Lasers for Raman Spectroscopy

Author: the photonics expert Dr. Rüdiger Paschotta

Raman spectroscopy is a technique of spectroscopy, used primarily for chemical analysis and various types of material characterization. In the simplest and most common case, the sample under investigation is irradiated with an intense light beam (usually a continuous-wave laser beam), and the small part of the power which is scattered by spontaneous Raman scattering is detected. The optical spectrum of that scattered light is often quite characteristic of a certain material; it typically contains some peaks where the Stokes shift is related to the frequency of a molecular vibration. However, there is usually far more light scattered without a frequency shift (Rayleigh scattering), which does not carry the desired information and must be filtered out.

Requirements on the Laser

The following features of a laser are typically required in Raman spectroscopy:

Wavelength and Optical Linewidth

The choice of laser wavelength can be influenced by several considerations:

• Short wavelengths are very much favored by the Raman scattering efficiency, which typically scales with the fourth power of the optical frequency.

• However, the simplest and cheapest laser solutions are available for longer wavelengths, e.g. in the near infrared. It is often necessary convert the light to shorter wavelengths with methods of nonlinear frequency conversion, in particular with frequency doubling, frequency tripling or frequency quadrupling. For continuous-wave light, as is commonly used, special measures are required to obtain a reasonably high conversion efficiency. For example, one uses resonant frequency doubling, which typically involves an automatic feedback scheme for maintaining resonance.
• Some samples, such as biological materials, can be damaged by light at short wavelengths.
• Short wavelengths, especially in the ultraviolet spectral regions, can cause safety and handling issues.
• The laser light may induce disturbing fluorescence in samples, and the resulting fluorescence background, which can severely limit the detection sensitivity achieved, is normally stronger for shorter laser wavelengths, and is usually stronger for wavelengths in certain (material-specific) regions.
• Available spectrometers and additional spectral filters work best in certain spectral regions.
• Resonance Raman spectroscopy requires a laser wavelength close to an absorption transition of the investigated medium.

Since such details can vary greatly from one application to another, a wide range of laser wavelengths from the infrared to the visible to the ultraviolet are used for Raman spectroscopy.

The spectral resolution achieved is limited by the laser's linewidth of the laser, i.e., by the width of the optical spectrum. Therefore, one usually uses narrow-linewidth lasers, emitting quasi-monochromatic light. The maximum allowable linewidth depends very much on the application. In spectroscopy, it is often given in terms of a spectroscopic wavenumber with units of inverse centimeters (cm⁻¹). Multiplying that quantity by the vacuum velocity of light gives the linewidth in terms of optical frequency. For example, if the linewidth limit is 1 cm⁻¹, that corresponds to ≈30 GHz. The conversion of that to a wavelength bandwidth can be done by multiplying with <$\lambda^2 / c$> (using the center wavelength); for example, 1 cm⁻¹ at 1 μm wavelength corresponds to ≈100 pm = 0.1 nm. In some cases, one may require a linewidth much smaller than that.

Output Power

The required optical output power also depends very much on the specific situation, in particular on the investigated substance, its concentration (e.g., in a solution, if it is not a pure material) and Raman cross-sections, and the required sensitivity and measurement time. It can range between a few milliwatts (for measurements on biological samples, for example) and the order of 1 W in some other cases.

Beam Quality

In many cases, the laser's beam quality should be quite high (ideally diffraction-limited), allowing tight focussing of the light on a sample. That is particularly relevant in Raman microscopy, where one also desires to reach a high spatial resolution.

Polarization

The polarization of the laser light is relevant in certain applications, specifically those involving anisotropic materials.

Laser Noise and Stability

Various laser parameters, in particular the center wavelength and output power, should remain rather stable during operation, as otherwise the accuracy and reproducibility of the spectroscopic measurements could be degraded. This is especially relevant when working with weak Raman scatterers, which require longer measurement times.

The impact of laser noise can be compensated to some extent, e.g. by monitoring the power fluctuations and processing the detector signals accordingly.

Types of Lasers Used for Raman Spectroscopy

As explained above, the laser requirements can differ very much depending on the use case. Therefore, different types of lasers may be used:

• Argon ion lasers have been used frequently in the past because they can provide high output powers at short wavelengths, in the visible or even ultraviolet. However, they are highly inefficient, thus require a lot of electrical power and effective cooling, and have a limited tube lifetime. Therefore, they have been largely replaced with other lasers, in particular with solid-state laser solutions.
• Some other gas lasers, such as helium–neon lasers, operate a lower output power levels and longer wavelengths, but are substantially cheaper to purchase and operate, and have good properties in terms of beam quality, linewidth and laser noise.
• Laser diodes are available with a wide range of emission wavelength, and are very compact and reliable sources. Although high beam quality and a narrow linewidth is more easily achieved at low power levels, there are optimized diode lasers with quite high output power and still high beam quality and narrow linewidth.
• Diode-pumped solid-state lasers are frequently used as they can offer high output powers, good beam quality and narrow linewidth. Frequency-doubled neodymium lasers emitting at 532 nm are often used, also devices emitting in the UV based on frequency tripling or quadrupling.
• Fiber lasers can also produce substantial output powers, and with more flexibility in the output wavelength. There are also fiber sources based on a low-power seed laser (possibly a laser diode) and a fiber amplifier.
• Quantum cascade lasers can be used when tunable mid-infrared radiation is required.

More to Learn

Encyclopedia articles:

• Raman spectroscopy
• narrow-linewidth lasers