Electrical tuning of elastic wave propagation in nanomechanical lattices at MHz frequencies

Abstract

Nanoelectromechanical systems (NEMS) that operate in the megahertz (MHz) regime allow energy transducibility between different physical domains. For example, they convert optical or electrical signals into mechanical motions and vice versa. This coupling of different physical quantities leads to frequency-tunable NEMS resonators via electromechanical non-linearities. NEMS platforms with single- or low-degrees of freedom have been employed to demonstrate quantum-like effects, such as mode cooling, mechanically induced transparency, Rabi oscillation, two-mode squeezing and phonon lasing. Periodic arrays of NEMS resonators with architected unit cells enable fundamental studies of lattice-based solid-state phenomena, such as bandgaps, energy transport, non-linear dynamics and localization, and topological properties, directly transferrable to on-chip devices. Here we describe one-dimensional, non-linear, nanoelectromechanical lattices (NEML) with active control of the frequency band dispersion in the radio-frequency domain (10-30 MHz). The design of our systems is inspired by NEMS-based phonon waveguides and includes the voltage-induced frequency tuning of the individual resonators. Our NEMLs consist of a periodic arrangement of mechanically coupled, free-standing nanomembranes with circular clamped boundaries. This design forms a flexural phononic crystal with a well-defined bandgap, 1.8 MHz wide. The application of a d.c. gate voltage creates voltage-dependent on-site potentials, which can significantly shift the frequency bands of the device. Additionally, a dynamic modulation of the voltage triggers non-linear effects, which induce the formation of a phononic bandgap in the acoustic branch, analogous to Peierls transition in condensed matter. The gating approach employed here makes the devices more compact than recently proposed systems, whose tunability mostly relies on materials' compliance and mechanical non-linearities.

Nanocontact Tailoring via Microlensing Enables Giant Postfabrication Mesoscopic Tuning in a Self‐Assembled Ultrasonic Metamaterial

“…The ability to tune the response of such materials also remains limited, which is a roadblock for interacting with different devices and phenomena, for device multifunctionality, and for spatially tailoring the materials' effective properties for strategies such as transformation acoustics or topological design . Prior lattices and acoustic metamaterials operating in the MHz–GHz regime have experimentally demonstrated tuning of < to ≈ of the bandgap center and stop band cutoff frequencies, respectively, and prior self‐assembled phononic crystals have demonstrated postassembly tuning of < of the bandgap center frequency. No prior studies have demonstrated the ability to tune a self‐assembled ultrasonic metamaterial in a spatially localized manner after fabrication of the crystal, which is particularly critical due to the sensitivity of most self‐assembly processes to the properties of the particles and the substrate.…”

“…The tunability, which we note is nonreversible (in contrast to refs. ), is achieved through the use of optical‐microlensing to tailor nanocontact features. Our self‐assembled metamaterials mitigate the aforementioned manufacturing scalability challenges, and have been previously demonstrated in the context of, for instance, surface acoustic wave (SAW) filters to have massive attenuations of −0.25 dB µm −1 (similar to recent phononic crystal filters with −0.31 dB µm −1 attenuation).…”

“…The ability to tune the response of such materials also remains limited, which is a roadblock for interacting with different devices and phenomena, for device multifunctionality, and for spatially tailoring the materials' effective properties for strategies such as transformation acoustics [5] or topological design. [6] Prior lattices and acoustic metamaterials operating in the MHz-GHz regime have experimentally demonstrated tuning of <20% [7] to ≈90% [8] of the bandgap center and stop band cutoff frequencies, microlensing, we illuminate the monolayer with high intensity optical light using a pulsed laser beam (60 µJ at maximum pulse energy, 532 nm optical wavelength, 430 ps pulse duration, and 1 kHz repetition rate) focused to a circular spot of 360 µm diameter. Each sphere converges the incident light to a bright spot on its focal plane, [19] which we posit locally heats, and subsequently melts or ablates, the titanium film under the contact, as is illustrated in Figure 1a,b.…”

Hexagonal Boron Nitride Phononic Crystal Waveguides

“…Hence, by assembling coupled resonators into long-range periodic structures, we can create phononic crystal structures and consequential phononic bands similar to the atomic lattices in crystalline solids and photonic crystal lattices, and their associated band structures. Such devices not only enable fundamental exploration of lattice-based solid-state phenomena including dispersive relation, energy transport, nonlinear dynamics, and topological states in the phononic domain, but also facilitate device functionalities, such as on-chip routing and filtering of radio-frequency (RF) acoustic waves [35,36]. The basic concept in Figure 1 is enabled and reinforced by the agile, additive features in device nanofabrication in the h-BN platform, evading conventional lithographic patterning and its associated chemical resist and contamination.…”

Spatially resolved optical excitation of mechanical modes in graphene NEMS

“…The combination of high-spatial-resolution optical drive and read-out enables full multimodal control of suspended 2D nanomechanical resonators for future NEMS applications. Our highresolution, all-optical approach could be combined with optical beam shaping and spatial light modulation to selectively address an arbitrary subset of resonators within large arrays, a feat not easily achievable with electrostatic gating, or could serve as a point source of propagating mechanical waves for use in 2D nanomechanical circuits 32 and waveguides 33 .…”