An Introduction into Raman Spectroscopy

This article provides a brief introduction into Raman spectroscopy and its related applications while also offering an overview of the physics, laser source requirements and detector properties required to properly evaluate a spectrum.

Critical Figures of Merit will also be discussed before the article outlines a range of Monocrom products suitable for use in Raman spectroscopy applications.

Raman spectroscopy – An overview

Raman spectroscopy is a characterization technique that works by irradiating a sample with a light source before analyzing its scattered light, particularly those photons scattered inelastically. Elastic scattering is referred to as Rayleigh scattering, however.

Photons suffering inelastic scattering interact with matter by either gaining or losing energy in the form of phonons. The portion of photons with resulting energy that is lower than the incident beam makes up the Stokes radiation, while photons gaining energy become part of the Anti-Stokes radiation.

The upper section of Figure 1.1 illustrates possible elastic and inelastic scattering transitions. Stokes and Anti-Stokes transitions each have different probabilities, and while the latter is quite raw, the signal may be enhanced using coherent excitation.

Raman spectroscopy measures frequency shift (expressed in wavenumbers, typically in units of cm−1). This value is independent of the excitation wavelength (λin), but in practice, several factors will result in one or another excitation wavelength being more or less appropriate. These factors will likely include the sample itself.

It is possible to measure Raman shift by analyzing either Stokes or Anti-Stokes radiation, but Stokes radiation displays an inherently higher intensity because the relative thermal population of energy levels is dictated by the Boltzmann factor:

(1.1)

Figure 1.1. Illustration of the energy shift suffered by photons inelastically scattered when the incident photon excites the material (a phonon is absorbed) or de-excites it (a phonon is released).1 Image Credit: Monocrom

In Equation 1.1:

  • ni represents the population of a specific state
  • kB is the Boltzmann constant
  • h is the Plank constant

h, together with the photon frequency (Vi = cvacin), gives the energy of that state i. Here cvac indicates the speed of light in vacuum.

The last variable missing is T, which is the temperature in Kelvin. Because of the intrinsically low quantum efficiency of this interaction (1 in 106 − 108 events), a high brightness light source is necessary.

Brightness (Br) can be expressed via:

(1.2)

Plas in this equation signifies the laser power, while Mo2 describes the beam quality in both axes (x and y), respectively. Cross-sections for Raman scattering range between 10−31 cm−2 and 10−25 cm−2. This is still five orders of magnitude lower than Rayleigh scattering.3

While the Raman Effect was first observed in 1928,4,5 the development of reliable Raman spectroscopy itself was not possible until the 1960s, thanks to the advent of the very first lasers. The intensity of Raman scattered light (Iscat) is directly proportional to incident beam intensity (I0) and reverse propotional to the fourth power of its wavelength (λin):6

Figure 1.2. Relationship between the three main elements involved in Raman Spectroscopy. The analyte and the laser source are closely interrelated. Besides, the detector is mainly a consequence of the wavelength, although the range of the detector plays an important role on the wavelength choice. Detector and analyte do not condition to each other directly. Image Credit: Monocrom

(1.3)

Lower wavelengths should be the best option in principle, but this is not the case. In order to understand this process, it is important to note the three key elements involved in Raman Spectroscopy:

  • The analyte
  • The laser source
  • The detector

Figure Fig. 1.2.1 outlines the relationship and mutual influence of these three elements.

The analyte

Raman Spectroscopy is a method for material characterization that makes use of the interaction between laser light and the rotational-vibrational energy-level structure of molecular compounds. This interaction allows users to acquire fingerprints of the material, useful for identification.

Raman spectroscopy can be used to identify certain materials and some relevant aspects of their molecular and lattice structure. The technique can also be used for quantification.

One common application of Raman Spectroscopy is in the identification of solid carbon materials. These materials demonstrate distinctive spectroscopic features in Raman scattering, based on whether the atoms are arranged to form graphite, diamond, amorphous carbon or even fullerenes, graphene or carbon nanotubes.

Material being characterized can exist in a solid, liquid or gaseous state, though it is not easy to detect gaseous samples using conventional techniques.

Several Raman-based techniques exist which are able to acquire an enhanced scattering response through the application of complementary strategies such as Surface Enhanced Raman Spectroscopy (SERS).

