Lasing at a Stationary Inflection Point

TL;DR

This paper introduces a laser concept based on the stationary inflection point (SIP) in active optical waveguides, demonstrating enhanced gain and lower thresholds near SIP compared to traditional band edge lasing.

Contribution

It advances the understanding of lasing at SIPs in active waveguides, showing the robustness of the frozen mode and the advantages over photonic band edge lasing.

Findings

Lasing near SIP exhibits a gain threshold scaling as the negative cube of waveguide length.

SIP-based lasing shows lower gain threshold than photonic band edge lasing.

Frozen mode properties are resilient to small perturbations and losses.

Abstract

The concept of lasers based on the frozen mode regime in active periodic optical waveguides with a 3rd-order exceptional point of degeneracy (EPD) is advanced. The frozen mode regime in a lossless and gainless waveguide is associated with a stationary inflection point (SIP) in the Bloch dispersion relation, where three Bloch eigenmodes coalesce forming the frozen mode. As a practical example, we consider an asymmetric serpentine optical waveguide (ASOW). An ASOW operating near the SIP frequency displays a large group delay of a non-resonant nature that scales as the cube of the waveguide length, leading to a strong gain enhancement when active material is included. Therefore, a laser operating in the close vicinity of an SIP has a gain threshold that scales as a negative cube of the waveguide length. We determine that this scaling law is maintained in the presence of small distributed losses, such as radiation associated with waveguide bends and roughness. In addition, we show that although gain causes a distortion in the modes coalescing at the SIP, the properties of the frozen mode are relatively resistant to such small perturbations and we still observe a large degree of exceptional degeneracy for gain values that bring the system above threshold. Finally, our study also reveals that lasing near an SIP is favored over lasing near a photonic band edge located in close proximity to the SIP. In particular, we observe that an SIP-induced lasing in an ASOW displays lower gain threshold compared to lasing near the photonic regular band edge (RBE), even though the SIP resonance has a lower quality factor than the RBE resonance.