The extension of the operating range on the low-frequency side down to <300 GHz in a terahertz nonlinear quantum cascade laser source based on intracavity frequency mixing in a long-wavelength (~13.7 μm), high-performance mid-infrared active region was achieved. The device was fabricated two section distributed feedback gratings with slightly different periods in order to obtain quite close dual mid-infrared laser pumps (λ₁ ~13.53 μm and λ₂ ~13.39 μm), achieving an emission at a frequency of 231 GHz as the difference frequency at room temperature. This is the lowest operating frequency in electrically pumped monolithic semiconductor laser sources operatable at room temperature.

Surface phonon polaritons (SPhPs) show great promise for tailoring light-matter interactions in systems below the diffraction limit. Investigating SPhP modes has mostly been pursued by measuring the energy resonances of these modes (i.e., eigenvalues). In other instances the study of SPhPs has been accomplished, by mapping electromagnetic fields (i.e., eigenstates) only at the material interface by atomic force assisted techniques, and in some limited cases measuring the three-dimensional fields using electron scattering. An accurate knowledge of SPhPs has been hindered by the lack of experimental techniques to map eigenstates in three dimensions, that are easy, cheap, and non-destructive. In this work we demonstrate the direct experimental measurement of infrared SPhPs eigenstates through three-dimensional Raman mapping. We apply this technique to map SPhPs in nanopillars of Indium Phosphide (InP). Furthermore, we demonstrate that SPhPs couple to bulk Raman modes through the material's polarizability and to a lesser extent via electron-phonon coupling. These observations provide a new method for measuring SPhP modes in nanostructured materials, as well as a novel way to investigate the physical phenomena involved in the coupling of bulk phonons to SPhPs.

High-index nanomaterials play a substantial role in the enhancement of optical effects based on electric and magnetic Mie resonances. While hexagonal boron nitride (hBN) has been heavily explored within the Reststrahlen bands (RB) as a natural hyperbolic phonon polariton material [1], close to the transversal optical modes outside the RB the dielectric constant has extremely high positive values. The latter provides the opportunity of producing dielectric resonators with very large dielectric constants. We report infrared Mie resonances of hBN nanodisks (NDs). Reflection and transmission spectra of hBN NDs of different sizes have been investigated to understand Mie resonances within the infrared range. We show the presence of a strong magnetic dipole resonance which energy and strength depends on the size and geometry of the hBN NDs as well as the substrate properties. Finite element modeling of the electromagnetic fields has been performed and is in excellent agreement with our experimental results. Numerical and experimental data have indicated that by selecting the proper substrate thickness and hBN NDs radius, much more prominent Mie resonances are achieved. Mie resonances provide an opportunity to easily manipulate light confinement for the design of optical devices such as nanoresonators, nanolasers, highly efficient metasurfaces and ultrafast metadevices.

[1] Caldwell, Joshua D., et al. "Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride." Nature communications 5.1 (2014): 5221.

⁺Author for correspondence: Milad.Nourbakhsh-1@ou.edu