NASA chooses Dutch terahertz detectors

Out of 32 proposals, NASA decided on the GUSTO mission as the project of choice to launch in December 2021. The terahertz sensors aboard the balloon-launched mission will be made in the Netherlands.

http://delta.tudelft.nl/artikel/nasa-chooses-dutch-terahertz-detectors/32957

NASA wants to untangle the complexities of the dust in between stars, as photographed here by the Hubble Space Telescope.

Dr. Jian Rong Gao was excited by the NASA press release last Friday. It says that the Gusto mission was selected as the mission to be built in the next few years.

'NASA has determined that Gusto (Galactic/extragalactic ULDB spectroscopic terahertz observatory) has the best potential for excellent science return with a feasible development plan,' says the press release.

The terahertz sensors on the mission will be made at Gao's lab at the section quantum nanoscience from the TU Delft Faculty of Applied Sciences (TNW) in collaboration with the Dutch organisation for space research SRON.

The sensors have been tried and tested in the STO2 mission (Stratospheric Terahertz Observatory) that was launched shortly before Christmas last year. The sensors are tuned to 1.4, 1.9 and 4.7 THz respectively to observe the presence of nitrogen, carbon and oxygen.

The 2021 mission will feature three eight-pixel cameras and other equipment necessary for the ultra-sensitive detection of terahertz radiation from the cosmic material between the stars. Dr. Christopher Walker of the University of Arizona will be the principal investigator on the project.

NASA says it has selected a science mission that will measure emissions from the interstellar medium. This data will help scientists determine the life cycle of interstellar gas in our Milky Way galaxy, witness the formation and destruction of star-forming clouds, and understand the dynamics and gas flow in the vicinity of the centre of our galaxy.

STO2 landed and data secured

The STO2 first light spectrum at 1.9 THz. Credit: Delft University of Technology

https://phys.org/news/2017-01-sto2.html

The STO2 telescope with Dutch detectors on board that circled around the South Pole in December 2016 to investigate gas clouds between the stars landed safely on 30 December.

At an altitude of 39 kilometers the NASA telescope circled along with the polar vortex for a period of three weeks. During that time STO2 picked up as much radiation as possible at the frequencies of 1.4 and 1.9 THz to find ionized nitrogen (NII) and ionized carbon respectively (CII) in a part of our Milky Way. These substances indicate the process of star formation from dust and gas.

Measuring oxygen

The 4.7 THz detector that would measure neutral atomic oxygen (OI) also worked. However, something went wrong in the system for the local oscillator that had to generate the required reference signal of 4.7 THz. An electrical component needed for the communication between this local oscillator and ground control became overheated by the sun. OI reveals that a star is actually being born. This is an observation that astronomers are keen to obtain, especially if that observation is being done for the first time beyond the earth's atmosphere, as would have been possible with STO2.

STO2 project leader for SRON and TU Delft researcher Jian-Rong Gao and his team are indeed disappointed about the absence of the 4.7 THz observations but on the other hand they are extremely happy with the large quantity of data for the other two frequencies. After an initial hiccup in the orientation mechanism of the telescope, the collection of that data proceeded really well. "Once the rough data have been processed to reveal spectral lines for CII and NII then STO2 will have drastically expanded the area mapped so far for these substances."

Mission continues

STO2 was launched from Antarctica on 9 December 2016. The polar vortex also ensures that the balloon missions land again at a location that can be reached along the South Pole Traverse, a sort of Antarctic 'motorway' between the South Pole and McMurdo. When the cooling fluid for the superconducting detectors (liquid helium) had been used up and the data was safely downloaded to computers on earth, STO2 landed on the South Pole Transverse. The telescope was picked up on 10 January so that it could be brought back to McMurdo.

STO2 is an exploratory mission under the leadership of the University of Arizona for astronomy in these terahertz frequencies. On 24 January 2017, NASA will visit the University of Arizona to decide about GUSTO. This is also a balloon mission but with a longer duration (about 100 days) and with more effective instruments on board. For NII, CII and OI, GUSTO will have cameras with eight pixels that will once again be developed by SRON and Delft University of Technology.

The teams of professor Alexander Tielens (Leiden University) and professor Floris van der Tak (SRON/University of Groningen) will contribute to the scientific analysis of the observations.

