350-GHz-Band 4 by 4 RTD Monostatic Radar Array for Sequential Multidirectional Ranging

Terahertz (THz) sensing offers significant potential for nondestructive evaluation, imaging, and metrology, but the cost and complexity of existing systems remain barriers to practical deployment. This letter presents a compact 4 by 4 monostatic radar array based on a single-RTD-per-pixel architecture, in which each resonant tunneling diode (RTD) functions as both a bias-tunable oscillator and a self-mixing detector. The RTD elements are sequentially addressed through a low-frequency switching network and share the same baseband control and readout electronics. A shared 3D-printed dielectric lens maps the RTD elements to spatially separated sensing directions. We experimentally verify the operation of all array elements and demonstrate sequential multidirectional ranging using four selected elements, followed by preliminary 16-pixel THz imaging. By combining self-mixing at each pixel with low-frequency selection and shared optics, the array avoids a separate THz receiver chain for each pixel and provides a compact architecture for electronically addressable THz sensing and imaging.