Diana E. Hunter

Typical degenerate femtosecond optical parametric oscillators adopt an astigmatism-compensating resonator employing few-100 µm, Brewster-cut periodically-poled crystals to minimize intracavity dispersion and maximize the parametric gain bandwidth, but these crystals are fragile and hard to manufacture, especially when multiple gratings are required. Here, we show how the opposite approach—using a mm-thick crystal and considerable intracavity material—can still achieve few-cycle pulses and multi-100-nm spectral bandwidths. Using plane mirrors to eliminate astigmatism, our design balances the normal dispersion of the gain crystal with anomalous dispersion from thick intracavity lenses. By confining poled regions to less than the physical crystal length, we achieve a broad parametric gain bandwidth while allowing the crystal length to become an independent parameter for engineering the intracavity dispersion. We experimentally demonstrate the concept in an MgO:PPLN optical parametric oscillator, synchronously pumped by a 500 MHz Ti:sapphire laser, resulting in 42 fs pulses at 1564 nm with a -20 dB bandwidth of 300 nm and an average output power of 112 mW. The plane-mirror design is compact, robust, stable, easily aligned, and readily adapted to a wide range of repetition rates. Degenerate optical parametric oscillators based on this approach offer a route to simple generation of phase-coherent light when pumped by a femtosecond frequency comb.

We demonstrate GPU-accelerated modelling of ultrafast optical parametric oscillators (OPOs) via the χ⁽²⁾ nonlinear envelope equation with 1265× improvement in execution time compared with a CPU-based approach. Incorporating an adaptive step-size algorithm and absorbing boundary conditions, our model is capable of simulating OPOs containing long (>10 mm) nonlinear crystals or significant intracavity dispersion with outputs generated in less than 1 minute, allowing the investigation of systems that were previously computationally prohibitive to explore. We implement real-world parameters such as optical coatings, material absorption, and non-ideal poling domains within quasi-phase matched nonlinear crystals, producing excellent agreement with the spectral tuning behaviour and average power from a previously reported prism-based OPO. Our digital twinning approach provides a low-cost iterative development platform for ultrafast OPOs.

We demonstrate a synchronously-pumped optical parametric oscillator (OPO) with a cavity formed from high refractive index inverted prisms, also known as Brewster mirrors. Exploiting a single total internal reflection, this is the simplest device capable of deviating a laser beam by 180^∘. The OPO produced a chirped signal output tunable from 1060 - 1570 nm with a maximum power of 114 mW. We investigate the geometric properties of ideal and imperfect Brewster mirror prisms and find the latter can provide ∼1000 × finer control of the signal wavelength when compared to cavity length tuning.

Long QT Syndrome Type 2 (LQTS2) is associated with inherited variants in the cardiac hERG K⁺ channel. However, the pathogenicity of hERG channel gene variants is often uncertain. Using CRISPR-Cas9 gene-edited hiPSC-derived cardiomyocytes (hiPSC-CMs), we investigated the pathogenic mechanism underlying the LQTS-associated hERG R56Q variant, and its phenotypic rescue by the type 1 hERG activator, RPR260243. These approaches enable characterization of the unclear causative mechanism of arrhythmia in the R56Q variant (an N-terminal PAS domain mutation that primarily accelerates channel deactivation) and translational investigation of the potential for targeted pharmacologic manipulation of hERG deactivation. Using perforated patch clamp electrophysiology of single hiPSC-CMs, programmed electrical stimulation showed that the hERG R56Q variant does not significantly alter the mean APD₉₀. However, the R56Q variant increases the beat-to-beat variability in APD₉₀ during pacing at constant cycle lengths, enhances the variance of action potential duration (APD₉₀) during rate transitions, and increases the incidence of 2:1 block. During paired S1-S2 stimulations measuring electrical restitution properties, the R56Q variant was also found to increase the variability in rise time and duration of the response to premature stimulations. Application of the hERG channel activator, RPR260243, reduces the APD variance in hERG R56Q hiPSC-CMs, reduces the variability in responses to premature stimulations, and increases the post-repolarization refractoriness. Based on our findings, we propose that the hERG R56Q variant leads to heterogeneous APD dynamics, which could result in spatial dispersion of repolarization and increased risk for re-entry without significantly affecting the average APD₉₀. Furthermore, our data highlight the antiarrhythmic potential of targeted slowing of hERG deactivation gating, which we demonstrate increases protection against premature action potentials and reduces electrical heterogeneity in hiPSC-CMs.

We demonstrate a synchronously-pumped optical parametric oscillator (OPO) cavity in which traditional dielectric mirrors are replaced by all-planar Brewster angle prism retroreflectors, also known as Pellin-Broca prisms. Exploiting total internal reflection, these prisms form a cavity supporting >350-fs chirped signal pulses that were externally compressible to sub-150-fs durations. This simple architecture produces wavelengths tuneable across 1100 − 1350 nm, suitable for basic multi-photon applications.