Terahertz (THz) radiation falls in between the infrared and microwave regions of the electromagnetic spectrum in the frequency range of 0.1–10 THz (wavelengths ranging from 0.03–3 mm). THz radiation, distinguished by its unique spectral positioning, possesses remarkable characteristics that enable diverse applications across multiple disciplines, including material composition identification^[1,2], nondestructive biological tissue testing^[3–5], advanced radar and remote sensing systems^[6–9], next-generation 6G communications^[10], and investigations of charge carrier ultrafast dynamics^[11–14]. Progress in THz technology remains critically dependent on advancing efficient wave generation/detection methodologies^[15], with radiation sources broadly classified into continuous-wave and pulsed configurations^[16]. Particularly, ultrashort-pulse sources have become instrumental in time-domain spectroscopic systems for material characterization^[17,18], while high-energy, low-repetition-rate pulsed sources prove essential for studying nonlinear phenomena in intense field environments^[19]. The optimization and expanded implementation of these radiation sources necessitate parallel advancements in detection methodologies^[20]. Currently, although numerous single-point detectors targeting pulsed intense field sources have been reported, array technology still requires further development^[21–23].

THz detectors can be divided into two categories: incoherent and coherent detectors. In comparison to incoherent detectors, coherent detectors have lower noise equivalent power (NEP) and faster response time than incoherent detectors because they can measure the amplitude and phase of THz radiation. However, these methods are more complex in terms of device design and rely primarily on optical pumping technology for coherent detection, which limits their applicability in laboratory research and makes them unsuitable for preparing array detectors. In contrast, incoherent detectors operating through power detection mechanisms present simplified structural designs with inherent array compatibility. Based on these principles, incoherent detectors can be categorized into thermal detectors, photon detectors, superconducting detectors, and electrical detectors. Among these options, thermal detectors such as bolometers^[24], pyroelectric detectors^[25], Gaolet detectors^[26], and photo-thermo-acoustic (PTA) metal film detectors^[27] are traditional but face challenges in achieving fast high-sensitivity inspection at room temperature^[24]. Superconducting detectors and photon detectors with sensitivities greater than 1017W/Hz and 1020W/Hz enable the capture of weak signals, while requiring operation at cryogenic temperatures that suppress thermal noise^[28,29]. Devices utilizing Schottky diodes and field-effect transistors (FETs) stand as representatives of electrical THz detectors, boasting swift response time at room temperature and the capability to be configured into arrays^[30–32]. However, these detectors are constrained by limited detection bandwidth^[33–36]. Furthermore, these conventional electrical array detectors demonstrate insufficient detection sensitivity when exposed to pulsed THz radiation characterized by low repetition rate, low average power, and high pulse energy.

When a sample absorbs THz radiation, the resultant heat generation elevates its temperature, while additional measurable physical effects—such as the optoacoustic effect—arise from electromagnetic radiation absorption. In recent years, THz optoacoustic technology has emerged as an innovative research tool, showing significant potential for rapid, high-sensitivity THz radiation detection at room temperature^[5,37]. Prior studies have explored carbon nanotube-polydimethylsiloxane (CNT-PDMS) nanocomposites for their high-frequency THz response in optoacoustic detection of pulsed radiation, utilizing microring structures to sense ultrasonic signals^[38]. Additionally, highly sensitive photoacoustic sensors based on micro-electro-mechanical system (MEMS) cantilever technology have been developed to detect weak THz radiation^[39]. However, these methods face manufacturing challenges for THz cameras due to high costs and complex fabrication processes. Another approach, integrating graphene foam with microphones, achieved THz detection but encountered difficulties in array configuration. Thus, developing room-temperature THz detectors capable of measuring ultrashort pulses with rapid response time and scalable integration into compact camera arrays remains an urgent priority.

In this paper, an array detector based on THz optoacoustics is developed, which is specifically designed for THz sources with low repetition frequency, low average power, and high pulse energy. The detector uses a single-point probe with excellent response speed (µs level) and extremely high sensitivity to THz pulses, with a noise equivalent detection energy (NEDE) of 598 pJ. This detector can effectively scan and image THz spots in three dimensions. The imaging resolution increases with the decrease of the crystal size of the piezoelectric probe. By employing parallel scanning technology, the detector significantly enhances imaging efficiency and reduces imaging time. Additionally, this detector exhibits high scalability, enabling it to be easily expanded into an array detector, thereby achieving real-time imaging of THz spots.

