Integration of plasmonic antenna on quantum cascade laser facets for chip-scale molecular sensing

Many important bio-molecules, such as proteins and pharmaceuticals, have their natural resonances in the mid-infrared (2 - 30μm) region of the optical spectrum. The primary challenge of sensing these molecules is to increase the interaction between them and light with such long wavelengths. This can be overcome by exploiting optical nano-antennas which can squeeze the optical mode into a volume much smaller than the operating wavelength. We present a novel antenna design based on hybrid materials composed of a coupled Au-SiO ₂-Au nanorod integrated on the facet of a quantum cascade laser (QCL) operating in the mid-infrared region of the optical spectrum. FDTD simulations showed that for sandwiched dielectric thicknesses within the range of 20 to 30 nm, peak optical intensity at the top of the antenna ends is 4000 times greater than the incident field intensity. The device was fabricated using focused ion beam milling. Apertureless mid-infrared near field optical microscopy (NSOM) showed that the device can generate a spatially confined spot within a nanometric size about 12 times smaller than the operating wavelength. Such high intensity, hot spot locations can be exploited to enhance the photon interaction for bio-molecules for sensing applications.