Raman spectroscopy is an analytical method that provides rich chemical information. It is particularly suitable for molecular fingerprinting of (bio)chemical systems by probing the sample’s vibrational states. It is also possible to gain information on the intermolecular interactions by controlling the environment of the sample through parameters such as its temperature. The simplicity of this method makes it possible to combine it with a microscopy technique allowing for sample imaging with chemical contrast.
Raman spectroscopy is based on inelastic scattering of light by an illuminated sample. The incident light puts the sample molecules in a virtual state and, upon decaying from this state, light is scattered. If the final state of the molecules is different from the initial one, the wavelength of the scattered light shifts. The shift could happen towards both longer or shorter wavelengths, which are referred to as Stokes shift or anti-Stokes shift. This difference in the wavelength is a direct measure of the energy difference between the states of the sample. When used in conjunction with complementary absorption and photoluminescence methods, Raman spectroscopy can provide the full picture of the sample's spectroscopic properties. It is also possible to monitor the strength of a specific Raman band to observe the dynamic process the sample experiences.
Inelastically scattered light from the sample is several orders of magnitude less intense than the elastically scattered light. Detecting such a weak signal is the major challenge doing Raman spectroscopy. To overcome this issue, the samples are typically laser illuminated. Laser illumination also allows precise determination of the resulting wavelength shift and its strength. An optical filtering scheme is needed before detection to eliminate the dominant contribution from elastically scattered light (without a shift in wavelength) and to select the wavelength of the scattered light to be detected. Between the two stages of Raman spectroscopy - sample illumination and scattered light detection - Zurich Instruments lock-in amplifiers become the bridge.
Three widely used approaches
Modulation transfer spectroscopy
Lock-in detection enables detection of a faint signal against a bright background. This requires the modulation of the incident light, which can be achieved through amplitude, frequency, or phase modulation with an optical modulator, as shown in the figure. This modulation is then transferred to the scattered light. The modulated signal is later extracted and separated from the background using a lock-in amplifier.
Advantage of this method
Ability to resolve weak Raman features buried in the noise or masked by the fluorescence background.
Non-linear pump-probe imaging
Using a pump-probe scheme, vibrational states can be coherently excited by applying pump and probe (e.g. Stokes if the shift is towards a longer wavelength) pulses with a resonant frequency difference. Synchronizing the pulse times with a well-defined phase relationship is key to triggering this multi-photon process. The stimulated Raman scattering (SRS) and coherent anti-stokes Raman scattering (CARS) methods fall into this category.
Upon modulating the laser beams, lock-in detection can provide stimulated Raman gain or stimulated Raman loss. As an alternative, a boxcar averager can capture all signal components from its short pulses, while rejecting all noise contributions not synchronized with the laser modulation.
Advantage of this method
Large signal-to-noise ratio that gives access to fast processes, such as catalytic reactions with up to video frame rate.
Scanning near-field optical microscopy (SNOM)
Bringing a metallic tip very close to the sample surface increases the Raman scattering efficiency thanks to near-field effects. In addition, the imaging resolution depends on the radius of the scanning tip and can be as low as 10 nm, i.e., significantly below the diffraction limit. This method is also referred to as tip-enhanced Raman spectroscopy (TERS). A phase-locked loop (PLL) is essential to keep the tip oscillation at resonance for maximum sensitivity.
Advantage of this method
High spatial resolution, hence it is used for nanoscale imaging with chemical contrast.
The Benefits of Choosing Zurich Instruments
- All measurement strategies listed above can be seamlessly implemented and tested in your experiment with the Zurich Instruments Lock-in Amplifiers.
- You can construct images directly from LabOne® using the LabOne DAQ tool while synchronously triggering the scanner with our lock-in amplifiers. Furthermore, integrated PLL/PID controllers provide the required control loops, e.g. for laser stabilization.
- With Zurich Instruments you can pursue both lock-in detection and boxcar averaging. In fact, both strategies can run simultaneously on the UHFLI Lock-in Amplifier and can thus be directly compared. For low repetition rates and experiments on a lower budget, the HF2LI (50 MHz) or the MFLI (500 kHz / 5 MHz) are attractive alternatives.
- You can achieve video-rate scanning speeds with the UHFLI, the only instrument on the market offering up to 5 MHz demodulation bandwidth for sub-µs pixel dwell times.
- Zurich Instruments' analog electronics offer multiple input stages to minimize the input noise and maximize the signal-to-noise ratio for periodic signals.
- The LabOne Plotter tool provides the time trace of the signal's amplitude to assist you in beam alignment.
- Fast digital data transfer through USB or GbE connections eliminates the need for a digitizer card to record your measurements. The data can be accessed and recorded in the LabOne user interface or through the application programming interfaces for Python, C, MATLAB®, LabVIEW™ and .NET.
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- Fimpel, P. et al. Boxcar detection for high-frequency modulation in stimulated Raman scattering microscopy. Appl. Phys. Lett. 112, 161101 (2018)
- Andreana, M. et al. Amplitude and polarization modulated hyperspectral Stimulated Raman Scattering Microscopy. Opt. Express 23, 28119–28131 (2015)
- Shi, L. et al. Electronic Resonant Stimulated Raman Scattering Micro-Spectroscopy. J. Phys. Chem. B 122, 9218–9224 (2018)