Super Bound States in the Continuum on a Photonic Flatband: Concept, Experimental Realization, and Optical Trapping Demonstration

In this Letter, we theoretically propose and experimentally demonstrate the formation of a super bound state in a continuum (BIC) on a photonic crystal flat band. This unique state simultaneously exhibits an enhanced quality factor and near-zero group velocity across an extended region of the Brillouin zone. It is achieved at the topological transition when a symmetry-protected BIC pinned at $k=0$ merges with two Friedrich-Wintgen quasi-BICs, which arise from the destructive interference between lossy photonic modes of opposite symmetries. As a proof of concept, we employ the ultraflat super BIC to demonstrate three-dimensional optical trapping of individual particles. Our findings present a novel approach to engineering both the real and imaginary components of photonic states on a subwavelength scale for innovative optoelectronic devices.

Figure 1

(a) Sketch of 1D photonic crystal slab having vertical symmetry. Distribution of the electric field of (b) the even symmetric and (c) the odd antisymmetric modes of the TE polarization in the structure (a). (d) Sketch of 1D photonic crystal “comb” structure. Distribution of the electric field of (e) the evenlike symmetric and (f) the oddlike antisymmetric modes of the TE polarization in the structure (d). (g)–(j) The dispersion relations for $k_y=0$ of (g) uncoupled even (blue) and odd (red) modes. (h) The coupling between oddlike and evenlike modes gives three BICs on a multivalley band. (i) The merging of the three BICs results in a super BIC on a flatband. (j) As the coupling coefficient $\alpha$ gets stronger, the band becomes parabolic.

Figure 2

(a)–(c) Numerical reflectivity spectra of the “comb” structure of filling factors $f=0.51$, 0.48, 0.45, obtained by rcwa simulations ($k_y=0$). (d)–(f) Comparison between the dispersion curves obtained from the rcwa simulations (dots) and from the analytical model (1) (solid line) for $k_y=0$. (g) Effective masses $m_x$ and $m_y$ at the $\mathrm{\Gamma }$ point of the upper band as function of the filling factor $f$ (rcwa simulations). (h) Dispersion surfaces correspond to the case of saddle surface ($f=0.50$), ultraflat band ($f=0.48$), and paraboloid surface ($f=0.46$) (calculated using the comsol software).

Figure 3

First row: quality factor of the upper band as a function of $k_x$ and $k_y$. Second row: orientation angle $\phi$ of the far-field polarization of the upper band. These results are obtained by using the analytical model based on the Friedrich-Wintgen formalism (1).

Figure 4

(a) Sketch of the sample design. (b) SEM image of a grating structure (here $f=0.39$). (c)–(e) Angle-resolved reflectivity measurements of three structures having three different filling factors $f$ that correspond to (c) before merging, (d) at the merging transition, (e) after the merging transition. To follow the same photonic band, the spectral range of 40 nm is centered at ${\lambda }_0=1043\mathrm{nm}$ (a), 1020 nm (b), 1007 nm (c).

Figure 5

(a),(b) Frames captured when the laser is on resonance (a), then off resonance (b) with the super BIC. The red (blue) circle corresponds to the laser spot at on (off) resonance with the super BIC. (c),(d) Coordinates of the particles extracted from the captured frames when the laser is on resonance (c), then off resonance (d) with the super BIC.