which the sample is exposed, enabling the measurement
of sensitive and combustible samples without the risk of
ignition. This combination of SERS and ROS sampling enables
the development of a new generation of sensitive handheld
units for the detection of explosives and other hazards in the
battlefield setting.
Additional advances in Raman have resulted from
the use of different lasers with unique properties and
advantages depending on their wavelength. The choice
of laser wavelength must be made carefully because the
intensity of Raman scattering decreases at the fourth power
of the laser wavelength. The tradeoff in Raman scattering
intensity and sample fluorescence must be considered when
selecting the best excitation wavelength for a particular Raman
measurement. Shorter laser wavelengths increase the intensity
of Raman scattering but usually result in a higher fluorescence
background from the sample. Alternatively, longer laser
wavelengths usually result in lower background fluorescence
but result in decreased Raman scattering intensity. Knowledge
of the characteristics of laser wavelengths used for Raman
measurements enables researchers to leverage the choice of
excitation wavelength to optimize their results for a given
sample.
NIR (near infrared) laser excitation using a 1064 nm laser
is often used for Raman measurements of compounds that
fluoresce. The suspicious white powders that raise security
alerts are notorious for high fluorescence at lower Raman
excitation wavelengths so they are often measured with a
1064 nm laser. While fluorescence is lower with 1064 nm
Raman, there are a few limitations to consider — including
the reduction in Raman-scattering intensity at the longer laser
wavelength, the need for a thermoelectric cooler (TEC) to
reduce noise in the indium gallium arsenide (InGaAs) detectors
used for NIR wavelengths, and additional eye safety concerns
to protect soldiers from the 1064 nm laser, which is not visible
to the naked eye. Work to mitigate the sensitivity, power, and
noise limitations is already under way with the development of
more sensitive detectors along with improvements in batteries
and the power management of field devices.
At the opposite end of the spectrum, ultraviolet (UV)
P r e c i s i o n E n g i n e e r i n g I s C o r e
Novotech is a global supplier of infrared Optics and Assemblies.
Infrared Germanium is another core to Novotech and weÕre stocked
in substantial inventories from ingots to optics to assemblies.
Other materials include: Silicon, ZnSe, and ZnS. Crystal growth,
fabrication and coating capabilities allow us to offer a
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materials solutions and quick deliveries.
ph: 978.929.9458
e: optics@novotech.net
www.novotech.net
17
Technology
Electronic Military & Defense Annual Resource, 4th Edition