A pump-probe system for a single femtosecond pulse duration

Abstract

The present invention discloses a pump-detection system for use within a single femtosecond pulse time, to address the problem that existing pump-detection systems are unable to accurately obtain physical changes in materials within the time scale of a pump light pulse excitation. Specifically, it includes a femtosecond laser light source, a beam splitter, a pump light transmission unit, a detection light transmission unit, and a photodetector; the femtosecond laser light source emits a pulsed laser that is divided into pump light and detection light by a beam splitter; the pump light transmission unit includes a first reflector, an optical delay device, a second reflector, a third reflector, and a field lens sequentially arranged along the pump light transmission optical path; the detection light transmission unit includes a 1 / 2 wave plate, a polarization beam splitter prism, a first convex lens, a hollow-core photonic crystal fiber, and a second convex lens sequentially arranged along the detection light transmission optical path. The pump-detection system of the present invention performs pulse width adjustment so that the detection light has a shorter pulse width than the pump light, and can measure changes in the physical properties of the material to be measured within a single pump pulse time.

Description

Technical Field

[0001] The present invention relates to a pump detection system, in particular to a pump detection system used within a single femtosecond pulse time. Background Art

[0002] With the rise of ultrafast laser technology in recent decades, research on the interaction between ultrafast lasers and materials has become a vibrant field. Femtosecond laser ablation of materials involves a complex set of physical processes. First, it requires studying the generation and propagation of the laser and the interaction between light and materials. Second, the material's transformation into a plasma at high temperatures requires studying the formation, expansion, light absorption, and scattering of the plasma. Finally, a complete description of this process, including phase transitions and heat transfer, is required. The complexity of these physical processes presents significant challenges in this research direction, and to date, a systematic solution has yet to be widely recognized by the scientific community. Most experimental observation methods focus on observing the timescale after a single pulse (on the order of hundreds of femtoseconds to nanoseconds). However, it is precisely the absorption of materials during a single pulse that drives the subsequent series of physical phenomena (on the order of sub-hundreds of femtoseconds). Chinese researchers have made significant efforts in this area, but while some progress has been made, they have yet to achieve precise detection of the duration of a single laser-excited material pulse. This has led to a lack of understanding of the absorption process of laser pulses and the properties of the ultrahigh-temperature plasma during the initial ablation phase.

[0003] The action of a single femtosecond laser pulse on a material is a nonlinear process with an extremely short time scale. The ionization of the material by the femtosecond laser lasts for about a few femtoseconds, while the duration of a single femtosecond pulse is only tens to hundreds of femtoseconds. It is not easy to carry out precise and accurate detection on such a short time scale.

[0004] In Chinese patent application number 201710470834.2, titled "A Dual-Beam Pump-Probe Experimental System," to accurately study complex microscopic motion processes within materials and capture transient changes, the pump light in the pump-probe system is split to increase the number or duration of transient reflectivity fluctuations, thereby increasing the chance and probability of detection. Specifically, the femtosecond pulse is split into two paths: pump light and probe light. The pump light excites the material, while the probe light detects the excited material. The spatial delay between the two beams is converted into a time delay between the pump and probe lights, thereby obtaining physical information about the material at different moments. However, this approach of increasing the duration of the action does not fundamentally solve the problem of accurate transient observation, and this multiple-pumping approach cannot reflect the general laws of interaction between pulsed lasers and materials. Furthermore, different power densities of pump light correspond to different ablation mechanisms. Under this beam-splitting scheme, the measurement of transient material properties is mainly concentrated in the time scale after the action of the pump light pulse, generally in the time scale of hundreds of femtoseconds to nanoseconds. In addition, the pump light and the probe light pulse are both obtained by beam splitting the same light pulse, so they have the same pulse width. Under this condition, if the same pulse width is used for measurement, the result is only Gaussian weighting with long-term accuracy. It is impossible to accurately obtain the changes in the physical properties of the material within the time scale of a pump light pulse excitation, resulting in the measurement results not reflecting the universal physical change process. Summary of the Invention

[0005] The purpose of the present invention is to provide a pump-detection system for a single femtosecond pulse time, so as to solve the technical problem that the existing pump-detection system cannot accurately obtain the physical changes of the material within the time scale of a pump light pulse excitation.

