A repository & source of cutting edge news about emerging terahertz technology, it's commercialization & innovations in THz devices, quality & process control, medical diagnostics, security, astronomy, communications, applications in graphene, metamaterials, CMOS, compressive sensing, 3d printing, and the Internet of Nanothings. NOTHING POSTED IS INVESTMENT ADVICE! REPOSTED COPYRIGHT IS FOR EDUCATIONAL USE.
http://onlinelibrary.wiley.com/doi/10.1002/adma.201503350/full Extreme sensitivity of room-temperature photoelectric effect for terahertz (THz) detection is demonstrated by generating extra carriers in an electromagnetic induced well located at the semiconductor using a wrapped metal–semiconductor–metal configuration. The excellent performance achieved with THz detectors shows great potential to open avenues for THz detection.
Part of the experimental setup for laser-based vector network analysis. A cluster of planar waveguides has been fabricated on a semiconductor. The waveguides are contacted at either end using two microwave probes. A laser beam is focused from the front onto the waveguide and generates in a photoconductive gap ultrashort voltage pulses. The voltage pulses are detected by a second laser beam which is focused from the back through the substrate onto the planar waveguide. Credit: PTB
Vector network analyzers (VNA) are among the most precise high-frequency measurement devices available today. Due to continuous development within the last decades VNAs are usable up to frequencies of 1 terahertz (1012 Hz) and complex error correction algorithms exist. However, VNAs are very expensive and require multiple frequency extenders in order to cover a wide frequency range. At the Physikalisch-Technische Bundesanstalt (PTB) a VNA has been developed which utilizes optoelectronic techniques based on femtosecond lasers. Such devices constitute a cost-effective alternative to conventional VNAs and might be used for high-frequency measurements in the future. The results have been published in the present issue of the renowned journal IEEE Transactions on Microwave Theory and Techniques.
The measurement principle of VNAs relies on the detection of power waves at discrete frequencies. Variation of the frequency allows frequency-resolved measurements. The measurement results are typically specified in terms of scattering parameters. In order to characterize a high-frequency device accurately, forward and backward propagating signals have to be separated, which is accomplished using directional couplers.
At PTB, researchers have shown that frequency-resolved scattering parameter measurements can also be realized using laser-based measurement techniques. For this purpose, a femtosecond laser emitting pulses approximately 100 femtoseconds (10-13 s) long in the near infrared is utilized. The laser beam is divided into a pump beam and a probe beam. The pump beam excites a so-called photoconductive switch integrated in a planar waveguide. This excitation leads to voltage pulses approximately 2 picoseconds long propagating on the planar waveguide. The probe beam is used to detect the electric field of the voltage pulses, employing the Pockels effect of the substrate on which the planar waveguide is fabricated. By changing the time delay between pump and probe beam through the use of a delay line, the shape of the voltage pulse can be accurately measured.
As main innovation, the PTB researchers have developed a laser-based measurement method allowing the separation of forward and backward propagating signals on the planar waveguide. This technique is the equivalent of directional couplers used in conventional VNAs. The separation, which requires the detection of voltage pulses at different positions on the planar waveguide, even works in the case of temporally overlapping forward and backward propagating signals. With the new optoelectronic time-domain measurement method, scattering parameter measurements on planar waveguides up to 500 GHz with a 500 MHz frequency spacing have been demonstrated. The method can also be utilized for the characterization of coaxial high-frequency devices and for the realization of a very precise voltage pulse standard.
More information: M. Bieler, H. Füser, and K. Pierz: Time-Domain Optoelectronic Vector Network Analysis on Coplanar Waveguides. IEEE Transactions on Microwave Theory and Techniques, vol. 63, no. 11, pp. 3775–3784, Nov. 2015
Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of −0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals.
A system for coupling teraherz (THz) radiation to a coaxial waveguide comprises an antenna that generates THz radiation having a mode that matches the mode of the waveguide. The antenna may comprise a pair of concentric electrodes, at least one of which may be affixed to or formed by one end of the waveguide. The radiation may have wavelengths between approximately 30 μm and 3 mm. The waveguide may comprise an inner core and an outer wall defining an annular region. A terahertz sensor system may comprise a terahertz antenna comprising first and second concentric electrodes, means for generating a field across the trodes and means for triggering the emission of terahertz radiation, a first waveguide having first and second ends, said first end being coupled to said antenna so as to receive at least a portion of said terahertz radiation, and a sensor for detecting said terahertz radiation
The edge of the Horsehead nebula, where it touches the empty space outside it, is rich in carbon. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
The carbon cycle is central to life on Earth. It describes how carbon flows between living organisms, and the ocean, atmosphere and rock of our planet, and is driven by the energy from our sun.
