Quantum Cascade Laser integrated with metal-dielectric-metal plasmonic antenna

In the near field region, optical antennas can generate local hot spots with high energy density. It can be very useful in increasing the photon-matter interactions for bio-sensing applications. There are several important bio-molecules having signature frequency (vibrational resonance) matching the mid infrared region of the optical spectrum. Thus mid-infrared antenna integrated with Quantum cascade laser (QCL) is highly desirable as it is currently considered to be one of the most efficient mid-infrared laser sources with a huge gamut of commercial applications. Here, we present a novel metal-dielectric-metal (MDM) based plasmonic nanorod antenna integrated on the facet of a room temperature working Quantum Cascade Laser. Simulations showed that at an optimized SiO₂ thickness of 20nm, the antenna can generate a local electric field with intensity 500 times higher than the incident field intensity. Further, it can increase the number of regions with local hot spots due to a higher number of geometrical singularities or sharp edges present in the MDM structure. This feature can be extremely useful, especially for bio-sensing applications. All device structures have been optimized based on 3d finite-difference timedomain (FDTD) numerical simulations. The antenna was fabricated on the facet of QCL using focused ion beam (FIB). The integrated plasmonic QCL has been measured using an apertureless mid-infrared near field scanning optical microscopy (a-NSOM). The measurement set-up is based on an inverted microscope coupled with a commercially available Atomic Forced Microscopy (AFM). We have experimentally found that such integrated nano antenna can generate a very narrow optical spot size, much below the diffraction limit, with high power density that matches well with the simulation results.