This technique sees the sample placed over a novel metal-coated substrate, for example, gold or silver. SERS allows any signal detected to be amplified by several orders of magnitude, allowing even single-molecule detection in instances where the metal layer is nanostructured.3,6,8

Resonance Raman (RR) spectroscopy6,7 is another widely recognized technique that involves irradiating the sample at a wavelength near one of its electronic transitions, resulting in true photon absorption and a highly enhanced Raman response.

It should be noted that in typical Raman scattering, the incident photon is not absorbed by the sample.

Coherent Anti-Stokes Raman Spectroscopy (CARS) is based on third-order susceptibilities. Acquiring a significant signal requires high enough laser intensities to ensure a two-photon absorption.

The literature outlines a range of other enhancement techniques, including Photo-Acoustic Raman Spectroscopy (PARS), Coherent Stokes Raman Spectroscopy (CSRS), or Stimulated Raman Gain Spectroscopy (SRGS).9

The laser source

The low quantum efficiency of Raman scattering means that two conditions are necessary in order for a proper light source to promote observable Raman scattered photons.

One of these conditions is high brightness Br (Equation 1.2), meaning that laser light sources remain the best choice. The other condition involves ensuring the wavelength used is as low as possible (Equation 1.3).

UV and visible lasers in the NUV-blue-green region are generally a good choice for investigating inorganic compounds,7 but organic matter and biological samples tend to show strong fluorescence in the visible region when irradiated at these wavelengths.

When this happens, the Raman signal may become ‘buried’ under intense, broad fluorescent emission background and related noise.

A simple means of avoiding this lies in shifting the selected incident wavelength out of the visible and NUV region towards the MUV or the NIR, though this does mean that the influence of other aspects must be considered; for example, the detector.

The range of Raman shift between 100 cm−1 and 4000 cm−1 covers almost the whole set of Raman-active species.7,9 When this range is translated into nanometers, the observable range is only a 26 nm wide spectral window (assuming λin = 248 nm).

This limited window is a significant challenge from the perspective of grating resolution and detection, and while UV-enhanced silicon detectors are one possible solution, the sensitivity will still be low.

Should λin = 1064 nm, the window of observation will be approximately 700 nm, requiring the use of germanium or InGaAs detectors. Figure 1.3 illustrates the relationship between excitation wavelength and Raman spectrum width.

Figure 1.3. Popular Raman excitation laser lines and their corresponding spectral observation window for the Raman scattering (Stokes radiation). The horizontal length of the side rectangles represent the spectral range in nanometers corresponding to a Raman shift going from 100 cm−1 to 4000 cm−1, while the height illustrates the relative scattering intensity according to the excitation wavelength (notice that the vertical left axis is in logarithmic scale). Additionally, relative spectral response of traditional detector technologies are superimposed to illustrate the link between laser source and detector. Image Credit: Monocrom

MUV wavelengths can also induce undesirable changes in many samples, for example, polymerization, ionization, or bond-breaking transitions.6

The detector

CCD cameras equipped with Si-based detectors offer excellent sensitivity, particularly in the red-NIR region. The optimal combination appears to be a laser source operating in range 400 – 550 nm range, coupled with a regular, room temperature CCD camera.

The use of MUV lasers presents two main drawbacks in terms of fluorescence. However: MUV lasers are expensive, while there are limited available options in the laser industry below 300 nm. The sensitivity of CCD cameras in the NUV-violet is also less than ideal for many applications.

When using NIR lasers, Raman scattered radiation will exhibit lower intensity as irradiation wavelength increases. Si-based detectors are also no longer a suitable option in instances where Raman scattered light is over 1 micron - other detector types must be used instead, for example, those based in InGaAs.

Overall, the best trade-off must be identified for the application in question.

The ongoing adoption of Raman spectroscopy by industry has resulted in the use of 785 nm lasers and CCD cameras becoming the gold standard.

This has led to the development of affordable, portable and even handheld Raman spectrometers, a stark contrast with the classic, bulky and costly laboratory equipment commonly used in the 1980s.

Lasers at 785 nm may be diode-based, meaning that they will be compact, affordable, efficient and widely available while offering excellent emission characteristics. This also means that silicon photosensitivity remains within acceptable levels, and issues with fluorescence are mitigated.

Raman shifts over 3000 cm−1 are still detectable under these conditions, but many cases will still require the use of different wavelengths and different detectors.

