatial configuration in a nonlinear medium through accurate interference of multiple coherent pump lasers, realizes compensation of quasi-phase matching conditions required by different nonlinear frequency conversion processes on demand, and thus can be adaptively optimized according to different target frequency conversion processes. Meanwhile, the gas medium is converted into a highly controllable nonlinear optical platform, which fundamentally improves the efficiency, flexibility and functional richness of frequency conversion, and promotes the application of nonlinear optical technology in a wider medium system.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of nonlinear optics and plasma physics, and specifically relates to a nonlinear frequency conversion method and system for plasma modulation quasi-phase matching. Background Technology

[0002] Nonlinear frequency conversion technology is a core technology for expanding laser wavelengths and obtaining novel frequency-coherent optical radiation. It is widely used in basic scientific research and modern photonics, demonstrating significant application value and technological importance. The key to achieving efficient energy conversion in mainstream nonlinear frequency conversion processes such as second harmonic (SH), sum-frequency, and difference-frequency conversion lies in meeting stringent phase-matching conditions. This ensures that the light waves participating in the nonlinear interaction maintain a stable phase relationship, achieving directional and continuous efficient transfer of optical field energy.

[0003] Currently, the mainstream approach in the industry relies on bulk nonlinear crystals for phase matching, primarily utilizing the inherent birefringence of the crystal or artificially fabricating quasi-phase-matching crystals with periodic polarization structures to achieve phase matching control. However, traditional birefringence phase matching and conventional periodic polarization quasi-phase matching methods have fixed control forms, complex fabrication processes, and poor phase matching flexibility, making it difficult to achieve dynamic and precise phase matching control. This fails to meet the flexible control requirements of multi-scenario and multi-frequency band nonlinear frequency conversion, greatly limiting the further application of nonlinear frequency conversion technology in cutting-edge fields such as the development of wide-band, tunable laser sources. Therefore, there is an urgent need to develop new quasi-phase-matching control methods to optimize the overall performance of nonlinear frequency conversion.

[0004] Furthermore, these solid-state nonlinear media possess inherent optical damage thresholds, making them unsuitable for high-peak-power, high-field laser environments, significantly limiting their applications in cutting-edge fields such as strong-field physics and high-power terahertz generation. In particular, for extremely nonlinear processes such as high-harmonic generation, the driving light intensity far exceeds the damage threshold of any solid, and the target wavelength (e.g., 13.5 nm in the extreme ultraviolet region) places even higher demands on phase-matching precision, which traditional crystal solutions are completely inadequate for. Therefore, developing a novel medium and mechanism capable of withstanding extreme strong fields and allowing for dynamic and precise programming of phase matching is crucial for achieving efficient and practical high-harmonic light sources, especially specific-wavelength EUV sources. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned problems by providing a nonlinear frequency conversion method and system for plasma modulation quasi-phase matching. This aims to improve the existing methods for achieving efficient nonlinear frequency conversion in centrosymmetric media such as gases, which lack the ability to dynamically and precisely control the nonlinearly interacting microstructures, resulting in low nonlinear conversion efficiency, poor stability, and the inability to perform adaptive optimization based on different target frequency conversion processes (such as difference frequency and sum frequency).

[0006] The technical solution adopted in this invention is as follows: a nonlinear frequency conversion method with plasma modulation quasi-phase matching, the method comprising the following steps: Step S100: The pump laser is provided by the laser generation unit, and the pump laser is split and controlled to form at least two coherent sub-pump beams; Step S200: The two subpump beams intersect and interfere in a predetermined region of the nonlinear medium, thereby inducing the formation of a plasma grating with a periodic arrangement structure; the period and modulation depth of the plasma grating are adjustable; Step S300: The laser generation unit provides the detection laser and guides the detection laser into the effective area of ​​the plasma grating, so that the detected fundamental light is converted into light of other frequencies; Step S400: The plasma refractive index is periodically modulated by a plasma grating. The reciprocal lattice vector brought by the plasma grating is used to compensate for the phase mismatch between the fundamental frequency light of the probe laser and the target frequency conversion light, so as to achieve quasi-phase matching. When the reciprocal lattice vector is equal to the difference between the wave vectors of the two frequencies, the fundamental frequency light is converted into the target frequency conversion light with enhanced intensity.

