阅读说明：本技术 一种超快激光光子时间拉伸下的薄膜热响应单脉冲探测方法 (Film thermal response single-pulse detection method under ultrafast laser photon time stretching ) 是由 王志 张露鹤 胡金耀 刘艳格 岳洋 刘波 张昊 于 2019-11-26 设计创作，主要内容包括：一种超快激光光子时间拉伸下的薄膜热响应单脉冲探测方法。包括：利用拉伸元件对超短飞秒激光进行时间尺度上的拉伸；通过光学元件将泵浦光和探测光集成为平行且间距可控的模块；将集成的泵浦光和探测光经聚焦透镜聚焦射入薄膜后反射光由耦合系统将探测光耦合进入单模光纤；对带有薄膜热反射率信息的脉冲光经光电探测器和高速示波器输出的高通量数据进行分析。本发明在探测光进入空间光路之前通过色散拉伸元件进行拉伸,大大提高了热反射率的探测精度；同时,用时间拉伸的方法对探测光进行拉伸可以避免原有逐点测量的繁琐,减小了实验的探测时间。本发明是一种针对连续探测薄膜热反射率的高通量、精确的实验方法。(A film thermal response single pulse detection method under ultrafast laser photon time stretching. The method comprises the following steps: stretching the ultrashort femtosecond laser on a time scale by using a stretching element; integrating the pump light and the probe light into parallel modules with controllable spacing through an optical element; the integrated pump light and the detection light are focused by a focusing lens and then enter a film, and then the reflected light couples the detection light into a single-mode optical fiber through a coupling system; and analyzing high-flux data output by the photoelectric detector and the high-speed oscilloscope of the pulsed light with the film thermal reflectivity information. According to the invention, before the detection light enters the space light path, the detection precision of the thermal reflectivity is greatly improved by stretching the dispersion stretching element; meanwhile, the detection light is stretched by a time stretching method, so that the original complexity of point-by-point measurement can be avoided, and the detection time of an experiment is shortened. The invention relates to a high-flux and accurate experimental method for continuously detecting the thermal reflectivity of a thin film.)

1. A film thermal response single pulse detection method under ultrafast laser photon time stretching. The transient thermal response of the thin film refers to the thermal response characteristic of the thin film material in a non-thermal equilibrium state by absorbing ultrafast laser pulses on a picosecond to nanometer time scale, and the characteristic is characterized by the change of the reflectivity of the material. The detection method is characterized in that the whole time scale is tested by using one detection pulse, and the detection method has the characteristic of high flux and comprises the following steps:

s1, dividing an ultrafast laser pulse emitted by a light source into two paths of light with different light intensities, wherein one path of light with strong light intensity is used as pump light for heating a sample, and the other path of light with weak light intensity is used as detection light for detection;

s2, performing photon time stretching on the detection light pulse by using a dispersion element to enable the duration of the detection light pulse to be larger than the thermal response time of the film material to be measured;

s3, simultaneously focusing the pump light and the probe light to the same position of the film to be detected;

and S4, coupling the detection light reflected from the film material into a high-speed photoelectric test system, and recording the change of the detection light pulse intensity after photon time stretching along with time to obtain the change of the film reflectivity along with time.

And S5, carrying out secondary stretching on the obtained detection light carrying the film reflectivity, amplifying the reflectivity change details of the pump light in the heat transport process after the pump light reaches the film on a time scale, and recording the change of the high-resolution detection light pulse intensity along with the time.

2. A test apparatus for implementing the method of claim 1, comprising: the system comprises an ultrafast laser source, an optical coupler, a dispersion stretching element, a time delay line, a space optical system and a high-speed photoelectric detection system. The femtosecond laser generated by the ultrafast laser source is divided into pumping light and detection light through an optical coupler, the pumping light is focused on a film sample to be detected through a spatial optical system, the detection light is focused on the film sample to be detected through the spatial optical system and is overlapped with the pumping light after passing through a dispersion stretching element and a time delay line, and then the detection light reflected by the sample is coupled into a high-speed photoelectric detection system, so that the high-speed and high-resolution measurement of the transient thermal response characteristic of the film material is realized.