Polar balloon STO2 to go the edge of space with Dutch instruments

Credit: Jian-Rong Gao
http://phys.org/news/2016-12-polar-balloon-sto2-edge-space.html

Stars and planets are born from clouds of molecules that coagulate and eventually fall apart again in the space between the stars in a galaxy. Astronomers still do not know exactly how this works.
That is why NASA's stratospheric balloon STO2 will be launched from Antarctica to the edge of space to measure cosmic far infrared radiation. At an altitude of 40 kilometers above Antarctica, the air is crystal clear. There is scarcely any water vapor, which often blocks this type of radiation at other locations in the atmosphere.

The NASA balloon that will carry the measuring instruments to this altitude will make use of the circular polar vortex, a stable airflow on which the balloon can circulate with for one or more rounds of about 14 days each.

This will allow scientists to carry out observations for a period of two weeks before they find the balloon at nearly the same location again. STO2 has been developed under the leadership of the University of Arizona and contains vital contributions from SRON Netherlands Institute for Space Research (Utrecht and Groningen) and tech university TUDelft. These are three receivers for 1.4, 1.9 and 4.7 terahertz respectively.

Spectra of radiation at these frequencies often disclose the presence of elements in space, including electrically neutral atomic oxygen. The localization of that last element in space, which can be achieved using the 4.7 terahertz receiver, is a long-cherished dream of astronomers. It is the first time a 4.7 terahertz receiver will be brought to the edge of space for an unrestricted view. Together with the Massachusetts Institute of Technology (MIT), the partners developed a reference source for radiation at this frequency. Electrically neutral atomic oxygen reveals us places in the gas clouds between stars that are particularly warm.

Credit: Delft University of Technology

This is a good indicator for stars that only just formed. This way we can directly find the birthplaces of new stars. STO2 is therefore an important scouting mission for future terahertz missions using a satellite in space. Far infrared radiation is sometimes also referred to as terahertz radiation. One terahertz is equivalent to a wavelength of 300 micrometers. The University of Arizona is scientifically in the lead of the mission. The teams of prof. dr. Alexander Tielens (Universiteit Leiden) and prof. dr. Floris van der Tak (SRON/Rijksuniversiteit Groningen) will help in the international scientific analysis of the observations. Thursday the team on Antarctica gets three hours of good weather conditions. If this is too short, nice launching weather will follow in the following days.

TU Delft-Terahertz scanning probe microscope

http://www.tudelft.nl/en/business/tu-delft-patent-portfolio/high-tech-systems-and-materials/terahertz-scanning-probe-microscope/

A wider range of materials can be researched

New materials require new instruments to understand and measure their complex electronic properties, in particular when they do not directly correlate with atomic positions.

By integrating a lens and an antenna in an atomic force microscope much higher frequencies can be applied to locally measure the complex impedance. Compared to previous solutions a wider range of materials can be researched with this microscope. >> Read more...  

Advantages:

• Frequencies up to several THz can be applied without high radiation losses
• The electronic properties of a wider range of materials can be measured, for instance, superconductors, polymers, magnetic materials and biological materials
• A wide frequency can be applied, from 10 GHz to several THz
• The assembly makes the cantilever a modular item which can be used with an existing Atomic Force Microscope
• 
  

TU Deflt-Terahertz generation from graphite

 (This work was performed by Gopakumar Ramakrishnan)

http://www.tnw.tudelft.nl/en/about-faculty/departments/imaging-physics/research/researchgroups/optics-research-group/research/thz-science-technology/thz-projects/thz-generation-from-graphite/

We illuminated graphite crystals with femtosecond laser pulses (wavelength centered around 800 nm, duration 50 fs) and found that they are capable of emitting THz pulses. This is somewhat unsual since the crystal symmetry of graphite prohibits second-order nonlinear optical processes. The crystals we used to measure the emission of THz pulses after illumination are so-called Highly Oriented Pyrolytic Graphite (HOPG) crystals. Rather than consisting of a single crystal, they consist of many microcrystallites with their c-axis oriented more or less in the same direction. An exaggerated illustration of this is shown below:

The degree of aligment of these crystallites is expresses in terms of the "Mosaic spread". Incidentally, other forms of graphite were also found to emit THz pulses upon illumination with femtosecond laser pulses, including pencil lead. A typical setup used to generate and detect the THz pulses is shown schematically here:

