Showing papers by "Richard D. Averitt published in 2007"

Active terahertz metamaterial devices

Abstract: Metamaterial structures are taught which provide for the modulation of terahertz frequency signals. Each element within an array of metamaterial (MM) elements comprises multiple loops and at least one gap. The MM elements may comprise resonators with conductive loops and insulated gaps, or the inverse in which insulated loops are present with conductive gaps; each providing useful transmissive control properties. The metamaterial elements are fabricated on a semiconducting substrate configured with a means of enhancing or depleting electrons from near the gaps of the MM elements. An on to off transmissivity ratio of about 0.5 is achieved with this approach. Embodiments are described in which the MM elements incorporated within a Quantum Cascade Laser (QCL) to provide surface emitting (SE) properties.

Electrically resonant terahertz metamaterials: Theoretical and experimental investigations

Abstract: We present a class of artificial materials that exhibit a tailored response to the electrical component of electromagnetic radiation. These electric metamaterials are investigated theoretically, computationally, and experimentally using terahertz time-domain spectroscopy. These structures display a resonant response including regions of negative permittivity ${ϵ}_{1}(\ensuremath{\omega})l0$ ranging from $\ensuremath{\sim}500\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$. Conventional electric media such as distributed wires are difficult to incorporate into metamaterials. In contrast, these localized structures will simplify the construction of future metamaterials, including those with negative index of refraction. As these structures generalize to three dimensions in a straightforward manner, they will significantly enhance the design and fabrication of functional terahertz devices.

Complementary planar terahertz metamaterials

Abstract: Planar electric split ring resonator (eSRR) metamaterials and their corresponding inverse structures are designed and characterized computationally and experimentally utilizing finite element modeling and THz time domain spectroscopy. A complementary response is observed in transmission. Specifically, for the eSRRs a decrease in transmission is observed at resonance whereas the inverse structures display an increase in transmission. The frequency dependent effective complex dielectric functions are extracted from the experimental data and, in combination with simulations to determine the surface current density and local electric field,provide considerable insight into the electromagnetic response of our planar metamaterials. These structures may find applications in the construction of various THz filters, transparent THz windows, or THz grid structures ideal for constructing THz switching/modulation devices.

Ultrafast optical switching of terahertz metamaterials fabricated on ErAs/GaAs nanoisland superlattices

Abstract: We demonstrate optical switching of electrically resonant terahertz planar metamaterials fabricated on ErAs/GaAs nanoisland superlattice substrates. Photoexcited charge carriers in the superlattice shunt the capacitive regions of the constituent elements, thereby modulating the resonant response of the metamaterials. A switching recovery time of 20 ps results from fast carrier recombination in the ErAs/GaAs superlattice substrates.

Enhanced Photosusceptibility near TC for the Light-Induced Insulator-to-Metal Phase Transition in Vanadium Dioxide

Abstract: We use optical-pump terahertz-probe spectroscopy to investigate the near-threshold behavior of the photoinduced insulator-to-metal (IM) transition in vanadium dioxide thin films. Upon approaching ${T}_{c}$ a reduction in the fluence required to drive the IM transition is observed, consistent with a softening of the insulating state due to an increasing metallic volume fraction (below the percolation limit). This phase coexistence facilitates the growth of a homogeneous metallic conducting phase following superheating via photoexcitation. A simple dynamic model using Bruggeman effective medium theory describes the observed initial condition sensitivity.

Dynamical frequency tuning of electric and magnetic metamaterial response

Abstract: A geometrically modifiable resonator is comprised of a resonator disposed on a substrate, and a means for geometrically modifying the resonator. The geometrically modifiable resonator can achieve active optical and/or electronic control of the frequency response in metamaterials and/or frequency selective surfaces, potentially with sub-picosecond response times. Additionally, the methods taught here can be applied to discrete geometrically modifiable circuit components such as inductors and capacitors. Principally, controlled conductivity regions, using either reversible photodoping or voltage induced depletion activation, are used to modify the geometries of circuit components, thus allowing frequency tuning of resonators without otherwise affecting the bulk substrate electrical properties. The concept is valid over any frequency range in which metamaterials are designed to operate.

