Ultrafast lasers are a type of laser, which is a laser with pulsed waves on the order of FS. Femtosecond (fs) is an extremely short unit of time, that is, 10-15s, only 1 quadrillion of a second, if 10fs is used as a geometric average to measure the universe, its lifetime is only 1 minute. In such a short period of time, we can expect that the pulse wave must have many interesting properties that will help us in our scientific experiments.
Lasers, as the name suggests, are "excited light", and the physical basis for producing them is stimulated radiation from atoms, a process that was first theoretically discovered by Albert Einstein in 1916. When the concept of stimulated radiation was first proposed, it did not receive the attention it deserved, although in 1924 a German scientist experimentally confirmed the existence of stimulated radiation. However, the great potential of the concept of stimulated radiation was really rediscovered after World War II, when people tried to expand the western wave from long waves to microwaves and even light waves, and found that only with the help of onlookers such as molecules and atoms could the coherent electromagnetic wave amplification of short wavelengths be realized, and Einstein's stimulated radiation was the physical mechanism to achieve this kind of dry amplification.
To produce a laser, two contradictions need to be resolved. The first is the contradiction between stimulated radiation and stimulated absorption. According to the Boltzmann distribution, there are always more atoms at lower energy levels than at higher energy levels in a thermally balanced atomic system, and when light interacts with the system, the absorption is more significant than the excited radiation, resulting in the attenuation of the light signal. Therefore, one of the basic conditions for the generation of laser light is to realize the inversion of the number of particles in the system. A ring that is already in the process of particle number reversal is called an activation medium and has the ability to amplify the light signal. In order for the number of particles to be reversed, an external energy source is required to act on the atomic system in an appropriate way (pump), and this energy source is called a pump source. Another contradiction to be solved by generating lasers is the contradiction between stimulated radiation and spontaneous radiation. In an atomic system, these two processes exist at the same time, competing with each other. In order to produce a laser, stimulated radiation needs to be in an advantageous position. Therefore, it is necessary to select an optical cavity with a suitable structure (or a sufficiently long activation medium), and the spontaneous radiation in the axis direction can obtain a high light field energy density through repeated gain, so as to obtain the output dominated by stimulated radiation.
Laser light is very different from ordinary light sources, it has the characteristics of high brightness, high directionality, high monochromacy and high coherence. It has a wide range of applications in processing, storage, medical, communications, radar, scientific research, national defense and other fields. As a special laser, femtosecond laser has undergone a process of quantitative change to qualitative change after the strengthening of various properties, and has more peculiar properties.
The interaction between ultrafast lasers and special forms of matter such as clusters, high-temperature and high-density plasma, and free electrons has also become a new research direction, which not only greatly broadens the in-depth development of this discipline, but also provides new solutions and new ways for the innovation and development of related important high-tech fields.
Recently, experimental studies have observed "hollow" atoms with a large number of inner shell holes generated by multiphoton excitation, which will open up a new way to achieve ultra-short-wavelength coherent radiation. For the first time, the interaction of ultrafast lasers with large clusters of atoms has successfully triggered benchtop fusion, thus pointing to the promise of a new concept of "benchtop" fusion. In addition, the study of the interaction between ultrafast lasers and clusters has the potential to serve as a bridge to help people understand the interaction between light and matter more completely.
When the light is stronger than (equal to) 1018 watt cm2, the interaction of the laser with the electron enters the super-relativistic strong field range. For the first time, it has been observed experimentally that free electrons are accelerated to relativistic energies of the order of megaelectron volts in a vacuum; The nonlinear Thomson scattering and the resulting approximately 300 femtoseconds, 0ultrafast hard X-ray pulses of 05 nm; Multiphoton nonlinear Compton scattering. Particularly striking is the first observation of the strong-field quantum electrodynamics phenomenon in which inelastic photon-photon scattering produces positive and negative electron pairs.
The generation and application of X-rays and light sources based on nonlinear Thomson scattering and Compton scattering, as well as the acceleration of electrons by ultra-strong ultrafast laser fields with subperiodic pulse width in vacuum, are also hot topics in the study of the interaction between ultrafast lasers and free electrons. In addition, in the wake field experiments generated by the interaction between ultrafast lasers and rarefied plasmas, it is also observed that the ultra-high gradient acceleration field is more than three orders of magnitude higher than the limiting acceleration electric field of the conventional high-energy particle accelerator, thus proposing a new scheme for the realization of miniaturization of high-energy particle accelerators.
