Optical filters: Unlocking the potential of Raman spectroscopy
Optical filters bring value to Raman spectroscopy (Credit: Artur Wnorowski/Shutterstock.com)
As a non-destructive analytical technique, Raman spectroscopy is being increasingly used to provide information on biological components, activities, and molecular structures. First discovered and trialled as a technique in the 1920s, the Raman effect was named after scientist Chandrasekhara Venkata Raman, who observed the effect in organic liquids in 1928, and won the Nobel Prize in Physics in 1930 for his discovery.
Even after almost a century, new applications are still emerging. In medicine, scientists at Medical University of Vienna, under the European Moon project, combined Raman spectroscopy with optical coherence tomography (OCT) to develop an eye scanner that is capable of adding molecular information to the visualisation of the internal structure of the eye, with the aim of enabling the earlier detection of neurode-generative and eye diseases, or even diabetes .
Further afield, the technique has even been applied to art history, with a collaboration between researchers from the School of Art and Archaeology, Zhejiang University in Hangzhou, China; Crafts Museum of China Academy of Art; Zhejiang University City College; and the Institute of Yangliuqing New Year Paintings. The team used Raman spectroscopy combined with macro X-ray fluorescence (MA-XRF) imaging and hyperspectral imaging in the visible range to identify the dyes and pigments in two historical New Year paintings, dating back to the late 19th century. 
As Raman spectroscopy has developed over the years, so too have the optical components required for its success. Brian Manning, Application Scientist at Chroma Technologies explains: “10 years ago, everyone was still using plasma lines, helium-neon lines, and krypton-argon lines. Those were considered to be monochromatic. Obviously, individual plasma lasers will give you multiple laser lines, but you can select which you're actually going to be using from moment to moment.” More recently, however, these are being replaced by diode lasers, says Manning.
“Diode lasers come in a gamut of qualities. You get what you pay for, in that the very most expensive lasers are very, very spectrally clean, meaning if you purchase a very expensive diode pump solid-state (DPSS) laser at 488nm, chances are good that's exactly what that laser is producing. However, laser prices have gone down.”
Why optical filters benefit Raman spectroscopy
Optical filters are an essential cog in the Raman spectroscopy wheel. For instance, the Raman scattering process can generate a relatively weak signal compared to the excitation light. Therefore, It is crucial to selectively filter out the intense excitation light from the detected Raman signal. Also, Raman spectroscopy often requires high spectral resolution to distinguish closely spaced Raman bands or to resolve subtle spectral features. Optical filters play a critical role here in achieving this resolution by selectively filtering out unwanted wavelengths and optimising signal-to-noise ratio.
Manning says: “Excitation filters or ‘cleanup filters’ will fix the output of the laser. So, regardless of what your laser is emitting spectrally, with an excitation filter in front of it, we can know exactly what's coming out of that laser. The Raman specific cleanup/excitation filters tend to be fairly narrow, plus or minus 1.5nm, typically centred at what we think is the laser wavelength in order to generate sufficient, rounded profiles of whatever the samples they're studying, and minimise overlapping with the signal that they're trying to capture.”
Barrier filters and emission filters can selectively transmit or block specific wavelengths of light. They too play a critical role in enhancing the signal-to-noise ratio, reducing background noise, and ensuring accurate spectral measurements. Exploring each type of filter in more detail, Manning says: “We've been trying to teach people to use the phrase “barrier filter” which originated historically because the function of that filter on the emission or detection side is to block the excitation wavelengths it's going to be used with. So the emission filter we sell, its primary function is to block the excitation light that would be emerging from this excitation clean up filter. The secondary aspect of that is to park that filter spectrally as close as we can to the laser wavelength itself, in order to collect wave numbers as close to the laser as possible, and in order to generate sufficient data for what you're actually trying to measure.”
Dichroic mirrors are designed to efficiently split the incoming light into two beams based on their wavelengths. The reflected beam typically contains the laser excitation wavelength and is directed toward the sample, while the transmitted beam carries the Raman-shifted wavelengths towards the detector. Manning continues: “In microscopy systems in particular, there may be a dichroic mirror that functions as a guidance optic which will reflect the excitation to the sample. It will transmit whatever Raman signal emerges from the sample to the detector for collection. In the grand scheme of things, even in wide-field epifluorescence microscopes, this is strictly just the guidance optic because although it does a little bit of blocking in its reflection range, it may limit the signal to a small extent 3 to 5% on transmission to the detector. Our workhorses, in order, are the barrier, and then the excitation filter.”
Optical filters and their benefits for fluorescence
A big challenge in Raman spectroscopy is dealing with fluorescence background and interference, and optical filters can be used to minimise this by selectively blocking the fluorescence wavelengths. In addition, different Raman experiments have varying requirements for spectral resolution, spectral range, and fluorescence rejection. Optical filters can be chosen and customised according to the specific needs of the experiment to optimise the Raman measurement.
This is something upon which Chroma Technology is well placed to advise, thanks to its 30+ years of experience as an OEM supplier of precise optical filters. As well as its existing range of filters for Raman spectroscopy, the company has the ability to fill in bespoke gaps of any customer or requirement.
Christopher Conca, OEM Sales Engineer at Chroma Technology explains: “There are customers out there who are in labs doing this for themselves — for the first or the millionth time — who want something that's a little bit different.” Those people may be looking for a Raman signal that is a given X wave number away from their laser’s emission. Conca starts by searching through the filters that Chroma has in stock for a solution that meets that need, and if nothing is already available, he takes them through the process of getting something custom-designed. “That's fairly straightforward once all parties have come to an understanding of what it is they're looking for,” he says, “it's a simple matter of translating what that is to our engineering staff, who will pull things into our CAD programmes and do the actual physics, looking at materials and methods to find what will work for the customer.”
An opto-mechanical engineer himself, Conca believes that the calibre of expertise at Chroma has aided the company’s ability to help customers with bespoke options. He says: “When someone comes to us, whether or not they have a lab, or perhaps have never worked with the technology before, we have the ability to give them that. Or, an OEM might say ‘here are the drawings of everything we've had so far, can you help us?’” The engineering group looks to see what they can offer, and Conca makes sure that jives with the customer’s expressed need. “If not,” explains Conca, “I will go to our engineering group and say, ‘I need this filter to do this rather than that, tell me if I can do this.’ In the end, what you really want to be able to do — and what we are able to do — is give the customer what they need and at the best price.”
Find out more about optical filters and the value they bring to Raman spectroscopy by downloading this white paper, which explores the fundamentals of optical filters, their types, and their indispensable role in Raman spectroscopy applications.