# Ultrafast Nonlinear Dynamics of Indium Tin Oxide Nanocrystals Probed via Fieldoscopy

## TL;DR

This study explores how indium tin oxide nanocrystals can be used for high-speed optical switching by measuring their response to ultrafast laser pulses.

## Contribution

The paper introduces fieldoscopy to reveal the first transient electric-field response of ITO nanocrystals under high-frequency optical excitation.

## Key findings

- Modulation remains reversible up to 1.2 mJ cm−2 with 10% depth and becomes irreversible beyond 3.3 mJ cm−2 with 20% depth.
- At higher fluences, the first cycle of optical pulses contributes less to the overall response.
- The modulation bandwidth spans from 2 to 2.5 µm, suitable for terahertz switching applications.

## Abstract

Scalable, high‐speed, small‐footprint photonic switching platforms are essential for advancing optical communication. An effective optical switch must operate at high duty cycles with fast recovery times, while maintaining substantial modulation depth and full reversibility. Colloidal nanocrystals, such as indium tin oxide (ITO), offer a scalable platform to meet these requirements. In this work, the transmission of ITO nanocrystals near their epsilon‐near‐zero wavelength is modulated by two‐cycle optical pulses at a repetition rate of one megahertz. The modulator exhibits a broad bandwidth spanning from 2 to 2.5 µm. Sensitive fieldoscopy measurements resolve the transient electric‐field response of the ITO for the first time, showing that the modulation remains reversible for excitation fluences up to 1.2 mJ cm−2 with a modulation depth of 10%, and becomes fully irreversible beyond 3.3 mJ cm−2, while reaching modulation depth of up to 20%. Field sampling further indicates that at higher excitation fluences, the relative contribution from the first cycle of the optical pulses is reduced. These findings are crucial for the development of all‐optical switching, telecommunications, and sensing technologies capable of operating at terahertz switching frequencies.

The ultrafast nonlinear dynamics of indium tin oxide nanocrystals excited with 1 MHz, 10 fs pulses, at their localized surface plasmon frequency, and near their epsilon‐near‐zero wavelength is probed via fieldoscopy. The study demonstrates fluence‐dependent switching behavior, advancing the understanding of high‐speed optical modulation in plasmonic semiconductor nanocrystals.