A method and device for measuring temperature based on nanosecond time-resolved plasma continuum spectrum

Abstract

This invention discloses a method and apparatus for measuring plasma continuous spectrum temperature based on nanosecond time-resolved plasma. The invention aims to address the lack of reliable temperature measurement methods during the early stages (1-10 ns) of plasma evolution in the process of ultrashort pulse laser ablation of materials. This invention utilizes a nanosecond-level time-resolved spectral acquisition platform (minimum gate width of 2 ns) to acquire plasma continuous spectra at different delay times. Using Planck's radiation law, nonlinear least-squares fitting is performed to inversely deduce the transient plasma temperature at each time point. This invention transforms the early continuous spectrum from an interference signal into a temperature measurement advantage, achieving for the first time reliable measurement of plasma temperature within a 1-10 ns window after laser ablation, with advantages of non-contact, non-invasiveness, and high time resolution.

Description

Technical Field

[0001] This invention relates to plasma diagnostic technology in the field of ultrafast laser-matter interaction, specifically a temperature measurement method and device based on nanosecond time-resolved plasma continuous spectrum. Background Technology

[0002] When using ultrashort pulse lasers (such as femtosecond and picosecond lasers) to perform micro- and nano-fabrication on various materials such as metals, semiconductors, and dielectrics, the laser-induced plasma temperature is a key physical parameter determining the thermal ablation mechanism, material removal efficiency, and processing quality. Accurately measuring the evolution temperature of plasma on nanosecond and even sub-nanosecond timescales is of significant guiding importance for optimizing processing parameters, suppressing thermal damage, and achieving high-precision microstructure fabrication. However, due to the extremely short duration of plasma existence (typically on the nanosecond scale), extremely high temperature (reaching thousands to tens of thousands of Kelvin), and the accompanying intense electromagnetic radiation and material ejection, traditional contact temperature measurement methods (such as thermocouples) are completely unsuitable, necessitating the development of high-time-resolution non-invasive diagnostic technologies.

[0003] Currently, established plasma diagnostic methods mainly include the Langmuir probe method and emission spectroscopy. The Langmuir probe method obtains local electron temperature and density by measuring the probe current-voltage characteristic curve. However, it is invasive, interferes with the initial plasma state, and requires a high vacuum environment, making it difficult to use for measuring rapidly expanding plumes under atmospheric or strong ablation conditions. In emission spectroscopy, traditional temperature measurement methods based on discrete spectral lines (such as the Boltzmann diagram method and the Saha-Boltzmann method) estimate the excitation temperature by analyzing the relative intensity of atomic or ion spectral lines. However, in the early stages of laser ablation plasma evolution (typically within 1-10 nanoseconds after the laser pulse), these discrete spectral lines are either indistinguishable due to strong Stark broadening and spectral line overlap, or completely submerged by strong, featureless continuous background radiation. Therefore, traditional emission spectroscopy temperature measurement methods relying on discrete spectral lines cannot provide reliable temperature data within the early nanosecond window.

[0004] In summary, existing methods such as the Langmuir probe method and discrete-line emission spectroscopy cannot meet the temperature measurement requirements in the early stages (1-10 ns) of laser ablation plasma evolution. The Langmuir probe is invasive and interferes with the plasma state; while traditional emission spectroscopy relies on discrete spectral lines, which become ineffective in the early nanosecond stages due to the spectral lines being submerged by continuous background radiation. Therefore, a non-contact measurement method capable of temperature diagnosis within the early nanosecond window of plasma evolution is urgently needed. Summary of the Invention

[0005] To address the aforementioned problems, the technical problem this invention aims to solve is to provide a temperature measurement method and device based on nanosecond time-resolved plasma continuous spectrum. This method utilizes a digital delay generator (DDG) to achieve precise synchronization between a laser and an ICCD, establishing a time-resolved plasma emission diagnostic platform to achieve high time-resolution spectral acquisition at the nanosecond level. The technical approach is as follows: an ultrashort pulse laser is focused onto the material surface, inducing the generation of transient plasma. This plasma emits strong continuous spectrum radiation in the early nanosecond stage (1-10 nanoseconds). The plasma emission spectrum is acquired by a spectrometer and ICCD at different delay times, and the continuous spectrum portion is extracted. Based on Planck's radiation law, the distribution of the continuous spectrum intensity emitted by the plasma in the early stage is fitted to Planck's formula, and the transient plasma temperature at the corresponding delay time is calculated. This achieves high time-resolved, non-invasive diagnosis of plasma temperature during ultrafast laser ablation.

