Near-field terahertz nonlinear optics with blue light

The coupling of terahertz optical techniques to scattering-type scanning near-field microscopy (s-SNOM) has recently emerged as a valuable new paradigm for probing the properties of semiconductors and other materials on the nanoscale. Researchers have demonstrated a family of related techniques, including terahertz nanoscopy (elastic scattering, based on linear optics), time-resolved methods, and nanoscale terahertz emission spectroscopy. However, as with nearly all examples of s-SNOM since the technique's inception in the mid-1990s, the wavelength of the optical source coupled to the near-field tip is long, usually at energies of 2.5 eV or less. Challenges in coupling of shorter wavelengths (i.e., blue light) to the nanotip has greatly inhibited the study of nanoscale phenomena in wide bandgap materials such as Si and GaN. Here, we describe the first experimental demonstration of s-SNOM using blue light. With femtosecond pulses at 410 nm, we generate terahertz pulses directly from bulk silicon, spatially resolved with nanoscale resolution, and show that these signals provide spectroscopic information that cannot be obtained using near-infrared excitation. We develop a new theoretical framework to account for this nonlinear interaction, which enables accurate extraction of material parameters. This work establishes a new realm of possibilities for the study of technologically relevant wide-bandgap materials using s-SNOM methods.

Automated summary

The coupling of terahertz optical techniques to scattering-type scanning near-field microscopy (s-SNOM) has opened up new possibilities for studying nanoscale phenomena in wide bandgap materials. In this study, researchers successfully demonstrated s-SNOM using blue light, which allowed for the generation of terahertz pulses directly from bulk silicon with nanoscale resolution. This breakthrough enables the extraction of spectroscopic information that is not obtainable with near-infrared excitation, providing a new realm of possibilities for studying wide-bandgap materials.