Research – Multifunctional Metamaterials (META)

Multi-disciplinary research in the META Lab blends experimental and computational nanophotonics, plasmonics, electronics, thermodynamics and mechanics. We are interested in exploring fundamental aspects of energy transfer between quantum emitters, propagating and trapped photons, thermal and acoustic phonons and electron plasma oscillations (plasmons) as well as in thermodynamics of light trapping and energy conversion and new material engineering. These studies are advancing development of new meso-scale multi-functional devices and materials for applications in light generation, optical information processing, atmospheric water capture, bio(chemical)sensing, heat management in textiles, and solar energy harvesting.

Sponsors:

Weaving space: live exhibit on now @STUDIO.nano

Space is a dangerous frontier – absence of gravity, harmful radiation & temperature extremes affect astronauts & electronics. To enable human space exploration, MIT-META Lab develops smart materials that provide radiation shielding, thermo-regulation, space navigation, energy harvesting & monitoring for integration into spacesuits, ships & habitats.

Visit MIT.nano center to see a virtual exhibit featuring our research on making new materials and devices to enable space exploration (first floor, next to One.MIT Silicon wafer location).

Mapping photocurrents on the micro-scale

Scanning microscopic photocurrent measurement setup custom-designed and built by a PhD student Morgan Blevins allows to probe anomalous photocurrents in topological materials and materials with broken inversion symmetry. Spatially-resolved mapping of surface reflectance and photocurrent profile allows to study exotic topological properties of Dirac and Weyl semimetals and to engineer self-biased low-noise photodetectors based on flexoelectric effect in 2D semiconductors and topological insulators.

Breaking Lorentz reciprocity with [pseudo-]magnetic fields

In conventional optical materials, light-matter interactions are reciprocal. This means that a light wave returns to its original state if it travels backwards through the medium following the same trajectory. However, reciprocity of light and energy transport is not a fundamental law, it can be broken by material losses, nonlinearity, or magnetic fields, enabling the “one-way” flow of optical energy and unlocking applications in energy harvesting, signal processing, and thermal emission control. Nonreciprocal surfaces do not obey the Kirchhoff’s law of thermal radiation, which postulates the equality of spectral directional emissivity and absorptivity. This has implications for the energy efficiency of devices that emit and absorb thermal radiation, such as solar thermal collectors and thermophotovoltaic cells, since they no longer have to emit as much energy as they absorb through any given spectral directional channel.

We experimentally observed room-temperature nonreciprocal reflection of mid-infrared light from planar highly doped InAs surfaces at low magnetic fields ranging from 0.07 T to 0.16 T. Prior experiments used larger magnetic fields ranging from 0.5 T to 1.5 T, which limited practical applications of this effect.

We theoretically predict strong nonreciprocity and violation of Kirchhoff’s law in a planar sub-wavelength-thin magnetophotonic crystal composed of dielectric and magnetic Weyl semimetal thin films. The optimized design eliminates the need for an external magnetic field and achieves a large nonreciprocal absorptivity contrast of 95.3%. Our work is a step toward the practical implementation of nonreciprocal thermal emitters and absorbers and applications such as remote magnetic field sensing.

Dragging/boosting surface waves with swift electron currents

In nonreciprocal optical systems, light behaves differently depending on the direction it travels. This phenomenon breaks the principle of Lorentz reciprocity and can be leveraged to eliminate backscattering noise and achieve unidirectional energy transport, enabling optical isolators and efficient light energy conversion systems. However, materials that exhibit strong magnetic or nonlinear responses necessary to achieve nonreciprocal light transport are rare. Magnetic fields cause interference with other on-chip components and complicate device integration. Thus, there is an unmet need for alternative external stimuli that are easy to implement in situ. We propose to meet this need by using plasmon Fizeau drag, the phenomenon in which moving charge carriers impart dragging (accelerating) force on counter- (co-)propagating surface plasmon polariton waves. Plasmon Fizeau drag can induce nonreciprocal surface modes and one-way energy transport, but – similar to the Doppler shift effect for propagating light waves emitted from objects moving at relativistic velocities – requires relativistic electron drift velocities to yield appreciable contrast between the dispersion characteristics of co-propagating and counter-propagating modes.

The high electron drift and Fermi velocities in 3D Dirac materials make them ideal candidates for the effect, however, both the theory of the Fizeau drag effect and its experimental demonstrations in these materials have been missing.

We developed a semiclassical theory of Fizeau drag in current-biased 3D Weyl and Dirac semimetals (W/DSMs), both under local and non-local approximation and with dissipative losses. We predict that under practical assumptions for loss, Fizeau drag in the DSM Cd₃As₂ opens windows of pseudo-unidirectional transport of surface plasmon polariton modes.