Figures of merit: Lasers and Raman spectroscopy

Other factors must be considered as well as the wavelength of the laser source. These are especially important when considering the joint perspective of the laser manufacturer and the Raman equipment integrator. These factors include:

  • Beam quality: TEM00 beams maximize spatial resolution when working with samples requiring an analysis of composition or structure spatial distribution.
  • Polarization: The laser beam should be linearly polarized in the branches of Raman Spectroscopy used to investigate the polarization degree of molecules.
  • Spectral linewidth: around 10 pm or less is required to ensure a suitable Raman spectra resolution. This is the smallest difference in cm−1 that it is possible to resolve between Raman features.
  • Spectral purity: Each side mode concerning the excitation wavelength must be suppressed. The main peak must therefore prevail over them at a level >60 dB. This level of purity is generally acceptable at 1-2 nm around the main peak, though this distance is reduced as the Raman shift moves into the sub −100 cm−1 level.
  • Frequency stability: Acquisition time is typically in the order of seconds or tens of seconds, meaning that maintaining a suitably still excitation wavelength: <10 pm drift over time and operation temperature.
  • Output power stability: The typical power range is between 10 and 1000 mW, though this does depend on the analyte and the excitation wavelength. Power stability is key to effective quantification and is linked to the integration time required to obtain a spectrum.
  • Isolation against optical feedback: Confocal microscopy configurations involve the use of over-coupled excitation and backscattered beams. Even the smallest portion of light backscattered into the laser source may result in power instabilities or laser degradation. The use of optical isolators can help avoid this.

Applications of Raman spectroscopy

Raman spectroscopy is becoming increasingly prevalent in biosciences, largely due to its non-invasive nature. While lasers used in Raman spectroscopy can accommodate many wavelengths, these are usually in the UV-VIS-NIR part of the electromagnetic spectrum.

It is possible to carry out remote identification of explosives using Q-Switched lasers. This can be achieved using diode pumped solid-state lasers working at 2nd, 3rd or 4th harmonic generation wavelengths. Raman spectroscopy has also been utilized in a range of other applications, including:

  • Archaeology and art
  • Medical diagnosis and biosciences
  • Chemical processes and polymers
  • The solar and semiconductor industries
  • Mineralogy and geology
  • Pharmaceuticals
  • Environmental science
  • Raman microscopy
  • Forensic analysis
  • Gemology teaching
  • Quality control
  • General research

Food safety

Signal enhancement techniques like SERS have enabled the development of online monitoring systems for food quality control and safety. The food industry has become more profit-driven over recent years.

The market has become more globalized, while consumers are increasingly concerned about the quality of mass-produced food.10

These consumer concerns are based on an evidenced link between harmful food, diseases and poor health.12,13 The connection between health and food quality is well documented, with poor quality food causing diseases and healthy food improving mental and physical health.10,11 Because of this, there is a high level of motivation to ensure high quality food products.

Raman spectroscopy (especially SERS) and other spectroscopy methods can be used to install robust quality control measures into a number of production processes;14,16 for instance, the detection of pesticides on fruit surfaces.15

Picture of the fiber coupled version of the S-series package. Also available as free-space version. Monocrom offers a broad variety of wavelength and output power combinations.

Figure 2.1. Picture of the fiber coupled version of the S-series package. Also available as free-space version. Monocrom offers a broad variety of wavelength and output power combinations. Image Credit: Monocrom

Monocrom products and Raman spectroscopy applications

Low power, single frequency diode laser

Monocrom’s S-series (Figure 2.1) is highly suited for the majority of common Raman spectroscopy set-ups. It comes equipped with free-space or SM-fiber output, as well as a diverse array of wavelength and output power combinations.

Single frequency versions offer a linewidth as low as a few tens of MHz as well as a high side mode suppression ratio (SMSR) - typically 50 dB.

The instrument’s SM-fiber coupling capabilities offer excellent beam quality, essential in ensuring a high brightness (Br, Equation 1.2) as well as a high spatial resolution. The latter is particularly important in Raman microscopy applications. These capabilities also offer the potential to employ PM-fibers for polarization-dependent Raman spectroscopy.

Most Raman spectroscopy applications require stable output power (Plas) and a stable wavelength (λin). These must remain in place over the integration time required to obtain a full spectrum. The S-series’ thermoelectric cooler (TEC) helps ensure this vital stability.

The footprint of the standard package is 100 x 100 mm2, though other custom packages may be manufactured if required.

High energy solid state laser with/without frequency conversion

Monocrom’s high-energy diode-pumped solid-state laser (HESSL, Figure 2.2) offers the potential to step into every coherent Raman spectroscopy application, as well as into remote Raman spectroscopy.

