Nanodevice Amplifies Tiny Signals

This is a schematic of the first-ever plasmomechanical oscillator (PMO). A cantilever, containing the gold cuboid nanoparticle, lies dead center. The device can lock onto and greatly amplify weak signals that oscillate at frequencies close to those of the PMO. (Source: B. Roxworthy, NIST)

Imagine a single particle, only one-tenth the diameter of a bacterium, whose miniscule jiggles induce sustained vibra­tions in an entire mechanical device some 50 times larger. By taking clever advantage of the inter­play between light, electrons on the surface of metals, and heat, researchers at the National Institute of Standards and Tech­nology NIST have for the first time created a plasmo­mechanical oscil­lator (PMO). It tightly couples plasmons to the mechanical vibra­tions of the much larger device it’s embedded in.

The entire system, no bigger than a red blood cell, has myriad techno­logical appli­cations. It offers new ways to minia­turize mechanical oscil­lators, improve communi­cation systems that depend on the modu­lation of light, dramati­cally amplify extremely weak mecha­nical and elec­trical signals and create exqui­sitely sensitive sensors for the tiny motions of nano­particles.

The device consists of a gold nano­particle, about 100 nanometers in diameter, embedded in a tiny cantilever made of silicon nitride. An air gap lies sandwiched between these components and an underlying gold plate; the width of the gap is controlled by an electro­static actuator – a thin gold film that sits atop the canti­lever and bends toward the plate when a voltage is applied. The nano­particle acts as a single plasmonic structure that has a natural, or resonant, frequency that varies with the size of the gap, just as tuning a guitar string changes the frequency at which the string rever­berates.

When a light source, in this case laser light, shines on the system, it causes electrons in the resonator to oscillate, raising the tempera­ture of the resonator. This sets the stage for a complex interchange between light, heat and mechanical vibrations in the PMO, endowing the system with several key proper­ties. By applying a small, direct-current voltage to the electro­static actuator that squeezes the gap shut, NIST researchers Brian Roxworthy and Vladimir Aksyuk altered the optical frequency at which the resonator vibrates and the inten­sity of the laser light the system reflects. Such opto­mechanical coupling is highly desirable because it can modulate and control the flow of light on silicon chips and shape the propa­gation of light beams traveling in free space.

A second property relates to the heat generated by the resonator when it absorbs laser light. The heat causes the thin gold film actuator to expand. The expan­sion narrows the gap, decreasing the frequency at which the embedded resonator vibrates. Conversely, when the tempera­ture decreases, the actuator contracts, widening the gap and increasing the frequency of the resonator. Crucially, the force exerted by the actuator always kicks the canti­lever in the same direction in which the canti­lever is already traveling. If the incident laser light is powerful enough, these kicks cause the cantilever to undergo self-sustaining oscil­lations with ampli­tudes thousands of times larger than the oscil­lations of the device due to the vibra­tion of its own atoms at room tempera­ture.

“This is the first time that a single plasmonic resonator with dimen­sions smaller than visible light has been shown to produce such self-sustaining oscil­lations of a mecha­nical device,” said Roxworthy. The team also demon­strated for the first time that if the electro­static actuator delivers a small mecha­nical force to the PMO that varies in time while the system under­goes these self-sus­taining oscil­lations, the PMO can lock onto that tiny variable signal and greatly amplify it. The researchers showed that their device can amplify a faint signal from a neigh­boring system even when that signal’s amplitude is as small as ten trillionths of a meter. That ability could translate into vast improve­ments in detecting small oscil­lating signals, Roxworthy says. (Source: NIST)

Reference: B. J. Roxworthy & V. A. Aksyuk: Electrically tunable plasmomechanical oscillators for localized modulation, transduction, and amplification, Optica 5, 71 (2018); DOI: 10.1364/OPTICA.5.000071

Link: Center for Nanoscale Science and Technology, National Institute of Standards and Technology NIST, Gaithersburg, USA

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