【摘要】 为在硅基光子平台上获得兼具高群折射率和可用带宽的慢光波导，提出了一种基于W1（移除一整排空气孔）线缺陷光子晶体波导的改进结构。该结构通过对紧邻波导的首排空气孔进行半截切除，在固定第一排孔径D′为0.6D(D为初始圆孔半径)的基础上，分别引入第二排空气孔位置偏移量s与孔径调节量r′两个结构自由度开展色散工程优化。数值结果表明：当位置偏移量s为0.12a(a为晶格常数)时，实现了平均群折射率高达47.63、慢光带宽为8.25 nm、对应归一化延迟带宽积为0.249的慢光性能；当孔径增大量r′为1.16r(r为初始圆孔半径)时，获得了平均群折射率为29.26、慢光带宽达16.59 nm、对应的归一化延迟带宽积约为0.307的慢光性能。进一步地，在波导两端设计方形孔形成波导集成的FabryPérot腔，通过透射谱自由光谱范围随波长的变化反演群折射率。实验结果表明，在s为0.12a与r′为1.16r两种优化结构中，均获得了平均群折射率为40的平坦慢光窗口，其可用带宽分别约为6.1 nm与9 nm，且慢光区位置及演化趋势与理论计算保持一致。上述结果验证了所提出的切孔与第二排孔协同调控色散工程的有效性，为光子集成回路中进一步集成非线性效应增强、光延时与脉冲表征等功能提供了可行路径。更多还原

【Abstract】 Objective Slow light photonic crystal waveguides(PCWs) have shown great potential in integrated optics, particularly for applications requiring enhanced light-matter interaction, optical delay, and compact nonlinear components. However, the trade-off between achieving high group index(n_g) and broad bandwidth remains a key challenge in chip-scale implementations. This work aims to address this issue by proposing a dispersion-engineered PCW design based on the W1 line-defect configuration. The approach combines first-row air-hole truncation and second-row cooperative modulation to realize wide flat-band slow-light transmission, while maintaining structural simplicity and fabrication compatibility. Furthermore, a square Fabry-Pérot(FP) cavity is introduced at the waveguide terminal, and is employed in both numerical analysis and experiments to validate the slow-light properties via spectral analysis, enabling accurate extraction of n_g variation.Methods The structure is designed on a 220-nm-thick silicon slab corresponding to the device layer of a standard silicon-on-insulator(SOI) platform, with a buried oxide underneath providing vertical index contrast in the fabricated devices. In plane, a triangular lattice with a period of 414 nm and an initial air-hole radius of 0.29a is patterned, and a W1 line defect is formed along the Γ-K direction by removing one row of holes. Different from a conventional W1, the first-row air holes next to the defect are laterally cut to form Dshaped truncated holes, and the remaining part is defined as D′. Reducing D′ increases the local effective index and shifts the guided band, so that a low-dispersion slow-light segment can appear before the intrinsic band edge(Figs. 1(b) and 1(c)). On this basis, a second degree of freedom is introduced by tuning the position or the radius of the second-row holes(Fig. 2): in one case, the holes are shifted toward the line defect; in the other case, their radius is enlarged. Both operations target further dispersion suppression around the designed wavelength. To verify the design on real devices, a pair of square air holes is added at the end of the optimized PCW to form an on-chip FP cavity of 40 ??m in length, and the n_g is retrieved from the spacing of adjacent resonances and compared with the numerical results(Fig. 3, Fig. 4).Results and Discussions Band-structure and n_g calculations show that truncating the first-row holes shifts the guided band toward lower normalized frequency, and more importantly, turns the previously steep rise of the n_g curve into a segment that becomes flat before the band edge is reached(Fig. 1(c)). When D′ is reduced to 0.6D or 0.5D, a practically usable slow-light window appears in the telecom band, in which the n_g variation is limited while the average value stays high. Based on this truncated configuration, shifting the second-row holes toward the defect strengthens the mode-lattice interaction and pushes the band to higher frequency; with a shift of s=0.12a, the sharp n_g peak is transformed into a flatter distribution with a low-dispersion bandwidth of 8.25 nm, a relatively high n_g of 47.63, and a corresponding normalized delay bandwidth product(NDBP) of 0.249, which is suitable for on-chip delay and nonlinear interaction(Figs. 2(a)-(c)). In the alternative route, enlarging the second-row radius to r′ =1.16r produces a red-shifted slow-light window; although the peak n_g is slightly lower than that of the shifted case, the flat slow-light bandwidth is broadened to 16.6 nm and the n_g is 29.3, resulting in an NDBP of ～0.307(Fig. 2(d)-(f)). Thus, the two optimizations are complementary: position shift favors higher n_g, while radius enlargement favors wider usable bandwidth. To confirm that these effects are not limited to simulations, both optimized PCWs were terminated by square mirrors to form FP cavities, and their transmission spectra were numerically obtained(Fig. 3). For the structure with a second-row position shift(s=0.12a), the simulated resonance spacing decreases toward the target band, and the FSR-extracted n_g is 45, which is in good agreement with the bandgap structure results, exhibiting a flat slow-light region with a bandwidth of 8.1 nm(Fig. 3(b)). For the structure with an enlarged second-row radius(r′ =1.16r), the same FSR analysis yields a red-shifted and slightly broader flat slow-light window, consistent with the calculated dispersion(Fig. 3(c)). Based on this, fabricated SOI devices were measured. SEM images confirm that the truncated first row, the tuned second row and the square cavity can all be fabricated with good uniformity(Fig. 4(a), Fig. 4(d)). The n_g retrieved from the measured FP spectra is 40, with a flat region of 6.1 nm, consistent with the simulation results in Fig. 3(Fig. 4(b), Fig. 4(c), Fig. 4(e), Fig. 4(f)).Conclusions This work numerically designed and experimentally demonstrated a slow-light PCW featuring a truncated first row of air holes jointly tuned with a second-row air-hole modification. The proposed structure preserves the simplicity and CMOS compatibility of the W1-type PCW. With the first-row truncation fixed at D′=0.6D, numerical results show that the position-offset design achieves a high group index slow light regime at s=0.12a, delivering ■=47.63 and Δλ=8.25 nm with a corresponding NDBP of 0.249. By contrast, increasing the second row hole radius to r′ =1.16r reduces ■ to 29.26 but expands the low-dispersion bandwidth to 16.59 nm, boosting the NDBP to ～0.307. By integrating a square FP cavity and retrieving n_g from the measured FSR, both optimized structures yield a flat slow-light window with ■=40, where Δλ is 6.1 nm for s=0.12a and 9 nm for r′=1.16r; the slow-light window position and its evolution trend are consistent with the theoretical predictions. The proposed approach provides a feasible structural route for integrating slow-light-enhanced nonlinear optics, optical delay, and ultrafast pulse characterization in photonic integrated circuits.更多还原