• Terahertz
• confocal microscopy
• quantum cascade laser
• THz
• QCL

The recent advances in generation, manipulation and detection of Terahertz (THz)
radiation have been accompanied by a growing interest in the application of THz
technologies to a wide range of fields spanning from imaging, remote sensing and
security controls, to spectroscopy and biomedical science.
An appealing reason for developing photonic techniques in this frequency range is the
capability for THz rays to penetrate common packaging materials (paper, plastic, wood,
ceramics).
The recent invention of quantum cascade lasers (QCL) emitting at THz frequencies is
enabling a major step forward in THz imaging technologies thanks to the availability of a
powerful, compact, solid-state, laser source. Once combined with bolometer array
camera detectors, they have allowed the first systems for stand-off THz image acquisition
operating in real time up to distances of more than twenty meters.
On the other side, there are also great expectations for THz microscopy, especially as a
potential diagnostic tool in quality inspection of various industrial productions or in
molecular biology at the cell level. Yet, this research direction is just in its infancy, with
most works focusing on near-field techniques.
In the present thesis work, I focused instead on trying to transfer the principle of optical
confocal microscopy to the THz, by implementing the first confocal imaging system
based on a QCL, in this case emitting at ~ 3 THz. The confocal spatial filtering can
sensibly reduce the depth of field of the microscope, as well as improve its resolution and
contrast, making it very useful for inspection of close-by surfaces, where different planes
have to be clearly distinguished, or where a depth information is crucial.
The use of a QCL source required various preparatory tests and measurements, to assess
the emission beam quality and to choose the best option for the spatial filter. In order to
reduce the emitting area and improving the profile regularity, hollow metallic waveguide
couplers and pinholes were examined. The waveguides were also fully characterized in
terms of propagation and coupling losses to QCLs in different configurations, proving
that very low-loss modes can be efficiently excited, an interesting result for the future
development of THz endoscopes.
The definitive confocal setup was finally assembled and characterized in terms of lateral
and axial resolution and several images of test objects were collected in order to show the
excellent performance of the instrument realized.