arXiv subjects

Bastian Ruhnke

Publications and source records attributed to Bastian Ruhnke.

6 recordsLinked to original sources

arXiv2026

$f$-2$f$ self-referencing for silicon nitride photonics via a heterogeneously integrated lithium niobate layer

Heterogeneous integration combines complementary material properties in a single photonic platform. Here, we demonstrate on-chip $f$-2$f$ self-referencing in a heterogeneously integrated Si$_3$N$_4$/LiNbO$_3$ platform, in which supercontinuum generation in a Si$_3$N$_4$ waveguide and second-harmonic generation in a LiNbO$_3$ layer are spatially separated and linked by adiabatic escalator couplers. Pumped by a 1560 nm mode-locked laser with 75 pJ on-chip pulse energy, we detect the carrier-envelope offset frequency with 30 dB signal-to-noise ratio in 300 kHz resolution bandwidth. A comparative measurement confirms that the second harmonic originates in the LiNbO$_3$ layer and indicates opportunity for further improvements through periodic poling. These results establish heterogeneous Si$_3$N$_4$/LiNbO$_3$ integration as a viable building block for self-referenced ultrafast pulse sources in low-loss Si$_3$N$_4$ photonics.

physics.optics↗

arXiv2026

Bright solitons in hybrid-dispersion photonic crystal microresonators

Bright dissipative Kerr solitons in optical microresonators provide chip-scale sources of ultrashort pulses and frequency combs. Their properties are defined by the cavity dispersion for which fundamentally conflicting requirements exist: short pulses and broadband spectra require weak dispersion, whereas strong dispersion is associated with predictable dynamics. Here, we resolve this conflict by introducing a localized strong-dispersion section spanning several modes around the pump resonance within an otherwise weakly dispersive system. We implement this hybrid-dispersion scheme in a photonic crystal microresonator and reveal a new soliton attractor of backward-propagating solitons, accessible at low pump power in a thermally stable manner within the blue-detuned regime. The conflicting requirements for broadband spectra and low-noise single-soliton formation are reconciled, even in microwave-repetition-rate resonators, which otherwise are prone to uncontrollable multi-soliton formation. These results highlight the potential to achieve previously incompatible characteristics in nonlinear photonic systems through hybrid-dispersion attractor shaping.

physics.optics↗

arXiv2026

Attosecond-level synchronisation of chip-integrated oscillators

Synchronised laser oscillators are essential for probing the fastest processes in chemistry, materials science, and biology down to atto-second timescales. Tight synchronisation is also crucial at scientific facilities such as free-electron lasers or radio-telescopes, and increasingly relevant to communication and information technologies in multi-node networks. Current synchronisation approaches based on mode-locked lasers achieve the required performance, but their complexity, cost, and size hinder deployment in multi-node networks. Here, we demonstrate attosecond-level synchronisation between chip-integrated microresonator soliton oscillators operating at either 25 or 300 GHz pulse repetition rate. For synchronisation, each oscillator receives over fibre a pair of continuous-wave lasers as a two-tone timing reference. The lasers power the microcombs and Kerr-nonlinear synchronisation results in integrated relative timing jitter below 400 as (1 kHz to 1 MHz), without any active stabilisation. This approach enables scalable precision timing for large facilities, data centres, disaggregated computing, navigation, and quantum networks; ultimately, it may lead to chip-integrated attosecond photonics.

physics.optics↗

arXiv2025

Quadrature squeezing in a nanophotonic microresonator

Squeezed states of light are essential for emerging quantum technology in metrology and information processing. Chip-integrated photonics offers a route to scalable and efficient squeezed light generation, however, parasitic nonlinear processes and optical losses remain significant challenges. Here, we demonstrate single-mode quadrature squeezing in a photonic crystal microresonator via degenerate dual-pump spontaneous four-wave mixing. Implemented in a scalable, low-loss silicon-nitride photonic-chip platform, the microresonator features a tailored nano-corrugation that modifies its resonances to suppress parasitic nonlinear processes. In this way, we achieve an estimated 7.8 dB of on-chip squeezing in the bus waveguide, with potential for further improvement. These results open a promising pathway toward integrated squeezed light sources for quantum-enhanced interferometry, Gaussian boson sampling, coherent Ising machines, and universal quantum computing.

physics.optics↗

arXiv2024

Supercontinua from integrated gallium nitride waveguides

Supercontinua are broadband spectra that are essential to optical spectroscopy, sensing, imaging, and metrology. They are generated from ultrashort laser pulses through nonlinear frequency conversion in fibers, bulk media, and chip-integrated waveguides. For any generating platform, balancing the competing criteria of strong nonlinearity, transparency, and absence of multiphoton absorption is a key challenge. Here, we explore supercontinuum generation in integrated gallium nitride (GaN) waveguides, which combine a high Kerr-nonlinearity, mid-infrared transparency, and a large bandgap that prevents two- and three-photon absorption in the technologically important telecom C-band, where compact erbium-based pump lasers exist. Using this type of laser, we demonstrate tunable dispersive waves and gap-free spectra extending to almost 4 micron in wavelength, relevant to functional group chemical sensing. Additionally, leveraging the material's second-order nonlinearity, we implement on-chip f-2f interferometry to detect the pump laser's carrier-envelope offset frequency, which enables precision metrology. These results demonstrate the versatility of GaN-on-sapphire as a new platform for broadband nonlinear photonics.

physics.optics↗

arXiv2024

Phase-stabilised self-injection-locked microcomb

Microresonator frequency combs (microcombs) hold great potential for precision metrology within a compact form factor, impacting a wide range of applications such as point-of-care diagnostics, environmental monitoring, time-keeping, navigation and astronomy. Through the principle of self-injection locking, electrically-driven chip-based microcombs with minimal complexity are now feasible. However, phase-stabilisation of such self-injection-locked microcombs, a prerequisite for metrological frequency comb, has not yet been attained. Here, we address this critical need by demonstrating full phase-stabilisation of a self-injection-locked microcomb. The microresonator is implemented in a silicon nitride photonic chip, and by controlling a pump laser diode and a microheater with low voltage signals (less than 1.5 V), we achieve independent control of the comb's offset and repetition rate frequencies. Both actuators reach a bandwidth of over 100 kHz, enabling phase-locking of the microcomb to external frequency references. These results establish photonic chip-based, self-injection-locked microcombs as low-complexity yet versatile sources for coherent precision metrology in emerging applications.

physics.optics↗