The principle of the THz optoacoustic array detector is depicted in Fig. 1(a). High-energy ultrashort THz pulses are irradiated onto the THz optoacoustic array detector. These pulses are absorbed by the optoacoustic conversion material at the front end of each detector within the array and subsequently converted into acoustic signals, which are then captured by the ultrasonic detector. When illuminated by a short electromagnetic pulse, the sample absorbs the electromagnetic energy and generates an optoacoustic wave. The optoacoustic waves then traverse the samples to the detector, and the final detected signal can be expressed as p=p0(r,t)×SIR(r,t)×EIR(t)=(βVs2CP)ηthμa(λ)F×SIR(r,t)×EIR(t).(1)

In Eq. (1), the spatial impulse response (SIR) is related to the shape and location of the transducer surface, while the electronic impulse response (EIR) is associated with the properties of the piezoelectric crystal and the probe circuit. Parameters β, Vs, and Cp, respectively, represent the thermal coefficient of volume expansion, the speed of sound, and the specific heat capacity of the absorption material. The factor μa represents the optical absorption coefficient and is determined by the absorption characteristics of the material at the given frequency of the electromagnetic wave. η_th defines the percentage of absorbed energy converted into heat, which is usually characteristic of a given material but is often considered to be 1. F stands for the optical fluence (the optical energy per unit area), depending on the characteristics of the electromagnetic wave. From Eq. (1), it is evident that the initial pressure is directly proportional to the incident electromagnetic pulse energy. Therefore, when a THz pulse is incident on a suitable optoacoustic material, the THz pulse energy can be directly detected through the generated optoacoustic wave.

The complete system for THz pulse detection based on ionic-liquid optoacoustics is presented in Fig. 1(b). The THz radiation source was generated by pumping a nonlinear crystal of lithium niobate (LiNbO3) with a femtosecond titanium-sapphire regeneration amplifier (Coherent, USA, wavelength 808 nm, pulse width 35 fs, repetition frequency 1 kHz) based on the tilted-pulse-front technique. The THz source exhibits a spectral range of 0.2–1.5 THz with a pulse energy of 4 µJ. The focused spot size is approximately 1.5 mm. A 10.16-cm-diameter 1-mm-thick silicon wafer, with black polyethylene pasted on both surfaces, was positioned in the THz optical path to block the pump light. In the THz optoacoustic material, the THz energy is converted into pressure waves through the optoacoustic effect. Flat piezoelectric ultrasonic transducers (Doppler, China) with central frequencies of 1 MHz (12-1P13-H and custom-made), 2 MHz (custom-made), or 5 MHz (custom-made) were employed to detect the generated THz optoacoustic signals. To investigate the temperature stability characteristics of the THz point detector, a temperature-control module incorporating a thermoelectric cooler and temperature sensor was developed, capable of precise temperature regulation from −10°C to 50°C. In addition, the THz detector was installed on an x-y-z platform (SMET-06A, LBTEK, China) to realize the space scanning of the detector. The detected signals were first amplified through a low-noise 50 dB amplifier (Usultratek), then digitized using a 20 MS/s data acquisition card (M2p. 5921-x4, Spectrum Instrumentation), and finally archived in a computer. For array-based detection configurations, the signals were simultaneously amplified and digitized by a multichannel data acquisition system (DAQ) (Model 2561, Tianjin Langyuan Technology Co., Ltd., China) before being transferred to the computer for subsequent analysis.

The ionic liquids 3-ethyl-1-methyl-1H-imidazol-3-ium bis [(trifluoromethyl) sulfonyl] amide (CAS: 174899-82-2, abbreviated as EMIMIm), 1-butyl-3-methylimidazolium hexafluorophosphate (CAS: 174501-64-5, abbreviated as BMIMPF6), and 1-methyl-1-propylpyrrolidin-1-ium bis (fluorosulfonyl)imide (CAS: 852620-97-4) were provided by Leyan Company (Shanghai, China). A peristaltic pump was employed to inject various types of ionic liquids into the microfluidic chip for liquid sample measurements, with the structural configuration and operational schematic of individual detectors within the array detection system illustrated in the block diagram of Fig. 1(b). The sample preparation process for the detector front-end began by mixing poly (vinylidene fluoride-co-hexafluoropropylene) (CAS: 9011-17-0) (Macklin, Shanghai, China) with acetone in a 1:7 mass ratio within a 50°C water bath. The mixture was magnetically stirred for 30 min until complete polymer dissolution, followed by the addition of EMIMIm ionic liquid and CNT powder. The resultant mixture underwent further magnetic stirring for 1.5 h at 50°C, after which it was transferred from the stirrer to a mold within a constant-temperature water bath. The mold was thermally cured at 70°C for 12 h, and the solidified sample was finally bonded to the piezoelectric detector front-end using AA352 UV-curable adhesive.

