title: Linear and nonlinear optical characterization of self-assembled, large-area gold nanosphere metasurfaces with sub-nanometer gaps.
abstract: We created centimeter-scale area metasurfaces consisting of a quasi-hexagonally close packed monolayer of gold nanospheres capped with alkanethiol ligands on glass substrates using a directed self-assembly approach. We experimentally characterized the morphology and the linear and nonlinear optical properties of metasurfaces. We show these metasurfaces, with interparticle gaps of 0.6 nm, are modeled well using a classical (without charge transfer) description. We find a large dispersion of linear refractive index, ranging from values less than vacuum, 0.87 at 600 nm, to Germanium-like values of 4.1 at 880 nm, determined using spectroscopic ellipsometry. Nonlinear optical characterization was carried out using femtosecond Z-scan and we observe saturation behavior of the nonlinear absorption (NLA) and nonlinear refraction (NLR). We find a negative NLR from these metasurfaces two orders of magnitude larger (n2,sat= -7.94x10-9cm2/W at Isat,n2= 0.43 GW/cm2) than previous reports on gold nanostructures at similar femtosecond time scales. We also find the magnitude of the NLA comparable to the largest values reported (β2,sat= -0.90x105cm/GW at Isat,β2= 0.34 GW/cm2). Precise knowledge of the index of refraction is of crucial importance for emerging dispersion engineering technologies. Furthermore, utilizing this directed self-assembly approach enables the nanometer scale resolution required to develop the unique optical response and simultaneously provides high-throughput for potential device realization.
authors.forename: Jake
authors.initials: J
authors.lastname: Fontana
authors.forename: Melissa
authors.initials: M
authors.lastname: Maldonado
authors.forename: Nicholas
authors.initials: N
authors.lastname: Charipar
authors.forename: Scott A
authors.initials: SA
authors.lastname: Trammell
authors.forename: Rafaela
authors.initials: R
authors.lastname: Nita
authors.forename: Jawad
authors.initials: J
authors.lastname: Naciri
authors.forename: Alberto
authors.initials: A
authors.lastname: Pique
authors.forename: Banahalli
authors.initials: B
authors.lastname: Ratna
authors.forename: Anderson S L
authors.initials: AS
authors.lastname: Gomes