Room-temperature THz photon detection via nonlinear upconversion with 2% full-system efficiency

Abstract: Sensitive detection of terahertz (THz) radiation is fundamental to progress in spectroscopy, advanced wireless communication, and the realization of emerging quantum technologies. However, the intrinsically low photon energies in the THz range combined with thermal background radiation tend to constrain detector performance when operating at ambient temperatures. Here, we demonstrate efficient room-temperature THz detection based on nonlinear upconversion in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA) to resolve frequencies from 1 to 7.5 THz. The system encompassing spectral filters and a single-photon counter achieves an overall detection efficiency of 2% for sum-frequency generated photons. This enables the detection of a train of 50 000 terahertz pulses carrying, on average, fewer than 0.04 photons per pulse, with a signal-to-noise ratio of unity. At a higher flux, when ~60 photons per pulse impinge on the BNA crystal, the per-pulse detection probability reaches 50%. After accounting for loss mechanisms in the setup, the nonlinear THz-to-near-infrared conversion efficiency in BNA exceeds 75%. These results demonstrate the feasibility of quantum experiments relying on single-photon-level THz detection via upconversion in nonlinear crystals in ambient conditions.

• The paper presents room-temperature THz detection using BNA, achieving a full-system efficiency of 2% and quantum sensitivity an order of magnitude better than previous devices.
• It employs ultrafast NIR gating and precise phase matching in a 450 μm BNA crystal to obtain 76% internal conversion efficiency over a broadband 1–7.5 THz range.
• The approach paves the way for practical quantum THz optics and spectroscopic applications, with potential improvements via optimized detection components.

Efficient Room-Temperature Terahertz Photon Detection via Nonlinear Upconversion in BNA

Introduction

The direct detection of THz radiation at room temperature remains a critical challenge due to the low photon energies in the THz regime being comparable to $k_B T$, complicating single-photon sensing. High-sensitivity detection is essential for quantum optics, advanced wireless systems, and spectroscopic studies in physics, chemistry, and biology. Existing solutions utilizing superconducting or quantum-dot-based devices operate at cryogenic temperatures, limiting their facilitization for practical application spaces where ambient operation is required.

Parametric upconversion offers an alternative by transforming low-energy THz photons to the near-infrared (NIR), where mature single-photon detectors permit shot-noise-limited detection. While upconversion in inorganic nonlinear crystals (e.g., GaP, LiNbO₃) has achieved progressively higher sensitivities and bandwidths, the search for further improvements has led to organic materials such as DAST, DSTMS, OH1, and notably, BNA (N-benzyl-2-methyl-4-nitroaniline), which offers strong nonlinear coefficients, favorable phase-matching, and low absorption in the THz and NIR range.

Experimental Architecture and Methods

The reported system employs a broadband, room-temperature THz detection mechanism leveraging sum-frequency generation (SFG) and difference-frequency generation (DFG) in a thick ($450$ $\mu$m) BNA crystal. THz pulses are generated via optical rectification in an auxiliary BNA crystal using ultrafast ($85$ fs) NIR laser pulses at $1035$ nm, matching Yb-based ultrafast laser technology.

Detection involves spatial and temporal overlapping of the incident THz field and a spectrally filtered NIR gating pulse within the BNA crystal. The upconverted (SFG/DFG) output is filtered and routed to a silicon single-photon counting module (SPCM) through a low-loss detection chain. The detection pathway is thoroughly characterized, accounting for Fresnel reflections, filter throughput, monochromator transmission, and the PDE of the SPCM, which is measured as $17.5\%$ at $1016$ nm.

Results

Room-temperature detection efficiency: The system achieves a full-system efficiency ($\eta_\mathrm{sys}$) of $1.98\%$ in converting incoming THz photons at $4$ THz into counted SFG photons, as measured at the SPCM. This efficiency is attained at a NIR gating-pulse fluence of $450$0 mJ/cm$450$1, below the damage threshold of BNA on sapphire.

Quantum sensitivity: The demonstrated sensitivity enables detection of fewer than $450$2 THz photons per pulse (over $450$3 pulses, $450$4) and a $450$5 per-pulse detection probability at a flux of $450$6 photons/pulse. This level of performance is quantitatively an order of magnitude superior to earlier room-temperature, free-space, solid-state THz upconversion detectors.

Internal conversion efficiency: After correction for all downstream losses, the in-crystal SFG photon conversion efficiency is $450$7, and the total (SFG+DFG) THz-to-NIR conversion is measured at $450$8. These values explicitly demonstrate the strong nonlinear interaction in BNA under the experimental conditions.

Bandwidth and spectral performance: The achievable operational bandwidth extends from $450$9 to $\mu$0 THz with a spectral resolution of $\mu$1 THz, determined by the gating pulse duration and instrument resolution. The spectra confirm broadband phase matching and are modulated by phonon absorption features intrinsic to BNA.

Loss and device limitations: Analysis identifies the SPCM PDE and monochromator as the dominant sources of detection loss. Future improvements are readily identified: shifting the upconverted signal to the visible would leverage peak PDEs ($\mu$2) of silicon SPCMs; minimizing monochromator losses by substituting with optimized filtering could further enhance overall quantum efficiency.

Implications and Prospects

The results validate the practicality and efficiency of organic nonlinear crystals, specifically BNA, for broadband, near-single-photon-level THz detection under ambient conditions. The explicit demonstration of $\mu$3 full-system and $\mu$4 in-crystal conversion efficiency addresses key impediments limiting room-temperature THz photon counting and establishes a performance benchmark.

From a fundamental perspective, the technique opens direct access to single-THz-photon quantum optics. This has ramifications for quantum-enhanced THz spectroscopy, quantum communication protocols utilizing THz carriers, and investigation of entangled or correlated THz states.

Practically, the method is well-suited to high-speed, high-sensitivity THz applications, including quantum-limited imaging, quantum state tomography, and photon-starved sensing. The broadband nature ensures suitability for both time-domain and frequency-resolved studies, while compatibility with Yb-based lasers supports deployment in compact, robust instrumentation.

The pathway for further advances is clear: enhancement of component throughput, migration of upconverted wavelengths to visible for optimal detector response, and integration with other nonlinear or quantum-compatible platforms could push effective single-photon THz detection into practical, scalable devices suitable for real-world quantum technologies.

Conclusion

This work empirically substantiates high-efficiency, room-temperature THz single-photon detection using nonlinear upconversion in BNA. The notably high internal conversion and system efficiency metrics, combined with broadband operation, provide a solid foundation for next-stage THz quantum experiments and photonic instrumentation. The framework is directly extensible by incrementally optimizing optical and detection chain elements, outlining a concrete trajectory to all-solid-state, room-temperature THz photon counting.