Free Electron Charge Density In Nonlinear Second And Third Harmonic Generation At Boundaries Between Metal Monolayers
Abstract
We propose to conduct experimental investigations highlighting the behavior of linear and nonlinear (harmonic generation) reflection and transmission from metallic surfaces that may be a few nanometers in thicknesses, and from structures that may contain boundaries that separate metallic media with different rest charge densities. By now, technological advances that drive device size reduction have reached the nanometer scale. Light-matter interactions at these nanoscales can display completely new phenomena, such as linear and nonlinear nonlocal pressure and viscosity effects, quantum tunneling and screening, while in most cases the models used to study them do not include these effects. The study of light interactions with thick metal layers (mirrors) at frequencies below their plasma frequencies reduces to the study of reflection due to the large negative dielectric constant and the absence of propagation modes. At nanometric scales, transmission through metallic structures is possible under different scenarios. Surface plasmons, for instance, can be excited at the surface between metal and dielectric materials increasing the field intensity provided by metal-dielectric interface resonances leading to new promising structures. The optical response of metallic materials has been explored intensely through different models. The Drude model, for example, assumes the metal is essentially a free electron gas, a cloud of free electrons that responds and is driven by an incident electric field. The resulting dielectric response depends on frequency and, at frequencies below the plasma frequency (usually lying in the UV region of the spectrum), becomes negative leading to strong reflection and absorption of radiation within a thickness usually referred to as the skin depth. At frequencies where the dielectric function approaches zero, the boundary conditions dictate an increase of the electric field amplitude, which in turn can enhance linear and nonlinear processes. In particular, surface excitation and harmonic generation has been explored intensively in the past. Although metals do not possess an intrinsic second order susceptibility, nonlinear harmonic generation can be obtained through the action of different processes, i.e. action of Lorentz force driven by the magnetic field, convection, magnetic dipole and electric quadrupole radiation terms, volume and surface contributions excited by polarization terms parallel or perpendicular to the incident plane, and quantum tunneling. Detailed numerical simulations by the Huntsville group have shown that in order to obtain a consistent interpretation of the observed experimental results the contribution of these different terms should be included. At optical frequencies, for instance, the effect of bound-electrons should be considered together with the free-electron contribution. As we intend to explore the effects of light-matter interaction while reducing the scale of the system to nanometer dimensions, it is clear that close inspection of the physics involved and the use of high-fidelity models are crucial. The main objectives of this project are: Objective 1 Study of the second and third harmonic generation in metal nanolayers Objective 2 Wavelength dependence of the THG and SHG emission Objective 3 Effect of the density gradient at metal-metal interface
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 22, 2018
- Source ID
- W911NF1810126
Entities
People
- Jose Trull Silverstre
Organizations
- Army Contracting Command
- Polytechnic University of Catalonia
- United States Army