Picosecond dynamics of hot carriers in infrared plasmonic metasurfaces

Abstract

The transient transmittance of plasmonic metasurfaces consisting of metal antennas over a semiconductor substrate was investigated, focusing on the role of carrier multiplication within the semiconductor through hot carrier generation by localized surface plasmons. Time-resolved far-infrared pump–probe spectroscopy monitored the dependence of the optical responses on the metasurface periodicity, the substrate temperature (ranging from 10 K to 300 K), and substrate dopant level (ranging from 2.9 × 10¹² to 3.3 × 10¹⁵ cm⁻³). The relative position of the excitation wavelength with respect to the plasmon resonance of the metasurface is crucial for the dynamic response. Transmittance changes up to 50% were achieved if the excitation was close to the plasmonic resonance. By decreasing the periodicity of the metasurface, the transmission minimum shifts towards shorter wavelengths. The periodicity also significantly affected the field enhancement of the metasurface, which plays a crucial role in the charge carriers' dynamics. Cooling the samples results in much larger transient transmission changes owing to the higher impact ionization rate, carrier multiplication factor, and longer diffusion length. A low initial substrate dopant concentration was found to be favorable for achieving a large transient response. The simulations from the developed diffusive impact ionization model validated all experimental results. These findings provide insight into the carrier dynamics and pave the way for developing electro-optical devices operating in the far-infrared spectral range.