![]() | This article may have too many section headers .(February 2020) |
This article needs additional citations for verification .(April 2014) |
Terahertz tomography | |
---|---|
Purpose | Imaging is done by terahertz radiation |
Terahertz tomography is a class of tomography where sectional imaging is done by terahertz radiation. Terahertz radiation is electromagnetic radiation with a frequency between 0.1 and 10 THz; it falls between radio waves and light waves on the spectrum; it encompasses portions of the millimeter waves and infrared wavelengths. Because of its high frequency and short wavelength, terahertz wave has a high signal-to-noise ratio in the time domain spectrum. [1] Tomography using terahertz radiation can image samples that are opaque in the visible and near-infrared regions of the spectrum. Terahertz wave three-dimensional (3D) imaging technology has developed rapidly since its first successful application in 1997, [2] and a series of new 3D imaging technologies have been proposed successively.
Terahertz imaging has advantages over the more expensive and shorter range X-ray scanners. A variety of materials are transparent to terahertz radiation, which allows it to measure the thickness, density, and structural properties of materials that are difficult to detect. Since terahertz is not ionizing radiation, the use of terahertz does not cause damage to living tissue, making terahertz a safe, non-invasive biomedical imaging technique. Moreover, because many materials have a unique spectral signature in the terahertz range, terahertz radiation can be used to identify materials. Terahertz imaging is widely used in the study of semiconductor material properties, biomedical cell imaging, and chemical and biological examination.[ citation needed ] Terahertz time domain systems (THz-tds) have made significant advances in 2D imaging. THz-tds is able to determine the sample complex dielectric constant, usually 0.1–4 THz, and provides information about the static characteristics of the sample over dozens of frequencies. [3] However, this technology has some limitations. For example, due to the lower power of the beam, the sensor must be more sensitive. Low image acquisition speeds may force a tradeoff between time and resolution.
Terahertz imaging can be useful for luggage and postal mail screening because it can identify substances on the basis of their characteristic spectra in this frequency band, such as explosives and illicit drugs; [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] for example, several liquid explosives can be distinguished by the change in dielectric response in the terahertz range as a function of alcohol percentage. [15] Although dangerous metal objects, such as knives, can be recognized by their shapes through certain pattern recognition algorithms, [16] it is impossible to see through metallic packages with terahertz waves. Thus, terahertz spectrometers cannot replace X-ray scanners, even though they provide more information than X-ray scanners for low-density materials and chemical separation. [17]
Terahertz systems are used for production control in the paper and polymer industries. [18] They can detect thickness and moisture content in paper [19] and conductive properties, moisture level, fiber orientation and glass-transition temperature in polymers. [20] [21] [22] [23]
Terahertz systems facilitate the detection of metallic and nonmetallic contamination in food. [24] For example, terahertz waves made it possible to detect metallic and nonmetallic foreign matter in chocolate bars, [25] since food with low water contents, such as chocolates, are almost transparent in the terahertz band. Terahertz tomography is also useful in the wine and spirits industries for quantifying moisture and analysing cork non-destructively.
Terahertz imaging can detect different isomers have different spectral fingerprints in the terahertz range, which enables terahertz spectroscopy to distinguish between stereoisomers—a crucial distinction in pharmacy, where one isomer may be the active compound and its enantiomer may be inactive or even dangerous. [26] Terahertz systems are also used for gauging tablet coating quality. [27]
Terahertz imaging enables non-destructive analysis of valuable artworks and can be conducted onsite. It can reveal hidden layers and via the transmittance of various pigments. [28] [29] It is also being investigated as a tool for 3D visualization. [30] [31]
Terahertz tomography can be divided into transmission and reflection mode. It acts as an extension of X-ray computed tomography (CT) to a different waveband. It mainly studies the establishment of process models such as refraction, reflection and diffraction when terahertz waves transmit samples, which has certain requirements for reconstruction algorithms. According to the different transmission delay of Terahertz wave reflected signal at different depths inside the sample, the depth information can be obtained by processing the reflected signal inside the sample to realize the tomography. Terahertz time-of-flight tomography (THz-TOF) and THz optical coherence tomography (Thz-OCT) are mainly used in implementation.
In diffraction tomography, the detection beam interacts with the target and uses the resulting scattered waves to build a 3D image of the sample. [32] The diffraction effect and the diffraction slice theorem shine light on the surface of the scattered object and record the reflected signal to obtain the diffraction field distribution after the sample in order to explore the surface shape of the target object. For fine samples with more complex surface structure, diffraction tomography is effective because it can provide a sample refractive index distribution. [33] However, there are also drawbacks: although the imaging speed of terahertz diffraction tomography is faster, its imaging quality is poor due to the lack of an effective reconstruction algorithm. In 2004, S. Waang et al. first used diffraction chromatography based on the THz-tds system to image polyethylene samples. [34]
Tomosynthesis is a technique used to create high-image tomography. The reconstruction can be done by several projection angles, which creates the image faster. This technique has low resolution but faster imaging speed. [35] This technique also has an advantage over terahertz CT. Terahertz CT is significantly affected by reflection and refraction, especially for wide and flat plate samples, which has a large incidence angle at the edge and severe signal attenuation. Therefore, it is difficult to obtain both complete projection data and substantial noise information simultaneously. However, terahertz fault synthetic tomography is not affected by refraction and reflection because of the small incidence angle during projection. It is an effective method for local imaging, rapid imaging, or incomplete sample rotation. In 2009, N. Unaguchi et al. in Japan used continuous terahertz solid-state frequency multiplier with frequency of 540 GHz to conduct TS imaging on three letters "T", "H" and "Z" at different depths of post-it notes. [36] The back projection method and wiener filter were used to reconstruct the spatial distribution of three letters.
