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Showing posts with label terahertz imaging. Show all posts
Showing posts with label terahertz imaging. Show all posts
Compact, chip-based lasers have conquered much of the electromagnetic spectrum, from ultraviolet to infrared, enabling technologies from digital communications and bar code readers to laser pointers and printers. But one key region of the spectrum remained untamed: the terahertz band, which lies between infrared light and microwaves. Engineers hankered for a ready source of terahertz radiation, which can penetrate opaque objects and probe chemical fingerprints inside. But compact terahertz lasers have only worked at ultralow temperatures, limiting them mostly to laboratory settings. There have been demonstrations, but not widely deployed. No longer. This week, researchers report creating a grain-of-rice–size terahertz laser on a chip that operates at 250 K, or –23°C, warm enough to be chilled by a plug-in cooler the size of a cracker.
One of the major challenges in THz imaging using array detectors is the responsivity non-uniformities across different pixels. This is caused by different pixels responding differently and non-linearly to the same incident field. Unfortunately, such responsivity non-uniformities cannot be corrected by the traditional amplitude normalization method, which only accounts for linear responsivity. Therefore, we propose a pixel calibration scheme that corrects a non-linear responsivity of individual pixels. This calibration scheme is particularly useful in a multi-frame sub-pixel imaging technique, which assumes that all pixels have the same characteristics. Using this imaging technique would allow us to achieve an image pixel size smaller than a detector pixel size, in much faster acquisition time compared with most existing systems employing a single-pixel detector. We demonstrate the performance of our pixel calibration approach through THz imaging of various objects by the multi-frame sub-pixel imaging technique. For a square aluminum piece as a test sample, total variation (TV) in the THz image is reduced by at least 12% after applying the pixel calibration. Finally, we present example uses of our proposed technique in moisture distribution imaging. The proposed technique holds promises towards rapid THz high-resolution imaging in non-destructive testing (NDT) and quality control (QC) applications.
Electrically tuneable high mobility charges on graphene yield an efficient electro-optical platform to control and manipulate terahertz (THz) waves. Real-world applications require a multiplex THz device with efficient modulation over a large active area. The trade-off between the efficient gating and switching speed, however, hinders the realization of these applications. Here, we demonstrate a large-format 256-pixel THz modulator which provides high-frame-rate reconfigurable transmission patterns. The time-domain and frequency-domain THz characterizations of graphene devices reveal the relaxation pathways of gate-induced charges and ion packing at graphene-electrolyte interface. The fundamental understanding of these limiting factors enables us to break the trade-off permitting switching frequencies up to 1 kHz. To show the promises of these devices, we demonstrate a single-pixel THz camera which allows spatial and spectroscopic imaging of large-area objects without any moving components. These results provide a significant advancement towards the achievement of non-invasive THz imaging systems using graphene-based platforms.
Holography has garnered an explosion of interest in tremendous applications, owing to its capability of storing amplitude and phase of light and reconstructing the full-wave information of targets. Spatial light modulators, metalenses, metasurfaces, and other devices have been explored to achieve holographic images. However, the required phase distributions for conventional holograms are generally calculated using the Gerchberg–Saxton algorithm, and the iteration is time-consuming without Fourier transform or other acceleration techniques. Few studies on designing holograms using artificial intelligence methods have been conducted. In this Letter, a three-dimensional (3D)-printed hologram for terahertz (THz) imaging based on a diffractive neural network (DNN) is proposed. Target imaging letters “THZ” with uniform field amplitude are assigned to a predefined imaging surface. Quantified phase profiles are primarily obtained by training the DNN with the target image and input field pattern. The entire training process takes only 60 s. Consequently, the hologram, that is, a two-dimensional array of dielectric posts with variational heights that store phase information, is fabricated using a 3D printer. The full-wave simulation and experimental results demonstrate the capability of the proposed hologram to achieve high-quality imaging in the THz regime. The proposed lens and design strategy may open new possibilities in display, optical-data storage, and optical encryption.
