Explanation
When the electric field of a propagating beam of light oscillates in a direction perpendicular to its direction of propagation, it is said to be a polarizedTechniques
To date, ATM techniques have utilized THz time-domain spectroscopy (THz-TDS) because of historical scarcity of strong THz sources and highly sensitive THz detectors that operate at room temperature. Many samples of interest contain large amounts of water that strongly absorb THz radiation, thus requiring a very strong THz source. This requirement is exacerbated when attempting to use highly sensitive THz detectors that conventionally require supercooling to liquid helium temperatures. Worse, the need for supercooling these detectors has made THz detection unavailable to many researchers around the world due to recent sharp rises in the price of liquid helium due to its scarcity. To circumvent THz detection hurdles, THz-TDS is utilized as it requires commonly available infrared detectors. In this case, an electro-optic (EO) crystal is used to detect changes in the THz after it has passed through a sample. The polarization properties of a synchronized infrared beam of light passing through the EO crystal are changed. This polarization change is detected by an infrared detector called a balanced detector that compares the magnitude of two perpendicular polarization components of the infrared beam. Until more powerful THz sources that provide a wide frequency range and more sensitive room temperature THz detectors are realized, THz-TDS is a reliable technique for ATM. The THz-TDS techniques used in ATM may be divided into two categories: rotated sample and stationary sample. Historically, the first technique involved rotation of the sample at the focus of a THz beam while the detector is placed far from the sample in theRotated Sample ATM
Original ATM techniques involve rotating the sample at the focal point of a linearly polarized THz beam using a mechanically rotated sample mount. For this reason, the configuration is typically a far-field instrument in which a balanced detector (sensitive to infrared light) is placed a considerable distance from the sample. In the terahertz time-domain spectroscopy configuration, both the infrared and THz beams are transmitted through an electro-optic (EO) crystal like ZnTe or GaP. Here, the infrared beam detects the change in birefringence of the EO crystal due to the THz beam. When a sample is placed in the THz beam, the polarized THz beam is perturbed and the resulting degree of birefringence in the EO crystal is changed. The resulting perturbation of the infrared beam is sensed at the balanced detector. Rotated sample ATM is very useful for large samples (0.1 to 1 cm). However, when measuring samples such as protein crystals that must be isolated inside a hydration chamber, for example, the sample cannot be easily rotated. Additionally, it is challenging to maintain the same location of a rotated sample at the precise focal point of a THz beam.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, , where 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, and the polarization angle of the ultrafast near-infrared (NIR) probe beam, , at the sample by the relationship The objective is to maintain equal magnitude of the THz electric field at the sample for all measurement angles, . This requires adjustment of for every .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-resisitivity 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, , 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.=THz Quarter Waveplate
= One strategy to provide full 360° rotation of THz polarization of equal electric field magnitude at the sample is to generate a circular state of polarization, then select particular linear polarization states from the circularly polarized beam with a THz polarizer. A circular polarization state may be generated by a quarter waveplate, however, common optical waveplates are typically designed for visible, near- and mid-infrared regions of the electromagnetic spectrum. A quarter waveplate designed for use in the THz frequency range consists of a right-angle silicon prism together with metal-coated planar mirrors as input/output. In particular, the silicon prism acts analogously to aApplications
Anisotropic terahertz microspectrosopy (ATM) has found applications inProtein dynamics
ATM is uniquely suited to measure resonant molecular vibrations in proteins. Molecular motions in proteins occur with frequencies in the terahertz range of the spectrum (0.3 THz to 3 THz). These structural changes include hinge motions in which two regions of molecules are connected together in a flexible way that bends like a mechanical hinge orReferences
{{Reflist Terahertz technology Spectroscopy Scientific techniques