3. The dispersion stretched element according to claim 2, which can be any optical element capable of stretching ultrashort pulse laser light in time by using dispersion effect, such as standard single mode fiber, dispersion compensation fiber, chirped fiber grating, and space grating.

4. The spatial optical system according to claim 2, wherein the pump light and the probe light are coupled to the same position on the surface of the thin film to be measured, and the polarization of the two beams of light is controlled and accurately positioned by optical elements such as a fiber collimator, a reflector, a polarization beam splitter, a wave plate, a manual displacement stage, and a lens.

5. The high-speed photoelectric detection system according to claim 2, comprising a high-speed photoelectric detector and a high-speed real-time oscilloscope, and capable of realizing high-speed signal detection.

6. The method for detecting the thermal response single pulse of the film under the condition of ultrafast laser photon time stretching as claimed in claim 1, wherein: the film is a film sample which is made of a material capable of generating non-thermal equilibrium response under the action of ultrafast laser pulse, has the thickness of generally not more than 1 mu m and represents the thermal response characteristic on a picosecond to nanometer time scale.

7. The high-speed measurement of transient thermal response characteristics of thin film materials of claim 2, wherein: the coupling of the detection light loaded with the heat reflection information into the single-mode optical fiber means that the detection light stretched by photon time is focused on a film, enters the single-mode optical fiber through a coupling lens after carrying reflectivity change information of a pulse period, and is subjected to photoelectric conversion by a photoelectric detector, and then time domain information is extracted by a high-speed oscilloscope for processing.

8. The high resolution measurement of transient thermal response characteristics of thin film materials of claim 2, wherein: the secondary stretching of the detection light carrying the film reflectivity means that the part with obvious reflectivity change in the detection light carrying the film thermal reflectivity is stretched in a secondary photon time mode, the details of the reflectivity change are amplified, and the high-resolution film thermal transport process can be analyzed on a fine time scale.

Technical Field

The invention relates to a film reflectivity single pulse detection method under femtosecond laser photon time stretching, which is widely applied to the aspects of researching high-flux information detection and film heat transport processes.

Background

The measurement of thermophysical quantities such as the heat transfer process of microscopic carriers of the micro-scale thin film device and the heat conduction between different materials cannot be directly characterized, but is reflected as the transient change of the thermal reflectivity of the sample material. When the size of the device is reduced to be equivalent to the characteristic scale of the device, the thermophysical property in the device shows obvious scale effect, so that the research on the heat transport process from the microscopic view of hot carriers is very necessary. Therefore, with the rapid development of the ultrafast laser technology, the micro-nano scale energy transportation problem is expanded towards the femtosecond nano scale. When the femtosecond nanoscale heat transfer mechanism is applied to the fields of microelectronic optoelectronic devices, material science, micromachining and the like, some challenging problems occur. The method comprises the steps of continuously detecting physical quantity-transient heat reflectivity capable of representing the thermophysical property of the material in a femtosecond scale, and obtaining high-throughput data of femtosecond resolution by using experimental means.

In the face of these challenges, finding an experimental method that can be implemented on the femtosecond scale is crucial to the material thermal diffusion research. The detection method for applying the photon stretching technology to transient heat reflection detection of the material is derived, the defects of large workload, long time consumption, low stability and the like of the original point-by-point measurement are overcome, and the method has important theoretical significance and application value for realizing the research of high-flux and transient dynamic characteristics of functional materials.

