Design of infrared microspectrometers based on phase-modulated axilenses

We design and characterize a novel axilens-based diffractive optics platform that flexibly combines efficient point focusing and grating selectivity and is compatible with scalable top-down fabrication based on a 4-level phase mask configuration. This is achieved using phase-modulated compact axilens devices that simultaneously focus incident radiation of selected wavelengths at predefined locations with larger focal depths compared to traditional Fresnel lenses. In addition, the proposed devices are polarization insensitive and maintain a large focusing efficiency over a broad spectral band. Specifically, here we discuss and characterize modulated axilens configurations designed for long-wavelength infrared (LWIR) in the $6~\mu$m--12~$\mu$m wavelength range and in the $4~\mu$m--6~$\mu$m mid-wavelength infrared (MWIR) range. These devices are ideally suited for monolithic integration atop the substrate layers of infrared focal plane arrays (IR-FPAs) and for use as compact microspectrometers. We systematically study their focusing efficiency, spectral response, and cross talk ratio, and we demonstrate linear control of multi-wavelength focusing on a single plane. Our design method leverages Rayleigh-Sommerfeld (RS) diffraction theory and is validated numerically using the Finite Element Method (FEM). Finally, we demonstrate the application of spatially modulated axilenses to the realization of compact, single-lens spectrometer. By optimizing our devices, we achieve a minimum distinguishable wavelength interval of $\Delta\lambda=240nm$ at $\lambda_0=8{\mu}m$ and $\Delta\lambda=165nm$ at $\lambda_0=5{\mu}m$. The proposed devices add fundamental spectroscopic capabilities to compact imaging devices for a number of applications ranging from spectral sorting to LWIR and MWIR phase contrast imaging and detection

Enhanced Nonlinearity of Epsilon-Near-Zero Indium Tin Oxide Nanolayers with Tamm Plasmon-Polariton States

Recently, materials with vanishingly small permittivity, known as epsilon-near-zero (ENZ) media, emerged as promising candidates to achieve nonlinear optical effects of unprecedented magnitude on a solid-state platform. In particular, the ENZ behavior of Indium Tin Oxide (ITO) thin films resulted in Kerr-type nonlinearity with non-perturbative refractive index variations that are key to developing more efficient Si-compatible devices with sub-wavelength dimensions such as all-optical switchers, modulators, and novel photon detectors. In this contribution, we propose and demonstrate enhancement of the nonlinear index variation of 30 nm-thick ITO nanolayers by silicon dioxide/silicon nitride (SiO2/SiN) Tamm plasmon-polariton structures fabricated by radio-frequency magnetron sputtering on transparent substrates under different annealing conditions. In particular, we investigate the linear and nonlinear optical properties of ITO thin films and resonant photonic structures using broadband spectroscopic ellipsometry and intensity dependent Z-scan nonlinear characterization demonstrating enhancement of optical nonlinearity with refractive index variations as large as in the non-perturbative regime. Our study reveals that the efficient excitation of strongly confined plasmon-polariton Tamm states substantially boost the nonlinear optical response of ITO nanolayers providing a stepping stone for the engineering of more efficient infrared devices and nanostructures for a broad range of applications including all-optical data processing, nonlinear spectroscopy, sensing, and novel photodetection modalities

Shaping, imaging and controlling plasmonic interference fields at buried interfaces

Filming and controlling plasmons at buried interfaces with nanometer (nm) and femtosecond (fs) resolution has yet to be achieved and is critical for next generation plasmonic/electronic devices. In this work, we use light to excite and shape a plasmonic interference pattern at a buried metal-dielectric interface in a nanostructured thin film. Plasmons are launched from a photoexcited array of nanocavities and their propagation is filmed via photon-induced near-field electron microscopy (PINEM). The resulting movie directly captures the plasmon dynamics, allowing quantification of their group velocity at approximately 0.3c, consistent with our theoretical predictions. Furthermore, we show that the light polarization and nanocavity design can be tailored to shape transient plasmonic gratings at the nanoscale. These results, demonstrating dynamical imaging with PINEM, pave the way for the fs/nm visualization and control of plasmonic fields in advanced heterostructures based on novel 2D materials such as graphene, MoS$_2$, and ultrathin metal films.Comment: 16 pages, 5 figures, 3 supplementary figure