The terahertz (THz) range (0.1-10 THz) lies between the microwave and infrared region in the electromagnetic spectrum and is characterized by low photon energy and strong penetration. The THz range covers the spectra attributed to the intermolecular vibrations and rotational energy levels of various organic and biological macromolecules. Terahertz technology realizes the integration of electronics and photonics, offering tremendous development potential in fields such as military applications, biomedical applications, and future communications. However, before mid-1980s, research on the nature of terahertz radiation could not be conducted owing to the absence of effective generation and detection methods with respect to electromagnetic radiation in the terahertz frequency range.

The subsequent rapid advancement in ultrafast laser pulse technology offered a stable and effective excitation source for generating terahertz pulses. However, owing to the low output power of existing terahertz-radiation sources and the high thermal-radiation background in the terahertz frequency range, new sources need to be developed to meet high requirements in terms of energy, bandwidth, and other performance characteristics.

Terahertz radiation can be generated through several methods, including optical methods, terahertz quantum cascade lasers, and solid electronic devices. This study primarily focuses on optical devices used for generating terahertz radiation. The femtosecond laser pulse, characterized by low repetition rates and high energy, can serve as the pump source, triggering strong nonlinear effects in various targets, thus generating strong terahertz radiation. This radiation affords manipulation and control over complex condensed-matter systems. Moreover, this method of generating terahertz radiation has been studied extensively, and the material state covers solid, gas, and liquid.

However, sources that can emit terahertz radiation having high energy and a broad frequency spectrum are still lacking. If a broadband strong terahertz source with stable output can be realized, it can greatly promote the development and practical process of terahertz technology in various fields. This study summarizes recent research progresses in generating intense broadband terahertz radiation using various materials excited via ultrafast femtosecond lasers, including studies on laser-induced terahertz radiation from nanometal films, gas plasma, and liquid plasma. The inherent physical mechanisms of each method are analyzed and discussed herein, affording numerous important exploration directions for research on terahertz-radiation sources.

Photoconductive antennas and nonlinear electro-optic crystals, which are routinely used as terahertz-radiation sources in laboratories, generate stable terahertz radiation when excited via ultrashort laser pulses. Terahertz radiation can be employed in research applications such as terahertz time-domain spectral imaging. Although the signal-to-noise ratio of terahertz radiation is considerably higher than that of the radiation in the traditional far-infrared Fourier spectrum, the detection and observed spectrum range of terahertz radiation is constrained, only covering a range of 0-3 THz. Recently, researchers have focused on generating terahertz waves from metal films. In 2007, Gregor et al. reported in Physical Review Letters that terahertz waves ranging from 0.2 to 2.5 THz could be generated using metal gratings with nanostructures. They postulated that these terahertz waves were generated owing to incoherent optical rectification, which was caused by the acceleration of photoelectrons by surface plasma evanescent waves. In 2011, Polyushkin et al. reported in Nano Letters the use of silver nanoparticle arrays to generate a terahertz pulse with a bandwidth of 0-1.5 THz. They suggested that terahertz waves were generated when photoelectrons were accelerated by the driving force created by the inhomogeneous plasma electric field. In 2014, Dai et al. reported in Optics Letters the generation of a terahertz wave with a bandwidth of 0-2.5 THz by exciting a gold film deposited on a titanium sapphire substrate using a two-color laser field. They attributed this generation process to the third-order nonlinear effect of metal, namely the four-wave mixing mechanism. We reported that femtosecond laser pulses can excite thin metal films to emit high-energy, broadband terahertz waves and studied the physical mechanism of this emission process from various aspects such as energy, material, frequency, and gas environment (Figs. 1-7).

In contrast to the abovementioned solid-medium generation methods, generating terahertz radiation using the femtosecond laser-excited air plasma provides several advantages, including ultrawideband, high intensity, remote detection feasibility, and no laser damage threshold. Using different pump lasers to excite plasma presents a novel research approach. Clerici et al. proposed a model in Physical Review Letters that could effectively predict the wavelength dependence of terahertz emission through experimental research. They demonstrated that the plasma current increases proportionally with the square of the pump wavelength, and the terahertz emission at 1800 nm is 30 times higher than that at 800 nm owing to the wavelength combination effect. We explored the characteristics of terahertz waves radiated from plasma excited using long-wavelength lasers (Figs. 8-12) and observed that the radiation ability of the terahertz waves is enhanced by pre-modulating the plasma, changing the excitation medium, and altering the ratio of the two-color light frequency (Figs. 13-19).

Reports on liquids being used as terahertz-radiation sources are scarce. In 2017, in Applied Physics Letters, Jin et al. reported the possibility of a terahertz wave being generated using a liquid water film. Simultaneously, Dey et al. reported in Nature Communications that ultrashort laser filaments in liquids could generate terahertz wave radiation. They discovered that the terahertz energy radiated by a liquid excited by the monochromatic field is an order of magnitude higher than that of the terahertz wave obtained by the two-color field scheme in the air. In 2018, Yiwen E et al. reported in Applied Physics Letters the mechanism of terahertz radiation generated by a water film based on the simulation they performed using a dynamic dipole that demonstrated the dependence of terahertz intensity on incident laser angle. Moreover, we conducted research on terahertz waves generated by exciting a liquid water film. We explored the characteristics of a terahertz wave generated via the laser excitation of a water film and water line (Figs. 20-25), and further increased the radiation efficiency of the terahertz wave.

Herein, we report that materials such as metal films, air, and liquid water films excited using ultrafast femtosecond lasers provide a novel approach for obtaining powerful terahertz radiation. The development of robust terahertz sources allows the exploration of the nonlinear characteristics of materials in the terahertz range and provides the experimental basis for observing the dynamic evolution of materials on the picosecond scale. The increasing development of terahertz technology and the practical application demand also constantly put forward new expectations for terahertz sources. Improving the understanding related to the terahertz-radiation mechanism in materials excited by laser pulses and identifying excellent terahertz sources will remain a long-term objective for many researchers.