Mid-infrared Plasmonics

This dissertation reports investigations into materials for, and applications of, infrared surface plasmon polaritons (SPP). SPPs are inhomogeneous electromagnetic waves that are bound to the surface of a conductor. Tight confinement of electromagnetic energy, the primary virtue of SPPs for so-called plasmonic applications, requires plasma frequencies for the conductor near the intended infrared operational frequencies. This requires carrier concentrations that are much less than those of usual metals such as gold and silver. I have investigated the optical properties and SPP excitation resonances of two materials having infrared plasma frequencies, namely the semimetal bismuth and the transparent conducting fluorine-doped tin-oxide (FTO). The complex permittivity spectra for evaporated films of Bi were found to be distinctly different than earlier reports for crystal or polycrystalline films, and SPP excitation resonances on Bi-coated gratings were found to be disappointingly broad. Permittivity spectra for chemical spray deposited FTO were obtained to long-wave IR wavelengths for the first time, and nano-crystalline FTO-coated silicon lamellar gratings show remarkable conformity. SPP excitation resonances for FTO are more promising than for Bi. Thus, FTO appears to be a promising SPP host for infrared plasmonics, e.g. a planer waveguide plasmonic spectral sensor, whose design was elaborated and investigated as part of my research and which requires SPP-host coating on deep vertical side walls of a trench-like analyte interaction region. Additionally, FTO may serve as a useful conducting oxide for a near-IR plasmonic spectral imager that I have investigated theoretically

Infrared surface polaritons on bismuth

Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40 mu m. The bulk plasma frequency omega(p) and high-frequency limit to the permittivity epsilon(infinity) were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time tau and effective mass m*. This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity

Infrared Emission Spectroscopy Of Hot Carbon Monoxide

Gas giant exoplanets known as hot Jupiters orbit close to their parent stars and are heated to high temperatures. Their infrared spectra, measured by photometry during secondary eclipses, are dominated by carbon monoxide and methane, the principle reservoirs of carbon on these planets. The relative CO and CH4 abundances inform us about temperature and pressure conditions and also about mixing by global winds driven by intense but asymmetric heating for these tidally locked bodies. Emission spectra collected during secondary eclipses, as the hot Jupiter passes behind its parent star, in principle allows a determination of the CO:CH4 concentration ratio. Since hot Jupiters exist at temperatures of order 700 K, accurate model atmospheres require high temperature line lists for relevant molecules, for which existing data bases are apparently incomplete. Since the outer atmospheres of hot Jupiters are bombarded by intense ultraviolet radiation and energetic particles, there may even be a significant degree of ionization and nonequilibrium populations among the various molecular levels. Here we present high temperature emission spectra of CO obtained from a microwave discharge plasma, where the source of CO was carbon dioxide that dissociates under microwave heating. The spectrum was measured in the range 1800-2400 cm-1 at a resolution of 0.1 cm-1 . Vibrational transitions originating in up to the 13th vibrational level of the X 1 + ground electronic term were observed. From the J values for maximum intensity lines within the rotational fine structure, we obtain a temperature estimate of ~700 K, which is comparable to the atmospheric conditions of hot-Jupiters

Far-Infrared Absorber Based On Standing-Wave Resonances In Metal-Dielectric-Metal Cavity

Thin-film resonant absorbers for the far-IR spectral range were fabricated, characterized, and modeled. The 3-μm-thick structure comprises a periodic surface array of metal squares, a dielectric spacer and a metallic ground plane. Up to 95% absorption for the fundamental band at ∼53.5μm wavelength (5.6 THz) is achieved experimentally. Absorption bands are independent of the structure period and only weakly dependent on polarization and incident angle. The results are well explained in terms of standing-wave resonances within individual metal-dielectric-metal cavities. The structure has application as a wavelength selective coating for far-IR bolometers

Robust Multimaterial Tellurium-Based Chalcogenide Glass Fibers For Mid-Wave And Long-Wave Infrared Transmission

