Reconfigurable Resonant Multimode Nonlinear Coupling for UV-to-infrared Frequency Generation

Abstract

On-chip coherent visible and near-infrared (NIR) light generation has broad applications in metrology, bio-sensing, and quantum information. High-Q microresonators are ideal candidates for generating light across such broad wavelength ranges via efficient second- ($χ^{(2)}$) and third-order ($χ^{(3)}$) nonlinear optical processes. However, harnessing these diverse nonlinearities simultaneously in a single microresonator remains elusive yet highly attractive both fundamentally and technologically. Here, we demonstrate coherent light generation from the ultraviolet to NIR in a silicon nitride microresonator pumped by a single continuous-wave telecom laser. This broad frequency generation arises from the interplay of $χ^{(2)}$ and $χ^{(3)}$ nonlinear processes. A cascade of nonlinear processes, including harmonic generation and optical parametric oscillation (OPO), is initiated by the photoinduced second harmonic generation enabled by all-optical poling. The dynamic reconfigurability of this $χ^{(2)}$ nonlinearity enables access to different transverse spatial modes at the second harmonic, enabling highly tunable OPO processes triggered by hybrid modal phase matching conditions and yielding milliwatt-level NIR light. This work sheds new insights into the fundamental physics of cooperative nonlinear multimode interactions in resonant systems and provides a versatile approach for reconfigurable OPOs, highlighting their potential to generate light at wavelengths beyond the reach of photonic integrated lasers.

Figures (5)

