A Brief Introduction into Raman Spectroscopy

This article is designed to provide a brief introduction to Raman spectroscopy, as well as its related applications. It also sets out to offer an overview of the physics behind Raman spectroscopy, its laser source requirements, plus the detector properties needed to properly assess a spectrum.

In addition, Critical Figures of Merit will be examined before the article goes on to outline a range of Monocrom products that are suitable for use in Raman spectroscopy applications.

Raman spectroscopy – An overview

Raman spectroscopy is what is known as a characterization technique. The process involves irradiating a sample with a light source before going on to analyze its scattered light, particularly photons that are scattered in an inelastic manner. It should be noted that elastic scattering is known as Rayleigh scattering.

Where photons are experiencing inelastic scattering, they interact with matter by gaining or losing energy through phonons. Photons with an energy result that is lower than that of the incident beam are a part of Stokes radiation. Conversely, photons gaining energy contribute to Anti-Stokes radiation.

Figure 1.1’s upper section outlines possible transitions of elastic and inelastic scattering. Stokes and Anti-Stokes transitions each have dissimilar probabilities, and while the latter is quite raw, the resulting signal may be augmented through the use of coherent excitation.

Raman spectroscopy involves the measurement of the frequency shift (expressed in wavenumbers, typically in units of cm−1). While this value is independent of the excitation wavelength (λin), several factors, including the sample itself, will result in one or another excitation wavelength being the most appropriate.

The Raman shift can be measured by analyzing either Stokes or Anti-Stokes radiation. Stokes radiation displays an inherently higher intensity, however, as the relative thermal population of energy levels is determined 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, and h is the Plank constant. Together with the photon frequency (Vi = cvacin), h gives the energy of that state. cvac represents the speed of light in vacuum.

The remaining variable is T, the temperature in Kelvin. Due to the intrinsically low quantum efficiency of this interaction (1 in 106 − 108 events), a light source with a high level of brightness is needed.

Brightness (Br) can be expressed via:

(1.2)

Here, Plas indicates the laser power and Mo2 describe the beam quality in both the axes (x and y), respectively. Cross-sections for Raman scattering can range from 10−31 cm−2 to 10−25 cm−2. It should be noted that this is five orders of magnitude lower than Rayleigh scattering.3

The Raman Effect was first observed in 1928,4,5, but the development of reliable Raman spectroscopy itself only became possible in the 1960s due to the advent of the very first lasers.

Intensity of Raman scattered light (Iscat) is proportional to incident beam intensity (I0) and 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)

While lower wavelengths should be the best option in principle, this is not, in fact, the case. To understand why it is crucial to bear in mind the three key elements involved in Raman spectroscopy:

  • The analyte
  • The laser source
  • The detector

Figure 1.21 outlines both the relationships and mutual influences of these three elements.

Raman Spectroscopy for Quantification Purposes

Raman spectroscopy is a technique for material characterization. It takes advantage of the interaction between the rotational-vibrational energy-level structure of molecular compounds and laser light. As a result of this interaction, users can acquire the fingerprints of the material, which are useful for identification.

Not only can Raman spectroscopy be used to identify specific materials and some pertinent aspects of their molecular and lattice structure, but it can also be used for quantification purposes.

Raman spectroscopy can be employed in the identification of solid carbon materials, and as such, materials demonstrate distinctive spectroscopic features in Raman scattering.

The distinctiveness enables atoms to form a range of structures, for example, diamond, graphite, graphene or carbon nanotubes, amorphous carbon or even fullerenes.

Though the material being characterized can exist in a range of states - solid, liquid or gaseous - it is not easy to detect gaseous samples using traditional techniques.

A number of Raman-based techniques exist where an enhanced scattering response can be acquired by using complementary strategies such as Surface Enhanced Raman Spectroscopy (SERS).

This involves the sample being placed over novel metal-coated substrates such as gold or silver. SERS allows for any signal detected to be amplified by several orders of magnitude. This facilitates the detection of even single-molecule detection in cases where the metal layer has a nanostructure.3,6,8

Resonance Raman (RR) spectroscopy6,7 is another widely known method, which involves utilizing a wavelength near to one of its electronic transitions to irradiate a sample.

