Latest Advances in Ultrafast Nano-Spectroscopy and Imaging Technologies: Applications of Tip-Based Microscopy

Research Background

In recent years, with the rapid development of optical microscopy technology, scientists have made significant progress in understanding nanoscale physical phenomena. However, traditional far-field optical microscopy is limited by the optical diffraction limit, making it difficult to achieve subwavelength spatial resolution. Meanwhile, the increasing demand for research on novel materials such as quantum materials, two-dimensional (2D) materials, and organic molecular materials has grown. Light-matter interactions in these materials often occur on extremely short timescales (femtoseconds to nanoseconds) and at very small spatial scales (nanometers to angstroms). Therefore, developing microscopy techniques that can simultaneously provide high spatial and temporal resolution has become a key focus of scientific research.

To overcome the limitations of traditional optical microscopy, scanning probe microscopy (SPM) has gradually emerged. In particular, SPM methods combined with ultrafast optical technologies, such as ultrafast scattering-type scanning near-field optical microscopy (Ultrafast s-SNOM), ultrafast nanofocusing, and ultrafast scanning tunneling microscopy (Ultrafast STM), have provided powerful tools for studying light-matter interactions at the nanoscale. These techniques not only reveal complex phenomena such as polaritons, quantum phase transitions, and many-body effects in materials but also capture dynamic processes across both time and space dimensions.

The review written by Zhao et al. aims to systematically summarize the working principles, latest developments, and applications of the above three ultrafast microscopy techniques in materials science, while also discussing future directions.

Source of the Paper

This review was co-authored by Zhichen Zhao, Vasily Kravtsov, Zerui Wang, Zhou Zhou, Linyuan Dou, Di Huang, Zhanshan Wang, Xinbin Cheng, Markus B. Raschke, and Tao Jiang. The authors are affiliated with institutions such as the MOE Key Laboratory of Advanced Micro-Structured Materials at Tongji University, ITMO University in Russia, and the University of Colorado Boulder in the United States. The paper was published in the journal eLight (Volume 5, Issue 1, 2025) and is accessible via DOI: 10.1186/s43593-024-00079-1.

Main Content and Analysis

1. Ultrafast s-SNOM Technology

Working Principles

Ultrafast s-SNOM is a non-invasive and versatile technique capable of probing carrier and lattice dynamics in various materials with high spatial and temporal resolution. Its core lies in using an atomic force microscope (AFM) probe with a nanoscale tip to achieve subwavelength spatial resolution through near-field optical effects. In experiments, pump-probe methods are typically employed, where the pump pulse excites the sample, and the probe pulse collects near-field signals. By precisely controlling the time delay between the pump and probe pulses, femtosecond-level temporal resolution can be achieved.

Applications and Discoveries

Ultrafast s-SNOM has demonstrated excellent performance in studying spatial heterogeneity and polariton propagation in materials. For example, this technique successfully revealed the dynamics of Dirac plasmons in graphene with varying layer numbers. Additionally, by studying vanadium dioxide (VO₂) thin films, ultrafast s-SNOM observed the evolution of nanoscale heterogeneity during photoinduced insulator-to-metal transition (IMT).

2. Ultrafast Nanofocusing Technology

Working Principles

Ultrafast nanofocusing technology is based on the nanofocusing effect of surface plasmon polaritons (SPPs). Specifically, by designing a spiral grating on a metal conical tip, incident light can be coupled into SPPs, achieving 3D mode compression and field enhancement at the tip apex. This method forms a bright point-like light source at the tip, enabling background-free nano-spectroscopy and nano-imaging.

Applications and Discoveries

This technique is particularly suitable for studying nonlinear optical effects such as second-harmonic generation (SHG), four-wave mixing (FWM), and coherent anti-Stokes Raman scattering (CARS). For instance, in monolayer graphene, ultrafast nanofocusing successfully revealed the ultrafast dynamics of electron-electron scattering and electron-phonon scattering. Moreover, this technique also exhibited excellent performance in imaging vibrational modes of carbon nanotubes.

3. Ultrafast STM Technology

Working Principles

Ultrafast STM technology introduces ultrafast electromagnetic pulses into the scanning tunneling microscope (STM) junction to modulate the local electric field, achieving femtosecond-level temporal resolution. Depending on the Keldysh parameter (γ), the tunneling process can be divided into photon-driven tunneling and field-driven tunneling mechanisms. By adjusting the carrier-envelope phase (CEP) of the pulses, coherent control of tunneling electrons can be achieved.

Applications and Discoveries

Ultrafast STM technology offers unique advantages in studying molecular vibrational dynamics and quantum coherence phenomena. For example, in studies of pentacene molecules, this technique successfully captured coherent oscillations during the tunneling process. Additionally, by combining terahertz (THz) pulses, ultrafast STM achieved quantum state imaging of single hydrogen molecules.

Research Significance and Value

This review not only systematically summarizes the working principles and latest developments of ultrafast s-SNOM, ultrafast nanofocusing, and ultrafast STM technologies but also discusses their wide-ranging applications in materials science. For example, these techniques provide unprecedented spatiotemporal resolution for studying polaritons, quantum phase transitions, and many-body effects in two-dimensional materials. Furthermore, these techniques show great potential in quantum information science, such as identifying defects and heterogeneities through nano-imaging to reveal electronic or phononic scattering processes leading to quantum decoherence.

Research Highlights

1. Technological Innovation: Ultrafast nanofocusing achieves efficient field enhancement and mode compression by designing spiral gratings.
2. Broad Applications: Ultrafast STM is not only applicable to conductive materials but can also be extended to the study of insulating materials.
3. Scientific Value: These techniques provide new perspectives for understanding light-matter interactions at the nanoscale, advancing both fundamental science and applied technology.

This review provides readers with a comprehensive overview of ultrafast microscopy technologies while also pointing out directions for future research.