Terahertz (THz) radiation has emerged as a crucial tool in various research domains, including matter manipulation, electron acceleration, and biological physics. The distinct properties of THz radiation are attributed to its unique frequency range between the far infrared and microwave regions, which covers the vibrational and rotational frequency of numerous materials, whereby THz radiation acts as an effective tool for resonant manipulation of such materials. Furthermore, the smaller wavelength of THz radiation than microwaves and the short duration of pulsed terahertz sources result in high peak field strength, presenting an enormous potential for nonlinear matter control and electron acceleration.

Terahertz spectroscopy serves as a fundamental component of terahertz application research. However, its unique frequency range presents challenges in detecting terahertz spectrum, as traditional electronic and optical methods are not readily applicable. Intuitively, terahertz energy can be measured with energy meters such as bolometers or pyroelectric detectors. Through the scanning by the terahertz bandpass filter, the terahertz spectrum can be measured. The most prevalent technique for terahertz spectrum detection is the scanning free-space electro-optical sampling method, which necessitates multiple shots to measure the terahertz time-domain waveform. The frequency spectrum can be obtained with a Fourier transformation of the time-domain waveform. However, for low repetition rate terahertz sources and irreversible processes, the scanning method is challenging to utilize, necessitating the development of single-shot terahertz detection techniques.

This study presents various single-shot terahertz time-domain waveform and frequency spectrum measurement techniques. The first section discusses single-shot time-domain waveform measurement methods, where the fundamental concept is to encode terahertz time-domain information into the properties of the probe laser, including spectral-encoding and spatial-encoding methods. The chirped ultrafast laser pulse is utilized to map the time information of terahertz radiation onto the spectrum of the probe laser, which is relatively easy to implement with minor modifications of standard scanning terahertz time-domain waveform measurement techniques. However, the time resolution of this technique is limited by the chirped probe pulse duration due to the uncertainty principle. To maintain time resolution at the transform limit of the probe laser, spectral interferometry techniques have been introduced, but the setup, alignment, and data processing required for this approach are more complex than those for standard spectral-encoding methods. Spatial-encoding methods represent another type of encoding, where terahertz time-domain information is encoded into the spatial distribution of the probe laser beam. The first type of spatial-encoding method is non-collinear spatial encoding, which employs an oblique incident probe, and different parts of the beam arrive at the electro-optical crystal at different moments. The setup of this method is relatively simple, and no complex alignment is required in the experiment. However, the time resolution and time window of this approach are contingent on the incident angle and conflict with each other, necessitating careful consideration of the incident angle. The terahertz focal spot may also impact the time window and introduce distortion in the time-domain waveform. The second type of spatial-encoding method utilizes echelons, which have a stair-like shape, to introduce a time interval between different parts of the probe beam. Reflective echelons are more appropriate for measuring terahertz pulses with short pulse duration and may achieve higher time resolution than transmissive echelons. We conducted four types of single-shot terahertz time-domain waveform measurements in our experiment, and their results are presented. However, the spectrum bandwidth using the methods mentioned above may still be limited by the response of the electro-optical crystal and the pulse duration of the probe laser, even if the time resolution is sufficiently high through careful design of the terahertz time-domain waveform measurement system.

The second section discusses single-shot terahertz frequency spectrum measurement methods. For terahertz radiation, such as terahertz sources based on laser-solid density plasma interactions, whose frequencies can be up to 30 THz, our research team developed two types of ultrawide bandwidth single-shot spectrometers. The first spectrometer employs multiple terahertz energy detectors with varying terahertz bandpass filters. The terahertz beam is split using silicon wafers, enabling the measurement of the terahertz spectrum in a single shot. This approach has a relatively simple optical design, but the spectrum bandwidth and resolution are restricted by the bandpass filter. To achieve high spectrum resolution with wide spectrum bandwidth, we designed and experimentally demonstrated a novel terahertz autocorrelator, which has ultrawide bandwidth with high frequency resolution.

This study provides an overview of several single-shot terahertz time-domain waveform and frequency spectrum measurement methods developed at the Key Laboratory of Optical Physics in the Institute of Physics, Chinese Academy of Sciences. The emphasis is placed on the design principles and characteristics of each method. These techniques are particularly applicable to strong terahertz sources with low repetition rates and are expected to significantly contribute to the characterization of terahertz radiation, an enhanced understanding of terahertz generation mechanisms, and the advancement of terahertz application research.