The linewidth (λin <0.1 nm) of the fundamental wavelength (λin = 1064 nm) transforms to <1.77 cm−1 - narrow enough for almost all requirements. The laser system is available in repetition rates of 1 < Vrep < 500 Hz up to Epulse = 1 J.

The high available output power means that the laser can also accommodate remote Raman spectroscopy. The high-energy solid-state laser delivers a high power (<2% at 8 h) as well as pulse-to-pulse (<1% rms) stability.

The pulse width can be set between 4 ns < τpulse < 25 ns, depending on the application in question. It has to be kept in mind that τpulse influences Epulse and vice versa.

The high fundamental pulse energy makes frequency conversion to the 2nd, 3rd or 4th harmonic easily possible with maintaining a high Epulse harmonic. The footprint of this laser system is 900 × 500 mm2, and this can also be adapted to customers’ needs as required.

Figure 2.2. Picture of the high energy solid state lasers. Image Credit: Monocrom

References

  1. J. Heath, N. Taylor: “Raman Microscopy”, John Wiley & Sons Ltd, 2017
  2. E Smith, G. Dent: “Modern Raman Spectroscopy – A practical Approach”, John Wiley & Sons Ltd, 2005
  3. X. Liu: “Organic Semiconductor Lasers and Tailored Nanostructures for Raman Spectroscopy”, Dissertation, Karlsruher Institute for Technology, 2015
  4. C. Raman, K. Krishnan: “A new type of secondary radiation”, Nature, Vol. 121, 501-502, 1928
  5. G. Landberg, L. Mandelstam: “Eine neue Erscheinung bei der Lichtzerstreuung in Kristallen”, Naturwissenschaften, Vol. 16, 557-558, 1928
  6. P. Vandenabeele: “Practical Raman Spectroscopy”, John Wiley & Sons Ltd, 2013
  7. E. Smith and G. Dent: “Modern Raman spectroscopy: a practical approach”, John Wiley & Sons Ltd, 2005.
  8. K. Kneipp: “Surface-enhanced Raman scattering”, Physics Today, Vol. 60, 40–46, 2007
  9. P. J. Larkin: “IR and Raman Spectroscopy – Principles and Spectral Interpretation”, Elsevier, 2011
  10. Z. You: “Application of Infrared Raman Spectroscopy in Analysis of Food Agricultural Products”, AIDIC Servizi S.r.l., Chemical Engineering Transactions, Vol. 59, 763-768, 2017
  11. J. Depciuch et al.: “Application of Raman spectroscopy and infrared spectroscopy in the identification of breast cancer”, Applied Spectroscopy, Vol. 70(2), 251-263, 2016
  12. A. A. Kadik et al.: “Application of IR and Raman spectroscopy for the determination of the role of oxygen fugacity in the formation of n–Ñ–Î–Í molecules and complexes in the iron-bearing silicate melts at high pressures”, Geochemistry International, Vol. 54(13), 1175-1186, 2016
  13. J. Yu et al.: “Recent applications of infrared and Raman spectroscopy in art forensics: a brief overview”, Applied Spectroscopy Reviews, Vol. 50(2), 152-157, 2015
  14. Z. Zhang: “Raman Spectroscopic Sensing in Food Safety and Quality Analysis”, University of Nebraska-Lincoln, 2017
  15. J. Chen, D. Dong, S. Ye: “Detection of pesticide residue distribution on fruit surfaces using surface-enhanced Raman spectroscopy imaging”, The Royal Society of Chemistry, Vol. 8, 4726-4730, 2018
  16. Y. S. Li, J. S. Church: “Raman spectroscopy in the analysis of food and pharmaceutical nanomaterials”, Elsevier, Journal of food and drug analyzes, Vol. 22, 29-48, 2014

About Monocrom

Monocrom is established in 1993 in Vilanova i la Geltrú, Barcelona area. Since then, their vision has been to grow as a global market leader in the semiconductor laser industry serving high-quality laser diode systems and custom laser solutions. A continued improvement and commitment to excellence are the basis of our business practices.

Monocrom is a UNE EN ISO 9001:2015 registered laser company located in one of Europe’s pioneering regions in photonics. Their portfolio of diode lasers comprises standard and custom-made solutions for the medical, industrial, and defense industries among others. Their laser systems are designed to meet the most demanding requirements in material processing, ophthalmology, dermatology and aesthetics, imaging, security, life science, and research applications.


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Last updated: Jun 11, 2021 at 6:38 AM

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