For imaging through opaque materials, x rays are an obvious choice. But their high energies can ionize atoms. At frequencies between the far-IR and microwave regions of the electromagnetic spectrum, terahertz radiation can penetrate materials such as paper, cloth, and plastics without damaging them. That property makes the frequency band useful for such applications as nondestructive testing, security screening, and biomedicine. But the frequency band is an elusive part of the spectrum: Terahertz photons lie below the reach of most optical technologies and just above the reach of electronics.
The upshot is that terahertz sources tend to have low power and their detectors, low sensitivity. One promising approach to circumvent those limitations is to use an atomic gas that converts terahertz-frequency photons into easily detectable optical-frequency ones. Rydberg atoms—atoms excited to a high principal quantum number n—turn out to be excellent sensors for the job. Lucy Downes, a doctoral student at Durham University, her advisers Kevin Weatherill and Charles Adams, and their colleagues have now demonstrated a terahertz imaging system using cesium vapor to make the conversion. The researchers produced optical photons at frame rates two orders of magnitude higher than the current state of the art in terahertz imaging.
The transitions between Rydberg states of alkali atoms have large dipole moments. And with a high vapor pressure at room temperature and easily accessible transition wavelengths, Cs is an advantageous choice for the demonstration. In their experiment, illustrated in panels a and b of the first figure, Downes and her colleagues place Cs gas in a quartz cell. IR radiation (yellow) focused on the cell produces a two-dimensional sheet of excited Cs atoms. Onto that sheet the researchers project a terahertz field (red) that has passed through an object of interest. The subsequent interactions of terahertz photons with Cs atoms excite the atoms to the n = 13 D5/2 level and give rise to fluorescence (green), whose emission is captured with an optical camera.
For their demonstration of high-speed terahertz imaging, the researchers captured subsequent video of dynamical processes—including a rotating optical chopper—at frame rates up to 3 kHz, as shown in the second figure. The white arrow highlights the movement of one spoke of the chopper between frames. (L. A. Downes et al., Phys. Rev. X10, 011027, 2020.)
A low-cost THz→IR converter based on a printed metasurface is fabricated. The converter is a thin membrane which absorbs the terahertz radiation at a single frequency using a grating of antennas, and converts it into thermal IR radiation. Here, we demonstrate THz imaging at 96 GHz using an IR camera equipped with the converter. The membrane can be upgraded to convert simultaneously multiple frequencies thus giving a possible path for terahertz multispectral imaging using an IR camera.
Researchers at the University of Durham have developed a method of terahertz imaging able to operate at 3,000fps.
Durham’s Lucy Downes said this was 100-times faster than any previous terahertz imager, speaking at the UK Knowledge Transfer Network’s Intelligent Imaging meeting in London on 29 April.
The technique uses laser-excited caesium atoms to perform terahertz-to-optical conversion, the light from which is captured with a standard camera.
Three infrared lasers are focused on a cell filled with caesium vapour. The lasers excite caesium atoms to a Rydberg state, at which point they can absorb a 0.55THz photon. The atoms then decay after around a microsecond and emit a green photon that can be detected with a visible camera.
The technique is only limited to how fast the atoms are excited and decay, which Downes said was potentially at frame rates of up to 1MHz. She said that this was the only method the group are aware of for imaging terahertz radiation in this way.
Downes noted a number of possible applications for the technology, such as beam profiling, mail sorting, and food inspection – for detecting plastic or foreign objects in food on fast production lines, for instance.
The terahertz imaging method is still at an experimental stage, but Downes noted that the laser technology has been miniaturised and it uses standard image sensors, so there is scope for packaging the components into a system.
