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Sub-wavelength microscopy techniques in the TeraHertz frequency range

  • Soylu, Gizem
Publication Date
Jul 24, 2020
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Imaging in the Terahertz frequency range at subwavelength resolution has gained a great interest for certain studies which cannot be carried out with other parts of the electromagnetic spectrum. However, classical optical schemes cannot be employed to obtain micrometre-range resolution for THz microscopy as diffraction limits the resolution to about 100 µm. In this thesis, we present two different original subwavelength THz microscopy techniques. In the first technique, the THz beam is screened by a thin metallic sheet in which a subwavelength hole has been made. The sample is placed against the sheet and moved over the hole to perform a raster image. The expected resolution is then equal to the hole size. The second technique presented in this thesis is based on generated a THz signal directly from the sample. When a laser beam is focused in the sample, the illuminated region, if non-centrosymmetric, can generate THz signals through optical rectification. The raster image is obtained by recording this THz signal while the laser beam is moved over the sample. The expected resolution is then close to the laser spot size.Both technique might involve weak THz signals. That is why we investigated on the possibility to measure them with a very sensitive detector, usually used for astronomy, named kinetic inductance detector (KID). This manuscript presents its principle as well as the study that was carried on. On a “classical” time domain spectroscopy setup, signal as low as 0.2 fW were thus recorded, demonstrating the interest of such detectors.The two last chapters are dedicated to the two microscopy techniques themselves. For the first one, a simulation model using a finite element model solver is used to design the most efficient aperture to enhance the transmission through a subwavelength hole. The results show that a conically tapered hole has a higher transmission than a classical cylindrical hole. Our attempts at using the KIDs camera for the first time for THz microscopy are discussed and first encouraging results are presented.Finally, the ORTI (Optical Rectification Terahertz Imaging) technique is investigated. An image with a 10 µm spatial resolution (λ/214 for 0.14 THz) was obtained while scanning the ferroelectric domains of a crystal of PPKTP. We show that the resolution of the image depends only on the laser spot size and not on the generated THz frequency. In addition, we showed that ORTI image can be used to scan a poly-crystalline sample as well as a crystal with different thickness areas. Lastly, the limitations of the spatial resolution of ORTI images are discussed in detail.

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