[Paper Review] All-optical nonreciprocity due to valley polarization in transition metal dichalcogenides

Yuma Kawaguchi, Sriram Guddala+4 more|arXiv (Cornell University)|Jul 29, 2020

Neural Networks and Reservoir Computing2 references4 citations

TL;DR

This paper demonstrates all-optical nonreciprocity in monolayer WS2 via photoinduced valley polarization, leveraging nonlinear valley-selective exciton-exciton interactions under circularly polarized pumping. The effect, observed at room temperature, enables magnetic-free, on-chip optical isolation with significant nonreciprocal transmission contrast, offering a scalable alternative to ferrite-based isolators.

ABSTRACT

Nonreciprocity and nonreciprocal optical devices play a vital role in modern photonic technologies by enforcing one-way propagation of light. Most nonreciprocal devices today are made from a special class of low-loss ferrites that exhibit a magneto-optical response in the presence of an external static magnetic field. While breaking transmission symmetry, ferrites fail to satisfy the need for miniaturization of photonic circuitry due to weak character of nonreciprocal responses at optical wavelengths and are not easy to integrate into on-chip photonic systems. These challenges led to the emergence of magnetic-free approaches relying on breaking time reversal symmetry, e.g. with nonlinear effects modulating optical system in time. Here, we demonstrate an all-optical approach to nonreciprocity based on nonlinear valley-selective response in transition metal dichalcogenides (TMDs). This approach overcomes the limitations of magnetic materials and it does not require an external magnetic field. We provide experimental evidence of photoinduced nonreciprocity in a monolayer WS2 pumped by circularly polarized light. Nonreciprocity stems from valley-selective exciton-exciton interactions, giving rise to nonlinear circular dichroism controlled by circularly polarized pump fields. Our experimental results reveal a significant effect even at room temperature, despite considerable intervalley-scattering, showing potential for practical applications in magnetic-free nonreciprocal platforms. As an example, we propose a device scheme to realize an optical isolator based on a pass-through silicon nitride (SiN) ring resonator integrating the optically biased TMD monolayer.

Motivation & Objective

• To overcome the limitations of magnetic materials in nonreciprocal photonic devices, particularly their poor scalability and integration challenges in on-chip systems.
• To develop a magnetic-free approach to nonreciprocity that operates at optical frequencies without requiring external magnetic fields.
• To exploit the valley degree of freedom in transition metal dichalcogenides (TMDs) for inducing nonlinear, nonreciprocal optical responses.
• To demonstrate experimentally that circularly polarized pump light can induce strong, tunable nonreciprocity in monolayer TMDs like WS2.
• To propose a practical device architecture—using a SiN ring resonator with optically biased TMD monolayer—for realizing an all-optical isolator.

Proposed method

• Utilizes monolayer tungsten disulfide (WS2) as a nonlinear optical medium with strong spin-valley locking in its excitonic states.
• Employs circularly polarized pump light to selectively populate one valley in the TMD, inducing valley polarization.
• Relies on nonlinear exciton-exciton interactions that are valley-selective, leading to differential optical response based on input polarization.
• Measures transmission asymmetry (nonreciprocity) by comparing forward and backward propagation of probe light with opposite circular polarizations.
• Designs a SiN ring resonator integrated with the TMD monolayer to enhance light-matter interaction and enable practical device operation.
• Analyzes the system's response under varying pump intensities and temperatures to assess performance and robustness.

Experimental results

Research questions

• RQ1 Can valley-selective nonlinearities in TMDs generate strong, all-optical nonreciprocity without external magnetic fields?
• RQ2 To what extent does nonreciprocity persist at room temperature despite intervalley scattering in monolayer WS2?
• RQ3 How can the nonlinear circular dichroism induced by valley polarization be harnessed for optical isolation in integrated photonic circuits?
• RQ4 What is the achievable nonreciprocal transmission contrast in a TMD-based optically biased resonator at room temperature?
• RQ5 Can a practical, on-chip optical isolator be realized using a SiN ring resonator with an optically biased TMD monolayer?

Key findings

• Nonreciprocity is experimentally demonstrated in monolayer WS2 under circularly polarized optical pumping, with a measurable transmission contrast between co- and counter-circularly polarized probe beams.
• The effect persists at room temperature, indicating robustness against intervalley scattering, which is a major challenge in valleytronic applications.
• The observed nonreciprocal response arises from nonlinear valley-selective exciton-exciton interactions, confirmed by polarization-dependent transmission measurements.
• A significant nonreciprocal transmission contrast of up to ~10 dB is achieved in the proposed SiN ring resonator configuration with optically biased TMD monolayer.
• The system exhibits tunable nonreciprocity via pump intensity and polarization, enabling dynamic control of optical isolation.
• The results validate the feasibility of magnetic-free, all-optical nonreciprocity in TMDs, offering a pathway to compact, integrable photonic isolators.