Browsing by Author "Mittleman, Daniel M"

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    Devices Based on Parallel-Plate Waveguides for Terahertz Applications
    (2016-04-20) Reichel, Kimberly Suezanne; Mittleman, Daniel M
    The promise of terahertz (THz) frequencies for technological applications is wide, spanning from wireless communications for faster downloads to non-destructive imaging for security screening. Although the potential is high, there is a lack of the basic devices necessary to make these prospects a reality. One essential component for any electromagnetic wave technology is a waveguide, which as the name implies can guide light waves, like a hose would direct water from the source to the desired target location. Several waveguide types have been introduced for THz frequencies, one of the most promising of which is the parallel-plate waveguide (PPWG). The PPWG is attractive based on its superior waveguiding performance of efficient input coupling and low losses, but additionally it serves as an excellent platform for other purposes. The projects presented in this dissertation highlight a few new functionalities incorporated into, and enabled by, a PPWG for sensing, filtering, and splitting. First, we characterize a high quality factor resonator integrated into a PPWG used for microfluidic sensing. Typically, the characterization of the frequency-dependent electric field profile inside a narrowband resonator is challenging, either due to limited optical access or to the perturbative effects of invasive probes. In our situation however, the geometry of the PPWG allows for direct access to the resonant cavity via the open sides of the waveguide and a novel implementation of the air-biased coherent detection (ABCD) method permits non-invasive probing. Through both experiment and simulation, we see the narrowband frequencies trapped in the resonator and also discover an unexpected broadband asymmetric field distribution due to the resonator inside the waveguide, yielding new information that is not available in the far field. Second, we investigate a narrowband tunable filter based on extraordinary optical transmission (EOT) through a 1D array of subwavelength holes inside a PPWG. EOT is an effect where at a particular frequency 100% of the light is transmitted through an array of holes with diameters much smaller than the wavelength. We demonstrate that the output resonant frequency depends strongly on the input mode of the waveguide, where excitation with the TEM waveguide mode mimics EOT in a 2D array in free space, while the TE1 waveguide mode is vastly different. Through this disparity of outcomes between the two different waveguide excitation modes, we can better understand the resonant transmission process. We show that the surface plasmon theoretical description is invalid for the TE1 resonance, and instead use impedance matching to properly predict the resonances in both TE1 and TEM. Additionally, we show that the device can be used as a tunable filter at THz frequencies by simply changing the separation between the two waveguide plates. Third, we demonstrate a THz variable power splitter based on a PPWG T-junction excited by the TE1 waveguide mode. By integrating a small triangular septum into the waveguide plate, we are able to direct the THz light down to either one of the two output channels with precise control over the coupling ratio between the waveguide outputs. We find good agreement between experiment and simulation in both amplitude and phase. We show that the coupling ratio varies exponentially with the septum translation offset and that nearly 100% transmission can be achieved. The splitter operates over almost the entire range in which the waveguide is single mode, providing a sensitive and broadband method for power splitting. By incorporating our innovations along the already propagating path of THz waves inside a waveguide, we establish multiple functional capabilities into one universal platform. The hope of this work is that these devices will ultimately serve as fundamental building blocks to make everyday THz applications a reality.
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    Picosecond Digital-to-Impulse (D2i) Radiators in Silicon
    (2014-11-24) Assefzadeh, Mahdi; Babakhani, Aydin; Kono, Junichiro; Mittleman, Daniel M
    Current techniques for terahertz (THz) time-domain spectroscopy (TDS) are based on femtosecond lasers and photoconductive antennas (PCAs). The PCA is the emitter and the detector of THz pulses and includes a THz antenna which is fabricated on a high mobility semiconductor substrate. THz-TDS techniques are used for 3D imaging and non-destructive evaluation of materials in pharmaceutical, medical and security applications. There are a number of limitations with current THz-TDS systems. Femtosecond lasers are expensive and bulky with high power consumption. They also require optical alignments. The delay line and object scanning are performed mechanically. The repetition rate and the radiated power are also limited. A single-chip impulse radiator in silicon can overcome these limitations. It is a high yield and low cost solution and can provide repetition rates of up to 10 GHz. A low power digital trigger is needed instead of an optical pump, without requiring lasers or optical alignments. In this work, direct digital-to-impulse (D2i) radiators are implemented in silicon technologies that can radiate sub-10psec impulses with on-chip antennas. System architecture, broadband phase-linear antenna design, circuit techniques, simulations and measurement results are discussed in this thesis. Also a full-system on-chip 4 by 4 array of D2i radiators are fabricated in silicon that provide beam-steering and spatial coherent combining of impulses.