Alexander V. Gavrilenko

Engineering of low-loss metal for nanoplasmonic and metamaterials applications

We have shown that alloying a noble metal (gold) with another metal (cadmium), which can contribute two electrons per atom to a free electron gas, can significantly improve the metal’s optical properties in certain wavelength ranges and make them worse in the other parts of the spectrum. In particular, in the gold-cadmium alloy we have demonstrated a significant expansion of the spectral range of metallic reflectance to shorter wavelengths. The experimental results and the predictions of the first principles theory demonstrate an opportunity for the improvement and optimization of low-loss metals for nanoplasmonic and metamaterials applications.

Effects of molecular adsorption on optical losses of the Ag (111) surface

The first principles density functional theory (DFT) is applied to study effects of molecular adsorption on optical losses of silver (111) surface. The ground states of the systems including water, methanol, and ethanol molecules adsorbed on Ag (111) surface were obtained by the total energy minimization method within the local density approximation (LDA). Optical functions were calculated within the Random Phase Approximation (RPA) approach. Contribution of the surface states to optical losses was studied by calculations of the dielectric function of bare Ag (111) surface. Substantial modifications of the real and imaginary parts of the dielectric functions spectra in the near infrared and visible spectral regions, caused by surface states and molecular adsorption, were obtained. The results are discussed in comparison with available experimental data.

Ethanol adsorption on the Si (111) surface: First principles study

Equilibrium atomic configurations and electron energy structure of ethanol adsorbed on the Si (111) surface are studied by the first principles density functional theory. Geometry optimization is performed by the total energy minimization method. Equilibrium atomic geometries of ethanol, both undissociated and dissociated, on the Si (111) surface are found and analysed. Reaction pathways and predicted transition states are discussed in comparison with available experimental data in terms of the feasibility of the reactions occurring. Analysis of atom and orbital resolved projected density of states indicates substantial modifications of the Si surface valence and conduction electron bands due to the adsorption of ethanol affecting the electronic properties of the surface.

Linear and nonlinear optical response of LiNbO 3 calculated from first principles

The dielectric function and second-harmonic generation spectrum of ferroelectric LiNbO3 are calculated from first principles. The calculations are based on the electronic structure obtained within density functional theory. The use of the GW approach to account for quasiparticle effects and the subsequent solution of the Bethe-Salpeter equation lead to a dielectric function in excellent agreement with measured data. The second harmonic generation spectrum calculated within the independent (quasi) particle approximation predicts strong nonlinear coefficients for photon energies above about 1.5 eV. The comparison with measured data suggests that the inclusion of self-energy effects in the nonlinear response improves the agreement with experiment.

Optical Second Harmonic Generation in Semiconductor Nanostructures

Optical second harmonic generation (SHG) studies of semiconductor nanostructures are reviewed. The second-order response data both predicted and observed on pure and oxidised silicon surfaces, planar Si(001)/SiO2 heterostructures, and the results related to the direct-current-and strain-induced effects in SHG from the silicon surfaces as well are discussed. Remarkable progress in understanding the unique capabilities of nonlinear optical second harmonic generation spectroscopy as an advanced tool for nanostructures diagnostics is demonstrated.

LiNbO 3 linear and nonlinear optical response from first-principles calculations

The dielectric function and second harmonic generation of ferroelectric LiNbO3 is calculated from first-principles. Thereby we start from the electronic structure calculated within the density functional theory. The use of the GW approach to account for quasiparticle effects and the subsequent solution of the Bethe-Salpeter equation leads to a dielectric function that is in excellent agreement with the available experimental results. Our second harmonic generation calculations rest on the independent particle approximation and predict strong non-linear coefficients, in particular in the energy range starting from 1.5 eV.

