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Balanced Air-Static-Biased Detection of Ultrabroadband Terahertz Waveforms
Alexander Holm Ohrt1, Oliver Nagy1, Robin Löscher2, Clara J. Saraceno2, Binbin Zhou1, and Peter Uhd Jepsen1
1Department of Electrical and Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
2Photonics and Ultrafast Laser Science, Ruhr-Universita ̈t Bochum, DE-44801 Bochum, Germany
We present a novel balanced air-biased coherent detection (ABCD) scheme for ultrabroadband terahertz (THz) waveform measurements. By rotating the bias electrodes by 90° relative to the conventional setup, the balanced detection scheme enables coherent detection at the full repetition rate of the laser system without requiring a lock-in amplifier or bias modulation using a signal generator. Due to its ability to reject shot-to-shot noise, as demonstrated in Fig. 1(b), the balanced detection scheme achieves a twofold increase in dynamic range (DR) and a fourfold increase in signal-to-noise ratio (SNR) compared to conventional ABCD. The balanced scheme allows for sub-second waveform acquisition with a continuously moving delay stage, demonstrated by acquiring 200 waveforms in just 100 seconds, shown in Fig. 1(e).
ABCD is a powerful tool for coherent ultrabroadband THz waveform detection [1], and in combination with two-color air-plasma THz generation, it allows for transient THz spectroscopy across an ultrabroad frequency range, enabling, for instance, the measurement of the ultrafast photocarrier dynamics in photovoltaic thin films [2]. The improvement in performance offered by the balanced ABCD scheme, as demonstrated in Fig. 1(c) and (d), makes it an ideal candidate for implementation in 2D ultrabroadband THz spectroscopy where signal quality and acquisition speed are of crucial importance. We find that conventional ABCD struggles with coherence in rapid acquisition settings with a continuously moving stage, resulting in unusably noisy waveforms with spectral artifacts. A schematic of the polarization state of the THz field ETHz, optical probe field Eω, and bias field Ebias, is shown in Fig. 1(a). We measure along two channels, A and B, that are orthogonal to each other but rotated 45° relative to the coordinate system in Fig. 1(a). The differential signal between the two channels yields a coherent measurement while common noise between the channels is canceled out. Balanced ABCD has been implemented before [3]; however, it still required lock-in amplification and bias modulation with signal generators. Our implementation is simpler, both optically and electronically, while delivering superior performance.
Figure 1: (a) Schematic of the polarization of the interacting fields in the balanced ABCD scheme. (b) Example of photodetector signals from waveform measurements in conventional and balanced ABCD, illustrating the noise-rejecting nature of the balanced scheme. (c) THz waveforms with different acquisition times using conventional and balanced ABCD. (d) Spectra of waveforms in (c). Shaded regions show the noise floor. (e) 2D map of 200 waveforms acquired in 100 seconds using balanced ABCD with a continuously moving delay stage.
In conclusion, our balanced ABCD scheme represents a significant advancement in the field of ultrabroadband THz waveform detection. It offers enhanced dynamic range and signal-to-noise ratio, simplifies the experimental setup, and enables faster data acquisition. These benefits make it a valuable tool for a wide range of applications in THz spectroscopy and beyond.
References
[1] N. Karpowicz, J. Dai, X. Lu, et al., Appl. Phys. Lett. 92, 011131 (2008)
[2] M. Sørensen, A. Gertsen, R. Fornari, et al., Adv. Funct. Mater. 33, 2370057 (2023)
[3] X. Lu and X.-C. Zhang, Appl. Phys. Lett. 98, 151111 (2011)