The THz optoacoustic signal amplitude of different materials at different THz energies is shown in Fig. 1(a). The significant linear positive correlation between the signal amplitude picked up by the Golay cell and the signal intensity of the THz energies is shown in Fig. 1(a). The significant linear positive correlation between the signal amplitude picked up by the Golay cell and the signal intensity of the THz ionic liquid detector was observed. As demonstrated in the figure, under identical THz energy irradiation conditions, the signal amplitude produced using ionic liquid as the optoacoustic conversion medium significantly exceeds that generated by water. Consequently, the THz ionic liquid detector exhibits superior detection sensitivity compared to the water-based detector. The THz ionic-liquid detector demonstrates sub-microsecond temporal resolution, with measured signal pulse widths <1μs [Fig. 2(b)], corresponding to a response time below 1 µs. In contrast, the response time of the Golay cell detector is as long as 0.1 s, which means that our detector is several orders of magnitude faster in terms of response speed compared to the Golay cell detector.

Previous research has attempted to combine PDMS with CNTs to create a high-performance THz optoacoustic conversion medium. Detectors constructed using this nanocomposite have demonstrated exceptional detection sensitivity. A more direct method, in which different concentrations of CNT powder are doped in PDMS solution and the doped liquid is used as a THz optoacoustic conversion material, is adopted. Experimental results indicate that CNT doping effectively enhances detection sensitivity [Fig. 2(c)]. In addition, in order to explore materials that can produce stronger acoustic signals, we tested the optoacoustic properties of other ionic solutions. We doped CNT powder into ionic liquids and discovered that as the doping concentration rose, so did the THz optoacoustic signal’s amplitude [Fig. 2(d)]. Further observations revealed that as the CNT concentration continued to increase, the amplification of the THz optoacoustic signal gradually approached saturation. Since THz sources are pulse sources, NEDE is a more appropriate measure to characterize the sensitivity of a THz photoacoustic detector^[36]. Here, the expression for NEDE can be expressed as NEDE=P/(S/N)=(4μJ)/(535mV/0.08mV)=598pJ, where P is the monopulse energy, S is the photoacoustic signal amplitude, and N is the noise amplitude. For a THz radiation source operating at a repetition rate of 1 kHz, a detection effect with a NEDE of 598 pJ (per pulse) is successfully attained.

This discovery not only simplifies the preparation process of the optoacoustic materials but also opens up new possibilities for THz detectors based on THz optoacoustics. Additionally, we have conducted tests on the stability of the detector. As shown in Fig. 2(e), the detector demonstrates excellent stability at different temperatures, with minimal disturbance in the signal as temperature varies. Considering the fluidity and instability of liquid materials, we prefer to use solidified samples as the optoacoustic conversion material. As a result, we chose a cured EMIMIm doped with a CNT (0.04 g/g) as the conversion material and selected a piezoelectric probe with an operating frequency of 1 MHz and a crystal surface size of 0.5mm×0.5mm as a single-point optoacoustic detector. The experimental results showed that as the sample size decreased, the intermediate frequency of the THz optoacoustic signal increased [Fig. 2(f)]. This discovery holds potential significance for enhancing the resolution of imaging using single-point or array detectors.

To investigate the detector’s capacity to capture THz pulses in space, the scanning imaging effect of 1, 2, and 5 MHz piezoelectric probes at the THz propagation focus was tested under the three conditions of 0.5, 1.0, and 3.0 mm diameters of EMI ion gel, by taking the signals’ peak-to-peak value. As shown in Fig. 3(a), the size of the THz spot captured by the detectors is related to the medium frequency of the detectors and the diameter of the samples. Taking the sample with a diameter of 0.5 mm and the probe with an intermediate frequency of 1 MHz as examples [the dotted line in Fig. 3(a)], Fig. 3(b) shows the spot curves measured in the x and y directions and the full width at half-maximum of the curve, respectively. The size of the spot decreases with the increase of the detector’s intermediate frequency and increases with the increase of the size of the ionic gel. The former confirms the ionic gel’s THz optoacoustic response frequency, whereas the latter shows that the gel’s size restricts the detector’s detection resolution. Here, we selected an ion gel with a diameter of 0.5 mm and tested the imaging resolution of a probe with a width of 0.5 mm and an intermediate frequency of 1 MHz. Figure 3(c) illustrates the physical and dimensional schematic of the cross-shaped metal sample, with the width of the metal strip (w) measuring 1.5 mm and an exposed length (l) of 5.2 mm. The near-field imaging results of the THz radiation transmitted through the sample are shown in Fig. 3(d), with a scanning step of 0.1 mm. The shape of the dark portion is consistent with that of the sample. As illustrated in Fig. 3(e), the edge spreading function (ESF) is derived from the detected voltage signal, and subsequently, the line spreading function (LSF) is calculated from the ESF. This allows for the determination of the system imaging resolution, which is known to be 0.50 mm.

