微观尺度的超快动力学研究主要关注电子和原子核的动态过程,对测量方法的时空分辨率有非常高的要求.近年来迅速发展的X射线自由电子激光(X-ray Free Electron Laser,XFEL)具有飞秒脉冲、高亮度、全相干等特点,为超快动力学研究提供了亟...微观尺度的超快动力学研究主要关注电子和原子核的动态过程,对测量方法的时空分辨率有非常高的要求.近年来迅速发展的X射线自由电子激光(X-ray Free Electron Laser,XFEL)具有飞秒脉冲、高亮度、全相干等特点,为超快动力学研究提供了亟需的装置,已经在物理、材料、化学和生命科学等领域取得了一系列革命性突破.本文结合实际应用体系,介绍了基于XFEL探测超快动力学的三类主要方法,即X射线吸收/发射光谱、非晶态样品的X射线散射和晶体样品的X射线衍射.除了探测装置,超快动力学的研究还需要高效的泵浦方法才能更好地启动结构演化,然后再与XFEL检测方法紧密结合,从而观测体系高时空分辨的超快动力学过程,帮助研究人员全面理解该动力学过程的微观机制.对于生物大分子,尤其是感光分子体系,基于XFEL的超快动力学研究为揭示其超快动力学过程和分子机制提供了新的思路和线索.展开更多
Picosecond ultrasonics,as a nondestructive and noncontact method,can be employed for nanoscale metallic film thickness measurements.The sensitivity of the system,which determines the measurement precision and practica...Picosecond ultrasonics,as a nondestructive and noncontact method,can be employed for nanoscale metallic film thickness measurements.The sensitivity of the system,which determines the measurement precision and practicability of this technique,is often limited by the weak intensity of the ultrasonic signal.To solve this problem,we investigate the distinct mechanisms involved in picosecond ultrasonic thickness measurement for two types of metals,namely tungsten(W)and gold(Au).For thickness measurement in W films,theory and simulation show that optimizing the pump and probe laser wavelengths,which determine the intensity and shape of the ultrasonic signal,is critical to improving measurement sensitivity,while for Au film measurements,where acoustic-induced beam distortion is dominant,the signal intensity can be optimized by selecting an appropriate aperture size and sample position.The above approaches are validated in experiments.A dual-wavelength pump-probe system is constructed based on a passively mode-locked ytterbium-doped fiber laser.The smoothing method and multipeak Gaussian fitting are employed for the extraction of ultrasonic time-of-flight.Subnanometer measurement precision is achieved in a series of W and Au films with thicknesses of 43-750 nm.This work can be applied to various high-precision,noncontact measurements of metal film thickness in the semiconductor industry.展开更多
A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal struc...A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyz- ing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of 4-150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures＇ effect on the laser-induced ionization or material ablation.展开更多
The creation and propagation of longitudinal acoustic phonons （LAPs） in high quality hematite thin films （α-Fe203） epitaxially grown on different substrates （BaTiO3, SrTiO3, and LaAlO3） are investigated using t...The creation and propagation of longitudinal acoustic phonons （LAPs） in high quality hematite thin films （α-Fe203） epitaxially grown on different substrates （BaTiO3, SrTiO3, and LaAlO3） are investigated using the femtosecond pump- probe technique. Transient reflection measurements （AR/R） indicate the photo-excited electron dynamics, and the initial decay less than 1 ps and the slow decay of -500 ps are attributed to the electron-LO phonon coupling and electron-hole nonradiative recombination, respectively. LAPs in α-Fe2O3 film can be created by ultrafast excitation of the ligand field state, such as the ligand field transitions under 800-nm excitation as well as the ligand to metal charge-transfer with 400- nm excitation. The strain modulations of the sound velocity and the out-of-plane elastic properties are demonstrated in α-Fe2O3 film on different substrates.展开更多
The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe...The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell （about 130 fs） than that in the conduction band of the CdSe core （about 400 fs）. From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating that ultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.展开更多
文摘微观尺度的超快动力学研究主要关注电子和原子核的动态过程,对测量方法的时空分辨率有非常高的要求.近年来迅速发展的X射线自由电子激光(X-ray Free Electron Laser,XFEL)具有飞秒脉冲、高亮度、全相干等特点,为超快动力学研究提供了亟需的装置,已经在物理、材料、化学和生命科学等领域取得了一系列革命性突破.本文结合实际应用体系,介绍了基于XFEL探测超快动力学的三类主要方法,即X射线吸收/发射光谱、非晶态样品的X射线散射和晶体样品的X射线衍射.除了探测装置,超快动力学的研究还需要高效的泵浦方法才能更好地启动结构演化,然后再与XFEL检测方法紧密结合,从而观测体系高时空分辨的超快动力学过程,帮助研究人员全面理解该动力学过程的微观机制.对于生物大分子,尤其是感光分子体系,基于XFEL的超快动力学研究为揭示其超快动力学过程和分子机制提供了新的思路和线索.
