# Electro-Optical Sampling of Single-Cycle THz Fields with Single-Photon Detectors

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. Standard THz-TDS Measurement

^{2}GaAs substrate with an interdigitated metal-semiconductor-metal electrode structure, which is modulated by an externally applied bias field.

## 3. Generation of Squeezed Vacuum and Measurement Calibration

_{1}= −75 mm, f

_{2}= +150 mm) was also inserted into the beam path to optimize the coupling. The radiation was then measured with InGaAs-based Single-Photon Avalanche Photodiodes (ID Quantique, ID230, Geneva, Switzerland). These detectors can record the arrivals of photons at a rate limited by a dead time as short as $2\mathsf{\mu}\mathrm{s}$ and with 25% quantum efficiency. The arrival time of the photons was digitalized with time-tagging electronics (PicoQuant, HydraHarp, Berlin, Germany).

_{π}) at the operating wavelength was 539 V, and the EOM was driven by a high-voltage amplifier (max 200 V, HVA200, Thorlabs) fed with a modulated signal.

## 4. THz Field Measurement with Single-Photon Detectors

## 5. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**(

**A**) THz time-domain spectroscopy using electro-optic sampling to characterize the THz field trace and spectrum using a classical balanced detection scheme. (

**B**) Power spectrum corresponding to the measurement in (

**A**). The blue curves are for an un-purged setup (blue curve) while the red curves show the result in a pure nitrogen atmosphere.

**Figure 2.**Calibration setup with EOM and experimental lock-in-like balanced detection using two single-photon detectors. (

**A**) Experimental scheme for calibration of detection using electro-optic modulation. (

**B**) Using ps-time tagging resolution of the photon counting software, a difference measurement was performed between counts of detectors D1 (red) and D2 (blue) using markers M inserted every 1/20 kHz to synchronize timing with the antenna modulation. (

**C**) Measured standard error of the detection method using EOM with single photon counts.

**Figure 3.**Terahertz detection using single photon detectors. (

**A**) Experimental scheme for measuring THz field using squeezed vacuum. The THz field generated from a photoconductive antenna (PCA) is overlapped to the squeezed vacuum pulse into a GaAs electro-optical crystal. The phase shift introduced by the THz pulse on the squeezed vacuum probe is measured by a balanced detection using single photon detectors. (

**B**) Measured phase shift (ΔΦ) induced by THz field (blue squares) overlapped with the phase shift from standard electro-optic sampling (red curve). The integration time for each data point was 105 min. (

**C**) Measured standard error of the measurement (blue crosses) compared to the expected standard error calculated considering the squeezed vacuum probe statistics (red curve) at increasing integration time. The good match indicates that the detection sensitivity is limited by the statistical properties of the probe state, and the THz field is not adding noise.

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**MDPI and ACS Style**

Shields, T.; Dada, A.C.; Hirsch, L.; Yoon, S.; Weaver, J.M.R.; Faccio, D.; Caspani, L.; Peccianti, M.; Clerici, M.
Electro-Optical Sampling of Single-Cycle THz Fields with Single-Photon Detectors. *Sensors* **2022**, *22*, 9432.
https://doi.org/10.3390/s22239432

**AMA Style**

Shields T, Dada AC, Hirsch L, Yoon S, Weaver JMR, Faccio D, Caspani L, Peccianti M, Clerici M.
Electro-Optical Sampling of Single-Cycle THz Fields with Single-Photon Detectors. *Sensors*. 2022; 22(23):9432.
https://doi.org/10.3390/s22239432

**Chicago/Turabian Style**

Shields, Taylor, Adetunmise C. Dada, Lennart Hirsch, Seungjin Yoon, Jonathan M. R. Weaver, Daniele Faccio, Lucia Caspani, Marco Peccianti, and Matteo Clerici.
2022. "Electro-Optical Sampling of Single-Cycle THz Fields with Single-Photon Detectors" *Sensors* 22, no. 23: 9432.
https://doi.org/10.3390/s22239432