The development of time-gated Raman spectrometer system by CoE Oulu group suggest CMOS SPAD based time-gating techniques could pave way to the better use of the high resolving power of Raman spectrometer techniques. Their potential fields of application are in agriculture, food and oil industries and in security control and crime investigations. Also, new type of instruments could possibly be developed.
Raman spectroscopy is a spectroscopic technique used to observe vibrational, rotational, and other low-frequency modes in a system. Raman spectroscopy is commonly used in chemistry to provide a fingerprint by which molecules can be identified.
In many practical measurements and applications, a Raman spectrum is masked by strong fluorescence background. However, since the lifetime of Raman scattered photons is much less (picosecond level) than that of the fluorescence photons (typically in the range of few nanoseconds or hundreds of nanoseconds), the fluorescence background can be suppressed if scattered photons are collected only simultaneously with the illuminating short laser pulse. The suppression efficiency is highest with short and intensive laser pulses (pulse width much smaller than fluorescence lifetime). In other words, by “time-gating” the recording on top of the laser pulse, the probability of the detection of the fluorescence photons is decreased since the fluorescence photons are emitted after delay characteristics to the material under study. Time gating suppresses also ambient light and the dark counts of the detector, improving thus the signal-to-noise ratio.
The CoE Oulu group has developed a time-gated Raman spectrometer system based on the above concept and shown that reverse biased pn-junction based single photon detectors realized in standard CMOS technologies can be used to record the Raman spectrum (US patent 8,917,388). More importantly, with these detectors the time gate window can be in sub-ns range even in large detector arrays, see below. Thus very efficient fluorescence suppression can be achieved even with ns-scale fluorescence time constants.
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