A repository & source of cutting edge news about emerging terahertz technology, it's commercialization & innovations in scanners (quality control, non-destructive inspection, diagnostics,& security) as well as in astronomy, communications, graphene, metamaterials, CMOS, compressive sensing and the move to solid state. NOTHING POSTED IS INVESTMENT,OR OTHER ADVICE REPOSTED COPYRIGHT IS FOR EDUCATIONAL USE. I am a retail investor in THz.
Terahertz (0.1 THz-30 THz) radiation reveals a wealth of information that is relevant for material, biological and medical sciences, with applications that span chemical sensing, high-speed electronics and coherent control of semiconductor quantum bits. To date, there have been no methods capable of controlling THz radiation at molecular scales. Here we report both generation and detection of broadband terahertz field from 10-nm-scale oxide nanojunctions. Frequency components of ultrafast optical radiation are mixed at these nanojunctions, producing broadband THz emission. These same devices detect THz electric fields with comparable spatial resolution. This unprecedented control, on a scale of four orders of magnitude smaller than the diffraction limit, creates a pathway toward THz-bandwidth spectroscopy and control of individual nanoparticles and molecules.
We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ∼20–160 kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ∼160 kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs. Furthermore, we show that the effective permittivity can be swept from negative to positive values with an increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.
M. Misra1, Y. Pan1, C. R. Williams1, S. A. Maier2, and S. R. Andrews1
1Department of Physics, University of Bath, Bath BA2 &AY, United Kingdom 2Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
We describe the design and performance of a freely positionable THz near field probe based on a hollow core photonic crystal fibre-coupled photoconducting dipole antenna with an integrated sub-wavelength aperture. Experimental studies of the spatial resolution are compared with detailed finite element electromagnetic simulations and imaging artefacts that are a particular feature of this type of device are discussed. We illustrate the potential applications with descriptions of time domain near field studies of surface waves on a metamaterial and multimode propagation in a parallel plate waveguide.
http://www.sciencemag.org/content/early/2013/05/15/science.1235399.abstract Polarization is one of the basic properties of electromagnetic waves conveying valuable information in signal transmission and sensitive measurements. Conventional methods for advanced polarization control impose demanding requirements on material properties and attain only limited performance. Here, we demonstrate ultrathin, broadband, and highly efficient metamaterial-based terahertz polarization converters that are capable of rotating a linear polarization state into its orthogonal one. Based on these results, we create metamaterial structures capable of realizing near-perfect anomalous refraction. Our work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities for applications in the technologically difficult terahertz frequency regime.
Terahertz (THz) radiation has uses from security to medicine, however sensitive room-temperature detection of THz is notoriously difficult. The hot-electron photothermoelectric effect in graphene is a promising solution: photoexcited carriers rapidly thermalize due to strong electron-electron interactions, but lose energy to the lattice more slowly. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating or dissimilar contact metals produces a net current via the thermoelectric effect. Here we demonstrate a graphene thermoelectric THz photodetector with sensitivity exceeding 100 V/W at room temperature and noise equivalent power (NEP) less than 100 pW/Hz^1/2, competitive with the best room-temperature THz detectors, while time-resolved measurements on similar devices indicate our graphene detector is more than seven orders of magnitude faster. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.
A novel plasmonic THz fiber is described that features two metallic wires that are held in place by the porous dielectric cladding functioning as a mechanical support. This design is more convenient for practical applications than a classic two metal wire THz waveguide as it allows direct manipulations of the fiber without the risk of perturbing its core-guided mode. Not surprisingly, optical properties of such fibers are inferior to those of a classic two-wire waveguide due to the presence of lossy dielectric near an inter-wire gap. At the same time, composite fibers outperform porous fibers of the same geometry both in bandwidth of operation and in lower dispersion. Finally, by increasing cladding porosity one can consistently improve optical properties of the composite fibers.
A new method of producing 3D images could improve cancer screening tests, scientists have said.
