Compact and low-threshold lasers are highly sought after for various applications in photonics, such as optical communications, chip-scale solid-state LIDAR, and quantum information processing. To achieve such lasers, it is important to enhance the interaction between light and matter by trapping light effectively in small optical cavities with high quality factors (Q) or small mode volumes (V-mode).
One approach to realizing low-threshold lasing is by using planar photonic crystals (PhCs) with defect-type modes or photonic bound states in the continuum (BICs). Although defect-type PhC lasers exhibit ultra-low thresholds due to their extremely small mode volumes, they suffer from instability caused by sensitivity to structural disorder.
In this context, BIC lasers, which possess topological robustness, are considered a promising alternative architecture. However, achieving high-Q radiative BIC (or quasi-BIC) modes in PhC slabs or gratings often requires extended lateral periodic structures to minimize in-plane light leakage. Consequently, their footprint is limited to hundreds of unit cells. Furthermore, BICs can only confine light in the vertical direction, making it challenging to further reduce their thresholds.
A research group led by Ying Yu and Siyuan Yu from Sun Yat-sen University has presented a novel approach to realizing continuous wave (CW) BIC lasers with low thresholds and small mode volumes. They combined O-band InAs/GaAs epitaxial quantum dot (QD) gain materials with mini-BIC cavities. By leveraging the three-dimensional confinement provided by the mini-BIC cavity and the QDs, they achieved an ultra-low threshold below 20 μW and a small size of approximately 2.5×2.5 μm². The results of their study have been published in the journal Light: Science & Applications.
Unlike traditional BICs, the mini-BIC structure not only confines light vertically but also traps it laterally using the photonic bandgap of a lateral heterostructure. This unique design eliminates the need for long-range periodicity, allowing for a smaller structural size. The mini-BIC consists of two sets of photonic crystals with different periods, A and B, as depicted in Figure 1a. Cavity A acts as a resonant microcavity for laser operation.
The discrete mode Mpq of the finite photonic crystal exists as a discrete state in k-space, where p and q are positive integers (e.g., M11). By precisely designing the state Mpq of cavity A to lie within the bandgap of cavity B, light can be trapped laterally. Figure 1b illustrates the resonant states M11 and M12/M21 located in the continuum above the light cone. To further enhance vertical light confinement, the discrete state can be engineered to converge with accidental BIC modes in k-space through fine-tuning of the lattice constant and hole radius in region A. This optimization also improves the Q factor of the preferred lasing mode.
In the experimental setup, the researchers first fabricated the mini-BIC structure on an InAs/GaAs quantum dot (QD) active membrane using micromachining techniques. The active membrane was then transferred to a glass substrate, and a layer of glass was applied to the membrane’s surface using an ultraviolet curing adhesive to ensure mirror-flip symmetry of the PhC slab. Through careful design, continuous-wave, single-mode lasing was achieved with a cavity size as small as 5×5 unit cells (~2.5×2.5 μm²) and a mode volume of 1.16(λ/n)³.
Figure 2a illustrates the device’s ultra-low single-mode lasing threshold of 17 μW (0.074 kW cm⁻²). Temperature-dependent laser performance tests revealed a maximum operating temperature of 343 K (70 ℃), with a characteristic temperature fitted as 93.9 K (Figure 2b). By precisely engineering the structural parameters, the mini-BIC lasers could be tuned across a wavelength range of 80 nm (Figure 2c).
Source: Chinese Academy of Sciences