Tiny Terahertz Laser for Imaging and Chemical Detection

https://www.techbriefs.com/component/content/article/tb/supplements/pit/briefs/28505

Terahertz radiation — the band of the electromagnetic spectrum between microwaves and visible light — has promising applications in medical and industrial imaging and chemical detection, among other uses. But many of those applications depend on small, power-efficient sources of terahertz rays, and the standard method for producing them involves a bulky, power-hungry, tabletop device.

A new technique boosts the power of tiny chip-mounted terahertz lasers by 88 percent. (Image: Demin Liu/Molgraphics)

MIT researchers have developed a new version of a chip-scale quantum cascade terahertz laser with distributed feedback. Until now, however, the device has had a major drawback: it naturally emits radiation in two opposed directions. Since most applications of terahertz radiation require directed light, that means that the device squanders half of its energy output. The researchers have now found a way to redirect 80 percent of the light that usually exits the back of the laser, so that it travels in the desired direction.

Bidirectional emission is a common feature of many laser designs. With conventional lasers, however, it's easily remedied by putting a mirror over one end. But the wavelength of terahertz radiation is so long, and the researchers’ new lasers — known as photonic wire lasers — are so small, that much of the electromagnetic wave traveling the laser's length actually lies outside the laser's body. A mirror at one end of the laser would reflect back only a tiny fraction of the wave's total energy. The researchers’ solution to this problem exploits a peculiarity of the tiny laser's design. A quantum cascade laser consists of a long rectangular ridge called a waveguide. In the waveguide, materials are arranged so that the application of an electric field induces a standing electromagnetic wave along its length. The standing wave is essentially inert and will not radiate out of the waveguide. To address that problem, the researchers cut regularly spaced slits into the waveguide, allowing the terahertz rays to radiate out. The slits are spaced so that the waves they emit reinforce each other — their crests coincide — only along the axis of the waveguide. At more oblique angles from the waveguide, they cancel each other out. So the researchers simply put reflectors behind each of the slits, a step that can be seamlessly incorporated into the manufacturing process that produces the waveguide itself. The reflectors are wider than the waveguide, and they're spaced so that the radiation they reflect will reinforce the terahertz wave in one direction but cancel it out in the other. Some of the terahertz wave that lies outside the waveguide still makes it around the reflectors, but 80 percent of the energy that would have exited the waveguide in the wrong direction is now redirected the other way.

“They have a particular type of terahertz quantum cascade laser, known as a third-order distributed-feedback laser, and this right now is one of the best ways of generating a high-quality output beam, which you need to be able to use the power that you're generating, in combination with a single frequency of laser operation, which is also desirable for spectroscopy,” said Ben Williams, an associate professor of electrical and computer engineering at the University of California at Los Angeles. “They've come up with a very elegant scheme to essentially force much more of the power to go in the forward direction. And it still has a good, high-quality beam, so it really opens the door to much more complicated antenna engineering to enhance the performance of these lasers.”

For more information, contact Larry Hardesty at hardesty@mit.edu, 617-253-4735.

Current generated when light hits a material reveals electrons behaving like an elusive particle

Scientists predicted and directly measured electrons in a semimetal. The electrons were behaving like elusive massless particles. Shining a circularly polarized light beam (pink spiral) onto a tantalum-arsenide semimetal (ball-and-stick crystal model) generates an electrical current (green arrow). Remarkably, the direction of the current flow changes by switching the light’s polarization from right-handed to left-handed, proving the handedness of exotic Weyl fermions. Credit: Massachusetts Institute of Technology

 https://phys.org/news/2018-02-current-material-reveals-electrons-elusive.html#jCp

A massless particle, a.k.a. Weyl fermion, predicted nearly 100 years ago, has been found in another corner of physics. Electrons in a semimetal can behave like these particles. They are either right-handed or left-handed—they are mirror images like our hands. Theory predicted that Weyl semimetals could produce handedness-dependent electrical current by shining circularly polarized infrared light onto it. Scientists then confirmed and measured this current. Changing from right- to left-handed light switched the direction of the current, meaning they could determine the handedness of these electrons.

