overview
We conduct research on various aspects of nanophotonics, the science of light-matter interactions at extremely small and discrete dimensions. Nanophotonics seeks to enhance the performance of light-based technologies while shrinking the footprint of devices — hence minimizing usage of limited resources and energy consumption. By examining the behavior of matter and energy at nanometer length and femtosecond time scales, we gain fundamental insights on light-matter interactions. This knowledge empowers us to develop transformative technologies in the energy, telecommunication and quantum sectors.
In our lab, we design, make and test artificially-engineered nanostructures to create hybrid materials with enhanced functionalities. An example of functionality is molding the flow of light (e.g., refraction, scattering or absorption) while modifying microscopic interactions in matter such as carrier-carrier, carrier-phonon, polariton formation, and strongly correlated interactions. These interactions control processes that are fundamental limits to the efficiency and reliability of energy-conversion and optoelectronic devices.
We probe these microscopic interactions with spatial, temporal and spectral specificity using advanced optical and electron-based microscopy tools. Our team is proficient in various optical microscopy techniques — ultrafast broadband absorption/emission transient spectroscopy, energy-momentum imaging, single-photon counting, Raman hyperspectral imaging, Hanbury Brown and Twiss interferometry techniques, and Hong-Ou-Mandel spectroscopy. Several team members are also proficient in fabricating nanostructured materials using electron-beam and photo lithography. To characterize the morphology and structural integrity of our nanostructured materials, we regularly use X-ray diffraction, scanning electron, transmission electron and electron energy-loss spectroscopic techniques. We are fortunate to partner with several staff scientists at Oak Ridge and Brookhaven National Laboratories to help us achieve our goals. We use state-of-the-art facilities at the Center for Nanophase Materials Science (TN) and the Center for Functional Nanomaterials (NY).
The intellectual merits of our research are to address these overarching scientific questions:
1. How a nanomaterial’s characteristics (crystal structure, size, morphology and interface) or its surrounding electromagnetic environment dynamically modify atomic and electronic degrees of freedom?
2. How the assembly of similar or dissimilar nanostructures result in emergent phenomena, and how to develop systematic methods and tools to probe and enhance such desired macroscopic linear and nonlinear effects.
3. How are intrinsic processes of solid-state materials are modified (1) during growth/assembly, and ultimately (2) during device operation?
Metamaterials
Metamaterials are artificial materials, engineered and constructed around basic nanostructure building blocks — similar to how natural materials are built up from atoms to form crystals. While natural materials derive their function from atoms, metamaterials derive their function from the type, shape, and orientation of the nanostructures. The unit cell geometry and spatial arrangement of the nanostructures provide extra ‘knobs’ to control functionality on a macroscopic scale. Tuning these parameters enable metamaterials to have unique properties not found in nature, like cloaking and superlensing.
Our activities in this area are centered around time-varying metamaterials: determining how to tune their optical properties dynamically. We use synchronized laser pulses to modify the carrier dynamics of the unit-cell nanostructure and probe the collective interactions that emerge within the metamaterial. Often, we investigate metamaterials that are two dimensional in nature, referred now as metasurfaces. Our group studies several classes of metasurfaces: all-dielectric, self-assembled, and those derived from disordered and amorphous nanostructures.
We think that dielectric metamaterials are an effective platform for ultrafast optical devices, with the possibility for ultrafast polarization-sensitive optoelectronics. Currently, we investigate how to tune all-dielectric metasurfaces dynamically by controlling intrinsic material’s properties like two-photon absorption, free carrier relaxation, lattice heating, and hot carrier cooling. In the past, we employed phase-change materials (PCM) to control the optical response of metallic metamaterials.
Scanning electron micrograph of a hybrid metamaterial originally used to derive optical magnetic response (7 x 7 μm2). Now, we employ nanostructured dielectric disk with electric and magnetic multipoles derived from Mie resonances to manipulate light-matter coupling.
quantum Nanosystems
Today’s silicon-based technologies are approaching physical limits set by dissipation, density, and speed. Quantum materials can offer an array of new electronic functionalities that extend and expand current technologies far beyond the capabilities of silicon. While all materials require quantum mechanics, not all materials are quantum materials. Quantum materials are solids with exotic physical properties, arising from the quantum mechanical properties of their constituent electrons, their strong interactions, and the presence of new forms of electronic order. With extreme sensitivity to external perturbations, quantum materials hold promise for emerging phenomena that will impact technologies ranging from sensors, to power management and transmission, to platforms for quantum computation.
Our activities revolve around studying quasiparticles hosted by quantum materials, and establishing appropriate methods to fabricate, and optically manipulate nanostructures of quantum materials. Our goal is to quantify how finite-sized structures, along with controlled levels of dopants and defects, modify fundamental quantum properties such as coherence, entanglement, and energy transduction pathways — fundamental properties which will be become the basis for transformative technologies. For example, by coherently exciting quantum nanostructured materials with femtosecond light pulses, we study how metastable states of matter are created and dictate unique electronic functionalities that do not exist in equilibrium. Photon counting and interferometric autocorrelation microscopy are used to understand the dynamics of bare excitons and exciton-polaritons in sub-wavelength nanostructures.
Ultrafast absorption and emission spectroscopy, and photon statistics microscopy to measure fs-ns dynamics and correlations in quantum nanostructured semiconductors
Energy nanoMaterials
The world demands for energy is expected to double to ~ 30 terawatts by the year 2050. Compounding this challenge is the need to protect our environment by increasing energy efficiency of devices and developing renewable energy sources. Given that more energy from the Sun strikes the Earth in one hour than all the energy we consume on the planet in a year, it is beneficial to develop novel solar technologies that would fulfill our energy demands. Within this context, developing ‘energy’ nanomaterials is an exciting and necessary approach to addressing these challenges.
Our activities focus on probing novel semiconductors like hybrid organic-inorganic perovskites and van der Waals heterostructures for energy conversion and light-based technologies. Using ultrafast microscopy, we study their optical properties at nanometer length and femtosecond time scales. Taking ultrafast snapshots of their electronic structure, we study how their carrier dynamics are modified, including the role of hot carriers. More recently, we have become vested in studying the role of grain boundaries and junctions on the overall free-carrier and exciton dynamics; these interfaces and discontinuities play a critical role in device efficiencies. We are fortunate to collaborate with Professor Mohite at Rice University, who share his expertise on all things hybrid perovskites.
In the past, we have also studied water-splitting nanostructured photocatalysts to create liquid fuels such as hydrogen. Hydrogen is a clean fuel that, when consumed in a fuel cell, produces only water. We studied how designing novel heterostructures enhance the water-splitting reaction. We also designed novel in situ ultrafast spectroscopy to probe the underlying fundamental mechanism of this efficiency boost.
Scanning electron micrograph of large-grain hybrid organic-inorganic perovskites (200 x 200 μm2). We also study the effect of finite-size structures and grain boundaries and interfaces on their optoelectronic linear and nonlinear properties.
Funding
We are grateful for the support from the Physics Department at the University of Alabama at Birmingham, the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), Alabama EPSCoR (EPScoR) and Oak Ridge Associated Universities (ORAU via the Ralph Powe Faculty Award).
