Robust lasing modes in coupled colloidal quantum dot microdisk pairs using a non-Hermitian exceptional point

Evan Lafalce,Chun Hao Lin,Marcus J Smith,Zhiqun Lin,Qingji Zeng,Vladimir V Tsukruk,Sidney T Malak,Jaehan Jung,Young Jun Yoon,Z Valy Vardeny

doi:10.1038/s41467-019-08432-6

Abstract

Evanescently coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points. It remains an open question how scalable and versatile such phenomena can be when carried over to other designs. Here we have studied the effect of gain/loss modulation in an evanescently coupled pair of microdisk optical resonators fabricated from solution-processed colloidal quantum dots. The emission spectra of these structures are sensitive to small imperfections, which cause frequency-splitting of the whispering gallery modes. Despite this inherent disorder, we found that when spatially modulating the optical pump to vary the gain differential between the coupled microdisks, the coupling drives the split parasitic intra-cavity modes into coalescence at an exceptional point of the resulting three-mode system. This unusual behavior is rationalized via a Hamiltonian that incorporates the intra-cavity coupling as well as the anisotropic inter-cavity coupling between modes in the microdisk pair.

Highlights

Coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points
Not limited to quantum systems, these concepts have been largely explored in classical optical systems, including those based on parity-time (PT) symmetry[5,6], that include microwave and optical microcavities[7,8,9,10], waveguides[11,12], and photonic crystals[13,14]
In the present work we investigate whispering-gallery modes (WGMs) from microdisk cavities formed from robust crosslinked assemblies of core/ alloyed-shell CdSe/Cd1−xZnxSe1−ySy colloidal quantum dots (CQDs) fabricated by a scalable lithographic process

Summary

Introduction

Coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points. A study of the emission spectra from microdisk pairs reveals that actively pumped interacting microlasers produce spectra more robust against the appearance of mode-splitting. By spatial variation of gain and loss in the coupled microcavity pairs, we show that the removal of the localized parasitic modes originates from their coalescence at an EP of the non-Hermitian multi-mode system.

Results

Conclusion