An isolated attosecond pulse generation method

Abstract

The present invention discloses a method for generating isolated attosecond pulses, belonging to the field of ultrashort pulse laser technology. The method is to introduce a long-pulse pump light on the basis of ionization gating, continuously inject the energy of the pump light into the ionization pulse, and generate attosecond pulses through the transfer of pump energy to attosecond pulse energy. The present invention first proposes a method for generating attosecond pulses from picosecond pulses. Since the energy of picosecond pulses can reach thousands of joules, at a conversion efficiency of about 0.1%, the corresponding energy of the generated attosecond pulses can reach several joules, which is 5-6 orders of magnitude higher than the existing microjoules.

Description

Technical Field

[0001] The present invention relates to the technology of ultrashort pulse lasers, and particularly to a method for generating isolated attosecond pulses. Background Art

[0002] With the development of the technology of ultrashort pulse lasers, the pulse width of the currently generated lasers has reached the attosecond level (10 -18 s). Among them, isolated attosecond pulses have important application values in ultrafast diagnosis. Due to the extremely short duration of the pulses, the electron transition processes in atoms, molecules, and solids can be directly diagnosed through isolated attosecond pulses.

[0003] The technology of isolated attosecond pulses has developed rapidly in the past 20 years. Currently, isolated attosecond pulses are mainly generated through two methods:

[0004] One is to generate attosecond pulses by high-order harmonic generation, and its process can be described by a three-step model: the first step, under the action of a strong laser electric field, the Coulomb potential barrier of the atom is depressed, and electrons can tunnel through the potential barrier through tunneling ionization; the second step, the free electrons after ionization are accelerated in the external laser field and obtain energy; the third step, after the electric field reverses, some electrons return to the vicinity of the atom and recombine with the parent ions, and at the same time, photons are radiated, and the photon energy is equal to the sum of the ionization energy of the electrons and the additional kinetic energy obtained by the electrons from the laser field. Since the photon energy is relatively high, the optical wave wavelength is generally in the extreme ultraviolet or soft X-ray band, corresponding to attosecond pulses in the time domain. However, since the laser pulse generally contains multiple cycles, the attosecond pulses generated in this way are a train of pulses. To obtain isolated attosecond pulses, various gating techniques have been developed, such as polarization gating, amplitude gating, and spatial gating. The main principle is to limit the generation time of the attosecond pulses within one laser cycle to obtain isolated attosecond pulses. Restricted by the conversion efficiency of high-order harmonics and the energy utilization rate of the gating techniques, the current efficiency of generating isolated attosecond pulses by high-order harmonic generation is at 10 -5 below, and the energy of the attosecond pulses is on the order of nanojoules (10 -9 J). Among many gating techniques, an effective technique is the ionization gating technique. The main principle is to ionize the gas with a strong short pulse, so that the pulse front completely ionizes the gas. In the completely ionized region, high-order harmonics cannot be generated due to phase mismatch. In this way, high-order harmonics are only generated in the pulse front region, and isolated attosecond pulses are thus generated.

[0005] In addition, the Max Planck Institute in Germany has developed a method for generating isolated attosecond pulses by spectral coherent beam combining. The main method is to perform coherent beam combining on four few-cycle pulses (5 fs - 10 fs) from ultraviolet to infrared. Through this method, a laser pulse of 380 as is generated, and the pulse energy reaches 20 μJ. The energy of this kind of attosecond pulse is much higher than that of the attosecond pulse generated by high-order harmonic generation. However, limited by the energy of few-cycle laser pulses, especially the energy of ultraviolet few-cycle laser pulses, it is difficult to increase the energy of the generated isolated attosecond pulses. Summary of the Invention

[0006] The object of the present invention is to provide a method for generating isolated attosecond pulses to solve the above problems.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for generating isolated attosecond pulses, on the basis of ionization gating, a long-pulse pump light is introduced, and the energy of the pump light is continuously injected into the ionization pulse, and attosecond pulses are generated through the transfer of pump energy to attosecond pulse energy.

[0008] The "long-pulse pump light" of the present invention refers to a laser with a duration of 1 ps - 100 ps.

