Enhancement and tunability of near-field radiative heat transfer mediated by surface plasmon polaritons in thin plasmonic films

The properties of thermal radiation exchange between hot and cold objects can be strongly modified if they interact in the near field where electromagnetic coupling occurs across gaps narrower than the dominant wavelength of thermal radiation. Using a rigorous fluctuational electrodynamics approach, we predict that ultra-thin films of plasmonic materials can be used to dramatically enhance near-field heat transfer. The total spectrally integrated film-to-film heat transfer is over an order of magnitude larger than between the same materials in bulk form and also exceeds the levels achievable with polar dielectrics such as SiC. We attribute this enhancement to the significant spectral broadening of radiative heat transfer due to coupling between surface plasmon polaritons (SPPs) on both sides of each thin film. We show that the radiative heat flux spectrum can be further shaped by the choice of the substrate onto which the thin film is deposited. In particular, substrates supporting surface phonon polaritons (SPhP) strongly modify the heat flux spectrum owing to the interactions between SPPs on thin films and SPhPs of the substrate. The use of thin film phase change materials on polar dielectric substrates allows for dynamic switching of the heat flux spectrum between SPP-mediated and SPhP-mediated peaks.Comment: 25 pages, 11 figure

Six-converter solar thermionic generator Final report, 10 Jan. 1967 - 31 Mar. 1968

Six converter solar thermionic generato

Current Saturation in Field Emission from H-Passivated Si Nanowires

International audienceThis paper explores the field emission (FE) properties of highly crystalline Si nanowires (NWs) with controlled surface passivation. The NWs were batch-grown by the vapor_liquid_solid process using Au catalysts with no intentional doping. The FE current_voltage characteristics showed quasi-ideal current saturation that resembles those predicted by the basic theory for emission from semiconductors, even at room temperature. In the saturation region, the currents were extremely sensitive to temperature and also increased linearly with voltage drop along the nanowire. The latter permits the estimation of the doping concentration and the carrier lifetime, which is limited by surface recombination. The conductivity could be tuned over 2 orders of magnitude by in situ hydrogen passivation/desorption cycles. This work highlights the role of dangling bonds in surface leakage currents and demonstrates the use of hydrogen passivation for optimizing the FE characteristics of Si NWs

Nanoantennas for visible and infrared radiation

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-ofstates engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of optical antennas based on the background of both well-developed radiowave antenna engineering and the emerging field of plasmonics. In particular, we address the plasmonic behavior that emerges due to the very high optical frequencies involved and the limitations in the choice of antenna materials and geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.Comment: Review article with 76 pages, 21 figure

Interfacing single photons and single quantum dots with photonic nanostructures

Photonic nanostructures provide means of tailoring the interaction between light and matter and the past decade has witnessed a tremendous experimental and theoretical progress in this subject. In particular, the combination with semiconductor quantum dots has proven successful. This manuscript reviews quantum optics with excitons in single quantum dots embedded in photonic nanostructures. The ability to engineer the light-matter interaction strength in integrated photonic nanostructures enables a range of fundamental quantum-electrodynamics experiments on, e.g., spontaneous-emission control, modified Lamb shifts, and enhanced dipole-dipole interaction. Furthermore, highly efficient single-photon sources and giant photon nonlinearities may be implemented with immediate applications for photonic quantum-information processing. The review summarizes the general theoretical framework of photon emission including the role of dephasing processes, and applies it to photonic nanostructures of current interest, such as photonic-crystal cavities and waveguides, dielectric nanowires, and plasmonic waveguides. The introduced concepts are generally applicable in quantum nanophotonics and apply to a large extent also to other quantum emitters, such as molecules, nitrogen vacancy ceters, or atoms. Finally, the progress and future prospects of applications in quantum-information processing are considered.Comment: Updated version resubmitted to Reviews of Modern Physic

Electric Space Propulsion Concepts Using Calcium Aluminate Electride Hollow Cathodes

