Confocal Raman Microscopy
fundamentals
Historically, the Indian scientist C. V. Raman
discovered in 1928 the - what was later called
Raman - effect by demonstrating inelastic
scattering of monochromatic light by highly
viscous fluids. Only two years later, Raman
was awarded the Nobel prize for his achievement. The Raman effect occurs if light impinges on a molecule and interacts with its
electronic bonds. The spontaneous Raman
effect is described as a three-level event,
in which an incident photon first excites
the molecule from ground state to a virtual
state. When the molecule relaxes, it emits a
photon and returns to a different rotational
or vibrational state. The difference in energy
between the original and the excited state
leads to a shift in the frequency of the emitted photon.
Raman microscopy and
its applications
Recording Raman spectra at single or multiple locations is a technqiue widely used
in condensed matter physics and chemistry
to study vibrational, rotational, or other
low-frequency modes of a system. Raman
spectra provide a fingerprint by which molecules and materials can be distinguished
from each other. Typically, the fingerprint
region of organic molecules is in the range
of 500 to 2000 cm-1, where one wavenumber
(cm-1) corresponds to an energy shift of
approximately 0.12 meV at 532 nm excitation
wavelength.
Other applications of Raman spectroscopy
include the determination of crystallographic orientation, gas analysis, material characterization, and observation of low energy
excitations such as phonons, magnons, and
superconducting gap excitations.
Confocal Raman microscopy
setup
In a modern confocal Raman microscope, a
laser source illuminates the sample as depicted in the figure below. Standard sources
include frequency doubled or tripled Nd:YAG
(532 nm, 355 nm) and HeNe (632.8 nm) lasers. The excitation is passed through a laser
line filter to block all undesired wavelengths,
and then focused on the surface using an
objective. To reject elastically scattered
light from the sample (Rayleigh scattering),
the backscattered light is spectrally filtered
through a dichroic mirror and a notch filter.
The remaining signal is then spatially filtered
through a blocking pinhole, and then propagates into a spectrometer, where the Raman
spectrum is recorded and analyzed with a
CCD camera.
attoRAMAN
Investigation of high-Tc superconductors
(pnictides, cuprates) and other new materials
such as graphene have led to a large demand
for Raman microscopy also at low temperatures
and in high magnetic fields. The attoRAMAN
exactly addresses these needs and allows
the user to record Raman images and Raman
spectra over a broad range of temperatures
(1.8 to 300 K), and at magnetic fields of up to
15 T. In materials with strong electron-phonon
coupling, such as graphene, the attoRAMAN is
a very efficient tool to study both mechanical
and electronic properties of a sample. A sophisticated software allows to analyze, sort, average, and postprocess spectra, enabling the user
to investigate finest details and fingerprints in
the Raman signature.
Spectrometer with CCD unit
Laser line filter
Blocking Pinhole
Laser
Out-of-Focus Light Rays
Blocking Pinhole
Out-of-Focus Light Rays
In-Focus Light Rays
Notch filter
Dichroic beamsplitter
Excitation Light Rays
Objective
Sample
Focal Plane
attoMICROSCOPY
Sophisticated Tools for Science
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