Spatio-spectral optical fission in time-varying subwavelength layers

TL;DR

This paper demonstrates spatio-spectral fission of ultra-fast pulses in a time-varying aluminium zinc oxide layer, revealing extreme nonlinear phenomena and advancing understanding of transient optical properties in transparent conducting oxides.

Contribution

It introduces a model accounting for space and time refraction in non-stationary layers, explaining spectral and energy separation during ultra-fast pulse propagation.

Findings

Observed spatial separation of spectrum and energy due to time-varying refractive index.

Provided a quantitative model for space and time refraction effects.

Revealed extreme nonlinear phenomena at subwavelength scales.

Abstract

Transparent conducting oxides are highly doped semiconductors that exhibit favourable characteristics when compared to metals, including reduced material losses, tuneable electronic and optical properties, and enhanced damage thresholds. Recently, the photonic community has renewed its attention towards these materials, recognizing their remarkable nonlinear optical properties in the near-infrared spectrum, a feature previously overlooked despite their long-standing application in photovoltaics and touchscreens. The exceptionally large and ultra-fast change of the refractive index, which can be optically induced in these compounds, extends beyond the boundaries of conventional perturbative analysis and makes this class of materials the closest approximation to a time-varying system, and a unique playground for studying a variety of novel phenomena within the domain of photon acceleration. Here we report the spatio-spectral fission of an ultra-fast pulse trespassing a thin film of aluminium zinc oxide with a non-stationary refractive index. By applying phase conservation to this time-varying layer, our model can account for both space and time refraction and explain in quantitative terms, the spatial separation of both the spectrum and energy. Our findings represent an example of extreme nonlinear phenomena on subwavelength propagation distances and shed light on the nature of several nonlinear effects recently reported not accounting for the full optical field distribution. Our work also provides new important insights into transparent conducting oxides transient optical properties which are critical for the ongoing research in photonic time crystals, on-chip generation of nonclassical states of light, integrated optical neural networks as well as ultra-fast beam steering and frequency division multiplexing