The crystal we used is a so-called rectangular cuboid crystal which we illuminated on all sides. Basically there are two kind of surfaces relevant here: the basal plane surfaces and the edge plane surfaces. The normal to the basal plane is parallel to the c-axis, whereas the normal to the edge plane is perpendicular to the c-axis. In the first experiments we used basal plane illumination like this:

This give rise to the emission of a THz pulses which look like this (left figure):

The figure on the right shows the dependence of the THz field amplitude on the pump power. At low powers, the dependence is clearly quadratic, indicative of a second-order nonlinear optical process. It's important to note that only a small signal is measured when the pump beam is perpendicularly incident on the sample. This is consistent with a picture in which the THz transient is generated by a current perpendicular to the basal plane surface (and thus parallel to the c-axis). To provide further evidence for this, we have also applied an in-plane magnetic field to the sample. If the idea about carrier transport along the c-axis is correct, a change in the direction of the current by the magnetic field should give rise to a change in the emitted THz field. In this case, the change is from "very little emission" to either a positive or a negative-going E-field.

The blue and the red curve on the right show the emitted THz electric field for two magnetic fields of opposite orientation. Clearly, reversing the magnetic field direction changes the polarity of the emitted THz pulse. This is not in contradiction with our earlier idea that after optical excitation, a current pulse along the c-axis is acquires an in-plane component due to the application of the magnetic field.
So then, do we understand everything? Well, not exactly. If we illuminate the edge plane surfaces, we also observe the emission of a THz pulse but in this case we can only speculate where this is coming from. One idea is that the THz pulse is emitted by carriers accelerated in static built-in potentials caused by stacking faults in the crystal, but this is speculation at this point. Finally, we already mentioned that pencil lead emits a THz pulse after illumination with a femtosecond laser pulse. In fact a pencil drawing on paper also emits THz light after illumination with a femtosecond laser pulse. This is illustrated in the drawing below where we plot the THz amplitude measured along a line across two lines drawn on paper:

Yield from graphene much higher than first thought

http://www.tnw.tudelft.nl/en/current/latest-news/article/detail/opbrengst-terahertz-licht-via-grafeen-stukker-hoger-dan-gedacht/

Generating terahertz light via graphene may soon be an interesting alternative to the current methods used to generate THz light. This is the conclusion reached by researchers in the Optics Research Group and recently published in an article in ACS Nano.

‘We have discovered that after illuminating it with an ultrashort (femtosecond) laser pulse, a single layer of graphene emits a short flash of terahertz light,’ says Prof. Paul Planken. Light with a terahertz frequency (around 1012 Hz) can be used for a wide range of practical applications, making this an interesting development for advancements in imaging and spectroscopy, for example. The main advantage is that light of this frequency is not harmful, unlike X-rays. ‘The light can be used to look inside and through packaging, for example, and to analyse works of art.’

Relatively strong

Terahertz light is normally generated via completely different methods, such as by using antennas fabricated on semi-conductors. The idea of illuminating ultrathin graphene layers has come up before, but it was thought that the THz light yield would be too low. Planken: ‘In absolute terms, the terahertz light is weak, but considering the fact that graphene is just 1 atom thick, it is actually relatively strong. The mechanism responsible for the partial conversion of laser light into THz light occurs when ultrafast electric impulses are directly generated in the graphene as, in a manner of speaking, the light particles in the laser pulse collide with the electrons in the graphene.’

Gold

‘Even more important is the discovery that when we lay the graphene on a layer of gold and then induce so-called surface plasmons (optical surface waves) on the surface of the gold using femtosecond laser pulses, the terahertz yield of the graphene rises by up to a factor of 300!’, continues Planken. This makes graphene an interesting possible alternative. ‘We had expected that there would be an enhancement factor if we induced plasmons, but we hadn’t expected it to be this significant.’ Researchers have not yet got to the bottom of the mechanism responsible for this huge increase, but the high pump light intensity in the graphene layer of the surface plasmons plays an important role.

Well before 2030

‘Just two years ago, the Nobel Prize winner Novoselov wrote that he did not expect to see a practical THz generator using graphene before 2030, but I now think that it could be developed, at least an optically pumped one, well before 2030. In any case, we will definitely know whether it’s a possibility within the next few years.’

