Nano-photonics and Nano-plasmonics

Ultrafast fluorescent decay induced by metal-mediated dipole–dipole interaction in two-dimensional molecular aggregates

Quantitative understanding of the ultrafast energy transfer between fluorescent nanoemitters and the environment is essential in nanophotonics and optoelectronics and beneficial to many industrial applications. For nanoemitters like single or colloidal dye molecules or quantum dots, their fluorescence decay near a metallic substrate can be described by a noninteracting single-dipole picture. In this work, we find a dominant fluorescent decay channel in a 2D molecular aggregate as a result of the strong and coherent dipole–dipole interaction mediated by a metallic substrate. This unique mechanism leads to an ultrafast fluorescent decay and 10-times greater energy dissipation rate than expected. Our finding opens up a unique way to manipulate energy transfer and to develop light-energy devices on the molecular level.

Acoustic Metamaterials

Acoustic bifunctional lenses with anisotropic materials and coordinate transformation method

Acoustic metamaterials promise new applications for steering and focusing sound waves, and reducing the ultrasound signature of the underwater objects. In this work in collaboration with Prof. Hongsheng Chen’s group in Zheng Jiang University, China, we designed and demonstrated bifunctional lenses based on anisotropic metamaterials that worked as a fisheye lens in one direction and a Luneburg lens in the other direction. In one direction, the spherical wave from a point source was converted to plane wave. In the other direction, the spherical wave was refocused to another point. Coordinate transformation method was used to derive anisotropic material properties required for bifunctional lenses. 3D printed Cross-shaped beams with different length in two perpendicular directions were used to fabricate the lens. The designed lenses have potential as key elements for acoustic imaging and directional sensing.

Micro and Nano Manufacturing

Metal Investment Casting

Metal investment casting is a process of converting a plastic or wax pattern into a metal replica. We are exploring the limits of this technology by closely studying each step of this process in order to determine the current obstacles in the production of arbitrary metal geometries in terms of parts complexity, resolution, volume, or finish. The use of metal investment casting is suited for demanding applications where metallurgic or surface smoothness properties are critical and not addressable by direct metal additive manufacturing techniques. We work at the frontline of plastic additive manufacturing to produce high resolution parts for use in investment casting tests. We also tackle upstream issues with geometry creation and downstream issues with investment handling, firing and cleaning for a holistic study of this process.

3D-printed microarchitected materials for efficient heat and mass transfer

Efficient transport and reaction for catalytic reactors are desirable in a broad array of implications for biological and environmental applications, or the automotive and power plant industry. Porous substrates with thinner cell/pore walls and higher cell/pore density enable faster activation of the catalyst due to low-heat capacity, larger catalytic surface area, and lower hydrodynamic resistance of working fluids. However, manufacturing of well-engineered structures with thin wall and higher cell/pore density remained a challenge. To overcome the limitations, we study manufacturing-friendly structural design and additive manufacturing process for microarchitected ceramic substrates with both a high surface area to volume ratio and low-heat capacity. Our central idea for achieving efficient ceramic substrates is leveraging on the geometrical benefits of 3D microlattices of thin-walled hollow-tubes.

Soft Materials

High-resolution, scalable 3D microprinting technologies with soft materials

Scalable lithographic printing without compromising resolution is still a challenging issue for the manufacture of complex microstructures from planar lithography to additive manufacturing. To address this issue, we explore advanced imaging technology to enable reconfigurable synthetic imaging leveraging interplay with a unique optical element and a conventional projection micro-stereolithography system. We envision new micro 3D printing techniques brings a new possibility on the microfabrication of complex microstructures for various emerging fields including soft robotics, tunable metamaterials, and tissue engineering.

Broadband solar management by tailoring light scattering of thermochromic hydrogel microparticles

Intelligent control of solar irradiance through windows is promising to reduce building energy consumption at least 1.055 quintillion Joule per year in the US. Light management with thermochromic smart windows holds practical significance due to their autonomous system and simplicity of the device. Yet, the conventional thermochromic material (VO2) faces scientific challenges with a high phase transition temperature (~68 ˚C) and poor thermochromic performance. Temperature-responsive hydrogel emerges as a promising alternative, where light management could be pursued by maneuvering the light scattering behaviors. This work reported the strategy to explore the thermochromic performance of co-polymerized hydrogel microparticles with the aid of tailoring particle size and internal structure. The solar modulation was effectively extended into IR region and an unprecedented solar transmittance modulation of 81.3% was achieved at ~32˚C by our hydrogel thermochromic device with great scalability and stability.

         PhD Dissertations