Raman Spectroscopy Lasers

Deeper Dive into Raman Spectroscopy Lasers

When a laser is incident on a sample, most of the photons will be scattered elastically and will not be subject to any energy change, known as Rayleigh Scattering. Raman scattering events are significantly more infrequent, only around 1 in a million incident photons, but the consideration of these inelastically scattered photons, where a change in frequency (or Stokes shift) can be observed, allows a range of information about the sample to be determined. The energy for this wavelength shift comes from a change in the energy state of a molecular bond or bonds. This is distinct from an interaction where the photon is absorbed by an atom and then re-emitted at a different wavelength, which is the domain of fluorescence spectroscopy.

The wavelength shift in the Raman scattered light corresponds directly to the current energy states of the molecular bonds in the sample so, as these are influenced not just by the atoms involved in those bonds but also by the total crystal structure and the strain the system is under, it is possible to interpret useful information from the Raman spectrum that can be difficult to obtain by other means. First demonstrated by C. V. Raman in 1928, this weak Raman effect has been historically difficult to discern. However, significant advances in laser sources, detectors, and optical techniques mean that Raman Spectroscopy and Microscopy are now widely used over a disparate range of applications; including biology, pharmaceuticals, semiconductors, forensics, security, and the analysis of art and museum artifacts.

Critical Laser Source Requirements Common to Raman Applications

• Wavelength – The strength of the Raman signal is directly dependent on the wavelength of the laser source, where lower wavelengths will produce stronger Raman signals, as well as allowing for higher spatial resolution. It is important, however, to balance this observation with the occurrence of background fluorescence, prevalent in many materials throughout the UV-visible spectrum, and the possibility of sample damage at high energy. These effects most often cause a compromise in the wavelength of the source used, where longer wavelengths, such as 532 nm, 785 nm, and 1064 nm, in combination with highly sensitive detectors, allow for the widest range of samples to be measured.
• Spectral Linewidth & Purity – The spectral linewidth of the laser source should also be considered, as it will limit the possible resolution of the Raman measurement as well as the minimum energy change that can be determined. It is important for the laser selected to have a linewidth below the overall resolution of the Raman spectrometer, on the order of picometers.  For high-resolution spectroscopy, this is critical and requires linewidths below 1 MHz. High spectral purity will also increase the signal-to-noise ratio from the measurement.
• Beam Quality – The beam quality is related to the possible spatial resolution. Here, single transverse mode beams (TEM00) are vital for confocal Raman Spectroscopy, in particular, allowing for high spatial control in all three axes, improving spatial resolution, and decreasing background effects.

Choosing a Laser for Raman Spectroscopy

Choosing the correct laser is especially important for Raman spectroscopy compared with other spectroscopic techniques because the Raman shift is directly related to the light source, and the measured spectroscopic data cannot be decoupled from the light source. The Raman Effect is very weak and is directly proportional to the power of the light source.

The strength of the Raman signal is also tied to the specific wavelength used. Longer wavelengths give a weaker Raman signal but have less (or no) fluorescence, while shorter wavelengths give exponentially stronger Raman signal but may also have much higher fluorescence that can drown out the Raman signal. A Raman light source must also have narrow linewidth and excellent wavelength stability and predictability, as it must always be within the tolerances of the filters. See our new Lasers 101 page for more in-depth information about the attributes of Lasers.

Today, diodes are the most popular choice for Raman excitation, but this was not always the case because traditional diode lasers have a high degree of uncertainty in their center wavelength and are typically specified with a +/- 5 nm tolerance. In Raman spectroscopy, this is completely unacceptable as precise knowledge of the starting wavelength is required to accurately measure the shift in the energy of the scattered photon.

RPMC can offer a useful Raman source with a volume Bragg grating (VBG)-based hybrid external cavity diode laser (HECL) in both multimode and single mode with +/- 0.5 nm or +/- 0.1 nm wavelength tolerance and very narrow linewidth. These sources are available in a variety of wavelengths, output configurations, and packaging configurations.

However, other laser types are well suited for Raman spectroscopy, which we will touch on below.

Multi-Mode vs Single-Mode Lasers for Raman Spectroscopy

Raman spectroscopy is one of the fastest growing and most diverse applications in all of laser spectroscopy.  As a result, it can be rather challenging at times to sift through the wide-ranging laser options all being marketed for Raman spectroscopy.  In this application note we will tackle one of the most common questions that arises when picking a laser for Raman spectroscopy; “Should I chose a single-spatial mode or multi-spatial mode laser for my application?”  On the surface, this seems like a simple question since Raman is a nonlinear optical effect and therefore the tighter the beam can be focused the higher the conversion efficiency.  Seemingly a single-mode laser would be preferable, but in practice there are other factors that can complicate the situation.

