Pedestal High-Contrast Gratings for Biosensing

High-contrast gratings (HCG) are an excellent candidate for label-free detection of various kinds of biomarkers because they exhibit sharp and sensitive optical resonances. In this work, we experimentally show the performance of pedestal HCG (PHCG), which is significantly enhanced in comparison with that of conventional HCG. PCHGs were found to provide a 11.2% improvement in bulk refractive index sensitivity, from 482 nm/RIU for the conventional design to 536 nm/RIU. The observed resonance was narrower, resulting in a higher Q-factor and figure of merit. By depositing Al2O3, HfO2, and TiO2 of different thicknesses as model analyte layers, surface sensitivity values were estimated to be 10.5% better for PHCG. To evaluate the operation of the sensor in solution, avidin was employed as a model analyte. For avidin detection, the surface of the HCG was first silanized and subsequently functionalized with biotin, which is well known for its ability to bind selectively to avidin. A consistent red shift was observed with the addition of each of the functional layers, and the analysis of the spectral shift for various concentrations of avidin made it possible to calculate the limit of detection (LoD) and limit of quantification (LoQ) for the structures. PHCG showed a LoD of 2.1 ng/mL and LoQ of 85 ng/mL, significantly better than the values 3.2 ng/mL and 213 ng/mL respectively, obtained with the conventional HCG. These results demonstrate that the proposed PHCG have great potential for biosensing applications, particularly for detecting and quantifying low analyte concentrations.

Photoanisotropic incoherent-to-coherent converter using a bacteriorhodopsin thin film

We have fabricated a novel optically addressed spatial light modulator by using bacteriorhodopsin immobilized in a polymer thin film. This incoherent-to-coherent converter is based on photoinduced anisotropy, as distinguished from previously reported devices based on photochroism in bacteriorhodopsin. This system is capable of a resolution of 114 lines/mm with a contrast ratio of 100:1.

Dark spatial solitons in bacteriorhodopsin thin films

We report on the observation of dark spatial solitons in thin solid films containing bacteriorhodopsin. Because of the high nonlinearity of the material, the dark spatial solutions can be easily observed even with a few milliwatts of power from a He-Ne laser. This result promises novel applications for optical switching and optical interconnects.