Ultra-low threshold continuous-wave quantum dot Mini-BIC laser

Ultra-low threshold and compact size lasers are very popular in photonic integrated circuits, designed to enable optical communications, chip-level solid-state lidar and quantum information applications. The general approach to implementing such lasers is to efficiently capture light and enhance the light-matter interaction by embedding the gain material in a low-wavelength or subwavelength scale optical cavity with a high mass factor and/or a small mode volume.

By introducing a defective PhC mode or a photon bound state into the continuum mode, low threshold laser emission is realized on a planar photonic crystal. For laser action, reported defect-type PhC lasers, although exhibiting an extremely small V mode and therefore having ultra-low thresholds, are unstable due to their sensitivity to structural disturbances. In this regard, BIC lasers, which may benefit from topological robustness, are one of the most promising alternative architectures. However, radiant BIC modes in PhC plates or gratings with a high Q factor typically require extended transverse periodic structures to reduce in-plane light leakage, thus essentially limiting their footprint to a few hundred units. In addition, BIC can only limit light in vertical directions.

The research group of Yu Ying and Yu Siyuan at Sun Yat-sen University proposed that by combining O-band InAs/GaAs epitaxial QD gain material with micro BIC cavity, a low threshold, small V-mode continuous-wave operated BIC laser was realized. Benefiting from the three-dimensional limitations of light and carriers provided by the mini-BIC cavity and QD, an ultra-low threshold of less than 20μW and a small size of ~2.5×2.5μm 2 are achieved. Unlike traditional Bics, Mini-Bics not only limit the vertical direction of light, but also use the photonic band gap of the transverse heterostructure to capture the transverse light. Therefore, it does not require long-term periodicity, allowing for a smaller structural size. The Mini-BIC structure consists of two sets of photonic crystals A and B with different periods, as shown in Figure 1a. Cavity A is used as the resonant microcavity of the laser. Since it is a finite photonic crystal, its mode exists as a discrete state M pq in K-space. By designing the state M pq of A so that it lies exactly within the band gap of B, light can be captured laterally. As shown in Figure 1b, M 11M 12 /M 21 is a resonant state in the continuous spectrum above the light cone. To further enhance the vertical optical restriction, by fine-tuning the lattice constant and hole radius of region A, the discrete state can be designed to converge with the accidental BIC mode in k space. The quality factor of the preferred laser mode can also be further improved.

Experimentally, the researchers first made the mini-BIC structure on InAs/GaAs QD active film by micromachining, and then transferred the active film to a glass substrate. Finally, UV curing adhesive is used to cover the film surface with a layer of glass to ensure the mirror flip symmetry of the PhC plate. By design, continuous wave, single-mode lasers can be achieved with cavity sizes as small as 5×5 cells. As shown in Figure 2a, the device exhibits 17 μW. Further temperature-dependent laser performance tests show that the maximum operating temperature can reach 343 K and the fitting characteristic temperature is 93.9 K, as shown in Figure 2b. By carefully designing the structural parameters, the wavelength range of the mini BIC laser has also been adjusted to 80 nm.

In summary, the researchers demonstrated a structure capable of efficiently capturing light and carriers in all three dimensions by fabricating miniaturized BIC cavities in an epitaxial QD gain film. They succeeded in achieving an efficient, high spectral quality, precise wavelength-designed micro-laser in the 1310 nm O-band wavelength range. Micro BIC lasers manufactured with membrane transfer technology can be flexibly implemented on different substrates, such as silicon or LiNbO 3. With the advantages of small footprint, ultra-low threshold and integrated adaptability, precise wavelength engineering, Mini BIC lasers provide a perspective light source for future PIC for high-capacity optical communication, sensing and quantum information.

Source: Laser Network