Harmonic generation in preionized liquid plasmas driven by intense midinfrared pulses

We investigated harmonic generation in preionized liquid ethanol and water driven by linearly polarized midinfrared (MIR) pulses at a center wavelength of 3100 nm. The liquid targets were preionized by a near-infrared pump pulse at 800 nm, enabling systematic control of the electron density in the liquids. At low pump intensities, the harmonic spectra remain unchanged. In contrast, at intermediate pump intensities, the spectra exhibit pronounced target- and delay-dependent modifications, reflecting changes in the optical properties of the ionized liquids. At high intensities, where the electron density exceeds the critical plasma density for MIR light, harmonic yields are strongly suppressed and spectral broadening disappears. Numerical simulations based on the nonlinear propagation model, incorporating plasma and Kerr effects, qualitatively reproduce the experimental observations above the critical plasma density and confirm the significant role of plasma shuttering in harmonic generation. The observation of narrow harmonic spectra provides clear evidence of evanescent-field-induced harmonic generation in preionized liquids. This highlights the potential of ionized liquid plasmas as a robust platform for exploring strong-field phenomena driven by intense and spatially nonuniform fields, beyond the damage thresholds of conventional materials.