Ultrafast all-optical processing functional devices with high response speed, low power consumption, and easy integration have now achieved significant applications in optical communications, quantum computing, optical sensing, and data processing. Ultrafast light modulation and efficient wavelength conversion can be realized by manipulating light-matter interaction, particularly through nonlinear optical effects. However, the weak nonlinearities of most optical materials, along with the phase mismatch issue, limit their application in signal processing.

Recently, a class of optical materials with epsilon-near-zero (ENZ) characteristics has attracted the attention of researchers, exhibiting significantly enhanced nonlinear optical response and reduced phase-matching requirements with increasing incident light intensity. Among the diverse types of ENZ materials, transparent conductive oxides (TCOs) have broken through the inherent limitations of mutual exclusion of "optical transparency" and "conductivity" in traditional materials, exhibiting the characteristics of metal-like, wide band gap, high light transmittance, and ease to tunability, which emerges as outstanding in the ENZ system. As the typical representative of TCOs, aluminum-doped zinc oxide (AZO) possesses the remarkable merit of non-toxicity, easy fabrication, high cost-effectiveness, and low loss, making it an ideal nonlinear optical material candidate for all-optical signal processing. Currently, researchers have extensively studied the preparation methods and nonlinear optical response of AZO materials and explored their applicability in optoelectronic devices. However, the broadband nonlinear optical response and harmonic generation capabilities still require urgent investigation.

The research group led by Prof. Chujun Zhao from Hunan University systematically investigated the ultrafast nonlinear optical modulation and efficient wavelength conversion performance of the AZO film inspired by the requirements in high-speed and broadband all-optical signal processing. The research results are published in Chinese Optics Letters, Vol. 23, Issue 9, 2025: Yuanyuan Gou, Yuan He, Tiantian Zhou, Yi Feng, Lili Miao, Chujun Zhao. Ultrafast optical response and efficient wavelength conversion in epsilon-near-zero aluminum-doped zinc oxide thin film[J]. Chinese Optics Letters, 2025, 23(9): 091901

In this work, the pump light (pulse width: 35 fs, repetition rate: 1 kHz) was generated by an optical parametric amplifier (OPA) pumped by a Ti: sapphire laser. The spectral information and power of the third-harmonic signal were collected by a spectrometer (Avantes, AvaSpec-ULS4096CL-EVO) and a photodetector (Thorlabs, PM100D), respectively, as shown in the figure below. The arrow labeled "z" indicates the moving direction of the AZO sample, which was used to locate the position of maximum harmonic generation. The other arrows indicate the directions of beam propagation. All experiments have been done in complete darkness to avoid interference from ambient light. The wavelength- and intensity-dependent properties of the third-harmonic generation (THG) in AZO film have been investigated by tuning the wavelength and the power, respectively.

The experimental results demonstrate that the AZO thin film exhibits picosecond response and broadband wavelength-dependent nonlinear absorption and refraction properties. In addition, the AZO film shows the strongest THG and the highest conversion efficiency 0.63×10⁻⁶ at the ENZ wavelength. This enhancement can result from the ENZ-induced field enhancement effect and reduced phase-matching condition. In the future, the research team will focus on exploring the underlying broadband ultrafast nonlinear optical mechanism of the ENZ material system and exploring the potential optoelectronic applications in all-optical signal processing.

Schematic diagram of the experimental setup for THG measurement.