Overcoming losses with gain in a negative refractive index metamaterial

On the basis of a full-vectorial three-dimensional Maxwell-Bloch approach we investigate the possibility of using gain to overcome losses in a negative refractive index fishnet metamaterial. We show that appropriate placing of optically pumped laser dyes (gain) into the metamaterial structure results in a frequency band where the nonbianisotropic metamaterial becomes amplifying. In that region both the real and the imaginary part of the effective refractive index become simultaneously negative and the figure of merit diverges at two distinct frequency points.Comment: 4 pages, 4 figure

Control and Dynamic Competition of Bright and Dark Lasing States in Active Nanoplasmonic Metamaterials

Active nanoplasmonic metamaterials support bright and dark modes that compete for gain. Using a Maxwell-Bloch approach incorporating Langevin noise we study the lasing dynamics in an active nano-fishnet structure. We report that lasing of the bright negative-index mode is possible if the higher-Q dark mode is discriminated by gain, spatially or spectrally. The nonlinear competition during the transient phase is followed by steady-state emission where bright and dark modes can coexist. We analyze the influence of pump intensity and polarization and explore methods for mode control.Comment: 5 pages, 4 figure

Gain and Plasmon Dynamics in Active Nanoplasmonic Metamaterials

Optical metamaterials are composite media that can be made to exhibit striking optical properties, some of which are not observed in nature, such as a negative refractive index. This advanced control over the electromagnetic response is enabled by subwavelength building blocks, most often based on metals. While metallic structural features provide the necessary resonant interaction with light, they also give rise to dissipative losses, which can interfere with the desired performance in applications. The incorporation of optical gain has emerged as a promising way to improve the loss-encumbered operation. It is this enhancement of metamaterials by gain, which is at the heart of this thesis. Three relevant topics will be considered: loss compensation, coherent amplification and lasing dynamics. The numerical studies presented here focus on the double-fishnet structure, a metamaterial exhibiting a negative refractive index at optical wavelengths. First, it is shown that loss compensation via optical gain is possible and that, in addition, it constitutes a practical means to overcome dissipative losses. Compensation of losses in combination with a negative refractive index is observed, disproving theoretical claims that rule out such behaviour. As a natural continuation, the characteristics above the threshold of amplification are investigated, i.e., when dissipative losses are overcompensated. By defining and analysing an effective rate balance, radiative outcoupling is found to be non-negligible. Hence, contrary to quasistatic predictions for nanoplasmonic metamaterials, a window of amplification opens. Beyond the regime of amplification, when gain exceeds both dissipative losses and radiative outcoupling, lasing instabilities occur. Nonlinear mode dynamics arise and it is shown that sole bright emission can be achieved despite the strong competition from a dark plasmonic mode. The numerical studies performed here shed new light on the complex physics arising from the nonlinear dynamic interaction of optical gain and resonant modes in nanoplasmonic metamaterials

Gain and plasmon dynamics in active nanoplasmonic metamaterials

Optical metamaterials are composite media that can be made to exhibit striking optical properties, some of which are not observed in nature, such as a negative refractive index. This advanced control over the electromagnetic response is enabled by subwavelength building blocks, most often based on metals. While metallic structural features provide the necessary resonant interaction with light, they also give rise to dissipative losses, which can interfere with the desired performance in applications. The incorporation of optical gain has emerged as a promising way to improve the loss-encumbered operation. It is this enhancement of metamaterials by gain, which is at the heart of this thesis. Three relevant topics will be considered: loss compensation, coherent amplification and lasing dynamics. The numerical studies presented here focus on the double-fishnet structure, a metamaterial exhibiting a negative refractive index at optical wavelengths. First, it is shown that loss compensation via optical gain is possible and that, in addition, it constitutes a practical means to overcome dissipative losses. Compensation of losses in combination with a negative refractive index is observed, disproving theoretical claims that rule out such behaviour. As a natural continuation, the characteristics above the threshold of amplification are investigated, i.e., when dissipative losses are overcompensated. By defining and analysing an effective rate balance, radiative outcoupling is found to be non-negligible. Hence, contrary to quasistatic predictions for nanoplasmonic metamaterials, a window of amplification opens. Beyond the regime of amplification, when gain exceeds both dissipative losses and radiative outcoupling, lasing instabilities occur. Nonlinear mode dynamics arise and it is shown that sole bright emission can be achieved despite the strong competition from a dark plasmonic mode. The numerical studies performed here shed new light on the complex physics arising from the nonlinear dynamic interaction of optical gain and resonant modes in nanoplasmonic metamaterials.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Biochar as an Electron Shuttle between Bacteria and Fe(III) Minerals

Biochar influences soil fertility, N₂O emissions, and atmospheric CO₂ budgets, and because of its quinone and aromatic structures, it is redox-active. Here we demonstrate that biochar concentrations of 5 and 10 g L^–1 stimulate both the rate and the extent of microbial reduction of the Fe­(III) oxyhydroxide mineral ferrihydrite (15 mM) by <i>Shewanella oneidensis</i> MR-1, while lower biochar concentrations (0.5 and 1 g L^–1) have a negative effect on ferrihydrite reduction. Control experiments showed that biochar particles and not biochar-derived water-soluble organic compounds are responsible for the stimulating and inhibiting effect. We also found that biochar changed the mineral product of ferrihydrite reduction from magnetite (Fe₃O₄) to siderite (FeCO₃). Our study suggests that biochar can influence soil biogeochemistry not only indirectly by changing the soil structure and chemistry but also by directly mediating electron transfer processes, i.e., by functioning as an electron shuttle