Nanoscale waveguide enables control of high-purity trions

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A semiconductor consists of two types of particles, electrons and holes (empty spaces where electrons should be). When an electron and a hole combine, they form an exciton, a bound state of these particles. Sometimes, an exciton can attract a third charged particle, resulting in a trio of particles called a trion.

Trions have several interesting optical and electrical properties, making them a prime candidate for use in optoelectronic devices. They have a significant advantage over electrons and holes, as they can be controlled using an electric field due to their lower binding energy.

Now, a group of researchers from the Pohang University of Science and Technology (POSTECH) have successfully controlled trions using a waveguide. The team achieved this by combining the science of excitons and plasmons, which are ripples or waves of electrons that are generated when light interacts with a material.

Nanoscale plasmonic waveguide for controlling trions

To create the trions, the team used a nanoscale plasmonic waveguide having a gap width of approximately 200 nm. A waveguide is a structure that confines and guides the propagation of light. In this instance, the use of a plasmonic guide allows the researchers to convert the light into plasmons and then transport it to a preferred location.

The researchers transferred molybdenum disulfide (MoS₂), which is a two-dimensional semiconducting material, onto the waveguide. Then, the team generated excitons by directing light onto the semiconducting material, making them flow towards the center of the waveguide, like a liquid going through a funnel.

The waveguide's plasmons assist in carrying the electrons from the metal part of the waveguide toward the semiconductor material. These transported electrons then join the excitons at the center of the waveguide, forming trions. In simpler terms, it's like assembling puzzle pieces to create a new three-particle structure.

The remarkable accomplishment of the research team extended beyond the creation of trions. They achieved an additional feat by gaining precise control over the location where these trions were created.

This was done by spatially controlling the plasmons using adaptive nano optics, allowing them to produce trions and plasmons at desired locations on the waveguide. Additionally, they were able to control the exciton to trion rate of conversion reversibly.

Their work opens up new avenues for the efficient creation of quasiparticles, such as trions, and facilitates high-efficiency photoconversion.

The study was published in Nature Communications.

Study abstract:

The generation of high-purity localized trions, dynamic exciton–trion interconversion, and their spatial modulation in two-dimensional (2D) semiconductors are building blocks for the realization of trion-based optoelectronic devices. Here, we present a method for the all-optical control of the exciton-to-trion conversion process and its spatial distributions in a MoS₂ monolayer. We induce a nanoscale strain gradient in a 2D crystal transferred on a lateral metal–insulator–metal (MIM) waveguide and exploit propagating surface plasmon polaritons (SPPs) to localize hot electrons. These significantly increase the electrons and efficiently funnel excitons in the lateral MIM waveguide, facilitating complete exciton-to-trion conversion even at ambient conditions. Additionally, we modulate the SPP mode using adaptive wavefront shaping, enabling all-optical control of the exciton-to-trion conversion rate and trion distribution in a reversible manner. Our work provides a platform for harnessing excitonic quasiparticles efficiently in the form of trions at ambient conditions, enabling high-efficiency photoconversion.