But a carbon cyclealso exists for our galaxy, and astronomers are opening new windows into space that let us watch this galacticcarbon ecosystemin action.
However, the light from carbon in space can be very hard to see because most of it is blocked by the Earth's atmosphere. But now a new telescope built in one of the most remote regions of our planet is letting us see cosmic carbon in a new light.
Game of millimetres
All elements in the universe emit light with a characteristic fingerprint in the form of emission lines. So just by teasing apart the spectrum of the light received from space, astronomers can determine what elements are out there.
Interstellar carbon comes in several forms. It is sometimes missing an electron, making it ionised. In this state it emits the brightest single spectral line produced by entire galaxies.
Carbon can also be found in atomic form as single atoms. Such atoms reside in the surfaces of molecular clouds, near to the interfaces with atomic gas. Or the carbon can be incorporated into molecules. Here it is primarily found as carbon monoxide, CO, the second most abundant molecule in the universe after hydrogen in the form of H₂.
Carbon monoxide emits in the millimetre portion of the electromagnetic spectrum. This can be readily studied, such as by the Mopra telescope in Australia, which is charting a new map of the molecular clouds of our galaxy.
The astronomical observatory at Ridge A, near the summit of the Antarctic plateau. The yellow box is the PLATO-R instrument module. The HEAT telescope is to the left and the solar cube to right. Credit: Craig Kulesa/University of Arizona
However, water absorbs the wavelengths of light emitted by ionised or atomic carbon, which makes it hard to see it from here on Earth. This means we must use airborne or space telescopes, which is an expensive proposition.
A small amount of terahertz radiation does penetrate to the ground at the driest locations on the Earth's surface. One example is the high Altiplano of Chile, where the giant ALMA radio telescope is being built. But the transmission is patchy and the signal variable.
Cold heightsThe very driest and coldest place on Earth surface is the summit of theAntarctic plateau. Here, through the long darkness of winter, the terahertz windows are opened. But this is a challenging environment to work in, to say the least.
Two decades of Antarctic development at the University of New South Wales, and three generations of autonomous laboratories have led to PLATO, the PLATeau Observatory. One module is now operating at Ridge A on top of the Antarctic plateau. With our partners at the University of Arizona building a new telescope to go with it, HEAT, the High Elevation Antarctic Terahertz telescope, we now are able to exploit the spectacularly dry, cold and stable conditions for astronomy.
HEAT can measure the terahertz lines of carbon. The telescope is fixed on the ice and records the signal as the sky rotates about it. HEAT is building up a map of carbon in the galaxy in strips, "day" by "day".
After two years of mapping the team have produced the first high resolution maps of carbon in the galaxy.
Craig Kulesa hard at work on the HEAT terahertz telescope at Ridge A near to the 4,000m summit of the Antarctic plateau. Credit: Craig Kulesa/University of Arizona
These maps need to be compared to other species in the interstellar medium. We use the Mopra telescope to see carbon monoxide. And the Parkes and Australia Telescope Compact Array (ATCA) for atomic hydrogen.
Using these tools we have been able to see where clouds of atomic carbon transition into clouds to molecular carbon. In one location, a filamentary molecular cloud over 200 light years in extent but no more than 10 light years across appears to be condensing out of a surrounding atomic substrate.
No clear sign of star formation is seen in this cloud. Its gas is incredibly cold and quiescent. It could be the first molecular cloud to be seen still in the process of formation.
We are also beginning to learn about a new component of the interstellar medium: the dark molecular gas. Here carbon exists but carbon monoxide is absent. Perhaps one-third of the molecular gas resides in this dark form.
One major element is still missing from this puzzle: the contribution from ionised carbon, because its emission occurs in an even harder part of the terahertz window to monitor. The next stage in our venture will open that window.
The USA will launch a balloon-borne telescope that will circumnavigate the Antarctic continent, driven by the winds of the polar vortex. It will be followed by a telescope built by China on the very summit of the Antarctic plateau, at the new Kunlun Observatory at Dome A.
Bit by bit, we're building the telescopes necessary to help shed light on carbon in space, and thus illuminate the grand carbon cycle that influences the evolution of the galaxy around us.
Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains big challenges to develop intense, coherent and tunable THz radiation sources that can cover the whole THz frequency region either by means of only electronics (both vacuum electronics and semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here we present a mechanism which can overcome these difficulties in THz radiation generation. Due to the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric, making transformation of SPPs into radiation immediately possible. The dual natural periodicity also brings significant excellences to the excitation and the transformation. The fundamental and hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate along the cyclotron trajectory together with the beam and gain energy from the beam continuously. The radiation density is enhanced over 300 times, up to 105 W/cm2. The radiation frequency can be widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for developing desired THz radiation sources to cover the whole THz frequency regime.