Terahertz (THz) technologies, generally defined as operating in the 0.1-10THz range, bridge the gap between electronic and photonic devices. Because THz radiation passes readily through materials such as plastics, paper and cloth it can be employed in non-destructive testing, and as it is non-ionising it is considered safe for security and biomedical applications. There is significant demand for high speed THz imaging across a range of applications but, despite ongoing efforts, fast full-field imaging remains an unfulfilled goal. Here we demonstrate a THz imaging system based upon efficient THz-to-optical conversion in atomic vapour, where full-field images can be collected at ultra-high speeds using conventional optical camera technology. For a 0.55 THz field we show an effective 1 cm 2sensor with near diffraction-limited spatial resolution and a minimum detectable power of 190 ± 30 fW s−1/2 per 40x40μm pixel capable of video capture at 3000 frames per second. This combination of speed and sensitivity represents a step change in the state of the art of THz imaging, and will likely lead to its uptake in wider industrial settings. With further improvements we expect that even higher frame rates of up to 1 MHz would be possible.
A new imaging system that uses a laser-excited, room-temperature atomic vapour to convert terahertz radiation to visible light has been created by researchers at the University of Durham in the UK. The system can acquire terahertz images rapidly and efficiently using a conventional high-speed camera and the new technique could make it easier to develop practical technologies that use terahertz radiation.
Terahertz radiation lies in the region of the electromagnetic spectrum between infrared light and microwaves. In principle, it has great promise for a wide range of applications including security screening, medical imaging and industrial quality control. However, generating and detecting electromagnetic radiation at 0.1-10 THz remains an ongoing challenge. Several competing techniques are used for different applications, but they all have disadvantages – and this lack of practical technologies is often referred to as the “terahertz gap”.
So why bother with terahertz radiation when the remaining electromagnetic spectrum is available? Durham’s Kevin Weatherill explains, “It’s a region in which many everyday materials such as paper, plastics and cloth are transparent so, as with X-rays, you can image things that are optically opaque. But being low-energy, the radiation is non-ionizing and therefore safe for biological and medical applications – though it still has a sufficiently short wavelength for reasonably high-resolution imaging.”
Slow and noisy
Several techniques have already been developed for terahertz imaging. Some systems use a single-pixel detector and build-up images by scanning a terahertz beam across the object – which is a slow process. “There are a small band of focal plane arrays or full-field sensors, which can take a 2D image in one shot,” says Weatherill, “Probably the state of the art is an array of microbolometers [thermal sensors]. Their frame rate is limited to about 30 Hz because the sensitivity is low, so you need to collect photons for a long time to see an image above the background noise.”
Weatherill and colleagues created their terahertz imaging system by filling a cell with caesium vapour and focusing three infrared lasers on it. Each laser is precisely tuned to one of three successive atomic transitions in caesium. When excited by these three lasers in succession, caesium atoms end up in a highly excited “Rydberg state”. Such an atom can then absorb a 0.55 THz photon, which puts it in a different Rydberg state that decays after about a microsecond. This decay involves the emission of a green photon, which can be then detected by a standard optical camera.
The 0.55 THz absorption is a sharp resonance, and terahertz radiation at other frequencies will not be detected. Therefore, unlike other techniques for collecting terahertz photons, the technique can reliably pick out a narrowband signal from broad spectrum thermal noise. The detection process is also about 100 times more sensitive than other techniques.
Two-colour imaging
The researchers acquired terahertz images at up to 3000 frames per second. They are now optimizing their equipment and believe that, in principle, it should be possible to collect data at frame rates up to 1 MHz. They are also keen to extend the research in other ways, such as detecting other terahertz frequencies and even two-colour terahertz imaging.
Durham’s Lucy Downes says, “I’m also very keen to try setting this up in reflection mode, so we can look for defects in the surfaces of bulk objects”.
Daniel Mittleman of Brown University in the US, says the most obvious applications of the imaging system are in the laboratory: “Things like explosions, shock wave tests, the fundamental physics of solids and fast, extreme phenomena are where you would need those kinds of frame rates, and any time the material is opaque to optics, terahertz could be an interesting alternative.”
For more commercial applications, he foresees challenges in creating practical devices. “Ultimately, it will be interesting to see how well they can package this for use outside a physics lab. If the applications are fundamental physics, that question becomes irrelevant. If they’re thinking about applications outside the laboratory, that question is relevant and I’ve no idea how to answer it.”
