Non-Hermitian Hamiltonians for linear and nonlinear optical response: A model for plexcitons.

In polaritons, the properties of matter are modified by mixing the molecular transitions with light modes inside a cavity. Resultant hybrid light-matter states exhibit energy level shifts, are delocalized over many molecular units, and have a different excited-state potential energy landscape, which leads to modified exciton dynamics. Previously, non-Hermitian Hamiltonians have been derived to describe the excited states of molecules coupled to surface plasmons (i.e., plexcitons), and these operators have been successfully used in the description of linear and third order optical response. In this article, we rigorously derive non-Hermitian Hamiltonians in the response function formalism of nonlinear spectroscopy by means of Feshbach operators and apply them to explore spectroscopic signatures of plexcitons. In particular, we analyze the optical response below and above the exceptional point that arises for matching transition energies for plasmon and molecular components and study their decomposition using double-sided Feynman diagrams. We find a clear distinction between interference and Rabi splitting in linear spectroscopy and a qualitative change in the symmetry of the line shape of the nonlinear signal when crossing the exceptional point. This change corresponds to one in the symmetry of the eigenvalues of the Hamiltonian. Our work presents an approach for simulating the optical response of sublevels within an electronic system and opens new applications of nonlinear spectroscopy to examine the different regimes of the spectrum of non-Hermitian Hamiltonians.

Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes.

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices.

Enantiospecific Response in Cross-Polarization Solid-State Nuclear Magnetic Resonance of Optically Active Metal Organic Frameworks.

We report herein on a NMR-based enantiospecific response for a family of optically active metal-organic frameworks. Cross-polarization of the 1H-13C couple was performed, and the intensities of the 13C nuclei NMR signals were measured to be different for the two enantiomers. In a direct-pulse experiment, which prevents cross-polarization, the intensity difference of the 13C NMR signals of the two nanostructured enantiomers vanished. This result is due to changes of the nuclear spin relaxation times due to the electron spin spatial asymmetry induced by chemical bond polarization involving a chiral center. These experiments put forward on firm ground that the chiral-induced spin selectivity effect, which induces chemical bond polarization in the J-coupling, is the mechanism responsible for the enantiospecific response. The implications of this finding for the theory of this molecular electron spin polarization effect and the development of quantum biosensing and quantum storage devices are discussed.

Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation.

As one of the successful approaches to GaAs surface passivation, wet-chemical nitridation is applied here to relate the effect of surface passivation to carrier recombination processes in bulk GaAs. By combining time-resolved photoluminescence and optical pump-THz probe measurements, we found that surface hole trapping dominates the decay of photoluminescence, while photoconductivity dynamics is limited by surface electron trapping. Compared to untreated sample dynamics, the optimized nitridation reduces hole- and electron-trapping rate by at least 2.6 and 3 times, respectively. Our results indicate that under ambient conditions, recovery of the fast hole trapping due to the oxide regrowth at the deoxidized GaAs surface takes tens of hours, while it is effectively inhibited by surface nitridation. Our study demonstrates that surface nitridation stabilizes the GaAs surface via reduction of both electron- and hole-trapping rates, which results in chemical and electronical passivation of the bulk GaAs surface.

Two-dimensional Fano lineshapes: Excited-state absorption contributions.

Fano interferences in nanostructures are influenced by dissipation effects as well as many-body interactions. Two-dimensional coherent spectroscopies have just begun to be applied to these systems where the spectroscopic signatures of a discrete-continuum structure are not known. In this article, we calculate the excited-state absorption contribution for different models of higher lying excited states. We find that the characteristic asymmetry of one-dimensional spectroscopies is recovered from the many-body contributions and that the higher lying excited manifolds have distorted lineshapes that are not anticipated from discrete-level Hamiltonians. We show that the Stimulated Emission cannot have contributions from a flat continuum of states. This work completes the Ground-State Bleach and Stimulated Emission signals that were calculated previously [D. Finkelstein-Shapiro et al., Phys. Rev. B 94, 205137 (2016)]. The model reproduces the observations reported for molecules on surfaces probed by 2DIR.

Understanding iridium oxide nanoparticle surface sites by their interaction with catechol.

