1. ELI-ALPS: First-level light source ultra-short pulse laser
Light sources such as free electron lasers and synchrotron radiation are very common internationally. These light sources provide light with unique parameters for users to use. A large number of scientists working on these light sources are laser experts, and many users come from the fields of chemistry, biology, condensed matter physics and other fields. These users reserve Beam Time at a large light source, and then the light source will give them a fixed time for them to conduct experiments.
Attosecond light sources based on high-order harmonics have so far been mainly run by individual laboratories and individual PIs. One of its advantages is its small Table-Top size. Scientists can conduct experiments in their own laboratories, so there is no beam time limit.
However, on the one hand, the further improvement of attosecond pulse parameters, along with the further enhancement of laser performance, has led to further increases in labor and capital costs; on the other hand, the complexity of attosecond time-resolved test equipment has also become higher, making it more difficult for non-laser professionals to conduct independent experiments.
There are two solutions to this problem. One is to transform a single PI group into a joint form of multiple research groups (research institutes/research centers), such as the Max Born Institute for Nonlinear Optics (MBI) in Berlin, Germany. The laser equipment, experimental equipment and users are all by themselves. The other is ELI-ALPS, which draws on free electron lasers and synchrotron radiation solutions to provide attosecond light sources for external users.
Figure 1 Ultrashort pulse lasers used by international research institutions. These lasers are used to generate high-flux and high-repetition-frequency attosecond light sources.
Figure 1 shows the parameters of ultrashort pulse lasers from several international research institutions. It can be seen that MBI has ultrashort pulse lasers with a repetition frequency of 100kHz and a single pulse energy of 100uJ. It should be pointed out here that the research units given in the table are not complete. For example, Lund University in Sweden also has lasers with similar parameters. For the pump detection experiment, scientists hope for high luminous flux, so they hope that the single pulse energy is as high as possible, which is the area in the upper left corner of Figure 1; for Coincidence Measurement, scientists hope for a weaker single laser intensity to obtain cleaner experimental data. However, too weak a laser will result in a very low signal-to-noise ratio. Therefore, a sufficient number of samples need to be recorded in the same time, that is, a high laser repetition frequency is required, which is the area in the lower right corner of Figure 1.
In general, the overall goal of the ELI-ALPS laser system is to increase the repetition frequency on the premise that the single pulse energy is sufficient. The motivation is simple, a higher repetition rate means more pulses per second, so the same experiment takes less time. For example, a test that can be completed in 100 days with a 1kHz system can be completed in one day using a 100kHz system. Figure 1 shows the general parameter space of different laser systems in ELI-ALPS, which will be introduced separately below.
1.1 High repetition rate laser system (high repetition rate laser system)
The single pulse energy of this laser is in the mJ level, and the repetition frequency is 100kHz. The specific parameters are shown in Table 1; the current parameters of similar commercial lasers are 1kHz and several mJ.
Figure 2 shows the structure diagram of this laser. The CEP-stable fiber oscillator provides an 80MHz laser with a central wavelength of 1030 nm and a single pulse energy of nJ. The acousto-optic modulator reduces the repetition frequency to 100kHz, and after pre-amplifier systems, LPF-large pitch fibers, 65um core diameter, The single pulse energy is amplified to 0.1uJ; after passing through the grating stretcher, it is broadened to 2ns. After passing through the 8-channel fiber amplifier, each LPF amplification channel uses an 80W pump laser. The 8-channel laser is finally coherently combined into one beam, and passes through the compressor to obtain a 200fs, 3mJ laser. After passing through the two-stage hollow fiber compressor, an ultra-short pulse laser with a period of 1mJ level and 100kHz can finally be obtained. In the future, the single pulse energy will be upgraded to 5mJ.
Table 1. Laser parameters of high repetition frequency lasers
Figure 2 High repetition frequency laser system
1.2 Single cycle laser system
The parameters of the second laser system are shown in Table 2. This system can finally obtain a laser with a single pulse of 45mJ period and a repetition frequency of 1kHz; the parameters of similar systems currently on the market are 1kHz and several mJ.
The structure is shown in Figure
3. Yb-based oscillator and regenerative amplifier form fs-CPA, which generates 1.5mJ (1030 nm) laser as the front stage; the oscillator splits a part of 1064 nm as the seed of the picosecond pump laser; 10% of the laser emitted by the regenerative amplifier is injected into fs-OPA to produce a CEP-stable idler of 1.3~1.5um as the seed of fs-NOPA; The output light of fs-NOPA is broadened to 75ps (0.5uJ) through a stretcher composed of a prism and a Dazzler; then it enters the multi-stage BBO-Based NOPA system and is amplified; after compression, a laser of 45mJ, less than 10fs, can be obtained.
Table 2. Parameters of single-cycle laser system
Figure 3. Single-cycle laser system structure diagram
1.3 High Field laser system
The laser parameters are shown in Table 3, the parameters are 2PW, 10Hz; currently there are many sets of PW systems domestically and internationally, most of which have low repetition frequencies. It should be pointed out here that ELI-ALPS focuses on atomic and molecular systems, that is, outside the nucleus, and the required laser intensity is not very high. Therefore, for PW-level lasers, the goal is to increase the repetition frequency rather than increasing the single pulse energy. Several other ELI pillars, such as Nuclear Physics, require higher laser power.
