UNIVERSITY OF UTAH
ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT
DISSERTATION DEFENSE FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Advisor: Ajay Nahata
CONTROLLING PROPAGATION PROPERTIES OF SURFACE PLASMON POLARITON AT TERAHERTZ FREQUENCY
Despite great scientific exploration since 1900’s, the terahertz range is one of the least explored regions of electromagnetic spectrum today. In the field of plasmonics, texturing and patterning allows for control over electromagnetic waves bound to the interface between a metal and the adjacent dielectric medium. These surface plasmon-polaritons (SPPs) display unique dispersion characteristics that depend upon the plasma frequency of the medium. In the long wavelength regime, where metals are highly conductive, such texturing can create an effective medium that can be characterized by an effective plasma frequency that is determined by the geometrical parameters of the surface structure. The terahertz (THz) spectral range offers unique opportunities to utilize such materials. While there has been significant work on developing coherent sources and detectors, relatively few other device technologies currently exist. A major issue that has constrained this development is the fact that most conventional dielectrics and semiconductors are highly lossy in this spectral range. Since metals are highly conductive, SPPs experience very low propagation loss when air acts as the adjacent dielectric medium.
This thesis describes a number of terahertz plasmonic devices, both passive and active, fabricated using different techniques. As an example, inkjet printing is exploited for fabricating two-dimensional plasmonic devices. In this case, we demonstrated the terahertz plasmonic structures in which the conductivity of the metallic film is varied spatially in order to further control the plasmonic response. Using a commercially available inkjet printers, in which one cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink, computer generated drawings of plasmonic structures are printed in which the individual printed dots can have differing amounts of the two inks, thereby creating a spatial variation in the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than bulk silver, while the carbon ink acts as a lossy dielectric at THz frequencies. Both inks sinter at room temperature immediately after contact with the plastic film. Using a periodic array of subwavelength apertures as a test structure, patterns printed with different fractional amounts of the two inks show dramatically different enhanced optical transmission properties. These differences arise from changes in the propagation loss properties as a function of conductivity. This data is used to design and fabricate aperture arrays in which the conductivity varies spatially. The resulting plasmonic effect is found to dramatically alter the spatial beam profile of the transmitted THz radiation, as measured by THz imaging. These plasmonic devices are passive devices.
The inkjet printing technique is limited to the two-dimensional structurers. In order to expand the capability of printing complex terahertz devices, which cannot otherwise be fabricated using standard fabrication techniques, we employed 3D printing. This technique is an additive manufacturing approach, which allows for the fabrication of complex terahertz devices using polymers. The printed structures are then coated with ~ 1 µm of gold on all sides, in order to make it a plasmonic device. Using this printing methodology, we fabricated both planar and non-planar terahertz waveguide devices, including 3D bends, 3D y-splitters and curved waveguides. For the purposes of comparison, we fabricated terahertz waveguide devices and compared them with the standard waveguide devices fabricated using laser ablation techniques with stainless steel films. We find excellent agreement between these two types of devices. At a stage where THz technology is still maturing, the development of devices where the propagation properties can be easily modulated holds great promise for the development of terahertz optoelectronic devices.
In the realm of active plasmonic devices, a wide range of innovative approaches have been developed utilizing a variety of materials including liquid crystals semiconductors, liquid metals, photochromic and electrochromic molecules and phase-change materials. One of the most heavily studied phase change materials for active plasmonic and metamaterial device implementations is vanadium dioxide, VO2, which undergoes a thermally-driven metal–insulator transition near room temperature associated with a structural change in its crystal symmetry. Phase transitions can lead to a variety of different macroscopic effects, which may be useful for active optical applications. As an example, shape memory alloys (SMAs) can be thermally cycled between different physical geometries. As an example, Nitinol, a nickel-titanium alloy, has been shown to be associated with a transformation between the martensite phase below the transition temperature and the austenite phase above the transition temperature. The two most commonly used approaches to control these transitions are referred to as one-way memory and two-way memory. In the former approach, an SMA that has been deformed returns to its original shape after being heated. This is most commonly demonstrated using wires, though numerous applications utilizing thin metal foils have also been shown. Two-way memory requires that the SMA undergo specific thermo-mechanical treatments, commonly referred to as training procedures, in order to thermally cycle between two different alloy geometries.
We discuss the use of SMAs for terahertz (THz) plasmonics that allows for switching between different physical geometries corresponding to different electromagnetic responses. We use Nitinol, a metal alloy of nickel and titanium composed of approximately equal atomic percentages, as the SMA medium that is structured to give the desired electromagnetic response. Nitinol has a DC conductivity of ~1.25 x 106 S/m for both phases which is similar to the value for stainless steel, making it well suited for THz plasmonic applications. As an SMA, it undergoes a structural transition between the martensite phase to the austentite phase that is bistable and reproducible at temperatures that are only slightly above room temperature. Using a two-way training protocol that we developed, we created samples that transition between either a one-dimensional (1D) or two-dimensional (2D) sinusoidally corrugated geometry and a flat substrate. In order to observe a plasmonic response, the foils are patterned either with a periodic array of subwavelength apertures or a single aperture and their transmission properties are measured using THz time-domain spectroscopy
Friday December 16, 2016
3-5 PM, ECE conference room
Merrill Engineering Building (MEB) room 2109
The public is invited