Iridium oxide (IrOx) is one of the best water splitting electrocatalysts, but its active site details are not well known. As with all heterogeneous catalysts, a strategy for counting the number of active sites is not clear, and understanding their nature and structure is remarkably difficult. In this work, we performed a combined study using optical spectroscopy, magnetic resonance and electrochemistry to characterize the interaction of IrOx nanoparticles (NPs) with a probe molecule, catechol. The catalyst is heterogeneous given that the substrate is in a different phase, but behaves as a homogeneous catalyst from the point of view of electrochemistry since it remains in colloidal suspension. We find two types of binding sites: centers A which bind catechol irreversibly making up 21% of the surface, and centers B which bind catechol reversibly making up 79% of the surface. UV-vis absorption spectroscopy shows that the A sites are responsible for the characteristic blue color of the NPs. Electrochemical experiments indicate that the B sites are catalytically active and we give the number of active sites per nanoparticle. We conclude by performing a survey of ligands used in solar cell architectures and show which ones bind well to the surface and which ones inhibit the catalytic activity when doing so, presenting quantitative guidelines for the correct handling of IrOx nanoparticles during their incorporation into multifunctional solar energy harvesting architectures. We suggest ligands binding on the surface oxygen atoms allow for large bound ligand densities with no detrimental effect on the catalytic activity.

Fano-Liouville spectral signatures in open quantum systems.

The scattering amplitude from a set of discrete states coupled to a continuum became known as the Fano profile, characteristic for its asymmetric line shape and originally investigated in the context of photoionization. The generality of the model and the proliferation of engineered nanostructures with confined states gives immense success to the Fano line shape, which is invoked whenever an asymmetric line shape is encountered. However, many of these systems do not conform to the initial model worked out by Fano in that (i) they are subject to dissipative processes and (ii) the observables are not entirely analogous to the ones measured in the original photoionization experiments. In this Letter, we work out the full optical response of a Fano model with dissipation. We find that the exact result for the excited population, Raman, Rayleigh, and fluorescence emission is a modified Fano profile where the typical line shape has an additional Lorentzian contribution. Expressions to extract model parameters from a set of relevant observables are given.

Continuum model for chiral induced spin selectivity in helical molecules.

A minimal model is exactly solved for electron spin transport on a helix. Electron transport is assumed to be supported by well oriented p(z) type orbitals on base molecules forming a staircase of definite chirality. In a tight binding interpretation, the spin-orbit coupling (SOC) opens up an effective π(z) - π(z) coupling via interbase p(x,y) - p(z) hopping, introducing spin coupled transport. The resulting continuum model spectrum shows two Kramers doublet transport channels with a gap proportional to the SOC. Each doubly degenerate channel satisfies time reversal symmetry; nevertheless, a bias chooses a transport direction and thus selects for spin orientation. The model predicts (i) which spin orientation is selected depending on chirality and bias, (ii) changes in spin preference as a function of input Fermi level and (iii) back-scattering suppression protected by the SO gap. We compute the spin current with a definite helicity and find it to be proportional to the torsion of the chiral structure and the non-adiabatic Aharonov-Anandan phase. To describe room temperature transport, we assume that the total transmission is the result of a product of coherent steps.

Controlling surface defects and photophysics in TiO2 nanoparticles.

Titanium dioxide (TiO2) is widely used for photocatalysis and solar cell applications, and the electronic structure of bulk TiO2 is well understood. However, the surface structure of nanoparticulate TiO2, which has a key role in properties such as solubility and catalytic activity, still remains controversial. Detailed understanding of surface defect structures may help explain reactivity and overall materials performance in a wide range of applications. In this work we address the solubility problem and surface defects control on TiO2 nanoparticles. We report the synthesis and characterization of ∼4 nm TiO2 anatase spherical nanoparticles that are soluble and stable in a wide range of organic solvents and water. By controlling the temperature during the synthesis, we are able to tailor the density of defect states on the surface of the TiO2 nanoparticles without affecting parameters such as size, shape, core crystallinity, and solubility. The morphology of both kinds of nanoparticles was determined by TEM. EPR experiments were used to characterize the surface defects, and transient absorption measurements demonstrate the influence of the TiO2 defect states on photoinduced electron transfer dynamics.

