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Research Description
Richard D. Averitt (Principal Investigator)
Research Interests: Ultrafast Optical Spectroscopy of Materials
Time-Integrated Optical Spectroscopy of Materials
Terahertz (far-infrared) Spectroscopy
Correlated Electron Materials
Metamaterials and Plasmonics |
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My
main research interest is using time-resolved and time-integrated
optical spectroscopic techniques to interrogate the fundamental and
applied properties of materials. My research spans from the
far-infrared through the visible and includes materials ranging from
correlated electron materials to metamaterials. Below
we discuss two main area of research. However, I don't see these as
essentially distinct from the point-of-view of spectroscopy or with
regards to the interaction with electromagentic radiation at the
phenomenological level of the constitutive relations in Maxwell's
equations. In fact, a
considerable amount of effort in the group during the coming years will
be to integrate metamaterials with complex quantum materials. The
reasons for this are twofold. On the one hand complex quantum materials
enable creating multifunctional metamaterials. On the other hand,
metamaterials will provide a novel means to interrogate and control
complex quantum materials. Stay tuned! |
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Optical Spectroscopy of Complex Materials
Complex
materials may be defined as having no dominant energy scale the
implication being that the charge, lattice, orbital, and spin degrees
of freedom conspire to determine their functional, and often emergent,
properties. This leads to rich macroscopic and mesoscopic behavior with
examples including colossal magnetoresistance, superconductivity,
multiferroicity, and electronic phase separation. Furthermore, advances
in the synthesis, growth, and integration of nanomaterials make
possible the design of nanoscale complex materials inspired by their
bulk counterparts. Optical spectroscopy is an important tool to
interrogate complexity in materials, naturally complementing techniques
such as angle-resolved photoemission or inelastic neutron scattering.
In particular, the beauty of optical studies of condensed phases is the
breadth of applicability. This is depicted in the Figure which displays
the spectral range and timescales of different phenomena occurring in
materials. Spectral coverage from approximately 0.001 – 4.0 eV is
especially important since many relevant excitations lie in this range.
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This
includes, as examples, gapped excitations related to superconductivity,
charge ordering, and hybridization phenomena; polaron, exciton, and
plasmon dynamics; or the coherent Drude response so intimately related
to metal-insulator transitions. For these reasons, optical spectroscopy
plays an important role in many areas of applied and fundamental
condensed matter physics. Examples include spintronics, Bose-Einstein
exciton condensation, plasmonics, dynamics in DNA, and semiconductor
heterostructures. Importantly,
ultrafast optical spectroscopy probes dynamics at the fundamental
timescales of electronic and atomic motion thereby providing an
important approach to investigate dynamical phenomena in complex
materials. We utilize time-resolved optical spectroscopy spanning from
the far-infrared through the visible to gain insights into the
functional response complex materials. This includes superconductors,
Heavy Fermions, manganites, and multiferroics. Our current work focuses
dynamical investigations of a host of transition metal oxides including
photoinduced phase transitions in vanadates and dynamical spin-phonon
coupling in multiferroics such as barium hexaferrite. |
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THz Metamaterial Absorber
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Terahertz Metamaterials
The initial impetus driving metamaterials research was the realization
and demonstration that a negative refractive index could be obtained by
creating patterned subwavelength composites consisting of highly
conducting metals such as gold or copper where the effective
permittivity and effective permeability are independently specified.
Additionally, metamaterials allow for tailoring the impedance in a
manner not easily achieved with naturally occurring materials. This
newfound approach to engineering the optical properties of materials
offers unprecedented opportunities to realize novel electromagnetic
responses from the microwave through the visible. This includes cloaks,
concentrators, modulators, with many more examples certain to be
discovered in the coming years.
Our work in this area is focused on creating novel metamaterial
composites which are resonant at far-infrared, or terahertz (THz)
frequencies. We are investigating metamaterials at THz frequencies for
two main reasons. |
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(a)
The length scale of THz radiation (1THz corresponds to a wavelength of
300 microns) is such that conventional lithography can be utilized to
create subwavelength composites. This allows us to investigate a host
of interesting electromagnetic phenomena in an efficient manner. For
example, with our collaborators, we have demonstrated resonant
metamaterial absorbers, voltage controlled modulators, and optically
tunable notch filters – all operating at THz frequencies. We employ an
approach using electromagnetic simulation, fabrication, and
electromagnetic characterization using terahertz time-domain
spectroscopy with a view of understanding the fundamental properties of
these electromagnetic composites.
(b) The
second reason we investigate metamaterials at THz frequencies is to
explore the possibility of filling the so-called “THz gap” using
devices such as those described above. There is considerable interest
in developing this portion of the electromagnetic spectrum for
applications given the unique characteristics of THz radiation which
includes the ability to transmit through materials that are opaque at
other frequencies. Thus, non-invasive imaging and spectroscopic
identification of illicit or hazardous materials are potential
applications. However, to realize this potential, substantial effort is
required to create fieldable sources and detectors along with the
development of component technologies that we take for granted in the
microwave, infrared, and visible portions of the electromagnetic
spectrum. Metamaterials will play an important role to fill this THz
technology gap.
Our current work in this area includes the development of
thermally-based metamaterial detectors, strongly resonant absorbers,
and reconfigurable metamaterials. We have ongoing collaborations with
Prof. Xin Zhang in the Dept. of Mechanical Engineering at BU, Prof.
Willie Padilla in the Physics Dept. at Boston College, Prof. Keith
Nelson at MIT, and various other collaborators from around the world.
A couple of more cool pictures from our work:
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Department Of Physics | Boston University | 590 Commonwealth Avenue, Boston, MA 02215
Phone: 617 353 2619 | Email: raveritt@physics.bu.edu |
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