[0007] Furthermore, in step S200, the period of the plasma grating is controlled by adjusting the angle between the two sub-pump beams to meet the quasi-phase matching condition for the frequency-converted light of a specific target.

[0008] Furthermore, in step S200, the modulation depth of the plasma grating is controlled by adjusting the polarization state and intensity of the sub-pump beam.

[0009] Furthermore, in step S300, the probe laser is synchronized or asynchronous with the sub-pump beam in time.

[0010] Furthermore, in step S400, two or more plasma gratings are cascaded along the optical path to form a cascaded enhancement structure, so that the signal intensity of the target frequency-converted light is cumulatively enhanced stepwise by each plasma grating during propagation.

[0011] Furthermore, the target frequency conversion light includes, but is not limited to, frequency-doubled and frequency-differential light.

[0012] Furthermore, the multiple plasma gratings in the cascaded enhancement structure are spatially staggered.

[0013] Furthermore, the nonlinear medium in step S200 is a gaseous medium.

[0014] Furthermore, the present invention also provides a nonlinear frequency conversion system for plasma modulation quasi-phase matching, the system comprising the following structure: The system includes a laser generation unit for providing pump laser and probe laser; a beam splitting and control unit for splitting and controlling the pump laser into at least two coherent sub-pump beams and guiding the at least two sub-pump beams to interfere within a predetermined region in the nonlinear medium to form a plasma grating; a probe light introduction unit for introducing the probe laser into the effective region of the plasma grating; and a signal collection and detection unit for collecting and detecting the target frequency-converted light generated by the plasma grating modulation.

[0015] Furthermore, the beam splitting and control unit includes optical elements and a displacement platform for adjusting the angle, polarization state, intensity, and optical path of the sub-pump beam.

[0016] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. This invention utilizes the precise interference of multiple coherent pump lasers to actively construct a plasma grating in a nonlinear medium, in which the period, modulation depth, and spatial configuration can be dynamically adjusted. This enables on-demand compensation for the quasi-phase matching conditions required for different nonlinear frequency conversion processes, thereby allowing for adaptive optimization based on different target frequency conversion processes.

[0017] 2. This invention transforms the gas medium into a highly controllable nonlinear optical platform, fundamentally improving the efficiency, flexibility, and functionality of frequency conversion, and promoting the application of nonlinear optical technology in a wider range of media systems. Attached Figure Description

[0018] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic diagram of the system architecture of the present invention; Figure 3 The second harmonic (SH) signal generated by the change in the pump light angle of this invention; Figure 4 This is a distribution diagram of the fluorescence intensity of the dual plasma grating of the present invention along the y-axis of the grating direction; Figure 5 This is a schematic diagram of the numerical calculation of SH intensity evolution under quasi-phase matching conditions according to the present invention; Figure 6This is a schematic diagram of the conversion efficiency of SH under the configurations of single-filament pump-probe, single-plasma grating pump-probe coplanar, and double-plasma grating pump-probe coplanar of the present invention.

[0019] Attached image labels: as shown Figure 2 As shown, 1. Pump; 2. Half-wave plate; 3. Glan prism; 4. Probe; 501. First lens; 502. Second lens; 503. Third lens; 6. Filter; 7. Flange; 8. Spectrometer; 9. Computer; 10. Plasma grating; 11. Plasma. Detailed Implementation

[0020] The present invention will now be described in detail with reference to the accompanying drawings.

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0022] The existing technology has the following drawbacks: solid nonlinear media have an inherent optical damage threshold, which cannot be adapted to high peak power and strong field laser environments, limiting their application in cutting-edge fields such as strong field physics and high power terahertz generation; for extreme nonlinear processes such as high-order harmonic generation, the driving light intensity far exceeds the solid damage threshold, and the target wavelength (such as the 13.5 nm extreme ultraviolet region) has extremely high requirements for phase matching accuracy, which traditional crystal solutions cannot meet at all; traditional crystal solutions cannot achieve dynamic and precise programming of phase matching while withstanding extreme strong fields, making it difficult to support the development of efficient and practical high-order harmonic light sources (especially specific wavelengths such as 13.5 nm EUV light sources).

