  We report on the proof-of-principle experiment of generating carrier-envelope
phase (CEP)-controllable and frequency-tunable narrowband terahertz (THz)
radiation from air-plasma filament prescribed by the beat of temporally
stretched two-color laser pulse sequence. The pulse sequence was prepared by
propagating the fundamental ultrafast laser pulse through a grating stretcher
and Michelson interferometer with variable inter-arm delay. By partially
frequency-doubling and focusing the pulse sequence, an air-plasma filament
riding a beat note was created to radiate THz wave with primary pulse
characteristics (center frequency and CEP) under coherent control. To reproduce
experimental results and elucidate complex nonlinear light-matter interaction,
numerical simulation has been performed. This work demonstrates the feasibility
of generating coherently controlled narrowband THz wave with high tunability in
laser-induced air plasma.
