Advances in Terahertz Imaging
147
3.1 THz Time-Domain Spectroscopy System Imaging
Terahertz radiation was initially measured with the terahertz time-domain spectroscopy system. Using nonlinear optical methods or with the help of photoconductive antenna, broadband terahertz pulses are generated and detected. Using this
method, it is possible to measure the amplitude as well as the phase of the THz
electric field in the time domain.
As an example to obtain an image with THz-TDS, Guerboukha et al. used a
transmission mode THz-TDS. They imaged a triangle constructed of high-density
polyethylene (HDPE), a metallic washer and a star constructed of paper [30]. The
three samples were placed on top of a sheet of paper which is surrounded by a metallic
aperture made of aluminium foil. A two-dimensional image of any object can be
obtained by raster-scanning the sample and by recording each point of the individual
traces by a pre-collimated THz beam [30]. This results in a three-dimensional data
cube, where the third dimension is time.
To obtain a hyperspectral image, the Fourier transform can be performed with
respect to time. In order to convert the obtained raw data of frequency or time into
some physically significant values, normalization is required [31–33]. Generally,
contrast of the image is enhanced by normalization.
Figure 2 (time domain) and Fig. 3 (frequency domain) present different cases
of normalizations [30]. Figure 2a shows typically acquired THz pulses across the
sample. These pulses can be delayed, attenuated or broadened during interaction
with the sample. The electric field amplitude at time t 0 = 27.6 ps is illustrated in
Fig. 2b. By this technique of visualization of the electric field, the dynamics of
irradiation is revealed at the picosecond scale. Again the normalized maximum peak
in time domain is shown in Fig. 2c. Here, the terahertz pulse which is measured in
air is used as the reference. The resultant image exhibits the reflection, scattering
or absorption losses in the material. In order to generate the image which is shown
in Fig. 2d, the time delay suffered by the main peak is mapped with respect to the
reference measurement. Thus, the optical path change across the sample, given as
(x, y) = n(x, y)d(x, y), can be mapped.
Fourier transform for the third dimension of the data cube gives the amplitude
and the phase of the spectrum. This is illustrated in Fig. 3a, b, respectively. A general
indication of the losses can be obtained from the amplitude. The amplitude generally
increases with the frequency. On the other hand, the sample refractive index and its
thickness are related to the phase. It is observed that spatial resolution is proportional
with the terahertz frequency. The amplitude and the phase at 0.475 THz are illustrated
in Fig. 3c, d, respectively.
The maximum losses in the material occur during the propagation through the
metallic washer, which is shown in the amplitude image. Again, the scattering losses
are observable at the borders of the HDPE triangle. Inside the HDPE triangle and
the star paper, phase image has better contrast compared to the amplitude image.
However, classical set-ups of THz-TDS for real-time operation of terahertz timedomain spectroscopy imaging systems suffer from two major problems. As already
147
3.1 THz Time-Domain Spectroscopy System Imaging
Terahertz radiation was initially measured with the terahertz time-domain spectroscopy system. Using nonlinear optical methods or with the help of photoconductive antenna, broadband terahertz pulses are generated and detected. Using this
method, it is possible to measure the amplitude as well as the phase of the THz
electric field in the time domain.
As an example to obtain an image with THz-TDS, Guerboukha et al. used a
transmission mode THz-TDS. They imaged a triangle constructed of high-density
polyethylene (HDPE), a metallic washer and a star constructed of paper [30]. The
three samples were placed on top of a sheet of paper which is surrounded by a metallic
aperture made of aluminium foil. A two-dimensional image of any object can be
obtained by raster-scanning the sample and by recording each point of the individual
traces by a pre-collimated THz beam [30]. This results in a three-dimensional data
cube, where the third dimension is time.
To obtain a hyperspectral image, the Fourier transform can be performed with
respect to time. In order to convert the obtained raw data of frequency or time into
some physically significant values, normalization is required [31–33]. Generally,
contrast of the image is enhanced by normalization.
Figure 2 (time domain) and Fig. 3 (frequency domain) present different cases
of normalizations [30]. Figure 2a shows typically acquired THz pulses across the
sample. These pulses can be delayed, attenuated or broadened during interaction
with the sample. The electric field amplitude at time t 0 = 27.6 ps is illustrated in
Fig. 2b. By this technique of visualization of the electric field, the dynamics of
irradiation is revealed at the picosecond scale. Again the normalized maximum peak
in time domain is shown in Fig. 2c. Here, the terahertz pulse which is measured in
air is used as the reference. The resultant image exhibits the reflection, scattering
or absorption losses in the material. In order to generate the image which is shown
in Fig. 2d, the time delay suffered by the main peak is mapped with respect to the
reference measurement. Thus, the optical path change across the sample, given as
(x, y) = n(x, y)d(x, y), can be mapped.
Fourier transform for the third dimension of the data cube gives the amplitude
and the phase of the spectrum. This is illustrated in Fig. 3a, b, respectively. A general
indication of the losses can be obtained from the amplitude. The amplitude generally
increases with the frequency. On the other hand, the sample refractive index and its
thickness are related to the phase. It is observed that spatial resolution is proportional
with the terahertz frequency. The amplitude and the phase at 0.475 THz are illustrated
in Fig. 3c, d, respectively.
The maximum losses in the material occur during the propagation through the
metallic washer, which is shown in the amplitude image. Again, the scattering losses
are observable at the borders of the HDPE triangle. Inside the HDPE triangle and
the star paper, phase image has better contrast compared to the amplitude image.
However, classical set-ups of THz-TDS for real-time operation of terahertz timedomain spectroscopy imaging systems suffer from two major problems. As already