When a powerful ultrashort laser pulse propagates, it exhibits unique spatiotemporal and spectral dynamics due to various nonlinear light-matter interactions. Unlike linear beam propagation, nonlinear pulse propagation involves a self-induced, intensity-dependent refractive index variation along the mode profile, resulting in self-focusing that tends toward eventual collapse if the beam’s power exceeds the critical power threshold for self-focusing. At high enough intensities, the beam ionizes the medium, generating plasma, which prevents collapse by defocusing the beam. When self-focusing is delicately balanced by plasma-defocusing and diffraction, a self-guided light structure known as a laser filament is formed. With applications ranging from new light source generation to atmospheric remote sensing, filamentation has made a substantial scientific and technological impact. Our group experimentally and theoretically investigates numerous properties of laser filamentation, including wavelength scaling of laser filamentation in solids and gases.

In the presence of an intense laser field, an electron can be ripped away from its parent atom via tunneling ionization. Next, the laser field accelerates the electron away from its parent ion, followed by a reversal in direction at half an optical cycle, and a final acceleration towards the parent ion leading to recombination and emission of an ultrashort burst of light at UV/x-ray wavelengths. This process, known as high-order harmonic generation (HHG), is useful for table-top UV/soft x-ray and attosecond pulse generation. Recently, we have successfully constructed a vacuum chamber and a flat-field XUV/soft x-ray spectrometer for HHG experiments using gaseous and solid targets to control and enhance HHG and eventually study structures of atoms, molecules, and solids.  

Femtosecond laser micromachining is the process of creating permanent structural changes in a material via exposure to a focused ultrashort pulse. During exposure to the pulse, multiphoton absorption or tunneling ionization occurs, and since the ionization process is nonlinear, material modifications can be highly localized. Furthermore, ultrashort laser pulses cause minimal thermal effects, which enables femtosecond laser micromachining to create clean micro- and nano-structures.

Ferroelectric materials are very important due to their applications such as random-access memory and piezoelectric microsensors. Recently, there is a growing interest in the elucidation of crystal and electronic structures in ferroelectric thin films like BiFeO3 [I-T Bae et al., Sci. Rep. 46498, 7 (2017)], a well-known multiferroic that exhibits both ferroelectric and antiferromagnetic properties. We can, via nonlinear optics, noninvasively characterize the material system in terms of point group symmetry. We are currently conducting experiments on thin film ferroelectric samples using second-harmonic generation spectroscopy. By varying the polarization of the incoming ultrashort pulse and/or the orientation of the thin film sample, we can map out the resulting second harmonic signal to the crystal structure of the material. In this way we can gain a more exact understanding of the properties of ferroelectric materials so that additional applications can be developed.