Broadband spectral mapping of photo-induced second-harmonic generation in silicon nitride microresonators

TL;DR

This work addresses the limited SHG bandwidth in high-Q Si3N4 microresonators by exploiting photo-induced χ^(2) nonlinearity with a novel pump-probe spectral mapping technique. By tracking detunings of the pump and SH resonances over a broad frequency range, the authors demonstrate highly detuned SHG with a substantial bandwidth and provide experimental evidence for the detuning condition δ_a < 0 and δ_b < 0 that enables all-optical poling. The approach reveals broadband SHG and multimode interactions without relying on strict doubly resonant conditions or group-velocity matching, highlighting potential for integrated χ^(2) photonics, coupling with χ^(3) processes, and on-chip optical clocks. These findings pave the way for monolithic integration of octave-spanning comb generation and f-2f self-referencing in Si3N4 microresonators, with implications for compact, scalable nonlinear photonic platforms.

Abstract

By employing a pump-probe technique for enhanced spectral mapping of the dynamics in nonlinear frequency conversion, we demonstrate that photo-induced second-harmonic generation (SHG) in silicon nitride (Si3N4) microresonators can persist when transitioning from the preferred doubly resonant condition--where the resonances of the optical harmonics are required to be matched--to a highly detuned state where the generated second harmonic is significantly shifted away from its corresponding resonance. This results in an unconventionally broad conversion bandwidth. Other intriguing phenomena, such as detuning-dependent all-optical poling and nonlinear multi-mode interaction, are also presented for the first time with direct experimental evidence. Our findings provide new insights into the physics of photo-induced second-order (χ^{(2)}) nonlinearity, highlighting its potential applications for nonlinear χ^{(2)} photonics in integrated Si3N4 platform

Figures (4)

• Figure 1: Characterization setup of the photo-induced second-harmonic using pump-probe technique. (a) Experimental setup for photo-induced SHG in Si$_3$N$_4$ microresonators and measuring the detunings of pump and SH resonances. ECDL: external-cavity diode laser; IM: intensity modulator; EDFA: erbium-doped fiber amplifier; FPC: fiber polarization controller; DM: dichroic mirror; PD: photodetector; BS: beam sampler; OSC: oscilloscope; LIA: lock-in amplifier. (b) Working principle of the pump-probe technique. The weak probe signals are up-converted via sum-frequency generation (SFG). The resulting beatings between the SFG signals illustrated within the dashed grey box at the SH resonance is picked up by the lock-in amplifier.
• Figure 2: Experimental (a-b) and theoretical (c-d) investigations of highly detuned second-harmonic generation involving TE$_{00}$ pump and TE$_{30}$ SH modes. (a) Measured lock-in response map when scanning the pump in and out of resonance. The response map is obtained by changing the pump laser wavelength in discrete steps, while the varying probe laser wavelength is adjusted accordingly. The triangular pump transmission, or thermal triangle, originates from pump-induced thermal and Kerr effects. AOP, all-optical poling. (b) Transmitted pump power and the generated SH power in the bus waveguide as functions of pump wavelength. (c) Theoretically retrieved lock-in response map using Eq. (\ref{['H_Omega']}), with the detunings extracted from (a-b) for $\delta_{\mathrm{a,b}}<0$. Before the AOP onset ($\delta_{\mathrm{a}}/\kappa_{\mathrm{a}}<-0.6, \delta_{\mathrm{b}}>0$), the SH detuning is set to change linearly. The pre-poling dynamics before the AOP onset shown in (a) are excluded for simplicity. The red (yellow) arrows denote the pump (SH) power, respectively. (d) Simulated transmitted pump power and the generated SH power in the bus waveguide.
• Figure 3: Lock-in response maps for SHG at different resonances in the same microresonator. (a) The case that the SH branch approaches the pump branch, and the SHG is prohibited when $\delta_{\text{b}}>0$. The red (yellow) arrows denote the pump (SH) power, respectively. (b) The case where SHG involves the high-order SH mode (TE${_{40}}$), as indicated by the broad SH branch.
• Figure 4: SHG involving multiple SH modes during a wavelength sweep in a pump resonance. (a) The SH1 branch is interrupted when the doubly-resonant conditions of the other two SH modes are successively satisfied. The SH3 branch is indistinct. (b) The SH power trace exhibits discontinuities when SH mode competition or hopping occurs. Insets, the scattering patterns of the generated visible lights (scale bars, 200 $µm$). The red (yellow) arrows denote the pump (SH) power, respectively. (c) Lock-in responses at the linear scale, sliced from (a). The pump wavelengths (from $\rm{I}$ to $\rm{VI}$) are respectively 1545.60, 1545.67, 1545.68, 1545.72, 1545.76, and 1545.80 nm. In stage III, SHG occurs in both SH1 and SH2 modes.