Electron confinement-induced plasmonic breakdown in metals.

Plasmon resonance represents the collective oscillation of free electron gas density and enables enhanced light-matter interactions in nanoscale dimensions. Traditionally, the classical Drude model describes plasmonic excitation, wherein plasma frequency exhibits no spatial dispersion. Here, we show conclusive experimental evidence of the breakdown of plasmon resonance and a consequent metal-insulator transition in an ultrathin refractory plasmonic material, hafnium nitride (HfN).

Confinement-induced nonlocality and casimir force in transdimensional systems.

We study within the framework of the Lifshitz theory the long-range Casimir force for in-plane isotropic and anisotropic free-standing transdimensional material slabs. In the former case, we show that the confinement-induced nonlocality not only weakens the attraction of ultrathin slabs but also changes the distance dependence of the material-dependent correction to the Casimir force to go as contrary to the ∼1/ dependence of that of the local Lifshitz force. In the latter case, we use closely packed array of parallel aligned single-wall carbon nanotubes in a dielectric layer of finite thickness to demonstrate strong orientational anisotropy and crossover behavior for the inter-slab attractive force in addition to its reduction with decreasing slab thickness.

Thickness-Dependent Drude Plasma Frequency in Transdimensional Plasmonic TiN.

Plasmonic transdimensional materials (TDMs), which are atomically thin metals of precisely controlled thickness, are expected to exhibit large tailorability and dynamic tunability of their optical response as well as strong light confinement and nonlocal effects. Using spectroscopic ellipsometry, we characterize the complex permittivity of ultrathin films of passivated plasmonic titanium nitride with thicknesses ranging from 1 to 10 nm. By measuring passivated TiN, we experimentally distinguish between the contributions of an oxide layer and thickness to the optical properties.

Charged Bosons Made of Fermions in Bilayer Structures with Strong Metallic Screening.

Two-dimensional monolayer structures of transition metal dichalogenides (TMDs) have been shown to allow many higher-order excitonic bound states, including trions (charged excitons), biexcitons (excitonic molecules), and charged biexcitons. We report here experimental evidence and the theoretical basis for a new bound excitonic complex, consisting two free carriers bound to an exciton in a bilayer structure. Our experimental measurements on structures made using two different materials show a new spectral line at the predicted energy with two different TMD materials (MoSe and WSe) with both n- and p-doping if and only if all the required theoretical conditions for this complex are fulfilled, in particular, only in the presence of a parallel metal layer that significantly screens the repulsive interaction between the like-charge carriers.

Monitoring Charge Separation Processes in Quasi-One-Dimensional Organic Crystalline Structures.

We perform the transient absorption spectroscopy experiments to investigate the dynamics of the low-energy collective electron-hole excitations in α-copper phthalocyanine thin films. The results are interpreted in terms of the third-order nonlinear polarization response function. It is found that, initially excited in the molecular plane, the intramolecular Frenkel exciton polarization reorients with time to align along the molecular chain direction to form coupled Frenkel-charge-transfer exciton states, the eigenstates of the one-dimensional periodic molecular lattice.