Bridge effects on light harvesting of a DBfA type polymer system

Plastic optoelectronic materials and thin film devices are very attractive in future optical sensor and solar energy applications due to their lightweight, flexible shape, high photon absorption coefficients, low cost, and environmental benefits. In this study, optoelectronic properties of D, D/fA blend, DfA, and a series of DBfA type of conjugated block copolymers has been investigated, where D is a donor type PPV conjugated block, B is a non-conjugated and flexible aliphatic hydrocarbon bridge chain containing different number of aliphatic methylene units, and fA is a fluorinated acceptor type PPV conjugated block. The optical absorptions of the D/fA blend, DfA, and DBfAs are typical overlaps of individual absorptions of D and fA blocks, while the solution steady state photoluminescence (PL) emission of D were quenched to different levels in blends and block copolymers, with DBfAs containing one methylene unit bridge (DB1fA) quenched most. This could be attributed to an intra-molecular photo induced electron transfer or charge separation in DBfA systems. Theoretical first principles study of the equilibrium atomic configuration of DfA reveals the existence of twisting angles between the D and fA blocks in DfA stable states which may account for a less PL quenching of DfA as compared to DB1fA. These results are important for designing and developing high efficiency polymer based optoelectronic systems.

Realistic electric field modeling of multilayered nanostructures by classic electrodynamics and first principles theory

Efficient engineering of metamaterials involves modeling of electric field profiles around these structures. Realistic modeling of the electric field in metamaterials requires accurate knowledge of optical constants of the compo- nents for which traditionally the bulk values are taken. Further progress in the developing of metamaterials is characterized by a reduction of the pattern size, dimensions of single layers in multilayered structures etc. It has been understood that optical functions in low-dimensional and nano-sized materials substantially differ from their bulk values increasingly affecting by quantum processes. In this work we develop a complex method for analytical modeling of electric field profiles in metamaterials including quantum processes in nano-sized multi-layered structures. In particular based on first principles density functional theory we obtained simple analytical functions allowing predictions the optical functions variations with the size reduction of single metamaterial components over a wide spectral region. It is shown that optical functions of nano-sized films substantially (by 50 percent and more) differ from those in bulk. The new calculated optical functions of the components are used for electric field profile modeling of nano-sized multilayered structures by nonlocal Green function technique including effects of spatial dispersion. Silicon, silicon dioxide, and water layers are used as an example. The method effectively incorporates real atomic structure reconstruction on surfaces and inner interfaces thus providing with a more realistic picture for modeling. By comparison with experiment it is demonstrated that our method predicts image potential of the nanostructures in better agreement with experiment than if using traditional classic electrodynamics approach neglecting the quantum effects. The results are discussed in comparison with literature.

Optical functions of nanocrystalline ZnO containing voidsand doped with Ga

The pulsed laser deposited nanocrystalline ZnO films doped by Ga up to six weight percent are studied by X-ray difraction and generalized spectro-ellipsometry. We report substantial atomic structure modification of heavy Ga-doped ZnO resulted and a concentration dependent increase of inter-planar distance. Measured dielectric function spectra show strong blue-shift of the samples studied. Equilibrium atomic configurations and electron energy structure of ZnO containing defects (voids and Ga impurities) are studied by the density functional theory (DFT) and generalized gradient approximation (GGA). Atomic geometries are obtained from the total energy minimization method. Optical functions are calculated within the random phase approximation including the quasi-particles corrections and plasma excitation effects. We report energetically favorable paths of the voids growth and aggregation in ZnO. Comparative analysis of experimental and theoretical results indicate that measured blue-shift in ZnO:Ga substantially exceeds the Burstein-Moss shift as used in many previous work to interpret concentration dependence of optical functions in heavy doped ZnO. We demonstrate that additional mechanisms, such as structural and alloying effect, should be involved for quantitative interpretation of optics of the nano-crystalline heavy-doped ZnO films.

Computations of Ground State and Excitation Energies of Poly(3-methoxy-thiophene) and Poly(thienylene vinylene) from First Principles

Ground state and excitation energies of poly(3-methoxy-thio-phene) (PMT) and poly(thienylene vinylene) (PTV) conjugated polymers are studied by first principles density functional theory (DFT). Two basic approaches of computational chemistry and physics are compared: time dependent DFT (TDDFT) of clusters and ab initiopseudopotentials within a standard DFT (PP-DFT) of infinite polymer chains. We demonstrate that series of excitation energies of PMT calculated by TDDFT with increased unit numbers converge well to the real experimentally measured energy gaps. Combination of TDDFT cluster method with PP-DFT approach for infinite chain provides single-gap quasiparticle correction value needed for optical calculations. Infinite chain model is used to calculate optical absorption of PTV.