Semrock filters, Optical filters

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1  Fluorescence Filters

Optical fluorescence occurs when a molecule absorbs light at wavelengths within its absorption band, and then nearly instantaneously emits light at longer wavelengths within its emission band. For analytical purposes, strongly fluorescing molecules known as fluorophores are attached to biological molecules and other targets of interest to enable identification, quantification, and even real-time observation of biological and chemical activity.  Fluorescence is widely used in biotechnology and analytical applications due to its extraordinary sensitivity, high specificity, and simplicity.

Most fluorescence instruments, including fluorescence microscopes, are based on optical filters.  A typical system has three basic filters: an excitation filter (or exciter), a dichroic beamsplitter (or dichromatic mirror), and an emission filter (or barrier filter). The exciter is typically a bandpass filter that passes only the wavelengths absorbed by the fluorophore, thus minimizing excitation of other sources of fluorescence and blocking excitation light in the fluorescence emission band. The dichroic is an edge filter used at an oblique angle of incidence (typically 45°) to efficiently reflect light in the excitation band and to transmit light in the emission band. The emitter is typically a bandpass filter that passes only the wavelengths emitted by the fluorophore and blocks all undesired light outside this band – especially the excitation light. By blocking unwanted excitation energy (including UV and IR) or sample and system autofluorescence, optical filters ensure the darkest background.

Most Semrock filter sets are high-brightness and high-contrast sets. These filter sets strike a balance between highest possible brightness while maintaining high contrast and are the best choice of filters under standard imaging conditions.  In situations where the signal level from a sample is low, filter sets with wider passbands enable maximum signal collection efficiency.  That is why studies such as the imaging of single molecules typically utilize a filter set with a wide passband or a long pass emission filter.  For these studies, it is important to maintain very low background autofluorescence signal by utilizing strict sample preparation protocol.

Filter sets with narrower passbands are preferred options when imaging a sample labeled with multiple fluorophores. Narrower passbands reduce crosstalk by allowing only the strongest portion of the fluorophore emission spectrum to be transmitted, reduce autofluorescence noise and thus improve the signal-to-noise ratio in high background autofluorescence samples. Such filter sets are ideal for samples with ample fluorescent signal level.



2  Raman Filters

Raman spectroscopy allows for the detection and identification of molecules through their unique vibrational and rotational energy level structure. As opposed to fluorescence methods, which require the addition of a separate fluorescing molecule as a “tag” attached to the actual molecule of interest, Raman spectroscopy allows direct detection of a molecule with no chemical alteration. Another important difference, however, is that the scattered Raman signal (as a percentage of the excitation power) is several orders-of-magnitude weaker than the corresponding fluorescence signal. Because of this, lasers are typically used as excitation sources to provide high power in a tightly focused spot, and very sensitive detectors are used to detect the very faint signal. Excellent filtering is therefore essential to block the very intense laser light while still allowing high transmission of the slightly wavelength-shifted Raman scattered signal.

Semrock stocks the widest selection of Raman-spectroscopy grade edge filters available, with edge wavelengths from 224 to 1064 nm. These filters are so steep and highly transmitting that they out-perform even the leading holographic notch filters, yet are less than half the price.  Now you can see the weakest signals closer to the laser line than you ever have before. With their deep laser-line blocking, ultra-wide and low-ripple passbands, proven hard-coating reliability, and high laser damage threshold, they offer performance that lasts.

For preventing laser light from reaching the detector and drowning out the relatively weak Raman signal, we offer a collection of both single-notch filters and multi-notch filters, which block one or more laser line while transmitting light on both sides.  For the most discriminating Raman measurements, eliminate laser spectral noise leakage by cleaning up your laser spectrum with a matched MaxLine™ laser clean-up filter. 

3  Laser-line Filters

The laser is unique among optical sources, having the unusual combination of narrow bandwidth, high intensity, extreme brightness, and optical coherence. Fifty years since its invention, it is used in a myriad of applications, from science & medicine to consumer products, entertainment, materials processing, and beyond. The optics industry, in general, has followed suit,  improving performance, reducing production costs and miniaturizing components, helping to bring complex optical systems out of the lab and into the mainstream. But even with all of these accomplishments, controlling and manipulating the spectral properties of light is still paramount. While there are many available technologies, the thin-film dielectric filter still has several advantages, including high transmission, deep blocking, steep spectral edges, and high power damage threshold. These features are why thin-film filters are still an integral component in laser & optical systems today.

Semrock filters are designed to perform many of the key spectral control functions common to most laser & optical systems. The MaxLine™ laser line filters and MaxDiode™ laser cleanup filters eliminate unwanted laser modes and spontaneous emission from the most common laser sources when spectral purity is critical. StopLine® notch filters eliminate only the laser light in applications where the laser light is undesirable, such as safety goggles for a laser surgeon. LaserMux™ laser combining filters allow multiple laser beams to be combined together and split apart with minimal distortion in multi-wavelength applications.

In Raman spectroscopy systems, which almost always use laser sources, RazorEdge® and EdgeBasic™ edge filters block the laser excitation light by a factor of over one million while still allowing the Raman scattered light at nearby wavelengths to pass with a high transmission, allowing these very faint signals to be detected with high signal-to-noise ratios. And our unsurpassed RazorEdge Dichroic filters let users direct a laser beam to a sample and see the scattered return while the laser light is eliminated, all with a single filter.

For those times where redirecting a beam of light is the main concern, the MaxMirror® has set the industry standard for high reflection for all polarizations, from the UV to the NIR all the way from 0° to 50° simlutaneously. These mirrors are found in almost all types optical systems, but are particularly useful in high-power laser systems and instruments requiring insensitivity to polarization. Finally, Semrock is continuing to extend our thin-film technology by introducing polarization products, such as Polarizing Bandpass filters. For systems requiring very high polarization discrimination, such as polarization imaging, these filters provide state-of-the-art performance from a more flexible technology than current alternatives.

 

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