We demonstrate high-quality non-destructive imaging using a broadband terahertz quantum cascade laser source based on Cerenkov difference-frequency generation. The source exhibited ultra-broadband terahertz emission spectra, as well as a single-lobed Gaussian-like far-field pattern at –30 °C. These features allowed us to build a compact imaging system with a high spatial resolution, from which a nearly theoretical minimum beam spot size was obtained. As a result, we achieve well-resolved, high-contrast images of objects obscured by opaque materials. We also achieved terahertz imaging with the THz DFG-QCL operated at room temperature.
Traditional terahertz (THz) equipment faces major obstacles in providing the system cost and compactness necessary for widespread deployment of THz applications. Because of this, the field of THz integrated circuit (THz IC) design in CMOS and SiGe HBT technologies has surged in the last decade. An interplay of advances in silicon process technology, design technique, and microelectronic packaging promises to narrow the gap between the requirements and the reality of system cost and performance of THz components. Furthermore, the scalability, reconfigurability, and signal processing features of silicon technology have initiated research in complex THz ICs that expand the functionality of THz systems; this has enabled new applications, methods, and algorithms. This paper reviews the progress in THz IC research and investigates several realizations of THz imaging and sensing applications with silicon-based components regarding their motivation, system performance, and challenges. THz computed tomography, broadband multicolor imaging, high-resolution FMCW radar imaging, subwavelength resolution near-field imaging, and compressed sensing are presented.
In this paper, the beam-steering characteristics of photoconductive dipole phased array antenna configuration at 1.95 THz is presented. The proposed array antenna configuration with frequency selective surface favourably improves its gain and directivity which is useful to upsurge the imaging capabilities to address the deliberations such as limited depth-of-field (DoF) and size-weight-and-power of the THz source for imaging applications. These are important considerations for applications like stand-off imaging and surveillance of moving targets where the high angular resolution as well as extended DoF are the important parameters for successful detection of concealed explosives. The projected planar profile and compact highly directive (2×2) small-gap photoconductive dipole phased array antenna can be castoff for the exposure of concealed explosives such as RDX, TNT, and HMX which illustrate their substantial spectral absorption fingerprints in terahertz (1.4–2.2 THz) regime of the spectrum. A simple method of beam-steering has been explored based on phase controlled optical excitation of highly directive small-gap photoconductive dipole array antenna. Further, the effects of uniform progressive phase shift on the beam-steering of uniform linear array (along x-axis) as well as planar array (x-axis and y-axis) is investigated.
In this paper, we introduce two convex-plano lenses for terahertz (THz) imaging applications. The first lens is determined to use an aspherical surface and compared with a similar spherical lens that has the same diameter, thickness, and back focal length. We demonstrate the superiority of aspherical surface by electromagnetic simulations and focal plane scanning experiments. The second lens has a relatively shorter focal length and has excellent focusing capability. By comparing with other two previous studies, we estimate that this lens may achieve high-resolution in THz imaging applications.
François Blanchard, Xin Chai, Tomoko Tanaka, Takashi Arikawa, Tsuneyuki Ozaki, Roberto Morandotti, and Koichiro Tanaka
Fig. 1. Experimental setup. (a) THz transmission scheme using wire-grid polarizer and a LN sensor. (b) Illustration of the sample used for near-field investigations. (c) Visible image of a gold ring array structure patterned on a In0.53Ga0.47As epilayer thin film.
Terahertz (THz) imaging is currently based on linear effects, but there is great interest on how nonlinear effects induced by terahertz radiation could be exploited to provide extra information that is unobtainable by conventional imaging schemes. In particular, at field strengths on the order of 100kVcm−1 to 1MVcm−1, transmission properties inside semiconductor materials are largely affected at the picosecond time-scale, which raise the prospect of interesting nonlinear imaging applications at THz frequencies. Here, we experimentally investigate a method to map the two-dimensional nonlinear near-field distribution of an intense THz pulse passing through a thin film-doped semiconductor. By inserting a metamaterial structure between the electro-optic sensor and the doped film, the nonlinear near-field dynamics shows a different and enhanced contrast of the sample when compared to its linear counterpart.
