Photonic-crystal slab for biodetection

The public-health and biosecurity communities need biosensors that are sensitive, operate in real time, and can be easily deployed. Many approaches are being pursued, including one by a group from Lawrence Livermore National Laboratory that uses a photonic crystal (PC) slab connected to two waveguides. The idea is for viruses or other tiny pathogens to randomly infiltrate the pores of a silicon PC, whose optical properties—specifically, the in-plane transmission spectrum’s band edge—change accordingly. First, the researchers used simulations to determine a PC geometry suitable for a specific virus, vaccinia, and for their laser’s wavelength. They then fabricated an appropriate 17 × 17 array of 280-nm pores and exposed it to a flux of polystyrene beads with two different sizes; those with 260-nm diameter entered the pores (see the figure, with empty and filled pores enlarged) while 320-nm ones did not. The measured band-edge redshifts were then used to calibrate the simulations and predict detection limits. Theoretically, as few as 10 vaccinia viruses could be detected with the PC, comparable to other biodetection methods. Advantages accrue, however, from the small sensor size, the ability to tune the geometry for different particles, and the ease of integration into lab-on-a-chip setups. The authors say that their random-binding scheme is more practical than methods that rely on binding organisms to single PC defects. (S. E. Baker et al., Appl. Phys. Lett. 97, 113701, 2010 http://dx.doi.org/10.1063/1.3487998 .)