Terahertz fault chromatography can reconstruct the 3D distribution of the refractive index by reflecting the terahertz pulse at different depths in the sample. The depth distribution information of the refractive index can be obtained by analyzing the time delay of the peak value of the reflected pulse. The longitudinal resolution of time-of-flight tomography depends on the pulse width of terahertz waves (usually in the tens of microns); therefore, the vertical resolution of flight time chromatography is very high. In 2009, J.Takayanagi et al. designed an experimental system that successfully used tomography on a semiconductor sample consisting of three sheets of superimposed paper and a thin two-micron thick layer of GaAs. [37]
The THz beam can be incorporated into 3D holography if the differentiation of each multiple scattered terahertz waves of different scattering orders is enabled. [38] With both intensity and phase distribution recorded, the interference pattern generated by object light and reference light encodes more information than a focused image. The holograms can provide a 3D visualization of the object of interest when reconstructed via Fourier optics. [39] However, it remains a challenge to obtain high quality images with this technique due to scattering and diffraction effects required for measurement. The high order scattering measurement usually results in poor signal to noise ratio (SNR). [40]
Fresnel lenses serve as a replacement for traditional refractive lenses [41] with the advantages of being small and lightweight. As their focal lengths depend on frequencies, samples can be imaged at various locations along the propagation path to the imaging plane, [42] which can be applied to tomographic imaging.
Synthetic aperture processing (SA) differs from traditional imaging systems when collecting data. In contrast to the point-to-point measurement scheme, SA uses a diverging or unfocused beam. [43] The phase information collected by SA can be adopted for 3D reconstruction.
Terahertz computed tomography records both amplitude and spectral phase information when compared to X-ray imaging. Terahertz CT can identify and compare different substances while non-destructively locating them.
Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye. There are three well-known branches of microscopy: optical, electron, and scanning probe microscopy, along with the emerging field of X-ray microscopy.
Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz), although the upper boundary is somewhat arbitrary and is considered by some sources as 30 THz. One terahertz is 1012 Hz or 1000 GHz. Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm = 100 µm. Because terahertz radiation begins at a wavelength of around 1 millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy. This band of electromagnetic radiation lies within the transition region between microwave and far infrared, and can be regarded as either.
The point spread function (PSF) describes the response of a focused optical imaging system to a point source or point object. A more general term for the PSF is the system's impulse response; the PSF is the impulse response or impulse response function (IRF) of a focused optical imaging system. The PSF in many contexts can be thought of as the extended blob in an image that represents a single point object, that is considered as a spatial impulse. In functional terms, it is the spatial domain version of the optical transfer function (OTF) of an imaging system. It is a useful concept in Fourier optics, astronomical imaging, medical imaging, electron microscopy and other imaging techniques such as 3D microscopy and fluorescence microscopy.
In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on both the amplitude and the phase of the terahertz radiation.
Medical optical imaging is the use of light as an investigational imaging technique for medical applications, pioneered by American Physical Chemist Britton Chance. Examples include optical microscopy, spectroscopy, endoscopy, scanning laser ophthalmoscopy, laser Doppler imaging, and optical coherence tomography. Because light is an electromagnetic wave, similar phenomena occur in X-rays, microwaves, and radio waves.
X-ray optics is the branch of optics that manipulates X-rays instead of visible light. It deals with focusing and other ways of manipulating the X-ray beams for research techniques such as X-ray crystallography, X-ray fluorescence, small-angle X-ray scattering, X-ray microscopy, X-ray phase-contrast imaging, and X-ray astronomy.
Electro-optic rectification (EOR), also referred to as optical rectification, is a non-linear optical process that consists of the generation of a quasi-DC polarization in a non-linear medium at the passage of an intense optical beam. For typical intensities, optical rectification is a second-order phenomenon which is based on the inverse process of the electro-optic effect. It was reported for the first time in 1962, when radiation from a ruby laser was transmitted through potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (KDdP) crystals.
Photothermal spectroscopy is a group of high sensitivity spectroscopy techniques used to measure optical absorption and thermal characteristics of a sample. The basis of photothermal spectroscopy is the change in thermal state of the sample resulting from the absorption of radiation. Light absorbed and not lost by emission results in heating. The heat raises temperature thereby influencing the thermodynamic properties of the sample or of a suitable material adjacent to it. Measurement of the temperature, pressure, or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic measurements.