Transient Thermoreflectance (TTR), a pump-probe technique. The Femtosecond laser is used as a light source of a transient heat reflection technology, namely a Femtosecond transient reflection technology (FTTR), so that the time resolution can be improved to a Femtosecond level, and the detection accuracy is greatly improved. In the FTTR technique, a pulse width of a pulse laser beam is generally several tens femtoseconds to several hundreds femtoseconds, and the pulse width is divided into two beams by a beam splitter prism. One beam of femtosecond pulse laser with stronger energy is used for heating a sample to be detected, and the other beam of femtosecond pulse laser with weaker energy is used for detecting the transient change of the reflectivity of the surface of the heated sample. The probe light pulse has a precisely set time delay with respect to the time at which the pump light pulse reaches the sample. During the measurement, the reflectivity can only be measured at one time, which is determined by the predetermined, precisely set lag time. When the reflectivity is measured at a series of time points, a curve of the reflectivity over a period of time after the laser pulse is applied can be constructed. After the instantaneous change of the reflectivity is obtained, the instantaneous change of the electron temperature after the action of a laser pulse is solved through the change function of the reflectivity along with the electron temperature. The setting of different lag times can be generally realized by changing the optical path difference of the two laser beams.

The FTTR technology realizes the measurement of thermophysical quantities such as the heat transfer process of microscopic carriers of a micro-sized thin film device on a femtosecond scale and heat conduction between different materials, however, in consideration of the set discontinuity of different lag times realized by changing the optical path difference of two beams of laser in the existing FTTR technology, the complexity of repeatedly adjusting the time lag of probe light relative to pump light in each measurement and the large error of fitting data obtained by multiple measurement results into continuous quantities are overcome, and the scheme of combining the photon time stretching technology and the FTTR technology is derived.

Photon time stretching, also known as dispersive fourier transform technology, is a high-throughput real-time information acquisition technology that has emerged in recent years. The DFT technology is used for mapping the spectrum of a pulse to a time domain waveform in a dispersion mode, and the shape of an actual spectrum can be simulated by the intensity envelope of the DFT technology. The limit of the bandwidth and the sampling rate of electronic equipment can be overcome by utilizing a photon time stretching technology, ultra-fast information measurement is realized, the imaging frame rate is mainly determined by the pulse repetition frequency of a mode-locked laser, and the imaging frame rate can reach dozens of MHz/s or even GHz/s which is incredibly. The photon time stretching technology is widely applied to the observation of transient physical processes such as ultra-high-speed microscopic imaging, microwave information analysis, spectral analysis, dissipative soliton structures, relativistic electron clusters, megawaves and the like.

In the existing device for controlling the optical path of the probe light, the device is changed to perform dispersion Fourier transform processing on the probe light in time, and pulses of 100fs are stretched to reach tens of ps to ns magnitude. Continuous measurement of the reflectivity of the sample film on the order of femtoseconds can be achieved as the pulse width of the probe light is sufficient to cover the entire heat transfer relaxation process as each pump pulse affects the film to begin generating heat transfer. The defects that the existing point-by-point measurement is large in workload and the optical path difference needs to be adjusted repeatedly are overcome, the resolution of the high heat reflectivity is improved on the existing picosecond TTR technology, and parameters such as the heat reflectivity of a sample film, the heat conductivity, the sample boundary conductivity and the like can be measured continuously on the femtosecond level.

In conclusion, the photon time stretching technology is combined with the femtosecond-level ultrashort pulse, the femtosecond transient heat reflection technology is utilized, the femtosecond-level continuous measurement is realized on the basis of the same light source, the defects of complexity and difficulty in operation of the existing point-by-point measurement are eliminated, and the resolution is improved on the PTTR technology in the last century. The technology can be used for measuring the thermal reflectivity of various thin-film materials and the thermal conductivity of material boundaries of various materials such as sapphire, MoS2 and GaAs. Can be widely applied to the measurement of heat reflection, heat conduction and electric-phonon coupling factors of various materials.

Disclosure of Invention

The invention aims to solve the technical problems of tedious point-by-point measurement, large operation amount and incapability of acquiring data at high flux of the conventional film heat reflection detection at present, and provides a film continuous detection method under femtosecond laser photon time stretching. According to the method, the femtosecond laser pulse is stretched to ps-ns magnitude in time and is incident on the film heated by the laser, so that the reflectivity change information can be carried, and the high-flux reflectivity data can be obtained by using a high-speed oscilloscope to perform data acquisition. The method has the characteristics of low operation complexity, convenient instantaneous data acquisition, high-flux data precision, high processing speed, accurate detection result and the like. The simple and accurate method can become an effective method for the film heat transport process and has strong application value.

