Transformative advancement in terahertz spectroscopy

Terahertz (THz) radiation, which occupies the frequency band between microwaves and infrared light, is essential in many next-generation applications, including high-speed wireless communications, chemical sensing, and advanced material analysis. To harness THz waves, scientists rely on functional devices like metasurfaces and resonant gratings, which exhibit sharp and effective resonance features. Characterizing and optimizing these high-performance devices, however, remains a technical challenge.

The difficulty stems from a fundamental trade-off when performing THz measurements: achieving high spectral resolution versus high spatial resolution. To accurately capture the narrow spectral fingerprints of certain gases and the features of devices with a high quality factor (Q), researchers need very high spectral resolution. On the other hand, to understand the fine details of how THz interacts with matter, capturing what happens very close to the device’s surface—the “near-field phenomena”—is essential. Existing THz spectroscopy methods cannot fulfill both requirements, forcing scientists to prioritize one over the other.

To tackle this long-standing limitation, a research team led by Jianqiang Gu from Tianjin University, China, has designed and experimentally demonstrated an innovative solution. Their report, published in Advanced Photonics, describes an approach called spatial-resolved asynchronous-sampling terahertz spectroscopy (SPRATS), which combines the best features of two distinct measurement techniques to bridge the gap between high spectral and spatial resolution.

The SPRATS system integrates two critical technologies, namely asynchronous optical sampling (ASOPS) and a photoconductive probe (PPB). ASOPS, which consists of mapping time delays through two lasers operating at offset repetition frequencies, provides high spectral resolution and fast scanning. Meanwhile, the PPB offers micrometer-level spatial resolution for near-field detection. The researchers managed to combine these two distinct measurement techniques through the meticulous optimization of system parameters and by implementing signal averaging techniques to improve the signal-to-noise ratio.

Using their setup, the team demonstrated a spatial resolution of 20 µm and a spectral resolution of up to 100 MHz, all while maintaining a robust dynamic range and respectable bandwidth. They applied this optimized SPRATS system to characterize a leaked guided mode resonance (GMR) in a grating, a device that traps light in a thin dielectric layer and releases it through controlled leakage to produce high-Q spectral features. By scanning the device’s electric field just 20 µm from the surface, the system directly verified the underlying resonance physics, matching theoretical simulations. This marked the first-ever in situ near-field mapping of a THz leaked-GMR.

Moreover, the system achieved a surprising level of accuracy in far-field characterization. When measuring the key performance indicators of the GMR device, SPRATS significantly outperformed a traditional ASOPS THz spectroscopy system. The researchers attribute this superior accuracy to the small detection area of the PPB, which allowed the system to selectively measure the “pure” signal transmitted through the center of the sample while avoiding signals diffracted from the edges, which typically degrade measurement quality. “Our SPRATS system demonstrates superior performance in spectral resolution and effectively extends the near-field research to the terahertz spectral modulation devices with high Q factor,” comments Dr. Gu.

The ability to simultaneously perform high-resolution spectral analysis and near-field observations significantly changes how researchers can study and optimize THz functional devices. The SPRATS system provides a vital tool for verifying complex theoretical models of light-matter interactions and is immediately applicable to many emerging THz technologies. “The proposed SPRATS system will undoubtedly propel research endeavors in the realms of highly sensitive terahertz sensing, terahertz nonlinear phenomena, and the research and development of high-Q terahertz devices,” concludes Dr. Gu.

For details, see the original Gold Open Access article by F. Huang et al., “In situ and in-depth characterization of high quality factor resonance by spatial-resolved asynchronous-sampling terahertz spectroscopy (SPRATS),” Adv. Photon. 8(1), 016002 (2025), doi: 10.1117/1.AP.8.1.016002