The quantum well thickness—defined by the number of inorganic layers ($n$)—is a pivotal structural parameter that governs the optoelectronic properties of two-dimensional (2D) perovskites. Despite the established role of quantum well thickness in modulating linear optical responses, the influence on the nonlinear optical conductivity remains largely unexplored. Herein, we report a systematic investigation of terahertz (THz) emission from ${\left(\mathrm{PEA}\right)}_2{\left(\mathrm{MA}\right)}_{n-1}\mathrm{P}{\mathrm{b}}_n{\mathrm{I}}_{3n+1}$ perovskite thin films with controlled quantum well thicknesses ($n=1-4$ and $\infty$) under normal-incidence excitation by a two-color laser field (800 + 400 nm). Our measurements reveal a pronounced inverse correlation between $n$ and THz emission efficiency, directly attributable to enhanced quantum and dielectric confinement at reduced layer numbers. Spectroscopic analysis confirms that the emission arises from a third-order nonlinear optical process, wherein transient photocurrents are generated via quantum interference between single-photon and two-photon absorption pathways. Polarization-resolved THz emission spectroscopy enables quantitative mapping of the nonlinear optical conductivity tensor across the 2D-to-3D structural transition. Notably, by structurally tuning the quantum well thickness $n$ and dynamically controlling the relative polarization angle of the two-color field, we achieve control over the magnitude and direction of the injection photocurrent—and consequently, the polarity of the emitted THz field. This work establishes THz emission spectroscopy as a powerful, contact-free probe of third-order nonlinear conductivity in low-dimensional quantum materials and underscores the promise of 2D perovskites as highly tunable platforms for next-generation THz emitters and nonlinear optoelectronic devices.

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• Nonlinear optics
• Photocurrent
• 2-dimensional systems
• Terahertz sources
• Terahertz spectroscopy
• Terahertz techniques