Research

Our research program involves studies of ultrafast dynamics and the development of ultrafast electron microscopic technology and device capable to resolve, in both space and time, atomic-scale motions, on the base of manipulating photon and electron, two elementary particles accessible in our daily life.

Ultrafast dynamics: When the state of a substance or material changes, there are, in the macroscopic view, changes of optical, thermal, electric, morphological, or other property, and, in the microscopic view, changes of motion status of electrons and nuclei which compose the substance. For condensed matters, microscopic changes primarily are electron state transition, phonon excitation, lattice distortion, atomic rearrangement, etc., and their interactions. The process from the onset of microscopic change to the establishment of new equilibrium, and the motion and mechanism involved, are defined as dynamics, which are parts of the nature of the substance, and play dominant roles in applications of the substance. Many of these dynamics occur in the range of femtoseconds (fs, 10^-15 s) to picoseconds (ps, 10^-12 s), a timescale comparable to the natural oscillation periods of atom and molecule, which is out of the resolvable temporal window for general electronic devices. Thus, such dynamics are named ultrafast dynamics.

The temporal resolution required for observing processes of ultrafast dynamics are mainly provided by fs laser pulses. The ultrashort pulse width of fs laser offers an imaging time window, which works like a shutter of camera, and a timing function, which works like a switch or trigger of camera, by perturbing the system under investigation with a laser pulse to initiate the dynamical process. Depending on the object of observation, the probe is made choice of pulses of laser, electron, or X-ray (the time window of X-ray pulse is independent from fs laser in many cases).

We work on ultrafast probes of electron and laser. We develop our own ultrafast microscopic technologies and apparatuses employing pulsed electrons with fs duration, including ultrafast electron diffraction and ultrafast electron microscopy. Together with the transient spectroscopy, these facilities are applied to observe, through both the electronic and atomic degrees of freedom, various ultrafast processes of carrier excitation and recombination, phonon evolution and its interaction with excited carrier, phase transition, and so on, in novel light conversion materials.

Multi-mode ultrafast electron diffraction (MMUED) uses electrons generated by fs laser pulses as the probe. Combining the temporal resolution by fs laser and the resolution of diffractive imaging in reciprocal space by the ultra-short wavelength of electron with high kinetic energy, MMUED is able to resolve changes of sub-millimeter (<10^-13 m) and sub-picosecond (<10^-12 s) in space and time. Multiple setups of MMUED make possible studies of a diversity of sample morphologies spanning bulk, thin film, nanoparticle, surface, adsorbate layer, and interface.
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Ultrafast transmission electron microscopy (UTEM) is also known as four-dimensional electron microscopy (4D EM), using pulsed electrons as the probe, similar to those of UED, to replace the DC electron beam in conventional electron microscope. The incorporation of the ultra-high temporal resolution of fs laser and the tremendous capability of spatial resolution and electron energy spectroscopy of conventional electron microscopy, renders the revolutionary concept and methodology for studies of materials, physics, chemistry, and biology. UTEM is capable to image in real space, reciprocal space, and electron energy spectrum, with a temporal resolution on the order of 100 fs (10^-13 s).
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Transient Absorption/Transient Reflection (TA/TR) spectroscopy observes the evolution process of excited carriers in material through measurements of the transmission/reflection change of the specimen, exploiting the direct interaction between photons and electrons, which enables TA/TR a temporal resolution possibly close to the laser pulse width. The broad-spectrum range (ultraviolet to far-infrared) of pump/probe allows TA/TR to directly track evolutions of carrier in different energy states, picturing the electronic degree of freedom for studies of ultrafast dynamics.
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