[0023] To address these challenges, existing technological approaches primarily revolve around external field assistance. For example, strong terahertz pulses or high-voltage DC electric fields are introduced; however, these methods often rely on large and complex external field sources, limiting the system's flexibility and integration. Dual-laser induced air plasma technology utilizes non-uniform plasma channels generated by two femtosecond lasers, leveraging their spatial symmetry breaking to produce nonlinear signals. While simplifying the device, this approach offers limited active control over the plasma state, making it difficult to finely optimize signal strength and stability. In recent years, the use of coherent laser interference to form periodic plasma gratings in gas has attracted attention. This structure itself constitutes a natural source of spatial asymmetry, possessing the potential to serve as a dynamically tunable grating for achieving QPM (Quadrature Plasma Mechanism). However, demonstrating specific frequency conversion phenomena (such as SH generation) using this structure still lacks a universally applicable methodological framework and efficient technical solutions in terms of how to achieve dynamic and precise control of key parameters such as the period, modulation depth, and spatial configuration of the plasma grating, how to systematically apply this platform to different nonlinear processes such as sum frequency and difference frequency, and how to effectively extend the nonlinear interaction length and achieve signal accumulation enhancement through the cascading and arrangement design of multiple gratings.

[0024] Therefore, the current development of nonlinear optics in gas media urgently requires an innovative method that can go beyond demonstrating specific phenomena and is dedicated to building a universal, flexible, and efficient dynamic control platform. The ability to flexibly reconstruct nonlinear interaction microstructures in gases on demand, thereby providing customized phase-matching solutions for various nonlinear frequency conversion processes, will lay a key technological foundation for the expansion of nonlinear optics from solid-state crystal systems to a wider range of gaseous and plasma-state media. Therefore, this invention proposes a plasma-modulated quasi-phase-matched nonlinear frequency conversion method. By controlling the interference of multiple pump beams, a periodically arranged plasma grating structure is formed in the nonlinear medium. The periodic modulation of the nonlinear coefficient of the medium is used to achieve QPM (Quasi-Phase Measuring) to compensate for the phase mismatch between the fundamental and harmonic waves (taking SH as an example). Each grating fringe unit generates a nonlinear frequency conversion signal, and the cascading of multiple units achieves a stepwise increase in signal intensity. This method employs a plasma grating pump-probe configuration, with the probe and pump beams strictly synchronized in time. By optimizing the Bragg diffraction angle, the modulation amplitude of the plasma can be effectively enhanced. The device and method proposed in this invention are versatile and cascadeable, providing a new technical approach for efficient and flexible nonlinear frequency conversion.

[0025] Example 1 like Figure 1 As shown, one embodiment of the present invention is a nonlinear frequency conversion method for plasma modulation quasi-phase matching, comprising the following steps: Step S100: The pump laser is provided by the laser generation unit, and the pump laser is split and controlled to form at least two coherent sub-pump beams; Step S200: The two subpump beams intersect and interfere in a predetermined region of the nonlinear medium, thereby inducing the formation of a plasma grating with a periodic arrangement structure; the period and modulation depth of the plasma grating are adjustable; The period of the plasma grating is controlled by adjusting the angle between two of its sub-pump beams. The modulation depth of the plasma grating is controlled by adjusting the polarization state and intensity of the sub-pump beam. Step S300: The laser generation unit provides the detection laser and guides the detection laser into the effective area of ​​the plasma grating, so that the detected fundamental light is converted into light of other frequencies, such as SH. Step S400: The refractive index of the plasma is periodically modulated using a plasma grating. The reciprocal lattice vector generated by the plasma grating compensates for the phase mismatch between the fundamental frequency light of the probe laser and the target frequency-converted light, achieving quasi-phase matching. When the reciprocal lattice vector equals the wave vector difference between the two frequencies, the fundamental frequency light is converted into the target frequency-converted light with enhanced intensity. Two or more plasma gratings are cascaded along the optical path to form a cascaded enhancement structure, enabling the signal intensity of the target frequency-converted light to be cumulatively enhanced stepwise through each plasma grating during propagation.