, the pre-laser is different from the traditional titanium sapphire solution. The fiber laser obtains 2mJ, 1030nm, sub-ps laser pulses; part of it produces white light supercontinuum, and uses the difference frequency method to obtain CEP-stable laser. The frequency is doubled to obtain a 1600nm laser. After OPA amplification, the frequency is doubled again to obtain an 800nm laser.
The first set of strong field laser system HF-PW, part of the laser in the front stage passes through the OPCPA system based on titanium sapphire, and finally a 10Hz, 2PW laser is obtained. The structure is shown in Figure 4.
Another laser system, HF-100, is still in the design stage and is likely to use a hybrid solution of OPCPA, polarization encoded CPA Scheme and thin disk Ti:Sapphire. See Figure 4 for its structure.
Table 3. Parameters of high-field laser system
Figure 4 Structure diagram of strong field laser system
1.4 Mid-infrared laser system
The parameters of this laser system are shown in Table
4. The output is 3um, 100kHz, and the single pulse energy is 150uJ. The structure is shown in Figure
5. The seeds of the fiber oscillator enter two amplification systems, one is a light-based CPA system (Yb FCPA) (300 fs, 200uJ), and the other is a diode-pumped all-solid-state laser (DPSSL) (1 ps, 2 mJ). The first set of amplifiers generates a CEP-stable 3um laser through difference frequency, and is then amplified by the second set of amplifiers to obtain 3um, 100kHz, 150 uJ's laser. At the same time, it also outputs CEP floating 1.4~1.75 um, sub-100fs, 100uJ laser.
Table 4 Mid-infrared laser system parameters
Figure 5 Mid-infrared laser system structure diagram
2. ELI-ALPS: secondary light source attosecond pulse
The goal of ELI-ALPS is to provide high repetition frequency, high throughput, and stable attosecond pulses for users, and does not pursue extreme parameters of attosecond pulses. As mentioned before, the current attosecond pulse photon energy can reach thousands of electron volts and the pulse width is tens of attoseconds; the target parameters of ELI-APLS are hundreds of electron volts and hundreds of attoseconds.
The structure of the ELI-ALPS laboratory is shown in Figure
6. High repetition frequency laser HR, mid-infrared laser MIR, single-cycle laser SYLOS, THz laser, and strong field laser HF are all located in separate rooms. The secondary attosecond light source and experimental area are mainly divided into three parts. (1) Gas higher harmonics (GHHG) using HR and SYLOS; (2) Electron sources (e-) and surface plasma high harmonics (SHHG) using SYLOS; (3) Utilize high-order harmonics of particle sources and surface plasmons of strong-field lasers. Figure 7 shows the primary light source, secondary light source and subsequent applications.
Table 5 shows a comparison of XUV and X-ray pulses generated by different devices. It can be seen that the advantage of the free electron laser FEL is high luminous flux, and the single pulse energy can reach the level of 100uJ; while the advantage of the high-order harmonic HHG is that the pulse is short, of the order of hundreds to tens of attoseconds.
Using the attosecond light source of ELI-ALPS, scientists can use various measurement methods and conduct ultrafast dynamics detection. These measurement devices include Reaction microscope (ReMi, also known as cold target recoil ion momentum spectrometer COLTRIMS), photoelectron emission microscope (PEEM), Angle-resolved photoemission spectroscopy (ARPES), Velocity map imaging spectrometers (VMI) and Magnetic bottle electron spectrometer, etc.
Figure 6 ELI-ALPS laser system and attosecond laboratory
Figure 7 ELI-ALPS primary laser light source, secondary attosecond light source and subsequent applications
Table 5 Comparison of XUV and X-Ray generated by synchrotrons (Sychrotrons), free electron lasers (FEL), existing gas higher harmonics (GHHG), and ELI-ALPS three beam lines
3. Summary
ELI-ALPS provides a unique first-level laser light source and second-level attosecond pulse, high repetition frequency, high throughput and stability. These characteristics will be beneficial to the development of ultrafast dynamics experiments.
Finally, the author would like to mention the domestic ultra-fast field. Beijing, Shanghai, Xi'an, Wuhan and other places are also building various attosecond light sources. Attosecond light sources (deep ultraviolet to soft X-rays) are based on high-performance ultrafast and ultra-strong lasers (visible to infrared bands) and matching optical components. Currently, foreign products account for a relatively high proportion of the domestic market.
In terms of scientific research, China will have user devices with the same or even better performance than those in advanced countries in the world, which will be provided to domestic and international scientists. However, this does not mean that China has caught up with developed countries in the ultra-fast field. Many core equipment still come from abroad. Perhaps there needs to be a balance in the construction process of large installations. The device needs to be built and put into use within a limited time, so in the selection of equipment, it is necessary to purchase mature foreign products that meet the performance requirements. On the other hand, just as ELI-ALPS has given many European laser companies opportunities to develop technology, these domestic large-scale installations should also be able to leave some space for domestic companies.