Dynamic tuning of high-<i>Q</i> quasi-bound states in continuum driven by liquid crystal

Optical systems based on bound states in the continuum (BIC) generally possess higher quality factor (<i>Q</i>) values and narrower operational linewidths than traditional photonic crystals or metasurfaces. The higher <i>Q</i> values offer extensive possibilities for high-performance optoelectronic devices. However, the narrower linewidths often pose challenges in practical applications, as fabrication errors during production inevitably lead to discrepancies between real optical devices and their ideal designs, which results in mismatches between actual and ideal operating wavelengths. To solve this problem, we explore the dynamic tuning effect of liquid crystal (LC) on quasi-bound states in the continuum (q-BIC) so as to compensate for wavelength shifts caused by fabrication errors. A photonic crystal slab with cross-shaped holes serves as the platform for generating q-BIC. Compared with the modulation induced by the tilt angles of incident light on q-BIC, LC has a less influence on the system’s <i>Q</i> factor when the same operational wavelength is shifted. For instance, shifting the central wavelength <i>λ</i><sub>0</sub> of q-BIC by 5.32 nm by using a tilted incident angle results in the <i>Q</i> factor decreasing to 24.16% (from 3809.05 to 920.28). Whereas shifting the central wavelength <i>λ</i><sub>0</sub> by 5.63 nm through the tilt angle <i>θ</i> of LC leads <i>Q</i> factor to increase 14.27% (from 3809.05 to 4352.65). This demonstrates the significant potential of LC dynamic tuning in high-<i>Q</i> and ultra-narrowband q-BIC devices. Finally, the mechanism of LC within the q-BIC system is discussed. The smaller influence of LC on the <i>Q</i> factor is attributed to its minimal disruption of the q-BIC system’s symmetry. Although LC also affects system symmetry within the cross-shaped holes, after adjusting the asymmetry parameters of the system, the <i>Q</i> factor and the LC tuning process can be well matched. The results of our research provides valuable references for carrying on extensive research on q-BIC.