Technology Advances Benefit Diverse Applications, Including the THz Domain
- Scientist at a Microscope
- Fig. 1: THz-Raman spectra of Carbamazepine show the additional Structural Fingerprint, which provides a clearer differentiation of the polymorphic forms. (Excitation wavelength 785 nm.)
- Fig. 2: Coherent provides a series of integrated modules and probes to enable the performance of any Raman spectrometer or microscope to be extended to the THz frequency range.
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The use of Raman spectroscopy as an essential tool in chemical analysis has grown dramatically over the past few decades, primarily due to technology advances in the lasers, optical filters, and spectrometer components that comprise these instruments. We survey a few of these key developments and their impact on applications, including portable/field use and real-time measurement of crystallinity and molecular structure using THz-Raman.
The Raman Advantage
The Raman effect occurs when light excites a sample and a small portion of the scattered photons are “Raman shifted” in frequency due to the inelastic scattering of those photons by the vibrational resonances of the chemical bonds in the sample. This results in a unique spectrum, called the “chemical fingerprint,” which can be correlated to the chemical composition of the sample and is similar to that from infrared absorption spectroscopy (FTIR or NIR). One major advantage of Raman is that it can be performed at an arbitrary excitation wavelength – so it can be implemented using wavelengths that efficiently transmit through water and glass, making it well-suited to field work, process measurements, and for pharmaceutical and life science applications. Conversely, the scattering effect is very weak (only 1 in 10-7 of incident photons are Raman shifted), and shifts are measured relative to the excitation wavelength. – putting unique demands on both the laser source and filters.
Lasers for Raman
Raman laser sources must first be frequency stabilized (generally to ~±0.1 cm-1 or ~7 pm @ 785 nm) to ensure repeatable measurement of shifts. Furthermore, they must be either single frequency or spectrally narrowed (i.e. linewidth <0.15 nm @ 785 nm, or ~2.4 cm-1) to ensure their bandwidth is much less than the spectrometer resolution and Rayleigh blocking filter cutoff.
The optimal wavelength choice depends on the application; as a rule, shorter wavelengths result in stronger Raman signals (signal strength varies with 1/l4), but may induce sample fluorescence, particularly in organic samples, drowning out important spectral features.
Longer wavelengths can reduce fluorescence, but the signal strength is much lower. The most popular wavelength range is 780-810 nm, which eliminates most fluorescence while enabling most Stokes-shifted signals to be efficiently detected within the range of silicon-based CCD and CMOS arrays (<~1000 nm).
- Single frequency CW visible lasers. Two of the most commonly used visible wavelengths are 488 nm and 532 nm, because of legacy gas and DPSS laser technologies. The Coherent Sapphire SF Series offers much more compact and efficient optically-pumped semiconductor laser (OPSL) technology at these wavelengths, utilizing an internal etalon to ensure single-longitudinal mode output.
- Stabilized NIR (and visible) laser diodes. Laser diodes are small, efficient and economical, but must be frequency stabilized for Raman. First-generation products relied on bulky external-cavity architectures, but next-generation diode lasers such as the SureLock from Coherent utilize a single miniature Volume Holographic Grating (VHG) filter to simultaneously “lock” the diode wavelength and narrow the spectrum, making them far more compact and cost-effective. These are available across a wide range of wavelengths from 405 nm to 1064 nm, and have enabled an entirely new generation of compact, affordable handheld, tabletop and micro-Raman spectrometers.
- CW ultraviolet lasers. If the Raman excitation wavelength is near to an electronic absorption (e.g. deep ultraviolet), Raman signals are enhanced by orders of magnitude. This Resonance Raman is used to sort the spectra of large biomolecules. UV excitation also results in eliminating most fluorescence and dramatically increases the Raman signal. The laser of choice is usually a frequency-doubled ion laser, such as the Coherent Innova FreD series with output at 244 nm or 257 nm.
VHG Filters – Revealing the THz-Raman Domain
Rayleigh blocking filters must deliver both exceptionally high optical density (>OD 6) while having narrow cutoff profiles in order to sufficiently block the excitation signal without also blocking important spectral information. In the 1990’s, the advent of holographic notch filters based on photosensitive gels greatly improved the performance of Raman spectrometers by combining very high blocking efficiency at the laser wavelength, a very narrow bandwidth, and high transmission at other wavelengths. Together with advancements in CCD and then CMOS cameras, this enabled compact, integrated handheld units, in-situ process probes, and microscope configurations.
In the past decade, engineers at Ondax (now Coherent) pioneered the development of a patented new generation of ultra-narrowband VHG notch filters using glass, rather than gels, as the substrate. Compared to earlier gel filters, these provide higher extinction ratios, greater environmental stability, and much sharper cut-on/cut-off characteristics – up to 10 times narrower. (Of note, the corresponding laser linewidth requirements for these filters are ~10 times narrower as well, or <0.015nm). This technology extends the signal capture range of traditional Raman systems down into the low frequency (low wavenumber) or THz spectral domain and beyond into the anti-Stokes region, where important structural details – including lattice or polymer structures, crystal orientation, rotational and phonon modes – can be clearly discerned.
Turnkey THz-Raman Modules
In addition to the “chemical fingerprint” associated with traditional Raman and FTIR systems, THz-Raman systems capture the “structural fingerprint” from the THz frequency domain, which has traditionally been difficult to directly access – see figure 1. This frequency range is associated with molecular / inter-molecular/ vibrations, including phonon modes, lattice modes, and/ rotational modes, and can identify/quantify ratios of polymorphs or provide real-time measure of the degree of crystallinity. The intensity of these signals are often 5-10 times stronger than the chemical fingerprint vibrational modes, significantly boosting signal strength.
To support the use of THz-Raman, Coherent offers key components such as VHG blocking and notch filters as well as compact SureLock lasers. Coherent also produces THz-Raman modules which provide a plug-and-play upgrade option to add THz capability to existing Raman spectrometers and microscopes (see figure 2), and fully integrated THz-Raman systems. These modules and systems include VHG filters, lasers and other components in a compact pre-aligned enclosure. They feature maximum optical throughput and exceptional attenuation (>OD 9) of the excitation source. Systems are available at most popular Raman excitation wavelengths, including 532 nm, 633 nm, 785 nm, 808 nm, 976 nm and 1064 nm.
Raman spectroscopy and microscopy deliver a wealth of compositional, structural and phase information about gas, liquid, solid and mixed materials including pharmaceuticals, crystals and biological samples—in real time, and without the need for sample preparation. Advances in key photonic components, modules and systems are enabling an ever widening of range of applications to realize the full potential of this very powerful analytic technique.
Randy Heyler, Torsten Rauch and Marco Arrigoni, Coherent Inc.