Electromagnetic (EM) metamaterials represent an important class of artificial materials composed of arrays of subwavelength unit-cell structures, which are also known as meta-atoms, with engineered effective optical properties, such as effective permittivity and permeability. Metamaterials provide a powerful platform to implement dynamic and nonlinear electromagnetic materials. Specifically, the effective permittivity and permeability can be tailored and reconfigured to construct metamaterial devices by modulating or actuating the constituent meta-atoms. By leveraging microelectromechanical system (MEMS) technology, active metamaterial devices, such as modulators, absorbers, and tunable waveplates, may be implemented. We developed myriad functional terahertz (THz) metamaterial devices based on MEMS actuators and optical excitation to manipulate the THz waves towards practical applications. In addition to far-field radiation, metamaterials exhibit extraordinary near-field properties yielding the capacity to tailor electric and magnetic field distributions. We studied the electron emission and nonlinear resonance response in THz metamaterials due to the electric-field enhancement effect. Furthermore, intelligent magnetic metamaterials for boosting the signal to noise ratio (SNR) of magnetic resonance imaging (MRI) have been developed. We employed the nonlinear response in metamaterials consisting sub-wavelength helical resonators and varactor-loaded split ring resonators to selectively enhance the magnetic field, thereby improving the SNR of MRI. Future practical applications of metamaterials will be explored and discussed.
Polarimetry is a well-developed technique in radar based applications and stand-off spectroscopic analysis at optical
frequencies. Extension to terahertz (THz) frequencies could provide a breakthrough in spectroscopic methods since the
THz portion of the electromagnetic spectrum provides unique spectral signatures of chemicals and biological molecules,
useful for filling gaps in detection and identification. Distinct advantages to a THz polarimeter include enhanced image-contrast
based on differences in scattering of horizontally and vertically polarized radiation, and measurements of the
dielectric response, and thereby absorption, of materials in reflection in real-time without the need of a reference
measurement. To implement a prototype THz polarimeter, we have developed low profile, high efficiency metamaterial-based
polarization control components at THz frequencies. Static metamaterial-based half- and quarter-wave plates
operating at 0.35 THz frequencies were modeled and fabricated, and characterized using a MHz resolution, continuous-wave
spectrometer operating in the 0.09 to 1.2 THz range to verify the design parameters such as operational frequency
and bandwidth, insertion loss, and phase shift. The operation frequency was chosen to be in an atmospheric window
(between water absorption lines) but can be designed to function at any frequency. Additional advantages of
metamaterial devices include their compact size, flexibility, and fabrication ease over large areas using standard
microfabrication processing. Wave plates in both the transmission and reflection mode were modeled, tested, and
compared. Data analysis using Jones matrix theory showed good agreement between experimental data and simulation.
Metamaterial and plasmonic composites have led to the realization that new possibilities abound for creating materials
displaying functional electromagnetic properties not realized by nature. Recently, we have extended these ideas by
combining metamaterial elements - specifically, split ring resonators - with MEMS technology. This has enabled the
creation of non-planar flexible composites and micromechanically active structures where the orientation of the
electromagnetically resonant elements can be precisely controlled with respect to the incident field. Such adaptive
structures are the starting point for the development of a host of new functional electromagnetic devices which take
advantage of designed and tunable anisotropy.
Recent advances in MEMS and focal plane array (FPA) technologies have led to the development of manufacturing microbolometers monolithically on a readout integrated circuit (ROIC). In this work, both numerical and finite element methods were performed to simulate the transient electrical and mechanical responses of resistive microbolometer FPAs made by several TCR (thermal coefficient of resistance) materials including a-Si, VOx and semiconducting YBCO. Numerical simulation shows that the pulsed bias readout mode in resistive microbolometer FPAs causes a non-steady-state of the system during the operation. As a result, NETD decreases with the increasing pulse width. In FPAs, the array size, frame rate, ROIC and mechanical reliability set the up-limit to the pulse width. The transient mechanical response for three microbolometer configurations was investigated using finite element modeling. The biased pulse results in membrane bending along the z-axis for the symmetric extended configuration (Type I), or twisting in three axes for the asymmetric extended configuration (Type II) due to the constraint force from the supporting arms. The square configuration (Type III) exhibits the smallest deformation and minimum shear stress at the sharp geometries. a-Si microbolometer generates higher shear stress than other microbolometers with the same square configuration.
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