Coherent Radiation From Nitrogen Molecular Ions Induced By Ultrafast Intense Laser Fields

With the rapid development of ultrafast laser technology,laser sources with short temporal duration,high energy,and broad spectrum are available.Ultrashort intense laser pulse is known as an advanced tool to study the interaction between light and matter.In this case,the interaction lies in the nonperturbative regime,where many interesting strong-field phenomena emerge such as above threshold ionization,laser plasma,and high-harmonic generation.Propagation of ultrashort intense laser pulses in transparent medium would generate laser filamentation,relating to the Kerr effect,the plasma effect,and other non-linear processes.In particular,the extreme interaction of femtosecond laser with atmosphere induces a laserlike radiation which is called air lasing.It has attracted much attention in the past decade.The newly found remote no-cavity lasing is based on the long-distance transmission of femtosecond laser pulse,with atmospheric molecules as its gain medium.Due to the distinct characteristics of narrow bandwidth,high brightness,and coherence,air lasing is found to have a variety of applications such like remote detection of molecules and environmental monitoring.According to the gain medium,air lasing is divided into atomic lasing,molecular lasing,and molecular ion lasing.So far,people have known well about the generation of atomic lasing and molecular lasing.However,the mechanism of the molecular ion laser,particularly the N₂⁺lasing,involving various electronic states,vibrational states,and rotational states of the ensemble of N₂⁺,is not unambiguously clear yet.Further investigation to clarify the underlying physical mechanism and to realize coherent control is highly deserved.By constructing spatiotemporally controlled ultrafast intense laser field,nitrogen molecules are ionized and coupled to excited states.This dissertation focuses on the study of generation of N₂⁺lasing,aiming at revealing the transitions among multi-electronic states of N₂⁺after the ionization from strong fields,and the induced coherent radiation.By using the polarization-skewed laser pulses,the new-born N₂⁺lasing is efficiently controlled and amplified.The main content of the results are summarized below.1.Probing the role of electron recollision in generation of N₂⁺lasing.By introducing the bicircular two-color(BTC)ultrafast laser field,the electron dynamic can be manipulated.Although the electron will return and collide with the parent ion in counter-rotating BTC field in contrast to the co-rotating case,we find similar intensity of the observed 428 nm N₂⁺lasing driven by BTC field of the two different rotating senses.The electron recollision scenario is straightforwardly excluded for generation of N₂⁺lasing in this experiment.Meanwhile,dipole allowed transitions among the multi-electronic states of N₂⁺are invoked to explain the dependence of 428 nm lasing on the relative strength of the BTC field.In addition,we obtain the circularly polarized air lasing by the BTC scheme based on the amplification from the seed pulse.2.Spatiotemporally precise manipulation of N₂⁺lasing.Driven by a waveform-controlled,polarization-skewed(PS)laser pulse,the leading and falling edges of the PS pulse are respectively utilized to ionize the nitrogen molecules and couple them to the excited states.By changing the relative strength of the leading and falling edges,both 391 nm and 428 nm lasing are found to be efficiently controlled.When changing the input laser energy,ionization of nitrogen molecules and post-ionized one-photon excitation are found to be in competition.The controlling of 428 nm lasing signal requires a higher input energy threshold.Moreover,the strength of the new-born N₂⁺lasing signal can be precisely manipulated by changing the phase of the PS pulse.3.High gain of N₂⁺ lasing.By combining a PS near-infrared(800 nm)laser and an infrared(IR,1580 nm)linearly polarized laser,we singly ionize the N₂ and deplete the population in the X²?_g⁺state of N₂⁺,leading to the noticeable population inversion between the B²?_u⁺and X²?_g⁺states of N₂⁺.As compared to the traditional linearly polarized laser pumping scheme,we obtain an enhanced intensity of N₂⁺lasing by 5?6orders of magnitude.We also study the influence of the input laser energy,wavelength and other experimental conditions on the amplified N₂⁺lasing.The high gain of N₂⁺lasing promotes the progress of air lasing in remote application.4.Dependence of N₂⁺ lasing on molecular orientation.By building up a non-collinear two-color laser field with a small crossing angle,pump-probe of N₂⁺lasing is achieved.When changing the polarization and relative delay of the pump and probe laser pulses,we find different dependences of 428 nm lasing on the polarization of the probe pulse at different delay time.The role of the seeded amplification in the N₂⁺lasing generation is examined by observing the time delay and seed polarization dependence.The result strengthens the essence of stimulated radiation generated N₂⁺lasing,dependent on the spatial orientation of the molecular axis with respect to the polarization of the seed pulse.