Volume holograms are holograms where the thickness of the recording material is much larger than the light wavelength used for recording. In this case diffraction of light from the hologram is possible only as Bragg diffraction, i.e., the light has to have the right wavelength (color) and the wave must have the right shape. Volume holograms are also called thick holograms or Bragg holograms.
A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.
A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research is usually defined as 0.1 to 10 THz.
Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.
Phase-contrast X-ray imaging or phase-sensitive X-ray imaging is a general term for different technical methods that use information concerning changes in the phase of an X-ray beam that passes through an object in order to create its images. Standard X-ray imaging techniques like radiography or computed tomography (CT) rely on a decrease of the X-ray beam's intensity (attenuation) when traversing the sample, which can be measured directly with the assistance of an X-ray detector. However, in phase contrast X-ray imaging, the beam's phase shift caused by the sample is not measured directly, but is transformed into variations in intensity, which then can be recorded by the detector.
Light sheet fluorescence microscopy (LSFM) is a fluorescence microscopy technique with an intermediate-to-high optical resolution, but good optical sectioning capabilities and high speed. In contrast to epifluorescence microscopy only a thin slice of the sample is illuminated perpendicularly to the direction of observation. For illumination, a laser light-sheet is used, i.e. a laser beam which is focused only in one direction. A second method uses a circular beam scanned in one direction to create the lightsheet. As only the actually observed section is illuminated, this method reduces the photodamage and stress induced on a living sample. Also the good optical sectioning capability reduces the background signal and thus creates images with higher contrast, comparable to confocal microscopy. Because light sheet fluorescence microscopy scans samples by using a plane of light instead of a point, it can acquire images at speeds 100 to 1,000 times faster than those offered by point-scanning methods.
TeraView Limited, or TeraView, is a company that designs terahertz imaging and spectroscopy instruments and equipment for measurement and evaluation of pharmaceutical tablets, nanomaterials, ceramics and composites, integrated circuit chips and more.
Terahertz spectroscopy detects and controls properties of matter with electromagnetic fields that are in the frequency range between a few hundred gigahertz and several terahertz. In many-body systems, several of the relevant states have an energy difference that matches with the energy of a THz photon. Therefore, THz spectroscopy provides a particularly powerful method in resolving and controlling individual transitions between different many-body states. By doing this, one gains new insights about many-body quantum kinetics and how that can be utilized in developing new technologies that are optimized up to the elementary quantum level.
Nano-FTIR is a scanning probe technique that utilizes as a combination of two techniques: Fourier transform infrared spectroscopy (FTIR) and scattering-type scanning near-field optical microscopy (s-SNOM). As s-SNOM, nano-FTIR is based on atomic-force microscopy (AFM), where a sharp tip is illuminated by an external light source and the tip-scattered light is detected as a function of tip position. A typical nano-FTIR setup thus consists of an atomic force microscope, a broadband infrared light source used for tip illumination, and a Michelson interferometer acting as Fourier-transform spectrometer. In nano-FTIR, the sample stage is placed in one of the interferometer arms, which allows for recording both amplitude and phase of the detected light. Scanning the tip allows for performing hyperspectral imaging with nanoscale spatial resolution determined by the tip apex size. The use of broadband infrared sources enables the acquisition of continuous spectra, which is a distinctive feature of nano-FTIR compared to s-SNOM. Nano-FTIR is capable of performing infrared (IR) spectroscopy of materials in ultrasmall quantities and with nanoscale spatial resolution. The detection of a single molecular complex and the sensitivity to a single monolayer has been shown. Recording infrared spectra as a function of position can be used for nanoscale mapping of the sample chemical composition, performing a local ultrafast IR spectroscopy and analyzing the nanoscale intermolecular coupling, among others. A spatial resolution of 10 nm to 20 nm is routinely achieved.
A virtually imaged phased array (VIPA) is an angular dispersive device that, like a prism or a diffraction grating, splits light into its spectral components. The device works almost independently of polarization. In contrast to prisms or regular diffraction gratings, the VIPA has a much higher angular dispersion but has a smaller free spectral range. This aspect is similar to that of an Echelle grating, since it also uses high diffraction orders. To overcome this disadvantage, the VIPA can be combined with a diffraction grating. The VIPA is a compact spectral disperser with high wavelength resolving power.
The time-domain counterpart of spatial holography is called time-domain holography. In other words, the principles of spatial holography is surveyed in time domain. Time-domain holography was inspired by the theory known as space-time duality which was introduced by Brian H. Kolner in 1994.
Anisotropic terahertz microspectroscopy (ATM) is a spectroscopic technique in which molecular vibrations in an anisotropic material are probed with short pulses of terahertz radiation whose electric field is linearly polarized parallel to the surface of the material. The technique has been demonstrated in studies involving single crystal sucrose, fructose, oxalic acid, and molecular protein crystals in which the spatial orientation of molecular vibrations are of interest.