Near-zero-index ultra-fast pulse characterization.

Transparent conducting oxides exhibit giant optical nonlinearities in the near-infrared window where their linear index approaches zero. Despite the magnitude and speed of these nonlinearities, a "killer" optical application for these compounds has yet to be found. Because of the absorptive nature of the typically used intraband transitions, out-of-plane configurations with short optical paths should be considered. In this direction, we propose an alternative frequency-resolved optical gating scheme for the characterization of ultra-fast optical pulses that exploits near-zero-index aluminium zinc oxide thin films. Besides the technological advantages in terms of manufacturability and cost, our system outperforms commercial modules in key metrics, such as operational bandwidth, sensitivity, and robustness. The performance enhancement comes with the additional benefit of simultaneous self-phase-matched second and third harmonic generation. Because of the fundamental importance of novel methodologies to characterise ultra-fast events, our solution could be of fundamental use for numerous research labs and industries.

Visible photon generation via four-wave mixing in near-infrared near-zero-index thin films.

Optical nonlinearities can be strongly enhanced by operating in the so-called near-zero-index (NZI) regime, where the real part of the refractive index of the system under investigation approaches zero. Here we experimentally demonstrate semi-degenerate four-wave mixing (FWM) in aluminum zinc oxide thin films generating radiation tunable in the visible spectral region, where the material is highly transparent. To this end, we employed an intense pump (787 nm) and a seed tunable in the NIR window (1100-1500 nm) to generate a visible idler wave (530-620 nm). Experiments show enhancement of the frequency conversion efficiency with a maximum of 2% and a signal-to-pump detuning of 360 nm. Effective idler wavelength tuning has also been demonstrated by operating on the temporal delay between the pump and signal.

Dynamic Control of Nanocavities with Tunable Metal Oxides.

Fabry-Pérot metal-insulator-metal (MIM) nanocavities are widely used in nanophotonic applications due to their extraordinary electromagnetic properties and deeply subwavelength dimensions. However, the spectral response of nanocavities is usually controlled by the spatial separation between the two reflecting mirrors and the spacer's refractive index. Here, we demonstrate static and dynamic control of Fabry-Pérot nanocavities by inserting a plasmonic metasurface, as a passive element, and a gallium doped-zinc oxide (Ga:ZnO) layer as a dynamically tunable component within the nanocavities' spacer. Specifically, by changing the design of the silver (Ag) metasurface one can "statically" tailor the nanocavity response, tuning the resonance up to 200 nm. To achieve the dynamic tuning, we utilize the large nonlinear response of the Ga:ZnO layer near the epsilon near zero wavelength to enable effective subpicosecond (<400 fs) optical modulation (80%) at reasonably low pump fluence levels (9 mJ/cm2). We demonstrate a 15 nm red shift of a near-infrared Fabry-Pérot resonance (λ ≅ 1.16 µm) by using a degenerate pump probe technique. We also study the carrier dynamics of Ga:ZnO under intraband photoexcitation via the electronic band structure calculated from first-principles density functional method. This work provides a versatile approach to design metal nanocavities by utilizing both the phase variation with plasmonic metasurfaces and the strong nonlinear response of metal oxides. Tailorable and dynamically controlled nanocavities could pave the way to the development of the next generation of ultrafast nanophotonic devices.