Robust single-mode operation is one of the most desirable features of compact semiconductor lasers, especially for integrated photonic applications that require spectral purity, energy efficiency, and stability under high-power operation. However, achieving such performance with a miniaturized footprint remains a persistent challenge due to intrinsic trade-offs among cavity size, quality factor (Q), and mode selectivity. Among various candidates, photonic crystal surface-emitting lasers offer numerous advantages, including high-power output, near-diffraction-limited low beam quality factor (M²) output, small divergence angles, low spectral dependence on temperature, compact device structures, and the ability to support ultrafast beam steering of the emitted light. However, mode competition in photonic crystal lasers leads to multimode emission under high-power operation. Therefore, achieving low-threshold, robust single-mode lasing over a wide pump range in miniature photonic crystal lasers remains a key challenge in the field of micro- and nanolasers.
Recently, research teams led by Professor Wei Bao at Rensselaer Polytechnic Institute and Professor Qing Gu at North Carolina State University proposed and experimentally demonstrated a photonic crystal laser design based on a merging bound state in the continuum (merging BIC). By engineering the distribution of several BICs in momentum space, this work achieved two key advances in a photonic crystal platform. First, robust single-mode lasing was realized in a finite-size device up to 80 times the threshold power. Second, an ultra-compact BIC laser with only a 5×5 periodic array was demonstrated through a combination of BIC and edge engineering. This work provides a new route for designing high-performance and ultra-compact on-chip single-mode lasers. The results are published in Light: Science & Applications , titled “Robust single-mode laser via merging bound state in the continuum.”
1. Merging BICs broaden the high-Q region
In the optical regime, bound states in the continuum (BICs) are optical states that remain confined despite existing within the radiation continuum. Ideal BICs have extremely low radiative loss, and therefore have long been regarded as a promising route toward low-threshold micro- and nanolasers. In practical finite-size devices, however, boundary scattering and fabrication imperfections turn ideal BICs into quasi-BICs and introduce additional loss. The researchers designed a suspended two-dimensional InGaAsP photonic crystal slab operating near the telecommunication wavelength. By tuning the air-hole geometry, several accidental BICs originally located at nonzero momenta gradually move toward the Γ point and merge with the symmetry-protected BIC at Γ. After merging, the high-Q region in momentum space is significantly broadened, which serves to reduce radiative loss and improve the tolerance of the device to perturbations.
2. Robust single-mode lasing in finite-size devices
To verify this design concept, the team fabricated suspended photonic crystal structures with a 20×20 array of unit cells. Experimental band-structure measurements showed that, in a finite-size photonic crystal, the originally continuous band is quantized into discrete modes. As the air-hole diameter gradually approaches the BIC merging condition, the lasing threshold is significantly reduced, confirming that BIC merging effectively suppresses radiative loss.
More importantly, when the pump power was further increased, the single-mode stability of different structures showed clear differences. Although the merging BIC condition reduces the loss of the target BIC mode, it also reduces the loss of nearby higher-order discrete modes, making mode competition stronger. In contrast, under the pre-merging condition, the threshold-gain contrast between the ground-state BIC mode and the nearest competing modes is maximized. As a result, single-mode operation can be maintained over a large pump range to far above the threshold. Experimentally, the pre-merging BIC device maintained strictly single-mode lasing up to 80 times the threshold power.
3. Edge engineering enables an ultra-compact BIC laser
The team further pushed the device toward miniaturization and experimentally realized an ultra-compact photonic crystal BIC laser with only 5×5 periods. Because edge leakage becomes stronger and the band structure becomes more discretized when the device size is reduced, the team adopted an edge-engineering design. The air-hole diameter at the device boundary was slightly reduced to improve the Q factor of the BIC mode. The experimental results show that this ultra-small device can still achieve lasing and maintain single-mode operation at 10 times the threshold power. Far-field interference patterns further confirm the vortex feature of the BIC mode. Unlike previous mini-BIC lasers that achieve footprint reduction by defining a BIC cavity core within a much larger photonic crystal heterostructure, the entire patterned photonic crystal region in this work contains only 5×5 periods, with a total area smaller than 15 μm².
4. Summary and outlook
This study demonstrates a new pathway for realizing low-threshold, robust single-mode and ultra-compact photonic crystal lasers by using merging BICs. Instead of simply pursuing the highest Q factor, the work emphasizes the role of mode competition in finite-size photonic crystals. In practical lasers, the optimal operating conditions depends not only on the loss of the target mode itself, but also on its mode selectivity relative to nearby competing modes. These results provide new physical insight into mode competition in finite-size BIC systems and offer a promising design strategy for high-stability, small-footprint and integrable on-chip laser sources.
Light: Science & Applications
Robust single-mode laser via merging bound state in the continuum