[0009] As a preferred technical solution, the specific steps are as follows: A long-pulse laser in the nanosecond range (1 ns - 20 ns) is equally divided into two beams by a beam splitter. Then, one of the two beams of laser passes through two light guiding mirrors and a focusing lens in sequence, and the other beam passes through a light guiding mirror and a focusing lens in sequence, and is focused into the gas to generate high-frequency sound waves; finally, a short-pulse laser in the femtosecond range is focused into the gas through a lens to ionize the sound wave to generate a plasma grating with a rapidly extending boundary, dynamically reflect the pump pulse, and a narrow cross region is formed between the reflection region and the frequency up-conversion region, and isolated attosecond pulses are generated in the cross region.

[0010] The "long-pulse laser in the nanosecond range" of the present invention refers to a laser with a duration of 1 ns - 20 ns; the "short-pulse laser in the femtosecond range" refers to a laser with a duration of 1 fs - 100 fs.

[0011] First, two laser beams are used to interfere in a gas, and a gas grating structure is generated by the electrostriction effect in the gas. Subsequently, a plasma grating with boundaries extending along the beam direction is generated by ionizing the gas grating with a short-pulse laser. Finally, a pump light whose propagation direction is opposite to the extension direction of the plasma grating is introduced. When the pump light and the plasma grating approach the Bragg condition, the pump light will be dynamically reflected to continuously inject energy into the ionization pulse. At the same time, when the ionization pulse ionizes the gas, a frequency up-conversion effect will occur in the ionization region. There is a very short crossover region between the reflection of the laser and the frequency up-conversion. In this region, the laser is continuously up-converted, and at the same time, the fundamental-frequency pump light is continuously injected, thereby generating an ultra-broadband attosecond pulse. The energy of the attosecond pulse comes from the long-pulse pump light with a duration of 1 ps - 100 ps.

[0012] One of the cores of the ultrashort pulse compression method of the present invention is to utilize the crossover region between the up-conversion effect of laser ionizing gas and the energy injection by pump light reflection. In this region, the ionization pulse is up-converted, and at the same time, the energy of the laser is supplemented. Among them, the reflectivity of the pump light can be adjusted by the grating period, and the up-conversion effect in the ionization region can be adjusted by the pulse width of the ionization pulse. Finally, by adjusting the reflectivity of the pump light and the up-conversion effect region to control the width of the crossover region, the generation of isolated attosecond pulses is achieved.

[0013] The fast-extending plasma grating involved in the present invention is generated by ionizing a gas grating with a short pulse. Among them, the gas grating is generated by the Brillouin effect in the gas. The Brillouin effect widely exists in solids, liquids, and gases. The gas grating (high-frequency acoustic wave) is a by-product of the Brillouin effect. Its main principle is that the laser interferes in the gas to generate a stable strong-weak distribution area, and then the electrostriction effect is used to generate a periodic strong-weak distribution of gas density, thereby forming a gas grating. When the included angle between the two laser beams is α, the period of the generated acoustic wave is λ / 2sin(α / 2), where λ is the wavelength of the laser. By adjusting the included angle α between the two beams of light, the period of the acoustic wave can be adjusted from λ / 2 to infinity. When the gas grating is ionized by a short pulse, the gas grating is converted into a plasma grating. Since the refractive index change of the plasma grating with the same amplitude is more than two orders of magnitude that of the gas grating, the generated plasma grating can totally reflect the pump light that satisfies the Bragg condition. At the same time, since the ionization pulse advances along the beam, the generated plasma grating has a boundary extending along the grating, so it can dynamically reflect the pump light and continuously inject energy into the ionization pulse.

[0014] Compared with the prior art, the advantages of the present invention are as follows: The present invention first proposes a method for generating attosecond pulses from picosecond pulses. Since the energy of picosecond pulses can reach several thousand joules, at a conversion efficiency of about 0.1%, the energy of the corresponding generated attosecond pulses can reach several joules, which is 5 - 6 orders of magnitude higher than the existing microjoules. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic diagram of the principle of the present invention;

[0016] Figure 2 is the optical path diagram of the implementation of the present invention;

[0017] Figure 3 is the simulation result diagram of the embodiment of the present invention.