This dissertation investigates the possibility of using compact and heaterless calcium aluminate electride hollow cathodes in different electric propulsion systems for space applications. As conventional hollow cathodes generally require a heater to reach the high operating temperatures necessary to thermally emit electrons, research on low temperature heaterless hollow cathodes as electron sources has been increasing. Efforts at Technische Universität Dresden have resulted in an operational hollow cathode design that can be reliably used for low current plasma discharges. Hollow cathodes are crucial components in electric propulsion systems to ionize the propellant and neutralize the extracted ion beam. The successful development of an operational hollow cathode opens the possibility of using the design in different low-power electric propulsion systems. As the electron emission properties of C12A7:e- are still not well understood, a volume-averaged hollow cathode model has been developed as part of this thesis to obtain an improved insight into the plasma processes governing the cathode discharge. The model consists of two computational domains in which the plasma properties are volume-averaged. A lumped-node thermal model coupled with the plasma model provides the cathode temperature distribution for different operating points. The model moreover provides the discharge voltage which can be directly compared to experimental data. The thermal model was compared to thermal measurements to derive adequate values for free model parameters. The discharge voltage fits well for a 1 A discharge but diverges from measurement data at higher currents. The model is a starting point for further modeling efforts and needs to be verified using extensive plasma diagnostics. The first electric propulsion system developed as part of this thesis is an electrothermal device that takes advantage of high particle temperatures in a hollow cathode discharge. A performance model and preliminary test series were used to derive design parameters for a prototype that was used for an extensive parameter study. The thruster reliably generates thrust over a current range between 1 A – 3 A. The thrust achieved with this device is in the high micronewton to low millinewton range. The specific impulse is on the order of 100 s, which is low for electric propulsion systems, and the high discharge voltages of approximately 50 V limit the achievable efficiency to <1%. The second thruster concept is a DC discharge gridded ion thruster using a C12A7:e- hollow cathode as the discharge cathode and the neutral gas inlet. An analytical discharge model combined with a particle-in-cell simulation for ion extraction by electrostatically biased grids was used to design a modular testing prototype. The concept requires a low discharge current on the order of 200 mA. Operating the cathodes in a milliamp discharge current range proved to be difficult and was accompanied by high discharge voltages. Extracting an ion beam from the testing prototype was not successful. The third propulsion system is a magnetoplasmadynamic thruster (MPDT) that takes advantage of a strong magnetic field generated by permanent magnets and an orthogonal current in a plasma discharge using a C12A7:e- hollow cathode. Conventional MPDTs require high current discharges to generate a sufficiently strong self-induced magnetic field. The developed concept is a design alternative to expand the operational envelope to lower powers. A major advantage is the comparatively easy scalability of the device. One prototype for the low amp current range was developed and successfully operated. The generated thrust is in the low millinewton range with a specific impulse up to 1,200 s. The test series highlighted thermal problems with the design. Consequently, a sub-amp version of the concept was developed. The thruster was successfully operated but required high mass flow rates, lowering the specific impulse and efficiency

Polaron Self-localization in White-light Emitting Hybrid Perovskites

Two-dimensional (2D) perovskites with general formula $APbX_4$ are attracting increasing interest as solution processable, white-light emissive materials. Recent studies have shown that their broadband emission is related to the formation of intra-gap color centers; however, the nature and dynamics of the emissive species have remained elusive. Here we show that the broadband photoluminescence of the 2D perovskites $(EDBE)PbCl_4$ and $(EDBE)PbBr_4$ stems from the localization of small polarons within the lattice distortion field. Using a combination of spectroscopic techniques and first-principles calculations, we infer the formation of ${Pb_2}^{3+}$, $Pb^{3+}$, and ${X_2}^-$ (where X=Cl or Br) species confined within the inorganic perovskite framework. Due to strong Coulombic interactions, these species retain their original excitonic character and form self-trapped polaron-excitons acting as radiative color centers. These findings are expected to be applicable to a broad class of white-light emitting perovskites with large polaron relaxation energy.Comment: 34 pages, 15 figures, 3 table