CBE Doctoral Dissertation Defense: “Reconfigurable Metasurfaces Based on Multistable Elastic Pixels” (Jed-Joan S. Edziah)

Abstract:

Metamaterials are engineered materials designed to manipulate and tailor electromagnetic (EM) waves. Metasurfaces are planar metamaterials that rely on inclusions whose optical properties and spatial arrangements are designed to interact with incident EM waves to yield desired reflected or transmitted waveforms. A reconfigurable metasurface in which the relative positions of inclusions can be controlled could yield a range of EM responses from a single device. Building such metasurfaces remains a major challenge. In current methods, one device state is typically defined by the spatial position of inclusions absent external forces or fields, and the other, volatile state (or states) are achieved via application of external forces or fields to alter inclusion position. Such reconfigurable metasurfaces require continuous energy input to maintain volatile device states, impacting device energy efficiency. A method to define distinct non-volatile device states using inclusions designed to have multiple equilibrium loci separated by energy barriers that are large compared to thermal energies could allow for efficient device operation and for reconfiguration. Each stable state would yield a non-volatile device configuration. A switching field could allow the inclusions to move from one stable location to another, defining distinct device states. The switching field could be removed, and the reconfigured state could persist until it became desirable to reconfigure the device again.

In this thesis, I define and develop the concept of a multistable elastic pixel (MEP). A MEP consists of an inclusion (i.e, a colloidal particle, disk, or chip) placed in a nematic liquid crystal (NLC) filled pore. In my research, rather than exploiting the birefringence of the NLC, the NLC’s elastic free energy is exploited to control the inclusion position within the pore. By changing inclusion positions in a metasurface comprising an array of MEPs, the EM response of the metasurface can be altered in a controlled fashion. The pore shape, anchoring energies, and those of the inclusions are designed to mold the NLC director field around spherical or disk-shaped inclusions. The NLC molecules within the pores are distorted from their preferred, uniform spatial organization. These distortions in the nematic director field define an elastic free energy landscape that depends on colloidal particle position. Inclusions within the MEPs move to equilibrium locations within the pore where the distortions, and hence the elastic free energy, are minimized. By designing pore shapes to have multiple equilibrium loci separated spatially by zones with elastic energy barriers, multiple inclusion docking sites and device states can be defined. In principle, the MEPs concept can be demonstrated in the homogenized limit (for inclusion sizes and periodicities much smaller than the incident wavelength) or in the diffractive limit (for inclusion sizes and periodicities similar to the incident wavelength). However, since the MEPs design relies on lithographic processes, for ease of fabrication, I focus on MEPs-based diffractive devices.

I develop and explore two bistable MEP designs, a ‘pillbox’ shaped pore and a ‘peanut’ shaped pore. Both pores feature curved ends connected either by straight walls (pillbox) or by a constricted region (peanut). I characterize the elastic energy landscape within these bistable MEP structures for colloids immersed in the NLC 4-Cyano-4′-pentylbiphenyl (5CB). I focus on inclusions (Ag-coated silica colloids) confined within pores fabricated atop a borosilicate substrate via photolithography. Pores were confined with a top borosilicate substrate via a spacer, such as a Cu electrode, which was used to apply an electric switching field. The required switching fields were on the order of 103-104 V/m. In the absence of the switching fields, inclusions remained in their stable locations. Energy landscapes were explored by displacing the colloidal inclusions from their equilibrium locations, and observing their trajectories as they returned to equilibrium. In the limit of negligible particle inertia, the trajectories are analyzed to reveal the forces and energy dissipated along a trajectory. Experiments agree with simulations of the NLC elastic free energy in the Landau-de Gennes framework, which show that a spherical or disk-shaped inclusion finds a minimum energy configuration near the curved ends of the pores, with an energy barrier to reconfiguration that is smallest for the straight-sided pillbox and becomes more significant for peanuts with narrower waists or greater antagonistic curvature. This design affords control of equilibrium inclusion locations and of the switching fields required to move between them. Furthermore, the NLC elastic energy landscape is highly non-linear. Topological defects can arise that alter the energy barriers to reconfiguration.

We develop diffraction-limited MEP-based devices that enable reconfigurable optical states. By arranging multiple MEPs on a surface, I design metasurfaces with nonvolatile, reconfigurable scattering cross-sections. I demonstrate our two-state device design in which inclusions are tuned from a lattice (State A) to a chain (State B) configuration. First, static ~10 μm Ag chip inclusions arranged in the State A and B configurations were fabricated on Si wafers using direct-write lithography and lift-off. These showed distinct far-field diffraction patterns in reflection mode under ~630 nm illumination in air, consistent with Fraunhofer theory. We fabricated arrays of MEP and circular pores within which static 9 μm Ag chip inclusions were deposited and assembled into NLC cells, which we refer to as MEP cells. The near-field optical responses of the two device states in the MEP cells were probed via reflected light microscopy using a partially coherent LED source filtered to emit green light. These responses were modeled by convolution with a circular kernel, whose outer radius r increases along z such that r = z · NA_Illumination. We successfully modeled the observed power distributions for each device state. States A and B exhibited clearly distinguishable near-field scattering signatures. Using the same cells, we also collected far-field diffraction patterns in reflection mode under normal-incidence 630 nm laser light with linear polarization filtering. The observed diffraction patterns corresponded to the unit cell periodicities of each state, again confirming distinct EM responses. Together, these results demonstrate that MEP-based architectures enable experimentally discrete optical states with both near-field and far-field distinctions.

To realize tunability, we demonstrate that chip inclusions can serve as reconfigurable elements in the two-state device. We showed that a single ~500 nm thick Ag chip inclusion can be electrically reconfigured within an MEP pore. The inclusion translated toward the positive electrode and reversed direction with polarity switching, similar to the behavior of colloidal inclusions, though no defect dynamics were observed. The switching voltage (100 V) was lower than that required for colloidal inclusions, likely due to reduced elastic forces, which may be attributed to the chip’s thinner edges. We also designed a magnetically and electrically tunable chip MEP unit; the magnetic functionality is intended to enable the use of a magnetic probe to fill small areas of MEP arrays, such as those utilized in demonstrating near-field optical signatures. Assembly of these chip inclusions into our MEPs is currently ongoing. Overall, our observations demonstrate that MEP-based metasurfaces can function as reconfigurable diffraction-limited devices in the visible range. Our demonstration of MEPs enables tunable and nonvolatile beam steering, which has significant applications in imaging, spectroscopy, and other laser-based technologies.

Zoom Link: https://upenn.zoom.us/j/98737258070?pwd=tqb0ucZcfWRCZiGaE8JzIZIFmAfAow.1
Meeting ID: 987 3725 8070
Password: 256841