Promising applications in photonics are driven by the ability to fabricate crystal-quality metal thin films of controlled thickness down to a few nanometers. In particular, these materials exhibit a highly nonlinear response to optical fields owing to the induced ultrafast electron dynamics, which is however poorly understood on such mesoscopic length scales. Here, we reveal a new mechanism that controls the nonlinear optical response of thin metallic films, dominated by ultrafast electronic heat transport when the thickness is sufficiently small. By experimentally and theoretically studying electronic transport in such materials, we explain the observed temporal evolution of photoluminescence in two-pulse correlation measurements that we report for crystalline gold flakes. Incorporating a first-principles description of the electronic band structure, we model electronic transport and find that ultrafast thermal dynamics plays a pivotal role in determining the strength and time-dependent characteristics of the nonlinear photoluminescence signal, which is largely influenced by the distribution of hot electrons and holes, subject to diffusion across the film as well as relaxation to lattice modes. Our findings introduce conceptually novel elements ruling the nonlinear optical response of nanoscale materials, while suggesting additional ways to control and leverage hot carrier distributions in metallic films.
Bibliografisk noteFunding Information:
We thank C. Wolff for stimulating discussions. This work has been supported in part by ERC (Advanced Grant 789104-eNANO), the Spanish MICINN (PID2020-112625-GB-I00 and Severo Ochoa CEX2019-000910-S), the Catalan CERCA Program, and Fundaciós Cellex and Mir-Puig. A.R.E. acknowledges support from the Generalitat de Catalunya, the European Social Fund (L’FSE inverteix en el teu futur)-FEDER. J.D.C. is a Sapere Aude research leader supported by Independent Research Fund Denmark (Grant No. 0165-00051B). N.A.M. is a VILLUM Investigator supported by VILLUM FONDEN (Grant No. 16498). The Center for Polariton-Driven Light–Matter Interactions (POLIMA) is funded by the Danish National Research Foundation (Project No. DNRF165).
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