Long-range transport of 2D excitons with acoustic waves

Abstract

Excitons are elementary optical excitation in semiconductors. The ability to manipulate and transport these quasiparticles would enable excitonic circuits and devices for quantum photonic technologies. Recently, interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices due to their long lifetime, large exciton binding energy, and gate tunability. However, the charge-neutral nature of the excitons leads to weak response to the in-plane electric field and thus inhibits transport beyond the diffusion length. Here, we demonstrate the directional transport of interlayer excitons in bilayer WSe2 driven by the propagating potential traps induced by surface acoustic waves (SAW). We show that at 100 K, the SAW-driven excitonic transport is activated above a threshold acoustic power and reaches 20 μm, a distance at least ten times longer than the diffusion length and only limited by the device size. Temperature-dependent measurement reveals the transition from the diffusion-limited regime at low temperature to the acoustic field-driven regime at elevated temperature. Our work shows that acoustic waves are an effective, contact-free means to control exciton dynamics and transport, promising for realizing 2D materials-based excitonic devices such as exciton transistors, switches, and transducers up to room temperature.

Surface acoustic wave induced phenomena in two-dimensional materials

“…Hence, the interaction between propagating SAWs and 2D materials attracts intense attention. Over the past decade, many intriguing acoustic-induced effects, including the acousto-electric effect, [81][82][83][84][85][86] acousto-galvanic effect, [86][87][88] acoustic Stark effect, 89,90 acoustic Hall effect and acoustic exciton transport, 91,92 have been experimentally reported by manipulating 2D materials with SAWs. In the meantime, the valley spin-acoustic resonance, 93 valley acousto-electric effect 94,95 and acoustic spin Hall effect 96 were theoretically proposed, awaiting for further experimental verifications.…”

“…For example, a biaxial strain pattern could be generated by either using orthogonal SAWs or selecting crystal cuts and propagation directions. 91 To control the directional transport of the charge neutral exciton flux, spatial tunings of exciton potential by mechanical strain 160,165 or an electric field [139][140][141][142]166 were realized and are summarized in Table 2. Travelling SAWs can dynamically utilize both strain fields and piezo-electric fields to manipulate and transport excitons in 2D semiconducting materials.…”

Interlayer Exciton Diode and Transistor

“…We demonstrated a highly directional IX diode, and an excitonic transistor using IX–IX repulsion to control the flow of excitons. While previous methods to drive anisotropic exciton transport in 2D semiconductors using strain engineering or surface acoustic waves have shown promising results, our approach using nanopatterned graphene creates to smallest physical channel for excitons, shows the highest measured IX speed, and is the first to show optically gated exciton transport. Our experimental results are in excellent quantitative agreement with our theoretical modeling showing that the electric fields generated by our nanopatterned graphene gates are accurate and reliable.…”

“…The charge neutrality of the exciton makes it insensitive to long-range Coulomb scattering mechanisms, offering the opportunity to realize transport with negligible Ohmic losses. 5,9,10 The >100 meV binding energy of excitons in van der Waals heterostructures shows potential for room temperature operation of such devices, 11 previously unattainable in similar coupled quantum well systems.…”

“…Two basic building blocks for excitonic circuits are diodes, which must demonstrate unidirectional flow of excitons, − and transistors, which must demonstrate controlled gating of excitons. − Excitons comprise a negatively charged electron bound to a positively charged hole resulting in a charge neutral quasiparticle with integer spin. The charge neutrality of the exciton makes it insensitive to long-range Coulomb scattering mechanisms, offering the opportunity to realize transport with negligible Ohmic losses. ,, The >100 meV binding energy of excitons in van der Waals heterostructures shows potential for room temperature operation of such devices, previously unattainable in similar coupled quantum well systems.…”