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We have developed GaInAsP semiconductor photonic crystal nanolaser biosensor, which detects the adsorption of proteins and DNAs from ultralow concentrations via the wavelength shift. In general, bio-chemical sensors based on photonic cavities are considered to detect the environmental refractive index change. In this study, however, we observed various evidence that the wavelength shift of the nanolaser sensor reflects not only the environmental index but also the surface charge of biomolecules and/or environmental ions. Different from the pH-sensitivity in the laser emission intensity we reported previously, the wavelength shift is extremely sensitive to the surface charge and ions so that we can detect the biomolecules from fM to aM. To investigate the mechanism, we exposed the device to plasma or DNA solution, and measured the zeta potential and flatband potential as well as the wavelength shift. From the consistent behaviors between them, we constructed a physical model that the surface charge modifies the Schottky barrier near the semiconductor surface, trap the excited carriers in the barrier, and produce the carrier plasma shift in the wavelength. Since the change of the flatband potential is already observed for fM DNA, we suppose a phenomenon such that a small number of charged molecules act as a trigger for changing the charged functional group on the device surface. These results also suggest the possibility that some other photonic sensors also involves similar electrochemical effects.
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