Single cycle optical nonlinearity of transparent conducting oxides

The exploration of light-matter interaction using intense, extremely short optical pulses has recently captured significant attention in the fields of condensed matter and photonics. While previous studies have predominantly focused on electron transitions between valence and conduction bands in solids, less emphasis has been given to electron dynamics within the conduction band in materials with high electron concentrations like metals and transparent conducting oxides (TCOs). The latter have recently attracted increased interest due to their notable nonlinear optical properties [1]. Initial experimental studies relied on qualitative phenomenological or macroscopic thermal models, providing valuable insights but lacking quantitative match. To bridge this gap, our recent theoretical works [2, 3] have provided a comprehensive quantitative description of the electronic, thermal, and optical response of Indium Tin Oxide (ITO), a prominent TCO. However, existing formulations for the study of non-equilibrium electron dynamics [4] do not formally describe correctly the dynamics under intense single-cycle illumination, essential for investigating the instantaneous turn-on of the optical response [5] and rapid turn-off dynamics [6].

 To close this knowledge gap, we model the full optical and electronic response of TCOs to a single-cycle intense pulse using the density matrix formulation. We unveil coherence-induced doubled frequency oscillations in the electron distribution and the emergence of transient inversion. Furthermore, we observe that stimulated emission approaches absorption levels at high intensities, yet, the system only reaches partial transparency due to the absence of population inversion in the remainder of the conduction band. Additionally, we observe the anticipated spectral shift and broadening associated with the instantaneous Kerr-like response. Our approach paves the way for investigations into time-varying photonics and coherent wave control on few-femtosecond timescales.

References

1.       M. Z. Alam, I. De Leon, R. W. Boyd, Science 116, 795 (2016); Kinsey, N.; DeVault C.; Boltasseva A.; Shalaev V. M., Nat. Rev. Mater. 2019, 4, 742.

2.       Sarkar, S.; Un, I.W.; Sivan, Y. Phys. Rev. Applied 2023, 19, 014005.

3.       Un, I.W.; Sarkar, S.; Sivan, Y. Phys. Rev. Applied 2023, 19, 044043.

4.       Del Fatti, N. et al. Phys. Rev. B 2000, 61, 16956; Saavedra, J. R. M. et al. ACS Photonics 2016, 3, 1637.

5.       Tirole, R.; Vezzoli, S.; Galiffi, E. et al. Nat. Phys. 2023, 19, 999.

6.       Lustig, E.; Segal, O.; Saha, S., et al. Nanophotonics, 2023, 12, 2221.