- The paper introduces a deterministic design method that replaces traditional trial-and-error optimization for creating ultrahigh-Q photonic crystal nanobeam cavities.
- Experimental results include measured Q factors up to 80,000 and strong waveguide transmission, with designs predicting on-resonance transmission exceeding 90%.
- This approach enhances photonic integration, paving the way for advancements in quantum information processing, photonic circuits, and biosensing applications.
Photonic Crystal Nanobeam Cavity Strongly Coupled to the Feeding Waveguide
The paper by Quan, Deotare, and Loncar presents a detailed paper on the deterministic design and experimental demonstration of ultrahigh-Q nanobeam photonic crystal (PhC) cavities that are strongly coupled to a feeding waveguide. These PhC cavities exhibit high quality (Q) factors significantly above 106, with mode volumes that are on the scale of the operational wavelength.
Design and Methodology
The authors introduce a design methodology that dispenses with the conventional trial-based parameter optimization characteristic of traditional PhC cavity design approaches. Instead, the design is deterministic, involving straightforward photonic band structure calculations, which greatly reduces computational overhead. The design of the nanobeam cavity aims to achieve high Q values by minimizing out-of-plane scattering losses, which are identified through Fourier space analysis. Specifically, Gaussian attenuation profiles within the mirror region of the cavity are shown to effectively reduce the fraction of energy lost to the radiation modes, in comparison to conventional exponential attenuation profiles.
To achieve the desired Gaussian attenuation, the authors propose "modulated Bragg mirrors" where the attenuation constant is varied linearly. In conjunction with maintaining constant phase velocity within each segment, the design ensures ultra-high Q factors while keeping the mode volumes compact. This results in strong coupling to the waveguide, allowing the device to reach on-resonance transmissions exceeding 90% in the engineered structures with computational predictions to guide the selection of nanobeam parameters effectively.
Experimental Results
Fabricated silicon devices exhibited a measured Q factor of 80,000 and a transmission of 73%. These values represent the highest transmission levels measured for deterministic wavelength-scale high-Q cavities. The strong match between the field in the waveguide and the cavity further exemplifies the effectiveness of the design methodology, providing practical advantages in photonic circuit integration.
Implications and Future Prospects
This work’s deterministic approach mitigates the need for extensive iterative optimization in designing high-Q cavities, making it feasible to produce these devices quickly and with less computationally intensive procedures. The strong coupling achieved paves the way for applications in quantum information processing, photonics integrated circuits, and non-linear optics, owing to the efficient light confinement and manipulation capabilities of the PhC cavities. Furthermore, the method holds potential for advancements in biosensing applications, where the sensitivities afforded by high-Q cavities can be leveraged to detect minute biochemical signals.
The integration of these high-Q nanobeam cavities into more complex photonic systems is proposed as a future step, as well as their adaptation to different materials to expand their operational wavelength range. This could further enhance their utility across a wider spectrum of photonics applications, from infrared to visible wavelengths, offering improvements in device efficiency and performance.
In conclusion, the deterministic method presented offers an efficient pathway to designing PhC nanobeam cavities, enhancing the feasibility and application scope of these devices in next-generation photonics technologies.