Nanoplasmonics in the mid-infrared

Surface plasmon polaritons (SPPs) are light waves propagating along an interface between a metal and a dielectric, e.g. gold and air. The resonant interaction of the electromagnetic waves and the collective oscillation of electrons in the metal plays an important role in the confinement, guiding and strong local enhancement of electromagnetic waves, which may be difficult or impossible to achieve using other technologies. Plasmonics have been widely used to enhance the interaction of electromagnetic radiation with chemical substances, as well as living cells. The mid-infrared spectral region is of particular interest for sensing applications, which is a strong driving force for ongoing research. In the visible and near infrared, plasmonic structures solve many fundamental problems in sensing, imaging and on-chip communication. Due to the different material properties of metals at longer wavelengths, this knowledge cannot be easily transferred to the mid-infrared, because surface waves are weakly bound to the metal-dielectric interface with an evanescent decay penetrating deep into the dielectric medium. In our previous research activities we presented an alternative way to overcome this issue by adding an additional thin dielectric layer on top of the metal. The aim of this research project is to provide a profound understanding of the fundamental properties of propagating and subwavelength confined SPPs in the mid-infrared, as well as to investigate their innovative potential for future integrated optics. In order to investigate the propagation properties of SPPs at mid-infrared frequencies, we will fabricate plasmonic structures at the Center for Micro- and Nanostructures at the TU Wien and will characterize the propagation using near-field microscopy. The project involves the analysis of waveguides and basic waveguide elements for integrated photonics, such as waveguide couplers and interferometers. We propose a metamaterial lens for propagating plasmons using an array of standing nanoantennas. Obeying Huygens principle, this metasurface can be used to engineer the optical wavefronts into arbitrary shapes via the phase of the scattered light from the individual antennas. The project further includes the investigation of plasmonic nanostructures to locally enhance and confine light beyond the classical diffraction limit. The direct coupling of light emitted by a quantum cascade laser to a plasmonic nanostructure will allow extremely high field intensities. This will be of great interest to study light matter interactions, e.g. of nonlinear or low dimensional materials, at high field intensities.

A monolithic platform to enable integrated mid-infrared sensors using lasers, detectors, plasmonics, frequency combs, etc. can be expected to have an enormous impact on our everyday life. Small and portable devices that capture data from the pollution in the air, infections of individual plants on a field, or our current physiological condition open up a variety of new possibilities. This FWF project enabled to proceed several major steps towards this goal, leading to a profound understanding of the fundamental physical principles governing plasmonic waveguides, nanoplasmonic structures as well as frequency comb formation in lasers both experimentally and from a theoretical point of view. A frequency comb is a form of light emitted by a laser source, that consists of a multitude of different frequencies, separated like the teeth of a comb by the same distance. Exploiting this concept for the detection of gases in the mid-infrared region of the spectrum allows to not only detect one species of molecules, which is the case for currently predominantly used techniques, but multiple at the same time. This is the case because different molecules can be distinguished by analyzing the frequency at which they absorb light, which is characteristic for each species. The semiconductor material systems we are using to build these frequency combs, quantum cascade lasers and interband cascade lasers, allow us to envision an entire spectroscopic measurement platform built on a single chip, where the generation, guiding and detection of the light can take place. This monolithic integration drastically reduces the size, ultimately resulting in a portable, possibly handheld and battery-powered sensor. Developing and understanding means to stabilize the fabricated frequency combs during operation against various unwanted influences, like temperature changes within the material or light that is scattered back into the device, is essential for the realization of said spectroscopic sensors. The interaction between different teeth of the comb can be seen in analogy to coupled pendulums. Starting with the simplest case of two pendulums, they can either be observed to oscillate in-phase, meaning in the same direction at the same time, or in anti-phase, corresponding to the opposite case, where they are moving in opposite directions. This concept can then be extended to a larger number of oscillators and is indeed an adequate description for our semiconductor frequency combs, as we have both experimentally and theoretically observed and confirmed. By applying alternating current, that matches the natural frequency of the optical comb, we can effectively stabilize it - a phenomenon known as coherent injection locking. By increasing the modulated current, we can force the comb to develop more teeth at different frequencies, thereby allowing the detection of even more chemical species when applying our gained knowledge to a sensor.