The morning, like every Thursday morning, began with a seminar by Delphine Debarre on "Image-based aberration correction in microscopy". The work was conducted at Oxford. The general idea was how to get rid of aberrations in different images. Sources of aberrations: optical system elements and specimen. From what I was able to gather, the way to correct for aberrations is to choose a proper "metric" for a given type of microscopy and use this to maximize the image. Basically:
image FFT = OTF * sample FFT
Where OTF is the optical transfer function and it depends on the aberration. Two forms of microscopy that this can be used for are structured-illumination microscopy and two-photon microscopy. I do not understand enough of this talk to write much more than this.
Before the seminar, Antoine and I talked more about the physics behind attaining this spatial profile. A few notable things that I have not talked about yet are the lifetime of the charge carriers and more about the Fabry-Perot effect. In terms of charge carriers, it is interesting to have an idea of their lifetime, as we are measuring both the THz beam with the carriers present and without the carriers present for each location that the laser diode spot is on the Si wafer. We want there to be a small relative excitation and de-excitation time relative to life time of the carriers, as we will be chopping the beam to have moments of essentially semi-metal and then semi-conductor. The lock-in will be used to analyze this signal. (More about this later).
The second notable effect is the Fabry-Perot effect, as I have mentioned in previous posts. One thing that I wanted to note about this effect is that the waveform which we see may not simply look like the initial pulse with a shifted phase and altered amplitude (i.e. it might not look simply as though it transmitted through the material) – the FP effect will cause this waveform to look like the sum of multiple waves. The majority of this is the part of the wave that is fully transmitted and then a number of internal reflections and then transmissions which all add together in our detected signal. It should be obvious that herein lays a problem, as we need to be able to tell how much of the wave is transmitted in the initial transmission, etc.
I also spent time trying to align the IR beam in the afternoon. I first aligned the height of the beam along a length of the optical bench and then put the laser in place to align it through the mirrors. I was able to align the beam decently well through all of the components, but I think maybe tweaking some of the mirrors would allow me to center it perfectly. I did not, however, change any of the mirrors – just the IR laser.
After aligning the beam, I was able to get into the lab with Dr. Gallot to look at the Si wafer that I will be using. We wanted to be sure that the THz beam did in fact transmit through the disc. For today, all we did was place the disc in the path of the THz beam and measure the waveform. We then took another reference signal. The two waveforms differed in amplitude and time. By measuring the thickness of the disc with calipers (0.524 mm), the distance between the two signals (0.63 mm), and the relative amplitudes of the two pulses, we were able to determine the index of refraction (3.405) and the absorption constant (0.021 cm^-1). We also measured the percent transmission, which was about 70%. Of course, these were all approximate measurements, but note that they are about what we expect for impure Si – ~ 3.4 index of refraction and about 70% transmission. Also note, this is not a pure wafer... either an n-type or p-type.
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