How to Choose a Raman Excitation Wavelength

Choosing a Raman Excitation Wavelength

How do you choose from the many Raman excitation wavelengths available?

Raman spectrometers can use a range of excitation wavelengths. Standard options from Austertech include 266, 532, 633, 785, 830, and 1064 nm. The right choice depends on the sample, fluorescence background, desired Raman intensity, and measurement needs.

Raman excitation wavelength overview

Ultraviolet — 266 nm
Advantages: High excitation efficiency and strong Raman scattering; can improve spatial resolution and reduce fluorescence interference. Typical uses include strongly fluorescent samples, petrochemical materials, and resonance Raman studies of biological samples such as DNA, RNA, and proteins.
Limitations: UV light can damage samples; the laser is more expensive and requires demanding optical filters and components.

Visible — 514, 532, or 633 nm
Advantages: Broad applicability across materials science, chemistry, inorganic chemical reactions, biomedicine, graphene and carbon-material resonance studies, and surface-enhanced Raman spectroscopy.
Limitations: Fluorescence signals can be strong and may obscure the Raman spectrum.

Near-infrared — 785, 830, or 1064 nm
Advantages: Usually produces less fluorescence interference; useful for fluorescence-prone samples, chemical products, biological tissue, and organic materials.
Limitations: Lower excitation efficiency generally produces weaker Raman signals and lower sensitivity than visible excitation.

Table 1. General wavelength-selection considerations

Why choose near-infrared excitation?

Near-infrared excitation is generally above 700 nm; common choices are 785, 830, and 1064 nm. It is often selected to suppress fluorescence. Fluorescence occurs after a sample absorbs light and then emits light, whereas Raman scattering is an inelastic scattering process and does not require the same absorption-and-emission sequence. Many samples have fluorescence absorption bands in the visible region, while relatively few absorb strongly in the near-infrared. As a result, near-infrared excitation can reduce fluorescence while allowing a Raman spectrum to be measured. It is often a practical solution when visible excitation produces a strong fluorescent background.

The trade-off is signal strength. Raman intensity decreases approximately with the fourth power of excitation wavelength under otherwise comparable conditions. In the comparison described here, 785 nm excitation gives roughly one-fifth of the Raman intensity of 532 nm, while 1064 nm gives roughly one-fifteenth. Detector response also tends to be lower in the near-infrared for many CCD systems. Therefore, collecting a spectrum with comparable quality may take longer than with visible excitation. Actual results also depend on the sample, optics, detector, laser power, and instrument design.

Why choose ultraviolet excitation?

UV excitation is generally below 350 nm, with 266 nm commonly used. Like near-infrared excitation, it can help avoid fluorescence overlap because fluorescence is often concentrated at longer, visible wavelengths. The fluorescence signal itself may be much stronger than the Raman signal, but wavelength separation can keep it from overwhelming the Raman region. Some biological molecules—including proteins, DNA, and RNA—can show resonance enhancement under UV excitation, increasing Raman signals and helping characterize molecular structure. UV light can also penetrate only a few nanometers into some semiconductor materials, which can support selective analysis of surface films. Its higher excitation efficiency can produce strong Raman scattering at relatively low laser power.

UV excitation has important limitations. Its thermal effects may burn or degrade a sensitive sample. The beam is invisible, the laser system is larger and more expensive, and operation is more complex. UV Raman instruments should therefore be operated by trained personnel with appropriate laser-safety controls and sample-damage checks.

How to select a wavelength

First consider whether fluorescence is likely to obscure the spectrum. Near-infrared excitation is often a good starting point for strongly fluorescent samples. Next consider whether the target material benefits from resonance enhancement, whether the sample tolerates laser heating, the required spectral range and resolution, measurement time, and the detector response. Test representative samples when possible. Raman lasers and spectrometers are generally designed as matched systems; changing the laser alone may not be compatible with an existing spectrometer. Fluorescence suppression and resonance enhancement are both important selection factors.

Austertech Raman instrument examples represented by Fulcrum Technology

Portable research Raman spectrometers:
Model | Excitation wavelength (nm) | Maximum laser power (mW) | Spectral range (cm⁻¹) | Resolution (cm⁻¹)
ATR3110-1064 | 1064 | 500 | 200–2600 | 12
ATR3110-830 | 830 | 550 | 200–3300 | 7
ATR31100-27 | 785 | 550 | 250–2700 | 4
ATR31100-35 | 785 | 550 | 200–3500 | 5
ATR31100-43 | 785 | 550 | 200–4300 | 6
ATR31100-633 | 633 | 80 | 200–3200 | 10
ATR31100-532 | 532 | 100 | 200–3700 | 7
ATR31100-266 | 266 | 20 | 200–5000 | 20

Entry-level handheld Raman spectrometers:
ATR6600 | 1064 nm | 500 mW maximum | 200–2500 cm⁻¹ | 12 cm⁻¹ resolution
ATR6500 | 785 nm | 350 mW maximum | 200–4000 cm⁻¹ | 8 cm⁻¹ resolution

Table 2. Example instrument specifications. Confirm the current configuration and specifications with the supplier before selecting a model.

Choose the excitation wavelength according to the properties of the samples you need to measure. If you are unsure, compare fluorescence background, signal strength, sample heating, and required spectral range using representative samples, then match the wavelength to a compatible instrument and detector.