[Paper Review] Observation and active control of a collective polariton mode and polaritonic band gap in few-layer WS2 strongly coupled with plasmonic lattices

Wenjing Liu, Yuhui Wang+8 more|arXiv (Cornell University)|Aug 20, 2018

Strong Light-Matter Interactions4 citations

TL;DR

This study demonstrates the observation and active electrical tuning of a collective polariton mode and a complete polaritonic band gap in few-layer tungsten disulfide (WS2) strongly coupled to plasmonic nanoparticle lattices. The collective mode arises from cooperative exciton-plasmon coupling enhanced by exciton-exciton interactions, with mode splitting increasing superlinearly with exciton oscillator strength, enabling active control via a field-effect transistor integration.

ABSTRACT

Two-dimensional semiconductors host excitons with very large oscillator strengths and binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS2 coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of a complete polariton band gap with energy scale comparable to the exciton-plasmon coupling strength. A coupled oscillator model reveals that the collective mode arises from the cooperative coupling of the excitons to the plasmonic lattice diffraction orders via exciton-exciton interactions. The emergence of the collective mode is accompanied by a superlinear increase of the polariton mode splitting as a function of the square root of the exciton oscillator strength. The presence of these many body effects, which are enhanced in systems which lack bulk polarization, not only allows the formation of a collective mode with periodically varying field profiles, but also further enhances the exciton-plasmon coupling. By integrating the hybrid WS2-plasmonic lattice device with a field-effect transistor, we demonstrate active tuning of the collective mode and the polariton band gap. These systems provide new opportunities for obtaining a deeper and systematic understanding of many body cooperative phenomena in periodic photonic systems and for designing more complex and actively controllable polaritonic devices including switchable polariton lasers, waveguides, and optical logical elements.

Motivation & Objective

• To investigate strong light-matter interactions in two-dimensional semiconductors coupled with plasmonic lattices.
• To explore the emergence of many-body effects such as collective polariton modes and polaritonic band gaps in low-dimensional systems.
• To demonstrate active electrical tuning of these collective modes and band gaps using a field-effect transistor configuration.
• To understand the role of exciton-exciton interactions in enhancing exciton-plasmon coupling in systems lacking bulk polarization.

Proposed method

• Integration of few-layer WS2 with periodic plasmonic nanoparticle lattices to enable strong exciton-plasmon coupling.
• Use of a coupled oscillator model to explain the origin of the collective polariton mode via cooperative coupling to diffraction orders.
• Employment of angle-resolved photoluminescence spectroscopy to map the polariton dispersion and identify the band gap.
• Fabrication of a hybrid WS2-plasmonic lattice device with a back-gated field-effect transistor for electrical tuning.
• Measurement of mode splitting scaling with the square root of the exciton oscillator strength to confirm many-body enhancement.
• Use of a field-effect gate to electrically modulate the carrier density and tune the polariton mode energy and band gap.

Experimental results

Research questions

• RQ1 How does the collective polariton mode emerge in few-layer WS2 coupled to plasmonic lattices, and what is its microscopic origin?
• RQ2 What role do exciton-exciton interactions play in enhancing the exciton-plasmon coupling strength and enabling collective behavior?
• RQ3 Can the energy and width of the polaritonic band gap be actively tuned via external electrical gating?
• RQ4 How does the mode splitting scale with the exciton oscillator strength, and what does this imply about many-body effects?
• RQ5 To what extent can the system’s optical response be controlled for applications in active polaritonic devices?

Key findings

• A collective polariton mode is observed near the exciton energy, arising from cooperative coupling of excitons to plasmonic lattice diffraction orders via exciton-exciton interactions.
• A complete polaritonic band gap forms with an energy scale comparable to the exciton-plasmon coupling strength, indicating strong coupling regime.
• The polariton mode splitting increases superlinearly with the square root of the exciton oscillator strength, confirming many-body enhancement.
• The collective mode exhibits periodically varying field profiles due to the periodicity of the plasmonic lattice and coherent coupling.
• Integration with a field-effect transistor enables active electrical tuning of both the collective mode energy and the polaritonic band gap.
• The system demonstrates a route to actively controllable polaritonic devices such as switchable lasers, waveguides, and optical logic elements.