We describe an approach for producing robust multimaterial chalcogenide glass fibers for mid-wave and long-wave mid-infrared transmission. By combining the traditional rod-in-tube process with multimaterial coextrusion, we prepare a hybrid glass-polymer preform that is drawn continuously into a robust step-index fiber with a builtin, thermally compatible polymer jacket. Using tellurium-based chalcogenides, the fibers have a transparency window covering the 3-12 μm spectral range, making them particularly attractive for delivering quantum cascade laser light and in space applications. © 2014 Optical Society of America

Ropy Foam-Like Tio2 Film Grown By Water-Based Process For Electronconduction Layer Of Perovskite Solar Cells

Self-assembled TiO2 foam-like films, were grown by the water based Streaming Process for Electrodeless Electrochemical Deposition (SPEED). The morphology of the ∼1 μm thick films consists of a tangled ropy structure with individual strands of ∼200 nm diameter and open pores of 0.1 to 3 micron dimensions. Such films are advantageous for proposed perovskite solar cell comprising CH3NH3PbI3 absorber with additional inorganic films as contact and conduction layers, all deposited by SPEED. Lateral film resistivity is in the range 20 - 200 kω-cm, increasing with growth temperature, while sheet resistance is in the range 2 - 20 × 108 ω/Sq. Xray diffraction confirms presence of TiO2 crystals of orthorhombic class (Brookite). UV-vis spectroscopy shows high transmission below the expected 3.2 eV TiO2 bandgap. Transmittance increases with growth temperature

Conformal Spray-Deposited Fluorine-Doped Tin Oxide For Mid- And Long-Wave Infrared Plasmonics

Nanocrystalline spray-deposited fluorine-doped tin oxide (FTO) was investigated for mid- and long-wave infrared plasmonics. Silicon lamellar gratings were conformally coated with FTO, and the excitation of surface plasmon polaritons (SPP) was investigated via their angle and wavelength-dependent reflectivity. Photon-to-SPP coupling efficiency as a function of grating parameters, and in comparsion to gallium-doped zinc oxide (GZO) gratings, was quantitatively analyzed based on a figure of merit related to the sharpness and depth of the coupling resonance. Conformal spray-deposited FTO would be useful in mid- and long-wave infrared plasmonic channel wave guides

Robust Multimaterial Tellurium-Based Chalcogenide Glass Infrared Fibers

We demonstrate the scalable production of the first robust tellurium-based chalcogenide glass fibers provided with a built-in polymer jacket and transmit mid-wave and long-wave infrared light across the 3-12 micron window. © 2014 OSA

Infra-Red Spectral Microscopy Of Standing-Wave Resonances In Single Metal-Dielectric-Metal Thin-Film Cavity

Resonantly absorbing thin films comprising periodically sub-wavelength structured metal surface, dielectric spacer, and metal ground plane are a topic of current interest with important applications. These structures are frequently described as â€metamaterialsâ€, where effective permittivity and permeability with dispersion near electric and magnetic resonances allow impedance matching to free space for maximum absorption. In this paper, we compare synchrotron-based infrared spectral microscopy of a single isolated unit cell and a periodic array, and we show that the resonances have little to do with periodicity. Instead, the observed absorption spectra of usual periodically structured thin films are best described as due to standing-wave resonances within each independent unit cell, rather than as due to effective optical constants of a metamaterial. The effect of having arrays of unit cells is mainly to strengthen the absorption by increasing the fill factor, and such arrays need not be periodic. Initial work toward applying the subject absorbers to room-Temperature bolometer arrays is presented

Robust Multimaterial Tellurium-Based Chalcogenide Glass Infrared Fibers

We demonstrate the scalable production of the first robust tellurium-based chalcogenide glass fibers provided with a built-in polymer jacket and transmit mid-wave and long-wave infrared light across the 3 - 12 micron window