Raman spectroscopy is a non-destructive analytical technique that probes molecular vibrations in a sample by leveraging the inelastic scattering of monochromatic light.
Raman scattering provides unique spectral information, enabling chemical compound identification, chemical reaction monitoring, and molecular structure analysis. The technique is used in various settings, including pharmaceuticals, materials science, geology, forensics, environmental science, and biology.
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Raman spectroscopy offers numerous benefits. It allows researchers to identify the compounds and molecules present in a sample, including pharmaceuticals, polymers, minerals, biomolecules, and organic and inorganic substances.
Its diverse capabilities make Raman spectroscopy a valuable tool in material science and analytical chemistry. Other benefits include its non-destructive nature and requirement for little to no sample preparation, making it ideal for investigating valuable or delicate samples.
In most cases, Raman spectroscopy can be conducted with little or no sample contact, preserving the sample’s integrity and minimizing the risk of cross-contamination. It also provides results in real-time or near-real-time, ideally suited to rapid analysis in many applications, including pharmaceutical quality control and process monitoring.
Raman spectroscopy can be applied to various samples, including solids, liquids, and gases. It can also be used to analyze small regions within a sample.
This is highly beneficial when identifying heterogeneous distributions, mapping molecular structures, and investigating surface properties.
Combining Raman spectroscopy with other analytical techniques is also possible to provide a more comprehensive understanding of a sample’s composition and structure. Raman spectroscopy can be used with infrared spectroscopy, mass spectrometry, or microscopy.
However, Raman signals can be weak or obscured by background interferences like fluorescence.
Optical Filters in Raman Spectroscopy
Optical filters allow specific wavelengths of light to be selectively transmitted or blocked depending on their spectral characteristics. These filters can be categorized into two types: edge filters and bandpass filters. Depending on the system in place, other optics may be necessary to utilize proper Raman measurements.
All Raman spectroscopic instruments employ edge and bandpass filters, but each serves a different purpose. Filters must have steep edges to function effectively as Raman filters, which is required to detect Raman signals with wavenumbers of 200cm-1 or lower.
Edge Filters
Edge filters, or longpass filters, are designed to transmit light with wavelengths longer than a defined cut-on wavelength.
Longpass edge filters are employed in Raman spectroscopy to block Rayleigh scattered light with the same wavelength as the incident laser light. Rejecting the intense Rayleigh scattering makes it possible to isolate and detect much weaker Raman-scattered photons.
These filters are generally used at normal incidence (0 °) to the incoming light path.
Bandpass Filters
Bandpass filters allow a specific and narrow range of wavelengths to pass through them, rejecting any wavelengths outside that band. These filters are also used at normal (0 °) incidences.
Various contemporary diode laser sources emit multiple wavelengths, generating less-than-monochromatic light. Bandpass filters are required in virtually any of these settings to act as laser “clean-up” filters, limiting these lasers’ spectral output.
Without appropriate “clean-up,” the lasers’ output can supersede or hide the measured Raman signal, creating an impure signal.
Dichroic Mirrors
Dichroics are generally installed at 45 ° in a microscope-based Raman system to redirect and reflect the laser output toward the investigated sample. They also ensure the signal generated is transmitted back to a detector.
These optics may contribute to the rejection of Rayleigh scattering in their reflection ranges, but far less efficiently than edge filters—they should not be relied upon for signal detection without an edge filter.
It is also important to optimize the transitional slope of dichroic mirrors between the laser’s maximum reflection and the transmission of the expected Raman signal to ensure a robust collection of anticipated wavenumbers.
Notch Filters
Notch filters are highly specialized optics with a narrow rejection range while exhibiting maximal transmission efficiencies for shorter and longer wavelengths.
These filters are used at normal incidence, enabling Raman signal detection in both Stokes and anti-Stokes directions while attenuating the incident laser excitation. They are widely used in Coherent anti-Stokes Raman imaging.
Factors Affecting Filter Selection
Several essential factors should be considered when selecting optical filters for Raman spectroscopy.
Laser Excitation Wavelength
Choosing the most appropriate laser excitation wavelength is central to Raman spectroscopy. Any optical filters utilized must be compatible with the laser’s excitation wavelength to block the Rayleigh scattering and transmit the Raman-shifted wavelengths effectively.
The choice of laser wavelength can result in a fluorescence signal on top of a Raman signal in some samples, confusing users. This fluorescence output can typically be reduced or eliminated by selecting a different wavelength or dual wavelengths or using special techniques.
