Scientists have successfully fabricated high-quality, defect-free optical nanofiber photonic crystal resonators, offering a significant advance in integrated photonics. Tomofumi Tanaka, Takahiro Suzuki, and Owen Mao, all from the Department of Applied Physics at Waseda University, alongside Ruddell et al, demonstrate a single-shot femtosecond laser ablation technique achieving quality factors exceeding 107. This breakthrough is particularly noteworthy because the research reveals thermo-optic effects dominate even with ultra-short pulse interrogation, paving the way for potential applications in high-speed quantum nodes for cavity QED and remarkably low-power in-line fiber optical switches , crucial components for future quantum networks.

The research team meticulously crafted these resonators by initially tapering standard single-mode optical fiber to a nanofiber diameter of 500nm and a waist length of 13mm using a flame-brush technique, optimising the shape for mechanical stability and adiabatic transition. Subsequently, a femtosecond laser, frequency-doubled to 400nm, was employed to ablate a periodic series of craters onto the nanofiber, forming a photonic crystal Bragg grating, a crucial step in defining the resonator’s properties. This precise control over the resonator structure minimises refractive index discontinuities, thereby reducing optical loss and enhancing performance.

This dominance of thermo-optic effects is particularly significant, suggesting potential for efficient thermal tuning and switching capabilities within these devices. This is achieved through strong confinement of light, both spatially and temporally, allowing for high intracavity intensities with moderate input power, enabling strong light-matter interactions. An example transmission spectrum of a fabricated nanofiber PhC cavity shows a reflection band spanning a few nanometers and containing narrow cavity resonances, confirming the resonator’s functionality. Researchers employed a 5-axis micrometer stage for precise nanofiber positioning during fabrication, ensuring optimal alignment with a 372.5nm period phase mask designed to create nanofiber Bragg gratings near 852nm.
The phase mask diffracts light into ±1 orders, forming an interference pattern that ablates the nanofiber, creating the periodic crater structure. By carefully controlling the laser parameters and alignment, the team successfully fabricated defect-free cavities with exceptional performance characteristics, paving the way for advanced photonic devices and quantum technologies. The use of a cylindrical lens pair and a cylindrical lens maximised the intensity of light along the nanofiber region, further enhancing the fabrication process.

Femtosecond laser fabrication of high-Q resonators enables novel

This innovative fabrication technique overcomes limitations of previous methods, such as contamination issues associated with focussed ion milling, and delivers a significant order of magnitude increase in quality factor compared to earlier works. The research team meticulously controlled the laser ablation process to create resonators with minimal defects, crucial for maintaining high Q-factors and enabling advanced quantum applications. To characterise the nonlinear optical properties, experiments employed both spontaneous parametric scattering (SPM) and cross-phase modulation (XPM) techniques, revealing thermo-optic effects as the dominant factor across the entire cavity bandwidth. Determining the precise cavity length proved challenging due to the large free spectral range and dispersion of the photonic crystal Bragg grating, precluding direct spectral measurement.

Researchers addressed this by estimating the cavity length to be on the order of a few mm, based on fabrication parameters and physical structure, subsequently estimating the cavity mode volume to be approximately 103 μm3. The study pioneered a method for establishing the cross-sectional mode area during nanofiber fabrication, providing a crucial parameter for resonator characterisation. Furthermore, the team determined the thermo-optic response cutoff frequency to be 24kHz, demonstrating the resonators’ rapid thermal dynamics and suitability for fast optical switching applications. This work represents a significant advancement in nanofiber photonics, offering a robust platform for exploring quantum phenomena and developing novel optical devices, potentially integrating into distributed quantum computing networks.

Thermo-optic dominance in high-Q nanofibre resonators

The team measured an intrinsic quality factor of Qi = 2.9 × 10^7, demonstrating exceptional resonator performance. This finding challenges conventional understanding of nonlinear optical behaviour in these structures. Results demonstrate a clear nonlinear response in cavity output power (Pout) for input pulse widths ranging from 147ns to 2s. The study observed bifurcation of the hysteresis loop, indicating bistable behaviour, and a new clockwise branch appeared at large Pin, as shown in detailed analysis of the time evolution of input and output power. Measurements confirm a larger than expected rise in Pout during the first half of the pulse, termed ‘overshoot’, which was more pronounced for longer pulse widths, strongly suggesting a thermo-optic origin.

This overshoot persisted even with 147ns pulses, the shortest achievable in this setup, solidifying the dominance of thermo-optic effects. The team further investigated nonlinear properties using cross-phase modulation (XPM), employing a pump wavelength of 946nm and a probe tuned to the target resonance at 852.35nm. A linewidth measurement of the target resonance revealed a full width at half maximum (FWHM) of 16.6MHz. By modulating the pump laser intensity, scientists observed the corresponding phase modulation of the probe light, allowing for frequency-dependent analysis of nonlinear effects. At low frequencies, the refractive index change was a combination of thermal and Kerr effects, but as the modulation frequency increased, the thermal contribution diminished. Data shows that the observed bistable curves did not merge after branching, contrary to theoretical models based solely on the Kerr effect. These resonators, with their high-Q and small mode volume, represent a significant advancement in integrated photonics.

High-Q Nanofiber Resonators Dominated by Thermo-Optics offer promising

These resonators demonstrate quality factors exceeding 10^7, representing a significant improvement over previously achieved values. The combination of high quality factors and small mode volumes positions these resonators as promising candidates for advanced applications in quantum technologies and optical switching. The authors acknowledge that fabrication limits currently constrain the maximum achievable quality factor, representing an area for future optimisation. Further research could explore the integration of these resonators into more complex quantum computing architectures and investigate their performance in diverse operating environments.