Figure 1. Representative setup for single photon counting Raman spectroscopy experiment. The laser beam is focused on the surface of the sample. The reflected light is collected by the same objective lens, directed through a dichroic mirror, and collected into a fiber-coupled SPD. The Time Tagger measures the time difference between the arrival times of photons and laser sync signals.
Raman spectroscopy is a powerful technique for analyzing the samples’ molecular composition, chemical structure, and chemical environment by examining the scattering of light. When a laser light source illuminates a sample, the majority of photons experience elastic (Rayleigh) scattering, while a small fraction undergoes inelastic (Raman) scattering, resulting from the interaction between the photons and the vibrational or rotational modes of the molecules within the sample.
This technique finds applications across fields such as biochemistry, pharmaceutical analysis, environmental monitoring, and materials research, where it provides insights into molecular structures and interactions. Despite its broad applications in molecular and materials science, Raman spectroscopy has intrinsic challenges, notably its limited sensitivity and vulnerability to fluorescence interference from the sample. Recent advancements have focused on improving sensitivity by enhancing Raman signal detection and isolating the Raman signal from background fluorescence.
To address these challenges, single-photon counting Raman spectroscopy combines the principles of Raman spectroscopy with single-photon counting techniques, enabling the detection of weak Raman signals even at low analyte concentrations [1]. This approach significantly improves the sensitivity and selectivity of Raman spectroscopy measurements, making it highly effective for trace analysis in complex samples. This method achieves sub-nanosecond temporal resolution by utilizing time-gated and time-correlated single-photon counting (TCSPC), effectively separating Raman signals from fluorescence and background noise.
A typical experimental setup for time-resolved Raman spectroscopy with single-photon detectors involves several key components for exciting the sample, filtering the signals, and detecting photons with precise timing.
A pulsed laser source is used to excite the sample and induce Raman scattering. The laser beam from the source is focused onto the sample using an objective lens that also collects the backscattered signal. A dichroic mirror and a notch filter are usually leveraged to separate the Raman signal from the excitation laser light and background fluorescence. An acousto-optic tunable filter (AOTF) is used to dynamically select specific Raman shifts, enabling flexibility in the targeted spectral range. Then, a highly sensitive single-photon detector, such as an avalanche photodiode (APD) or a superconducting nanowire single-photon detector (SNSPD), captures the Raman photons. The timing electronics synchronize photon detection with the laser source at extremely high temporal resolution.
The Swabian Instruments’ Time Tagger plays a critical role in the setup, recording the arrival time of each photon with picosecond-level resolution. Such a high temporal resolution enables time-gated and time-correlated single-photon counting (TCSPC), which is essential for isolating weak Raman signals from competing fluorescence emission and, therefore, improving the signal-to-noise ratio.
Common challenges in single-photon counting Raman spectroscopy include:
Swabian Instruments' Time Taggers offer time jitter down to 1.5 ps, enabling highly accurate measurements when paired with single-photon detectors for Raman spectroscopy. This ultra-low jitter ensures precise timing, enhancing sensitivity to weak Raman signals and enabling detailed analysis at very low analyte concentrations. This resolution makes even subtle Raman shifts more detectable, making it ideal for high-precision, trace-level analyses.
Swabian Instruments' Time Taggers offer advanced capabilities for time-resolved Raman spectroscopy through precise photon counting and flexible time gating. By leveraging high-resolution time-tagging and customizable gating windows, these instruments effectively isolate Raman signals by excluding longer-lived fluorescence emissions. This targeted gating significantly reduces fluorescence interference, improving the clarity and reliability of Raman spectra and enhancing the detection of weak Raman signals.
Swabian’s Time Taggers support channel-specific hardware signal delays and adjustable dead times, adding flexibility to photon detection in complex measurements. These features allow each detection channel to be finely tuned, optimizing signal acquisition for multi-channel setups and enabling efficient, high-fidelity data collection across various experimental conditions.
Swabian Instruments' Time Taggers combine exceptional performance with a powerful software engine, allowing you to perform multiple real-time measurements with ease—either through a few lines of code in your favorite programming language (Python, Matlab, LabVIEW, C#/C++, Mathematica, .NET) or with a few clicks in the intuitive TimeTaggerLab GUI. Key Features:
[1] Y. Li, et. al., Photon counting Raman spectroscopy: a benchmarking study vs surface plasmon enhancement. Optics Express 32, 16657 (2024).
At Swabian Instruments, we’re excited to announce a new application note in collaboration with the Awschalom Group at the University of Chicago “Optically Detected Magnetic Resonance. Quantum Spin Probe of Single Charge Dynamics.” This collaborative work showcases the power of advanced quantum measurement techniques using our Pulse Streamer 8/2 and Time Tagger 20 .
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Photon detection distinguishability is increasing interest in advancements in photonics and quantum technology. At Swabian Instruments, we are proud to lead this innovation with our Time Taggers , as detailed in our latest abstract presented at the Optica Latin America Optics and Photonics Conference, which took place in Puerto Vallarta, Mexico, in November 2024.
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