Ultrafast switching of surface plasmonic conditions in nonplasmonic metals

Abstract

We demonstrate that ultrafast carrier excitation can drastically affect electronic structures and induce brief surface plasmonic response in non-plasmonic metals, potentially creating a plasmonic switch. Using first-principles molecular dynamics and Kubo-Greenwood formalism for laser-excited tungsten we show that carrier heating mobilizes d electrons into collective inter and intraband transitions leading to a sign flip in the imaginary optical conductivity, activating plasmonic properties for the initial non-plasmonic phase. The drive for the optical evolution can be visualized as an increasingly damped quasi-resonance at visible frequencies for pumping carriers across a chemical potential located in a d-band pseudo-gap with energy-dependent degree of occupation. The subsequent evolution of optical indices for the excited material is confirmed by time-resolved ultrafast ellipsometry. The large optical tunability extends the existence spectral domain of surface plasmons in ranges typically claimed in laser self-organized nanostructuring. Non-equilibrium heating is thus a strong factor for engineering optical control of evanescent excitation waves, particularly important in laser nanostructuring strategies.

Ultrafast electron dynamics and orbital-dependent thermalization in photoexcited metals

“…As a result, the electron temperature stays above the lattice temperature during several picoseconds and the framework of a two-temperature model (TTM) can be used to describe laser-induced processes in metals [10,11]. Beyond a standard TTM, a fast electron thermalization is also assumed to describe the effect of high electron temperature on electronic, optical and thermal properties of metals [12][13][14][15][16]. The modification of electronic structure due to increase of the effective electron temperature can change the metal melting temperature [17,18], induce solid-solid phase transformation [19][20][21][22], and affect electron-phonon coupling dynamics [23,24].…”

On the Insignificant Role of the Oxidation Process on Ultrafast High-Spatial-Frequency LIPSS Formation on Tungsten

“…Assuming that at this low fluence and high electron–phonon coupling strength, the electron thermal energy diffusion is weak, so the energy confinement can roughly be estimated by the optical penetration depth for tungsten at 800 nm. This is given by = 25 nm, where k = 2.9 is the extinction coefficient for photoexcited W [ 58 ]. For this small penetration depth, a small liquid layer, and thus small HSFL period, is expected.…”

“…It is expected that the introduction of additional scattering inhomogeneities on the surface with an average distance lower than the initial roughness would result in a higher concentration of scattering centers. This favors dipole–dipole coupling for nonradiative fields, reducing the pattern period [ 58 ]. Topographical features in the SEM ( Figure 1 and Figure 2 ), for different gaseous atmospheres are different arising from the polycrystallinity of the samples.…”

Drude-Lorentz Model for Optical Properties of Photoexcited Transition Metals under Electron-Phonon Nonequilibrium

“…The observed material reflectivity and thus the amount of laser energy absorbed in the skin depth is defined by the collective electronic response to the laser field through intraband and interband transitions in sub-bands of the crystal state [8][9][10][11][12]. The time-resolved complex refractive index of the laser-irradiated surface has been previously reported by a dual-angle reflectometry technique, which was applied to study the optical dynamics of d-band electrons in transition metals near the threshold fluence [13][14][15][16]. To better determine the refractive index without Fresnel equations, a direct measurement of optical properties has been recently proposed by an ultrafast pump-probe method combining ellipsometry with reflectometry [17,18].…”

“…The nonequilibrium data were obtained using ab initio molecular dynamics and the Kubo-Greenwood formalism [14,24]. These calculations, combined with Kramers-Kronig relations, give access to the complex optical permittivities of the considered metals for several electron temperatures from 300 K to 30,000 K. They serve as reference points for the fitting process for real and imaginary parts of the permittivities, optimizing parameters such as oscillator and collision frequencies.…”