[0018] Figure 1 Among them: a, gas grating; b, plasma grating; c, ionization laser pulse; d, pump pulse; e, isolated attosecond pulse; 1, long pulse laser; 2, first beam splitter; 3, first light guiding mirror; 4, second light guiding mirror; 5, third light guiding mirror; 6, first focusing lens; 7, second focusing lens; 8, short pulse laser focusing lens, 9, second beam splitter. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be further described below in conjunction with the accompanying drawings. Embodiment

[0020] A method for generating an isolated attosecond pulse, the principle of which is as Figure 1 shown:

[0021] First, when the ionization laser pulse c enters the gas grating a, the gas grating a is ionized into a plasma grating b. Since the ionization laser pulse c propagates as a light beam, the boundary of the generated plasma grating b extends with the light beam; the plasma grating b with the boundary extending with the light beam dynamically reflects the pump pulse d, and injects the reflected energy into the ionization laser pulse c; at the same time, when the ionization laser pulse c ionizes the gas grating a, an up-conversion light is generated, and an intersection region is formed between the up-conversion light region and the reflection region of the pump pulse, and an isolated attosecond pulse e is generated in this intersection region.

[0022] Based on the above principle, the specific implementation is shown in the Figure 2 optical path diagram:

[0023] Figure 2 In, the nanosecond-scale long pulse laser 1 is equally divided into two beams after passing through the first beam splitter 2, and then one of the two beams of laser passes through the first light guiding mirror 3, the third light guiding mirror 5 and the first focusing lens 6 in sequence, and the other beam passes through the second light guiding mirror 4 and the second focusing lens 7 in sequence, and is focused into the gas to generate high-frequency sound waves; finally, the femtosecond-scale ionization laser pulse c is focused into the gas through the short pulse laser focusing lens 8 to ionize the sound waves to generate a plasma grating b with a rapidly extending boundary, which dynamically reflects the pump pulse d, and the reflection region and the frequency up-conversion region generate up-conversion light to form an isolated attosecond pulse e, and the isolated attosecond pulse e is finally led out through the second beam splitter 9.

[0024] The generation of attosecond pulses using this method has been verified by particle model simulations. The simulations are carried out using the international open-source particle simulation program Epoch. The obtained simulation results are as Figure 3 shown. The simulation results show that a pump laser with a pump intensity of 4×10 13 W / cm 2 , 1.6 ps (10 -12 s), a wavelength of 1 μm, and a diameter of 100 μm (6th-order super-Gaussian), and an ionization pulse of 30 fs, a wavelength of 800 nm, and 100 μm (6th-order super-Gaussian) are used; the background gas is hydrogen with an average density of 0.1nc (about 5 atm) and a modulation degree of 50%; after a reaction of 240 μm, the image of the 3D attosecond pulse obtained is as Figure 3 shown:

[0025] Figure 3 Among them, the left figure is the 3D distribution map of the attosecond pulse, the upper right figure is the pulse width distribution map, and the lower right figure is the time waveform map of the pulse focal spot center. The intensity of the attosecond pulse is 35 times that of the pump laser intensity, the full-width at half-maximum is about 330 as, the conversion efficiency is about 0.1%, and the laser attosecond pulse energy obtained in the simulation is 1.45 μJ. However, only a laser diameter of 100 μm is used in this simulation. If the laser diameter is enlarged to the order of 10 cm, an attosecond pulse energy output of the order of joules can be achieved.

[0026] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. An isolated attosecond pulse generation method, characterized in that: Based on ionization gating, a long pulsed pump light is introduced, and the energy of the pump light is continuously injected. Combining the ionization pulse with the frequency up-conversion effect of the ionization pulse, by adjusting the grating period, a narrow crossover region is generated by the reflection of the pump light and the frequency up-conversion effect. Through the transfer of pump energy to attosecond pulse energy, attosecond pulses are generated. The specific steps are as follows: A long pulsed laser in the nanosecond range is equally divided into two beams by a beam splitter. Subsequently, one of the two beams of laser passes through two light guiding mirrors and a focusing lens in sequence, and the other beam passes through a light guiding mirror and a focusing lens in sequence, and is focused into the gas to generate high-frequency sound waves. Finally, a short pulsed laser in the femtosecond range is focused into the gas through a short pulsed laser focusing lens to ionize the sound wave to generate a plasma grating with a rapidly extending boundary. The plasma grating dynamically reflects the pump pulse, and there is a narrow crossover region between the reflection region and the frequency up-conversion region, and isolated attosecond pulses are formed in the crossover region.

2. The method for generating isolated attosecond pulses according to claim 1, wherein The reflectivity of the pump pulse is adjusted by the grating period, and the up-conversion effect in the ionization region is adjusted by the pulse width of the ionization pulse.

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