Tunable Graphene Phononic Crystal.

Jan N Kirchhof,Sebastian Heeg,Victor Deinhart,Kristina Weinel,Sviatoslav Kovalchuk,Katja Höflich,Kirill I Bolotin

doi:10.1021/acs.nanolett.0c04986

Abstract

In the field of phononics, periodic patterning controls vibrations and thereby the flow of heat and sound in matter. Bandgaps arising in such phononic crystals (PnCs) realize low-dissipation vibrational modes and enable applications toward mechanical qubits, efficient waveguides, and state-of-the-art sensing. Here, we combine phononics and two-dimensional materials and explore tuning of PnCs via applied mechanical pressure. To this end, we fabricate the thinnest possible PnC from monolayer graphene and simulate its vibrational properties. We find a bandgap in the megahertz regime within which we localize a defect mode with a small effective mass of 0.72 ag = 0.002 mphysical. We exploit graphene’s flexibility and simulate mechanical tuning of a finite size PnC. Under electrostatic pressure up to 30 kPa, we observe an upshift in frequency of the entire phononic system by ∼350%. At the same time, the defect mode stays within the bandgap and remains localized, suggesting a high-quality, dynamically tunable mechanical system.

Highlights

A phononic crystal (PnC) is an artificially manufactured structure with a periodic variation of material properties, for example, stiffness, mass, or stress.[1]
Our device design of a tunable, two-dimensional PnC consists of the following key elements
We choose a much smaller unit cell compared to typical silicon nitride (SiN)-PnCs (∼100 μm size) and use helium focused helium ion beam milling (FIB)-milling to pattern the PnC.[42]

Summary

Introduction

A phononic crystal (PnC) is an artificially manufactured structure with a periodic variation of material properties, for example, stiffness, mass, or stress.[1]. Δf of these so-called defect modes are especially high.[15,16] In particular, resonances with Q > 8 × 108 have been observed at room temperature in silicon nitride (SiN) PnCs.[15−17] In these devices, the quality factor exceeds the empirical Q ∼ m1/3 rule,[17−19] and the vibrational periods overcome the thermal decoherence time limit of τ = hQ/kBT.[15,17] This, in turn, enables the study of quantum effects in resonators of macroscopic size, all at room temperature.[20,21]

Methods

Results

Conclusion