The technology uses four single-pixel detectors and has been developed at University of Glasgow's physics and astronomy department. It allows for the creation of 3D pictures without expensive digital cameras.
The technique is called 3D computational imaging, or ghost imaging, and can detect wavelengths that digital cameras cannot pick up. It could be used to look under the skin to detect cancer or other medical conditions quickly.
Oil industry firms could also use the technology to detect gases hidden from the naked eye that leak from the ground near oil wells.
Professor Miles Padgett, Kelvin chair of Natural Philosophy at the university, said: "Single-pixel detectors in four different locations are used to detect light from a data projector which illuminates objects with a rapidly shifting sequence of black-and-white patterns similar to crossword puzzles.
"Four detectors give images, each of which contain shadows, giving us clues about the 3D shape of the object. Combining the four images using a well-known technique known as shape-from-shade allows us to create a full 3D image of the object."
Details of the discovery appear in a report entitled 3D Computational Imaging With Single-Pixel Detectors in the current Science journal.
Research assistant Matthew Edgar, who contributed to the paper, said: "A more portable version of the system could be created quite easily, making it much more practical to use outside the lab. It could be used to look for the tell-tale gases which leak from the ground where oil can be found, for example, or it could be tuned into the terahertz range to probe just below the skin to search for tumours or other medical conditions."
The technology is in its early stages and commercial partners are being sought to help develop it.
Baoqing Sun, lead author of the paper, said: "This means that single-pixel detectors which cost just a few pounds each are now capable of producing images across a far wider spectrum than 3D imaging systems currently on the market which cost tens of thousands of pounds."
In1771, the young Spanish painter Francisco Goya travelled to Rome to learn and be inspired by the many great artists there. In that year, he is thought to have painted “The Sacrifice to Vesta” depicting a sacrifice to the goddess of fire. The work is important because it shows how Goya was developing artistically at a formative stage in his career.
There is little dispute that Goya is the author of this work– the artist’s skill and style are clear and various experts have confirmed the attribution. However, the painting lacks a signature, a feature that would leave nothing to debate.
Today, Cristina Seco-Martorell at the University of Barcelona in Spain and a few buddies say they have discovered Goya’s signature hidden behind a layer of varnish in the picture. And the trick they used to make this discovery is an entirely type of analysis using terahertz waves.
Terahertz radiation occupies the part of the electromagnetic spectrum between the infrared and the microwave. Until recently, it had been largely neglected because of the difficulty in producing and detecting terahertz waves.
All that has changed in recent years thanks to a new generation of cameras capable of producing and sensing this type radiation.
Seco-Martorell and buddies imaged the painting by dividing it into 1 mm square pixels. They created their image by recording the terahertz reflection from each pixel and then assembled them into a full image of the painting.
The resulting images are much more revealing than those created using infrared or x-rays. Terahertz waves can be reflected from various layers within a painting giving a three-dimensional picture of how the artwork was put together. At the same time, terahertz waves are selectively absorbed and reflected by different types of pigments and materials and this gives spectroscopic information about the nature of the paints that were used.
The big surprise in this analysis is the discovery of a signature in the bottom right-hand corner of the painting. Seco-Martorell and co say their images clearly reveal the G and the “ya” of Goya’s signature, although the “o” is not visible.
This signature does not show up on an X-ray of the picture that was taken in 2007. Seco-Martorell and pals say this is easily explained. “It is likely that the signature was written using a pencil (basically carbon) and that the painting was covered by a top layer of finishing varnish that turned dark over time, hiding the signature to optical inspection,” they say.
And since the atomic weight of carbon in the signature and in the surrounding canvas and paint is very similar, an x-ray would not have picked up the difference.
The signature clearly removes any remaining doubt about the author of this work. It also establishes terahertz imaging is an important new tool for analysing paintings.
