Showing papers by "Richard D. Averitt published in 2014"
TL;DR: In this article, a review of recent developments in this emerging field is presented and examples include the cuprates, manganites, and vanadates, as well as other transition metal oxides.
Abstract: Advances in the synthesis, growth, and characterization of complex transition metal oxides coupled with new experimental techniques in ultrafast optical spectroscopy have ushered in an exciting era of dynamics and control in these materials. Experiments utilizing femtosecond optical pulses can initiate and probe dynamics of the spin, lattice, orbital, and charge degrees of freedom. Major goals include (a) determining how interaction and competition between the relevant degrees of freedom determine macroscopic functionality in transition metal oxides (TMOs) and (b) searching for hidden phases in TMOs by controlling dynamic trajectories in a complex and pliable energy landscape. Advances in creating intense pulses from the far-IR spectrum through the visible spectrum enable mode-selective excitation to facilitate exploration of these possibilities. This review covers recent developments in this emerging field and presents examples that include the cuprates, manganites, and vanadates.
158 citations
TL;DR: In this paper, photo-excited free carriers in silicon pads placed in the capacitive gaps of split ring resonators are designed and fabricated to operate in reflection, and large modulation depth (38% and 91%) in two absorption bands (with 97% and 92% peak absorption) is demonstrated.
Abstract: Development of tunable, dynamic, and broad bandwidth metamaterial designs is a keystone objective for metamaterials research, necessary for the future viability of metamaterial optics and devices across the electromagnetic spectrum. Yet, overcoming the inherently localized, narrow bandwidth, and static response of resonant metamaterials continues to be a challenging endeavor. Resonant perfect absorbers have flourished as one of the most promising metamaterial devices with applications ranging from power harvesting to terahertz imaging. Here, an optically modulated resonant perfect absorber is presented. Utilizing photo-excited free carriers in silicon pads placed in the capacitive gaps of split ring resonators, a dynamically modulated perfect absorber is designed and fabricated to operate in reflection. Large modulation depth (38% and 91%) in two absorption bands (with 97% and 92% peak absorption) is demonstrated, which correspond to the LC (0.7 THz) and dipole (1.1 THz) modes of the split ring resonators.
93 citations
TL;DR: Time-resolved mid-infrared (mid-IR) near-field spectroscopy of the narrow bandgap semiconductor InAs shows the potential to become an important building block of optically controlled plasmonic devices operating at infrared frequencies.
Abstract: We report on time-resolved mid-infrared (mid-IR) near- field spectroscopy of the narrow bandgap semiconductor InAs. The dominant effect we observed pertains to the dynamics of photoexcited carriers and associated surface plasmons. A novel combination of pump− probe techniques and near-field nanospectroscopy accesses high momentum plasmons and demonstrates efficient, subpicosecond photo- modulation of the surface plasmon dispersion with subsequent tens of picoseconds decay under ambient conditions. The photoinduced change of the probe intensity due to plasmons in InAs is found to exceed that of other mid-IR or near-IR media by 1−2 orders of magnitude. Remarkably, the required control pulse fluence is as low as 60 μJ/cm 2 , much smaller than fluences of ∼1−10 mJ/cm 2 previously utilized in ultrafast control of near-IR plasmonics. These low excitation densities are easily attained with a standard 1.56 μm fiber laser. Thus, InAsa common semiconductor with favorable plasmonic properties such as a low effective masshas the potential to become an important building block of optically controlled plasmonic devices operating at infrared frequencies.
90 citations
TL;DR: In this article, an electrolyte-based voltage tunable vanadium dioxide (VO2) memory metasurface was demonstrated for large spatial scale, low voltage, non-volatile switching of terahertz (THz) resonances.
Abstract: 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.
55 citations
TL;DR: In this article, the authors demonstrate that three different symmetries are spontaneously broken in the vicinity of the VO2 phase transition: (1) Monoclinic-tetragonal (rutile) crystal symmetry breaking due to the structural phase transition, (2) in-plane (x-y plane) rotational symmetry and (3) out-of-plane mirror symmetry breaking at the film cross-section due to substrate-induced epitaxial strain.
