We spectroscopically investigated the energy gap of the correlated antiferromagnetic insulator LaMnPO1-xFx (x = 0.0 and 0.04) as a function of temperature and pressure, separately, in conjunction with many-body electronic structure calculations. These results show that the electronic structure in all measured regimes is well described by a model that includes both Mott-Hubbard interactions and Hund's rule coupling. Moreover, we find that by applying external pressure, thereby reducing the effective Mott-Hubbard interaction and Hund's coupling, the energy gap in LaMnPO1 xFx can be fully closed, yielding a metallic state.

We report the Drude oscillator strength D and the magnitude of the bulk band gap E_{g} of the epitaxially grown, topological insulator (Bi,Sb)_{2}Te_{3}. The magnitude of E_{g}, in conjunction with the model independent f-sum rule, allows us to establish an upper bound for the magnitude of D expected in a typical Dirac-like system composed of linear bands. The experimentally observed D is found to be at or below this theoretical upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, direct comparison of the measured D to magnetoresistance measurements of the same sample supports assignment of the observed low-energy conduction to topological surface states.

We have investigated the electronic structure and carrier dynamics of the topological insulator Bi2-xSbxTe3-ySey, for x = 0.5, y = 1.3 and x = 1, y = 2, using infrared spectroscopy. Our results show that both of these BSTS alloys are highly insulating in the bulk, with analysis of the infrared data indicating an upper limit to the carrier density of 4.4 x 10(17) cm(-3). Furthermore, analysis of the interband transitions of Bi1.5Sb0.5Te1.7Se1.3 revealed distinct band-structure critical points, which suggest high crystallographic order of our crystals. Motivated by the low carrier density and crystallographic order identified in these compounds, we searched for the Landau level transitions associated with the surface states through magneto-optical measurements in the far infrared range. We failed to observe any indications of the Landau level resonances at fields up to 8 T in sharp contrast with our earlier finding for a related Bi1-xSbx alloy. We discuss factors that may be responsible for suppressed magneto-optics response of these single crystals.

We characterized plasmon propagation in graphene on thin films of the high-κ dielectric PbZr0.3Ti0.7O3 (PZT). Significant modulation (up to ±75%) of the plasmon wavelength was achieved with application of ultrasmall voltages (< ±1 V) across PZT. Analysis of the observed plasmonic fringes at the graphene edge indicates that carriers in graphene on PZT behave as noninteracting Dirac Fermions approximated by a semiclassical Drude response, which may be attributed to strong dielectric screening at the graphene/PZT interface. Additionally, significant plasmon scattering occurs at the grain boundaries of PZT from topographic and/or polarization induced graphene conductivity variation in the interior of graphene, reducing the overall plasmon propagation length. Lastly, through application of 2 V across PZT, we demonstrate the capability to persistently modify the plasmonic response of graphene through transient voltage application.

We have investigated the electronic structure and carrier dynamics of the topological insulator $\mathrm{Bi}{}_{2\ensuremath{-}x}\mathrm{Sb}{}_{x}\mathrm{Te}{}_{3\ensuremath{-}y}\mathrm{Se}{}_{y}$, for $x=0.5,y=1.3$ and $x=1,y=2$, using infrared spectroscopy. Our results show that both of these BSTS alloys are highly insulating in the bulk, with analysis of the infrared data indicating an upper limit to the carrier density of $4.4\ifmmode\times\else\texttimes\fi{}{10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. Furthermore, analysis of the interband transitions of $\mathrm{Bi}{}_{1.5}\mathrm{Sb}{}_{0.5}\mathrm{Te}{}_{1.7}\mathrm{Se}{}_{1.3}$ revealed distinct band-structure critical points, which suggest high crystallographic order of our crystals. Motivated by the low carrier density and crystallographic order identified in these compounds, we searched for the Landau level transitions associated with the surface states through magneto-optical measurements in the far infrared range. We failed to observe any indications of the Landau level resonances at fields up to 8 T in sharp contrast with our earlier finding for a related $\mathrm{Bi}{}_{1\ensuremath{-}x}\mathrm{Sb}{}_{x}$ alloy. We discuss factors that may be responsible for suppressed magneto-optics response of these single crystals.

We report on infrared (IR) optical experiments on Bi2Te3 and Mn-doped Bi2Te3 epitaxial thin films. In the latter film, dilute Mn doping (4.5%) of the topologically nontrivial semiconductor host results in a time-reversal-symmetry-breaking ferromagnetic order below T-C = 15 K. Our spectroscopic study shows that both materials share the Bi2Te3 crystal structure, as well as classification as bulk degenerate semiconductors. Hence the Fermi energy is located in the Bi2Te3 conduction band in both materials, and furthermore, there is no need to invoke topological surface states to describe the conductivity spectra. We also demonstrate that the Drude oscillator strength gives a simple metric with which to distinguish the possibility of topological surface state origins of the low frequency conductance, and we conclude that in both the pristine and Mn-doped Bi2Te3 samples the electromagnetic response is indeed dominated by the bulk material properties, rather than those of the surface. An encouraging aspect for taking advantage of the interplay between nontrivial topology and magnetism, however, is that the temperature dependence of the Mn-doped Bi2Te3 film suggests bulk charge carriers do not play a significant role in mediating ferromagnetism. Thus, a truly insulating bulk may still be suitable for the formation of a ferromagnetic ground state in this dilute magnetic topological semiconductor.

We demonstrate an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface. Large spatial scale, low voltage, non-volatile switching of terahertz (THz) metasurface resonances is achieved through voltage application using an ionic gel to drive the insulator-to-metal transition in an underlying VO2 layer. Positive and negative voltage application can selectively tune the metasurface resonance into the “off” or “on” state by pushing the VO2 into a more conductive or insulating regime respectively. Compared to graphene based control devices, the relatively long saturation time of resonance modification in VO2 based devices suggests that this voltage-induced switching originates primarily from electrochemical effects related to oxygen migration across the electrolyte–VO2 interface.

