Browsing by Author "Assefzadeh, Mahdi"

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    Broadband Terahertz Signal Generation and Radiation Based on Direct Digital-to-Impulse Radiating Arrays in Silicon
    (2018-04-16) Assefzadeh, Mahdi; Babakhani, Aydin; Knightly, Edward
    Broadband terahertz (THz) signal generation and radiation has unique applications in 3-D hyper-spectral imaging, molecular sensing, and high-speed wireless communication. THz waves interact with the rotational and vibrational transitions of molecules with applications in material detection and biomedical sensing. They also penetrate through non-metallic and non-polar mediums that can be used to image concealed objects for security purposes. Conventional terahertz pulse generation techniques are based on the optical excitation of a III-V photoconductive antenna with a femtosecond optical laser pulse. This method of THz pulse generation and detection is widely used in THz time-domain spectroscopy systems. Although THz-TDS is a powerful technique, its dependence on bulky, expensive and power-hungry femtosecond lasers, optomechanical components, and costly photoconductive antennas compromises its speed, accessibility and scalability. In this dissertation, an on-chip laser-free direct digital-to-impulse (D2I) architecture is introduced that is capable of radiating a THz pulse by creating and exciting a broadband radiating resonator consisting of an on-chip antenna and a broadband matching network. This novel method converts a digital trigger edge to a radiated THz pulse with a high timing accuracy. A broadband matching network and an ON/OFF impulse-shaping technique are designed to maximize the amplitude of the pulse and minimize ringing. This method achieves a high DC-to-radiated efficiency by turning off the current switch shortly after turning it on. The deep nonlinear switching mechanism results in numerous harmonics from GHz to THz. Based on the high timing accuracy of the radiated THz pulses in D2I, a novel trigger-based beamforming architecture is introduced that enables broadband pulse beamforming in which all frequency content is steered simultaneously. This is in contrast with conventional phased-array architectures that have a limited bandwidth, where an RF signal is time-delayed. One of the main challenges of sampling a picosecond pulse in the time domain is ensuring that both the receiver and its antenna are broadband and have a linear phase response. Pyramidal horn antennas cannot be used to receive picosecond pulses, due to their limited bandwidth and nonlinear phase response. In addition, commercially available sampling oscilloscopes have a 3-dB bandwidth of less than 70 GHz, therefore cannot be used to sample a pulse with a FWHM of \textasciitilde 2 ps. To address this problem, we propose a direct time-domain measurement scheme based on femtosecond-laser-based THz sampling systems. Having a high-power, broadband frequency-comb source is critical in imaging and spectroscopy applications. By applying a periodic trigger signal, the D2I architecture radiates an impulse train in the time domain, which has a frequency-comb spectrum with a spacing of $1/T$. To perform spectroscopy, T can be controlled to sweep the whole spectrum. Further in this thesis, we will present our THz pulse radiating chips applied in imaging and spectroscopy experiments. This is the first time that a fully electronic chip is capable of generating and radiating harmonic tones at frequencies higher than 1 THz. Owing to the high scalability of the D2I architecture, combined with the broadband pulse beam-forming method, large arrays of D2I radiators can be built to radiate high-power, steerable narrow beams. In this thesis, ultra-short pulse radiating sources will be presented that were used to demonstrate laser-free broadband gas spectroscopy and THz imaging.
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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.