A miniature mid-infrared spectrometer that combines the advantages of actuated microelectromechanical systems (MEMS) with the sensitivity of plasmonics has been built by scientists from Boston University (ACS Photon. 3, 14–19; 2016). The device, a few hundred micrometres long and wide, consists of a gold film featuring a subwavelength hole array that is suspended above a gold reflector by an actuated polysilicon frame. Electrostatic actuation of the frame changes the gap between the gold film and the reflector, thus modifying the optical path length and the spectral response of the Fabry–Perot interferometer that they form.

“The additional mechanical degree of freedom provided by the MEMS will enable dynamic tuning of the spectral response of the system, making this device a powerful tool for differential measurements and spectrometry,” commented Thomas Stark from Boston University.

Credit: AMERICAN CHEMICAL SOCIETY

In recent years, subwavelength-sized holes in metal films have proved to be useful in various sensing applications due to their ability to greatly enhance the strength of the local electromagnetic field via the excitation of surface plasmons. This plasmonic enhancement can dramatically boost the sensitivity of absorption and Raman spectroscopy, allowing the techniques to reach new regimes of operation. However, the broad spectral width of a plasmon resonance means that it can be difficult to distinguish between closely spaced spectral absorption features.

In the Boston team's MEMS device (pictured), a polysilicon frame supported a suspended gold film, in which an array of rectangular holes was fabricated using focused ion beam milling. The polysilicon frame was suspended 21.67 μm above the gold reflector. The size of the rectangular holes and the thickness of the gold film were 1,200 nm × 500 nm and 100 nm, respectively. The team says that the suspended design has two advantages. First, surrounding the nanoscale holes with a homogeneous dielectric background provides higher quality plasmonic resonances. Second, the use of the polysilicon frame prevents the suspended gold film from being strained during the actuation of the MEMS device.

By tuning the distance between the gold film and the reflector from 1.7 μm to 21.67 μm, the spectral response of the MEMS device was modulated. Combining this modulation with phase-sensitive detection methods makes it possible to enhance the device's signal-to-noise ratio.