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Single-photon time-stretch infrared spectroscopy

This paper presents a broadband mid-infrared single-photon time-stretch spectrometer that achieves high-resolution, single-photon sensitivity at room temperature by nonlinearly upconverting dispersed MIR photons to the near-infrared band for detection with low-noise silicon detectors.

Original authors: Ben Sun, Kun Huang, Huijie Ma, Jianan Fang, Tingting Zheng, Yongyuan Chu, Hairun Guo, Yan Liang, E Wu, Ming Yan, Heping Zeng

Published 2026-05-29

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CC BY 4.0

Original authors: Ben Sun, Kun Huang, Huijie Ma, Jianan Fang, Tingting Zheng, Yongyuan Chu, Hairun Guo, Yan Liang, E Wu, Ming Yan, Heping Zeng

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to listen to a whisper in a very noisy room. That is what scientists face when they try to analyze light in the mid-infrared (MIR) range. This type of light is incredibly useful for identifying chemicals, diagnosing diseases, or inspecting materials because different molecules "sing" at specific infrared frequencies. However, the sensors we usually use to catch this light are like old, crackly radios: they are noisy, slow, and often need to be frozen to work properly.

This paper introduces a clever new way to listen to that whisper using a technique called "Single-Photon Time-Stretch Spectroscopy." Here is how it works, broken down into simple steps:

1. The Problem: The "Noisy Room"

Traditional infrared sensors are like trying to hear a pin drop while a jet engine is running. They generate too much internal noise (static) and are slow to react. This makes it impossible to detect very faint signals, like a tiny amount of a gas or a single molecule, without freezing the equipment to near absolute zero.

2. The Solution: The "Translator"

Instead of trying to build a better microphone for the noisy room, the team decided to translate the message into a different language that our existing, high-quality microphones understand perfectly.

The Translation (Upconversion): They take the invisible mid-infrared light and mix it with a powerful laser beam. This process acts like a translator, instantly converting the "foreign" infrared light into near-infrared light (which is close to visible red light).
The Benefit: Now that the light is in this new "language," they can use standard, high-speed silicon detectors (the same kind found in your digital camera) which are very quiet, fast, and work at room temperature.

3. The Stretch: The "Rubber Band" Trick

Once the light is translated, they need to figure out exactly which frequencies were in the original beam. Usually, you would need a giant, expensive prism to split the light into a rainbow.

Instead, they use a Time-Stretch technique:

Imagine the light pulse is a short, tight rubber band.
They send this light through a very long, special fiber optic cable (8 kilometers long!).
Because of the physics of the cable, different colors of light travel at slightly different speeds. The blue end of the rainbow arrives first, and the red end arrives later.
The short rubber band gets stretched out into a long, slow-moving line. Now, the color of the light is directly linked to time. Blue light arrives at 1:00, red light arrives at 1:05.

4. The Detective Work: "Coincidence Counting"

Now that the light is stretched out over time, they use a super-sensitive detector to count the photons (particles of light) one by one.

They use a "stopwatch" that starts when the laser pulse is fired and stops when a photon hits the detector.
By recording exactly when each photon arrives, they can reconstruct the original "rainbow" of the infrared light.
Because they are counting individual photons, they can detect signals so faint that they are essentially counting single particles of light.

Why This is a Big Deal

The authors achieved three major things that make this a breakthrough:

Extreme Sensitivity: They can detect light levels as low as 0.14 photons per pulse. This is like hearing a single whisper in a stadium.
High Resolution: They can distinguish between colors that are incredibly close together (0.5 wavenumbers), allowing them to identify specific chemicals with great precision.
Simplicity: Unlike other methods that require complex arrays of thousands of sensors or freezing equipment, this system uses a single detector and works at room temperature.

The Bottom Line

The paper describes a new "translator" that takes difficult-to-detect infrared light, converts it into a friendly format, stretches it out so we can read it like a timeline, and counts it one by one. This allows scientists to see and identify materials with incredible clarity and sensitivity, using simple, room-temperature equipment.

What the paper claims it can do:
The authors state this technology is ready for immediate use in material science (analyzing what things are made of) and life sciences (studying biological samples). They specifically mention its potential for trace detection (finding tiny amounts of substances), remote sensing (measuring things from a distance), and biomedical examination. They do not claim it is currently a medical device for diagnosing patients, but rather a powerful new tool for scientific analysis in those fields.

