Today (Wednesday) I spent the afternoon getting set up with my desk and new French laptop which has all the keys in different places. Yesterday does not count for work since we had a nice long picnic and then could not get our badges for lab access/cafeteria.
The rest of the day consisted of talking about THz generation and detection with Dr. Guilhem Gallot, who I will be working under for the next two months. The material we talked about helped out with my understanding how the basic laser set up of my lab works. The generation and detection works just how I mentioned in the powerpoint presentation last Friday. Generation happens with two electrodes placed on a semiconducting sheet held at a potential difference. A pulsed fs laser creates charge carriers in the semiconductor (transition from valence to conducting band). These charges accelerate from the two electrodes and we get THz radiation.
The radiation is not really "aimed" in a single direction and so a lens is used to focus the beam. Detection works in a very similar way. We have two metal electrodes set on a semiconductor (by the way, this is usually GaAs, and is typically prepared in low-temperature conditions) which look almost like an "H" with a gap in the horizontal bar connecting the two vertical bars (of the H). This is not kept at a potential difference. What we do is have a reference THz beam which is aimed directly at the incoming beam we want to measure. This whole setup is on a device which essentially steps to take a range of measurements. The measured wave and reference wave add together and we have interference. The "stepper platform" allows us to scan point-by-point what this new wave is and then the computer builds our signal for us.
Also note the detector measures a photocurrent which is amplified with a lock-in amplifier.
We also talked a little about THz spectroscopy and how we would get something such as an absorption spectrum by removing the reference spectrum from the measured. This can be used for solids, liquids, and gasses, and seems pretty straight-forward. Along with this introduction to THz spectroscopy, we mentioned the Fourier analysis which goes into the data aquisition.
Near-field was the last thing that we talked about. Basically, we use near-field to attain better spatial resolution of our sample. It works by blocking the laser with a sheet that has a pinhole aperture that allows only some of the light through. We then place the sample very close to this aperture and we use the evanescent light waves to probe the sample. The draw back to this technique is loss of a lot of the beam due to the small aperture.
Finally, we talked about what I should be doing for part of my time this summer. We mentioned using one of the three setups in the lab to try and look at cells (of what type, it was not mentioned). The idea is to deposit a layer of cells on a Si wafer and measure the absorption and transmission of THz radiation through said cells. I also am asked to look at a paper which describes a technique used for profiling a THz beam using optical transmission modulation in Si.
Right now it stands that I would like to get a lot of reading/reviewing done before getting in the lab. My main focus is on understanding techniques in the spatial profiling of the THz beam and on the Fourier analysis.
The radiation is not really "aimed" in a single direction and so a lens is used to focus the beam. Detection works in a very similar way. We have two metal electrodes set on a semiconductor (by the way, this is usually GaAs, and is typically prepared in low-temperature conditions) which look almost like an "H" with a gap in the horizontal bar connecting the two vertical bars (of the H). This is not kept at a potential difference. What we do is have a reference THz beam which is aimed directly at the incoming beam we want to measure. This whole setup is on a device which essentially steps to take a range of measurements. The measured wave and reference wave add together and we have interference. The "stepper platform" allows us to scan point-by-point what this new wave is and then the computer builds our signal for us.
Also note the detector measures a photocurrent which is amplified with a lock-in amplifier.
We also talked a little about THz spectroscopy and how we would get something such as an absorption spectrum by removing the reference spectrum from the measured. This can be used for solids, liquids, and gasses, and seems pretty straight-forward. Along with this introduction to THz spectroscopy, we mentioned the Fourier analysis which goes into the data aquisition.
Near-field was the last thing that we talked about. Basically, we use near-field to attain better spatial resolution of our sample. It works by blocking the laser with a sheet that has a pinhole aperture that allows only some of the light through. We then place the sample very close to this aperture and we use the evanescent light waves to probe the sample. The draw back to this technique is loss of a lot of the beam due to the small aperture.
Finally, we talked about what I should be doing for part of my time this summer. We mentioned using one of the three setups in the lab to try and look at cells (of what type, it was not mentioned). The idea is to deposit a layer of cells on a Si wafer and measure the absorption and transmission of THz radiation through said cells. I also am asked to look at a paper which describes a technique used for profiling a THz beam using optical transmission modulation in Si.
Right now it stands that I would like to get a lot of reading/reviewing done before getting in the lab. My main focus is on understanding techniques in the spatial profiling of the THz beam and on the Fourier analysis.
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