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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Anisotropic terahertz microspectroscopy | 2/3 | https://en.wikipedia.org/wiki/Anisotropic_terahertz_microspectroscopy | reference | science, encyclopedia | 2026-05-05T10:03:39.205651+00:00 | kb-cron |
==== Instrument Design ==== An ATM designed with a rotated sample is typically a far-field measurement configuration using a time-domain spectroscopy strategy. A high power infrared laser is typically used. Its beam is split by a beamsplitter into two optical paths: a probe beam and a THz generation beam. The THz generation beam typically receives the greater fraction of NIR power in order to maximize the power of the THz light commonly generated by a voltage-pulsed photoconductive antenna. The generated THz light is collected through a hyper-hemispherical silicon lens and passed to an off-axis parabolic mirror that collimates the THz beam for polarization by a THz polarizer that is often made of a simple wire-grid. The linearly polarized THz beam is then focused by a second off-axis parabolic mirror onto the sample. The THz beam transmitted through the sample is again collected by a third off-axis parabolic mirror, collimated onto a fourth parabolic mirror that then focuses the beam onto an electro-optic (EO) crystal whose birefringence is perturbed by the strength of the THz beam. The NIR probe beam is passed through the EO crystal to probe the induced degree of birefringence caused by the THz beam and passed to a detection module that often consists of an NIR quarter wave plate, a Wollaston prism that spatially separates orthogonal polarization states of the probe beam into two optical paths that are individually detected at a balanced detector. The resulting signal reported by the balanced detector is a measure of the difference in magnitude of these two orthogonal components of the NIR probe beam and therefore a direct correlation of the degree of birefringence induced in the EO crystal by the THz beam passed through the sample.
=== Stationary Sample ATM === Previously called "ideal ATM" and "polarization-varying ATM," stationary sample ATM (SSATM) involves rotation of the linearly polarized state of the THz beam in a time-domain spectroscopy (TDS) configuration parallel to the interrogated material sample. In a SSATM configuration, the THz beam polarization is rotated through 360° in a plane perpendicular to the propagation direction of the beam. Measurements of the sample's anisotropy is measured at several THz polarization angles. At least two methods to achieve THz polarization rotation for SSATM have been demonstrated: 1) by using a THz quarter waveplate (THz-QWP) together with an infrared polarizer and 2) by rotating the photoconductive antenna. In the case of employing a THz-QWP and an infrared polarizer, the magnitude of the measured signal,
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{\displaystyle {\rm {Sig}}(\tau ,\alpha _{\rm {THz}},\phi _{\rm {NIR}})}
, where
τ
{\displaystyle \tau }
is a time delay between THz generation and the detected pulses in a THz-TDS system is dependent on the relative polarization angle of the THz light,
α
T
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z
{\displaystyle \alpha _{\rm {THz}}}
and the polarization angle of the ultrafast near-infrared (NIR) probe beam,
ϕ
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{\displaystyle \phi _{\rm {NIR}}}
, at the sample by the relationship
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sin
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{\displaystyle {\rm {Sig}}(\tau ,\alpha _{\rm {THz}},\phi _{\rm {NIR}})\propto \left|\left[\cos(\alpha _{\rm {THz}})\sin(2\phi _{\rm {NIR}})+2\sin(\alpha _{\rm {THz}})\cos(2\phi _{\rm {NIR}})\right]\right|.}
The objective is to maintain equal magnitude of the THz electric field at the sample for all measurement angles,
α
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{\displaystyle \alpha _{\rm {THz}}}
. This requires adjustment of
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{\displaystyle \phi _{\rm {NIR}}}
for every
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{\displaystyle \alpha _{\rm {THz}}}
.
==== Instrument Design ==== A SSATM instrument is typically designed in a time-domain spectroscopy configuration in which a high power infrared laser beam is divided into two optical paths by a beamsplitter. The first optical path often receives a greater fraction of the optical power of the laser to maximize the output power of generated THz light. THz light is often generated with a voltage-pulsed photoconductive antenna, collected with a hyper-hemispherical silicon lens, collimated using an off-axis parabolic mirror that is then passed through a THz polarizer, made circular by a THz quarter waveplate constructed of two planar mirrors and a right-angled high-resistivity silicon prism to form circularly polarized light. A second THz polarizer selects from the circularly polarized THz light the angle at which each measurement is made once the light reaches a sample located at a focal point of the beam and mounted in direct contact with an electro-optic crystal often made of either ZnTe or GaP. The second optical path includes a retroreflector mirror mounted on a delay stage that adjusts the time-of-flight of the NIR beam to match the delay time,
τ
{\displaystyle \tau }
, of the THz light at the sample. The NIR beam is linearly polarized and chopped at a frequency suitable for detection, directed to the EO crystal to measure the change in its birefringence due to the degree of THz absorption by the sample. The NIR beam is reflected by the sample/EO crystal interface and directed to the detection module that often consists of an NIR quarter waveplate, a Wollaston prism that spatially selects perpendicular polarization states of the light toward two detectors in a balanced detector. The detected signal is a measure of the difference of the magnitude of the two perpendicular polarization states and corresponds to the degree of birefringence induced in the EO crystal by the THz light as-perturbed by the sample.