Terahertz Emission Microscopy

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In a terahertz time-domain spectroscopy, the spatial resolution is limited by the wavelength of the THz radiation, which is about 300μm at 1THz. To overcome this limitation, researchers at Okayama University (Okayama, Japan) have developed a laser-THz emission microscope (LTME) for non-destructive imaging of semiconductor devices. In this technique, the spatial resolution is limited by the shorter wavelength of the femtosecond laser that is used to generate the THz radiation. This microscope has a sensing plate made of a stack of Thin SiO2 (275 nm) and Si (170nm) layers deposited on a sapphire substrate. When a femtosecond laser hits the silicon layer from the substrate side, a THz pulse with the peak wavelength of 0.3 THz is generated and emitted from this plate. When a chemical reaction happens on the sensing plate (active area is 10 mm2), the chemical/electrical potential of the sensing plate changes accordingly. This leads to a change in the magnitude of the local field and as a result the intensity of the generated THz pulse. Scanning the pump laser across the sensing plate produces an image of the respective chemical reaction. Currently this technique has a spatial resolution of 50μm which can be improved by optimizing the femtosecond laser focusing.  In addition, this system can be used to perform label-free immunoassays, in which the system sensitivity is independent of the sample material’s molecular weight.  An Immunoassay is a biochemical test that measures the presence or concentration of a macromolecule in a solution through the use of an antibody or immunoglobulin. In a conventional method immunoassays employ a variety of different labels to allow for detection of antibodies and antigens. Labels are typically chemically linked or conjugated to the desired antibody or antigen.

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