The Detection Of Structural Dymamics In Aluminium Film By Ultrafast Electron Diffraction

The optical pump-probe technology basing on femtosecond laser pulse has the capacity of ultrafast temporal resolution and the electron diffraction has the capacity of ultrahigh spatial resolution. Ultrafast electron diffraction (UED) employs both of the two, so it is a technology which is capable of both ultrafast temporal and ultrahigh spatial resolution.Drived by the great potential of ultrafast electron diffraction, we have built an UED system with sub-picosecond temporal resolution and sub-milli-angstrom spatial resolution. Using this UED system we have performed a series of important experiments, such as the ultrafast structural dynamics (phonon generation and lattice thermal motions ) of Al thin film excited by femtosecond laser pulse was detected, the electronic Grüneisen constant of Al is measured at room temperature, the ultrafast behavior of charge separation field of plasmas generated by femtosecond laser pulse interaction with Ag needle target was observed as well.We overcame several technical obstacles in designing our new system with the experience of the first state of UED model. Stabilization of E-gun is important to the experiment, and the second part of chapter 2 gives a brief introduction of the ultrahigh vacuum system and the all in one design. E-gun is the heart, we present a e-gun which has improved electron pulse(less than 400μm diameter and 400 femtosecond width), based on the compact design and 12MV/m DC acceleration field. The next part shows the imaging system, which is the eye of the UED. In order to get the single electron detection sensitivity and good Signal-to-Noise Ratio (SNR), it contains three parts: phosphor screen, image intensifier and scientific-level CCD. We also use the commercial power supply to give a certification for the safety and stabilization of HV supply.Several successful experiments have been done till now. Chapter 4 persents both coherent phonon and lattice thermal motions of polycrystalline aluminum thin-film initiated by femtosecond laser pulse. The result shows that in the free-standing film under open-boundary conditions, a one-dimensional standing wave is formed with the corresponding vibration period of 7.2ps. The rise of temperature is 88K in 2.6ps under 5.0 mJ/cm2 pump influence, the corresponding coupling constant of electron and phonon is 840fs, which is consistent well with the previous work. All of the results show us that our UED system has the 400fs temporal resolution, sub-milli-angstrom spatial resolution. Chapter 5 gives a description of a new method to measure the electronic Grüneisen constant of Al at room temperature, overcoming the traditional method limited by low temperature. This offeres a unique path to determine electronic Grüneisen constant in ferromagnetic transition metals, this parameter of which is not achievable with the conventional low temperature approach.Optical-field ionized plasmas were also investigated using pump-probe method by our system. In chapter 6 we have realized 4D electron shadow imaging to probe the initial stage of laser-plasma formation at picosecond time-resolution during the first 200ps. It shows that there exist two transient electric fields near the irradiated target surface, which either focus or defocus the probe electron beams. During this process about 8×107suprathermal electrons are ejected with speed 1.2×107m/s under the laser intensity 1014 W/cm2. The experiment suggests that 4D electron shadow imaging is a novel tool for real-time diagnosis of ultrafast plasma dynamics.