Cultural heritage objects are increasingly being investigated using advanced non-destructive optical measurement
techniques. Holographic and speckle interferometry based instrumentation allow dimensional measurement of
objects at the tens of nanometer scale. For the structural diagnostics of artwork, double-exposure techniques are
often used to locate defects, delaminations, voids and other structural features. Shearography is a speckle
interferometry configuration that uses a close-to-common-path shearing interferometer configuration to give a direct
sensitivity to displacement gradient at the object surface. This configuration is particularly useful for measurements
outside the optical laboratory, as the stability requirements are much reduced compared with holography techniques.
Terahertz imaging is a new category of sensor, used to investigate materials using electromagnetic radiation in the
0.1 to 10 THz frequency range. At these frequencies many materials become semi-transparent, so bulk structural
diagnostics can be performed. Typically terahertz imaging is performed using a scanning pixel, or multi-pixel,
sensor. In this manuscript shearography is first used to identify areas of interest of possible structural anomalies in
the artwork. These regions of interest are then studied in more detail using the terahertz imaging instrument.
Together the two instruments provide an analysis of both the surface and bulk structural features. The approach is
demonstrated experimentally using a wooden panel painting.
We report on multi-channel detection of ultrashort THz pulses by a linear array of 16 photoconductive dipole antennas.
The dipole antennas built on low-temperature grown GaAs are excited by a line focus of fs-pulses. By the parallel
detection of a complete line of ultrashort THz pulses, the measurement speed of THz ultrashort pulse time domain
systems can be accelerated by an order of magnitude. For demonstration, the THz beam profile along the line detector is
determined, and its spectral dependence of the electric field distribution is compared and verified by wave-optical
A THz time domain imaging system is optimized and analyzed with ZEMAX. The requirements to the optical design of
time domain imaging systems in the THz spectral region are deduced. A system is presented, which is diffraction-limited
for wavelengths down to 838 μm and field points up to ±4 mm. In the optical system a 90° off-axis parabolic mirror is
combined with an aspheric plastic lens. The lens was made from ZEONEX E48R®, and it was manufactured by ultraprecision
machining. A resolution test of the system shows that on time domain analysis of the pulse maximum on-axis 1 LP/mm can be resolved with an intensity contrast of 0.22. The resolution of the outermost field point is 0.67 LP/mm with an intensity contrast of 0.23. An outlook of an optimized system for imaging a field of ±10 mm in x- and y-direction
Imaging of styrofoam with the help of ultrashort Terahertz pulses is investigated. With a combination of pulse amplitude
and time delay imaging it is possible to speed up the measurement about two orders of magnitudes.
The applicability of moth-eye structures to THz components is investigated. With the help of RCWA and effective medium theory, optimal structural parameters for one-dimensional and two-dimensional periodical surface-relief gratings are deduced. The required structural parameters are in such order of magnitude that they can be manufactured by ultra-precision machining directly into the surface of the substrate material. Benefiting is that plastic materials, which are preferred materials in THz spectral region, can be accurately manufactured by ultra-precision machining. The application of the moth-eye structures follows directly the primary shaping of the components by conventional manufacturing methods like turning and milling so that no additional materials are necessary. A comparison between several structures fabricated on planar plastic probes is given.
InN, a novel semiconductor material, is used as THz surface emitter. The material is irradiated with fs-laser pulses at 1060 nm and 800 nm and the emitted ultrashort THz pulses are measured by phase sensitive detection. Pulsforms, amplitudes and spectra are compared to the THz emission of p-doped InAs, the standard material for THz surface emission.