This will give not just true photon absorption but also a highly enhanced Raman response. In typical Raman scattering, the incident photon will not suffer absorption by the sample being irradiated.

Coherent Anti-Stokes Raman Spectroscopy (CARS) utilizes third-order susceptibilities, and the acquisition of a significant signal necessitates a high enough laser intensity to guarantee a two-photon absorption.

In addition to the above, an array of other enhancement techniques are available, including Coherent Stokes Raman Spectroscopy (CSRS), Photo-Acoustic Raman Spectroscopy (PARS), and Stimulated Raman Gain Spectroscopy (SRGS).9

Raman Scattering  

Due to the low quantum efficiency of Raman scattering, two conditions are required to allow an appropriate light source to promote observable Raman scattered photons.

The first of these conditions is high brightness (Br, Equation 1.2), under which laser light sources remain the optimum option. The second condition requires using a wavelength that is as low as possible (Equation 1.3).

UV lasers, as well as visible lasers in the NUV-blue-green region, are regarded favorably when it comes to investigating inorganic compounds.7 However, biological samples and organic matter can display a strong fluorescence in the visible region when they are irradiated at such wavelengths.

This means it is possible that the Raman signal can become ‘buried’ under intense, broad fluorescent emission background and related noise.

A straightforward approach to avoiding this involves shifting the chosen incident wavelength from the visible and NUV region towards the MUV or the NIR. This does mean that the effect of other influences, such as the detector, must be taken into account.

A Raman shift between 100 cm−1 and 4000 cm−1 can encompass almost the entire set of Raman-active species.7,9 Assuming λin = 248 nm, the observable range is only a 26 nm wide spectral window when the range is translated into nanometers.

Such a restricted window is a major challenge when it comes to grating resolution and detection, and though UV-enhanced silicon detectors are one possible answer to this, the sensitivity will still be low.

If λin = 1064 nm, then the window of observation will be in the region of 700 nm, which will necessitate the use of InGaAs or germanium. Figure 1.3 outlines the relationship between the excitation wavelength and the 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 often lead to unwanted changes in samples, including ionization, polymerization, or bond-breaking transitions.6

The detector

A CCD camera equipped with a Si-based detector offers first-rate sensitivity, especially in the red-NIR region. A standard, room temperature CCD camera in combination with a laser source operating in the 400 – 550 nm range is an optimal solution.

Utilizing MUV lasers results in two central drawbacks in terms of fluorescence: MUV lasers are generally quite costly, and market options below 300 nm are limited in terms of availability. Additionally, CCD cameras in the NUV-violet range are less ideal for a range of applications requiring high sensitivity.

Where NIR lasers are utilized, Raman scattered radiation will display lower intensity as the irradiation wavelength increases. If Raman scattered light is over 1 micron, then Si-based detectors are no longer a suitable option.

Other detector types, such as those based in InGaAs, should be utilized instead. In general, the best trade-off must be identified for the application under consideration.

The increasing use of Raman spectroscopy within industry has led to a combination of 785 nm lasers and CCD cameras becoming the industry standard.

Cost-efficient portable and handheld Raman spectrometers are now on the market, a significant evolution from the large and expensive laboratory options commonly available in the 1980s.

As lasers at 785 nm can be diode-based, they are likely to be affordable, efficient, compact, and widely available on the market while also offering exceptional emission characteristics.

Silicon photosensitivity will remain within acceptable levels while any issues with fluorescence are alleviated. Though Raman shifts over 3000 cm−1 can still be detected under these conditions, this tends to require the use of other wavelengths and detectors.