We investigate near-field infrared spectroscopy and superfluid polariton imaging experiments on conventional and unconventional superconductors. Our modeling shows that near-field spectroscopy can measure the magnitude of the superconducting energy gap in Bardeen-Cooper-Schrieffer-like superconductors with nanoscale spatial resolution. We demonstrate how the same technique can measure the c-axis plasma frequency, and thus the c-axis superfluid density, of layered unconventional superconductors with a similar spatial resolution. Our modeling also shows that near-field techniques can image superfluid surface mode interference patterns near physical and electronic boundaries. We describe how these images can be used to extract the collective mode dispersion of anisotropic superconductors with subdiffractional spatial resolution.

This dissertation reports infrared (sub-THz through visible/UV) spectroscopy studies of two novel semiconductor systems: dilute (ferro)magnetic semiconductors (DMSs) and topological insulators (TIs). The first part addresses work on thin films of the canonical DMS, (Ga,Mn)As. In this system there is general consensus that the ferromagnetic mechanism is mediated by itinerant holes introduced by the Mn doping. However, the details of this exchange have been widely contested, and center on the character of the electronic states in the vicinity of the Fermi level. Through detailed infrared studies, we show the important role played by Mn impurity states in the transport dynamics and ferromagnetic interaction. In the second part, I focus on epitaxial films of the TI Bi₂Te₃, as well explore Mn or Sb doped Bi₂Te₃ films. In the former case, Mn doping leads to a ferromagnetic transition below Tc=15 K. Contrasting with the spectroscopic hallmarks of carrier mediated ferromagnetism in the canonical DMS (Ga,Mn)As, we show charge carriers do not play an important role in mediating ferromagnetism in this dilute magnetic topological semiconductor. While both pristine and Mn-doped Bi₂Te₃ are revealed to have significant concentrations of bulk charge carriers, we show that Sb doping in Bi₂Te₃ is highly effective in lowering the Fermi level with respect to the conduction band edge to reduce bulk carriers. Moreover, the Drude spectral weight in Sb doped Bi₂Te₃ is sufficiently small as to be consistent with the response of topological Dirac surface state (SS) charge carriers. This latter assertion is evidenced by establishing both the experimental bulk optical band gap, and theoretical sum rule constraints on the Drude oscillator strength of the SS response

We have investigated the electronic structure of Bi2Se3 epitaxial thin films with thicknesses between 15 and 99 quintuple layers (QL) on a Si substrate using a combination of variable angle spectroscopic ellipsometry and infrared transmission spectroscopy. The results we have obtained are consistent with a Fermi level that shifts relative to the conduction band as a function of sample thickness. We also present evidence that the bulk energy gap in these thin films is as much as 0.1 eV smaller than the value of 0.3 eV predicted by band structure calculations and confirmed by photoemission experiments. The thickness dependence of material properties in Bi2Se3 observed in this work reveals thickness to be a parameter that can be tuned to control and possibly optimize the bulk properties of Bi2Se3 thin films.

We report on the magnetic and the electronic properties of the prototype dilute magnetic semiconductor Ga1-xMnx As using infrared (IR) spectroscopy. Trends in the ferromagnetic transition temperature T-C with respect to the IR spectral weight are examined using a sum-rule analysis of IR conductivity spectra. We find nonmonotonic behavior of trends in T-C with the spectral weight to effective Mn ratio, which suggest a strong double-exchange component to the FM mechanism, and highlights the important role of impurity states and localization at the Fermi level. Spectroscopic features of the IR conductivity are tracked as they evolve with temperature, doping, annealing, As-antisite compensation, and are found only to be consistent with a Mn-induced IB scenario. Furthermore, our detailed exploration of these spectral features demonstrates that seemingly conflicting trends reported in the literature regarding a broad mid-IR resonance with respect to carrier density in Ga1-xMnx As are in fact not contradictory. Our study thus provides a consistent experimental picture of the magnetic and electronic properties of Ga1-xMnx As.

We have fabricated electric double-layer field-effect devices to electrostatically dope our active materials, either x = 0.015 Ga1-xMnxAs or x = 3.2 x 10(-4) Ga1-xBexAs. The devices are tailored for interrogation of electric field-induced changes to the frequency-dependent conductivity in the accumulation or depletion layers of the active material via infrared (IR) spectroscopy at room temperature. The spectra of the (Ga,Be)As-based device reveal electric field-induced changes to the IR conductivity consistent with an enhancement or reduction of the Drude response in the accumulation and depletion polarities, respectively. The spectroscopic features of this device are all indicative of metallic conduction within the GaAs host valence band (VB). For the (Ga,Mn)As-based device, the spectra show enhancement of the far-IR itinerant carrier response and broad mid-IR resonance upon hole accumulation, with a suppression of these features in the depletion polarity. These latter spectral features demonstrate that conduction in ferromagnetic (FM) Ga1-x MnxAs is distinct from genuine metallic behavior due to extended states in the host VB. Furthermore, these data support the notion that a Mn-induced impurity band plays a vital role in the electrodynamics of FM Ga1-x MnxAs. We add that a sum-rule analysis of the spectra of our devices suggests that the Mn or Be doping does not lead to a substantial renormalization of the GaAs host VB.

We have performed broad-band zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi0.91Sb0.09.The zero-field results allow us to measure the value of the direct band gap between the conducting La and valence Ls bands.Under applied field in the Faraday geometry (k || H || C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence ω ∝ √ H.We discuss the ramification of this observation for the surface and bulk properties of topological insulators.