The technical scheme adopted by the invention is as follows:

a film thermal response single pulse detection method under ultrafast laser photon time stretching comprises the following steps:

step 1, processing a femtosecond light source into a beam of pump light with higher power and a beam of probe light with lower power by using an optical fiber coupler and a dispersion element; the specific process is as follows:

the femtosecond pulse laser light source is adopted, the light source is divided into two beams of light by a fiber coupler with higher coupling ratio, wherein one beam with higher power is used as pumping light for heating a sample film, and the other beam with lower power is used as detection light for acquiring heat reflectivity data.

And step 2, performing photon time stretching on the detection light pulse by using a dispersion element. The dispersion Fourier transform technology is utilized to stretch the detection light from the pulse width of about 100fs to ps-ns magnitude, so that the duration time of the detection light pulse is longer than the thermal response time of the detected film material, and the complete reflectivity information of the loaded film is provided.

Step 3, simultaneously focusing the pump light and the probe light to the same position of the film to be detected; the specific process is as follows:

the pumping light and the detecting light are adjusted into two beams of collimated and parallel light by a space optical element, and the two beams of light are focused to the surface of the sample by the same focusing lens at the same time. An element capable of controlling the distance between the pump light and the detection light is required to be added when the two beams of light are adjusted, so that the two beams of light can meet the purpose of separating and collecting the detection light through the angle after the two beams of light are reflected by the sample.

Step 4, collecting and analyzing the detection light loaded with the film reflectivity information; the specific process is as follows:

firstly, probe light reflected from a film material is coupled into a single mode fiber through a coupling system, the single mode fiber is connected with a high-speed photoelectric test system, the change of the pulse intensity of the probe light after photon time stretching along with time is recorded, the change of the reflectivity of the film along with time is obtained, and the thermal response characteristic of the film can be analyzed.

And 5, performing secondary stretching on the obtained detection light, performing secondary photon time stretching on the part with obvious light intensity change, and performing high-resolution film thermal transport process analysis.

The dispersion stretching element can be any optical element capable of stretching the ultrashort pulse laser by using a dispersion effect, such as a standard single-mode optical fiber, a dispersion compensation optical fiber, a chirped fiber grating and a space grating.

The ultrafast pulse laser is characterized in that the pulse width is less than 1 ps.

The coupling system needs to achieve the function of collecting the detection light into the single-mode optical fiber by combining elements such as a coupling lens, an adjusting frame, a diaphragm and the like, the parameters of the coupling lens are matched with the diameter of a mode field of the optical fiber, and the coupling system comprises all elements capable of coupling the space light into the single-mode or multi-mode optical fiber, such as a focusing lens, a lens group, an objective lens and the like.

The high-speed photoelectric detector and the high-speed oscilloscope can complete instantaneous high-flux data acquisition for detection light, and experimental data with femtosecond high resolution can be acquired by utilizing high bandwidth and high sampling rate.

The invention has the advantages and positive effects that:

the invention provides a method for continuously detecting a film under femtosecond laser photon time stretching, which is widely applied to the research aspect of high-throughput analysis of the film heat transport process. The method provided by the invention combines the photon time stretching technology with FTTR, and overcomes the defects of complicated and complicated operation that the time delay of two beams of light needs to be changed by controlling the spatial optical path difference each time in the original measuring method. In the existing FTTR technology, the relative reflectivity change of each point within the reflectivity change time of a sample is measured by changing the time delay of probe light relative to pump light by a stepping motor for many times within the time range in which each pump light pulse reaches a sample thin film to change the reflectivity of the sample, as shown in schematic diagram 4; according to the method, the detection light is subjected to time stretching, and the duration of the detection light covers the reflectivity change time of the sample, so that the reflectivity change quantity of the sample can be continuously measured in real time in one period, which is shown in a schematic diagram 5; meanwhile, in the light path design, the invention adopts the mode of combining the optical fiber and the space light path on the original full-space light path mode, thereby improving the stable control on the time scale of the detection light. In addition, for the detection light loaded with the sample reflectivity change information, secondary time stretching can be carried out, and the sensitivity of the sample reflectivity can be improved. Therefore, the method is an efficient and accurate calculation method for the film thermal reflectivity change and the thermal transport process analysis, and has important application value.