CO2 Preactivation in Photoinduced Reduction via Surface Functionalization of TiO2 Nanoparticles.

Salicylate and salicylic acid derivatives act as electron donors via charge-transfer complexes when adsorbed on semiconducting surfaces. When photoexcited, charge is injected into the conduction band directly from their highest occupied molecular orbital (HOMO) without needing mediation by the lowest unoccupied molecular orbital (LUMO). In this study, we successfully induce the chemical participation of carbon dioxide in a charge transfer state using 3-aminosalicylic acid (3ASA). We determine the geometry of CO2 using a combination of ultraviolet-visible spectroscopy (UV-vis), surface enhanced Raman scattering (SERS), (13)C NMR, and electron paramagnetic resonance (EPR). We find CO2 binds on Ti sites in a carbonate form and discern via EPR a surface Ti-centered radical in the vicinity of CO2, suggesting successful charge transfer from the sensitizer to the neighboring site of CO2. This study opens the possibility of analyzing the structural and electronic properties of the anchoring sites for CO2 on semiconducting surfaces and proposes a set of tools and experiments to do so.

Identification of binding sites for acetaldehyde adsorption on titania nanorod surfaces using CIMS.

The interaction of acetaldehyde with TiO(2) nanorods has been studied under low pressures (acetaldehyde partial pressure range 10(-4)-10(-8) Torr) using chemical ionization mass spectrometry (CIMS). We quantitatively separate irreversible adsorption, reversible adsorption, and an uptake of acetaldehyde assigned to a thermally activated surface reaction. We find that, at room temperature and 1.2 Torr total pressure, 2.1 ± 0.4 molecules/nm(2) adsorb irreversibly, but this value exhibits a sharp decrease as the analyte partial pressure is lowered below 4 × 10(-4) Torr, regardless of exposure time. The number of reversible binding sites at saturation amounts to 0.09 ± 0.02 molecules/nm(2) with a free energy of adsorption of 43.8 ± 0.2 kJ/mol. We complement our measurements with FTIR spectroscopy and identify the thermal dark reaction as a combination of an aldol condensation and an oxidative adsorption that converts acetaldehyde to acetate or formate and CO, at a measured combined initial rate of 7 ± 1 × 10(-4) molecules/nm(2) s. By characterizing binding to different types of sites under dark conditions in the absence of oxygen and gas phase water, we set the stage to analyze site-specific photoefficiencies involved in the light-assisted mineralization of acetaldehyde to CO(2).

Photoinduced kinetics of SERS in bioinorganic hybrid systems. a case study: dopamine-TiO2.

The reported observation of SERS on semiconductors has confirmed the feasibility of distinguishing the charge-transfer mechanism from the electromagnetic one responsible for the enhancement of the signal in metal nanoparticles. Experimental investigation of the well characterized dopamine-TiO(2) system revealed an unexpected dependence on coverage and size. We propose here a theoretical model applicable to SERS on semiconducting substrates that explains this remarkable behavior. The model is based on a competition mechanism arising from the formation of an electron gas in the conduction band of the semiconductor due to the photoexcitation of a charge-transfer complex. Taking into account the two competing effects, a linear increase in the Raman intensity arising from increasing coverage and a quenching effect due to the photon absorption by the electron gas, provides excellent agreement between our model and the experiment for 5 nm nanoparticles. Discrepancies for the case of 2 nm nanoparticles are attributed to quantum confinement, an effect that is investigated elsewhere.

Challenges in distinguishing superexchange and hopping mechanisms of intramolecular charge transfer through fluorene oligomers.

The temperature dependence of intramolecular charge separation in a series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene FL(n) (n = 1-4) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FL(n) bridge. In toluene, the temperature dependence of the charge separation rate constants for PTZ-FL(n)-PDI, (n = 1-4) is relatively weak and is successfully described by the semiclassical Marcus equation. The activation energies for charge separation suggest that bridge charge carrier injection is not the rate limiting step. The difficulty of using temperature and length dependence to differentiate hopping and superexchange is discussed, with difficulties in the latter topic explored via an extension of a kinetic model proposed by Bixon and Jortner.