Two-Photon Absorption of Metal-Assisted Chromophores

A Multi-Scale Approach to Simulate the Nonlinear Optical Response of Molecular Nanomaterials

Nonlinear optics is essential for many recent photonic technologies. Here, a novel multi-scale approach is introduced to simulate the nonlinear optical response of molecular nanomaterials combining ab initio quantum-chemical and classical Maxwell-scattering computations. In this approach, the first hyperpolarizability tensor is computed with time-dependent density-functional theory and incorporated into a multi-scattering formalism that considers the optical interaction between neighboring molecules. Such incorporation is achieved by a novel object: the Hyper-Transition(T)-matrix. With this object at hand, the nonlinear optical response from single molecules and also from entire photonic devices can be computed, including the full tensorial and dispersive nature of the optical response of the molecules, as well as the optical interaction between different molecules as, for example, in the lattice of a molecular crystal. To demonstrate the applicability of the novel approach, the generation of a second-harmonic signal from a thin film of an Urea molecular crystal is computed and compared to more traditional simulations. Furthermore, an optical cavity is designed, which enhances the second-harmonic response of the molecular film up to more than two orders of magnitude. This approach is highly versatile and accurate and can be the working horse for the future exploration of nonlinear photonic molecular materials in structured photonic environments.

Degenerate and non-degenerate two-photon absorption of coumarin dyes

Two-photon absorption (2PA) spectroscopy is a robust bioimaging tool that depends on the determined cross-sections (σ^2PA). The absorption of both photons occurs simultaneously with equivalent (degenerate) or different (non-degenerate) photon energies, D-2PA and ND-2PA, respectively. The former has been investigated experimentally and computationally for many systems, while the latter remains relatively unexplored computationally and limited experimentally. In this study, response theory using time-dependent density functional theory (TD-DFT) and the 2-state model (2SM) have been utilized to investigate σ^D-2PA and σ^ND-2PA for the excitation to the lowest energy singlet state (S₁) of coumarin, coumarin 6, coumarin 120, coumarin 307, and coumarin 343. Solvents involved were methanol (MeOH), chloroform (ClForm), and dimethylsulfoxide (DMSO), where the latter leads to the largest σ^2PA. Values of σ^2PA are largest for coumarin 6 and lowest for coumarin, which illustrates the effect of substituents. The 2SM clarifies how the largest cross-sections correspond to molecules with the largest transition dipole moments, μ₀₁. In general, σ^D-2SM computations agree with σ^D-2PA. Moreover, σ^ND-2SM are in qualitative agreement with σ^ND-2PA with comparable enhancement relative to σ^D-2PA. Overall, σ^ND-2PA are larger than σ^D-2PA where the increase is in the range of 22% to 49%, depending on the coumarin as well as the relative energies of the two photons. This work aids in future investigations into various fluorophores to understand their photophysical properties for ND-2PA.

Regulation of optical properties for fluorescent triphenylamine-silver hybrid based on SPR effect

In this study, a two photon absorbing (TPA) material consisting silver nanoparticles and triphenylamine-thiol derivative (TBS) has been prepared through interfacial coordination effect according to soft-hard-acid-base principle. The interfacial structure and morphology of the hybrid are researched in detail. Linear and nonlinear optical properties of the hybrid are studied. Upon interfacial coordination, the hybrid shows red-shifted UV-Vis absorption, causing from enhanced electronic drawing strength due to existence of Ag atom. The results also indicate that the surface Plasmon resonance (SPR) effect of Ag nanoparticles (~6 nm) brings about enhancement in single photon fluorescence emission and two photon absorption. Compared with free TBS, Ag-TBS hybrids show higher TPA cross-section (δ), which is 8784 GM for TBS and up to 103876 GM for Ag-TBS hybrid, showing ~12 fold increase. Due to excellent TPA property, the hybrids have good application in the field of optical power limiting and its limiting threshold is 0.49 J/cm^-2. This type of interfacial coordination induced hybrid provides a promising strategy to regulate linear optical properties and optimize nonlinear performance.

A Discrete Interaction Model/Quantum Mechanical Method for Simulating Plasmon-Enhanced Two-Photon Absorption

In this work, we extend the discrete interaction model/quantum mechanical (DIM/QM) method to simulate plasmon-enhanced two-photon absorption (PETPA). The metal nanoparticle is treated atomistically by means of electrodynamics, while the molecule is described using damped cubic response theory within a time-dependent density functional theory framework. Using DIM/QM, we study the PETPA of para-nitroaniline ( p-NA) with a focus on the local and image field effects, the molecular orientation effects, and the molecule-nanoparticle distance effects. Our findings show that the enhancement is more complex than the simple | E|⁴ enhancement mechanism, where | E| is the local field at the position of the molecule. Because of specific interactions with the nanoparticle, we find that a TPA dark state of p-NA can be significantly enhanced through a coupling with the plasmon excitation. The results presented in this work illustrate that the coupling between molecular excitations and plasmons can give rise to unusual and complex behavior in nonlinear spectroscopy that cannot simply be understood by considering the optical properties of the individual molecules and nanoparticles separately. The method presented here provides detailed insights into the enhancement of nonlinear optical properties of molecules coupled to plasmonic nanoparticles.

Decoration of gold nanoparticles with cysteine in solution: reactive molecular dynamics simulations

The dynamics of gold nanoparticle functionalization by means of adsorption of cysteine molecules in water solution is simulated through classical reactive molecular dynamics simulations based on an accurately parametrized force field. The adsorption modes of the molecules are characterized in detail disclosing the nature of the cysteine-gold interactions and the stability of the final material. The simulation results agree satisfactorily with recent experimental and theoretical data and confirm previous findings for a similar system. The covalent attachments of the molecules to the gold support are all slow physisorptions followed by fast chemisorptions. However, a great variety of binding arrangements can be observed. Interactions with the adsorbate caused surface modulations in terms of adatoms and dislocations which contributed to strengthen the cysteine adsorption.

Two-photon absorption of ligand-protected Ag15 nanoclusters. Towards a new class of nonlinear optics nanomaterials

Theoretical and experimental two-photon absorption cross sections of thiolated small silver cluster Ag₁₅L₁₁ exhibiting extraordinary large TPA.