The study of numerical character for femtosecond pulse shaping

This paper describes the generation of shaped femtosecond multiple pulses by using the phase-only Dammann filters in 4f femtosecond shaper and gives the experimental result of femtosecond pulse characterization by the frequency- resolved optical gating （FROG） technique. With the theoretical simulation, it concludes that the quality of the generated output array is relevant to the number of pixels and the spacing between the components.

Control of the photoionization/photodissociation processes of cyclopentanone with trains of femtosecond laser pulses

A train of three equally spaced femtosecond laser pulses is employed to control the photoionization/photodissociation processes of cyclopentanone. With the increase of pulse separation, a strong modulation of product ion yield is observed. More than ten-fold changes of ion yield ratio between different products can be realized. The experimental observations further explain the compositions and formation pathways of peaks in the mass spectra. The controlling mechanisms are also discussed.

Femtosecond laser-induced periodic structures:mechanisms, techniques, and applications

Over the past two decades,femtosecond laser-induced periodic structures(femtosecond-LIPSs)have become ubiquitous in a variety of materials,including metals,semiconductors,dielectrics,and polymers.Femtosecond-LIPSs have become a useful laser processing method,with broad prospects in adjusting material properties such as structural color,data storage,light absorption,and luminescence.This review discusses the formation mechanism of LIPSs,specifically the LIPS formation processes based on the pump-probe imaging method.The pulse shaping of a femtosecond laser in terms of the time/frequency,polarization,and spatial distribution is an efficient method for fabricating high-quality LIPSs.Various LIPS applications are also briefly introduced.The last part of this paper discusses the LIPS formation mechanism,as well as the high-efficiency and high-quality processing of LIPSs using shaped ultrafast lasers and their applications.

Angular distributions of CH_3I fragment ions under the irradiation of a single pulse and trains of ultrashort laser pulses

The angular distribution of CH3I is investigated experimentally using a single Fourier transform-limited laser pulse and a pulse train, where a 90-fs 800-nm linearly polarized laser field with a moderate intensity of 2.8 × 10^13 W/cm2 is used. The dynamic alignment is demonstrated in a single pulse experiment. Moreover, a pulse train is used to optimize the molecular alignment, and the alignment degree is almost identical to that with the single pulse. The results are analysed by using chirped femtosecond laser pulses, and it demonstrates that the structure of pulse train rather than its effective duration is crucial to the molecular alignment.

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Improving the up-conversion luminescence efficiency crucial in several related application areas. In this work, of rare-earth ions via the multi-photon absorption process is we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er3＋ ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components. So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. More- over, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width （or laser spectral bandwidth）, final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earth ions.

Simulating the resonance-mediated(1+2)-three-photon absorption enhancement in Pr3+ions by a rectangle phase modulation

Enhancing the upconversion luminescence of rare earth ions is crucial for their applications in the laser sources,fiber optic communications,color displays,biolabeling,and biomedical sensors.In this paper,we theoretically study the resonance-mediated(1+2)-three-photon absorption in Pr^(3+) ions by a rectangle phase modulation.The results show that the resonance-mediated(1+2)-three-photon absorption can be effectively enhanced by properly designing the depth and width of the rectangle phase modulation,which can be attributed to the constructive interference between on-resonant and near-resonant three-photon excitation pathways.Further,the enhancement efficiency of resonance-mediated(1+2)-threephoton absorption can be affected by the pulse width(or spectral bandwidth)of femtosecond laser field,final state transition frequency,and absorption bandwidths.This research can provide a clear physical picture for understanding and controlling the multi-photon absorption in rare-earth ions,and also can provide theoretical guidance for improving the up-conversion luminescence.

Identification of isomers and control of ionization and dissociation processes using dual-mass-spectrometer scheme and genetic algorithm optimization

Identification of acetone and its two isomers, and the control of their ionization and dissociation processes are performed using a dual-mass-spectrometer scheme. The scheme employs two sets of time of flight mass spectrometers to simultaneously acquire the mass spectra of two different molecules under the irradiation of identically shaped femtosecond laser pulses. The optimal laser pulses are found using closed-loop learning method based on a genetic algorithm. Compared with the mass spectra of the two isomers that are obtained with the transform limited pulse, those obtained under the irradiation of the optimal laser pulse show large differences and the various reaction pathways of the two molecules are selectively controlled. The experimental results demonstrate that the scheme is quite effective and useful in studies of two molecules having common mass peaks, which makes a traditional single mass spectrometer unfeasible.