In 1960, the first laser, the ruby laser, opened the door for the study of ultrafast processes. In 1961, Q-switching technology realized the first short laser pulse output with a pulse width of tens of nanoseconds on a ruby laser. The pulse width of the laser pulse was even shortened to 10 nanoseconds. The pulse width that can be obtained by Q-switching technology can only be To reach the nanosecond level, this is due to the limitation of the laser cavity length (2L/c, L is the laser cavity length, and C is the speed of light.
The phase-locking technology developed in 1964 formed the multi-longitudinal model of the lasers independently oscillating into time order. The mode-locking technology was the first to realize the active mode-locked nanosecond green laser pointer pulse output on the helium-neon laser. Two years later, the first picosecond laser pulse output was realized on rubidium glass laser. In the mid-1960s, the development of ruby laser mode locking and neodymium glass laser mode locking began to study the picosecond phenomenon in the picosecond time domain. In 1976, a saturable dye absorber was used in the broadband tunable dye laser medium system to achieve sub-picosecond ultrashort laser pulse output for the first time.
In the 1980s, a revolutionary change occurred in ultrafast spectroscopy. The concept of Collision Pulse Mode Locking (CPM) introduces a dye laser, and the picosecond laser pulse is compressed into the femtosecond (fs) time domain to generate a 100 fs pulse. Then a 30 fs pulse appeared. This is achieved by a ring laser coupled with a dye amplifier chain, working at a wavelength of 620 nm. The appearance of Kerr gate technology promotes the development of ultrafast spectroscopy including ultrafast fluorescence spectroscopy. The application of chirped pulse compression technology compresses the pulse width to 20 fs or even 6 fs.
It is particularly worth pointing out that Ti:Sapphire lasers play a very important role in the development of ultrafast processes. Ti:Sapphire materials are an important gain medium for ultrashort pulse oscillators and amplifiers. It can output ultrafast pulses with a pulse width of 4 to 5 fs at 800 nm. Fast pulse. The materials that can achieve 20 sub-femtosecond output in the near-infrared frequency region are Cr4+: YAG, Cr3+: LiSAF, Cr4+: forsterite (M92Si04). Let us compare and estimate the energy density of a femtosecond laser: a femtosecond laser pointer with a pulse width of about 20 fs produces 1J energy, and the peak energy flow of this laser focus reaches 1020W/cm2.
From the emergence of ruby lasers, with the help of important pulse Q-switching, mode-locking and compression technologies, ultra-fast processes have experienced and realized nanoseconds (1ns=10-9s), picoseconds (1ps=10-12s), femtoseconds (1fs= 10-15s) and attosecond (1as=10-18s) development process. When using terawatt (1012w) laser excitation, it can achieve subattosecond (10-19s) ultrashort pulse output. Theoretically, it has been proved that if a petawatt (1015w) laser is used for excitation, it can generate zeptosecond (10-21s) and subzeptosecond (10-22s) laser pulses.