In recent years, the interaction between ultrafast lasers and high-temperature and high-density plasmas, especially the study of new phenomena and laws of high nonlinearity caused by relativistic effects, has also attracted great attention from the international academic community. Although new phenomena such as the formation of plasma channels have been observed to be generated by ultrafast lasers with huge light pressure, which pushes the critical density to move forward, and thus forms plasma channels, the interaction between ultrafast lasers of the order of 1018 and 1020 watts and centimeters and high-temperature and high-density plasma, such as the "hole-in-plasma" effect, the generation of superthermal electrons, and the control and transport of energy spectrum, still need to be studied in depth. Obviously, the study of the interaction between ultrafast laser and high-temperature and high-density plasma is not only one of the important research contents in this field, but also has the potential to provide a basis for the development of related high-tech fields such as laser nuclear fusion.
The discovery and in-depth study of the high-order harmonic phenomenon excited by ultrafast laser field not only provides an effective way to obtain fully coherent light sources in the vacuum ultraviolet region (VUV) and extreme ultraviolet region (XUV) bands, but also puts forward new ideas and methods for the generation of subfemtosecond or even attosecond extreme ultrafast short-wavelength coherent radiation, so that it is possible to break through the femtosecond barrier, create extremely ultrafast attosecond photonic technology for mankind, and create attosecond spectroscopy, A new discipline of attosecond physics and even attosecond science and technology, and a future high-tech field.
A major breakthrough has been made in the research of high-order harmonic emission in ultrafast laser fields, and higher-order harmonics have entered the "water window" band. At present, the research on new concepts and methods for generating extreme ultrafast coherent radiation in the order of subfemtosecond or even attosecond is becoming more and more active. In the research of short-wavelength X-ray band lasers, the existing X-ray laser mechanism cannot achieve a breakthrough with a wavelength of less than 2 nanometers, and the emergence of ultrafast laser provides the possibility of realizing ultra-short-wavelength coherent radiation based on new mechanisms such as inner shell transition. At present, the research on the new mechanism of photoionizing and ultrashort wavelength coherent radiation in the inner shell driven by ultrafast laser has also become a new hot spot in this field.
Ultrafast laser technology provides innovative means and methods for the development of interdisciplinarity. Ultrafast laser technology also provides innovative means and methods for the development of cutting-edge interdisciplinary disciplines such as ultrafast chemical kinetics, microstructured materials science, ultrafast information photonics and life sciences. For example, the femtosecond produced by the ultrafast laser itself and its interaction with matter, and even the extreme ultrafast coherent light source technology in the XUV and X-ray bands that may be of the order of subfemtosecond and attosecond provides a powerful means for human beings to study and apply various ultrafast processes, which will enable human beings to further understand the energy transfer and information transfer processes within the microscopic world at a deeper level, and then may realize the artificial control of certain physical, chemical and biological processes, and promote the science of microstructured materials. The research and development of interdisciplinary fields such as ultrafast chemical kinetics have produced breakthrough cutting-edge research results with significant impact.
In recent years, the research progress in the application of femtosecond laser to chemical reaction kinetics has been particularly remarkable. Xavier (A.).h.Zewail was awarded the 1999 Nobel Prize in Chemistry for his work in the development of femtosecond spectroscopy and the study of transition states with extremely short lifetimes during chemical reactions. The above progress also brings new hope for the use of ultrafast and intense lasers to control chemical reactions. Selectively breaking or forming some small molecule chemical bonds has been successful, but the complex system of large molecules has not been broken. The combination of ultrafast and intense laser technology and near-field optical microscopy can carry out multi-dimensional control of the interaction between laser and molecules, which is a powerful means to study "single-molecule physics" or "single-molecule chemistry", and has the potential to be used to "tailor" biological macromolecules.
Remarkable progress has also been made in the preparation of ultrafast intensity lasers in the preparation of material microstructures and the study of ultrafast kinetic behaviors, including the development and application of new detection methods for ultra-high spatiotemporal spectral resolution. For example, the optical pump-ultrafast X-ray diffraction probe measurement technology has been applied to the ultrafast lattice dynamics of single crystals, and the ultra-high spatiotemporal resolution of picoseconds-milliangstrom has been achieved. Micro-polymerization and micropolymerization have made it possible to use ultrafast and intense lasers to obtain material processing accuracy that is better than the diffraction limit and smaller than the wavelength of light, which has brought new applications in three-dimensional high-density data storage. Recent experiments have also confirmed that the recording density can be increased to 1014 bits centimeter3 by intermittently irradiating glass containing rare earth element samarium particles at micron intervals using femtosecond intense lasers, and multi-wavelength overlapping recording technology.
Relatively speaking, ultrafast laser science is a very young new discipline, on the eve of a major breakthrough, and its important role and potential go far beyond what is described in this article. Looking to the future, Chinese scientists are expected to make important contributions in this frontier field of modern physics and even modern science. This is not only a challenge, but also a rare opportunity.