The detection of handedness of electrons in a Weyl semimetal opens new experimental possibilities for studying and controlling these elusive massless particles and their quantum weirdness. Their quantum behavior can lead to novel optical phenomena. One example is photocurrents (electrical current induced by light). Another example is detection of photons (quantized packets of light) from the mid-infrared optical spectrum to lower frequencies (terahertz). Infrared detection is vital for night vision and heat imaging. Terahertz detection is useful for package-penetrating devices. In addition, the right- and left-handedness in a semimetal could be used like zeroes and ones in conventional computing. The result? Novel pathways to store and carry data.

An elusive massless particle with charge and spin ½, a.k.a. Weyl fermion, was predicted nearly 100 years ago. It still has not been observed in particle physics. However, scientists have predicted and observed electrons in the semimetal tantalum arsenide (TaAs) behaving just like the elusive particle. The particles have handedness determined by whether the directions of spin and motion of the particle are parallel or anti-parallel. In other words, the electrons in TaAs make up a novel topological phase called a Weyl semimetal. Therefore, electrons in a Weyl semimetal are the low-energy siblings of Weyl fermions in particle physics. Theory predicted that Weyl semimetals could support significant photocurrents due to the combination of specific symmetry breaking, finite chemical potential, and finite tilts of the Weyl energy spectrum. Recently, a team of scientists from multiple institutions set out to test this theory.

In two publications, the scientists first predicted and then reported the direct optical observation of the induced photocurrent and therefore the handedness of Weyl fermions in the semimetal TaAs. In these experiments, researchers observed for the first time that the photocurrent reaches a maximum value for right circularly polarized light. Switching the light to left circularly polarized minimized the total photocurrent. These observations will lead to additional experiments, because the theory also suggests that Weyl materials that lack a point of inversion symmetry could be used to develop highly sensitive detectors for mid- and far-infrared light.

MIT-EFFICIENT TERAHERTZ SOURCES BASED ON DIFFERENCE-FREQUENCY GENERATION IN TRIPLY-RESONANT PHOTONIC RESONATORS

http://tlo.mit.edu/technologies/efficient-terahertz-sources-based-difference-frequency-generation-triply-resonant

Intellectual Property

Efficient terahertz sources based on difference-frequency generation in triply-resonant photonic resonators

Issued US Patent

US Patent 8,285,091

Applications

Applications for this technology span a variety of industries that would benefit from having a small-footprint and efficient THz source operating at room temperature, including non-destructive material characterization, biology and medical imaging, environmental monitoring, homeland security (security screening), and ultrafast computing.

Problem Addressed

The current technologies are not as adept in efficient conversion at low powers. Current technologies for efficient THz generation require either operation at cryogenic temperature or, if one needs to operate at room temperature, intricate set-ups and powerful lasers (both possibilities lead to not practical and not compact THz sources).

Technology

The invention is a system for efficient terahertz (THz) generation based on difference-frequency generation in a triply-resonant photonic resonator. The system includes a photonic resonator comprising at least one nonlinear material that enables THz generation via difference-frequency generation (DFG) of two near-infrared (NIR) or optical beams.  This photonic resonator is coupled evanescently to at least one NIT or optical waveguide and is embedded or placed in the proximity of a second photonic resonator, whose resonant frequency is in the THz waveguide.

Advantages

• Allows for efficient nonlinear frequency conversion at low powers
• Provides a THz source that, while being compact and efficient, can operate at room temperature (i.e., it meets all the requirements of a truly practical THz source)
• The THz source has the potential of enabling a broader use of THz radiation

Tiny terahertz laser could be used for imaging, chemical detection

New design boosts the power output of the best-performing chip-scale terahertz laser by 80 percent.

http://news.mit.edu/2017/tiny-terahertz-laser-imaging-chemical-detection-0808

Larry Hardesty | MIT News Office

Terahertz radiation — the band of the electromagnetic spectrum between microwaves and visible light — has promising applications in medical and industrial imaging and chemical detection, among other uses.

But many of those applications depend on small, power-efficient sources of terahertz rays, and the standard method for producing them involves a bulky, power-hungry, tabletop device.

