Gonzalo Alvarez-Pérez

arXiv2026

Plasmonic-cavity Modulator for the Mid-IR with a Semi-transparent and Nonlinear Heavily-doped Semiconductor Mirror

We present a free-space plasmonic modulator based on a single heavily-doped semiconductor layer. We investigate its ability to modulate both the linear and nonlinear response at mid-infrared frequencies slightly below the plasma frequency of the semiconductor. We demonstrate electric control of the linear transmittance and reflectance, and of the efficiency of third-harmonic generation with a field-effect gate structure. We discuss further performance optimization of the device in terms of modulation speed and depth towards a fast modulator with very simple active material requirements. Our results establish a viable route toward practical plasmonic modulators and mixers operating in the mid-infrared atmospheric window available for free-space communications at wavelengths between 8 and 12 um.

physics.optics↗

arXiv2026

In-plane vector-field imaging of propagating surface phonon polaritons

Polariton interferometry through optical near-field microscopy has become a powerful tool in nanophotonics, enabling direct spatial access to the propagation characteristics of strongly confined, evanescent polariton modes. Scattering-type near-field optical microscopy has matured as the prime tool for such studies, yet mostly the out-of-plane components of the optical near fields are probed, owing to the elongated geometry of the nanotip. Here, we demonstrate a complementary far-field nonlinear microscopy approach which allows to selectively probe in-plane polariton field components. Accessing the full vector field is interesting when studying complex mode patterns such as hyperbolic polaritons or skyrmions, where the in-plane field components are typically only inferred from the out-of-plane component but not measured directly. To this end, we use nonlinear infrared-visible wide-field sum-frequency generation microscopy, where the short visible wavelength of the nonlinear signal provides the high spatial resolution to access evanescent modes in the infrared. The symmetry selection rules of the nonlinear process further enable polarization-selective imaging of both in-plane polariton field components through spatial interferometry. The concept is demonstrated experimentally using surface phonon polaritons at the AlN-air interface launched by a gold antenna. A simple, semi-analytical model reproduces the peculiar propagation patterns. Hyperspectral imaging with a tunable narrowband laser further gives access to the polariton dispersion. The wide-field methodology holds high promise for in-depth and high-throughput studies of infrared nanophotonic structures.

physics.optics↗