Figures of merit: Lasers and Raman spectroscopy

The wavelength of the laser source is not the only factor that needs to be taken into consideration. These factors are of high importance when it comes to considering the joint perspective of the Raman equipment integrator and the laser manufacturer and. Relevant factors include:

  • Beam quality: TEM00 beams can maximize spatial resolution where samples require an analysis of structure, spatial distribution or composition.
  • Spectral linewidth: Approximately 10 pm or less is needed in order to ensure the Raman spectra resolution is usable. This is the smallest difference in cm−1 at which it remains possible to resolve between Raman features.
  • Polarization: Where Raman Spectroscopy is used to investigate the polarization degree of molecules, the laser beam should be linearly polarized.
  • Spectral purity: Side modes relating to the excitation wavelength need to be suppressed, which means that the main peak must prevail over the side modes at a level >60 dB. While this level of purity is usually acceptable at 1-2 nm around the main peak, this distance is reduced where the Raman shift begins to move into the sub −100 cm−1 range.
  • Frequency stability: Acquisition time is typically in the order of seconds or tens of seconds, meaning that maintaining a suitably still excitation wavelength of <10 pm drift over time and operation temperature is vital.
  • Isolation against optical feedback: A confocal microscopy configuration involves utilizing over-coupled backscattered and excitation beams. As the smallest portion of light backscattered into the laser source can lead to power instabilities or laser degradation, it is advisable to use optical isolators to avoid this.
  • Output power stability: While the typical power range is usually between 10 and 1000 mW, this can depend on the analyte and excitation wavelength in question. For effective quantification, power stability is a key factor and is linked to the integration time needed to acquire a spectrum.

Applications of Raman spectroscopy

Thanks to its non-invasive nature, the use of Raman spectroscopy is becoming increasingly widespread in biosciences. Lasers used in Raman spectroscopy generally operate in the UV-VIS-NIR part of the electromagnetic spectrum but can, in fact, accommodate many wavelengths.

For example, Q-Switched lasers can be employed in applications involving the remote identification of explosives by using diode-pumped solid-state lasers working at 2nd, 3rd or 4th harmonic generation wavelengths. Other applications where Raman spectroscopy has also been utilized include:

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

Food safety in Raman Spectroscopy

By utilizing signal enhancement techniques such as SERS, it is possible to develop on-line monitoring systems for variables like food quality control and food safety. As the food industry has become more profit-driven and the market more globalized, consumers, in turn, have become increasingly aware of and concerned about the quality of mass produced food.10

Consumer concerns come from evidence-based links between harmful food, diseases, and poor health.12,13 In fact, the link between health and food quality has been thoroughly documented - poor quality food leads to disease, whereas healthy food can lead to improvements in mental and physical health.10,11

Consequently, industries are highly motivated to ensure food products are produced to high standards.

Spectroscopy methods, including Raman spectroscopy (and especially SERS), can enable robust quality control measures in a range of production processes;14,16 such as 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

The Monocrom S-series (Figure 2.1) is well-suited for most widely used Raman spectroscopy set-ups. It offers free-space or SM-fiber output, as well as a diverse array of output power and wavelength combinations.

The single frequency models allow for a linewidth as low as a few tens of MHz as well as offering a high side mode suppression ratio (SMSR) – typically in the area of 50 dB.

The Monocrom S-series’ SM-fiber coupling capabilities offer excellent beam quality. This is vital for ensuring not just a high level of brightness (Br, Equation 1.2) but also a high spatial resolution, with the latter being particularly important when utilized in Raman microscopy applications.

These capabilities also offer the potential to use PM-fibers for polarization-dependent Raman spectroscopy.

In general, Raman spectroscopy applications require both stable output power (Plas) and a stable wavelength (λin). These must remain stable over the integration time required to obtain a full spectrum - the S-series’ thermoelectric cooler (TEC) assists with ensuring this 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) gives users the option to leverage every rational Raman spectroscopy application, as well as into remote Raman spectroscopy.

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

Due to the high available output power, the laser can also accommodate remote Raman spectroscopy. The high-energy solid state laser can deliver a high power (<2% at 8 h) plus pulse-to-pulse (<1% rms) stability.

Pulse width can be set between 4 ns < τpulse < 25 ns, depending on the application in question, it is important to take into account 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. While the footprint of the laser system is 900 × 500 mm2, it can 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 8, 2021 at 10:42 AM

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