[0006] The technical solution of this invention is as follows: On the one hand, this invention proposes a method for temperature measurement of plasma continuum based on nanosecond time-resolved resolution, comprising the following steps: (1) An ultrashort pulse laser is focused on the sample surface, inducing transient plasma emission spectra. A single laser pulse from an ultrashort pulse laser is vertically focused onto the sample surface after passing through a dichroic mirror and a focusing objective. The sample can be placed on a three-dimensional displacement platform, which is used to move the sample to a new position after each laser pulse acquisition. The laser pulse induces a dense, electrically neutral plasma composed of electrons, ions, and aggregated nanoparticles at the focal spot. This plasma evolves rapidly on an ultrafast timescale and emits a broadband optical signal including a continuous spectrum.

[0007] (2) Collect nanosecond time-resolved spectra at different delay times using a time-resolved spectral acquisition platform. A time-resolved spectral acquisition platform is constructed, which includes a focusing objective, a dichroic mirror, a reflecting mirror, a 4-f optical relay system, filters, a spectrometer, and an enhanced charge-coupled device (ICCD). The ICCD is coupled to the spectrometer, with a gate width in the nanosecond range (minimum gate width is 2 ns), and has a built-in digital delay generator (DDG). The DDG is used to precisely synchronize the laser trigger signal and the gate signal of the ICCD.

[0008] After laser irradiation, the light signal emitted by the plasma is collected by the focusing objective lens, passes through the dichroic mirror, is reflected by the mirror, and then enters the 4D laser beam. An optical relay system and filters are used, which are then coupled into the spectrometer. To eliminate interference from reflected laser pulses, filters are placed in the spectral collection optical path. Different delay times (e.g., 1 ns, 5 ns, 10 ns) and fixed nanosecond-level gate widths (e.g., 2 ns) are set using the DDG (Diverter-Delayed Gauge), and plasma emission spectra within each delay time window are collected. The spectra at each delay time are then accumulated and averaged multiple times to reduce experimental measurement errors. Furthermore, to maintain data integrity and avoid accumulated surface damage, the sample is moved to a new location after each acquisition to ensure that each measurement reflects the original laser emission. Material interactions.

[0009] (3) Obtain the continuous spectrum and perform spectral correction. Extract 1 from the collected spectral data The continuous spectrum region within a 10 ns time window was selected, avoiding the locations of potentially weak discrete spectral lines. The extracted continuous spectrum was corrected, with factors including the spectrometer's grating diffraction efficiency, the dichroic mirror transmittance, and the ICCD's quantum efficiency. By introducing a pre-calibrated instrument response function, the raw spectral intensity was converted into true radiative intensity, providing accurate input data for subsequent temperature fitting.

[0010] (4) The plasma temperature is inversely calculated by performing nonlinear least squares fitting using Planck's formula. Based on the corrected continuous spectrum data, after background subtraction, the continuous spectrum intensity can be expressed as wavelength. and temperature The transient plasma temperature at the corresponding delay time is obtained by using Planck's radiation law for nonlinear least-squares fitting. Planck's formula is: In the formula, Let be Planck's constant. At the speed of light, Boltzmann's constant, and These are the fitting parameters for effective emissivity and temperature, respectively. The wavelength of the optical signal (i.e., the wavelength value corresponding to each data point in the continuous spectrum data).

[0011] During the fitting process, the temperature is estimated by minimizing the sum of squared residuals. : in, For the first The wavelength corresponding to each data point To obtain the spectral radiance in the experiment, The fit strength is calculated using Planck's formula. For indexing data points, Let be the total number of data points. Solving this makes... smallest The value represents the plasma temperature at that delay time; the temperatures at different delay times are arranged in chronological order, and a curve showing the temperature evolution over time is plotted.

[0012] The method of this invention can be applied to plasma temperature diagnosis in ultrafast laser precision machining, laser-induced breakdown spectroscopy, and the study of material ablation mechanisms.

[0013] In another aspect of the present invention, a nanosecond time-resolved plasma continuous spectrum thermometry device for implementing the above method is proposed, comprising: an ultrashort pulse laser and a time-resolved spectral acquisition platform, wherein the time-resolved spectral acquisition platform comprises a dichroic mirror, a focusing objective lens, a reflecting mirror, a 4-f optical relay system, a filter, a spectrometer and an enhanced charge-coupled device, and the enhanced charge-coupled device has a built-in digital delay generator. The laser beam emitted by the ultrashort pulse laser is reflected by the dichroic mirror and then focused onto the sample surface by the focusing objective to induce transient plasma. The light signal emitted by the transient plasma is collected by the focusing objective, transmitted through the dichroic mirror, reflected and redirected by the mirror, and then passes sequentially through the 4-f optical relay system and the filter before entering the spectrometer. After being dispersed by the spectrometer, it is detected by the enhanced charge-coupled device (ECD). The digital delay generator receives the trigger signal from the ultrashort pulse laser and outputs a gating signal to the photocathode of the ECD to control the acquisition timing of the ECD.