An interesting corollary is that this technique has even greater potential. The spectroscopic capabilities of terahertz imaging ought to allow accurate identification of pigments and other materials used in the painting. For example, in the images above, the areas of greater reflectivity probably indicate pigments with a higher metallic content.
However, identifying the exact chemicals and pigments involved is only possible by comparing the results to a database of known chemical signatures and this does not yet exist for artistic materials such as pigments. So there is work for the future.
In the meantime, it’s not hard to imagine that art historians around the world will want to get their hands on terahertz imaging machines in the hope of identifying other unexpected features in famous paintings.
Two independent groups of researchers come up with a new kind of laser
By Neil Savage
Posted
Image: NaturePolariton Pillars: The 2-micrometer-wide pillars are key to confining polaritons in a new kind of electrically powered laser.
A new type of laser has the potential to be much more energy efficient than conventional lasers, according to two groups of scientists who separately came up with very similar designs for it.
Known as a polariton laser, the device isn’t, strictly speaking, a laser at all. Conventional lasers work through stimulated emission of radiation: Electrons in a laser cavity are raised to a high-energy state, and when they drop to a lower state, they emit the excess energy as photons, producing a coherent beam of light.
This new device, however, is based on the stimulated scattering of polaritons. A polariton is a “quasiparticle,” a mixture of an electron-hole pair (also known as an exciton) and a photon, which can exist only within a crystal. When energy is pumped into the system, the exciton-polaritons absorb it and then quickly release it as photons—the stimulated scattering that creates the laser beam. In a conventional laser, the majority of electrons must be in a high-energy state before lasing can begin. Such a “population inversion” isn’t required with polariton lasers, so it takes less energy to run them.
Since 1996, when the concept was first described, researchers have built polariton lasers that used light from other lasers to pump energy into the system. Now two groups, working independently, have built devices that run on electricity, a key step in turning any laser from a laboratory curiosity into something useful.
“This is a big deal,” says Pallab Bhattacharya, a professor of engineering at the University of Michigan, whose paper describing his team’s work appears online in Physical Review Letters on Wednesday. “A real device is one that is electrically injected. This makes it a practical device.”
“It is not a laser in the common sense, but it shares a lot of characteristics with the conventional laser,” says Sven Höfling, a researcher at the University of Würzberg, in Germany. He and colleagues from Iceland, Japan, Russia, Singapore, andthe United States published a paper on their work in Thursday’s issue of Nature.
Illustration: NatureReflected Glory: The polariton laser cavity contains for distributed Bragg reflectors (DBRs) surrounding a quantum well (QW) made of indium gallium arsenide.
One major difference between conventional and polariton lasers is that the lasing threshold—the amount of energy it takes to stimulate the light emission—is orders of magnitude lower in polariton lasers. Bhattacharya says these early prototypes have a threshold current of 12 amperes per square centimeter and will presumably improve. By comparison, he says, it took years of research to make advanced experimental lasers based on quantum dots—tiny clumps of semiconductor material—with a similar threshold current.
Polariton lasers can also be switched on and off much faster than conventional lasers can, Battacharya says, which allows signals to be encoded onto the laser beam very quickly. That means the laser might be useful for low-power (and therefore less-expensive) optical telecommunications and light amplification. Polariton lasers might also be used to trigger lasing at terahertz wavelengths. They could be used to build cheaper, more compact terahertz lasers, which could be a safer alternative to X-ray scanners in spectroscopy and security applications.
Alexey Kavokin, a physicist at the University of Southampton, in England, who is familiar with Höfling’s results, says polariton lasers might also be used in optical logic circuits. Because they can switch from on to off or the reverse in mere picoseconds, and because the polarization of their light might be controlled, polariton lasers would make excellent AND and NOT gates, he says. But these lasers have limits. For example, Kavokin says, they aren’t candidates for high-power applications, such as cutting and welding, because pumping more energy into them destroys the polaritons and thus ends the lasing effect.