Technical Summary: Single-Photon Time-Stretch Infrared Spectroscopy

Problem Statement
Sensitive mid-infrared (MIR) spectroscopy is critical for applications ranging from industrial inspection to biomedical diagnosis. However, conventional MIR spectrometers face significant limitations in photon-scarce scenarios (e.g., trace detection, remote sensing) due to the high dark current, slow response times, and low operating temperatures required by standard infrared sensors like narrow-bandgap semiconductors or thermal bolometers. Furthermore, scanning-free MIR spectrographs relying on focal plane arrays (FPAs) suffer from limited spectral resolution due to low pixel density, while single-photon array detectors are technically challenging and expensive. Although superconducting nanowire detectors offer single-photon sensitivity, they require cryogenic cooling and are currently limited to wavelengths below 2 µm. Existing indirect strategies, such as frequency upconversion, have demonstrated broadband MIR spectrometers, but previous time-stretch implementations in the MIR region have been hindered by the lack of low-loss dispersive media and high-sensitivity detectors, often failing to reach the single-photon detection level due to spontaneous parametric noise in difference-frequency generation (DFG) processes.

Methodology
The authors propose and implement a broadband MIR single-photon time-stretch spectrometer that circumvents the limitations of direct MIR detection by transferring MIR information to the near-infrared (NIR) band. The system operates through three primary stages:

Broadband MIR Generation: A nanophotonic silicon nitride (Si $_3$ N $_4$ ) waveguide, pumped by a femtosecond erbium-doped fiber laser, generates a supercontinuum covering 2.4–4.2 µm.
High-Fidelity Spectral Upconversion: The MIR signal is nonlinearly converted to the NIR band via sum-frequency generation (SFG) in a chirped-poling lithium niobate (CPLN) crystal. Crucially, the pump source (a Yb-doped fiber laser) is passed through a chirped fiber Bragg grating (CFBG) to create a chirped pulse. This "chirped-pumping" strategy ensures that the short MIR pulse overlaps only with a narrow spectral portion of the pump, enabling high-fidelity spectral transduction with a narrow effective pump bandwidth (~0.011 cm $^{-1}$ ) and suppressing noise.
Time-Stretch and Coincidence Counting: The upconverted NIR photons are sent through 8 km of low-loss single-mode fiber (SMF) to induce group velocity dispersion (GVD), temporally stretching the pulse. The arrival times of these dispersed photons are registered using a time-correlated single-photon counting (TCSPC) module synchronized with the pump pulses. A low-timing-jitter avalanche photodiode (APD) detects the photons. The resulting coincidence histogram between the pump trigger and the detected photons reconstructs the MIR spectrum in the time domain.

Key Contributions

Architecture: The work introduces a time-stretch architecture for MIR spectroscopy that utilizes a single-pixel detector, avoiding the complexity and cost of single-photon array detectors.
Noise Suppression: By employing a chirped-pump SFG process rather than DFG, the system achieves a noise-free coherent conversion, enabling operation at the single-photon level.
Resolution and Sensitivity: The system achieves a spectral resolution of approximately 0.5 cm $^{-1}$ and a detection sensitivity down to 0.14 photons/nm/pulse.
Room-Temperature Operation: Unlike superconducting alternatives, the entire system operates at room temperature, utilizing mature NIR fiber and detector technologies.

Experimental Results
The system was characterized using a polystyrene film sample. The time-stretch spectrum, reconstructed from the coincidence histogram, showed excellent agreement with reference Fourier-transform infrared (FTIR) measurements at a resolution of 0.5 cm $^{-1}$ . The spectral resolution was found to be limited primarily by the pump bandwidth and the detector's timing jitter (measured at 29 ps FWHM). In low-light conditions, the signal-to-noise ratio (SNR) demonstrated a square-root dependence on integration time and input pulse energy. At an accumulation time of 900 seconds, the system achieved a sensitivity of 0.14 photons/nm/pulse. The effective number of spectrally resolvable elements was calculated to be up to 1775, significantly exceeding the channel counts of conventional array-based dispersive spectrometers.

Significance
The paper claims that this work establishes an effective path toward ultra-sensitive, high-resolution MIR spectroscopy at room temperature. By combining frequency upconversion with time-stretch spectroscopy, the system overcomes the stringent requirements for high-definition array detectors and the trade-off between spectral coverage and resolution. The authors state that the demonstrated features—broadband spectral coverage (2.4–4.2 µm), sub-wavenumber resolution, single-photon sensitivity, and room-temperature operation—provide a practical alternative for applications in material analysis, chemical sensing, and life sciences, particularly in scenarios where photon flux is severely limited. The work represents a landmark in high-resolution MIR single-photon spectrometers, offering high optical throughput and single-pixel simplicity.