The band gap of a semiconductor or dielectric, relative to the photon energy of incident light, is commonly taken to define whether the material is transparent or opaque. However, it is well known that extreme nonlinear optics can distort this definition across ultrashort timescales. Here, using single-shot two-color pump-probe microscopy at a free-electron laser facility, we show that an ultrashort midinfrared pulse can create nanosecond-long inhomogeneous optical patterns seeming to comprise below-band gap opacity and enhanced transparency. We explain this peculiar observation as stemming from ultrafast transient self-focusing, which in turn leads to long-lived free-carrier-induced opacity and defocusing. Our results, demonstrating that nonlinear optical effects in the infrared spectral range can give rise to surprisingly persistent material changes, highlight the importance of full-field pump-probe imaging techniques in rationalizing inhomogeneous phenomena that may otherwise be misinterpreted or overlooked entirely.

Close

PrevNext

View figure in article

• Light-matter interaction
• Nonlinear optics
• Phase transitions
• Free-electron lasers
• Semiconductors
• Ultrafast femtosecond pump probe