Drawings

FIG. 1 is a schematic diagram of a method for continuous detection of thin films under time stretching by femtosecond laser photons according to the present invention.

In the figure: 1. a polarization maintaining fiber coupler; 2. pump light; 3. detecting light; 4. a dispersive stretching element; 5. a time delay line; 6. a fiber collimator; 7. a polarizing beam splitter; 8. a mirror; 9. a focusing lens; 10. a sample film; 11. a fiber optic coupling system; 12. a photodetector; 13. a high-speed oscilloscope; 14. an optical system.

Fig. 2 is a schematic diagram of an embodiment of an optical system module. The whole is realized by FiberBench and a wall plate carrying an optical element.

In the figure: 1. a reflector capable of adjusting the horizontal position; 2. a pump light input end wall plate; 3. a detection light input end wall plate; 4. a wave plate; 5. a polarizing beam splitter; 6. a focusing lens wall panel; 7. a sample and a sample adjusting rack; 8. FiberBench base plate.

Fig. 3 is a graph showing a pulse width comparison of the femtosecond pulse laser before and after being stretched by the dispersion element.

In the figure: 1 is an unstretched femtosecond pulse; 2 is the pulse after photon time stretching by the dispersion compensation fiber; 3 and 4 are the full width at half maximum of the original femtosecond pulse, about 100 fs; and 5 is the pulse width after photon time stretching, which is about 10 ns.

Fig. 4 is a schematic diagram of the relative relationship between pump light, sample reflectivity change and probe light on a time scale in the conventional femtosecond laser pumping-detection technology.

In the figure: 1 is the variation of the normalized intensity of the pump light pulse with time within 6 ps; 2 is the change of the reflectivity of the sample along with the time after the sample is excited by the pump light; 3, detecting the change of the light pulse normalized intensity along with time; 4 is the time delay of the probe light relative to the pump light; and 5 is the corresponding reflectivity change of the sample detected by the detecting light in the case of the 4.

Fig. 5 is a schematic diagram of the relative relationship of pump light, sample reflectivity change and stretched detection light on a time scale in the femtosecond laser pumping-detection technology realized by the method.

In the figure: 1 is the variation of the normalized intensity of the pump light pulse with time within 6 ps; 2 is the change of the reflectivity of the sample along with the time after the sample is excited by the pump light; 3, the change of the pulse normalized intensity of the detection light after photon time stretching along with the time; and 4 is a time range corresponding to the change of the reflectivity of the sample which can be detected when the detection light is stretched to the condition.

Detailed Description

The present invention will be further explained by taking the continuous measurement of transient heat reflectivity change of 120nm gold film using femtosecond laser with center wavelength 1560nm and pulse width 100fs as light source, and the attached drawings are only used for example purposes and are not intended to limit the application scope of the present invention.

Firstly, the output light of the laser is divided into two paths by a polarization-maintaining fiber coupler with the power ratio of 9: 1, in the embodiment, femtosecond pulse laser with the center wavelength of 1560nm is selected, the repetition frequency is 50MHz, the pulse width is 100fs, the peak power is 50mW, and the single-axis and slow-axis work is carried out. The polarization maintaining fiber coupler divides a light source into two beams, one beam of light with strong energy is used as pumping light, and the other beam of light with weak energy is used as detection light. The average power of the pump light is 30mW and the average power of the probe light is 3mW measured by a power meter.