Observation of Competing Order in a High-T c Superconductor Using Femtosecond Optical Pulses

Abstract: We present studies of the photoexcited quasiparticle dynamics in ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (Tl-2223) using femtosecond optical techniques. Deep into the superconducting state (below 40 K), a dramatic change occurs in the temporal dynamics associated with photoexcited quasiparticles rejoining the condensate. This is suggestive of entry into a coexistence phase which, as our analysis reveals, opens a gap in the density of states (in addition to the superconducting gap), and furthermore, competes with superconductivity resulting in a depression of the superconducting gap.

Properties of planar electric metamaterials for novel terahertz applications

Abstract: Planar electric metamaterials are experimentally studied in transmission and reflection utilizing terahertz time-domain spectroscopy. Electrically resonant behavior is observed and provides an estimate of the frequency-dependent transmissivity, reflectivity, and absorptivity of metamaterial composites. Numerical simulations are in good agreement with the measured results and provide additional information helpful in understanding their electromagnetic response. Our results and approach help define the boundaries of a metamaterials-based design paradigm and should prove beneficial in future terahertz applications, particularly with respect to novel filtering, modulation, and switching devices. In addition, this work clarifies some of the mechanisms that limit efficient metamaterials operation at higher-frequencies.

Dynamical/Tunable Electromagnetic Materials and Devices

Abstract: A composite material that is responsive to either electromagnetic or thermal radiation. The composite has a controllable structure that is either dynamically or tunably responsive to such radiation and comprises a metamaterial. Sensors, such as a bolometer, that incorporate the composite are also described.

Phase inhomogeneities in the charge-orbital-ordered manganite Nd0.5Sr0.5MnO3 revealed through polaron dynamics

Abstract: Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperature-dependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.

Terahertz Metamaterial Devices

Abstract: Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or photonic properties at terahertz frequencies. This results in formidable challenges for creating the components needed for generating, detecting, and manipulating THz waves. Considering the promising applications of THz radiation, it is important overcome such material limitations by searching for new materials, or by constructing artificial materials with a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties that derive from their sub-wavelength structure. The potential of metamaterials for THz radiation originates from a resonant electromagnetic response which can be tailored for specific applications. Metamaterials thus offer a route towards helping to fill the so-called "THz gap". In this work we discuss novel planar THz metamaterials. Importantly, the dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials. We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted in the creation of efficient THz switches and modulators of potential importance for advancing numerous real world THz applications.

Split-Ring Resonator Enhanced Terahertz Antenna

Abstract: An electric split-ring resonator is used to enhance the performance of a photoconductive dipolar antenna based terahertz transmitter. Enhancement is observed near and limited to the resonant frequency of the electric split-ring resonator.

Active metamaterials: A novel approach to manipulate terahertz waves

Abstract: Compared to the neighboring infrared and microwave regimes, the terahertz (1 THz = 1012 Hz) regime is still in need of fundamental technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or photonic properties at terahertz frequencies. This results in formidable challenges in the generation, detection, and creation of devices to efficiently control and manipulate THz waves. Considering the promising potential applications of THz radiation, we need to overcome such material obstacles by actively searching for new materials, or by constructing artificial materials with a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties that derive from their sub-wavelength structure. The potential of metamaterials for THz applications originates from their resonant electromagnetic response, which significantly enhances their interaction with THz radiation. Thus, metamaterials offer a route towards helping to fill the so-called "THz gap". In this work we design a series of novel planar THz metamaterials. Importantly, the critical dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials. We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted in the invention of efficient THz switches and modulators which will be of importance for advancing numerous real world THz applications.

Properties of Novel Terahertz Electric Metamaterials

Abstract: Planar electric metamaterials are studied with terahertz time-domain spectroscopy in transmission and reflection Energy absorption of 5-20% due to Ohmic losses within the metal patterning is observed at resonant frequencies Finite-element simulations verify experimental results

Metamaterials for Novel Terahertz and Millimeter Wave Devices

Abstract: We present experimental results of metamaterials operating at terahertz and mm-wave frequencies. Metamaterials consist of a single layer of 200 nm thick gold on a doped or undoped semiconducting substrate. By optical and electronic doping of supporting semiconducting substrates we show external control of planar arrays of metamaterials, characterized with terahertz time domain spectroscopy. Both methods yield meta-material / semiconductor devices which can be utilized as switches or modulators, enabling modulation of THz transmission by 50 percent. Experiments are supported by simulations and results agree well. Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly play a key role in future demonstrations of novel high-performance devices.

