[Paper Review] Parity-time-symmetric coupled microring lasers operating around an exceptional point

Hossein Hodaei, Mohammad‐Ali Miri+5 more|arXiv (Cornell University)|Sep 21, 2015

Quantum Mechanics and Non-Hermitian Physics29 references126 citations

TL;DR

This paper demonstrates stable single-mode lasing in spectrally multi-moded microring lasers by exploiting parity-time (PT) symmetry and operating near an exceptional point. By differentially pumping two coupled microring resonators—where one is gain-assisted and the other lossy—the system undergoes a phase transition at the exceptional point, forcing lasing to occur exclusively in the high-Q PT-broken mode, enabling continuous wavelength tuning of over 3 nm via temperature control.

ABSTRACT

The behavior of a parity-time (PT) symmetric coupled microring system is studied when operating in the vicinity of an exceptional point. Using the abrupt phase transition around this point, stable single-mode lasing is demonstrated in spectrally multi-moded micro-ring arrangements.

Motivation & Objective

• To experimentally demonstrate single-mode lasing in inherently multi-moded microring lasers using PT-symmetric coupling.
• To investigate the role of exceptional points in non-Hermitian photonic systems for mode stabilization.
• To achieve continuous wavelength tuning in single-mode PT-lasers using the thermo-optic effect in semiconductors.
• To control lasing behavior by tuning the gain-loss contrast and coupling strength between microring resonators.

Proposed method

• Modeling the coupled microring system using temporal coupled-mode theory, with eigenfrequencies derived from Eq. (1): 𝜔𝑛(1,2) = 𝜔𝑛 + i(𝛾𝑎𝑛+𝛾𝑏𝑛)/2 ± √𝜅𝑛² − ((𝛾𝑎𝑛−𝛾𝑏𝑛)/2)².
• Fabricating InGaAsP-based microring pairs with controlled separations (50–300 nm) to tune the coupling strength 𝜅𝑛 via near-field overlap.
• Using a knife-edge to differentially block pump light, selectively introducing loss in one ring while maintaining gain in the other to traverse the exceptional point.
• Measuring coupling strength via wavelength splitting in uniformly pumped configurations, confirming exponential dependence on ring separation.
• Employing temperature tuning (270–300 K) to exploit the thermo-optic coefficient (𝑑𝑛/𝑑𝑇 ~10⁻⁴ K⁻¹) for continuous spectral tuning.
• Monitoring emission spectra and intensity profiles using a CCD camera and high-resolution spectrometer (0.4 nm resolution).

Experimental results

Research questions

• RQ1 Can PT-symmetric coupling in microring lasers enforce single-mode operation in inherently multi-moded systems?
• RQ2 How does the system behave when traversing an exceptional point, and what role does the PT-broken mode play in lasing stability?
• RQ3 To what extent can wavelength tuning be achieved in PT-symmetric microring lasers without mode hopping?
• RQ4 How does the coupling strength and gain-loss contrast affect the transition through the exceptional point?

Key findings

• At the exceptional point (|𝛾𝑎𝑛−𝛾𝑏𝑛| = 2𝜅𝑛), the two supermodes coalesce in both real and imaginary parts, leading to a sharp phase transition.
• After passing through the exceptional point, only the PT-broken mode—localized in the gain ring—exhibits sufficient amplification to lase, achieving single-mode operation.
• The linewidth of the single-mode lasing was measured at ~10 GHz, confirming high spectral purity.
• Continuous wavelength tuning of 3.3 nm was achieved over a 30 K temperature range (270–300 K), with single-mode operation preserved.
• Further temperature increase induced a free spectral range jump, enabling an additional 3.3 nm tuning range due to gain spectrum shift.
• The coupling strength was experimentally measured to be on the order of ~10¹² s⁻¹, with exponential dependence on ring separation, consistent with simulations.