Spectral Control of Coupled InP Nanolasers around Exceptional Points through Selective Excitation

Abstract

Tunable on-chip nanolaser sources are indispensable for advances in optical data transmission and computing. One way to tune emission wavelength is through use of coupling between two nanolasers, and to exploit their non-Hermitian properties, such as single mode emission and exceptional points as a function of non-uniform gain distribution [1]. We experimentally study mode interactions between coupled, epitaxially-grown InP microdisks, with diameters in the micron range that support resonant whispering gallery modes (Fig. 1a) [2]. Through selective excitation, via a digital micromirror device (DMD), arbitrary pumping powers on each disk (P1, P2) can be reached (Fig. 1b). This enables the full exploration of the coupled mode landscape that is theoretically described by coupled mode theory [3] and includes PT-symmetric and PT-broken regimes, lasing gaps, and exceptional points. In specific regions of the mode landscape, a reversed pump dependence is observed, resulting in a counter-intuitive lasing gap [3]. When one of the microdisks is pumped above threshold (P1) and the pump on the second disk (P2) is increased from zero, hence increasing the total pump power, the lasing intensity decreases until the system is below threshold and in the lasing gap (Fig. 1c). Further increase in P2 is needed to close the lasing gap and drive the system above threshold at a shifted wavelength. When the power on the constantly pumped disk (P1) is increased, the lasing gap closes and the system reaches a virtual exceptional point.