305 research outputs found
Structural Control of Metamaterial Oscillator Strength and Electric Field Enhancement at Terahertz Frequencies
The design of artificial nonlinear materials requires control over the
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 nonresonant closed
conducting rings. An in-plane, lateral shift of a half unit cell between the
SRR and closed ring arrays results in a decrease 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 and we propose a
qualitative inductive coupling model to explain the observed electromagnetic
reponse.Comment: 11 pages, 5 figure
Decoupling Crossover in Asymmetric Broadside Coupled Split Ring Resonators at Terahertz Frequencies
We investigate the electromagnetic response of asymmetric broadside coupled
split ring resonators (ABC-SRRs) as a function of the relative in-plane
displacement between the two component SRRs. The asymmetry is defined as the
difference in the capacitive gap widths (\Delta g) between the two resonators
comprising a coupled unit. We characterize the response of ABC-SRRs both
numerically and experimentally via terahertz time-domain spectroscopy. As with
symmetric BC-SRRs (\Delta g=0 \mu m), a large redshift in the LC resonance is
observed with increasing displacement, resulting from changes in the capacitive
and inductive coupling. However, for ABC-SRRs, in-plane shifting between the
two resonators by more than 0.375Lo (Lo=SRR sidelength) results in a transition
to a response with two resonant modes, associated with decoupling in the
ABC-SRRs. For increasing \Delta g, the decoupling transition begins at the same
relative shift (0.375Lo), though with an increase in the oscillator strength of
the new mode. This strongly contrasts with symmetric BC-SRRs which present only
one resonance for shifts up to 0.75Lo. Since all BC-SRRs are effectively
asymmetric when placed on a substrate, an understanding of ABC-SRR behavior is
essential for a complete understanding of BC-SRR based metamaterials
Coupling Between An Optical Phonon and the Kondo Effect
We explore the ultra-fast optical response of Yb_{14}MnSb_{11}, providing
further evidence that this Zintl compound is the first ferromagnetic,
under-screened Kondo lattice. These experiments also provide the first
demonstration of coupling between an optical phonon mode and the Kondo effect.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Let
Detection of coherent magnons via ultrafast pump-probe reflectance spectroscopy in multiferroic Ba0.6Sr1.4Zn2Fe12O22
We report the detection of a magnetic resonance mode in multiferroic
Ba0.6Sr1.4Zn2Fe12O22 using time domain pump-probe reflectance spectroscopy.
Magnetic sublattice precession is coherently excited via picosecond thermal
modification of the exchange energy. Importantly, this precession is recorded
as a change in reflectance caused by the dynamic magnetoelectric effect. Thus,
transient reflectance provides a sensitive probe of magnetization dynamics in
materials with strong magnetoelectric coupling, such as multiferroics,
revealing new possibilities for application in spintronics and ultrafast
manipulation of magnetic moments.Comment: 4 figure
Magnetic exchange interaction between rare-earth and Mn ions in multiferroic hexagonal manganites
We report a study of magnetic dynamics in multiferroic hexagonal manganite
HoMnO3 by far-infrared spectroscopy. Low-temperature magnetic excitation
spectrum of HoMnO3 consists of magnetic-dipole transitions of Ho ions within
the crystal-field split J=8 manifold and of the triangular antiferromagnetic
resonance of Mn ions. We determine the effective spin Hamiltonian for the Ho
ion ground state. The magnetic-field splitting of the Mn antiferromagnetic
resonance allows us to measure the magnetic exchange coupling between the
rare-earth and Mn ions.Comment: accepted for publication in Physical Review Letter
Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide
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 Tc 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.Comment: accepted for publication in Physical Review Letter
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