基金supported by the National Natural Science Foundation of China(Grant No.52075383)the National Key Research and Development Program of China(Grant Nos.2022Y FF0708300,2022YFF0706002).
文摘Picosecond ultrasonics,as a nondestructive and noncontact method,can be employed for nanoscale metallic film thickness measurements.The sensitivity of the system,which determines the measurement precision and practicability of this technique,is often limited by the weak intensity of the ultrasonic signal.To solve this problem,we investigate the distinct mechanisms involved in picosecond ultrasonic thickness measurement for two types of metals,namely tungsten(W)and gold(Au).For thickness measurement in W films,theory and simulation show that optimizing the pump and probe laser wavelengths,which determine the intensity and shape of the ultrasonic signal,is critical to improving measurement sensitivity,while for Au film measurements,where acoustic-induced beam distortion is dominant,the signal intensity can be optimized by selecting an appropriate aperture size and sample position.The above approaches are validated in experiments.A dual-wavelength pump-probe system is constructed based on a passively mode-locked ytterbium-doped fiber laser.The smoothing method and multipeak Gaussian fitting are employed for the extraction of ultrasonic time-of-flight.Subnanometer measurement precision is achieved in a series of W and Au films with thicknesses of 43-750 nm.This work can be applied to various high-precision,noncontact measurements of metal film thickness in the semiconductor industry.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11004111 and 61137001)the Natural Science Foundation of Tianjin City,China (Grant No. 10JCZDGX35100)+1 种基金the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100031120034)the Fundamental Research Funds for the Central Universities of China
文摘A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyz- ing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of 4-150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures＇ effect on the laser-induced ionization or material ablation.
基金Project supported by the National Natural Science Foundation of China(Grant No.11174195)
文摘The creation and propagation of longitudinal acoustic phonons （LAPs） in high quality hematite thin films （α-Fe203） epitaxially grown on different substrates （BaTiO3, SrTiO3, and LaAlO3） are investigated using the femtosecond pump- probe technique. Transient reflection measurements （AR/R） indicate the photo-excited electron dynamics, and the initial decay less than 1 ps and the slow decay of -500 ps are attributed to the electron-LO phonon coupling and electron-hole nonradiative recombination, respectively. LAPs in α-Fe2O3 film can be created by ultrafast excitation of the ligand field state, such as the ligand field transitions under 800-nm excitation as well as the ligand to metal charge-transfer with 400- nm excitation. The strain modulations of the sound velocity and the out-of-plane elastic properties are demonstrated in α-Fe2O3 film on different substrates.
基金This work was supported by the National Natural Science Foundation of China （No.11074003） and the Key Program of Educational Commission of Anhui Province of China （No.KJ2010AI32）. For the help of Prof. J. L. Zhao at Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences in sample preparation is greatly appreciated.
文摘The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell （about 130 fs） than that in the conduction band of the CdSe core （about 400 fs）. From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating that ultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.
