Optical antenna metasurfaces have attracted substantial attention in recent years, as they may enable new classes of planar optical elements. However, actively tuning nanoantenna resonances, whether dielectric or plasmonic, remains an unresolved challenge. In this work, we investigate tuning mid-infrared (MIR) Mie resonances in semiconductor subwavelength particles by directly modulating the permittivity with free charge carriers. Using femtosecond laser ablation, we fabricate spherical silicon and germanium particles of varying sizes and doping concentrations. Single-particle infrared spectra reveal electric and magnetic dipole, quadrupole, and hexapole resonances. We first demonstrate size-dependent Si and Ge Mie resonances spanning the entire mid-infrared (2-16 μm) spectral range. We subsequently show doping-dependent resonance frequency shifts that follow simple Drude models. Taking advantage of the large doping dependence of Si and Ge MIR permittivities, we demonstrate a huge tunability of Mie resonance wavelengths (up to ~ 9 μm) over a broad 2-16 μm MIR range. This tuning range corresponds to changes of permittivity as large as 30 within a single material system, culminating in the emergence of plasmonic modes at high carrier densities and long wavelengths. We also demonstrate dynamic tuning of intrinsic semiconductor antennas using thermo-optic effects. These findings demonstrate the potential for actively tuning infrared Mie resonances, thus providing an excellent platform for tunable metamaterials.
Dielectric optical antenna resonators have recently emerged as a viable alternative to plasmonic resonators for metamaterials and nanophotonic devices, due to their ability to support multipolar Mie resonances with low losses. In this work, we experimentally investigate the mid-infrared Mie resonances in Si and Ge subwavelength spherical particles. In particular, we leverage the electronic and optical properties of these semiconductors in the mid-infrared range to design and tune Mie resonators through free-carrier refraction.
Si and Ge semiconductor spheres of varying sizes of 0.5-4 μm were fabricated using femtosecond laser ablation. Using single particle infrared spectroscopy, we first demonstrate size-dependent Si and Ge Mie resonances spanning the entire mid-infrared (2-16 μm) spectral range. Subsequently we show that the Mie resonances can be tuned by varying material properties rather than size or geometry. We experimentally demonstrate doping-dependent resonance frequency shifts that follow simple Drude models of free-carrier refraction. We show that Ge particles exhibit a stronger doping dependence than Si due to the smaller effective mass of the free carriers. Using the unique size and doping dispersion of the electric and magnetic dipole modes, we identify and demonstrate a size regime where these modes are spectrally overlapping. We also demonstrate the emergence of plasmonic resonances for high doping levels and long wavelengths. These findings demonstrate the potential for tuning infrared semiconductor Mie resonances by optically or electrically modulating charge carrier densities, thus providing an excellent platform for tunable electromagnetic metamaterials.
We report the demonstration of single mode AgCl<sub>x</sub>Br<sub>1-x</sub> channel waveguides for mid-infrared range. The waveguides were made by the deposition of AgCl<sub>x</sub>Br<sub>1-x</sub> layers on top of a Ag/Ti/SiO<sub>2</sub>/Si substrate, followed by photolithographic and lift-off processing. We showed that these waveguides operate in a single mode for the 6-14 μm band. The propagation losses of 20 dB/cm were measured at λ=10.6 μm using the cut-back method. We discuss the possible propagation losses mechanisms and show that the waveguide sidewall roughness is likely the major contributor for these losses. Using this fabrication process we have also realized Y-couplers and splitters. The development of these waveguides is a crucial step towards realizing on-chip AgCl<sub>x</sub>Br<sub>1-x</sub> mid-infrared integrated optical circuits which will be used for applications such as chemical sensing and spectro-interferometry for planet detection.
In the astrophysical context of the search for Earth-like extrasolar planets, an important research effort has been done for
the realization of single-mode integrated optics devices for mid-infrared space-based interferometry. Preparatory projects
like FKSI , where rejection of high order modes is required to a level better than 40dB, will need photonic devices
that achieve modal filtering and beam combination in the mid-IR band. In this context, we present results on midinfrared
planar integrated optic beam combiners characterized at LAOG using chalcogenide and silver halide materials.
We show results on FTS measurements, allowing to determine the single mode spectral domain, as well as interference
fringes obtained from Y-junctions realized on these materials.
Modal filters are necessary to the proposed high-performance mid-infrared nulling interferometers, because they can
help achieve deeper interferometric nulls. Silver halide fibers of composition AgCl<sub>x</sub>Br<sub>1-x</sub>(0<x<1) are leading
candidates for these purposes, due to their high transparency in 4-20 μm spectral range. We have fabricated silver
halide fibers with small cores and small differences between the refractive indices of the core and the clad. These
operated as single mode fibers. An outer absorbing layer was applied to strip off cladding modes and reduce the
minimum fiber length needed for modal filtering. Short sections of such fibers exhibited round and symmetrical
optical field mode distributions with losses of 10-15dB/m at λ=10.6μm. We have tested the modal filtering
properties of such short fiber sections. We found that the presence of unsuppressed cladding modes at the output is
the main factor limiting the fiber's performance as a modal filter. This effect can be mitigated by appropriate
aperturing of the output. With a properly sized aperture, a 10.5 cm long fiber can suppress the power in nonfundamental
modes by a factor larger than 17000. If an aperture is not used, the suppression is reduced by a factor of