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.

Fig. 1

Experimental setup and waveguides. a Experimental set-up. TLS, tuneable laser source; MZM, Mach-Zehnder modulator; EDFA, erbium-doped fibre amplifier; BPF, fibre band pass filter; LF, lensed fibre; CCD, digital camera; BF, free-space block filter assembly; PD, power detector; SH, second harmonic; TH, third harmonic. b Pump (magenta line) and second harmonic (blue line) spectra at the waveguide output for a coupled peak power of 90 W, after the second harmonic growth. No significant pump broadening is observed. c Scanning electron microscope picture of a waveguide cross-section (scale bar: 0.5 μm). d Simulation of the pump profile, injected on the fundamental transverse magnetic mode

Fig. 2

Second harmonic enhancement over time under pulsed pumping. a Growth curves of the second harmonic generation (SHG) average power over time in waveguide (i), for pump wavelengths of 1539 nm (magenta line), 1544 nm (blue line) and 1549 nm (black line). The coupled peak power is 90 W. The pictures show the light scattered at the end facet, coming either from second harmonic generation (SHG, red light) or third harmonic generation (THG, green light) The edge of the chip is indicated by the dashed white line (scale bar: 20 μm). The labels (A)-(D) indicate the instant and for which pumping wavelength the picture was taken. b Power of the transverse electric (TE) or transverse magnetic (TM) SHG component over time when the pump polarization is switched, at the constant wavelength of 1544 nm, from the TE to TM mode, and back to TE in waveguide (i). At each switching point the polarizer is first rotated by 90°, entailing a measured SHG power drop, then the pump polarization is aligned parallel to the polarizer axis, triggering the growth of the second harmonic component. Blue line: power of TE SH under TE pump. Magenta line: power of TM SH under TM pump. c SHG growth over time, for pump wavelengths of 1537 (cyan line) and 1550 (black line) nm in waveguide (ii). The coupled peak power is varied between 60 (solid line) and 90 W (dash line). The inset shows the light scattered at the end facet once saturation is reached, with very little green emission

Fig. 3

Quasi-phase-matching evidence under continuous-wave probing. a Detected second harmonic power as a function of the probe wavelength in waveguide (i) for a continuous-wave (CW) coupled probe power of 350 mW. The waveguide was previously pumped at 1539 nm (magenta line), 1542 nm (cyan line), 1544 nm (blue line) and 1549 nm (black line). Phase-matching peaks are observed around each pump wavelength. b Detected second harmonic power as a function of CW probe wavelength for 130 mW of coupled power in waveguide (ii), previously pumped at either 1537 nm and 90 W coupled power (blue line) or 1550 nm and 60 W coupled power (magenta line)

Fig. 4

Grating dynamics and evaluation of its parameters. a Illustration of the χ ⁽²⁾ grating inscription in a SiN waveguide. After irradiation, a spatially periodic DC field builds up with a periodicity that is twice the coherence length between the pump and second harmonic mode. ω _p indicates the pump frequency. b Persistence measurement showing the continuous wave probe (blue triangles) and second harmonic (green circles) power over more than 80 h of operation. Both quantities were measured at the waveguide output. c Second harmonic power (estimated in waveguide (i)) as a function of the coupled continuous wave probe power. The probe is centred at 1544 nm, and the grating was previously inscribed at the same wavelength. The squares are experimental points while the red line is a linear fit with a slope of 2. The observed saturation at high power comes from coupling instabilities