Both groups’ lasers were built using gallium arsenide, and both use a magnetic field to make the scattering more efficient. Both also operate only at extremely low temperatures, on the order of 30 kelvins. Höfling says the next step in the research will be to try to build an electrically pumped polariton laser that operates at room temperature; optically pumped room-temperature versions already exist. The lasers are still at a fairly basic stage of research, he says, and it could be a long time before anyone builds a commercial polariton laser.
Both Höfling and Bhattacharya were surprised to learn of each other’s papers, which were released in the same week. “That’s a tremendous coincidence, and a big deal. We validate each other,” Bhattacharya says. “The two taken together, it’s a big boost for the field.”
The frequency and fluence dependent transient photoconductivity in ternary CdSSe nanobelts is investigated using time-resolved terahertz spectroscopy. Carrier density and mobility are extracted by modeling the measured complex photoconductivity using Drude-Smith model. Within the first few picoseconds of excitation, both carrier density and mobility reach their maximum values and then decay gradually over tens to hundreds of picoseconds. The decay of free carriers is mainly attributed to fast surface trapping and structural-defect mediated recombination. The surface trapping saturates rapidly with increasing excitation fluence attributable to the low trapping density on the nanobelt surface caused by self-passivation of surface defects during the growth process.
Skin cancer accounts for nearly half of all cancers in the United States. More than 2 million cases of non-melanoma skin cancer are found in this country each year.
Around 40-50% of Americans who live to the age of 65 will develop skin cancer at least once.
Terahertz radiations are known to be safe but due to the lack of commercially available continuous wave terahertz sources, most medical research in terahertz imaging thus far has been focused on terahertz pulsed imaging (TPI).
Technology
UMass Lowell researchers have developed a Terahertz/Optical-based imaging system for cancer detection, especially in skin.
Continuous wave terahertz imaging (CWT) has the potential to differentiate between nonmelanoma skin cancers and normal skin. Use of cross-polarized terahertz reflection to enhance contrast helps in to clearly identify the cancerous area of the sample by eliminating Fresnel reflection and imaging deeper into the tissue volume.
Combination of Terahertz with Polarized light imaging at optical frequencies allows high sensitivity to cancer tissue provided by terahertz imaging as well as high resolution imaging of tissue morphology and chromophores, afforded by optical imaging techniques.
Cross-Polarized terahertz Image
Cross-Polarized Optical Image
Histopathology
Squamus Cell Carcinoma Imaging comparison with histopathology
Applications
Tumor surgical margin detection in
Squamous Cell Carcinoma
Basal Cell Carcinoma
Competitive Advantages
Optical imaging offers higher resolution, which enables identification tissue morphology.
Combining terahertz and optical interrogation of skin lesions shows promise for accurate delineation of skin cancers
Significant improvement of the terahertz image quality due to the rejection of Fresnel reflections.
Cross-polarized imaging has long been a staple in optical domain, but has never been used in terahertz biomedical imaging, most likely due to erroneous assumption that terahertz radiation does not penetrate into the biological tissue deep enough to make cross-polarized imaging valuable.
Significant improvement of terahertz image quality the image quality due to probing deeper into the tissue and collecting signals from larger volumes without contamination of the detected radiation by specular reflection.
Terahertz imaging is sensitive to water content and offers intrinsic contrast between normal and cancerous skin
References
Joseph CS, Yaroslavsky AN, Neel VA, Goyette TM, Giles RH. Continuous wave terahertz transmission imaging of nonmelanoma skin cancers.Lasers Surg Med. 2011 Aug;43(6):457-62. doi: 10.1002/lsm.21078.
Joseph CS, Patel R, Neel VA, Giles RH, Yaroslavsky AN. Imaging of ex vivo nonmelanoma skin cancers in the optical and terahertz spectral regions Optical and Terahertz skin cancers imaging.J Biophotonics. 2012 Sep 14. doi: 10.1002/jbio.201200111
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