Standoff detection of an electric field by bidirectional nitrogen lasing

Abstract

We report on standoff detection of DC electric field by bidirectional cavity-free lasing emission of neutral nitrogen molecules excited by intense circularly polarized femtosecond laser pulses. We observed that both the backward and forward 337.4 nm coherent lasing emission present a monotonous dependence on the strength of a remotely applied DC field up to ~ 1 kV/cm field strength. Moreover, this method shows a dependence on the polarity of the external DC field, providing a sensitive method for remote characterization of the electric field amplitude and direction. We attribute the underlying mechanism of lasing signal modulation to the electric field-induced electron acceleration/deceleration, which results in a variation of the kinetic energy of the free electrons and a modulation of the population inversion responsible for the nitrogen molecules lasing. The polarity sensitive detection anisotropy is interpreted by the symmetry breaking of the electron motion in the plane perpendicular to the laser propagation due to the injection of a weak second harmonic laser field produced in the quarter-wave plate for circularly polarized pump laser. Numerical simulations based on the two-dimensional time-dependent Schrodinger equation and the classic Newton equation for electron kinetic energy support our interpretation. This study provides a proof-of-principle method for standoff detection of electric fields based on nitrogen lasing, which can be potentially useful for atmospheric and metrological applications.