The polarization modulation strategy of the femtosecond laser field was shown to be a well-established method to control up-conversion luminescence in rare-earth ions. In this work, we further extend the polarization control behavior from weak to intermediate femtosecond laser fields. We experimentally show that the polarization control efficiency of the up-conversion luminescence in the Sm3+-doped glass will be affected by the femtosecond laser intensity, which decreases with the increase of the laser intensity. We theoretically propose a fourth-order perturbation theory to explain the experimental observation, which includes the two-photon and four-photon absorptions, and the destructive interference between the two-photon and four-photon absorption will result in the suppression of the polarization control efficiency due to their different polarization control degrees. These experimental and theoretical results provide a new insight into understanding the polarization control process of up-conversion luminescence in rare-earth ion doped luminescent materials under an intermediate femtosecond laser field, and also can open a new opportunity for the polarization control application in some related areas.

Improving up-conversion luminescence efficiency of rare-earth ions is always a research hotspot because of its important applications in laser source, color display, photoelectric conversion and multiplexed biolabeling. Herein, we first utilize a combined two-color laser field with the laser wavelengths of 800 and 980 nm to further enhance the up-conversion luminescence in an Er3+/Yb3+-codoped glass sample. We show that the green up-conversion luminescence intensity by the combined two-color laser field can be greatly enhanced by comparing it with the sum of that induced by the two individual laser fields. We also show that the luminescence enhancement can be attributed to the cooperative up-conversion excitation process by the energy transfer from Yb3+ to Er3+ ions via the 980 nm laser field excitation and then the excited state absorption via the 800 nm laser field excitation. These studies present a clear physical picture for the up-conversion luminescence generation and enhancement in an Er3+/Yb3+-codoped glass sample, which are very helpful for properly designing the laser field to generate or improve the up-conversion luminescence efficiency in various lanthanide-codoped luminescent materials.

The upconversion luminescence of rare-earth ions has attracted considerable interest because of its important applications in photoelectric conversion, color display, laser device, multiplexed biolabeling, and security printing. Previous studies mainly explored the upconversion luminescence generation through excited state absorption, energy transfer upconversion, and photon avalanche under the continuous wave laser excitation. Here, we focus on the upconversion luminescence generation through a nonresonant multiphoton absorption by using the intense femtosecond pulsed laser excitation and study the upconversion luminescence intensity control by varying the femtosecond laser phase and polarization. We show that the upconversion luminescence of rare-earth ions under the intense femtosecond laser field excitation is easy to be obtained due to the nonresonant multiphoton absorption through the nonlinear interaction between light and matter, which is not available by the continuous wave laser excitation in previous works. We also show that the upconversion luminescence intensity can be effectively controlled by varying the femtosecond pulsed laser phase and polarization, which can open a new technological opportunity to generate and control the upconversion luminescence of rare-earth ions and also can be further extended to the relevant application areas.

We theoretically demonstrate the effect of the intensity ratio of the two-color laser field on the terahertz generation based on a transient photocurrent model. We show that the terahertz generation depends on the intensity ratio of the two-color laser field at a given total laser intensity, and the optimal intensity ratio for the maximum terahertz generation will decrease with the increase of the total laser intensity. We also show that the final ionization degree can be used to explain the optimal intensity ratio change at different laser intensities. Furthermore, we utilize the increasing rate of electron density and the electron drift velocity to illustrate the physical mechanism of the maximum terahertz radiation generation.

The multi-photon dissociation and Coulomb explosion of ethyl bromide C2H5Br under near-infrared (800 nm) femtosecond laser field are experimentally investigated by a DC-sliced ion imaging technique. The sliced images of fragment ions C2H5+, Br+, CH3+, CH2Br+, H2+ and C2H3Br+ are obtained, and their dissociative pathways are assigned by observing their corresponding kinetic energy release (KER) and angular distribution. It is shown that low-KER components of these fragment ions result from multi-photon dissociation of singly charged parent ion C2H5Br+, while high-KER components come from Coulomb explosion of doubly charged parent ion C2H5Br2+. It is also shown that the precursor species [C2H5+…Br+] has a longer lifetime than [C2H3Br+…H2+] and [CH3…CH2Br+]. In addition, the probable H2 and H2+ elimination channels are theoretically simulated by Gaussian 09 software packages, and the results show that the former is an asynchronous process while the latter is a synchronous process.

