Visual observation, the most ubiquitous form of light-matter interaction, happens in the human time scale. In order to perform observation of dynamics in progressively faster time scales, for instance, protein motions, proton transfer, rotational, vibrational, electronic dynamics, we need tools that are equivalent or faster.
Ultimately, if one wants to investigate electronic dynamics which are extremely fast, laser pulses whose duration is at the order of femtoseconds or attoseconds would be desired. One femtosecond is one millionth billionth of a second (1 fs = 10-15 s) and one attosecond is one thousandth of one femtosecond (1 as = 10-18 s).
In our group, we are seeking to generate, measure, and develop new methodologies in creation of ultrashort laser pulses. All of our studies are based on nonlinear optics and strong-field laser physics. Below are among few of our research directions.

Generation and measurement of ultrafast (femtosecond-attosecond) laser pulses

The time it takes a bound electron to respond to the electromagnetic force of light sets a fundamental speed limit on the dynamic control of matter and electromagnetic signal processing. Time-integrated measurements of the nonlinear refractive index of matter indicate that the nonlinear response of bound electrons to optical fields is not instantaneous; however, a complete spectral characterization of the nonlinear susceptibility tensors—which is essential to deduce the temporal response of a medium to arbitrary driving forces using spectral measurements—has not yet been achieved. Here we demonstrate that intense optical attosecond pulses synthesized in the visible and nearby spectral ranges allow sub-femtosecond control and metrology of bound-electron dynamics. Vacuum ultraviolet spectra emanating from krypton atoms, exposed to intense waveform-controlled optical attosecond pulses, reveal a finite nonlinear response time of bound electrons of up to 115 attoseconds, which is sensitive to and controllable by the super-octave optical field. Our study could enable new spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids, as well as light-based electronics operating on sub-femtosecond timescales and at petahertz rates.

Related publications:

Generation and measurement of high-order harmonics from condensed media

At the regime of low electric field strength, light-matter interaction results into only linear response of the media. When the electric field strength gets stronger, approaching the Coulomb force governing the dynamics of electrons, there would be perturbation to the standard Hamiltonian. The response of the quantum systems in this case will include a nonlinear response, a significant deviation from the linear response commonly found in daily life. When the electric field strength is increased to be equal to, or higher than the Coulomb potential, this is the regime of strong-field physics. In this interesting situation, the media response strongly to the incident electric field. As a result, ultrafast driven electronic dynamics give rise to the radiation of high energy photons, at multiples of the driving electric field frequency. These are high-order harmonics generated (HHG) from strong-field light-matter interaction.

Related publications: