Methods and systems for detecting radiation for example, terahertz radiation, with the aid of acoustic signal generation and detection include: directing an optical beam into a volume of gas; ionizing at least a portion of the volume of gas with the optical beam to produce a plasma; and detecting an acoustic signal produced from an interaction of a radiation wave with the plasma. The methods and systems are particularly adapted for remote detection of chemicals, biological substances, and explosives, among others. The capability of the methods and systems can be enhanced by employing multi-color laser excitation to produce the plasma and varying the time delay between the multi-color pulses.