For more than 20 years, Qing Hu, a distinguished professor of electrical engineering and computer science at MIT, and his group have been working on sources of terahertz radiation that can be etched onto microchips. In the latest issue of Nature Photonics, members of Hu’s group and colleagues at Sandia National Laboratories and the University of Toronto describe a novel design that boosts the power output of chip-mounted terahertz lasers by 80 percent.

As the best-performing chip-mounted terahertz source yet reported, the researchers’ device has been selected by NASA to provide terahertz emission for its Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO) mission. The mission is intended to determine the composition of the interstellar medium, or the matter that fills the space between stars, and it’s using terahertz rays because they’re uniquely well-suited to spectroscopic measurement of oxygen concentrations. Because the mission will deploy instrument-laden balloons to the Earth’s upper atmosphere, the terahertz emitter needs to be lightweight.

The researchers’ design is a new variation on a device called a quantum cascade laser with distributed feedback. “We started with this because it was the best out there,” says Ali Khalatpour, a graduate student in electrical engineering and computer science and first author on the paper. “It has the optimum performance for terahertz.”

Until now, however, the device has had a major drawback, which is that it naturally emits radiation in two opposed directions. Since most applications of terahertz radiation require directed light, that means that the device squanders half of its energy output. Khalatpour and his colleagues found a way to redirect 80 percent of the light that usually exits the back of the laser, so that it travels in the desired direction.

As Khalatpour explains, the researchers’ design is not tied to any particular “gain medium,” or combination of materials in the body of the laser.

“If we come up with a better gain medium, we can double its output power, too,” Khalatpour says. “We increased power without designing a new active medium, which is pretty hard. Usually, even a 10 percent increase requires a lot of work in every aspect of the design.”

Big waves

In fact, bidirectional emission, or emission of light in opposed directions, is a common feature of many laser designs. With conventional lasers, however, it’s easily remedied by putting a mirror over one end of the laser.

But the wavelength of terahertz radiation is so long, and the researchers’ new lasers — known as photonic wire lasers — are so small, that much of the electromagnetic wave traveling the laser’s length actually lies outside the laser’s body. A mirror at one end of the laser would reflect back a tiny fraction of the wave’s total energy.

Khalatpour and his colleagues’ solution to this problem exploits a peculiarity of the tiny laser’s design. A quantum cascade laser consists of a long rectangular ridge called a waveguide. In the waveguide, materials are arranged so that the application of an electric field induces an electromagnetic wave along the length of the waveguide.

This wave, however, is what’s called a “standing wave.” If an electromagnetic wave can be thought of as a regular up-and-down squiggle, then the wave reflects back and forth in the waveguide in such a way that the crests and troughs of the reflections perfectly coincide with those of the waves moving in the opposite direction. A standing wave is essentially inert and will not radiate out of the waveguide.

So Hu’s group cuts regularly spaced slits into the waveguide, which allow terahertz rays to radiate out. “Imagine that you have a pipe, and you make a hole, and the water gets out,” Khalatpour says. The slits are spaced so that the waves they emit reinforce each other — their crests coincide — only along the axis of the waveguide. At more oblique angles from the waveguide, they cancel each other out.

Breaking symmetry

In the new work, Khalatpour and his coauthors — Hu, John Reno of Sandia, and Nazir Kherani, a professor of materials science at the University of Toronto — simply put reflectors behind each of the holes in the waveguide, a step that can be seamlessly incorporated into the manufacturing process that produces the waveguide itself.

The reflectors are wider than the waveguide, and they’re spaced so that the radiation they reflect will reinforce the terahertz wave in one direction but cancel it out in the other. Some of the terahertz wave that lies outside the waveguide still makes it around the reflectors, but 80 percent of the energy that would have exited the waveguide in the wrong direction is now redirected the other way.

“They have a particular type of terahertz quantum cascade laser, known as a third-order distributed-feedback laser, and this right now is one of the best ways of generating a high-quality output beam, which you need to be able to use the power that you’re generating, in combination with a single frequency of laser operation, which is also desirable for spectroscopy,” says Ben Williams, an associate professor of electrical and computer engineering at the University of California at Berkeley. “This has been one of the most useful and popular ways to do this for maybe the past five, six years. But one of the problems is that in all the previous structures that either Qing’s group or other groups have done, the energy from the laser is going out in two directions, both the forward direction and the backward direction.”