[0026] Among them, the target frequency conversion light includes, but is not limited to, frequency doubling and difference frequency light; and the multiple plasma gratings of the cascaded enhancement structure are spatially misaligned.

[0027] This embodiment overcomes the traditional limitation of achieving efficient nonlinear frequency conversion in centrosymmetric media such as air. By actively writing a periodic plasma grating through multi-beam interference, a dynamically tunable QPM mechanism is introduced and verified for the first time in this type of medium. The reciprocal lattice vector provided by this grating can accurately compensate for wave vector mismatch caused by plasma dispersion, enabling signals such as SH to coherently accumulate along the propagation direction, improving the conversion efficiency by more than an order of magnitude compared to a single-filament reference. This marks a new stage in nonlinear frequency conversion technology, expanding from static matching that relies on fixed crystals and is easily damaged, to strong-field matching that can be dynamically programmed in gases without damage threshold limitations.

[0028] Example 2 like Figure 2 As shown, the present invention provides an experiment on cascaded enhanced SH generation based on a dynamically tunable plasma grating; (1) Establishment of single-filament pump-probe configuration benchmark; Experimental setup such as Figure 2As shown in (a). Figure 2 (a) Experimental setup for generating SH using a pump 1-probe 4 with laser-induced air plasma; the Yb Femto Super laser system outputs pulses with a center wavelength of 1030 nm, a pulse width of 230 fs, and a repetition rate of 5 kHz. The emitted laser is split into two paths by a beam splitter: one path serves as the probe light, with a fixed pulse energy of 0.8 mJ, determined by the focal length... The fused silica lens 501 focuses the light to form a precisely controlled weak filament in air, maximally suppressing plasma fluorescence and supercontinuum background noise generated by the probe light itself. For example... Figure 2 As shown in (b) Figure 2 (b) For the polarization configuration, the polarization state of the probe light is fixed as S-polarization by the combination of half-wave plate 2 and the polarizer (electric field vector E). probe Along the +z direction of the coordinate system). Another path serves as the pump light, its energy continuously adjustable within a range of 1.5 mJ via half-wave plate 2 and Glan prism 3, and determined by the focal length. Lens 502 focuses the beam. The propagation directions of the pump beam and probe beam are spatially orthogonal (e.g., orthogonal along the x-axis and y-axis, respectively), ensuring that the two beams intersect in the focal region, forming a typical pump-probe spatial configuration, such as... Figure 2 As shown in (c) Figure 2 (c) is a single-beam pump 1-probe 4 orthogonal configuration. The polarization angle of the pump light relative to the probe plane... It is clearly defined that when the electric field vector E of the pump light... pump When parallel to the plane containing the two beams (i.e., the xy plane), it is denoted as... (P-polarization); when E pump When perpendicular to this plane (along the +z direction), it is denoted as (S-polarization). The relative time delay between two beams of light. Controlled by a precision electric translation stage equipped with a reflector, the zero point ( The position where two pulses coincide in time at the spatial focal point is defined as the location where the two pulses coincide in time.

[0029] The generated SH signal is collected along the original propagation direction of the probe light (+y direction). The collection optical path includes a focal length. A large-aperture lens 503 is used for efficient coupling of signal light. To filter out strong fundamental frequency light (1030 nm) and some plasma radiation, a suitable neutral density filter 6 and a low-pass filter with a center cutoff wavelength of approximately 550 nm are sequentially placed in the optical collection path. The filtered signal light is coupled into a multimode fiber and introduced into a spectrometer for detection. The exposure time of the spectrometer 8 is typically set to 200 μs, and each data point is the average of multiple acquisitions to improve the signal-to-noise ratio.

[0030] Quantitative calculations show that the SH energy conversion efficiency of this single-filament configuration under optimal conditions is approximately 0.043‰. While this value verifies the possibility of SH generation induced by system asymmetry in non-coaxial geometry, the root cause of its low efficiency lies in its inability to compensate for the severe wave vector mismatch (Δk) caused by plasma dispersion, resulting in the SH wave failing to accumulate effectively during propagation due to destructive interference. The performance data of this configuration provides an important quantitative benchmark for the subsequent enhancement effects of this embodiment.

[0031] (2) The plasma grating configuration realizes dynamic quasi-phase matching QPM by constructing and utilizing the plasma grating to actively introduce QPM conditions; like Figure 2 As shown in (d), the plasma grating pump-probe orthogonal coplanar configuration fundamentally changes the method of pump light preparation: it is split into two coherent sub-beams (denoted as beam A and beam B) with approximately equal energy. This is achieved by precisely adjusting the incident angle α between these two beams and utilizing the focal length... Lens 2 is made confocal, and the two beams interfere in the focal region. The periodic spatial modulation of the interference light field intensity causes non-uniform ionization of the air, thereby writing a plasma grating 10 with a well-defined spatial period Λ in the focal region. This grating period is precisely determined by the Bragg condition: ,in The pump light wavelength is 1030 nm. By changing the angle... It can achieve the control of grating period Continuous and dynamic regulation.

[0032] Keeping the probe beam parameters the same as in the benchmark test, it is precisely guided into the effective region of the plasma grating. The core experiment of the system is: fixing the total pump energy, scanning the angle between the two pump beams. (i.e., scanning grating period) The generated SH signal intensity was recorded simultaneously. Experimental results are as follows: Figure 3 The figure shows that the intensity of the second harmonic SH varies with the incident angle. The change is not monotonous, but rather occurs from a specific angle (in this embodiment). A sharp resonance peak is observed at the point ( ). The appearance of this resonance peak is direct and conclusive evidence of QPM occurrence. At the resonance angle, the corresponding grating period At this point, the reciprocal lattice vector provided by the plasma grating This precisely compensates for the wave vector mismatch between the fundamental frequency light and the SH light in the plasma medium, thus satisfying the phase matching condition: .

[0033] To further verify the physical correctness of this QPM process, a set of closed-loop checks were performed. First, based on the QPM theoretical formula... The optimal period determined by the experiment Substituting these values, we can deduce the refractive index difference required to achieve a match. Next, starting from the basic model of plasma physics, the refractive index of plasma is determined by the electron density Ne: ,in The plasma frequency. The reverse calculation yields... By substituting the values ​​into the model, the corresponding plasma frequency can be calculated. Thus, the electron density is obtained. This electron density value falls within the typical range for femtosecond laser-induced air plasma. Finally, this... Substitute the values ​​back into the refractive index formula and calculate independently. and The obtained difference Δn' is in high agreement with the Δn value initially derived from the QPM formula. This complete logical chain—from observation to parameter derivation, model verification, and closed-loop self-consistency—strongly demonstrates that the prepared plasma grating period is precisely designed and can effectively compensate for the intrinsic dispersion of the plasma under specific electron densities, thus realizing dynamic and tunable QPM. More importantly, the entire QPM process is realized entirely in transient plasma induced by femtosecond high-intensity lasers. This dynamic medium itself originates from and can withstand strong fields, making the QPM mechanism revealed in this invention naturally applicable to strong field conditions, overcoming the fundamental defect that traditional nonlinear crystals inevitably suffer optical damage under similar light intensities. Under these optimized conditions, the SH conversion efficiency of the single plasma grating configuration is improved to approximately 0.131‰, an order of magnitude improvement over the single-filament reference.

[0034] (3) The effective working length is extended by cascading dual plasma gratings to construct cascaded dual plasma gratings; Two plasma gratings are arranged sequentially in space along the propagation direction of the probe light (+y axis). This can be achieved through various optical path designs, such as: using time-division multiplexing to excite gratings at two adjacent spatial locations sequentially using the same pump light path; or using beam-splitting technology to divide the pump light into two groups, independently controlling and simultaneously exciting two spatially separated gratings, such as... Figure 4 The key technical requirement is to ensure the period of the two gratings. Each was independently optimized near the QPM condition, and they were spatially precisely aligned along the optical path.

