Collaborative Research: Nanoprobes for mapping the spatiotemporal evolution of ultrafast optical vector near field

Advances in optical nanotechnology have enabled a wide range of applications, such as increased sensitivity for the detection of just a few molecules with plasmonic nanoparticles. To quantify the performance of optical devices and develop new capabilities, it is essential to measure the behaviors of ultrafast optical fields with nanoscale spatial resolution and femtosecond scale temporal resolution. To address the challenge, this program engenders a novel type of nanoprobe, integrated with a custom near-field scanning microscope system, for comprehensive characterization of the ultrafast optical near field. The research program will enhance understanding by combining the expertise of three researchers in different research areas at three universities. The exciting areas in ultrafast optics and nano-optics will provide excellent education opportunities for graduate and undergraduate as well as K-12 students in the lab, in the classroom, and through outreach activities. Graduate and undergraduate students will be trained through the research activities such as nanoprobe fabrication, application of near-field scanning optical microscope system, optical measurements, and numerical simulation and retrieval in a collaborative setting across the three universities. Results will be incorporated into courses. In keeping with prior projects of the researchers, women and underrepresented groups will be encouraged and expected to participate in the program.

The goal of this program is to develop a nanoprobe based characterization method that can map the spatiotemporal evolution of ultrafast optical vector near field in nanometer-femtosecond scale. A nanoprobe, which consists of a second order nonlinear nanocrystal perched on a nanowire or a near-field scanning optical microscope (NSOM) probe, will be integrated with a custom-built NSOM system to achieve sample-probe distance control and nanoscale spatial resolution. The nonlinear response of the nanocrystal (i.e., second harmonic generation -SHG) can be exploited to characterize both the amplitude and the phase profiles of the local ultrafast field as well as the spatiotemporal evolution through the collinear SHG frequency resolved optical gating (FROG) holography. Due to the presence of a strong 'local oscillator' and the reliance on homodyne detection, FROG holography will also improve the measurement sensitivity. Finally, polarized SHG from the nanoprobe is utilized to probe the polarization of the local ultrafast optical field. Since the second harmonic signal has a distinct wavelength, it is insensitive to any background noise generated by the reflection or scattering of the fundamental field. Further, the second order nonlinear tensor is determined by the material properties such as the crystal structure and is largely independent of the particle morphology, leading to a more controllable nanoprobe sensor. Knowledge of the local spatiotemporal fields enhances the capability to quantify spectroscopic signals from plasmonic structures, a long-standing challenge in nanospectroscopy.