[0014] Furthermore, the device also includes a control computer, which is signal-connected to the ultrashort pulse laser and the enhanced charge-coupled device, respectively, for controlling the output laser pulses of the ultrashort pulse laser and setting the delay time of the digital delay generator.

[0015] Furthermore, the ultrashort pulse laser is a femtosecond laser.

[0016] Furthermore, the device also includes a three-dimensional displacement stage on which the sample is placed, and the three-dimensional displacement stage is used to move the sample to a new position after each laser pulse acquisition.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (1) This invention utilizes the transient plasma generated by ultrashort pulse laser acting on the material surface as a signal source, and directly uses the continuous spectrum signal that dominates the plasma emission spectrum to fit Planck's radiation law to infer the temperature, without relying on the identification and extraction of discrete atomic or ion spectral lines. In the early stage of plasma evolution (1 (10 nanoseconds), discrete atomic / ion spectral lines are completely submerged by continuous background radiation, causing traditional line ratio temperature measurement methods to completely fail. However, this invention transforms this obstacle into a diagnostic advantage, fundamentally avoiding temperature measurement deviations caused by spectral line overlap, self-absorption, and strong broadening, and achieving reliable temperature measurement within the early nanosecond window, which is unattainable by standard methods. (2) This invention employs a digital delay generator (DDG) to precisely control the synchronization and delay between the ICCD and the laser, constructing a plasma emission diagnostic platform with nanosecond-level (minimum gate width of 2 ns) time resolution. This platform can acquire plasma spectra at multiple different delay times within the nanosecond scale, capturing the fine process of early temperature evolution. Thanks to the high gain characteristics of the ICCD, this system is not only suitable for ultrashort pulse and single pulse temperature measurement scenarios, but can also be easily extended to long pulse and multi-pulse continuous temperature diagnosis. (3) The present invention adopts a non-contact, non-invasive emission spectral detection method, which does not interfere with the state of the plasma itself. It is suitable for temperature measurement under extreme transient environments and can be extended to various solid material systems such as metals, semiconductors, dielectrics and phase change materials, and has good versatility. Attached Figure Description

[0018] Figure 1 This is a flowchart of the workflow of the present invention based on the nanosecond time-resolved plasma continuous spectrum temperature measurement method. Figure 2 This is a schematic diagram of the device for the nanosecond time-resolved plasma continuous spectrum temperature measurement method of the present invention. Among them, 1 is a femtosecond laser, 2 is a dichroic mirror, 3 is a focusing objective lens, 4 is a sample, 5 is a three-dimensional displacement stage, 6 is a reflecting mirror, 7 is a first lens, 8 is a second lens, 9 is a filter, 10 is a spectrometer, 11 is an ICCD, and 12 is a control computer. Detailed Implementation

[0019] See Figure 2 The present invention provides a device for measuring the temperature of plasma continuous spectrum based on nanosecond time-resolved plasma, comprising a time-resolved spectral acquisition platform, wherein the time-resolved spectral acquisition platform consists of a dichroic mirror 2, a focusing objective lens 3, a reflecting mirror 6, a 4-f optical relay system composed of a first lens 7 and a second lens 8, a filter 9, a spectrometer 10, and an ICCD 11; the minimum gate width of the ICCD 11 is 2 ns, and it has a built-in digital delay generator (DDG).

[0020] The device also includes an ultrashort pulse laser (femtosecond laser 1 in this embodiment), a three-dimensional displacement stage 5, a sample 4, and a control computer 12 and corresponding software connected to the femtosecond laser 1 and ICCD 11 via a data cable.

[0021] The following combination Figure 1 The flowchart shown illustrates a detailed description of specific embodiments of the present invention. A method for temperature measurement based on nanosecond time-resolved plasma continuous spectrum includes the following steps: 1. Pulsed laser-excited plasma: Sample 4 is placed on a three-dimensional displacement stage 5, and the position of the stage 5 is adjusted so that the laser focus accurately acts on the surface of sample 4. The control computer 12 controls the femtosecond laser 1 to output a single-pulse laser, the single-pulse energy and repetition frequency of which are tunable. This laser beam is reflected and redirected by the dichroic mirror 2, and after being focused by the focusing objective lens 3, it is vertically focused onto the surface of sample 4. At the focal spot, due to the extremely high peak power density of the femtosecond laser, sample 4 is instantaneously vaporized and ionized, inducing the generation of a dense, electrically neutral plasma composed of electrons, ions, and aggregated nanoparticles. This plasma evolves rapidly on an ultrafast timescale and emits a broadband optical signal including a continuous spectrum.