[0035] After locking in this optimal QPM period, the SH intensity exhibits a clear stepwise increase with propagation distance y, as shown in... Figure 5As shown, this is a typical characteristic of QPM dynamics in nonlinear optics. The process is strictly governed by the coupled-wave equations describing nonlinear coupling:

[0036] in . Figure 5 The curve is the equation under the QPM condition ( The numerical solution is obtained using experimentally determined Λ and the calculated value. , As input parameters, comparing single-grating and dual-grating configurations, the latter, due to the increased effective interaction length, significantly improves the saturation intensity of SH, intuitively demonstrating the ability to control nonlinear conversion efficiency through plasmonic gratings.

[0037] In this configuration, the behavior of the nonlinear process changes. In the first grating, the probe light begins to generate SH radiation through nonlinear coupling. When this portion of the SH light propagates into the second grating region along with the remaining fundamental light, the periodic modulation provided by the second grating continues to act on this optical field combination, driving further frequency conversion and energy transfer. This process is equivalent to extending the effective operating length of the QPM from the scale of a single grating (on the order of approximately hundreds of micrometers) to the cumulative scale of two gratings.

[0038] Performance comparison experiments such as Figure 6 As shown, the most convincing evidence is presented. In the coplanar configuration of the dual-plasma grating, the conversion efficiency of SH reaches approximately 0.288‰. This value is not only about 6.7 times the single-filament reference efficiency (0.043‰), but also about 2.2 times the single-plasma grating efficiency (0.131‰). This result conclusively demonstrates that multiplying the effective nonlinear action length by cascading multiple QPM units is an effective strategy to overcome efficiency bottlenecks and achieve cumulative signal strength amplification.

[0039] This embodiment illustrates the technical path from establishing a benchmark to achieving a breakthrough in dynamic QPM, and then further improving performance through cascade design. The entire scheme integrates the active preparation of plasma periodic structures, precise control of QPM conditions, in-depth understanding of nonlinear dynamics, and flexible application of multidimensional degrees of freedom, systematically verifying the effectiveness, superiority, and universal potential of the method in this embodiment.

[0040] The plasma grating constructed in this embodiment allows for real-time and precise control of key parameters (such as period Λ, modulation depth, and spatial configuration) through the pump light's angle, polarization, intensity, and time sequence. This high degree of flexibility and adjustability is reflected in the following: by changing the interference angle, the grating period can be continuously adjusted to adapt to the specific phase-matching conditions required for different wavelengths or different nonlinear processes (such as frequency doubling and sum-frequency); by controlling the pump light's polarization state and intensity, the modulation depth and symmetry breaking intensity of the grating can be effectively controlled, thereby optimizing the efficiency of nonlinear interactions; and various spatial configurations, such as coplanar, non-coplanar, and even cascaded configurations, can be achieved, providing multiple design options for balancing interaction length, collection efficiency, and application requirements.

[0041] Meanwhile, the dual-grating cascaded configuration proposed in this embodiment spatially connects multiple QPM units, effectively doubling the effective interaction length of the nonlinear frequency conversion process and achieving a step-wise cumulative amplification of signal strength. Experiments have shown that the conversion efficiency of the dual-grating configuration is 2.2 times that of the single-grating configuration. This provides an effective technical path to solve the core problem of short nonlinear interaction lengths in gaseous media, laying the foundation for future higher-order cascading and greater efficiency improvements.

[0042] Example 3 The strong-field compatible, dynamically programmable QPM implemented in this embodiment provides an original solution to address the core challenges in current photonics technology. By cascading the plasma grating configuration of this invention, it is expected to perform phase-matched control of the high-order harmonic generation process, significantly improving its conversion efficiency and beam quality, and providing a new technical route for obtaining high-power, high-repetition-rate extreme ultraviolet radiation (a key light source for semiconductor lithography).