Secondly, one path of the detection light is connected with a dispersion compensation optical fiber, so that the pulse light of 100fs is subjected to photon time stretching, a stretching element uses the dispersion compensation optical fiber of 1.5km, and the dispersion value is-100 to-170 ps/km/nm. In the case of a femtosecond pulsed laser with a center wavelength of 1560nm, the dispersion value is measured by the experiment to be about 146.2ps/km nm, and the pulse width is stretched from-100 fs to about 10 ns. The power of the stretched detection light is weakened, and after the detection light is stretched, the detection light is amplified by an optical fiber amplifier so as to meet the power requirement of back-end detection.

The pump light and probe light are then integrated into a collimated and mutually parallel optical system. Firstly, pumping light and detecting light are mutually connected by using wall plates carrying optical fiber collimators, and the two wall plates can ensure that the emergent points of the two beams of light have the same height in the horizontal direction. The two beams of light are kept in a parallel and mutually perpendicular state by adjusting the pitching angles of the three pitching adjusting nuts of the collimator. After the parallel collimation of the two beams of light is finished, respectively placing 1/4 wave plates, 1/2 wave plates and 1/4 wave plates at the spatial positions where the detection light passes through, and enabling the polarization direction of the detection light to be the horizontal direction as much as possible by adjusting the angles of the three wave plates; a reflector is arranged on a spatial path of the pumping light, the pumping light is reflected by the reflector and then passes through a Polarization Beam Splitter (PBS) with Tp: Ts > 99: 1 together with the detection light, and the schematic diagram is shown in figure 2. The pump light polarized in the vertical direction and the detection light polarized in the horizontal direction can be combined into the same beam of light after passing through the PBS at the same time, and then the pump light and the detection light are focused to the same point of the film to be detected through the same focusing lens. The reflector of the pump light is placed on a one-dimensional manual displacement table, the horizontal position of the displacement table is adjusted, two beams of light which pass through PBS become parallel and are converged for several millimeters, and the two beams of light can be separated by a certain angle after being reflected by a film.

The two beams of light which are well adjusted and the focusing lens are integrated into a part of a whole, and the positions in three directions are adjustable. Ensuring that the detection light emits to the sample film along the central axis of the focusing lens, and ensuring that the detection light vertically enters the coupling system by adjusting the horizontal position of the whole body and the pitching and horizontal angles of the sample film; the distance of the pump light from the central axis of the focusing lens is adjusted to ensure that the separation angle between the sample film and the detection light is large enough after the sample film is reflected, so that the error of the pump light coupled into the single-mode fiber on the experimental result is reduced.

In order to realize that the real-time change of the reflectivity generated after the film is influenced by the pump light can be reflected on the detection light, the requirement that when a pump light pulse is applied on the film, the detection light which is stretched to cover the reaction time of the film at a time is required to be met,

this requires matching the pulse arrival times of the two beams.

After the spatial position debugging of the pumping light and the detection light is finished, the detection light carrying the film reflection information and the pumping light for exciting the film thermal reaction are collected through a coupling system after being reflected by the film. Two beams of light are coupled into a single-mode optical fiber by a coupling system provided with a diaphragm, a coupling lens and an optical fiber adapter, and time domain information of the two beams of light is observed by a high-speed photoelectric detector and a high-speed oscilloscope. The starting positions of two beams of light pulses are changed by increasing the length of the single-mode optical fiber before the probe light enters the space optical path until the starting position of the stretched probe light is approximately synchronous with the pump light.

The time-synchronized pump light is filtered by a polaroid only allowing the horizontal polarization direction to pass through, only the probe light is left for data acquisition, and as the film heat transport time is finished within dozens of ps, the duration time is too short for ns-magnitude pulses, and the resolution of the acquired reflectivity information is insufficient. To analyze the instantaneous change of the film reflectivity within tens of ps, the output probe light needs to be stretched for the second time, and then a high-speed oscilloscope is used for observing the probe light after the second stretching, so that the heat transport process of the measured film can be analyzed on the ps scale.