Ultrabroadband characterization of microwave-to-terahertz supercontinua driven by ultrashort pulses in the mid-infrared

Ultrafast nonlinear dynamics driven by high-peak-power ultrashort mid-infrared (mid-IR) pulses gives rise to ultrabroadband radiation whose spectrum spans over multiple decades, stretching from the terahertz (THz) to the microwave range. We show that, despite its enormous, multidecade bandwidth, an accurate spectral and spatial-mode characterization of this radiation is possible via a suitable combination of mutually complementary signal-analysis methods borrowed from ultrafast optics, radio electronics, THz photonics, and microwave engineering. This analysis reveals intense subgigahertz radiation emitted as a part of mid-IR-driven supercontinuum generation and shows that microwave-to-THz field waveforms can almost reach the diffraction limit in their beam focusability, yielding field strengths above 3 MV/cm.