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Ashish Agarwal
Ph.D. Student
agashish(at)umich(dot)edu

M.S. EE, University of Michigan, 2009
M.S. ChE, University of Michigan, 2006
B.S. ChE, Indian Institute of Tech., 2005
 

Research Projects

Negative Index Materials

One of the most interesting types of metamaterials that has been actively researched in this decade is Negative Index Material (NIM). NIM requires the electric permittivity and magnetic permeability of a material to be simultaneously negative over a frequency range, in order to have negative refractive index. A planar slab of NIM has the unusual ability to act as a lens by focusing the light from an object to form an image.  The lens can not only focus the propagating component of light but also amplify the evanescent waves from the object allowing resolution to be higher than the diffraction limit. Superlens will find application in manufacture of smaller and faster chips, data storage devices of higher capacity, superior high resolution optical microscopes.

Naturally occurring materials do not have negative refractive index because magnetic polarization usually tends to occur only at low microwave frequencies and negative permittivity is not observed over this frequency range.  The two most popular approaches to artificially fabricate NIM has been using lithography based metal-resonator structures and photonic crystals. Both the techniques are limited because of the metallic losses in metal based structure, surface imperfections and size limit of structural features from lithography. My project aims to fabricate negative index materials by using self-assembly of nanoparticles as a process to create complex geometries and structures of higher order.  Self-assembly allows the structural features to be of extremely small dimensions and overcome losses through embedded gain materials. I synthesize 0D and 1D building blocks at nanoscale by wet chemistry and perform series of surface modifications to help the particles self-assemble into uniform 2D structures. I am currently researching the role of surface charge density, medium and stabilizers on the self-assembly of gold nanorods. Understanding of the self-assembly process will provide us the ability to perform necessary changes to the structure to achieve negative permeability from geometry.

Images: TEM images of gold nanorod pairs. Assembly was achieved by stacking of negatively charged polymer between the positively charged gold nanorods. Increasing polymer concentration increases the number of rods in the pair.  Gold as a metal has negative permittivity at visible frequencies and anti-parallel current mode in the pair of rods leads to a diamagnetic response which gives negative permeability, hence NIM.

Photoacoustic imaging of cells with gold nanorods as contrast agents

The project in collaboration with Biomedical Engineering department studies the potential of gold nanorods to target cells and provide contrast for photoacoustic imaging. The elongated “rod” shape of these nanoparticles provides a mechanism to tune their plasmon peak absorption wavelength. Their plasmon absorption peak is designed to be at 700 nm for increased penetration depth into biological tissue. A series of surface modifications are performed to conjugate the protein onto the surface of gold nanorods. The conjugated protein on the nanorod provides it the specificity to target the required cell in the body. We are currently using targeted gold nanorods as contrast agents for studying cancer cells and inflammatory joint diseases.

Images: 2D cross-sectional PAT of a rat tail joint. (A) Image based on intrinsic contrast which was taken before the administration of contrast agent. Images taken after (B) the first and (C) the second administration of Etanercept conjugated gold nanorods. For each administration, 0.025 ml agent with a 10 picomolar concentration was injected intra-articularly through the arrows in the images. (D) Histological photograph of a similar cross section in a rat tail joint showing the morphological feature of synovium and other articular tissues.

Publications