• Figure 1: UV-to-NIR coherent light generation in a high-Q Si$_3$N$_4$ microresonator from a single telecom pump.a. Illustration of optically reconfigurable second-harmonic generation in a Si$_3$N$_4$ microring resonator: the photo-induced nonlinearity from all-optical poling self-organizes into a QPM $\chi^{(2)}_\text{eff}$ grating whose spatial profile is determined by the nonlinear interference between the TE00 pump and distinct SH modes (TE00, TE10, TE20...) enabling efficient SHG across various SH modes. b. A hybrid optical parametric oscillation with two harmonically spaced pumps ($\omega$-2$\omega~$ hOPO) can generate signal ($\omega_{\mathrm{s}}$) and idler ($\omega_{\mathrm{i}}$) spectrally located between the pump frequencies. Dynamical switching between different SH modes alters the phase-matching condition for the $\omega$-2$\omega~$ hOPO process through re-configuration of the nonlinear grating, providing wide tunability of the signal and idler frequencies across the NIR range. c. Simultaneous second-, third-, and fourth-harmonic generation achieved via intermodal phase-matched $\chi^{(3)}$ (green arrows) or $\chi^{(2)}_\text{eff}$-mediated (blue arrows) processes. d. Visible light generation beyond pump harmonic processes triggered by the $\omega$-2$\omega~$ hOPO-generated signal and idler waves through further nonlinear interactions such as stimulated four-wave mixing, sum-frequency generation, and second-harmonic generation yielding yellow, orange, and blue colors. e. Top-view images of the visible light scattered from the microresonator.
• Figure 2: Experimental investigation of AOP-enabled SHG and cascaded nonlinear processes.a. Schematic of the experimental setup. The second and third dichroic mirrors (DMs) are removable to allow for either signal and SH power detection or broadband UV-to-NIR spectra acquisition. ECDL, external-cavity diode laser; EDFA, erbium-doped fiber amplifier; FPC, fiber polarization controller; PD, photodetector; OSA, optical spectrum analyzer. b. On-chip pump transmission, SH power, and signal (800-1100 nm) power during a wavelength sweep across the pump resonance at 1,587.3 nm. c. Two-photon microscopy images of the nonlinear $\chi^{(2)}_\text{eff}$ gratings along the ring circumference. The microresonator was poled at pump wavelengths corresponding to spectral positions II and IV in panel b. Both gratings feature a period of $\Lambda=15.5$ µm, confirming the generation of SH in the TE40 mode. d. UV-to-NIR spectral map obtained by tuning the pump wavelength across the same resonance shown in b. e. Slices of the spectral map in d at pump wavelengths of 1,587.489, 1,587.508, 1,587.521, and 1,587.546 nm (positions I to IV). Two distinct signal/idler pairs are observed at approximately 904.4/1,275.7 nm and 965.2/1,171.5 nm. The orange (599.6 nm), yellow (575.2 nm), and blue (452.2, 482.6, 482.8 nm) spectral lines are indicated by corresponding colored arrows. They however have poor extraction and out-coupling efficiency to the OSA.
• Figure 3: Simulation analysis of dispersion and phase-matching for the$\omega$-2$\omega~$hOPO.a. Effective refractive indices ($n_\text{eff}$) for TE and TM modes up to the seventh order. Avoided mode crossing and mode hybridization occur at the intersections of specific TE and TM modes. b. Group velocity dispersion ($\beta_2$) for TE and TM modes. Most modes exhibit normal dispersion in the NIR range, while anomalous dispersion is confined to the TE30, TE40, and TE50 modes within the SH and signal (gray-shaded area) bands. Avoided mode-crossings cause strong local perturbation of $\beta_2$. c. Graphical identification of signal and idler wavelengths for the $\omega$-2$\omega~$ hOPO, driven by the TE00 pump at 1587.5 nm (violet dot) and the corresponding TE40 SH (green dot). The dispersion curves $\beta (\nu)$ are transformed to $\Delta \beta (\Delta \nu)$ (solid lines) using the relations $\Delta\beta = \beta - (\beta_\mathrm{pump} + \beta_\mathrm{SH})/2$ and $\Delta\nu = \nu - (\nu_\mathrm{pump} + \nu_\mathrm{SH})/2$. This sets the midpoint (gray dot) $((\nu_\mathrm{pump} + \nu_\mathrm{SH})/2, (\beta_\mathrm{pump} + \beta_\mathrm{SH})/2)$ as the origin. The dashed lines represent the mirror-symmetric counterparts of the solid ones, given by $-\Delta\beta(-\Delta\nu)$. Energy and momentum conservation restrict the possible signal/idler pairs to residing at the centrosymmetric intersections of these solid and dashed curves. The identified pair consists of a TE30 signal (dark gray circle) at 330.2 THz (907.9 nm) and a TE10 idler (light violet circle) at 236.3 THz (1,268.7 nm).
• Figure 4: Coarse and fine tuning of OPO signal and idler wavelengths in the NIR range.a. Simulated $\omega$-2$\omega~$ hOPO spectra with different pump wavelengths and SH modes. The pump wavelength is tuned over 60 nm range, while the SH mode is changed from TE00 to TE60. No $\omega$-2$\omega~$ hOPO pair is found for SH in the TE00 and TE20 modes. The amplitude of the spectral lines are defined by the product of the modal overlap factor $\Gamma$ and the coupling coefficient to the bus waveguide $\kappa_\mathrm{ex}$. The excitation of TE40 SH enables up to three different combinations of modes, two of which were observed experimentally. b. Overlaid experimental spectra recorded while tuning the pump wavelength (1535-1580 nm) and chip temperature (20-80$^{\circ}\mathrm{C}$). The focus of the output achromatic lens is always optimized for the SH coupling.
• Figure 5: Other cascaded nonlinear processes in the $\omega$-2$\omega~$ hOPO.a. Coexistence of two hOPO pairs for the pump/SH/signal/idler wavelengths at 1570.2/785.1/892.0/1266.6 nm and 1570.2/785.1/938.8/1182.9 nm, with the modes inferred as TE00/TE40/TE30/TE10 and TE00/TE60/TE50/TE10, respectively. b. Observation of a $\omega$-2$\omega~$ hOPO followed by a MI comb at the signal band. The MI comb has a span of 20 nm. The pump/SH/signal/idler modes are TE00/TE50/TE40/TE10 with corresponding wavelengths of 1567.1/783.5/933.4/1186.1 nm.