The Filter’s Optical Properties
Optical filters must offer high transmission efficiency for the targeted Raman spectral region. These filters must also provide a steep cut-on to reject the lasers and transmit low-wavenumber Raman output.
Signal-to-Noise Ratio (SNR)
A filter should ideally improve the SNR by limiting background noise. The high-value SNR necessary in Raman filter sets can be achieved via deep blocking characteristics and high transmissions.
Sourcing Optical Filters for Raman Spectroscopy
When selecting an optical filter supplier, ensuring they can effectively meet the filter requirements of the Raman spectroscopy application or project is important.
Capability
A good optical filter supplier should offer the entire range of required optical filters, whether these feature higher transmission values, steep transitions, or advanced blocking to keep out unwanted photons. The filters must also be capable of collecting the proper wavenumbers to identify all expected samples efficiently.
Experience and Expertise
An optical filter supplier should demonstrate a proven track record of expertise in optical filter design and manufacturing.
An experienced supplier should have a comprehensive understanding of Raman spectroscopy applications’ unique challenges and requirements and, ideally, be able to offer tailored solutions to meet an application’s specific needs. It is also vital that a potential supplier offer first-rate customer support, technical expertise, and value for money.
Customization
Raman spectroscopy setups can be notably different based on factors such as laser excitation wavelength, experimental requirements, and sample characteristics. A good optical filter supplier should offer customization options to adapt its range of filters to a specific application, ensuring optimal performance in every case.
Reputation and Customer Reviews
When selecting an optical filter supplier, research the supplier’s reputation, examine existing customer reviews and testimonials, and assess existing customers’ overall satisfaction with the company’s filters and services.
About Chroma Technology
Chroma Technology is at the forefront of the optical filter industry. The company manufactures Raman filter sets designed for OEM and research applications. These filter sets are comprised of highly durable and extremely precise optical filters using first surface hard coat sputter technology.
The company has been supplying optical solutions for over 30 years, working with industries ranging from the life sciences to agriculture and manufacturing, including security, inspection, and aerospace.
Chroma Technology’s reputation is built on a long history of dedicated customer service, including free applications and technical support.
Chroma Technology’s Optical Filter Solutions for Raman Spectroscopy
Chroma Technology offers a comprehensive portfolio of Raman filters and sets designed for the most used Raman wavelengths, including UV (266 nm), visible (488 nm, 532 nm, and 633 nm), and NIR (633 nm, 785 nm, and 1064 nm).
Standard Raman Filters and Sets
Chroma Technology’s range of standard filters offers a robust combination of high transmission suitable for detecting weak signals, broad transmission bands, outstanding image quality, deep blocking, and ultra-steep edges.
Thanks to its expertise and experience in precision deposition technology and thin-film design, the company's optical filters are some of the most advanced available.
Chroma Technology’s standard Raman filter sets feature a narrow-band laser clean-up filter coupled with a laser dichroic paired with a longpass emission filter. Each filter possesses sheer spectral slopes. Together, these filters enable the detection of even the faintest Raman signals.
Cost-Effective Raman Sets
Chroma Technology also offers a selection of cost-effective versions of its popular 785 nm and 532 nm Raman sets, designed to deliver exceptional performance at an affordable price.
Its range of cost-effective Raman sets boasts identical or similar designs to its standard catalog sets, ensuring no compromise in quality and accuracy. These sets are designed to offer slightly relaxed specifications, ideally suited to applications that do not require overengineering, especially in the flatness of dichroic beamsplitters and the steepness of longpass filters.
Several OEM customers in China have successfully used these filters in portable and handheld Raman devices employed in various settings, such as security checks, food safety, explosives and drug detection, and laboratory applications.
Materials science researchers also use these ultra-steep filters in Raman microscopes.
Customization Capability
Chroma Technology’s expert team supports customers looking for a filter solution outside of its standard or cost-effective ranges or when the customer is unsure of the best solution for their application.
The company’s team is available to support customers through the customization process. Simply provide a drawing or details on the desired application outcome. Once Chroma Technology’s engineering team has the required specifications, it can design a custom set for the application.
The team can also troubleshoot a filter set or application that is not functioning correctly, using its breadth of experience to troubleshoot a problem without overengineering a solution.
Acknowledgments
Produced from materials originally authored by Chroma Technology Corp.
This information has been sourced, reviewed, and adapted from materials provided by Chroma Technology Corp.
For more information on this source, please visit Chroma Technology Corp.