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The Influence of Laser Wavelength on Raman Spectroscopy

Just as in the more familiar case of Rayleigh scattering, the efficiency of the Raman effect is inversely proportional to the wavelength raised to the 4th power. Meaning that for every 16% the laser wavelength is reduced the number of photons inelastically scattered doubles, and a 50% reduction in wavelength causes a 16-fold increase in the signal strength. Therefore, it would seem evident that a shorter wavelength laser is always the best option, but when we factor in the other two considerations, things get complicated. The biggest challenge with Raman spectroscopy is its relatively week signal strength compared to the fluorescence, and most complex molecules exhibit auto-fluorescence when excited with ultraviolet and visible light. As a result, most organic and biological samples must be excited in the near-infrared to avoid the Raman signal being drowned out by the fluorescence background. The strange dichotomy created by these competing effects, where on the one hand decreasing the wavelength will produce greater signal intensity while simultaneously increasing the background noise (auto-fluorescence) is the single most significant contributor to the reason why there are so many different laser wavelengths utilized in Raman spectroscopy.

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Single Longitudinal Mode DPSS Lasers for Ultra-Low Frequency Raman

Raman spectroscopy allows for the investigation of vibrational modes of a molecule, by measuring the frequency shift of scattered laser light.  This works because the difference in energy between the incident wavelength and the scattered wavelength is equal to the amount of energy required to induce a particular vibrational mode.  As a result, higher frequency vibrations such as CH and NH stretches are shifted further from the laser wavelength than lower frequency modes like aromatic ring vibrations allowing you to correlate the spectral bands to the molecular bond structure of the molecule.  When attempting to measure ultra-low-frequency vibrations (< 150 cm^-1), such as lattice modes, it can become quite challenging to measure the spectrum of the scattered light because of the proximity to the laser.  In this blog post, we will examine how holographic filters can be used in conjunction with single frequency lasers to get around these difficulties.

Read the full article here.

Raman Spectroscopy: Why are Picosecond Pulses Superior to Femtosecond

Raman-based spectroscopy, on the other hand, is an entirely different nonlinear technique, relying on the frequency shift experienced by laser radiation incident on a molecule, related to its rotational and vibrational modes. Not being related to electronic transitions, the Raman shift is relative concerning the irradiating wavelength, and therefore, unless pursuing coherent excitation, laser source tunability is not required. Raman-based spectroscopy, being a nonlinear process, usually requires ultrashort pulse generation (e.g., practical TPA set-ups usually require < 300 fs pulse duration). On the other hand, since the Raman gain is generally higher than TPA cross-sections, cheaper and simpler picosecond lasers could be efficiently employed in incoherent Raman spectroscopy. Furthermore, in Raman spectroscopy, the spectral resolution is related to the laser source bandwidth. Therefore, the narrower bandwidths of picosecond lasers represent a remarkable advantage over their femtosecond counterparts. For pulsed laser sources, spectral bandwidths and pulse durations are related by a Fourier-transform relationship, descending from the time-energy Heisenberg uncertainty principle. More precisely, the minimum spectral bandwidth is inversely proportional to the pulse duration. The shorter the pulse duration, the larger the bandwidth for a given temporal pulse profile. For instance, for a pulse duration of ~1 ps at the wavelength of ~1 μm, the minimum FWHM spectral bandwidth is ~1 nm. For pulses ten times longer at the same wavelength (~10 ps), the minimum spectral bandwidth is 10 times narrower (~0.1 nm)

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How Does Concatenation Enhance Raman Spectroscopy?

However, this results in reduced sensitivity of low-cost silicon CCD detectors at higher wavenumbers, making it difficult (or impossible) to observe the “stretch” portion of the Raman spectra (i.e. 2000 – 4000 cm-1). This reduced sensitivity is particularly true for excitation wavelengths greater than 760 nm. In addition, longer wavelength excitation lasers result in a reduced Raman excitation cross-section, making it challenging to observe and perform quantitative analysis in the “fingerprint” portion of the Raman spectra (i.e. 0 – 2000 cm-1). Raman concatenation overcomes these difficulties and allows a user to visualize the entire Raman spectra from 0 to 4000 cm-1 by utilizing two lasers with a single-grating-spectrometer and single probe.

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How Will This Homogenized ‘Stub Laser’ Save you Money & Space?

It is well known that the output beam characteristic of a multimode laser diode is inherently non-uniform, due to both spatial and temporal variations of the mode profile [Figure 1], that result from thermal lensing and filamentation [1]. These non-uniformities (‘hot-spots’ and ‘dark-spots’) can lead to deleterious effects for many applications, including solid-state laser pumping [2], Raman spectroscopy of sensitive materials [3], laser speckle contrast imaging [4], and laser illumination [5].

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How Can We Help?

With over 25 years experience providing narrow linewidth, wavelength stabilized, excellent beam quality Raman spectroscopy lasers in key wavelengths to researchers and OEM integrators working in various markets and applications, and 1000s of units fielded, we have the experience to ensure you get the right product for the application. Working with RPMC ensures you are getting trusted advice from our knowledgeable and technical staff on a wide range of laser products.  RPMC and our manufacturers are willing and able to provide custom solutions for your unique application.

If you have any questions, or if you would like some assistance please Contact Us here. Furthermore, you can email us at info@rpmclasers.com to talk to a knowledgeable Product Manager.

Alternatively, use the filters on this page to assist in narrowing down the selection of Raman spectroscopy lasers for sale. Finally, head to our Knowledge Center with our Lasers 101 page and Blogs, Whitepapers, and FAQ pages for further, in-depth reading.

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