Ultrafast Nonreciprocal Beam Steering and Frequency Conversion in Epsilon-Near-Zero Materials

Time-gradient materials and metasurfaces have opened up new applications for manipulating light actively, and have unlocked new optical phenomena. In particular, ultrafast optical driving of epsilon-near-zero (ENZ) materials has attracted great interest as it allows for fast refractive index changes, arising from large nonlinearity, sub-picosecond timescale response, in turn enabling all-optical dynamic frequency-conversion and beam-steering applications. <br/>Here we report the observation of nonreciprocal beam steering and frequency shifting on hundreds of femtosecond time-scales. Ultrafast pump-probe measurements were applied to indium-tin-oxide (ITO) bulk material and metasurfaces, with ENZ wavelength in the near infrared range. A wavelength and time-dependent study demonstrates maximum frequency of 18nm and refraction angle shifting of 0.7 degrees when permittivity approaches zero as predicted, which also allows for THz modulation speeds. The results indicate that ENZ materials and metasurfaces are promising for future applications of frequency conversion and dynamic optical steering.