On-chip optical parametric oscillation into the visible: generating red, orange, yellow, and green from a near-infrared pump

Optical parametric oscillation (OPO) in a microresonator is promising as an efficient and scalable approach to on-chip coherent visible light generation. However, so far only red light at < 420 THz (near the edge of the visible band) has been reported. In this work, we demonstrate on-chip OPO covering > 130 THz of the visible spectrum, including red, orange, yellow, and green wavelengths. In particular, using a pump laser that is scanned 5 THz in the near-infrared from 386 THz to 391 THz, the signal is tuned from the near-infrared at 395 THz to the visible at 528 THz, while the idler is tuned from the near-infrared at 378 THz to the infrared at 254 THz. The widest signal-idler separation we demonstrate of 274 THz corresponds to more than an octave span and is the widest demonstrated for a nanophotonic OPO to date. Our work is a clear demonstration of how nonlinear nanophotonics can transform light from readily accessible compact near-infrared lasers to targeted visible wavelengths of interest, which is crucial for field-level deployment of spectroscopy and metrology systems.Comment: 6 pages, 5 figure

A universal frequency engineering tool for microcavity nonlinear optics: multiple selective mode splitting of whispering-gallery resonances

Whispering-gallery microcavities have been used to realize a variety of efficient parametric nonlinear optical processes through the enhanced light-matter interaction brought about by supporting multiple high quality factor and small modal volume resonances. Critical to such studies is the ability to control the relative frequencies of the cavity modes, so that frequency matching is achieved to satisfy energy conservation. Typically this is done by tailoring the resonator cross-section. Doing so modifies the frequencies of all of the cavity modes, that is, the global dispersion profile, which may be undesired, for example, in introducing competing nonlinear processes.Here, we demonstrate a frequency engineering tool, termed multiple selective mode splitting (MSMS), that is independent of the global dispersion and instead allows targeted and independent control of the frequencies of multiple cavity modes. In particular, we show controllable frequency shifts up to 0.8 nm, independent control of the splitting of up to five cavity modes with optical quality factors $\gtrsim 10^5$, and strongly suppressed frequency shifts for untargeted modes. The MSMS technique can be broadly applied to a wide variety of nonlinear optical processes across different material platforms, and can be used to both selectively enhance processes of interestand suppress competing unwanted processes.Comment: 13 pages, 8 figure

Scalable and Robust Beam Shaping Using Apodized Fish-bone Grating Couplers

Efficient power coupling between on-chip guided and free-space optical modes requires precision spatial mode matching with apodized grating couplers. Yet, grating apodizations are often limited by the minimum feature size of the fabrication approach. This is especially challenging when small feature sizes are required to fabricate gratings at short wavelengths or to achieve weakly scattered light for large-area gratings. Here, we demonstrate a fish-bone grating coupler for precision beam shaping and the generation of millimeter-scale beams at 461 nm wavelength. Our design decouples the minimum feature size from the minimum achievable optical scattering strength, allowing smooth turn-on and continuous control of the emission. Our approach is compatible with commercial foundry photolithography and has reduced sensitivity to both the resolution and the variability of the fabrication approach compared to subwavelength meta-gratings, which often require electron beam lithography.Comment: 10 pages, 6 figure

Sub-Doppler spectroscopy of quantum systems through nanophotonic spectral translation of electro-optic light

An outstanding challenge for deployable quantum technologies is the availability of high-resolution laser spectroscopy at the specific wavelengths of ultranarrow transitions in atomic and solid-state quantum systems. Here, we demonstrate a powerful spectroscopic tool that synergistically combines high resolution with flexible wavelength access, by showing that nonlinear nanophotonics can be readily pumped with electro-optic frequency combs to enable highly coherent spectral translation with essentially no efficiency loss. Third-order (\c{hi}(3)) optical parametric oscillation in a silicon nitride microring enables nearly a million optical frequency comb pump teeth to be translated onto signal and idler beams; while the comb tooth spacing and bandwidth are adjustable through electro-optic control, the signal and idler carrier frequencies are widely tuneable through dispersion engineering. We then demonstrate the application of these devices to quantum systems, by performing sub-Doppler spectroscopy of the hyperfine transitions of a Cs atomic vapor with our electro-optically-driven Kerr nonlinear light source. The generality, robustness, and agility of this approach as well as its compatibility with photonic integration are expected to lead to its widespread applications in areas such as quantum sensing, telecommunications, and atomic clocks.Comment: 17 pages, 5 figure

Stably accessing octave-spanning microresonator frequency combs in the soliton regime

Microresonator frequency combs can be an enabling technology for optical frequency synthesis and timekeeping in low size, weight, and power architectures. Such systems require comb operation in low-noise, phase-coherent states such as solitons, with broad spectral bandwidths (e.g., octave-spanning) for self-referencing to detect the carrier-envelope offset frequency. However, stably accessing such states is complicated by thermo-optic dispersion. For example, in the Si3N4 platform, precisely dispersion-engineered structures can support broadband operation, but microsecond thermal time constants have necessitated fast pump power or frequency control to stabilize the solitons. In contrast, here we consider how broadband soliton states can be accessed with simple pump laser frequency tuning, at a rate much slower than the thermal dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4 microresonators, including the generation of a multi-soliton state with a pump power near 40 mW and a single-soliton state with a pump power near 120 mW. We also develop a simplified two-step analysis to explain how these states are accessed in a thermally stable way without fast control of the pump laser, and outline the required thermal properties for such operation. Our model agrees with experimental results as well as numerical simulations based on a Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover, it also explains an experimental observation that a member of an adjacent mode family on the red-detuned side of the pump mode can mitigate the thermal requirements for accessing soliton states