Thejaswi Tumkur

Control of spontaneous emission in a volume of functionalized hyperbolic metamaterial

We have developed a simple method to fabricate lamellar metal-dielectric hyperbolic metamaterials on flat, flexible, and curvilinear substrates, which allows for functionalization of dielectric layers by dye molecules. The control of spontaneous emission of dye molecules with hyperbolic metamaterials has been studied in two different sample configurations, and the effect has been found to be much stronger when emitters are placed inside the metamaterial rather than on its surface.

Reduced reflection from roughened hyperbolic metamaterial.

We show that roughened surfaces of hyperbolic metamaterials scatter light preferentially inside the media, resulting in a very low reflectance. This phenomenon of fundamental importance, demonstrated experimentally in arrays of silver nanowires grown in alumina membranes, is consistent with a broad-band singularity in the density of photonic states. It paves the road to a variety of applications ranging from the stealth technology to high-efficiency solar cells and photodetectors.

Permittivity evaluation of multilayered hyperbolic metamaterials: Ellipsometry vs. reflectometry

Metal-dielectric nanolaminates represent a class of hyperbolic metamaterials with uniaxial permittivity tensor. In this study, we critically compare permittivity extraction of nanolaminate samples using two techniques: polarized reflectometry vs. spectroscopic anisotropic ellipsometry. Both Au/MgF2 and Ag/MgF2 metal-dielectric stacks are examined. We demonstrate the applicability of the treatment of the multilayered material as a uniaxial medium and compare the derived optical parameters to those expected from the effective medium approximation. We also experimentally compare the effect of varying the material outer layer on the homogenization of the composite. Additionally, we introduce a simple empirical method of extracting the epsilon-near-zero point of the nanolaminates from normal incidence reflectance. The results of this study are useful in accurate determination of the hyperbolic material permittivity and in the ability to tune its optical properties.

Strong coupling of surface plasmon polaritons and ensembles of dye molecules.

We demonstrate the strong coupling of dye molecules to surface plasmon polaritons (SPPs) excited in the Kretschmann geometry and propagating at the interface of silver and dye-doped polymer. The dispersion curve of such a system, studied in the reflectometry experiments, is split into three branches and demonstrates an avoided crossing - the signature of a strong coupling. We have further studied the excitation spectra of the dye emission and found that the positions of the excitation peaks have a good match with the points in the dispersion curve determined by the reflectometry. At the same time, the analysis of the spectra of the plasmon-mediated spontaneous emission, decoupled to the prism and acquired at multiple collection angles, has resulted in a quite different dispersion curve exhibiting a non-trivial splitting into multiple branches. This suggests that the same plasmonic environment couples differently to absorbing and emitting dye molecules.

Thermal radiation of lamellar metal-dielectric metamaterials and metallic surfaces

We studied angular distributions and spectra of thermal radiation of lamellar metal/dielectric metamaterials with hyperbolic dispersion and compared them with the corresponding characteristics of simple metallic films and pairs of metallic and dielectric layers. The spectra of thermal radiation, in the mid-infrared part of the spectrum, were nearly flat and featureless, in a good agreement with the model predictions. The angular distributions of thermal emission deviated from the predictions of the Kirchoff’s law and closely followed the Lambert’s law, ∝cosθ. The thermal radiation properties of hyperbolic metamaterials were not much different from those of simpler metallic structures.

Control of spontaneous emission with functionalized multilayered hyperbolic metamaterials

The behavior of spontaneous emission of emitters embedded inside metamaterials with hyperbolic dispersion has been investigated. A simple technique has been developed to fabricate lamellar metal-dielectric hyperbolic metamaterials on substrates which can be flat, flexible or curvilinear in geometry. Moreover, this method opens up the possibility of functionalizing the dielectric layers by dye molecules. Utilizing this technique, we study the spontaneous emission kinetics of emitters placed either on top, or embedded inside hyperbolic metamaterials. While we observe a reduction in the radiative lifetimes in both cases, owing to the singularity in the density of photonic states, the effect is much stronger when the dye molecules are inside the metamaterial, rather than on its surface.

Strong coupling of localized surface plasmons and ensembles of dye molecules

We have studied strong exciton-plasmon coupling in the films of silver nanoislands and layer-by-layer deposited films of gold nanoparticles coated by highly concentrated R6G dye. The experimental results agree with the analytical model.

Plasmon-mediated emission in the strong coupling regime

We demonstrate the strong coupling of SPPs and localized plasmons to dye molecules, characterized by splitting of the dispersion curve. We further observe an anomalous behavior in the emission of molecules strongly coupled to plasmons.

Thermal radiation of hyperbolic metamaterials and metallic surfaces

We have studied angular distribution and spectra of thermal radiation of lamellar metal/dielectric metamaterials with hyperbolic dispersion and have found them to be reasonably close to the corresponding properties of simple metallic films.

Control of chemical reactions in the vicinity of hyperbolic metamaterials and metallic surfaces

We show that photo-oxidation of organic semiconducting films can be controlled by geometry and composition of metallic and metal/dielectric substrates, in agreement with increase of the chemical reaction rate by the density of photonic states.