Polarization-based tachometer for measuring spinning rotors

• Published in: Optics express Vol. 26; no. 24; pp. 31839 - 31849
• Main Authors: Prost, Emilien, Hertz, Edouard, Billard, Franck, Lavorel, Bruno, Faucher, Olivier
• Format: Journal Article
• Language: English
• Published: United States Optical Society of America - OSA Publishing 26.11.2018
• Subjects: Chemical Physics; Optics; Physics
• ISSN: 1094-4087; 1094-4087
• DOI: 10.1364/OE.26.031839

We report on the polarization analysis of shortpulse ultraviolet radiation produced by third-harmonic generation in a gas of coherently spinning molecules. A pulse of twisted linear polarization imprints a unidirectional rotational motion to the molecules leading to an orientation of their rotational angular momenta. A second pulse, time-delayed with respect to the first one, circularly polarized in the plane of rotation of the molecules, acts as a driving field for third-harmonic generation. The angular momentum and energy conservation applied to this process foresees the generation of two Doppler-shifted circularly-polarized harmonics of opposite handedness. Our analysis reveals that spinning molecules enable the generation of a well polarized third-harmonic radiation exhibiting a high degree of ellipticity. Tracking the orientation of the latter allows a time-capture of the molecular axis direction from which the average angular velocity of the rotating molecules is inferred. This method provides a user-friendly polarization-based tachometer for measurement of the rotational speed of spinning nonlinear rotors.