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Spintronics is the area of research that aims at utilizing the quantum mechanical spin degree of freedom of electrons in solid-state materials for information processing and data storage application. Since the discovery of the giant magnetoresistance, the field of spintronics has attracted lots of attention for its numerous potential advantages over contemporary electronics, such as less power consumption, high integration density and non-volatility. The realization of a spin battery, defined by the ability to create spin current without associated charge current, has been a long-standing goal in the field of spintronics. The demonstration of pure spin current in ferromagnet/nonmagnetic material hybrid structures by ferromagnetic resonance spin pumping has defined a thrilling direction for this field. As such, this dissertation targets at exploring the spin and magnetization dynamics in ferromagnet/oxide/semiconductor heterostructures (Fe/MgO/GaAs) using time-resolved optical pump-probe spectroscopy with the long-range goal of understanding the fundamentals of FMR-driven spin pumping. Fe/GaAs heterostructures are complex systems that contain multiple spin species, including paramagnetic spins (GaAs electrons), nuclear spins (Ga and As nuclei) and ferromagnetic spins (Fe). Optical pump-probe studies on their interplay have revealed a number of novel phenomena that has not been explored before. As such they will be the major focus of this dissertation. First, I will discuss the effect of interfacial exchange coupling on the GaAs free-carrier spin relaxation. Temperature- and field-dependent spin-resolved pump-probe studies reveal a strong correlation of the electron spin relaxation with carrier freeze-out, in quantitative agreement with a theoretical interpretation that at low temperatures the free-carrier spin lifetime is dominated by inhomogeneity in the local hyperfine field due to carrier localization. Second, we investigate the impact of tunnel barrier thickness on GaAs electron spin dynamics in Fe/MgO/GaAs heterostructures. Comparison of the Larmor frequency between samples with thick and thin MgO barriers reveals a four-fold variation in exchange coupling strength, and investigation of the spin lifetimes argues that inhomogeneity in the local hyperfine field dominates free-carrier spin relaxation across the entire range of barrier thickness. These results provide additional evidence to support the theory of hyperfine-dominated spin relaxation in GaAs. Third, we investigated the origin and dynamics of an emergent spin population by pump power and magnetic field dependent spin-resolved pump-probe studies. Power dependent study confirms its origin to be filling of electronic states in GaAs, and further field dependent studies reveal the impact of contact hyperfine coupling on the dynamics of electron spins occupying distinct electronic states. Beyond above works, we also pursue optical detection of dynamic spin pumping in Fe/MgO/GaAs heterostructures in parallel. I will discuss the development and progress that we have made toward this goal. This project can be simply divided into two phases. In the first phase, we focused on microwave excitation and optical detection of spin pumping. In the second phase, we focused on all-optical excitation and detection of spin pumping. A number of measurement strategies have been developed and executed in both stages to hunt for a spin pumping signal. I will discuss the preliminary data based upon them.

This thesis first presents a 26-channel channelizer based on the mammalian cochlea and covering the 20-90 MHz band. Each channel has a 6-pole frequency response with a constant absolute bandwidth of 1.4 MHz at 20-30 MHz, and a constant fractional bandwidth of 4.5±0.6% at 30-90 MHz, and is built entirely using lumped elements. Measurements show an S₁₁ < -12 dB at 20-90 MHz, a loss of 4-7 dB, > 40 dB isolation between the channels, and agree well with simulations. The applications areas are in communication systems with very high levels of interferes and in defense systems. In another project, tunable lumped-element bandstop filters for the UHF-band cognitive radio systems are presented. The 2-pole filters are implemented using lumped elements with both single- and back-to-back silicon varactor diodes. The single diode filter tunes from 470 to 730 MHz with a 16-dB rejection bandwidth of 5 MHz and a filter quality factor of 52-65. The back-to-back diode filter tunes from 511 to 745 MHz also with a 16-dB rejection bandwidth of 5 MHz and a quality factor of 68- 75. Both filters show a low insertion loss of 0.3-0.4 dB. Nonlinear measurements at the filter null with [delta]f = 2 MHz show that the back-to-back diode filter results in 12-dBm higher third order intermodulation intercept point (IIP3) than the single diode filter. A scaling series capacitor is used in the resonator arm of the back-to-back diode filter and allows a power handling of 25 dBm at the 16 dB rejection null. The cascaded response of two tunable filters is also presented for multi-band rejection applications, or for a deeper rejection null (> 36 dB with 0.6 dB loss at 600 MHz). The topology can be easily extended to higher-order filters and design equations are presented. The third project presents on-chip slot-ring and horn antennas for wafer-scale silicon systems. A high efficiency is achieved using a 100 [mu]m quartz superstrate on top of the silicon chip, and a low loss microstrip transformer using the silicon backend metalization. A finite ground plane is also used to reduce the power coupled to the TEM mode. The slot-ring and 1- [lambda]²/₀ horn achieve a measured gain of 0-2 dBi and 6- 8 dBi at 90-96 GHz, respectively, and a radiation efficiency of ̃50%. The horns achieve a high antenna gain without occupying a large area on the silicon wafer, thus resulting in a low cost system. The designs are compatible with either single or two-antenna transceivers, or and with wafer scale imaging systems and power-combining arrays. To our knowledge, this is the highest gain on-chip antenna developed to-date. Finally, differential on-chip microstrip and slot-ring antennas for wafer-scale silicon systems are presented. The antennas are fed at the non- radiating edge which is compatible with differential coupled-lines, and are built on a 0.13-[mu]m CMOS process with a layout which meets all the metal density rules. A high radiation efficiency is achieved using a 100 [mu]m quartz superstrate placed on top of the silicon chip. Both antennas achieve a measured gain of ̃3 dBi at 91-94 GHz, with a -10 dB S₁₁ bandwidth of 7-8 GHz and a radiation efficiency of >50%. The designs are compatible with single and multi-element transceivers, and with wafer-scale imaging systems and power combining arrays. To our knowledge, this is the first demonstration of high- efficiency on-chip differential antennas at millimeter- wave frequencies