Two-color photoionization has been widely used as a versatile tool for intense, broadband terahertz (THz) radiation generation. In this scheme, an ultrashort laser's fundamental and its second harmonic pulses are co-focused into a gas of atoms or molecules, transforming them into plasma by photoionization. This process produces an intense THz pulse emitted in the forward direction. The main focus of this dissertation is to provide a physical understanding of such THz generation and investigate its generation mechanism at both microscopic and macroscopic levels.

First, we examine the generation process by measuring the relative phase between two-color (fundamental and second harmonic) laser fields and the resulting THz field simultaneously. We discover that a relative phase of π/2 yields maximal THz outputs, consistent with a semi-classical plasma current model. We find that this optimal relative phase is independent of laser intensities, gas species, and two-color laser amplitude ratios. We also measure concurrent near-field photocurrents. All these measurements verify laser-produced plasma currents as a microscopic source for THz generation.

We also investigate THz radiation from an ensemble of aligned air molecules in two-color laser fields. Our experiments show that THz radiation is strongly affected by molecular (nitrogen and oxygen) alignment. We explain this phenomenon in the context of the plasma current model combined with alignment-dependent ionization.

Phase-matching is essential to achieve high-efficiency nonlinear frequency conversion. We discover THz generation by two-color photoionization in elongated air plasmas (filamentation) is naturally phase-matched in the off-axis direction, resulting in donut-shaped radiation profiles in the far field. Because of this off-axis phase-matching, THz yields increase almost linearly with the filament length, scalable for further THz energy enhancement.

Lastly, we study the polarization of emitted THz radiation. In the case of in-line focusing geometry, we observe the polarization evolves from linear to elliptical with increasing plasma length. This ellipticity arises from two combined effects--successive polarization rotation of local THz plasma sources, caused by laser phase and polarization modulations, and the velocity mismatch between laser and THz, which produces an elliptical THz pulse from a series of time-delayed, polarization-rotating local THz fields.