Georgia Tech has constructed an 80 channel infra-red spectrometer with 0.6nm resolution using only 1mm2 of silicon and a CCD imager.
All of the optical components are etched into the 230nm thick silicon layer of a silicon-on-insulator wafer - the semiconductor is transparent at the wavelengths of interest, which is around 1,550nm.
Crucial to the device are 4µm diameter disc-shaped optical resonators, one per channel (see grey photo), each with a 1.2µm hole in the middle.
The 4µm, 1.2µm and 230nm dimensions are chosen so that, when light of the correct wavelength is couple in, the light resonates around with energy concentrated in a narrow strip of silicon just inside the outer edge of the disc.
Electron beam lithograph is used to define the discs.
Its accuracy allows each disc to differ in diameter by 1nm from its immediate neighbour, resulting in 84 different resonances spaced at 0.6nm wavelength intervals.
Physically, the discs are spaced at 10µm intervals along the edge of the 400nm-wide input waveguide (the S-shaped line in the orange photo) which passes close to, but does not touch, each disc (too small to be seen in the photo).
Evanescent field coupling jumps the gap between waveguide and discs, feeding a small amount of light into each one.
The gap affects the Q of the resonators, which is set at around 5,000 so that their bandwidth fits into the 0.6nm wavelength spacing.
A separate output waveguide passes close to the other side of each disc, and each output waveguide terminates in a scattering structure which projects light out of the surface if the wafer so that it can be viewed by the CCD imager - the scattering structures form the long thin rectangle at the centre of the orange photo.
In operation, the scattering sites light up at an intensity proportional to the energy in their band in the input signal.
The technology, said the University, could cover wavelengths from 1-3µm, and potentially be extended to visible wavelengths using silicon nitride rather than silicon for the waveguides.
The work was lead by Professor Ali Adibi.
"The micro-spectrometer we designed may allow individuals to replace the big, bulky, high-resolution spectrometers with a large bandwidth they are currently using with an on-chip spectrometer the size of a penny," noted Adibi. "Our device has the potential to be a high-resolution, lightweight, compact, high-speed and versatile micro-spectrometer with a large dynamic range that can be used for many applications."
His team is now working on a version with up to 1,000 resonators, hoping to achieve 0.15nm resolution with a spectral range of 150nm on a 200µm2 footprint.
The spectrometer architecture was described in the journal Optics Express.