Uncorrelated position and velocity distribution of the electron bunch at the photocathode from the residual energy greatly limit the transverse coherent length and the recompression ability. Here we first propose a femtosecond pulse-shaping method to realize the electron pulse self-compression in ultrafast electron diffraction system based on a point-to-point space-charge model. The positively chirped femtosecond laser pulse can correspondingly create the positively chirped electron bunch at the photocathode (such as metal–insulator heterojunction), and such a shaped electron pulse can realize the self-compression in the subsequent propagation process. The greatest advantage for our proposed scheme is that no additional components are introduced into the ultrafast electron diffraction system, which therefore does not affect the electron bunch shape. More importantly, this scheme can break the limitation that the electron pulse via postphotocathode static compression schemes is not shorter than the excitation laser pulse due to the uncorrelated position and velocity distribution of the initial electron bunch.

We experimentally demonstrate hydrogen migration in photoionization and photofragmentation processes of cyclohexane (C6H12) under a near-infrared (800 nm) intense femtosecond laser field by the dc-slice imaging technique, and the observation of fragment ions CH3+, C2H5+, and C3H7+ can be regarded as direct evidence that chemical bond rearrangement processes associated with hydrogen migration occur in the dissociative ionization process of cyclohexane. We measure the sliced images of fragment ions CH3+, C2H5+, and C3H7+ and calculate their corresponding kinetic energy release (KER) and angular distributions, and it is confirmed that high-KER components of these fragment ions result from two-body Coulomb explosion of the doubly charged parent ion C6H122+ whereas low-KER components result from dissociative ionization of the singly charged parent ion C6H12+. Moreover, we measure the maximally attainable relative yields of fragment ions CH3+, C2H5+, and C3H7+, which approach 4.5%, 4.0%, and 3.0%, respectively.

Coulomb explosion and dissociative ionization of 1,2-dibromoethane are experimentally investigated in a near-infrared (800 nm) femtosecond laser field by dc-slice imaging technology. The sliced images of the fragment ions C2H4Br+, Br+, C2H4+, Br2+ and CH2Br+ are obtained, and their corresponding kinetic energy releases (KERs) and angular distributions are calculated. It is confirmed that the high-KER components come from Coulomb explosion of 1,2-C2H4Br22+, while the low-KER components come from dissociative ionization of 1,2-C2H4Br2+. Furthermore, the dissociation pathway leading to C2H4+ and Br2 is theoretically simulated, and the results show that the singly charged precursor overcomes an energy barrier to dissociate via an asynchronous concerted mechanism after undergoing isomerization.

We theoretically and experimentally demonstrate the control of the intermediate state absorption in (1+2) resonance-mediated multiphoton absorption process by shaping the femosecond laser pulse. A theoretical model is proposed to investigate the intermediate state absorption of (1+2) resonance-mediated three-photon absorption process in the molecular system, and an analytical solution is obtained on the basis of time-dependent perturbation theory. Our theoretical results show that the intermediate state absorption can be enhanced by controlling the laser spectral phase due to final state absorption reduction, and this absorption enhancement efficiency increases with the increase of the laser intensity. These theoretical results are experimentally confirmed in IR144 dye by varying the laser spectral phase with a sinusoidal modulation function.

Femtosecond-induced resonance-enhanced multi-photon ionization (REMPI) photoelectron spectroscopy of the Rydberg states is faced with low spectral resolution due to the broadband laser spectrum. In this paper, we theoretically demonstrate that a high-resolution (2 + 1) REMPI photoelectron spectrum of the Rydberg states in a sodium (Na) atom can be obtained by shaping a femtosecond laser pulse with a spectral phase step modulation. Our results show that, by using a phase-shaped femtosecond laser pulse, some narrowband holes or peaks in the REMPI photoelectron spectrum can be created, and the positions of these holes or peaks are correlated with the eigenenergies of the Rydberg states. Thus, by observing these holes or peaks in the REMPI photoelectron spectrum, both the high-resolution REMPI photoelectron spectrum and the energy-level diagram of the Rydberg states can be obtained.

In this paper, a theoretical model is proposed to investigate the molecular rotational state populations pumped by multiple laser pulses through an impulsive Raman process based on second-order perturbation theory and an analytical solution for the dependence of the rotational state populations on the time delays and the relative amplitudes of the multiple laser pulses has been achieved. The results indicate that the molecular rotational state populations can be controlled by precisely manipulating the time delays and the relative amplitudes, which can be significantly enhanced or completely suppressed, and so the molecular rotational wave packet and field-free molecular alignment can be efficiently manipulated.

In this paper, a theoretical model is proposed to investigate the molecular rotational state populations pumped by multiple laser pulses through an impulsive Raman process based on second-order perturbation theory and an analytical solution for the dependence of the rotational state populations on the time delays and the relative amplitudes of the multiple laser pulses has been achieved. The results indicate that the molecular rotational state populations can be controlled by precisely manipulating the time delays and the relative amplitudes, which can be significantly enhanced or completely suppressed, and so the molecular rotational wave packet and field-free molecular alignment can be efficiently manipulated.