Cicely Rathmell, MSc

Raman spectroscopy offers a number of benefits for testing and characterization, with the selection of system components and measurement parameters offering great flexibility. In this technical tip, we’ll focus on the choice of Raman laser excitation wavelength.

	Raman scattering requires a very high intensity light source or long spectral acquisition times in order to generate enough photons for detection. The source should also be monochromatic to interrogate the narrowest Raman features and with the greatest resolution. Since the amount of Raman scattering is proportional to the intensity of the light source, lasers are a natural choice, particularly as they can be focused very precisely into a small sample or through a window.

	Raman signal is proportional to 1/Î»4 of the excitation wavelength. While this makes it tempting to use a short wavelength laser, autofluorescence generated in the sample can easily degrade signal-to-noise and make Raman peaks difficult to resolve. Organic samples are particularly prone to autofluorescence, driving excitation to red or NIR wavelengths (660â830 nm). The effect is much less pronounced for inorganic materials, including carbon nanotubes and fullerenes, more commonly studied with 532 nm laser excitation.

					Figure 1: Modular Raman systems begin with the choice of excitation laser and offer flexibility in measuring different materials.

	Excitation at 1064 nm virtually eliminates autofluorescence, but also reduces the Raman signal significantly, further complicated by the reduced sensitivity of NIR spectrometers. UV excitation is also tempting, as the full Raman fingerprint region can be collected prior to the onset of autofluorescence at ~330 nm, but again technical limitations prevail-UV lasers and filters are more expensive and lower performance, while the high energy of UV light can damage samples quickly during measurement.

	Raman spectroscopy requires laser sources with spectral purity and wavelength stability. Our Raman lasers use an innovative stabilization technology that provides extremely narrow wavelengths that remain locked at the desired excitation wavelength regardless of ambient temperature changes. These high-power excitation sources combine integrated laser drivers and thermoelectric coolers in a compact footprint. Lasers are available for various excitation wavelengths and can be used as turnkey (with built-in laser drivers and thermoelectric cooling) or integrated (designed for use in OEM systems) packages.

	830 Douglas Avenue, Dunedin, FL 34698
	tel. (727) 733-2447, fax (727) 733-3962

Articles

Raman spectroscopy offers a number of benefits for testing and characterization, with the selection of system components and measurement parameters offering great flexibility.

Tech Tip: Choosing Raman Excitation Wavelengths

Even in today's digital world, some of our most valuable documents are still printed on paper. Banknotes, passports, and professional licenses have high value and are at risk for counterfeiting, tampering, and forgery. The security printing industry has responded to the challenge by developing a multitude of new anti-counterfeiting and protection techniques, many of which are based on incorporation of novel optical features enabled by spectroscopy.

	Inks and pigments with optically unique properties can provide quick visual verification of authenticity, and can be difficult to replicate. These range from iridescent or color-shifting inks containing tiny flakes of mica that cause a change in color with viewing angle to inks that are poorly read by the illumination sources in traditional scanners used for copying.

	Fluorescent inks are often used to overlay documents with special words or images that appear only when illuminated by UV light or light of another specific wavelength. Fluorescent pigments may also appear as fibers incorporated into the paper itself.

	To test this, we assembled an Ocean Optics spectrometer system to look at fluorescence from the European Union flag security feature on a 20-Euro note. When viewed with UV light, fluorescence from the security feature should cause the blue background on the flag to appear green, while the yellow stars should appear orange (Figure 1).


	Figure 1: Fluorescence from a security feature on a 20-Euro bill. 

	We configured a modular fluorescence system consisting of a reflection probe, a 385 nm LED for excitation and a high sensitivity spectrometer for detection. With the probe pointed at the flag, a broad fluorescence peak was observed from ~490–560 nm, corresponding to the expected green color for the background when viewed with UV light. Fluorescence from the stars was visible in the same spectrum as a strong peak from 610–620 nm.

	While a counterfeit bill might be able to replicate some Euro security features accurately enough to fool the eye at a glance, a full spectral measurement compared against an authentic Euro fluorescence spectrum could very quickly discern a fake.

	Optical security features and analysis techniques are an increasing part of safeguarding and evaluating document security. Compact spectroscopy systems are an important part of every step in the authentication process, from development to analysis in the field. Ocean Optics technologies can be adapted for integration into robust analysis systems and complemented by user-friendly pass/fail software to help professionals in this burgeoning field that protects our economic systems, our possessions and even our identities.

 presented at Optical Document Security-The Conference on Optical Security & Counterfeit Deterrence. Optical Document Security, 2014, last accessed on-line July 21, 2015.


	830 Douglas Avenue, Dunedin, FL 34698
	tel. +1(727) 733-2447, fax: +1(727) 733-3962
	Website: 

Inks and pigments with optically unique properties can provide quick visual verification of authenticity, and can be difficult to replicate.

Cicely Rathmell, MSc

Articles

Tech Tip: Choosing Raman Excitation Wavelengths

Spectroscopy’s New Role in Document Security