Giant Second Harmonic Generation Enhancement in a High-Q Doubly Resonant Hybrid Plasmon-Fiber Cavity System.

A high-quality plasmon-fiber cavity in a doubly resonant configuration can exhibit second-harmonic generation (SHG) with over 5 orders of magnitude enhancement compared to gold nanoparticles on a fused silica substrate. Through coupling to a fiber cavity with the proper diameter, a high-quality (Q ≈ 160) resonance can be achieved in combination with a single gold nanoparticle. In a classical picture, where the incident electric field travels coherently Q times around the fiber during the nonlinear process, the high Q of the coupled mode aids in highly efficient SHG. We accomplish two feats: First, we analyze the Q factor dependence of the SHG efficiency, proving the expected Q4 dependence and thus confirming coherent E-field amplification in the fiber cavity. Second, we carefully adjust the fiber size further and tune the plasmon response of a gold nanoparticle to a high-Q cavity mode. We make sure that the second harmonic wavelength is simultaneously in resonance with a higher order fiber cavity mode, fulfilling the doubly resonant condition. As a result, a giant SH response with conversion efficiency up to 1.6 × 10-5 is detected upon a pump intensity of 5 × 108 W/cm2 for 100 fs pump pulses around 840 nm incident wavelength. Additionally, the importance of the doubly resonant condition is proven by detuning the size of the fiber, which leads to a drastic drop in SHG efficiency. This disparity of the SHG efficiency can be observed even by eye, when monitoring the intensity changes of the visible SH light during detuning.