RESUMO
The light emission characteristics of GaInAsP semiconductors in aqueous solutions are modified by the surface charge, depending on the balance between radiative and nonradiative recombination. This Letter demonstrates the application of a GaInAsP photonic crystal band-edge laser in sensing a solution pH, which reflects the surface charge. The sensitivity is enhanced by a hybrid of fractal honeycomb and close-packed structures with a large surface-to-volume ratio and a high quality factor (Q), which allows a low threshold carrier density. We experimentally obtained a maximum pH sensitivity of 1.9 dB/pH near the threshold and a signal-to-noise ratio of 52/pH (pH resolution of 0.019) at a pump power 2.4 times the threshold where the intensity noise diminished.
RESUMO
In general, biochemical sensors based on photonic cavities are used to detect changes in the refractive index of the environment. In this study, however, a GaInAsP semiconductor photonic-crystal nanolaser sensor that we recently developed was found to detect not only the environmental refractive index but also the surface charge. In contrast to the pH sensitivity we reported previously, this is an ultra-sensitive detection mechanism capable of identifying proteins and deoxyribonucleic acids (DNA) at a femtomolar-order or lower concentrations. When the device is exposed to plasma or DNA solutions, the laser wavelength simultaneously changes with the zeta potential and the flat-band potential of the semiconductor surface. This indicates that the charged functional groups on the surface, which are formed by these treatments, modify the Schottky barrier near the semiconductor surface, trap the excited carriers in the barrier, and change the refractive index of the semiconductor via the carrier effects. These findings also suggest that some other photonic sensors may also exhibit similar electrochemical and optoelectronic effects.