Realizing remarkable tunability in optical properties without sacrificing speed is critical to obtain all optical ultrafast devices. In this work, we investigate the ultrafast temporal behavior of optically tunable epsilon-near-zero (ENZ) metamaterials, operating in the visible spectral range. To perform this the ultrafast dynamics of the hot electrons is acquired by femtosecond pump-probe spectroscopy and studied based on a two-temperature model (2TM). We show that pumping with femtosecond pulses changes the effective permittivity of the metamaterial by more than $400\%$. This significant modulation is more pronounced in the ENZ region and we confirm this by the 2TM. The realized ultrafast modulation in effective permittivity, along with the ultrashort relaxation time of 3.3 ps, opens another avenue towards ultrafast photonic applications.

Metallic nanoparticles can localize the incident light to hot spots as plasmon oscillations, where the intensity can be enhanced by up to four orders of magnitude. Even though the lifetime of plasmons is typically short, it can be increased via interactions with quantum emitters, e.g., spaser nanolasers. However, molecules can bleach in days. Here, we study the lifetime enhancement of plasmon excitations due to the coupling with longer-lifetime dark plasmon modes. We apply an analytical model based on harmonic oscillators to demonstrate that a coupled system of bright and dark plasmon modes decays more slowly than the bright mode alone. Furthermore, exact solutions of the three-dimensional Maxwell equations, i.e., finite-difference time domain, demonstrate that the lifetime of the coupled system significantly increases at the hot spot, which is not predictable by far-field response. The decay of the overall energy of such a coupled system, which can be extracted from experimental absorption measurements, is substantially different from the decay of the hot spot field. This observation enlightens the plasmonic applications in which the hot spot intensity enables the detection of the optical responses.

Controlling the hybridization is a very powerful tool to manipulate the modes in a single nanostructure. We investigate the hybridization between localized and propagating surface plasmons in a nanostructure system where a thin metal layer strongly interacts with a nanodisk array. Hybrid plasmon resonances are observed in the reflection spectra obtained from finite-difference time domain simulations and experimental measurements in the visible-near-infrared region. We demonstrate how the geometrical parameters of the nanostructure can be utilized to bring these plasmon modes in the strong coupling regime. The hybrid plasmon modes exhibit anticrossing with a Rabi splitting of $\sim 0.1\mathrm{eV}$ , which is the signature of strong coupling. Near-field profiles of the hybrid modes exhibit a mixture of localized and propagating plasmon characteristics, with propagating modes excited on both sides of the metal film. Our design promises richer implementations in light manipulation towards novel photonic applications compared to the systems with thick metal films.

Plasmonic nanostructures enhance nonlinear response, such as surface enhanced Raman scattering (SERS), by localizing the incident field into hot spots. The localized hot spot field can be enhanced even further when linear Fano resonances take place in a double resonance scheme. However, hot spot enhancement is limited with the modification of the vibrational modes, the breakdown of the molecule, and the tunneling regime. Here, we present a method which can circumvent these limitations. Our analytical model and solutions of 3D Maxwell equations show that: enhancement due to the localized field can be multiplied by a factor of 10 ² –10 ³ . Moreover, this can be performed without increasing the hot spot intensity which also avoids the modification of the Raman modes. Unlike linear Fano resonances, here, we create a path interference in the nonlinear response. We demonstrate on a single equation that enhancement takes place due to cancellation of the contributing terms in the denominator of the SERS response. Our method can be implemented on an atomic force microscope tip, decorated (or “contaminated”) with appropriate quantum emitters.

We experimentally demonstrate that two coupled metal nanostructures (MNSs), a silver nanowire and bipyramid, can produce ∼30 times enhanced second harmonic generation compared to the particles alone. We develop a simple theoretical model, presenting the path interference effects in the nonlinear response of coupled MNSs. We show that the reason for such an enhancement can be the occurrence of a Fano resonance due to the coupling of the converter MNS to the long-lived mode of the attached MNS.

Extraordinary optical transmission has been utilized in many optical applications, but the plasmonic losses hinder their full potential. To obtain enhanced transmission, one of the loss compensation methods is to introduce gain. However, an enhanced transmission or even eliminated absorption does not guarantee plasmonic loss compensation. Here, we reveal the distinction between the transmission enhancement mechanisms in gain-assisted plasmonic arrays. To uncover the underlying mechanisms of the modified transmissions, we calculate the effective electric permittivity by employing a self-consistent gain model. We demonstrate that a large transmission enhancement in a plasmonic system composed of periodic nanocavities and coaxially placed nanoislands, is led by the loss compensation, which manifests itself as narrowing in effective permittivity. In contrast, a slight transmission enhancement in a plasmonic array without the nanoislands arises from the background amplification.