Abstract
We propose a class of lasers based on a fourth-order exceptional point of degeneracy (EPD) referred to as the degenerate band edge (DBE). EPDs have been found in parity-time-symmetric photonic structures that require loss and/or gain; here we show that the DBE is a different kind of EPD since it occurs in periodic structures that are lossless and gainless. Because of this property, a small level of gain is sufficient to induce single-frequency lasing based on a synchronous operation of four degenerate Floquet-Bloch eigenwaves. This lasing scheme constitutes a light-matter interaction mechanism that leads also to a unique scaling law of the laser threshold with the inverse of the fifth power of the laser-cavity length. The DBE laser has the lowest lasing threshold in comparison to a regular band edge laser and to a conventional laser in cavities with the same loaded quality $(Q)$ factor and length. In particular, even without mirror reflectors the DBE laser exhibits a lasing threshold which is an order of magnitude lower than that of a uniform cavity laser of the same length and with very high mirror reflectivity. Importantly, this novel DBE lasing regime enforces mode selectivity and coherent single-frequency operation even for pumping rates well beyond the lasing threshold, in contrast to the multifrequency nature of conventional uniform cavity lasers.
Highlights
Demonstration of a low-threshold laser operating at a single frequency is an important quest in the optical and physical sciences
We report the evolution of the lasing action inside the degenerate band edge (DBE) laser using finite-difference timedomain (FDTD) algorithm as well as the lasing threshold analysis [in Fig. 1(a)] in Sec
We have demonstrated a novel regime in low-threshold lasers based on exceptional point of degeneracy referred to as the degenerate band edge (DBE) in a pair of coupled periodic waveguides
Summary
Demonstration of a low-threshold laser operating at a single frequency is an important quest in the optical and physical sciences In this regard, the use of periodic structures with engineered dispersion diagram is a popular and effective way to enhance the interaction between the gain medium and the electromagnetic wave and tailoring the lasing characteristics of active structures. The DBE has led to observing giant gains in optical cavities [47]; here we leverage a general EPD concept, for the first time, to propose the new regime of lasing, resulting in low-threshold and single frequency operation of the degenerate band edge laser.
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