This technology relates to detecting terahertz radiation using an optical beam in a volume of gas. Ionizing a portion of the volume of gas with the optical beam produces a plasma and fluorescence is produced from an interaction of a radiation wave with the plasma. Information contained in the characteristics of the detected fluorescence can be used to characterize the radiation wave. Terahertz (THz) waves occupy a segment of the electromagnetic spectrum between the infrared and microwave bands. As such, they can be used for imaging and sensing in ways not possible with conventional technologies such as X-ray and microwave. Because THz radiation transmits through almost anything that is not metal or liquid, the waves can see through most materials that might be used to conceal explosives or other materials, such as packaging, corrugated cardboard, clothing, shoes, backpacks, and book bags.