Research

Time Resolved Spectroscopy of Complex Materials

Optical Spectroscopy of Complex Materials

Quantum 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.

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.

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.

(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.