# Direct Classic Route of Generating Mono-Color EM-Pulse with Attosecond-Level Duration
## 1 Introduction  
Demands from applications promote the progress in preparing/generating ultrashort-duration, down to wavelength-level or shorter, laser pulse [1; 2; 3; 4; 5; 6; 7; 8; 9; 10], which in turn promotes new applications areas appearing [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. For applications emphasizing high energy density, spectral broadening technique, which relies on optical property of bulk material [21; 22; 23; 24] and fine adjusting optical elements [25; 26; 27], can supply single \(\upmu\)m-level wavelength laser pulse with femtosecond(fs)-level duration and mJ-level energy. For applications emphasizing optics probing and optical manipulation of quantum system, the pulse is not required to be high-energy-density but to be of a shorter duration down to attosecond(as)-level. Current technique routes for achieving as-level duration are from (1) high-order harmonics generation (HHG) from laser-irradiated atoms in gaseous medium [1; 2; 3; 4; 28; 29; 30; 31; 32; 33]. (2) HHG effect in laser-plasma interaction [34; 35]. (3) The interaction between laser and electron beam free-electron-laser (FEL) facility [36; 37; 38]. In these routes, Fourier components with shorter wavelength (below half of incident central wavelength) are produced from the interaction of the incident pulse with matter mentioned above. The ability of the pulse being shortened is determined by the efficiency of producing these Fourier components. All these routes face the same difficulty, the efficiency. In the laser-atom route, the amplitude of higher-order harmonic component decreases exponentially with respect to the order. For example, the intensity of 50-order component is \(10^{-12}\) of that of the fundamental component [39]. Merely enhancing the intensity of the fundamental component is of little effect on this fact because the laser-atom route demands the survival of the involved quantum system, which does not favor the application of more intense fundamental components. The other two routes are based on the interaction of the fundamental component with classic electrons. Besides preparing plasmas, manipulating bulk plasma electrons to do a collective oscillation with a frequency multiple folds of the incident laser also demands the incident laser to be high-energy-density enough. The route based on FEL facility is also not optimistic in the efficiency because its background physics is associated with high-energy relativistic electrons and large-sized undulator magnetic field configuration, which are rather expensive for most university-level small research groups. The commonality of these routes is to use easily-prepared electromagnetic (EM) Fourier components, such as visible and near-infrared laser with \(\upmu\)m-level wavelength (from 1 \(\upmu\)m to 0.4 \(\upmu\)m), as \"raw material\" to produce, through different technique routes, EM Fourier components with wavelength \(<\)0.1 \(\upmu\)m, which act as building components of desired pulse with as-level duration. Namely, producing the EM pulse with as-level duration is along an indirect way through the EM pulse with longer duration, or first producing a EM pulse with longer duration and then producing a EM pulse with as-level duration from this longer duration \"raw material\" supplied by a commercial available laser. It is instructive and interesting to consider initially directly producing such a desired EM pulse with as-level duration. Retrospecting the history of laser, as well as its elder brother maser, we can find that the generation of EM pulse at visible and near-infrared band starts from flexible utilization of AC electric and magnetic field at power frequency (about tens Hz). A laser apparatus, due to electron structure of its kernel components/elements (i.e., gain medium), can convert power frequency input to optics-frequency output.