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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Anisotropic terahertz microspectroscopy | 3/3 | https://en.wikipedia.org/wiki/Anisotropic_terahertz_microspectroscopy | reference | science, encyclopedia | 2026-05-05T10:03:39.205651+00:00 | kb-cron |
===== 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 a Fresnel rhomb with a single total internal reflection on the longer face of the prism and is a passive broadband component that permits a wide frequency sweep during measurements.
== Advantages == A few advantages of ATM over other related microspectroscopy techniques include the orientation of the THz electric field at the sample and the ability to readily measure materials that are sensitive to environmental conditions like hydration, cryo-cooling, and evacuation.
=== THz polarization orientation at the sample === A key characteristic of ATM is the orientation of the polarized electric field of THz light at the sample. In particular, unlike other microspectroscopy techniques like scattering scanning near-field optical microscopy (s-SNOM), the electric field of the interrogating THz field is parallel to the surface of the sample. In s-SNOM, the shape of the oscillating metallic probe tip directs the THz polarization into a direction predominantly perpendicular to the sample surface.
=== Environmentally sensitive sample materials === Living organisms typically consist of large quantities of water. Many anisotropic materials of interest are biological in nature and as such require hydration during spectroscopic measurements. While some limited novel techniques to measure properties of materials inside a hydrated sample chamber have been recently reports, the primary design requirement of ATM is that the material is accessible through a window that is transparent to THz light such as quartz. Similarly, samples requiring cryo-cooling or low pressure vacuum environment are readily interrogated in ATM using THz-transparent window materials.
== Applications == Anisotropic terahertz microspectroscopy (ATM) has found applications in structural biology and molecular fingerprinting of DNA and proteins. The technique is also suitable for drug discovery and studying THz frequency properties of thin film solid state materials. Special attention is given to molecular motions in proteins where many structural changes occur at 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 by a flexible molecular structure that bends like a mechanical hinge or elbow. ATM is uniquely capable of measuring the spatial direction in which hinge motions occur because of its use of linearly polarized electric fields.
=== Protein 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 or joint and other conformational changes that occur within systems of protein molecules. Protein molecules are typically surrounded by water molecules and are arranged in random orientations. For this reason, it is common to arrange protein molecules in crystal form such that their orientations all the same. In particular, in a protein crystal the dipole of all protein molecules are naturally aligned. This allows us to perform microspectroscopy with polarized THz light and ascertain the spatial orientation of vibrations within molecules.
== References ==