CCD as THz Imager

IEEE Spectrum: The SwissFEL laser team led by Christoph Hauri at the Paul Scherrer Institute near Zurich shows that one can use a commercial megapixel CCD to capture THz-band images. "It was previously known that low frequency radiation, when intense enough, could lead to dramatic changes in the semiconductor band structure. (For the intense light source they turned to a new and uniquely powerful source the SwissFEL team developed.) Long terahertz wavelengths force electrons to tunnel through the bandgap and the charge carriers start multiplying, leading to huge sensitivity."

The open access paper "High-performing nonlinear visualization of terahertz radiation on a silicon charge-coupled device" by Mostafa Shalaby, Carlo Vicario, and Christoph Hauri with the experimental results has been published in Nature Communications:

(a) Time-dependent electric field of the THz pulse retrieved using air-biased-coherent-detection technique. (b) The corresponding broadband amplitude spectrum. A narrowband portion centered around 10 THz is obtained by application of a band pass filter (BPF). (c) The detected images (normalized) on the (4.65-μm pixel size) CCD obtained with the 10 THz BPF in different slices around the focus with z being the propagation direction. The corresponding real images reconstructed taking the natural logarithm (as discussed later). The beam consists of several smaller beams that focus in different planes. The images obtained with a (23.5-μm pixel size) bolometric imager are shown. All images are plotted at the same scale on dimension of 200 × 200 μm. In the focal plane (z=0 μm), two spots are visible with a separation of 51 μm. While the pixel size is smaller than 51 μm in both cameras and so the two spots were obvious in both images, the high-resolving image from the CCD provides a significantly more detailed and accurate beam profile.