The correlated polar semimetal Ca3Ru2O7 exhibits a rich phase diagram including two magnetic transitions (TN =56 K and TC =48 K) with the appearance of an insulating-like pseudogap (at TC ). In addition, there is a crossover back to metallic behavior at T∗=30 K, the origin of which is still under debate. We utilized ultrafast optical pump optical probe spectroscopy to investigate quasi- particle dynamics as a function of temperature in this enigmatic quantum material. n conjunction with density functional theory, our experimental results synergistically reveal the origin of the T-dependent pseudogap. Further, our data and analysis indicate that the T∗ emerges as a natural consequence of T-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low fluence ultrafast optics as a sensitive probe of low energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.
Electromagnetic metamaterials are typically comprised of subwavelength metal or dielectric resonators that, when fashioned as two or three-dimensional composites, result in novel optical and photonic functionalities. Importantly, the enhanced local electric and magnetic fields of these resonators are accessible leading to strong interactions upon integration with quantum materials. Ultimately, we seek to create emergent photonic composites where the whole is more than the sum of the parts. The possibilities are nearly endless with a host of quantum materials ranging from semiconductors to transition metal oxides to superconductors offering unique possibilities. This is especially true at terahertz frequencies where the electrodynamic response of quantum materials often manifest in dramatic fashion. In this talk, we will focus on terahertz quantum metamaterials (TQMs) highlighting recent examples and emphasizing that TQMs offer a two-way street to both create technologically relevant composites and to investigate fundamental condensed matter physics under extreme conditions.
Polarimetry is a well-developed technique in radar based applications and stand-off spectroscopic analysis at optical
frequencies. Extension to terahertz (THz) frequencies could provide a breakthrough in spectroscopic methods since the
THz portion of the electromagnetic spectrum provides unique spectral signatures of chemicals and biological molecules,
useful for filling gaps in detection and identification. Distinct advantages to a THz polarimeter include enhanced image-contrast
based on differences in scattering of horizontally and vertically polarized radiation, and measurements of the
dielectric response, and thereby absorption, of materials in reflection in real-time without the need of a reference
measurement. To implement a prototype THz polarimeter, we have developed low profile, high efficiency metamaterial-based
polarization control components at THz frequencies. Static metamaterial-based half- and quarter-wave plates
operating at 0.35 THz frequencies were modeled and fabricated, and characterized using a MHz resolution, continuous-wave
spectrometer operating in the 0.09 to 1.2 THz range to verify the design parameters such as operational frequency
and bandwidth, insertion loss, and phase shift. The operation frequency was chosen to be in an atmospheric window
(between water absorption lines) but can be designed to function at any frequency. Additional advantages of
metamaterial devices include their compact size, flexibility, and fabrication ease over large areas using standard
microfabrication processing. Wave plates in both the transmission and reflection mode were modeled, tested, and
compared. Data analysis using Jones matrix theory showed good agreement between experimental data and simulation.
Metamaterial and plasmonic composites have led to the realization that new possibilities abound for creating materials
displaying functional electromagnetic properties not realized by nature. Recently, we have extended these ideas by
combining metamaterial elements - specifically, split ring resonators - with MEMS technology. This has enabled the
creation of non-planar flexible composites and micromechanically active structures where the orientation of the
electromagnetically resonant elements can be precisely controlled with respect to the incident field. Such adaptive
structures are the starting point for the development of a host of new functional electromagnetic devices which take
advantage of designed and tunable anisotropy.
In this paper we present our recent developments in terahertz (THz) metamaterials and devices. Planar THz metamaterials and their complementary structures fabricated on suitable substrates have shown electric resonant response, which causes the band-pass or band-stop property in THz transmission and reflection. The operational frequency can be further tuned up to 20% upon photoexcitation of an integrated semiconductor region in the split-ring resonators as the metamaterial elements. On the other hand, the use of semiconductors as metamaterial substrates enables dynamical control of metamaterial resonances through photoexcitation, and reducing the substrate carrier lifetime further enables an ultrafast switching recovery. The metamaterial resonances can also be actively controlled by application of a voltage bias when they are fabricated on semiconductor substrates with appropriate doping concentration and thickness. Using this electrically driven approach, THz modulation depth up to 80% and modulation speed of 2 MHz at room temperature have been demonstrated, which suggests practical THz applications.
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.
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
Tunable electromagnetic metamaterials can be designed through the incorporation of semiconducting materials.
We present theory, simulation, and experimental results of metamaterials operating at terahertz frequencies.
Specific emphasis is placed on the demonstration of external control of planar arrays of metamaterials patterned
on semiconducting substrates with terahertz time domain spectroscopy used to characterize device performance.
Dynamical control is achieved via photoexcitation of free carriers in the substrate. Active control is achieved by
creating a Schottkey diode, which enables modulation of THz Transmission by 50 percent, an order of magnitude
improvement over existing devices. 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.
We report the photoluminescence spectrum of well characterized, epitaxially grown single crystal C60 thin films. The highly regular and reproducible spectral features observed can be explained by a simple molecular model which takes into account enhanced coupling of an excited C60 molecule with its nearest neighbor. This model provides an identification of all the observed spectral features at low temperature including a qualitative understanding of the relative peak height ratios, as well as the observed temperature dependence of the PL spectrum.