Third harmonic generation and microscopy, enhanced by a bias harmonic field

We present strong enhancements of the signal yield in third harmonic generation microscopy by seeding the optically nonlinear sample with some intensity at the third harmonic wavelength, in addition to the driving fundamental beam. By applying a third harmonic bias pulse with a power of less then 0.1% of the fundamental beam, we boost the signal yield by more than a factor of 3000, compared to the conventional third harmonic signal from the microscopy sample. The signal enhancement is most pronounced at low laser intensity and/or weak nonlinear susceptibilities. This makes the concept particularly suitable to improve the signal-to-noise ratio from samples with weak signals, e.g., as typical for applications of nonlinear microscopy. Moreover, we demonstrate improved spatial resolution in beam propagation direction by more than an order of magnitude. This exploits the dependence of our approach for enhanced third harmonic generation from inevitable dispersion in the sample. The improvement is most evident in cases, where the sample geometry allows only weak focusing.