Gaurav Bahl

Barium titanate (BTO) has been experiencing a surge of interest for integrated photonics technologies because of its large nonlinear optical coefficients, especially the Pockels coefficient, and in part due to newly available thin-film substrates. In this work, we report a redeposition-free dry etching process for monolithic BTO-on-insulator photonics that produces very low-roughness and high-verticality (≈ 60 ^o ) waveguides. Using this, we experimentally demonstrate the first, to the best of our knowledge, monolithic BTO-on-insulator microresonators with intrinsic quality factor ( Q _i ) >1 million, representing the highest intrinsic quality factor reported in any BTO-based integrated platform to date. Waveguide propagation losses as low as 0.32 dB/cm are also reported. We additionally demonstrate Mach–Zehnder amplitude modulators with V _π L =0.54 V⋅cm and effective electro-optic coefficient r _eff =162 pm/V.

Spatiotemporal modulation approaches have been often employed as alternatives for producing optical nonreciprocity without magneto‐optic materials. Unidirectional inter‐modal scattering, enabled by either acousto‐optic or electro‐optic (EO) modulation, is a promising method in this category as it can directly modify optical dispersions and even enables linear nonreciprocal photonic devices in the strong coupling limit. Although EO approaches are often preferred for their practicality, it is challenging to generate the large spatiotemporal momentum required for inter‐modal phase matching without EO drive schemes involving multiple drive stimuli. Here, we demonstrate highly selective nonreciprocal inter‐modal EO scattering enabled by a single high‐index radiofrequency (RF) traveling wave stimulus. Our experimental demonstration is performed on a thin‐film lithium niobate‐integrated photonics platform in which we engineer a slow‐wave radiofrequency (SWRF) transmission line with an effective RF index that natively generates the required RF momentum while simultaneously maintaining strong RF‐optical mode overlap. By additionally engineering the interaction length, we achieve a directional dB nonreciprocal scattering contrast. The SWRF architecture provides a scalable route to magnetic‐free nonreciprocity and establishes momentum‐engineered RF waves as a powerful tool for next‐generation, fully integrated nonreciprocal photonic systems.

Synthetic materials that emulate tight-binding Hamiltonians have enabled a wide range of advances in topological and non-Hermitian physics. A crucial requirement in such systems is the engineering of nonreciprocal couplings and synthetic magnetic fields. More broadly, the development of these capabilities in a manner compatible with quantum-coherent degrees of freedom remains an outstanding challenge, particularly for superconducting circuits, which are highly sensitive to magnetic fields. Here, we demonstrate that —a nonreciprocal coupling with exactly matched magnitude but nonreciprocal π phase contrast—can be realized between degenerate states using only spatiotemporal modulation. We first show the existence of continuous exceptional surfaces in modulation parameter space where coupling with arbitrarily large magnitude contrast can be achieved, with robust volumes of π phase contrast contained within. We then demonstrate that intersection of these volumes necessarily gives rise to continuous surfaces in parameter space where pure gyration is achieved. Our experiments are performed using microwave superconducting resonators that are modulated using dc superconducting quantum interference device arrays, and achieve isolation and pure gyration coupling with only superconducting circuit elements. However, our method is fully agnostic to physical implementation (classical or quantum) or frequency range and paves the way to realizing large-scale nonreciprocal metamaterials in any platform.

Asymmetric energy exchange interactions, also known as Hatano-Nelson type couplings, enable the study of non-Hermitian physics and associated phenomena like the non-Hermitian skin effect and exceptional points (EPs). Since these interactions are by definition nonreciprocal, there have been very few options for implementations in integrated photonics. In this work, we show that asymmetric couplings are readily achievable in integrated photonic systems through time-domain dynamic modulation. We experimentally study this concept using a two-resonator photonic molecule produced in a lithium niobate on insulator platform that is electro-optically modulated by rf stimuli. We demonstrate the dynamic tuning of the Hatano-Nelson coupling between the resonators, surpassing the asymmetry that has been achieved in previous work, to reach an EP for the first time. We are additionally able to flip the relative sign of the couplings for opposite directions by going past the EP. Using this capability, we show that the through-chain transport can be configured to exhibit both giant (∼60 dB) optical contrast as well as photonic gyration or nonreciprocal π phase contrast.

The mechanical motion of permanent magnets is crucial in applications such as motors, wireless power transfer, communication systems, and biomedical devices. These systems often rely on either resonant oscillatory motion or continuous rotational motion of permanent magnets, with performance and stability influenced by factors like temperature variations, material properties, and nonlinear effects. In this paper, we introduce an adaptive driving circuit using a negative impedance converter (NIC) that autonomously sustains resonance or continuous rotation in magneto-mechanical systems without the need for sensors or active tuning. The NIC-based driving circuit also incorporates a saturable gain to regulate and maintain the desired oscillation amplitude and rotational speed using simple DC voltage control. Experimental results validate the system's ability to maintain resonance or continuous rotation under varying conditions, demonstrating the robustness and practicality of this driving circuit.

On-chip generation of optical frequency combs using nonlinear ring resonators has enabled numerous applications of combs that were otherwise limited to mode-locked lasers. Nevertheless, on-chip frequency combs have relied predominantly on single-ring resonators. In this study, we experimentally demonstrate the generation of a novel class of frequency combs, the topological frequency combs, in a two-dimensional lattice of hundreds of ring resonators that hosts fabrication-robust topological edge states with linear dispersion. By pumping these edge states, we demonstrate the generation of a nested frequency comb that shows oscillation of multiple edge state resonances across ≈40 longitudinal modes and is spatially confined at the lattice edge. Our results provide an opportunity to explore the interplay between topological physics and nonlinear frequency comb generation in a commercially available nanophotonic platform.

Undesirable light scattering is a fundamental cause for photon loss in nanophotonics. Rayleigh backscattering can be particularly difficult to avoid in wave‐guiding systems and arises from both material defects and geometric defects at the subwavelength scale. It has recently been shown that systems exhibiting chiral dispersion due to broken time‐reversal symmetry (TRS) can naturally mitigate Rayleigh backscattering, yet this has never been explored in integrated photonics. Here we demonstrate the dynamic suppression of disorder‐induced Rayleigh backscattering in integrated photonics even when defects are clearly present. Our experiments are performed using lithium niobate on insulator resonators in which TRS is broken through an electrically‐driven acousto‐optic interaction. We experimentally observe near‐complete suppression of Rayleigh backscattering within the resonator by measuring the optical states and through direct measurements of the back‐scattered light. We additionally provide a new and intuitive generalization argument that explains this suppression of backscattering as a form of topological protection in synthetic space.