Sensing Flexural Motion With Quantum Dots

Chladni plate’s mode of vibration visualized by grains of sand collected at the nodes. Now, researchers discovered a similar effect with much smaller vibrating objects excited by light waves. (Source: S. Carter et al.)

In the late 18th century, Ernst Chladni, a scientist and musician, disco­vered that the vibra­tions of a rigid plate could be visua­lized by covering it with a thin layer of sand and drawing a bow across its edge. With the bow movement, the sand bounces and shifts, collecting along the nodal lines of the vibration. Chladni’s disco­very of these patterns earned him the nickname, “father of acoustics.” His disco­very is still used in the design and construc­tion of acoustic instruments, such as guitars and violins.

Recently, inves­tigators have discovered a similar effect with much smaller vibra­ting objects excited by light waves. When laser light is used to drive the motion of a thin, rigid membrane, it plays the role of the bow in Chladni’s original experi­ment and the membrane vibrates in resonance with the light. The resul­ting patterns can be visualized through an array of quantum dots (QDs), where these tiny structures emit light at a frequency that responds to movement.

In addition to being a modern take on an old pheno­menon, the new discovery could lead to the deve­lopment of sensing devices as well as methods for controlling the emission charac­teristics of QDs. Since the light frequency emitted by the QDs is corre­lated with the movement of the underlying membrane, new devices for sensing motion, such as acce­lerometers, can be envisioned. A reverse appli­cation is also possible since the motion of the under­lying membrane can be used to control the frequency of light emitted by the QDs.

The tiny devices in the work reported here consist of a 180-nano­meter thick slice of semi­conductor, suspended like a tram­poline above a solid substrate. An array of QDs, analogous to the sand in the acoustic example, are embedded in the slice, whose thick­ness is less than one-tenth of one percent that of a human hair. A second probe laser is used to visualize the resulting resonances. The QDs absorb the probe light and emit a second light pulse in response, which is picked up by a detector and routed to a display. The resul­ting patterns are remarkably like those visua­lized in Chladni’s original acoustic experi­ment, even though the new device is driven entirely by light.

One possible appli­cation of this discovery, accor­ding to Sam Carter of the Naval Research Lab, is to sense subtle forces produced by nearby dense objects. “Concealed nuclear materials could be detec­table,” he said, “since dense materials like lead are used to shield the devices.” The highly dense shielding needed for nuclear materials causes small gravi­tational anomalies and tiny move­ments that might be detec­table by a device based on the principle disco­vered here. The investigators plan to continue their work by looking at elec­tronic spin. It is hoped that tech­niques to measure the effect on spin will increase the sensi­tivity of the devices. (Source: AIP)

Reference: S.G. Carter.: Sensing flexural motion of a photonic crystal membrane with InGaAs quantum dots, Appl. Phys. Lett. 111, 183101 (2017); DOI: 10.1063/1.4995069

Link: Naval Research Laboratory, Washington, USA

Speak Your Mind