Depth Analysis of Semi-Transparent Media by a Time-Correlated CMOS SPAD Line Sensor-Based Depth-Resolving Raman Spectrometer

Abstract

Raman spectroscopy has proved to have potential in deep surface analytical applications. We present here, to the best of our knowledge, the first time depth analysis of semi-transparent media by a depth-resolving Raman spectrometer based on an adjustable time-correlated CMOS SPAD (singlephoton avalanche diode) line sensor that can measure the depth of target samples embedded in a centimeter-scale semi-transparent medium simultaneously with a normal Raman depth profiling operation and suppress the fluorescence background by means of adjustable picosecond time gating. The variability of the depth derivation was measured to be ± 0.43 cm at depths ranging from 2 to 9 cm. In addition, the advantages of the adjustable picosecond time gating in terms of depth derivation and fluorescence background suppression performance were shown by comparing gate widths ranging from 100 ps to 13 ns. We believe that the technology concerned could pave the way for a new kind of compact, practical depth-resolving Raman spectrometer for deep subsurface analytical applications. Index Terms-Depth-resolving Raman spectrometer, depth analysis, time-correlated single photon counting (TCSPC), CMOS single-photon avalanche diode (SPAD).

Mineralization of dental tissues and caries lesions detailed with Raman microspectroscopic imaging

“…The fluorescence suppressed time-resolved Raman spectrometer can be used to rapidly acquire comparable spectra with the continuous wave Raman spectroscopy. Furthermore, time-resolved CMOS SPAD-based Raman can be used to collect depth-resolved chemical information 34 . Our Fig.…”

Spatially offset Raman spectroscopy for biomedical applications

“…35,36 Alternative approaches include time gated fluorescence suppression. [37][38][39][40] Micro-SORS The use of micro-SORS which combines SORS with microscopy to probe very thin layers on a micrometre to sub-mm scale has also been demonstrated. The technique has been applied extensively to the analysis of paint in cultural heritage and also exemplified in analysis of wheat seeds and in other areas.…”

Chemical imaging of human teeth by a time-resolved Raman spectrometer based on a CMOS single-photon avalanche diode line sensor

“…The spectral accuracy of the sensor has been shown to be within the spectral resolution (∼6 cm −1 ) with various standard Raman samples, such as 50/ 50 mixture of toluene/acetonitrile, TiO 2 and ethanol. 31,36,37 A detailed description of the 16 × 256 CMOS SPAD sensor with the integrated 256-channel 3-bit on-chip TDC is given in the article by Nissinen et al 31 The reference Raman set-up was a commercial confocal CW Raman imaging microscope (Thermo Fisher Scientific, USA, model DXR™2xi) equipped with a 10× objective (Olympus Plan N, 10×/0.25 NA). The CW laser of the reference spectrometer has a wavelength, average power and spot size of 532 nm (same as used with the time-resolved CMOS SPADbased Raman spectrometer), 10 mW and 2.6 µm, respectively.…”

“…Furthermore, timeresolved CMOS SPAD sensors enable to implement novel functionalities for Raman spectroscopy, such as Raman radar and Raman depth-profiling operations, in a practical and compact way, which might in the future open novel application possibilities for Raman spectroscopy in various fields of science. 36,37 In this paper, we demonstrate the capability of a fluorescence-suppressed Raman spectrometer based on a timeresolved 16 × 256 CMOS SPAD line sensor with an integrated 256-channel 3-bit on-chip time-to-digital converter (TDC) 31 to form chemical images of extracted human teeth. This is the first time, to the best of our knowledge, that CMOS SPADbased Raman spectroscopy is used to chemically image human teeth.…”