Biophotonics_Concepts_to_Applications

disperses light into a spectrum) and then are registered by a CCD detector array that
has a large number of pixels (>1000) in the spectral axis in order to acquire a
high-resolution Raman spectrum. Commonly used laser wavelengths in Raman
spectroscopy include 532 nm (a green diode-pumped Nd:YAG laser) and 785 nm
(an AlGaAs diode laser), which enable the achievement of a lateral resolution of
better than half the wavelength (i.e., 250–350 nm). This sub-cellular resolution is
similar to that attainable influorescence imaging.

Example 9.9What are some optical sources that can be used for Raman

spectroscopy?

Solution: Some common lasers and their selected emission wavelengths that

are applicable to Raman spectroscopy include the following:

• Ar (488 nm, 514.5 nm)


• Kr (530.9 nm, 647.1 nm)


• He–Ne (623 nm)


• Diode lasers (782 nm, 830 nm)


• Nd:YAG (1064 nm)

A general illustration of a microscope system used for Raman spectroscopy is
shown in Fig.9.15. The main microscope components are the following:

• An excitation light source, for example, a 532-nm or a 785-nm laser


• Various reflecting and light-collecting and focusing optical elements; lens #1
  can be a cylindrical lens to illuminate the sample with a slice of light or the lens
  can be removed so that only a spot of light falls on the sample


• An excitation opticalfilter that passes only the selected spectral band (≈40 nm
  wide) needed for absorption by thefluorophore being used


• A dichroic mirror that blocks light from the excitation wavelengths and passes to
  the detector only the spectral region in which thefluorescent emission occurs


• A spectrometer that spreads the wavelengths across the face of a detector, such
  as a photodiode array, CCD, or PMT

Laser Sample

Spectrometer CCD

Lens #1

Lens #2

Microscope

objective

Fig. 9.15 Diagram of a Dichroic mirror
microscope system used for
Raman spectroscopy

9.6 Raman Spectroscopy 277