We illustrate the unique performance advantage of this dynamic mechanism of inducing nonreciprocity by designing a flat, sub-wavelength-thin infrared absorber with 100% peak absorptance, a high nonreciprocal absorptance contrast of ~87%, and high in situ spectral tunability. The performance can be attributed in part to coupling to Berreman modes, which become nonreciprocal due to the shifting of the epsilon-near-zero (ENZ) point as a function of angle of incidence.

Finally, application of current bias gives rise to other exotic high-momentum waves – hyperbolic modes – in Dirac semimetals. These effects allow to dynamically tune the radiative heat exchange between two material interfaces separated by a small vacuum gap by simply switching the voltage on and off or reversing its polarity. We predict that modulating the magnitude and direction of the current bias in Cd₃As₂, in combination with gating, can achieve on/off switching ratios of up to 22 for gaps of 100 nm, higher than previously explored gate-tunable systems.

Sculpting photonic materials properties by stress and strain gradients

Metal thiophosphates (MTPs) are a new family of intermediate-bandgap (1.3–3.5 eV) 2D materials that exhibit diverse electronic, magnetic, and nonlinear optical properties, and show promise for applications in energy harvesting, storage, and photo-detection. These properties are strongly influenced by the transition metal element within the MPTs. Sulfur vacancies and other defects in MTPs allow defect-state-to-valence-band transitions leading to visible light emission at sub-band gap energies. PL measurements under a variety of external stimuli can shed the light on the structural and electronic properties evolution in these materials and reveal material candidates to achieve either high tunability or high stability under extreme conditions.

In collaboration with the Institute of Physics in Warsaw, the US Air Force Research Lab, and Technology de Monterrey, we showed experimentally that defect-mediated photoluminescence in AgScP₂S₆ can be enhanced and spectrally-shaped by structural defects in the material. These defects form during material growth, originating from dislocations buried under surface planes, and exhibit varying thickness and inhomogeneous localized strain distribution. Our data also show that photoluminescence can be further enhanced and tuned via thermal annealing – which increases the density of sulfur vacancies – and by temperature-induced strain gradients.

In turn, Zinc phosphorus trisulfide (ZnPS₃), demonstrated remarkable structural stability under extreme pressures and cryogenic temperatures. PL measurements and Raman spectroscopy revealed a fully-reversible pressure-induced phase transition starting at ~7 GPa, after which ZnPS₃ demonstrates stability up to 24.5 GPa. Ab-initio DFT calculations support these observations and predict a semiconductor-to-semimetal transition at 100 GPa. Cryogenic X-ray diffraction measurements revealed that ZnPS₃ has a high mean thermal expansion coefficient of about 4.4 × 10⁻⁵ K⁻¹, among the highest reported for 2D materials. This unique combination of tunable electronic properties under low pressure and high thermal sensitivity makes ZnPS₃ a strong candidate for sensing applications in extreme environments

Engineering polymers for solid-state cooling and heating technologies

Building refrigeration is one of the most energy-intensive technology sectors, representing about 20% of overall energy consumption, has low efficiency (below 60%), and is accompanied by significant greenhouse gas emissions. We aim to advance an alternative thermoregulation technology – solid-state cooling/heating based on elastocaloric and twistocaloric effects in composite polymer fibers and yarns. Our objective is to develop strain-activated fibers and textiles for temperature control, dynamic structural changes, energy storage, and thermal conductivity modulation.

Monomaterial SVETEX textiles for passive thermoregulation

Conventional fabrics absorb body heat and perspiration, providing fertile ground for bacterial growth. Furthermore, conventional textile production pollutes water with dangerous toxins, and 73% of fabrics end up in landfills. We are developing smart sustainable fibers and mono-material multi-functional textiles that passively regulate temperature via control of radiation, thermal conduction, and evaporation, inhibit bacterial growth, save energy and water during fabrication and usage, and can help to reduce and re-use plastic waste. _{Video credit: Prof. Skylar Tibbits; knitting credit: Lavender Tessmer}

Atmospheric water harvesting

Many countries have arid climates in significant parts of their territory, which severely inhibits their land development and creates harsh humanitarian conditions for their populations. Atmospheric water harvesting (AWH) technology, which extracts moisture from the ambient air to generate water, is a promising strategy to realize decentralized water production in the arid areas and in regions where large-scale installations are impractical by economic or security reasons. However, most existing sorption-based AWE prototypes exhibit prohibitively high energy consumption, associated with their high desorption heat, which renders water release an energy-intensive process. Our new harvesting technology facilitates both the sorption and the desorption processes, and already achieved forty-five-fold reduction of the energy needed to extract the same mass of water from an AWH hydrogel relative to the standard single-stage evaporation-condensation technique.

Developing photonic sensors with high sensitivity & easy readout

Bio(chemical) sensors play an outsized role in medical care, biological research, drug development, national security, and environmental monitoring. We are developing new technologies to enable reliable sensors for detection of new viral and bacterial pathogens and environmental pollutants by combining photonic amplification, biological recognition & nano-mechanical forces on nano-photonic and nano-plasmonic chips.