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.

Popular Abstract

When measuring temperatures in every day life we use thermometers. For large objects a standard thermometer is sufficient to get an idea of how the temperature is distributed. If the temperature is not constant, but there exist a slow heat flow, the thermometer may also determine which direction the heat flows by measuring the temperature drop in one area and rise in an adjacent area. Temperature and heat flow measurements are important for many practical applications. For example when turning on the oven you don't want your nearby cabinets to be as hot as the inside of the oven, and when driving the car you don't want the seating area to be the same temperature as the engine.

When the objects of interest get smaller, the thermometer has to get smaller as well to be able to resolve the details of the temperature distribution. And when the heat flow fluctuates very fast, the standard thermometer doesn't have time to cool down to make measurements fast enough. For these situation more sophisticated measuring instruments have been constructed. One example of these is the thermocouple where a thin string measures the temperature. This has the advantage of being smaller and thus both being able to resolve finer detail, and also cooling faster to be able to characterize faster fluctuations.

When the objects of interest reach the atomic scale, all of these methods fall short. One such object is the transistor in a modern CPU. One such CPU may contain billions of transistors, and the surface of the whole CPU can be on the order of 100 mm^2. This means that modern transistors may be as small as hundreds of nanometers! Not only that, but the CPU might perform billions of actions which produce heat every second.

To be able to characterize how heat is generated and how it flows in these systems, even more sophisticated methods have to be constructed. In this project a measurement technique is constructed which uses ultrashort laser pulses with 70 fs (0.00000000000007 s) duration. These pulses are shot at the material in pairs, where the first pulse makes the material vibrate, and the second pulse sees how the material vibrates at a short time later.

In this manner phenomena on the order of ps (000000000000.1 s) can be measured, which can give us insight into the behaviour of the generation of heat and of heat flow on very small scales.

• author: Nilsson, Eric
• supervisor: Jörgen Larsson
• organization: Atomic Physics; Department of Physics
• course: PHYM01 20191
• year: 2019
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• language: English
• id: 8992995
• date added to LUP: 2019-09-05 14:42:39
• date last changed: 2019-09-05 14:42:39