Picosecond Raman spectroscopy with a fast intensified CCD camera for depth analysis of diffusely scattering media

Abstract

A spectroscopic depth profiling approach is demonstrated for layers of non-transparent, diffusely scattering materials. The technique is based on the temporal discrimination between Raman photons emitted from the surface and Raman photons originating from a deeper layer. Excitation was carried out with a frequency-doubled, 3 ps Ti:sapphire laser system (398 nm; 76 MHz repetition rate). Time-resolved detection was carried out with an intensified CCD camera that can be gated with a 250 ps gate width. The performance of the system was assessed using 1 mm and 2 mm pathlength cuvettes with powdered PMMA and trans-stilbene (TS) crystals, respectively, or solid white polymer blocks: Arnite (polyethylene terephthalate), Delrin (polyoxymethylene), polythene (polyethylene) and Teflon (polytetrafluoroethylene). These samples were pressed together in different configurations and Raman photons were collected in backscatter mode in order to study the time difference in such media corresponding with several mm of extra net photon migration distance. We also studied the lateral contrast between two different second layers. The results demonstrate that by means of a picosecond laser system and the time discrimination of a gated intensified CCD camera, molecular spectroscopic information can be obtained through a turbid surface layer. In the case of the PMMA/TS two-layer system, time-resolved detection with a 400 ps delay improved the relative intensity of the Raman bands of the second layer with a factor of 124 in comparison with the spectrum recorded with a 100 ps delay (which is more selective for the first layer) and with a factor of 14 in comparison with a non-gated setup. Possible applications will be discussed, as well as advantages/disadvantages over other Raman techniques for diffusely scattering media.

Raman signal enhancement via elastic light scattering

“…It has been used for a wide variety of applications including cancer diagnosis [1,2], the detection of hazardous materials [3], and industrial quality control. Raman spectroscopy has previously been carried out in turbid media [4,5]; however, the effect elastic scattering has on the process has yet to be investigated. In other words, is there a way to use elastic scattering to optimize Raman generation?…”

Recent advances in the development of Raman spectroscopy for deep non‐invasive medical diagnosis

“…The concept was developed from earlier temporal-domain Raman counterparts [5,[17][18][19][20] which utilised time-correlated single-photon counting [17] or picosecond Raman Kerr gating (the latter developed originally for fluorescence rejection from Raman spectra [4,[21][22][23]. A CCD gated method has also been demonstrated recently for subsurface probing [24] and its potential for combining with SORS has been demonstrated [25]. Although beneficial in some situations the temporal concepts are restricted by higher instrumental complexity and cost.…”

Looking inside Catalyst Extrudates with Time-Resolved Surface-Enhanced Raman Spectroscopy (TR-SERS)

“…The use of a short detector gate allows selective collection of Raman photons from a certain depth within the sample and eliminates a considerable amount of fluorescent background from the surface as well as from the bulk. 17,19,20 Combinations of SERS, SORS, and TRRS are also possible. Previous work has shown that a slight improvement of the selectivity for photons from the bulk versus the surface can be achieved by combining TR detection with a SORS excitation geometry.…”

“…Surface-enhanced Raman spectroscopy (SERS), spatially offset Raman spectroscopy (SORS), and time-resolved Raman spectroscopy (TRRS) are all advanced Raman techniques that have been introduced, in part, to overcome the problem of fluorescence interference in a variety of applications. [14][15][16][17][18][19][20] Through signal enhancement and selectivity for material in the bulk versus the surface, these methods could also help in studying the inside of catalyst extrudates in more detail.…”

“…Recently it has been shown that SERS used in combination with SORS can provide selectively enhanced depth measurements through diffusive surface layers for biomedical applications. [24][25][26] Time-resolved Raman spectroscopy by means of a Kerr gate 16 or an intensified charge-coupled device (CCD) camera 20,[27][28][29] offers an alternative for sub-surface analysis inside a strongly scattering sample. With this approach the distinction between Raman photons emanating from the surface versus those generated inside the sample is made on the basis of their distribution in time.…”