[0006] The inventive concept of this invention is to compress the original probe light pulse width to accurately reflect the transient physical processes occurring during the action of a single pulse on the material under test. This allows for precise detection of the action process using a narrower probe light pulse width, while maintaining the original pump light pulse width. This directly addresses the issue of the short duration of the pump light action and the difficulty in detecting the action process. By measuring the transient reflectivity of the material surface, the transient absorption of light by the material under test during the initial phase of laser action is accurately reflected, providing a direct understanding of the core initial processes of the entire light-matter interaction.

[0007] In order to achieve the above-mentioned purpose and realize the above-mentioned inventive concept, the present invention provides a pump-detection system for a single femtosecond pulse time, which is special in that it includes a femtosecond laser light source, a beam splitter, a pump light transmission unit, a detection light transmission unit and a photodetector;

[0008] The femtosecond laser light source emits pulsed laser which is divided into pump light and probe light by a beam splitter;

[0009] The pump light transmission unit includes a first reflector, an optical delay device, a second reflector, a third reflector, and a field mirror, which are sequentially arranged along the pump light transmission optical path; the pump light is reflected by the first reflector, enters the optical delay device, is delayed by the optical delay device, and is emitted to the second reflector, and is reflected by the second reflector and the third reflector, and enters the field mirror, and is focused by the field mirror and incident on the surface of the material to be tested;

[0010] The detection light transmission unit includes a 1 / 2 wave plate, a polarization beam splitter prism, a first convex lens, a hollow-core photonic crystal fiber, and a second convex lens, which are sequentially arranged along the detection light transmission path. The detection light enters the first convex lens after power adjustment through the 1 / 2 wave plate and the polarization beam splitter prism. The first convex lens couples the adjusted detection light pulse into the hollow-core photonic crystal fiber for nonlinear spectrum broadening. The light beam is then collimated through the second convex lens and emitted to the surface of the material to be tested. The photodetector receives the light beam reflected by the surface of the material to be tested.

[0011] Furthermore, the detection light transmission unit further includes a first GTI mirror pair and a second GTI mirror pair arranged on the optical path between the second convex lens and the material to be tested;

[0012] The first GTI mirror pair and the second GTI mirror pair are sequentially arranged along the detection light emitted after being collimated by the second convex lens, and are used to perform dispersion compensation compression on the detection light emitted after being collimated.

[0013] Furthermore, the detection light transmission unit further includes a fourth reflector, a fifth reflector and a sixth reflector;

[0014] The fourth reflector is arranged on the optical path between the beam splitter and the 1 / 2 wave plate, and is used to reflect the detection light split by the beam splitter to the 1 / 2 wave plate;

[0015] The fifth reflector and the sixth reflector are arranged on the optical path between the second convex lens and the first GTI mirror pair, and are used to continuously reflect the collimated detection light so that it is incident on the first GTI mirror pair.

[0016] Furthermore, it also includes a laser beam expander;

[0017] The laser beam expander is arranged on the optical path between the femtosecond laser light source and the beam splitter, and is used to expand and collimate the pulsed laser.

[0018] Furthermore, the laser beam expander is a laser beam expander that expands the pulsed laser spot to twice its size.

[0019] Furthermore, the optical delay device includes a seventh reflecting mirror, an eighth reflecting mirror and a stepping motor;

[0020] The seventh reflector and the first reflector are parallel to each other;

[0021] The eighth reflector and the second reflector are parallel to each other;

[0022] The seventh reflector and the eighth reflector are perpendicular to each other, and both the seventh reflector and the eighth reflector are installed at the active end of the stepping motor.

[0023] Furthermore, the beam splitter is a 5:5 beam splitter.

[0024] Furthermore, the hollow-core photonic crystal fiber is a kagome-type hollow photonic crystal fiber.

[0025] Furthermore, the nonlinear refractive index of the hollow core photonic crystal fiber is 0.625·10-23m 2 / W, the fiber length is 3m, the fiber inner diameter is 40um, the effective mode field diameter is 31um, and the numerical aperture NA is 0.015.

[0026] Furthermore, the group delay dispersion of the two GTI mirrors in the first GTI mirror pair is -550 fs. 2 ;

[0027] The group delay dispersion of the two GTI mirrors in the second GTI mirror pair is -100fs. 2 .

