In the past few years, Raman spectroscopy has generated a great deal of interest as an analytical method suitable for a variety of markets such as:
- industrial screening
- life sciences and materials applications
- point of care analysis
- research and development
Recently, the size of the Raman systems has been further reduced and they come with more enhanced sensitivity. These systems have also become multifunctional, thanks to high sensitivity cameras, compact laser sources, and ultra-light compact spectrometers. This article shows how better performance is achieved with a 405 nm laser, as opposed to more conventionally used longer wavelengths lasers.
Not absent, just buried
One standard concern in Raman spectroscopy is that inherently weak Raman signals are often obscured by the effect of fluorescence. This effect is usually reduced by using lasers in the NIR (785-1064 nm) range. However, the intensity of the Raman scattering is inversely proportional to the fourth power of the laser wavelength, and as a result the extended NIR wavelengths will tend to decrease the intensity of the Raman scattering.
In addition, the sensitivity of the silicon sensors reduces in the wavelength region > 800 nm and therefore these wavelengths will need a costly InGaAs type sensor in addition to complicated configuration of the device.
Shorter wavelengths can also be used to prevent the effect of fluorescence, and as a result instruments employing 532 nm have become increasingly popular in the past few years. Conversely, at 532 nm laser excitation, several materials are still present for which the fluorescence caused by the absorption of laser light conceals the weaker Raman signal.
Figure 1. Typical material fluorescence spectrum.
For such types of materials, for example polyimide, using even shorter wavelengths might be a better solution. When the laser wavelength is shifted further to 405 nm, the Raman signal is reinforced (1/λ4 dependence) and it ultimately becomes less vulnerable to the effects of fluorescence.
Therefore, once the Raman signal is strengthened, the effects of fluorescence can be lowered and a silicon sensor can then be employed for the visible range. A 405 nm laser can also be used to achieve a composite analysis of photoluminescence and Raman peaks.
Figure 2. Raman spectrum of polyimide being easily resolved when using 405 nm excitation. The Raman signal is buried in fluorescence for 532 nm and 785 nm laser excitation.
Lasers for Raman spectroscopy
To date, 785 nm offers the best balance between Raman scattering efficiency in the majority of situations, eliminating fluorescence, laser light absorption, and thus the heating effects and limits to the detector sensitivity.
These features make it the most popular wavelength employed for Raman spectroscopy. However, based on the application and the material being studied, the advantage of adopting wavelengths that may yet become common in Raman spectroscopy should also be taken into account.
Broad linewidths (>1 nm) are found in today’s commercially available semiconductor diode lasers at 405 nm, but excellent wavelength stability, the narrow linewidth characteristics, and low noise are required to acquire Raman signals and to overcome individual Raman bands.
While the use of volume Bragg grating (VBG) technology may help achieve a semiconductor diode laser with narrow linewidth 405 nm, stable performance can only be achieved with careful alignment of the VBG element and sensitive temperature control.
The Cobolt 08-NLD 405 nm laser is developed based on n VBG stabilization of a 405 nm diode. The proprietary HTCure™ manufacturing technique of Cobolt integrates thermo mechanical stability with high-precision alignment, and can be used to achieve stable wavelength locking resulting in of <20 nm (<1.2 cm-1) linewidths at 30 mW output powers across a wide temperature range.
Figure 3. Typical spectrum of the Cobolt 08-NLD laser (FWHM <20 pm), output power 30 mW.
Figure 4. Wavelength stability of the Cobolt 08-NLD 405 nm over 50C (<15 pm pk-to pk).
An internal optical isolator can also be used to reduce the risk of reflected light caused by the sample returning via the system, and therefore risking damage to the filters and laser guarantee >75 dB side mode suppression for >2 nm wavelengths from the peak.
These key performance characteristics are handled by the Cobolt 08-NLD 405 nm laser in a compact footprint, while guaranteeing reliability through Cobolt’s proprietary HTCure™ manufacturing technology.
Figure 5. Cobolt 08-NLD laser.
By using a stable, narrow linewidth 405 nm laser for Raman spectroscopy, the influences of fluorescence inherent to particular samples can be reduced to obtain a stronger Raman signal, and also traditional silicon detectors that come standard in Raman systems can be used.
This not only simplifies the set-ups but also enables more cost-competitive systems. In addition, a better signal-to-noise ratio and a potentially higher sensitivity are expected to lead to shorter acquisition times.
Cobolt develops, manufactures and supplies diode-pumped solid-state lasers (DPSSLs) and Diode Laser Modules in the visible, invisible and near infrared spectral ranges. The company provides a broad range of market-adapted laser products built on a wavelength flexible, power-scalable and robust technology platform.
The lasers are particularly suitable for OEM integration, but do also comply with applicable standards and directives for use as stand-alone devices in laboratory environment. Cobolt is committed to supplying innovative laser products that meet or exceed the market’s expectations concerning quality, reliability and performance. The lasers are designed and manufactured to ensure a high level of reliability, and operation of the company using qualified and established processes assures the quality of the company’s products.
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