Terahertz time-domain spectroscopy (THz-TDS) systems effectively overcome the limitations of conventional imaging methods in terms of penetration depth and biosafety, demonstrating broad application prospects. However, traditional THz-TDS systems are constrained by scanning speed and diffraction limit, failing to achieve signal acquisition with both high speed and high spatial resolution. Photoconductive probes with subwavelength antenna structures offer an effective approach for high-spatial-resolution characterization. Nevertheless, when point-by-point high-resolution scanning is performed on large-scale samples, enormous pixels generate massive time-domain datasets. Moreover, the terahertz time-domain signal acquisition at each pixel is inherently limited by the translation speed and delay length of the mechanical delay line, exacerbating the conflict between scanning speed and delay range. The electronically controlled optical sampling (ECOPS) technique achieves nonlinearly varying time delays through periodic sinusoidal voltage modulation of the repetition frequency offset between two femtosecond laser pulses, enabling high-speed sampling. However, the nonlinear sampling under sinusoidal modulation requires additional measurement devices and complex post-processing for time axis extraction, which increases system complexity and cost and compromises sampling accuracy. This challenge drives the development of novel ECOPS compatible with photoconductive probes to achieve convenient, stable, and high-speed terahertz time-domain signal acquisition. In this study, piecewise voltage function modulation was introduced into the ECOPS, which was then integrated with photoconductive probes, facilitating the development of a rapid terahertz probe system with spatial resolution at the subwavelength level.

We proposed and built a terahertz characterization system integrating piecewise voltage modulated-ECOPS with photoconductive probes. The system comprises two femtosecond fiber lasers, a fiber optical module, terahertz emission/detection modules, a trigger module, and a repetition frequency control module. By locking the repetition frequency of two lasers to identical values and injecting a periodic piecewise voltage function into the phase-locked loop of one laser, linearly varying time delays were achieved. The linear time axis was calibrated using a 1 mm thick high-resistivity silicon wafer. To investigate the effect of different steady-state voltages and modulation frequencies of piecewise voltage functions on the terahertz time-domain signals and frequency-domain amplitude spectra, we set the modulation frequency at 500 Hz and adjusted the steady-state voltages to 0.7, 0.8, and 0.9 V. Subsequently, we fixed the steady-state voltage at 0.9 V and changed the modulation frequencies to 500, 400, and 250 Hz. Comparative analyses between asynchronous optical sampling (ASOPS) and ECOPS were performed under identical acquisition time. To validate the rapid and high-resolution field characterization capability, two-dimensional scans of terahertz spots and metallic cross structures were conducted over a 5.1 mm×5.1 mm area with a 300 μm step size. The imaging frequencies were at 0.55 THz and 0.60 THz.

The ECOPS terahertz probe system with piecewise voltage modulation enables high-speed scanning with high spatial resolution. By truncating the top and bottom of sinusoidal waveform, the piecewise voltage function can enable the repetition frequency offset unchanged in period of steady-state voltage, thereafter achieving linearly time-domain sampling (Fig. 2). Measuring the time interval between transmitted main terahertz pulses and secondary reflections through a 1 mm high-resistivity silicon wafer, we can deduce the delay extension factor for determining repetition frequency offset and time window length (Fig. 3). Under 500 Hz modulation frequency and 0.9 V steady-state voltage, the system achieves waveform acquisition rate of 1 kHz, bandwidth of 1.4 THz, time window of 134.226 ps, and dynamic range of 41 dB (Fig. 4). The time-domain signal and frequency-domain amplitude spectrum at different modulation frequencies and steady-state voltages reveal that steady-state voltage primarily governs time window length and time-domain peak-to-peak value, while modulation frequency controls the time window without significantly altering spectral characteristics (Fig. 5). Compared to ASOPS, ECOPS achieves superior noise suppression through increased signal averaging within identical acquisition times (Fig. 6). Although ECOPS exhibits lower frequency resolution than ASOPS, it enhances bandwidth and dynamic range by sacrificing time window length. Two-dimensional scans of terahertz spots and metallic cross structures demonstrate the intensity and phase distributions (Fig. 7 and Fig. 8), with single-pixel acquisition time reduced to 100 ms.

We propose and build a terahertz characterization system combining ECOPS with photoconductive probes, realizing high-speed linear sampling through periodic piecewise voltage modulation. Compared to ECOPS based on sinusoidal modulation, this approach eliminates complex auxiliary measurements and post-processing of the time-domain signal required for nonlinear sampling. Experimental results demonstrate a bandwidth of 1.4 THz, a time window of 134.226 ps, and a dynamic range of 41 dB at 1 kHz waveform acquisition rate, with single-pixel scanning time compressed to 100 ms in two-dimensional imaging. The results under different modulation frequencies and steady-state voltages reveal that modulation frequency manipulates the time window, while steady-state voltage regulates the time window length and peak-to-peak value of the time-domain signal. In the future, in order to further enhance the imaging rate of the system, it can be considered to store and correctively align a single signal from each forward/backward sampling separately to achieve the simultaneous leveraging of forward/backward signals. Moreover, the continuous translation function based on the electronically controlled two-dimensional translation stage can be developed to achieve high-speed continuous scanning.