[0028] Beneficial effects of the present invention:

[0029] 1. Compared with the traditional pump-detection system with the same pulse width, the present invention has performed pulse width adjustment, so that the detection light has a shorter pulse width than the pump light. It can not only measure the accurate changes in the physical properties of the material to be measured within a pump pulse time, but also detect changes outside the pump pulse time by changing the delay.

[0030] 2. The present invention also incorporates a first GTI mirror pair and a second GTI mirror pair in the probe light transmission unit. These mirror pairs compensate for the dispersion of the stretched probe light. By combining them with hollow-core photonic crystal fiber, they achieve nonlinear compression of the probe light, resulting in a shorter pulse width than the pump light. A stepper motor delay line then adjusts the spatial optical path length of the two beams, resulting in a temporal delay between the two beams. This allows for accurate and precise measurement of transient physical properties within the action time of a single pulse.

[0031] 3. The optical delay device used in the present invention is driven by a stepping motor. The minimum single-step step of the stepping motor is 100 nm, which can provide a smaller step distance and thus obtain a higher time resolution.

[0032] 4. The present invention adopts Kagome-type hollow photonic crystal fiber, which is a new type of microstructured fiber that does not rely on bandgap light guidance. It has a flexible structural design, a high damage threshold, low loss (the loss in the high-transmittance area can be as low as ~40dB / km), supports broadband transmission (100-500nm), and can effectively modulate the fiber dispersion and nonlinear effects by changing the gas filled in the fiber core and adjusting the gas pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a timing diagram of the pump-probe pulses within the action time of a single femtosecond pulse in an embodiment of the present invention;

[0034] Figure 2 1 is a schematic structural diagram of an embodiment of a pump-probe system for a single femtosecond pulse time according to the present invention;

[0035] Figure 3 is a GDD curve diagram of the GTI mirror for different bands in an embodiment of the present invention;

[0036] Figure 4 1 is a diagram showing the autocorrelation curve of the compressed pulse and the distribution of the light spot pattern in an embodiment of the present invention.

[0037] Figure Number:

[0038] 1-Femtosecond laser light source, 2-Pulsed laser, 3-Laser beam expander, 4-Beam splitter, 5-Probe light, 6-Pump light, 7-Fourth reflector, 8-1 / 2 wave plate, 9-Polarization beam splitter prism, 10-First convex lens, 11-Hollow core photonic crystal fiber, 12-Second convex lens, 13-Fifth reflector, 14-Sixth reflector, 15-First GTI mirror pair, 16-First reflector, 17-Optical delay device, 18-Third reflector, 19-Field mirror, 20-Material to be tested, 21-Photodetector, 22-Second reflector, 23-Second GTI mirror pair, 24-Seventh reflector, 25-Eighth reflector. DETAILED DESCRIPTION

[0039] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0040] The principles of the present invention are as follows:

[0041] like Figure 1 As shown, by splitting the main laser pulse into pump and probe beams, the nonlinear effects of the hollow-core photonic crystal fiber on the probe beam are used to effectively broaden the spectrum of the ultrashort pulse. Combined with external dispersion compensation, nonlinear compression is achieved, resulting in a pulse width shorter than that of the pump beam. A stepper motor delay line then adjusts the spatial optical path length of the two beams, resulting in a temporal delay between the two beams. This allows for accurate and precise measurement of transient physical properties within the action time of a single pulse.

[0042] A pump-probe system for single femtosecond pulse duration, such as Figure 2 As shown, it includes a femtosecond laser light source 1, a laser beam expander 3, a beam splitter 4, a pump light transmission unit, a detection light transmission unit and a photodetector 21; the pump light transmission unit includes a first reflector 16, a second reflector 22, an optical delay 17, a third reflector 18 and a field lens 19; the detection light transmission unit includes a 1 / 2 wave plate 8, a polarization beam splitter prism 9, a first convex lens 10, a hollow core photonic crystal 11 and a second convex lens 12, and also includes a first GTI mirror pair 15 and a second GTI mirror pair 23 arranged on the optical path between the second convex lens 12 and the material to be tested 20, and also includes a fourth reflector 7, a fifth reflector 13 and a sixth reflector 14;