[0022] Meanwhile, the broadband light signal emitted by the plasma is collected by the same focusing objective 3. Since the wavelength of the light signal is different from the wavelength of the excitation laser, it can be transmitted through the dichroic mirror 2 and enter the subsequent spectral collection optical path.

[0023] 2. Time-resolved spectral acquisition and correction: (1) The ICCD11 is coupled with the spectrometer 10 as the core of the time-resolved spectral acquisition platform. Before spectral acquisition, the synchronous output interface of the femtosecond laser 1 is connected to the external trigger input of the digital delay generator (DDG) built into the ICCD11 via a coaxial cable, and the delay time of the DDG and the gate width parameters of the ICCD11 are preset. During acquisition, for each laser pulse output by the femtosecond laser 1, its synchronous output interface simultaneously sends a TTL trigger signal to the DDG. After receiving the trigger signal, the DDG starts timing according to the preset delay time. When the timing ends, it immediately outputs a gate opening pulse to the photocathode of the ICCD11, so that the ICCD11 is in the acquisition state for a fixed gate width of 2 ns, thereby realizing the precise synchronization of the laser trigger signal and the gate signal of the ICCD11 using the DDG built into the ICCD11. By setting different delay times and a fixed gate width of 2 ns for the DDG, the plasma emission spectrum within each delay time window is acquired.

[0024] The light signal emitted by the plasma is collected by the focusing objective lens 3, first passes through the dichroic mirror 2, is reflected and redirected by the reflecting mirror 6, and then collimates and focuses through a 4-f optical relay system composed of the first lens 7 and the second lens 8. After stray light is filtered out by the filter 9, it is coupled into the entrance slit of the spectrometer 10. The spectrometer 10 disperses the polychromatic light into a spectrum arranged by wavelength, and the ICCD 11 coupled to the exit port of the spectrometer 10 performs photoelectric conversion and signal enhancement. Finally, the spectral data at each delay time is recorded and stored by the control computer 12. To reduce experimental measurement errors, the spectrum at each delay time is acquired multiple times and averaged. Furthermore, to avoid the influence of accumulated surface damage on the measurement results, the sample is moved to a new position after each laser pulse acquisition to ensure that each measurement reflects the original laser-material interaction.

[0025] (2) Extract 1 from the collected spectral data Within a 10 ns time window, the continuous spectrum region was selected, avoiding potentially weak or discrete spectral lines. The extracted continuous spectrum was corrected, with correction factors including the grating diffraction efficiency of spectrometer 10, the transmittance of dichroic mirror 2, and the quantum efficiency of ICCD 11. The original spectral intensity was converted into actual radiant intensity to provide accurate input data for subsequent temperature fitting. The specific correction method involved obtaining the efficiency or transmittance curves for the corresponding wavelength bands from the product datasheets of each component, multiplying each curve point-by-point along the wavelength to obtain the overall instrument response function. Dividing the measured spectral intensity by this response function yielded the corrected actual radiant intensity, providing accurate input data for subsequent temperature fitting.

[0026] 3. Planck fit to retrieve plasma temperature: The continuous spectrum intensity of the output after correction in step 2 is used as a function of wavelength and temperature. A nonlinear least-squares fit is performed using Planck's radiation law to obtain the transient plasma temperature at different delay times. The initial value of the delay time is determined by the optical path difference between the laser pulse and the ICCD11, and its specific value needs to be determined through system delay calibration. The delay step size is determined based on the laser pulse width and the plasma evolution lifetime. In this embodiment, the laser pulse width is approximately 171 fs. To fully capture the rapid evolution process in the early stages of the plasma, the delay step size is set to 1 ns. Simultaneously, the plasma continuous spectrum gradually decays after 10 ns, while atomic / ion discrete spectral lines begin to appear and dominate. At this point, the continuous spectrum signal intensity weakens and is more susceptible to interference from discrete spectral lines, leading to a significant reduction in Planck fitting efficiency. Therefore, in this embodiment, several delay times are uniformly selected within the range of 1 ns to 10 ns for acquisition to ensure the purity of the continuous spectrum signal and the reliability of the fitting results. By fitting the above data, plasma temperature values ​​corresponding to delay times of 1 ns, 5 ns, and 10 ns can be obtained. By arranging the temperatures at each time point in order of delay time, a curve of temperature evolution over time can be plotted, which can be used to analyze the cooling rate and energy relaxation process of the early plasma.

