Ultrafast Switching
Located conveniently at SLAC National Accelerator Laboratory, we have the unique opportunity to get access to the 28GeV electron beam produced by the 2 mile long linear accelerator. The accelerator crew are able to compress the electron bunch both in space and time at a facility called Final Focus Test Beam (FFTB), so we can enjoy the very intense electric and magnetic field in the vicinity of the subpicosecond microfocused electron pulses. We apply this EM field pulse in our ultrafast switching experiments to study the extreme time domain behavior of different materials, bearing an interest in both the fundamental physics and implications for future technological applications.
Observed and simulated magnetic pattern in a thin metal film, generated by a strong ultrafast electromagnetic field pulse parallel to the film surface generated by a relativisic electron beam at SLAC.
Spin Injection
While electronics keeps being the driving force of the daily changing landscape of the modern world, recent developments have shown increasing evidences that the proposed new electronics base on the electron spin, i.e. spintronics, could be the candidate as a fundation for the next generation electronic devices, both in storage and computation. Our spin injection project focuses on a prototype spin-based memory device, where a spin polarized current is used to reverse a magnetic nano-bit. We reported in 2006 the first direct observation of the detailed magnetic switching process in space and time using advanced pump-probe x-ray microscopy. Base on the same technique, we are now looking into a variety of spin-injection based devices with the same goal of delivering insights by seeing the nanoworld moving at a picosecond pace.
Coherent Scattering and X-ray Lensless Imaging
With the advent of the next generation ultrafast ultrabriliant x-ray sources, the dream of performing cinematography in the nanoscale wonderland is not too far away. Lensless imaging has shown the potential of being able to deliver the diffraction-limited resolution from a single x-ray laser pulse, yet not worrying about possible beam damage to the optics, simply because as the name of the technique indicates, no imaging optics is required. Instead, the technique takes the recorded diffraction pattern from the object of interest in the far field and obtains an image of the object computationally.
The efforts in our group have been focused on lensless holography. Unlike the popular approach using iterative algorithms to decipher the diffraction patterns, holography enjoys the advantage of the encoded phase information in the recorded holograms and is able to deliver the object reconstruction instantly in a noniterative deterministic fashion.
Sometimes, image reconstruction is difficult due to the circumstances of certain experiments. However, the coherent scattering pattern still contains valuable imformation about the system being studied. We are able to monitor the variation of the coherent scattering speckles as we change the sample environment, e.g. temperature, current, voltage, magnetic field. This adds on top of traditional x-ray spectroscopy technique an important new aspect: access to length scale information.
X-ray lensless spectroholography deliver images of magnetic worm domain patterns in a Co/Pd multilayer thin film with 50 nm resolution