Andreas Pusch

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.

Intermediate band solar cells: Recent progress and future directions

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

Coherent amplification and noise in gain-enhanced nanoplasmonic metamaterials: a Maxwell-Bloch Langevin approach.

Nanoplasmonic metamaterials are an exciting new class of engineered media that promise a range of important applications, such as subwavelength focusing, cloaking, and slowing/stopping of light. At optical frequencies, using gain to overcome potentially not insignificant losses has recently emerged as a viable solution to ultra-low-loss operation that may lead to next-generation active metamaterials. Maxwell-Bloch models for active nanoplasmonic metamaterials are able to describe the coherent spatiotemporal and nonlinear gain-plasmon dynamics. Here, we extend the Maxwell-Bloch theory to a Maxwell-Bloch Langevin approach-a spatially resolved model that describes the light field and noise dynamics in gain-enhanced nanoplasmonic structures. Using the example of an optically pumped nanofishnet metamaterial with an embedded laser dye (four-level) medium exhibiting a negative refractive index, we demonstrate the transition from loss-compensation to amplification and to nanolasing. We observe ultrafast relaxation oscillations of the bright negative-index mode with frequencies just below the THz regime. The influence of noise on mode competition and the onset and magnitude of the relaxation oscillations is elucidated, and the dynamics and spectra of the emitted light indicate that coherent amplification and lasing are maintained even in the presence of noise and amplified spontaneous emission.

Gain and plasmon dynamics in active negative-index metamaterials

Photonic metamaterials allow for a range of exciting applications unattainable with ordinary dielectrics. However, the metallic nature of their meta-atoms may result in increased optical losses. Gain-enhanced metamaterials are a potential solution to this problem, but the conception of realistic, three-dimensional designs is a challenging task. Starting from fundamental electrodynamic and quantum mechanical equations, we establish and deploy a rigorous theoretical model for the spatial and temporal interaction of lightwaves with free and bound electrons inside and around metallic (nano-) structures and gain media. The derived numerical framework allows us to self-consistently study the dynamics and impact of the coherent plasmon–gain interaction, nonlinear saturation, field enhancement, radiative damping and spatial dispersion. Using numerical pump–probe experiments on a double-fishnet metamaterial structure with dye molecule inclusions, we investigate the build-up of the inversion profile and the formation of the plasmonic modes in a low-Q cavity. We find that full loss compensation occurs in a regime where the real part of the effective refractive index of the metamaterial becomes more negative compared to the passive case. Our results provide a deep insight into how internal processes affect the overall optical properties of active photonic metamaterials fostering new approaches to the design of practical, loss-compensated plasmonic nanostructures.

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 nanofishnet structure. We report that lasing of the bright negative-index mode is possible if the higher-

A highly efficient CMOS nanoplasmonic crystal enhanced slow-wave thermal emitter improves infrared gas-sensing devices

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Self-focusing of femtosecond surface plasmon polaritons

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.

Quantum Cascade Photon Ratchets for Intermediate-Band Solar Cells

The application of plasmonics to thermal emitters is generally assisted by absorptive losses in the metal because Kirchhoff’s law prescribes that only good absorbers make good thermal emitters. Based on a designed plasmonic crystal and exploiting a slow-wave lattice resonance and spontaneous thermal plasmon emission, we engineer a tungsten-based thermal emitter, fabricated in an industrial CMOS process, and demonstrate its markedly improved practical use in a prototype non-dispersive infrared (NDIR) gas-sensing device. We show that the emission intensity of the thermal emitter at the CO2 absorption wavelength is enhanced almost 4-fold compared to a standard non-plasmonic emitter, which enables a proportionate increase in the signal-to-noise ratio of the CO2 gas sensor.

Control of terahertz nonlinear transmission with electrically gated graphene metadevices

We study the propagation of femtosecond pulses in nonlinear metal-dielectric plasmonic waveguiding structures by employing the finite-difference time-domain numerical method. Self-focusing of plasmon pulses is observed for defocusing Kerr-like nonlinearity of the dielectric medium due to normal dispersion. We compare the nonlinear propagation of plasmon pulses along a single metal-dielectric interface with the propagation within a metal-dielectric-metal slot waveguide and observe that nonlinear effects are more pronounced for the single surface where longer propagation length may compensate for lower field confinement.

Dynamical calculation of third harmonic generation in a semiconductor quantum well

We propose an antimonide-based quantum cascade design to demonstrate the ratchet mechanism for incorporation into the recently suggested photon ratchet intermediate-band solar cell. We realize the photon ratchet as a semiconductor heterostructure in which electrons are optically excited into an intermediate band and spatially decoupled from the valence band through a type-II quantum cascade. This process reduces both radiative and nonradiative recombination and can thereby increase the solar cell efficiency over intermediate-band solar cells. Our design method uses an adaptive simulated annealing genetic algorithm to determine the optimum thicknesses of semiconductor layers in the quantum cascade, allowing efficient transport (via phonon emission) of the electrons away from the interband active region.