Fabrication of high-Q defect-free optical nanofiber photonic crystal resonators

TL;DR

The paper addresses the challenge of creating defect-free, high-Q nanofiber photonic crystal cavities suitable for quantum photonics. It introduces a single-shot femtosecond laser ablation technique on tapered nanofibers to fabricate defect-free Bragg gratings that form high-Q cavities, with measured resonances exhibiting $Q>1e7$ and intrinsic values up to $Q_i$ in the tens of millions. Nonlinear characterization via self-phase and cross-phase modulation reveals that thermo-optic effects dominate across the cavity bandwidth, with a cutoff near $24~\text{kHz}$ corresponding to a time constant of $6.6~\mu\text{s}$. This combination of high quality factor and small mode volume indicates potential for fast cavity QED nodes and low-power inline fiber switches, offering a promising platform for quantum networking and nonlinear integrated photonics.

Abstract

We demonstrate the fabrication of defect-free optical-nanofiber photonic-crystal Fabry-Perot resonators with quality factors exceeding 10^7 using single-shot femtosecond laser ablation. An investigation of the nonlinear optical properties reveals that thermo-optic effects dominate within the entire cavity bandwidth, even when interrogating with pulses one order of magnitude shorter than the 6.6 us thermal cutoff time. The combination of high-Q and small mode volume of these resonators could facilitate the creation of high-speed quantum nodes for cavity QED based quantum computing and networking, as well as low-power in-line fiber optical switches.

Figures (5)

• Figure 1: Experimental setup for fabricating PhC cavities on nanofibers by femtosecond laser ablation. HWP: half-wave plate, GTP: Glan-Taylor polarizer, BBO: barium borate crystal, PM: phase mask, ONF: optical nanofiber. Lens focal length units are given in mm. Inset: SEM image of a section of a PhC cavity fabricated using this setup.
• Figure 2: (a) Experimental setup used for measuring SPM in a nanofiber PhC cavity. AOM: acousto-optic modulator, QWP: quarter-wave plate, HWP: half-wave plate, PD: photodetector, FG: function generator, SG: signal generator. (b) Transmission spectrum of the PhC cavity used for SPM measurements. Red arrow indicates the target resonance.
• Figure 3: Results of the SPM experiment shown in Fig. 2. (a)-(c) Input pulse (black dots) and corresponding cavity output (colored dots). The input pulse widths and detunings are (a) 147 ns and $\delta=1.78$, (b) 1.02 $\mu$s and $\delta=1.6$, and (c) 2.02 $\mu$s and $\delta=1.71$. (d-f) Cavity output versus cavity input corresponding to (a-c).
• Figure 4: (a) Experimental setup used for measuring XPM in nanofiber PhC cavities. EOM: electro-optic modulator, PD: photodetector. (b) Transmission spectrum of the PhC cavity used in XPM measurements. Red arrow indicates the target resonance to be investigated. (c) Linewidth measurement (gray dots) of the target resonance in (b). Black line is the fitted Lorentzian linewidth, with a FWHM of 16.6 MHz.
• Figure 5: Result of the XPM measurement. Black line is the measured frequency response with both the probe and pump lasers, while gray line is the measured frequency response of only the pump laser. Orange horizontal line corresponds to the expected Kerr effect plateau, blue dotted line corresponds to the cutoff frequency of the photothermal effect at 24 kHz, and red dashed line corresponds to the cavity linewidth of 16.6 MHz.