Determining the Molecular Dipole Orientation on Nanoplasmonic Structures

Abstract

We developed a theoretical method to investigate the effects of the orientation of a molecular monolayer on plasmonic systems. Molecular layers strongly alter the plasmonic resonance of nanoparticles, affecting their ability to couple to other nanoparticles and quantum emitters. The ability to understand how the coating impacts the optical properties of the nanostructures is critical for the application of plasmonics in areas such as light detection, sensing, and plasmon-enhanced solar energy conversion. We extend the three-dimensional finite-difference time-domain method to include molecular layers with induced dipoles at an arbitrary orientation relative to the nanostructure’s surface. Numerical calculations show how the orientation of molecular dipoles affects the plasmon resonance of both tetrahedral and ellipsoidal nanoparticles. Finally, we demonstrate how the layer impacts the coupling between ellipsoidal nanoparticle and a colloidal quantum dot.

Modeling the Chiral Imprinting Response of Oriented Dipole Moments on Metal Nanostructures

“…In this section we extend a previously developed method for modeling oriented dipole moments to include chiral materials within FDTD. 39 FDTD solves Maxwell's equations on a grid…”

“…In this section we extend a previously developed method for modeling oriented dipole moments to include chiral materials within FDTD . FDTD solves Maxwell’s equations on a grid where E is the electric field, H is the magnetic field, D is the electric flux density, and B is the magnetic flux density.…”

“…ADE recasts the constitutive relations as where in the frequency domain P Metal , P Molecule , and M Molecule are defined as The polarization and magnetization fields are then inverse Fourier transformed, and all of the resulting derivatives are expanded with a central-difference approximation to generate the update equations, defined for chiral polarizations and chiral and achiral magnetizations as Here Δ t is the time step size, n is the current time step, i , j , and k are grid point coordinates in the x , y , and z directions, and E i +1/2, j +1/2, k +1/2 n , H i,j,k n –1/2 , and H i +1/2, j +1/2, k +1/2 n –1/2 are determined through spatially averaging the field components. The update equations for the nonchiral components of P̆ Metal and P̆ Molecule are defined in refs and , respectively. , …”

“…The update equations for the nonchiral components of P ̆Metal and P ̆Molecule are defined in refs 39 and 41, respectively. 39,41 Figure 1 displays the modified Yee Cell for anisotropic chiral material. For isotropic materials the chiral polarization and magnetization terms are calculated at their respective electric and magnetic field component locations represented by the red triangles and blue arrows, respectively.…”