Two-Dimensional Photonic Crystal Microcavities in Nanocrystalline Diamond
The paper under discussion elucidates the fabrication and characterization of two-dimensional photonic crystal microcavities in nanocrystalline diamond, with a focus on potential applications for cavity quantum electrodynamics (QED) at room temperature. This paper is significant due to the unique properties of negatively charged nitrogen-vacancy (N-V) centers in diamond, which possess long spin lifetimes and are compatible with single photon sources.
Fabrication and Structural Insights
Nanocrystalline diamond films were grown using microwave plasma-enhanced chemical vapor deposition (CVD) on SiO2/Si substrates. The resultant diamond films exhibited a grain size between 10 and 30 nm and material thickness ranging from 140 to 160 nm. The photonic crystal cavities were engineered by employing electron beam lithography and reactive ion etching (RIE) techniques, finalized with oxygen RIE transfer into the diamond substrate. The distinctive aspect of these structures is the L7 cavity design, which features seven missing holes in a triangular lattice, resonating with the zero phonon line of N-V centers at approximately 637 nm.
Optical Characterization and Simulation
The optical properties of these cavities were analyzed using micro-photoluminescence (μ-PL) techniques, which employed a 532 nm continuous wave (CW) laser for excitation. Experimental observations revealed luminescence attributable to defect states within the nanocrystalline diamonds, demonstrated by multi-modal behavior at specified wavelengths. Finite-Difference Time-Domain (FDTD) simulations were executed to predict mode frequencies, acquiring coherence with experimental data upon wavelength adjustment for thickness variation.
Polarization and Mode Behavior
The polarization characteristics of the emitted photons were scrutinized, revealing anomalous behavior primarily for the fundamental mode, e1. In experimental observations, expected polarization characteristics were contradicted, with measurable polarization in both x and y directions for the e1 mode. Such anomalies highlight potential scattering losses, attributed to manufacturing inconsistencies or inherent variations in nanocrystalline material structures.
Quality Factors and Implications
The reported quality factors (Qs) for the photonic crystal cavities were notable, reaching up to 585 for the fundamental mode. Interestingly, discrepancies between simulated and experimental Q values for the e1 mode were significant, posing simulation estimates as high as 9100, contrasting with the measured value. This difference emphasizes the role of material scattering losses and defects in optical performance, suggesting potential improvements through single crystal diamond utilization to augment Q.
Conclusion and Future Directions
This exploration into nanocrystalline diamond photonic crystal cavities provides substantial insights into both fabrication challenges and theoretical possibilities for enhanced cavity QED applications. The findings underscore the limitations imposed by material imperfections, while simultaneously laying groundwork for future research emphasizing single crystal diamond structures to potentially achieve superior quality factors. Further investigation into material uniformity and cavity design optimization remains essential for advancing the applicability of diamond-based photonic devices in quantum computing and beyond.