[0043] A femtosecond laser light source 1 emits an incident femtosecond pulse laser 2 with a pulse width of 279 fs, which is expanded and collimated to twice the spot size by a laser beam expander 3, and then split into a pump light 6 and a probe light 5 by a 5:5 beam splitter 4. The pump light 6 is adjusted in optical path length by a first reflector 16 and an optical delay device 17, and then focused by a third reflector 18 and a field lens 19 to act on the surface of a material to be tested 20. The power of the detection light 5 is controlled by the 1 / 2 wave plate 8 and the polarization beam splitter prism 9. The first convex lens 10 couples the adjusted detection light 5 pulse into the hollow-core photonic crystal fiber 11 for nonlinear spectral broadening. The light beam is then collimated by the second convex lens 12 and emitted. The light beam is then transmitted to the first GTI mirror pair 15 and the second GTI mirror pair 23 through the fifth reflector 13 and the sixth reflector 14 for dispersion compensation compression. The compressed narrow pulse (36fs) is obliquely incident on the surface of the material to be tested 20, and enters the photodetector 21 after reflection from the surface of the material to be tested 20.

[0044] The laser beam expander 3 includes two lenses, the first lens has a long focal length, and the second lens has a short focal length. The two focal points overlap, thereby achieving the purpose of beam expansion and collimation.

[0045] The optical delay device 17 includes a seventh reflector 24, an eighth reflector 25 and a stepper motor; the seventh reflector 24 is parallel to the first reflector 16; the eighth reflector 25 is parallel to the second reflector 22; the seventh reflector 24 and the eighth reflector 25 are perpendicular to each other, and the seventh reflector 24 and the eighth reflector 25 are both installed at the active end of the stepper motor, so the distance between the seventh reflector 24 and the first reflector 16 and the distance between the seventh reflector 24 and the eighth reflector 25 can be adjusted simultaneously by the stepper motor.

[0046] like Figure 3 As shown, the hollow photonic crystal fiber is a Kagome-type hollow photonic crystal fiber, a new type of microstructured fiber that does not rely on bandgap light guidance. The Kagome hollow fiber used in this system has a core diameter of 40μm, an effective mode field diameter of 31μm, and a numerical aperture (NA) of 0.015. The vacuum level when the system is not operating is 3mbar, and the fiber length is 3m. To achieve effective coupling, the focused spot diameter is close to the fiber's mode field diameter, while the focused numerical aperture is smaller than it. For an incident 3.6mm collimating lens, a lens with an f=100mm is used for coupling.

[0047] like Figure 3 As shown, the first GTI mirror pair 15 and the second GTI mirror pair 23 are coated with layers of different thicknesses, creating standing waves at specific laser wavelengths in specific layers to adjust the dispersion of femtosecond pulses. The group delay dispersion of both GTI mirrors in the first GTI mirror pair 15 is -550 fs²; the group delay dispersion of both GTI mirrors in the second GTI mirror pair 23 is -100 fs². The reflectivity of both GTI mirror pairs 15 and 23 for light between 950 nm and 1150 nm is 99.9%.

[0048] The present invention performs nonlinear compression on the pulse width of the detection light, which is described in detail below. Figure 4As shown, the average power of the probe light is 6 W, the repetition rate is 100 kHz, the pulse width is 279 fs, and the single pulse energy is 60 μJ. The injected power is modulated by a half-wave plate 9 and a polarization beam splitter prism 9. Under the conditions of an injected signal of 3 W, a pulse width of 279 fs, a repetition rate of 100 kHz, and an internal fiber pressure of 1.013 bar, the probe light spectrum is broadened to 60 nm. The nonlinear refractive index used is 0.625·10-23 m2 / W, and the fiber length is 3 m. The broadened spectrum is compressed using two pairs of GTI mirrors (Layertec GmbH), which have a high reflectivity of 99.9% in the 950 nm to 1150 nm band, ensuring a compression efficiency of nearly 70% for the entire system. The compressed pulse width is measured using an autocorrelator, yielding a compressed pulse output of 36 fs.

