Mode selection of near-infrared topological bulk state photonic crystal lasers (<italic>invited</italic>)

ObjectiveTopological bulk lasers utilize the band-inversion-induced reflection mechanism to achieve high-performance single-mode lasing with a high side-mode suppression ratio. However, current research primarily focuses on the fundamental mode with directional vertical emission. To further achieve precise design and on-demand selection of lasing modes, particularly higher-order modes, a mode selection scheme for near-infrared lasers based on the topological bulk state localization mechanism is investigated. The study aims to explore the characteristics and control mechanisms of high-order modes in the topological bulk platform and to realize the selection between high-directionality Gaussian beam emission (l=1 order) and polarization singularity ring emission (l=0 and l=2 orders) through precise device parameter design.MethodsA topological bulk laser structure is designed, consisting of an internal topological photonic crystal region surrounded by an external trivial photonic crystal region. The lattice constant is set to 820 nm, and the side length of the triangular holes is 200 nm to match the emission wavelength of the InGaAsP multi-quantum well gain medium. The scaling factor of the external trivial region is kept fixed (ηtrivial=0.95), while an array of devices with varying internal topological region scaling factors (ηtopological) is designed and fabricated. The core principle is band alignment: by regulating the scaling factor of the internal triangular holes, the bandgap size and position of the topological region are adjusted. When the bandgap of the trivial region aligns with the frequency of a specific order eigenmode (l=0, 1, 2) in the topological region, the lasing condition for that specific mode is met, thereby achieving selective excitation. The feasibility of this method is verified through finite element numerical simulations, which simulate the band alignment of dipole and quadrupole modes for different scaling factors and the near-field and far-field patterns of the full structures. Experimentally, a series of InGaAsP quantum well microcavity lasers were fabricated using electron beam lithography, inductively coupled plasma etching, and wet etching processes. The devices were optically pumped at room temperature using a 1064 nm pulsed laser. The far-field radiation patterns and lasing spectra were measured using a custom micro-photoluminescence measurement setup equipped with a k-space imaging lens, an infrared CCD camera, and a spectrometer.Results and DiscussionsSimulation results of the band structures indicate that as the topological scaling factor varies, the bandgap of the topological region shifts, aligning with the l=2, l=1, and l=0 modes sequentially (Fig.2). The finite element analysis of the full cavity eigenmodes reveals the distinct near-field distributions and corresponding far-field radiation patterns in the momentum space for different mode orders (Fig.3). Experimental measurements were performed on a series of fabricated devices with the topological scaling factor ranging from 1.02 to 1.06. The experimental results show a clear and deterministic evolution in the far-field lasing modes as the topological scaling factor increases: the far-field emission pattern exhibits a transition from a ring shape corresponding to the l=2 order at the scaling factor of 1.02, to a solid point shape corresponding to the l=1 order at the scaling factor of 1.03, and back to a ring shape corresponding to the l=0 order at scaling factors greater than or equal to 1.04 (Fig.4). Specifically, the l=1 mode demonstrates high vertical directionality, while the l=0 and l=2 modes exhibit characteristics of polarization singularities with null intensity at the center. Furthermore, all devices demonstrated stable single-mode operation with high side-mode suppression ratios in their emission spectra (Fig.4). To further verify the vector beam characteristics of the l=0 and l=2 modes, a linear polarizer was introduced into the far-field imaging path. Both simulation and experimental results demonstrate that the ring-shaped beam patterns split into a synchronously rotating two-lobe distribution as the polarization angle rotates, explicitly confirming the spatially variant polarization characteristic of the generated vector beams (Fig.5).ConclusionsA mode selection method for near-infrared lasers based on topological bulk state localization is proposed and successfully demonstrated. By precisely tuning the geometric scaling factor of the topological photonic crystal, the method realizes the on-demand selection of high-directionality point emission for the l=1 order and polarization singularity ring emission for the l=0 and l=2 orders. The comprehensive study reveals that the topological bulk platform supports not only the fundamental mode but also higher-order modes, and these modes can be selectively excited through the band alignment mechanism. This work provides a robust and effective new physical approach for the design of on-chip integrated structured light sources, higher-order mode lasers, and novel topological photonic devices. The findings offer potential applications in high-capacity optical communication, optical manipulation, and quantum information processing.