Lightsabers (“laster swords”) for improving photodetector speed and responsivity

摘要

The micrometer scale of optics is significantly larger than the nanometer scale of modern electronic devices. To produce photodiodes yielding both superior speed and responsivity, a critical challenge is to confine the incident light efficiently to an active region having a small (subwavelength) area. In recent years, plasmonics has been applied as a means to confine light to subwavelength areas. In this case, the plasmonic structure converts the incident (far-field) light into near fields in order to achieve the sub-wavelength confinement. However, the surface plasmons are a near-field phenomenon such that the electromagnetic energy does not penetrate deeply. Further, surface plasmon resonances are generated only over narrow range of frequencies. Thus, the question arises: can we avoid the conversion to near fields and propagate the light into the semiconductor over a sub-wavelength area? When desired, can we propagate broadband electromagnetic energy into the sub-wavelength area to provide efficient broadband photodiodes? The latter may especially be desirable if the common silicon semiconductor is replaced with a more broadband semiconductor such as graphene. Here, it is proposed that a propagating sub-wavelength beam of light called a photonic nanojet and resembling a lightsaber or “laser sword” can be used to focus light onto the small active area of a photodiode. Exploratory three-dimensional, Maxwell's equations finite-difference time-domain (FDTD) simulations are conducted and demonstrate that the nanojets can confine light to an area comparable to a nanostructured dipole antenna while propagating multiple wavelengths into the semiconductor, even over a broad range of frequencies when desirable.