Direct observation of exceptional points in coupled photonic-crystal lasers with asymmetric optical gains.

Kyoung-Ho Kim,Hong-Gyu Park,Min-Soo Hwang,You-Shin No,Jae-Hyuck Choi,Ha-Reem Kim

doi:10.1038/ncomms13893

Abstract

Although counter-intuitive features have been observed in non-Hermitian optical systems based on micrometre-sized cavities, the achievement of a simplified but unambiguous approach to enable the efficient access of exceptional points (EPs) and the phase transition to desired lasing modes remains a challenge, particularly in wavelength-scale coupled cavities. Here, we demonstrate coupled photonic-crystal (PhC) nanolasers with asymmetric optical gains, and observe the phase transition of lasing modes at EPs through tuning of the area of graphene cover on one PhC cavity and systematic scanning photoluminescence measurements. As the gain contrast between the two identical PhC cavities exceeds the intercavity coupling, the phase transition occurs from the bonding/anti-bonding lasing modes to the single-amplifying lasing mode, which is confirmed by the experimental measurement of the mode images and the theoretical modelling of coupled cavities with asymmetric gains. In addition, we demonstrate active tuning of EPs by controlling the optical loss of graphene through electrical gating.

Highlights

Counter-intuitive features have been observed in non-Hermitian optical systems based on micrometre-sized cavities, the achievement of a simplified but unambiguous approach to enable the efficient access of exceptional points (EPs) and the phase transition to desired lasing modes remains a challenge, in wavelength-scale coupled cavities
The exceptional points (EPs), observed in the non-Hermitian parity–time (PT)-symmetric physical systems that possess multi-well potentials of energy gain or loss[1,2,3,4,5,6,7,8,9,10,11,12,13,14], have been recently explored in coupled optical cavities consisting of two identical micrometre-sized cavities, by controlling intercavity coupling and asymmetric gain or loss[15,16,17,18,19]
Several counter-intuitive lasing behaviours were observed in coupled cavities, including the suppression and enhancement of Raman lasing in coupled microtoroids with controlled asymmetric cavity loss[17], reversed pump dependence of lasing in coupled quantum-cascade microdisks under unbalanced electric pumping[18], and single-mode lasing in coupled microrings under asymmetric optical pumping[19]

Summary

Introduction

Counter-intuitive features have been observed in non-Hermitian optical systems based on micrometre-sized cavities, the achievement of a simplified but unambiguous approach to enable the efficient access of exceptional points (EPs) and the phase transition to desired lasing modes remains a challenge, in wavelength-scale coupled cavities. We demonstrate coupled photonic-crystal (PhC) nanolasers with asymmetric optical gains, and observe the phase transition of lasing modes at EPs through tuning of the area of graphene cover on one PhC cavity and systematic scanning photoluminescence measurements. As the gain contrast between the two identical PhC cavities exceeds the intercavity coupling, the phase transition occurs from the bonding/anti-bonding lasing modes to the single-amplifying lasing mode, which is confirmed by the experimental measurement of the mode images and the theoretical modelling of coupled cavities with asymmetric gains. Scanning photoluminescence (PL) measurements show the bonding/anti-bonding lasing modes, single-amplifying lasing mode and the combination of these lasing modes are excited in the coupled cavities with no graphene, large-area and smallarea graphene covers, respectively These observed lasing modes are clearly identified using the measured mode images and the theoretical asymmetric gain model of non-Hermitian coupled cavities.

Methods

Results

Conclusion