Ziqi Li is an Associate Professor at Beihang University. He received his Ph.D. in Optics from Shandong University and previously worked as a Research Fellow at Nanyang Technological University. His research lies at the intersection of terahertz photonics, ultrafast spectroscopy, and low-dimensional quantum materials, with a particular focus on terahertz radiation generation, nonequilibrium carrier dynamics, and ultrafast light–matter interactions that underpin emerging terahertz functionalities. His recent work also extends to strong-field terahertz excitation for transient manipulation of nonequilibrium material states.

He develops and applies far-field and near-field terahertz spectroscopic, emission, and imaging techniques—including terahertz time-domain spectroscopy, optical pump–terahertz probe spectroscopy, terahertz emission spectroscopy, and terahertz scattering-type scanning near-field optical microscopy—to probe ultrafast processes across coupled temporal, spectral, spatial, and field dimensions. These approaches enable the investigation of carrier relaxation, photoconductivity dynamics, interfacial charge transfer, nonlinear terahertz emission, nanoscale electromagnetic responses, and field-driven transient states in emerging low-dimensional and quantum materials.

By combining microscopic mechanism studies with device-oriented exploration, his work aims to address fundamental physical bottlenecks and enabling technologies for integrated terahertz functional devices, including compact terahertz emitters, ultrafast optoelectronic components, active terahertz materials, and nanoscale terahertz platforms. He has published more than 50 peer-reviewed papers in journals such as Applied Physics Reviews, Nano Letters, and Advanced Materials, with over 1,600 citations, and holds three granted invention patents, and service as a reviewer for journals such as Science Advances, Optica, and ACS Nano, etc. His honors include the Wang Daheng Optics Award, selection as a Young Scientist at the Lindau Nobel Laureate Meeting, and the Most Cited Paper Award from Applied Physics Reviews.

His Email: drziqili@buaa.edu.cn

Terahertz radiation is an electromagnetic spectral region located between millimeter waves and the far infrared, typically referring to electromagnetic waves with frequencies from approximately 0.1 THz to 10 THz. Toward lower frequencies, it connects microwaves, millimeter waves, and macroscopic electronics; toward higher frequencies, it connects the far infrared, infrared optics, and photonics. It is therefore often regarded as a bridge between electronics and photonics. The term “terahertz” is simply a frequency-based designation: 1 THz=10¹² Hz, meaning that the electromagnetic field oscillates 10¹² times per second. Since visible light is more commonly described by wavelength in everyday contexts—for example, green light has a wavelength of approximately 500 nm—the unit THz alone does not immediately provide an intuitive physical picture. However, frequency and wavelength are strictly related by c=fλ. Thus, 1 THz in vacuum corresponds to a wavelength of approximately 300 μm, or 0.3 mm, which is about 600 times longer than the wavelength of green light. In addition, 1 THz corresponds to an oscillation period of 1 ps, a single-photon energy of 4.136 meV, and a wavenumber of 33.36 cm^-1. Terahertz radiation is therefore not a mysterious new type of radiation, but rather a non-ionizing electromagnetic wave with wavelengths from the millimeter to tens-of-micrometers range, picosecond-scale oscillation periods, and millielectronvolt-scale photon energies. Its uniqueness lies precisely in the fact that it connects the frequency/bandwidth language of electronics with the wavelength/energy/wavenumber language of spectroscopy.

The importance of terahertz science does not simply come from its position “in between” electronics and photonics. More fundamentally, this frequency range naturally matches many of the most essential low-energy degrees of freedom in materials. Intraband transport of free carriers, scattering and mobility in semiconductors, superfluid response and superconducting gaps, low-frequency optical phonons in polar crystals, interlayer vibrations and plasmons in two-dimensional materials, and collective low-frequency vibrations in molecular crystals and biological macromolecules often fall within or near the terahertz energy range. Terahertz spectroscopy therefore does not merely add another spectral window; it provides direct access to the low-energy electrodynamics of materials. It connects future high-speed communication and high-frequency electronics through the language of frequency and bandwidth, while also connecting condensed matter physics, materials spectroscopy, and quantum materials research through the language of energy, wavenumber, and spectral resonances. In essence, studying terahertz phenomena means developing ways to observe intrinsic material responses that are low in energy, ultrafast in time, and often highly heterogeneous in space.

From an application perspective, terahertz radiation combines low photon energy, strong material sensitivity, and moderate penetration capability. Because the energy of a single terahertz photon is only in the millielectronvolt range, terahertz radiation is non-ionizing and does not directly break chemical bonds as X-rays can. At the same time, it is highly sensitive to free carriers, polar phonons, hydration layers, molecular conformations, defects, interfaces, and low-energy collective modes. As a result, terahertz technologies hold significant potential for nondestructive testing, security imaging, biomedical sensing, semiconductor and two-dimensional-material characterization, studies of superconducting and correlated materials, optoelectronic device dynamics, and future 6G/high-frequency communication. Its unique value lies in its ability to “see” low-energy responses inside materials and at interfaces, while also tracking the evolution of carriers, excitons, phonons, and phase transitions on picosecond time scales.

The core experimental platform in our group is an ultrafast terahertz scattering-type scanning near-field optical microscope (ultrafast THz s-SNOM). Conventional far-field terahertz spectroscopy is limited by the long wavelength of terahertz radiation, with spatial resolution typically in the tens to hundreds of micrometers range; it therefore measures an averaged response over the illuminated spot. However, in many frontier materials—especially two-dimensional materials, moiré systems, phase-transition materials, heterostructures, and nanoscale devices—the key physics often occurs in local regions on nanometer-to-micrometer length scales. THz s-SNOM uses a metallic tip to confine far-field terahertz radiation into a nanoscale near field around the tip apex, thereby overcoming the far-field diffraction limit. This enables high-resolution probing of local conductivity, dielectric response, polaritons, phase-transition domains, edge states, and spatial inhomogeneity while preserving the intrinsic sensitivity of terahertz radiation to low-energy electrodynamics. Simply put, far-field terahertz spectroscopy tells us “what the sample responds like on average,” whereas ultrafast THz s-SNOM further reveals “where these responses originate in real space, how they are distributed, and how they evolve on ultrafast time scales.”

Based on this platform, we will focus on developing two complementary experimental capabilities: terahertz emission spectroscopy and optical-pump–terahertz-probe spectroscopy. Terahertz emission experiments investigate how materials actively radiate terahertz waves after ultrafast optical excitation, enabling studies of transient photocurrents, surface and interface electric fields, spin currents, nonlinear photoelectric effects, symmetry breaking, and ultrafast charge separation. Optical-pump–terahertz-probe measurements first drive materials into nonequilibrium states using femtosecond laser pulses, and then use terahertz pulses to read out their transient low-frequency responses. This allows us to track photogenerated carriers, excitons, interlayer charge transfer, mobility changes, defect trapping, phonon coupling, superconductivity quench and recovery, and correlated phase-transition dynamics. Together, these approaches allow us to systematically study ultrafast processes, low-energy excitations, and local electromagnetic responses in low-dimensional and quantum materials from two complementary perspectives: how materials generate terahertz radiation and how materials respond to terahertz radiation.