156
A. Saha
losses occurring due to absorption, but in THz-CT, for non-uniform samples having
a large refractive index, losses occurring due to reflection and refraction become
important.
However, it is challenging to perform real-time THZ-CT as it is required to measure the sample angular rotation also. But to reconstruct the internal structure of an
object, the time-domain/spectral information is not required for THz-CT. Alternatively, techniques can be used where only the transmitted amplitude/intensity THz
radiation is required.
3.5 THz Near-Field Imaging
Far-field imaging system can show resolution for only features of an object which are
comparable to or larger than the wavelength of the THz beam used. To have resolution
for subwavelength features of the object, near-field region imaging system has to be
incorporated. Thus, detection of the evanescent fields at small distances from the
object is required. This imaging technique is called THz near-field imaging [64–67].
Different techniques have been used for subwavelength THz imaging.
A pair of apertures (pinholes) is used in the confocal THz near-field imaging
technique. This acts as a spatial filter to restrict terahertz radiation passing out of
the cone of light. This increases the resolution for both lateral and depth features
[68–71]. The pinhole diameter can be reduced to improve the lateral resolution. But,
the diameter can be reduced only up to a certain threshold limit, below which no
signal is collected at all.
On the other hand, more throughput light intensity can be obtained in solid-lens
immersion approach. In this technique, a lens is used which is designed specially
to generate a smaller spot size compared to the diffraction limit in the evanescent
field region following the lens. Chernomyrdin et al. [72] used a combination of a
truncated sphere and an aspheric lens, and at 0.5 THz, they achieved a resolution
of λ/3.1, which is obviously better than the resolution (λ/1.2) of a simple aspheric
lens. However, in this case, the properties of the material of the immersion lens
fundamentally limit the spatial resolution.
The sample is placed in contact with a subwavelength detector in the directcontact method. Here, the detector is a femtosecond optical beam which investigates
the change of polarization in a nonlinear crystal. In this case, it is possible to measure
the THz electric field directly in the near-field region since the crystal is in direct touch
with the sample. This technique can be used for real-time near-field THz imaging.
Recently, technology of SiGe heterojunction bipolar transistor has been used by
Grzyb et al. [73] for direct-contact near-field imaging. Most of the techniques for
real-time near-field imaging depend on the scanning of subwavelength apertures
mechanically, which makes it difficult for the acquisition of multiple images.
A. Saha
losses occurring due to absorption, but in THz-CT, for non-uniform samples having
a large refractive index, losses occurring due to reflection and refraction become
important.
However, it is challenging to perform real-time THZ-CT as it is required to measure the sample angular rotation also. But to reconstruct the internal structure of an
object, the time-domain/spectral information is not required for THz-CT. Alternatively, techniques can be used where only the transmitted amplitude/intensity THz
radiation is required.
3.5 THz Near-Field Imaging
Far-field imaging system can show resolution for only features of an object which are
comparable to or larger than the wavelength of the THz beam used. To have resolution
for subwavelength features of the object, near-field region imaging system has to be
incorporated. Thus, detection of the evanescent fields at small distances from the
object is required. This imaging technique is called THz near-field imaging [64–67].
Different techniques have been used for subwavelength THz imaging.
A pair of apertures (pinholes) is used in the confocal THz near-field imaging
technique. This acts as a spatial filter to restrict terahertz radiation passing out of
the cone of light. This increases the resolution for both lateral and depth features
[68–71]. The pinhole diameter can be reduced to improve the lateral resolution. But,
the diameter can be reduced only up to a certain threshold limit, below which no
signal is collected at all.
On the other hand, more throughput light intensity can be obtained in solid-lens
immersion approach. In this technique, a lens is used which is designed specially
to generate a smaller spot size compared to the diffraction limit in the evanescent
field region following the lens. Chernomyrdin et al. [72] used a combination of a
truncated sphere and an aspheric lens, and at 0.5 THz, they achieved a resolution
of λ/3.1, which is obviously better than the resolution (λ/1.2) of a simple aspheric
lens. However, in this case, the properties of the material of the immersion lens
fundamentally limit the spatial resolution.
The sample is placed in contact with a subwavelength detector in the directcontact method. Here, the detector is a femtosecond optical beam which investigates
the change of polarization in a nonlinear crystal. In this case, it is possible to measure
the THz electric field directly in the near-field region since the crystal is in direct touch
with the sample. This technique can be used for real-time near-field THz imaging.
Recently, technology of SiGe heterojunction bipolar transistor has been used by
Grzyb et al. [73] for direct-contact near-field imaging. Most of the techniques for
real-time near-field imaging depend on the scanning of subwavelength apertures
mechanically, which makes it difficult for the acquisition of multiple images.