Abstract
A numerical study of second-harmonic generation (SHG) in cavity resonator integrated grating filters (CRIGFs) shows extraordinary strong conversion rates in planar corrugated waveguides based on lithium niobate layers, not exceeding 625 µm in length. The key idea is to couple to the dark mode of the structure at the pump frequency that has a Q-factor much stronger than the bright mode commonly used. This is achieved by shifting the grating coupler between the enclosing Bragg mirrors, resulting in an asymmetric structure. Three asymmetric structures are proposed: without any matching, with impedance matching and with indirect phase matching through an additional pair of gratings. They offer respectively increasing conversion rates of η=4.31 × 10−3 W-1, 0.149 W-1 and 2.38 W-1, which compared to a symmetric CRIGF (η=7.12 × 10−6 W-1) offer respectively more than 2, 4 and 5 orders of magnitude improvement of the conversion rate.
Highlights
Nonlinear optical frequency conversion and, in particular, second-harmonic generation (SHG), plays an important role in laser systems, infrared imaging, and signal detection
We show that shifting the central grating couplers (GCs) within the FP resonator, thereby introducing an asymmetry, facilitates the excitation of a dark mode with high Q-factor while preserving the incidence close to normal, and maintaining strong focusing of the incident light
We demonstrate a mode-amplitude enhancement inducing more than 5 orders of magnitude improvement in conversion efficiency η compared to the more conventional symmetric cavity resonator integrated grating filters (CRIGFs) design
Summary
Nonlinear optical frequency conversion and, in particular, second-harmonic generation (SHG), plays an important role in laser systems, infrared imaging, and signal detection. Weak conversion rates allow for the so-called undepleted pump approximation that neglects the inverse up-conversion in which the signal is sufficiently strong to influence the pump field In this approximation, the generated SHG power is proportional to the square of the pump power or equivalently to the 4th power of the waveguide-mode amplitude at the pump frequency, pointing out the importance of efficient light coupling. In almost all the previous studies, the structures are designed to make use of the so-called bright waveguide modes, for which electric field extrema are aligned with the center of the grooves and bumps It is well-known that dark modes [20] have Q-factors largely exceeding those of bright modes, because the former are weakly excited the imaginary part of their propagation constant is small. Η improves by more than 5 orders of magnitude (compared to the symmetrical CRIGF)
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