There are many applications which require high sensitivity spectral detection. In some cases, you need the wavelength range to be extended to cover all necessary spectral fingerprints. We are proposing a broadband spectrometer for ultrasensitive detection based on plasmonic hyperbolic metamaterials and diffraction gratings. Using variety of materials in fabrication of the hyperbolic metamaterials, we can cover the wide spectral range from near UV (~250 nm) to IR (~2 μm). In our spectrometer, the diffraction gratings have two functions. One is coupling the incident light source with the plasmonic guiding modes, which have a very high effective refractive index (≥8.1), much higher than the refractive index of germanium (4.05), the natural material with the highest refractive index. While a prism can also be used for coupling guiding modes with incident light, a diffraction grating is the only way to excite the guiding modes because of the plasmonics modes with very high effective refractive index. The second function of the diffraction gratings is their natural role in spectrometers. We demonstrated based on numerical simulations that we could reach high detection spectral sensitivity using compact diffraction gratings combined with hyperbolic metamaterials; the huge “n-meter” spectrometer is not necessary.
Plasmonic structures for biomedical sensing are in use for a long time. However, there is a fundamental limitation of their sensitivity due to low effective refractive index of layered plasmonic structures. We are proposing a hyperbolic metamaterial (HMM) structure which is a combination of surface plasmon Polaritons (SPPs) and long-range surface plasmon Polaritons (LRSPPs) modes. The result of the interaction between these modes leads to plasmonic modes with ultra-high effective refractive index. We calculated and optimized plasmonic HMM structure with effective refractive index equal to 8.1, i.e. twice as much as that of germanium, a natural material with the highest refractive index. We simulated these structures for gold, silver, copper and aluminum. The best way to use these structures for protein sensing is to use diffraction gratings – there is no natural material which can be used as a prism. By optimizing layer parameters and diffraction grating we were able to build a model of the structure with sensitivity as 10-9 for refractive index. We are hoping to achieve sensitivity up to 10-11, so this structure can be used for different protein sensing application including detection of metastatic cells spreading the human body.