Abstract: Epitaxial strain can play an important role in controlling the local phase dynamics of transition metal oxides. With scattering-type scanning near-field optical microscopy, we visualize the three dimensional landscape of phase inhomogeneity in strained VO2 films grown on [100]R TiO2 substrates. We demonstrate that three different symmetries are spontaneously broken in the vicinity of the VO2 phase transition: (1) Monoclinic-tetragonal (rutile) crystal symmetry breaking due to the structural phase transition, (2) in-plane (x-y plane) rotational symmetry breaking due to the formation of periodic strain domains, and (3) out-of-plane (z-axis) mirror symmetry breaking at the film cross-section due to substrate-induced epitaxial strain.
38 citations
TL;DR: In this article, a split ring resonator (SRR) array stacked above an array of closed conducting rings is proposed to increase the strength of the MM oscillator by a factor of 4 and a 40% change in the amplitude of the resonant electric field enhancement in the SRR capacitive gap.
Abstract: The design of artificial nonlinear materials requires control over internal resonant charge densities and local electric field distributions. We present a MM design with a structurally controllable oscillator strength and local electric field enhancement at terahertz frequencies. The MM consists of a split ring resonator (SRR) array stacked above an array of closed conducting rings. An in-plane, lateral shift of a half unit cell between the SRR and closed ring arrays results in an increase of the MM oscillator strength by a factor of 4 and a 40% change in the amplitude of the resonant electric field enhancement in the SRR capacitive gap. We use terahertz time-domain spectroscopy and numerical simulations to confirm our results. We show that the observed electromagnetic response in this MM is the result of image charges and currents induced in the closed rings by the SRR.
21 citations
13 Mar 2014
TL;DR: In this article, the authors reported a highly flexible dynamic perfect absorber at terahertz (THz) frequencies, which was patterned on thin GaAs patches, which were fashioned in an array on a 5μm polyimide substrate via transfer printing technique.
Abstract: This paper reports our recent progress on a highly flexible dynamic perfect absorber at terahertz (THz) frequencies. Metamaterial unit cells were patterned on thin GaAs patches, which were fashioned in an array on a 5μm polyimide substrate via transfer printing technique, and the backside of the substrate was coated with gold film as ground plane. Optical-pump THz-probe reflection measurements show that the absorptivity at resonance frequency of 1.58THz can be tuned up to 57% through photo-excitation of free carriers in GaAs layers in presence of 800nm pump beam. Our flexible tunable MM perfect absorber exhibits potential applications in energy harvesting, imaging and stealth coating.
5 citations
07 Jul 2014
TL;DR: In this article, the authors demonstrate air breakdown in SiNx metamaterials with THz electric fields and highly nonlinear responses in 3D silicon metammaterials with 3D THz magnetic fields.
Abstract: We report THz electric and magnetic field-induced nonlinear responses in metamaterial structures. We demonstrate air breakdown in SiNx metamaterials with THz electric fields, and highly nonlinear responses in 3D silicon metamaterials with THz magnetic fields.
1 citations
01 Apr 2014
TL;DR: In this paper, the essential components for terahertz (THz) polarimetry were developed to enhance the ability to study previously unexploited spectral responses in the THz frequency range of molecules to allow for identification of chemical or biological threat analytes.
Abstract: : Polarimetry is the analysis of the polarization state of radiation following interaction with a sample. It has distinct advantages in comparison to techniques that solely measure changes in amplitude. There has been virtually no polarimetry work at terahertz (THz) frequencies because, until recently, it has been difficult to create components to control the polarization and the amplitude of THz radiation at modulation rates sufficient for potential high-sensitive applications. Our project attempted to develop the essential components such that THz polarimetry may enhance the ability to study previously unexploited spectral responses in the THz frequency range of molecules to allow for identification of chemical or biological threat analytes.