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Confocal Raman Microscopy fundamentals attoMICROSCOPY
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 attoRAMAN
Investigation of high-T c 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 .
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 .
Laser line filter Blocking Pinhole
Laser
Out-of-Focus Light Rays
Excitation Light Rays
Sample
Spectrometer with CCD unit
Blocking Pinhole Out-of-Focus Light Rays
In-Focus Light Rays Notch filter
Dichroic beamsplitter
Objective
Focal Plane