In addition to two-dimensional imaging of the spot focus, the detector can also detect the THz propagation path. The three-dimensional spot scanning results of the detector are illustrated in Fig. 4, where each spot is 2 mm apart in the THz beam transmission direction. As shown in Fig. 4(a), the spatial coordinate system with the focal point as the origin was established, and the spot images from 6 mm in front of the focal point to 8 mm behind the focal point were scanned at a 2 mm interval. The beam shows the propagation process from divergence to focus and then divergence. The imaging results of the light spots at 8 locations are shown in Fig. 4(b), in which the radius of the light spots decreases from large to small and then increases again, and the light intensity in the center of the light spots changes from weak to strong and then decreases. According to the change rate of the spot radius and spot intensity, the gradient of the focusing process of the spot is obviously greater than that of the divergence process. The results show that the detector has excellent performance in the field of three-dimensional detection of THz radiation.

The piezoelectric ultrasound transducer can precisely image THz spots in space thanks to the single-point scanning technique. Figure 5(a) shows the results of scanning using a piezoelectric probe with a crystal size of 0.5mm×0.5mm and an IF of 1 MHz, where the scan step size is as fine as 0.1 mm, and a 40pixel×40 pixel image is constructed. However, the processing of large amounts of data and the inherent limitations of mechanical motion during single-point scanning have become major constraints on scanning efficiency. To solve this problem, the multipoint parallel scanning strategy is realized using the cost advantage of piezoelectric ultrasonic transducer and mature technology, and the scanning cycle time is greatly shortened. As shown in Figure 5(b), when synchronized operation is performed with four piezoelectric THz probes, the imaging results are the same as a single point scan, with a fourfold increase in imaging rate. If the number of probes is further increased, the imaging rate will continue to increase. In addition, the detection method of piezoelectric probes is naturally adapted to the array layout, which opens a new way for the development of THz cameras that can operate efficiently at low repetition rates. On this basis, we use a single pixel of 0.5mm×0.5mm and a total 8×8 array to directly image the THz distribution in space, eliminating the scanning step. As shown in Fig. 5(c), the single acquisition images of the array are highly consistent with the single-pixel and multipoint scanning results. It is worth noting that the size of the single-point detector in the array has a direct impact on the imaging resolution. Therefore, we reduced the crystal size of the single-point detector to 0.3mm×0.3mm and used a 10×10 array to image the THz spot. As shown in Fig. 5(d), a significant improvement in imaging resolution is achieved compared to Fig. 5(c). The maximum imaging resolution that an array detector can achieve with a single spot scan is exactly the upper limit of its imaging resolution.

Each pixel unit in the array detector functions as an independent THz single-point detector, demonstrating exceptional detection performance at room temperature. Its response speed is extremely fast, reaching the microsecond level (around 1 µs), significantly outperforming traditional pyroelectric detectors and Golay cell detectors, which are slower by several orders of magnitude. Additionally, this detector boasts a low NEDE, which can be further enhanced by optimizing the THz optoacoustic materials and improving the detection sensitivity of the ultrasonic detector.

The imaging rate can be greatly improved by integrating multiple point detectors because the scaling of parallel detection is easy. A practical demonstration utilizing an array detector for real-time THz spot imaging showcases its cost-effectiveness and high spatial resolution. Theoretically, there is no intrinsic limit to the resolution of the potential highest resolution of the detector array, but practical constraints arise from manufacturing ultrasound array detectors with smaller individual elements and high sensitivity. Since ultrasonic array manufacturing technology has been maturely applied in industry, costs are effectively controlled, making it promising for practical industrial applications.

Although high-field THz sources, particularly crystal-based THz parametric sources, are considered as the future trend, the issue of heat accumulation in crystals leads to instability in light sources. The spot imaging results depicted in Figs. 3–5 reveal subtle variations in the shape of each light source, thereby emphasizing the indispensability and significance of the THz array detector investigated for practical applications.

In conclusion, we developed an array detector based on THz optoacoustics for THz sources with low repetition frequency, low average power, and high pulse energy. The detector generates ultrasonic signals by responding to the energy of a single THz pulse, rather than relying on continuous thermal integration. This means that the detector can be used to measure the single-shot source. The detector has an excellent response speed of about 1 µs and extremely high sensitivity, with an NEDE of 598 pJ. By scanning the three-dimensional space, the detector can effectively image the propagation path of the THz beam. In addition, the imaging efficiency is significantly improved by the array arrangement of the detector. The imaging resolution can be improved by reducing the size of the piezoelectric probe crystal. Finally, we expanded the detector to an array detector to achieve fast imaging of THz radiation with low repetition rate and high pulse energy. Our work complements the shortcomings of current THz camera systems and provides a new approach for the development of THz cameras.