[0049] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed in the present invention shall be covered by the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

Claims

1. A pump-probe system for a single femtosecond pulse, characterized in that: It comprises a femtosecond laser light source (1), a beam splitter (4), a pump light transmission unit, a detection light transmission unit and a photodetector (21); The femtosecond laser light source (1) emits a pulsed laser (2) which is split into a pump light (6) and a probe light (5) by a beam splitter (4); The pump light transmission unit comprises a first reflector (16), an optical delay device (17), a second reflector (22), a third reflector (18) and a field mirror (19) which are sequentially arranged along the pump light transmission optical path; the pump light (6) is reflected by the first reflector (16) and then enters the optical delay device (17); after being delayed by the optical delay device (17), it is emitted to the second reflector (22); after being reflected by the second reflector (22) and the third reflector (18), it enters the field mirror (19); after being focused by the field mirror (19), it is incident on the surface of the material to be tested (20); The detection light transmission unit comprises a 1 / 2 wave plate (8), a polarization beam splitting prism (9), a first convex lens (10), a hollow core photonic crystal fiber (11) and a second convex lens (12) which are sequentially arranged along the detection light transmission optical path; the detection light (5) enters the first convex lens (10) after undergoing power adjustment through the 1 / 2 wave plate (8) and the polarization beam splitting prism (9); the first convex lens (10) couples the adjusted detection light (5) pulse into the hollow core photonic crystal fiber (11) to perform nonlinear spectrum broadening, and then collimates the light beam through the second convex lens (12) and emits the light beam to the surface of the material to be tested (20); the photodetector (21) receives the light beam reflected from the surface of the material to be tested (20); The detection light transmission unit further includes a first GTI mirror pair (15) and a second GTI mirror pair (23) arranged on an optical path between the second convex lens (12) and the material to be tested (20); The first GTI mirror pair (15) and the second GTI mirror pair (23) are sequentially arranged along the detection light (5) emitted after being collimated by the second convex lens (12), and are used to perform dispersion compensation compression on the detection light (5) emitted after being collimated.

2. The pump-probe system for a single femtosecond pulse according to claim 1, characterized in that: The detection light transmission unit further includes a fourth reflecting mirror (7), a fifth reflecting mirror (13) and a sixth reflecting mirror (14); The fourth reflector (7) is arranged on the optical path between the beam splitter (4) and the half-wave plate (8), and is used to reflect the detection light (5) split by the beam splitter (4) to the half-wave plate (8); The fifth reflector (13) and the sixth reflector (14) are arranged on the optical path between the second convex lens (12) and the first GTI mirror pair (15), and are used to continuously reflect the collimated detection light (5) so that it is incident on the first GTI mirror pair (15).

3. The pump-probe system for a single femtosecond pulse according to claim 1 or 2, characterized in that: Also included is a laser beam expander (3); The laser beam expander (3) is arranged on the optical path between the femtosecond laser light source (1) and the beam splitter (4), and is used for expanding and collimating the pulsed laser (2).

4. The pump-probe system for a single femtosecond pulse according to claim 3, characterized in that: The laser beam expander (3) is a laser beam expander that expands the spot of the pulsed laser (2) to twice its size.

5. The pump-probe system for a single femtosecond pulse according to claim 4, characterized in that: The optical delay device (17) includes a seventh reflecting mirror (24), an eighth reflecting mirror (25) and a stepping motor; The seventh reflector (24) and the first reflector (16) are parallel to each other; The eighth reflector (25) and the second reflector (22) are parallel to each other; The seventh reflector (24) and the eighth reflector (25) are perpendicular to each other, and both the seventh reflector (24) and the eighth reflector (25) are mounted on the active end of the stepping motor.

6. The pump-probe system for a single femtosecond pulse according to claim 5, characterized in that: The beam splitter (4) is a 5:5 beam splitter.

7. The pump-probe system for a single femtosecond pulse according to claim 1, characterized in that: The hollow-core photonic crystal fiber (11) is a kagome-type hollow photonic crystal fiber.

8. The pump-probe system for a single femtosecond pulse according to claim 7, characterized in that: The nonlinear refractive index of the hollow-core photonic crystal fiber (11) is 0.625·10-23m 2 / W, the fiber length is 3m, the fiber inner diameter is 40um, the effective mode field diameter is 31um, and the numerical aperture NA is 0.

015.

9. The pump-probe system for a single femtosecond pulse according to claim 1 or 2, characterized in that: The group delay dispersion of the two GTI mirrors in the first GTI mirror pair (15) is -550 fs. 2 ; The group delay dispersion of the two GTI mirrors in the second GTI mirror pair (23) is -100 fs. 2 .

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