Metamaterials and their THz applications

Abstract: We present experimental results of metamaterials operating at terahertz and mm-wave frequencies Metamaterials consist of a single layer of 200 nm thick gold on a doped or undoped semiconducting substrate By optical and electronic doping of supporting semiconducting substrates we show external control of planar arrays of metamaterials, characterized with terahertz time domain spectroscopy Both methods yield meta-material / semiconductor devices which can be utilized as switches or modulators, enabling modulation of THz transmission by 50 percent Experiments are supported by simulations and results agree well Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly play a key role in future demonstrations of novel high-performance devices

Ultrafast Observation of the Coexistence of Antiferromagnetism and Superconductivity in a High-T c Superconductor

Abstract: Ultrafast quasiparticle dynamics of the high-Tcsuperconductor Tl2Ba2Ca2Cu3Oywere probed using the all-optical pump-probe technique. Our results are consistent with the coexistence of antiferromagnetism and superconductivity at low temperatures.

Novel terahertz electric metamaterials

Abstract: Planar electric metamaterials and their inverse structures are demonstrated to show complementary electrical resonant behavior in terahertz time-domain spectroscopy. Simulations and measured data agree well and illustrate potential applicability of these materials for terahertz devices.

Electrical Control of Terahertz Metamaterials

Abstract: The metamaterials resonant response significantly enhances THz-wave/material interaction. We demonstrated real-time switchable THz metamaterial via external voltage bias to modify the metamaterial capacitive elements, thereby achieved effective control and manipulation of freely propagating terahertz waves.

Opto-electronic control of terahertz metamaterials

Abstract: We demonstrate external control of metamaterials operating at terahertz frequencies. Through photodoping of semiconducting substrates, used to support metamaterial arrays, we show ultrafast switching times. New metamaterial "grids" are presented, which may be formed by the union of electric metamaterials arrays. Metamaterial grids are then utilized to form a Schottky contact are used to demonstrate voltage switching of the metamaterials resonance. Both devices presented may be utilized to form novel devices at terahertz frequencies and also scaled to other energy regimes of interest.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ultrafast Carrier Dynamics in an InAs/InGaAs Quantum-Dots-in-a-Well Photodetector

Abstract: Differential transmission spectroscopy is used to measure carrier dynamics in a quantum-dots-in-a-well heterostructure. This provides fundamental insight into carrier relaxation from three to two to zero dimensions and has significant implications for dots-in-a-well-based mid-infrared photodetectors.

Dynamic Coupling-decoupling Crossover in the Current-driven Vortex State in Tl 2 Ba 2 CaCu 2 O 8 Probed by the Josephson Plasma Resonance

Abstract: Employing terahertz time-domain spectroscopy, we have measured the Josephson plasma resonance in Tl2Ba2CaCu2O8 high-Tc thin films, and studied the current-driven coupling-decoupling crossover in the driven vortex lattice.

Dynamically Frequency Tunable Terahertz Metamaterials

Abstract: We present novel terahertz metamaterials that feature frequency tunable resonant responses. Tunability is achieved by selective patterning of semiconductor regions within split-ring resonator structures. Simulations reveal non-linear frequency tuning as a function of conductivity.

Giant magnetoelastic effect in multiferroic Ba0.6Sr1.4Zn2Fe12O22

Abstract: We report a giant magnetoelastic effect in multiferroic Ba0.6Sr1.4Zn2Fe12O22 measured by ultrafast pump-probe spectroscopy. Coherent phonon excitation allows to measure the field- induced changes in the speed of sound and the corresponding elastic stiffness.

Active metamaterials: A novel approach to manipulate terahertz waves: Invited paper

Abstract: Compared to the neighboring infrared and microwave regimes, the terahertz (1 THz = 1012 Hz) regime is still in need of fundamental technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or photonic properties at terahertz frequencies. This results in formidable challenges in the generation, detection, and creation of devices to efficiently control and manipulate THz waves. Considering the promising potential applications of THz radiation, we need to overcome such material obstacles by actively searching for new materials, or by constructing artificial materials with a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties that derive from their sub-wavelength structure. The potential of metamaterials for THz applications originates from their resonant electromagnetic response, which significantly enhances their interaction with THz radiation. Thus, metamaterials offer a route towards helping to fill the so-called "THz gap". In this work we design a series of novel planar THz metamaterials. Importantly, the critical dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials. We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted in the invention of efficient THz switches and modulators which will be of importance for advancing numerous real world THz applications.