“It’s very difficult to generate this terahertz power, and then once you do, you’re throwing away half of it, so that’s not very good,” Williams says. “They’ve come up with a very elegant scheme to essentially force much more of the power to go in the forward direction. And it still has a good, high-quality beam, so it really opens the door to much more complicated antenna engineering to enhance the performance of these lasers.”

The new work was funded by NASA, the National Science Foundation, and the U.S. Department of Energy.

OT-SpectroscopyNOW blog-Phosphorene: Two-dimensional Raman

 Phosphorene: Two-dimensional Raman

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http://www.spectroscopynow.com/raman/details/highlight/14de3301024/Last-Months-Most-Accessed-Feature-Phosphorene-Two-dimensional-Raman.html

Flat out phosphorus

Raman spectroscopy and transmission electron microscopy have been used by an international team to investigate the phosphorus analogue of graphene, the two-dimensional phosphane, known as phosphorene.

Phosphorene has potential applications in a new class of semiconducting transistor for that perennial aspiration, the ever faster and more powerful computer of the future. Unfortunately, while phosphorene can conduct electrons its ability to do so is anisotropic, meaning it depends on which way you orient it relative to the system as to whether it does so or not. Thus, a quick and simple way to determine the orientation of the material was needed for experimental setups and now, a team comprising researchers from the Massachusetts Institute of Technology, the Rensselaer Polytechnic Institute (RPI) in Troy, New York state, Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania, has done just that. There approach accurately determines orientation by examining the interaction between light and electrons within phosphorene or other thin layers of black phosphorus.

Calculated approach

Materials scientists have been studying phosphorene intently since it was first isolated in 2014. RPI's Vincent Meunier and his team confirmed the structure of phosphorene that same year. "This is a really interesting material because, depending on which direction you do things, you have completely different properties," explains Meunier, a phenomenon that might of course be exploited in devices. "But because it's such a new material, it's essential that we begin to understand and predict its intrinsic properties."

Meunier and colleagues have now built on the theoretical modelling and prediction of the properties of phosphorene using Rensselaer's supercomputer in the Center for Computational Innovations (CCI). On the basis of their calculations, they have home in on certain features of this novel material that will ultimately help physicists and materials scientists better understand it and thence technologists make use of those properties.

Writing in the journal ACS Nano Letters, the team initially set out to refine an existing technique for determining the orientation of the crystal using Raman spectroscopy. The team were reviewing their Raman data and spotted a few unexplained inconsistencies. So, they next turned to obtaining images of the orientation of their crystalline samples using Transmission Electron Microscopy (TEM), and lined these up with the "images" gleaned from the Raman results. As a topographic technique, TEM offers a definitive determination of the orientation of a crystal, but takes a lot more effort than recording a Raman spectrum. Nevertheless, the comparison revealed that electron-phonon interactions alone did not account for the orientation of the crystal. And the reason why led the way to yet another anisotropy of phosphorene - that of interactions between photons of light and electrons in the crystal.

Intrinsic anisotropy

The Raman spectrum should be intrinsic to the material and thus show the anisotropy of phosphorene. "But, it turns out that if you shine the light in different directions, you get different results, because the interaction between the light and the electrons in the material - the electron-photon interaction - is also anisotropic, but in a non-commensurate way," explains Meunier. The team suspected that phosphorene was anisotropic with respect to electron-photon interactions, but hadn't quite anticipated the significance of the property. "Usually electron-photon anisotropy doesn’t make such a big difference, but here, because we have such a particular chemistry on the surface and such a strong anisotropy, it's one of those materials where it makes a huge difference," Meunier adds.

Fundamentally, the discovery reveals a limitation in what current interpretation of Raman spectra can achieve in studying these materials. "It turns out that it's not so easy to use Raman vibrations to find out the direction of the crystal," Meunier explains. "But, and this is the beautiful thing, what we found is that the electron-photon interaction (which can be measured by recording the amount of light absorbed) - the interaction between the electrons and the laser - is a good predictor of the direction. Now you can really predict how the material will behave as a function of excitement with an outside stimulus."

Meunier worked with Mildred Dresselhaus of the Massachusetts Institute of Technology, as well as colleagues at Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania.