Topological nanorainbow laser

Abstract

Multiwavelength (or multifrequency) coherent light sources with spatial isolation are essential for diverse applications. Nevertheless, regulating the multiple emission wavelengths in an ultracompact scale is challenging. In this work, we realize a topological nanorainbow laser by exploiting synthetic dimensions in parameter space as a freedom to manipulate the optical resonances in a photonic crystal gain system. The system has spectrally isolated and spatially dispersed topological synthetic modes with near-diffraction-limited mode volumes and appropriately designed high-quality factors. We achieve high-performance nanorainbow lasing with low threshold, broadband spectrum, large spontaneous emission factor (β), and milliwatt output power, simultaneously. These emission features are predetermined by the ultrasmall mode volumes and appropriately designed high-quality factors of the spectrally and spatially isolated topological synthetic modes. This work enables high-performance, spatially multiplexed multiwavelength emission for on-chip photonics, facilitating advances in broadband signal processing, optical computing, and beyond.

Fig. 1.. The design of topological rainbow structures with synthetic dimension.

(A) Schematic diagram of a topological nanorainbow laser. (B) Structure design based on a triangular PhC lattice. The purple dashed line denotes the interface of two PhC domains, and L denotes total cavity length. The inset illustrates the unit cell of deformed PhC and a supercell with typical synthetic state profile. (C) Frequency evolution of synthetic states within the bandgap as the function of translational distance Δ and local momentum ${\mathbf{k}}_{\mathbf{2}}$ . The curves outline the topological synthetic bands with different fixed values of ${\mathbf{k}}_{\mathbf{2}}$ . (D) The calculated spectrally and spatially resolved eigenstates of a finite rainbow structure with $L=25a$ . The spatial intensity profiles are captured along the interface. The dashed curves delimit the maximum and minimum values of $\mathrm{\Delta }$ for a frequency in synthetic band, which are extracted by projecting the synthetic band in (C) onto the ( $\mathrm{\Delta }$ , f) plane. (E) Typical magnetic field profiles of the topological synthetic states in the rainbow structure

Fig. 2.. The characteristics of a topological nanorainbow laser with L=25a.

(A) Scanning electron microscope (SEM) image of a fabricated topological nanorainbow laser, whose interface length is 25a. The inset shows truncated airholes in the middle. (B) Calculated Q factors and mode volumes of the eigenstates. The inset shows one typical magnetic field $\mid H\mid$ profile of the synthetic state. (C) A typical emission spectrum measured with a pump power density of 34.1 kW/cm². The light gray shading in (B) and (C) denotes the bulk band region of PhC slab. (D) The light-in-light-out (LL) curve of the laser (black dots) and the emission linewidths (green circles) estimated by Lorentzian fitting the first emission peak at 1576 nm. Inset: The logarithmic plot of the LL curve. The LL curve can be well fitted through rate equation analysis (yellow curve) and exhibits a mild S-shaped curve with an estimated spontaneous emission enhancement factor β of ~0.45. (E) The emission pattern above the lasing threshold captured by an infrared camera. a.u., arbitrary units. (F) The theoretically calculated emission pattern by superimposing the electric fields of all the topological synthetic eigenmodes on the device surface. The white rhomboid outlines the geometry boundaries of the rainbow structure.

Fig. 3.. The emission property comparison with different synthetic PhC interface lengths.

(A) The measured emission spectra and (B) LL curves for topological nanorainbow lasers with $L=25a,40a,\text{and}55a$ . All the emission spectra were measured at the pump power density of ~22.0 kW/cm².

Fig. 4.. The characteristics of a topological nanorainbow laser with multiple synthetic dimension periods.

(A) SEM image of a fabricated topological nanorainbow laser with L = 40a and rotating angle $\mathrm{\theta }=13°$ . The right-top inset shows a structural schematic for rotating angles, and the right-bottom inset shows truncated airholes at the structure center. (B) Typical eigenmode profiles of the topological synthetic states. (C) The emission pattern captured at the pump power density of 13.64 kW/cm² and the theoretically calculated emission pattern. The white rhomboid outlines the geometry boundaries of the laser. (D) The corresponding emission spectrum (top) measured by guiding the light in free space to a spectrometer equipped with a single-point detector. The spatial mapping of the emission peaks at different hotspots marked by circles and alphabets in (C) is measured by coupling the signal to a high-resolution spectrometer using fiber. (E) The LL curve of the laser (black dots) and the emission linewidths (estimated by Lorentzian fitting the first emission peak at 1567.5 nm; green circles). The inset shows the logarithmic plot of the LL curve with an estimated β of ~0.40.