Figure 1: Raman spectroscopy apparatus.
In fact, each Raman device consists of a laser that excites the sample and a detector that collects the emitted signal. Additional optics are integrated into the system to focus the beam and optimize the settings to improve signal quality. A simple Raman spectroscopy setup can include an ND:YAG laser, mirrors on two kinematic mounts, two right-angle prisms, and an achromatic lens. As shown in Figure 2, the light emitted by the laser is aligned on two mirrors and folded by a right-angled prism 90°. The achromatic lens then focuses the light onto the sample. The light scattered out of the sample hits a second prism, which deflects it into a beam collector. The achromat then collects the scattered light and focuses it onto the detector for collection.
Laser power considerations
For many sample types, the use of near-infrared (NIR) excitation lasers is advantageous because many species will fluoresce at wavelengths close to ultraviolet light, and this fluorescence will mask any scattering measurements, making it nearly impossible to record any meaningful data. This does not mean that higher wavelengths represent better systems;One thing to consider when using NIR is higher noise and cost. In general, a good balance between performance, fluorescence, and cost is a 785nm NIR laser. One problem with using near-infrared lasers is that they tend to emit higher power. In order to perform Raman spectroscopy efficiently with such a laser, it is critical that the optics used in the system have a laser damage threshold high enough to be compatible with the specific laser source being used. However, it is important to understand that due to the statistical nature of laser damage testing, this threshold is not the power at which damage will never occur. Conversely, the laser damage threshold is defined as the limit where the probability of damage is less than the critical risk level. This depends on several factors such as beam diameter, number of test points per sample, and number of samples tested to determine the specifications.
Signal-to-noise ratio
In terms of performance, the high signal-to-noise ratio (SNR) facilitates the detection of inefficient Raman shifts. A good example of why high SNR is important is the analysis of fluorescent samples, which is notoriously difficult to study using Raman spectroscopy. As mentioned earlier, the signal generated by the fluorescent material becomes the main source of noise, overriding Raman scattering. Since simply increasing the power of the laser also increases the fluorescence signal, the fluorescence is usually mitigated by changing the wavelength at which the laser is excited.
Detector selection
One way to improve the extracted signal is to use a laser with a shorter wavelength and therefore a higher photon energy to ensure that the energy of the light is higher than the energy of the energy gap between the ground and excited states of the electron. The responsivity of the detector should also be considered. For example, when using a 532nm laser, the resulting scattered photons will be distributed in the visible range, therefore, detectors with high quantum efficiency in the visible spectrum, such as charge-coupled devices (CCDs), should be selected. However, indium gallium arsenide (InGaAAs) detectors are ideal when using near-infrared lasers, such as ND:YAG with a wavelength of 1064 nm, due to their high responsiveness in the near-infrared region. This is an easy way to increase the signal on the detector without compromising the overall design.
Thin back-thin CCDs are ideal for low-light detection in Raman spectroscopy because of their quantum efficiency of up to 90% at peak wavelengths. These detectors have high quantum efficiency in the visible and ultraviolet spectrum because the incident light directly interacts with the active region of the sensor. NIR-backed thin CCDs with improved quantum efficiency in the NIR and red wavelength regions can also be used to add longer wavelength SNRs.
Filters
In Raman spectroscopy experiments, a clean excitation signal is an important part of ensuring accurate measurements of scattering data. To ensure that only the desired signal is detected, high-performance band-pass and long-pass filters complement each other well. When incorporated into the system (Figure 3), the high transmission and narrow bandwidth of the bandpass filter eliminate noise and ensure that only the desired laser line reaches the sample. Then, after the laser interacts with the sample, a long-pass filter is introduced to allow a longer wavelength than the excitation wavelength to pass through, which is characteristic of Stokes Raman scattering.
Figure 2: A band-pass filter is used to filter the beam before it enters the system, and a long-pass filter is used to ensure that only Stokes Raman scattering passes through the detection.