Ultrafast Transient coherent Raman Microscopy(TRaM)
Abstract: The aim of this project was to construct a measurement setup to be able to measure the decoherence time of optical phonons. Optical phonons lose coherence by decaying to intermediate phonons and eventually to acoustic phonons which can carry heat, which is why measuring the coherence time of optical phonons is important to characterizing heat flow on short time scales. Ultrashort laser pulses were produced with a Ti:Sapphire laser oscillator and a Michelson interferometer was constructed to split the pulses into pairs and introduce a time delay between the two pulses. To measure the decoherence time the first pulse excites optical phonons in the material and the second pulse probes how much coherence is left in the system by measuring the visibility of the spectral interference fringes on the Raman peak. By varying the delay on the interferometer, the coherence for different pulse delays can be measured and thereby also the amount of coherence left after this time. The measurements were done using a Raman microscope which focused the beam into a 10 μm small spot size on a diamond sample. The motivation for constructing this particular measurement setup is that a Ti:Sapphire laser oscillator with a sufficiently short pulse duration was already constructed close to a Raman microscope.
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