Claims

1. A method for temperature measurement of plasma continuum based on nanosecond time-resolved spectroscopy, characterized in that, Includes the following steps: By focusing an ultrashort pulse laser onto the sample surface, transient plasma emission spectra are induced. Using a time-resolved spectral acquisition platform, a digital delay generator is used to precisely synchronize the laser trigger signal and the gating signal of the enhanced charge-coupled device. The gate width is set to the nanosecond level, and different delay times are set. The plasma emission spectra within each delay time window are acquired to obtain nanosecond time-resolved spectra. Select a continuous spectrum region from the acquired spectral data and perform instrument response correction on the continuous spectrum; The corrected continuous spectrum was fitted nonlinearly using Planck's radiation law to inversely deduce the transient plasma temperature at the corresponding delay time. The transient plasma temperatures at each delay time were arranged in chronological order to obtain the temperature-time evolution curve.

2. The method for measuring the temperature of plasma continuum based on nanosecond time-resolved resolution according to claim 1, characterized in that: The time-resolved spectral acquisition platform includes an enhanced charge-coupled device (CCD), a spectrometer, a focusing objective, a dichroic mirror, a reflecting mirror, a 4-f optical relay system, and filters; the enhanced CCD has a minimum gate width of 2 ns and a built-in digital delay generator. The light signal emitted by the transient plasma is collected by the focusing objective lens, transmitted through the dichroic mirror, reflected and redirected by the mirror, and then enters the spectrometer after passing through the 4-f optical relay system and the filter. After being dispersed by the spectrometer, it is detected by the enhanced charge-coupled device.

3. The method for measuring the temperature of plasma continuum based on nanosecond time-resolved resolution according to claim 1, characterized in that: To reduce experimental measurement errors, the plasma emission spectrum within each delay time window was accumulated and averaged multiple times, and the sample was moved to a new position after each laser pulse acquisition to avoid cumulative surface damage.

4. The method for measuring the temperature of plasma continuum based on nanosecond time-resolved spectroscopy according to claim 1, characterized in that: The instrument response correction includes grating diffraction efficiency correction, dichroic mirror transmittance correction, and enhanced charge-coupled device quantum efficiency correction.

5. The method for measuring the temperature of plasma continuum based on nanosecond time-resolved resolution according to claim 1, characterized in that: When using Planck's radiation law for nonlinear least-squares fitting, the sum of squared errors between the experimentally measured continuous spectrum radiation intensity and the theoretical radiation intensity calculated by Planck's formula is used as the objective function. The transient plasma temperature at the corresponding delay time is obtained by minimizing the objective function.

6. The method for measuring the temperature of plasma continuum based on nanosecond time-resolved resolution according to claim 1, characterized in that: The method is applicable to plasma temperature diagnosis in ultrafast laser precision machining, laser-induced breakdown spectroscopy, and material ablation mechanism research.

7. A nanosecond time-resolved plasma continuous spectrum thermometry device for implementing the method as described in any one of claims 1-6, characterized in that, include: An ultrashort pulse laser and a time-resolved spectral acquisition platform, wherein the time-resolved spectral acquisition platform includes a dichroic mirror, a focusing objective lens, a reflecting mirror, a 4-f optical relay system, a filter, a spectrometer, and an enhanced charge-coupled device, and the enhanced charge-coupled device has a built-in digital delay generator; The laser beam emitted by the ultrashort pulse laser is reflected by the dichroic mirror and then focused onto the sample surface by the focusing objective to induce transient plasma. The light signal emitted by the transient plasma is collected by the focusing objective, transmitted through the dichroic mirror, reflected and redirected by the mirror, and then passes sequentially through the 4-f optical relay system and the filter before entering the spectrometer. After being dispersed by the spectrometer, it is detected by the enhanced charge-coupled device (ECD). The digital delay generator receives the trigger signal from the ultrashort pulse laser and outputs a gating signal to the photocathode of the ECD to control the acquisition timing of the ECD.

8. The apparatus according to claim 7, characterized in that: It also includes a control computer, which is signal-connected to the ultrashort pulse laser and the enhanced charge-coupled device, respectively, for controlling the output laser pulse of the ultrashort pulse laser and setting the delay time of the digital delay generator.

9. The apparatus according to claim 7, characterized in that: The ultrashort pulse laser is a femtosecond laser.

10. The apparatus according to claim 7, characterized in that: It also includes a three-dimensional displacement stage, on which the sample is placed, and the three-dimensional displacement stage is used to move the sample to a new position after each laser pulse acquisition.

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