The instrument-maker’s quest for smaller, faster, more sensitive, more flexible sensors is never-ending. The hunt drives them to smaller and smaller scales, where novel phenomena offer new ways of thinking about—and measuring—nature. There may be Faustian bargains along the way, though: at very small scales, they sometimes lose the ability to capture data that is easily available at larger scales.
In the present instance, extraordinary optical transmission (EOT) is a highly efficient light transmission through arrays of nanoholes. It’s an order of magnitude more efficient than conventional transmission through apertures of equivalent size, thanks to (in the words of its discoverers) "the coupling of light with plasmons—electronic excitations—on the surface of the periodically patterned metal film.” But EOT is only possible with perforations on the order of tens of nanometers in diameter, arrayed in a grid whose period is significantly shorter than one wavelength of the light to be studied.
Diffraction gratings—the workhorses of spectrometry for centuries—only work if the slits are significantly farther than one wavelength apart.
Now, though, researchers at the University of Alabama at Huntsville have devised at least one way of having their cake and eating it too, via a “super nanograting.” Haisheng Leong and Junpeng Guo, members of Huntsville’s electrical and computer engineering department, have built a surface plasmon resonance spectrometer (SPRS): a 300-micrometer-square silicon chip coated with a 50 nm of gold and then electron-beam etched with about half a million 140-nm-diameter holes distributed in a 420 nm grid. The duo converted this straightforward EOT sieve into a novel dual-scale grating by the simple expedient of omitting every fifth row from the grid. The result is equivalent to a diffraction grating with a 2100 nm pitch…but with the order-of-magnitude-better light-transmission and plasmon-sensitivity characteristics of a nanoarray.
Surface plasmons are oscillating valence electrons in the surface between a metal and a dielectric. Surface plasmon resonance fascinates sensor designers because the resonance wavelength is very sensitive to conditions at the interface. The phenomenon has been exploited, for example, to detect biomolecules (blood glucose, for example) clinging to the conductor surface.
The Huntsville device both transmits and diffracts light. And changes in the surface plasmon resonance frequency also change the frequency of the diffracted light. The diffraction grating, naturally enough, splits the light and throws each frequency onto a particular region of a simple CCD sensor. This allows the researchers to directly detect and quantify resonance changes in the material on the SPRS’s surface. (In fact, the Huntsville SPRS shows two diffraction peaks: one reflects the tightly confined oscillations at the gold-silicon surface, and the other the more weakly constrained resonance at the gold-air boundary. Both resonances change frequency if liquids are applied to the chip’s surface, but the Huntsville research focused on the stronger, tightly confined gold-silicon signal.)
In experiments reported in Optics Express, Leong and Guo tested a prototype SPRS. They found distinctly different resonance patterns when the device was exposed to methanol, isopropyl alcohol, and air alone. Methanol boosted the main diffraction peak wavelength by about 16 nm, and isopropyl alcohol increased it by 31 nm, for example.
The finding could pave the way for a new generation of spectrometer detector, a class based on metal nanohole arrays and capable of simultaneously sensing surface plasmon resonance and quantifying spectra.
Image: J. Guo / University of Alabama, Huntsville