The article entitled Plasmon enhanced terahertz emission from single-layer graphene published inACS Nano is the result of collaboration between the Optics Research Group and the research group of Prof. Dai-Sik Kim and other researchers at Seoul National University and Ajou University in South Korea.

Breakthrough in super-terahertz detection technology for astronomy

http://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=4414695783364213278

An international team of researchers led by SRON and TU Delft have successfully demonstrated a superconducting heterodyne receiver with an unprecedented high sensitivity at 4,7 THz in frequency or 63 micrometers in wavelength. The so called super-terahertz heterodyne receiver - about 85 times more sensitive than its predecessor, operating near the quantum noise limit - is able to operate at the exact frequency that is needed to enable the detection of the astrophysically important neutral atomic oxygen (OI) line. This is very good news for astronomers who need to study the oxygen lines from different astronomic sources to trace star formation and galactic evolution. It is also an important milestone in the development of the technology for the candidate balloon mission GUSSTO (NASA). The research results appeared this month in Applied Physics Letters.

This photo shows the key contributors; Jenna Kloosterman (right), Darren Hayton (middle) and Ren Yuan. The photo was taken at the laboratory of the TU Delft, where the measurements started

A heterodyne receiver can convert a (high-frequency) signal from space into a lower frequency without losing any information. Similar to FM radio, this mixing of frequencies makes the reception clearer and the signal from space can be amplified better. The heterodyne receiver can offer line detection with a nearly quantum noise limited sensitivity and an unparalleled high spectral resolution. The new superconducting heterodyne receiver is based on a novel solid-state terahertz quantum cascade laser to generate the local signal that is to be mixed with the signal from space (local oscillator, operating at 4,7 THz). A superconducting hot electron bolometer is used as a mixing detector..

A terahertz quantum cascade laser (QCL) is actually a tiny semiconductor chip that is based on a repeated stack of semiconductor multiple quantum well heterostructures, where laser emission is achieved through the use of intersubband transitions. Although a QCL has been demonstrated as a local oscillator in the lab before, it is the first time that the QCL is applied at 4,7 THz with an exact targeting frequency. The new QCL is based on a so-called third-order distributed feedback grating design. The grating has a double purpose in realizing both controllable single-frequency emission and a good output beam. The laser used was developed by a research group at the MIT in USA.   

The detector used is a hot electron bolometer that consists in essence of a superconducting Niobium-Nitride nanobridge and an on-chip planar antenna. Such a detector was developed by SRON in a close collaboration with the Kavli Institute of Nanoscience at the TU Delft.  

Interstellar medium
The observations of key astronomic cooling lines allow astronomers to trace star formation and galactic evolution. The fine-structure line of electrically neutral atomic oxygen (OI) at 4.7 THz is the dominant cooling line of warm, dense, and neutral atomic gas. In strongly UV irradiated photodissociation regions (PDRs), the OI line flux is generally larger than that of the carbon CII line, making it an ideal diagnostic for probing the physical conditions in regions of massive star formation and galactic centers. The spectrally resolved OI line is necessary to untangle the complexities of the interstellar medium.

The NASA balloon mission GUSSTO

The success of the newly developed QCL-bolometer receiver technology, not only demonstrated by the sensitivity, but also supported by a beautiful methanol line spectrum, is an important milestone in demonstrating the 4.7 THz technology for GUSSTO, which stands for Galactic/Xgalactic Ultra long duration balloon Spectroscopic Stratospheric THz Observatory. GUSSTO was selected by NASA as one of five Explorer Mission of Opportunity proposals. Its Phase A-Concept Study has been completed. Now NASA should announce the Phase–B decision in February this year. The success also demonstrates the technology for follow-up instruments of the molecule hunter HIFI aboard the Herschel Space Telescope (ESA).

Scientific paper
The key results were obtained at SRON by a group of young scientists: Darren Hayton (SRON), Jenna Kloosterman (University of Arizona), Ren Yuan (TU Delft and Purple Mountain Observatory), and Wilt Kao (MIT).  This joint effort was coordinated by Jian-Rong Gao(SRON/TU Delft) The research results were published in a scientific paper that appeared inApplied Physics Letters, vol. 102, 011123(2013), authored by J. L. Kloosterman, D. J. Hayton, Y. Ren, T. Y. Kao, J. N. Hovenier, J. R. Gao, T. M. Klapwijk, Q. Hu, C. K. Walker, and J. L. Reno.