Large second harmonic generation enhancement in Si3N4 waveguides by all-optically induced quasi-phase-matching

Abstract

Efficient second harmonic generation in integrated platforms is usually achieved by resonant structures, intermodal phase-matching or quasi-phase matching by periodically poling ferroelectric waveguides. However, in all these structures, it is impossible to reconfigure the phase-matching condition in an all-optical way. Here, we demonstrate that a Watt-level laser causes a periodic modification of the second-order susceptibility in a silicon nitride waveguide, allowing for quasi-phase-matching between the pump and second harmonic modes for arbitrary wavelengths inside the erbium band. The grating is long-term inscribed, and leads to a second harmonic generation enhancement of more than 30 dB. We estimate a χ (2) on the order of 0.3 pm/V, with a maximum conversion efficiency of 0.05% W−1. We explain the observed phenomenon with the coherent photogalvanic effect model, which correctly agrees with the retrieved experimental parameters.

Self-organized nonlinear gratings for ultrafast nanophotonics

“…Interestingly, in some cases, we see strong temporal oscillations in the intensity of the second harmonic. Similar oscillations were also observed in other studies of silicon nitride waveguides 9,10 . In our waveguides, the oscillations typically become faster when the laser intensity is increased ( Supplemental Fig.…”

“…However, decreasing the power below 40 mW allows the second harmonic to build up once again. This behavior indicates that the SHG results from the formation of a SONG through the coherent photogalvanic effect 7,9,10 .…”

Photo-induced second-order nonlinearity in stoichiometric silicon nitride waveguides

“…The measured conversion efficiency corresponds to a high equivalent second-order susceptibility, χ ( eq 2) = 3.7±0.2 pm/V. Although highly Si-rich SiN thin films might be expected to yield a somewhat larger value as concluded from photoelectron spectroscopy ( χ ( eff 2) = 11.8 pm/V [25]), the second order susceptibility that we find is larger than in SiN thin films ( χ ( eff 2) = 2.5 pm/V [24]), is one order of magnitude larger than in stress-released Si 3 N 4 waveguides ( χ ( eff 2) = 0.3 pm/V [27]) and more than two orders of magnitude larger than in Si 3 N 4 ring resonators ( χ ( eff 2) < 0.04 pm/V) [26].…”

“…On the other hand, there have been four reports on second-order response in related amorphous SiN-type materials. Specifically, second-harmonic generation has been observed in SiN films (fabricated using plasma enhanced chemical vapor deposition [24] or fabricated with RF sputtering [25]), in Si 3 N 4 ring resonators (fabricated with plasma enhanced chemical vapor deposition at lower temperatures [26]) where the nanoscale structure of the waveguide breaks the inversion symmetry and modal phase matching is employed, and in Si 3 N 4 waveguides fabricated using stress release patterns [27] where, similar to our work, the coherent photogalvanic effect breaks the inversion symmetry. However, we find an equivalent secondorder response that is an order of magnitude larger.…”

Broadband 200-nm second-harmonic generation in silicon in the telecom band

“…Most CMOS materials are either centrosymmetric (semiconductors) or isotropic (oxides) and therefore do not exhibit a bulk second-order susceptibility (χ (2) ), except at interfaces where the inversion symmetry is broken-a fact that is used for sensitive interface and surface characterizations [17][18][19] . There have been efforts to observe bulk χ (2) effects by inducing stress [20][21][22] or optical poling 23 in the material; however, the net conversion efficiency is low either due to a weak χ (2) or the lack of phase matching between the signal and the pump. Recently, by employing an electric-field-induced second harmonic 24,25 , and quasi-phase matching (QPM) scheme 26,27 , we demonstrated a strong χ (2) susceptibility (41 pm/v) and efficient SHG up to 13%/W at 1.15 µm 28 .…”