27/02/18: Kaloyan Zlatanov: Optically stimulated second and third harmonic generation

Coherent nonlinear microscopy (CNM) is widely used in three-dimensional imaging of transparent samples, particularly for biological tissues [1]. Examples for parametric and label free CNM processes are second harmonic generation (SHG) and third harmonic generation (THG). However, working under far off-resonant excitation conditions usually leads to low optical conversion efficiencies. Hence, the typically low signals may lead to a bad signal-to-noise ratio and low image contrast. There are several approaches to enhance the signal yield in CNM, e.g. using dispersion to optimize the phase-matching integral [2]. One promising concept is the “Enhancement of Second-Order Nonlinear-Optical Signals by Optical Stimulation” [3]. The basic idea is to stimulate a nonlinear signal by already “seeding” with some  radiation at the harmonic frequency. The previous and first demonstration of the concept for SHG yielded signal enhancements more than 10  in a biologically relevant medium. We present strong enhancements of third harmonic generation by optical stimulation in a microscopy setup. The effect is most pronounced at low laser intensity and weak nonlinear susceptibilities, making it suitable in harmonic microscopy.

[1] S. Yue, M.N. Slipchenko, J.-X. Cheng, “Multimodal nonlinear optical microscopy”, Laser Photon. Rev. 5, 496–512 (2011).
[2] Christian Stock, Kaloyan Zlatanov, and Thomas Halfmann, “Dispersion-enhanced third-harmonic microscopy”, Optics Communications Vol. 393, 289-293 (2017).
[3] A. J. Goodman and W. A. Tisdale, “Enhancement of Second-Order Nonlinear-Optical Signals by Optical Stimulation”,  Phys. Rev. Lett. 114, 183902 (2015).