[0043] Specifically, taking the generation of 13.5 nm (approximately 92 eV) extreme ultraviolet radiation as an example, the principle and feasibility of the method in this embodiment are illustrated. When a femtosecond laser with a wavelength of 1030 nm is used to drive the generation of higher harmonics, 13.5 nm light corresponds to approximately the 76th harmonic. The generation efficiency of higher harmonics is limited by the macroscopic phase mismatch accumulated during the recombination process of ions and electrons. The dynamic plasma grating provided in this embodiment has a period... It can be precisely set as needed. The grating provides a reciprocal lattice vector ( It can be used to compensate for the wave vector mismatch between the driving light and the qth harmonic: Through theoretical calculations and grating period design, QPM conditions can be optimized for specific harmonic orders (such as the 76th order), thereby overcoming the macroscopic phase mismatch bottleneck and theoretically achieving an order-of-magnitude improvement in harmonic conversion efficiency for that order.

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of plasmonic modulation quasi-phase-matched nonlinear frequency conversion, characterized in that, The method includes the following steps: Step S100: The pump laser is provided by the laser generation unit, and the pump laser is split and controlled to form at least two coherent sub-pump beams; Step S200: The two subpump beams intersect and interfere in a predetermined region of the nonlinear medium, thereby inducing the formation of a plasma grating with a periodic arrangement structure; the period and modulation depth of the plasma grating are adjustable; Step S300: The laser generation unit provides the detection laser and guides the detection laser into the effective area of ​​the plasma grating, so that the detected fundamental light is converted into light of other frequencies; Step S400: The plasma refractive index is periodically modulated by a plasma grating. The reciprocal lattice vector brought by the plasma grating is used to compensate for the phase mismatch between the fundamental frequency light of the probe laser and the target frequency conversion light, so as to achieve quasi-phase matching. When the reciprocal lattice vector is equal to the difference between the wave vectors of the two frequencies, the fundamental frequency light is converted into the target frequency conversion light with enhanced intensity.

2. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 1, characterized in that, In step S200, the period of the plasma grating is controlled by adjusting the angle between two of the sub-pump beams.

3. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 1, characterized in that, In step S200, the modulation depth of the plasma grating is controlled by adjusting the polarization state and intensity of the sub-pump beam.

4. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 1, characterized in that, In step S300, the probe laser is synchronized with the sub-pump beam in time and space to optimize the interaction effect of each pulse.

5. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 1, characterized in that, In step S400, two or more plasma gratings are cascaded along the optical path to form a cascaded enhancement structure, so that the signal intensity of the target frequency-converted light is cumulatively enhanced stepwise by each plasma grating during propagation.

6. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 5, characterized in that, Target frequency conversion light includes, but is not limited to, frequency harmonics and frequency or difference frequency light.

7. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 5, characterized in that, Multiple plasma gratings in the cascaded enhancement structure are spatially staggered.

8. The nonlinear frequency conversion method for plasma modulation quasi-phase matching according to claim 1, characterized in that, The nonlinear medium in step S200 is a gaseous medium.

9. A plasma-modulated quasi-phase-matched nonlinear frequency conversion system, using the plasma-modulated quasi-phase-matched nonlinear frequency conversion method according to any one of claims 1-8, characterized in that, The system includes the following structure: The laser generation unit is used to provide pump laser and probe laser; A beam splitting and control unit is used to split and control the pump laser into at least two coherent sub-pump beams, and guide the at least two sub-pump beams to interfere in a predetermined region in a nonlinear medium to form a plasma grating. The probe light introduction unit is used to guide the probe laser into the active area of ​​the plasma grating; The signal collection and detection unit is used to collect and detect the target frequency-converted light generated by plasma grating modulation.

10. The plasma-modulated quasi-phase-matched nonlinear frequency conversion system according to claim 9, characterized in that, The beam splitting and control unit includes optical elements and a displacement platform for adjusting the angle, polarization state, intensity and path length of the sub-pump beam.

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