Terahertz laser leaves the lab

 

A scanning electron microscope (SEM) image of a terahertz quantum cascade laser (QCL) device. Credits: Qing Hu and colleagues

https://physicsworld.com/a/terahertz-laser-leaves-the-lab/

Isabelle Dumé

A new type of high-power quantum cascade laser that works without bulky cooling equipment could usher in a host of novel imaging applications by making it easier to generate terahertz radiation outside the laboratory. The laser, which was developed by researchers at the Massachusetts Institute of Technology in the US and the University of Waterloo, Canada, can operate at temperatures of up to 250 K – some 40 K higher than the previous record, and attainable with a compact cooler rather than a specialist cryogenic system.

Terahertz (THz) radiation falls between the infrared and microwave regions of the electromagnetic spectrum, with wavelengths in the range of 3 mm – 30 µm. While many molecules absorb light at these wavelengths (making it possible to use THz radiation to identify their molecular “fingerprint”), THz radiation passes straight through everyday materials such as paper, cloth and plastics. This means that THz radiation, like X-rays, can be used to “see” inside objects that are opaque to visible light. Unlike X-rays, however, photons in the THz region have relatively low energies, making THz radiation non-ionizing and therefore safe for biological and medical use. A further benefit is that THz radiation has a shorter wavelength than microwave radiation, which means it can create higher-resolution images.

Underused

While all of this sounds good on paper, radiation between 0.1 and 10 THz is underused in practice due to the lack of practical sources and detectors in this range. Generating THz beams that are intense enough to be useful is also a challenge. Quantum cascade lasers (QCLs) are one option, as their microscopic structures can be tuned to produce coherent THz radiation. However, these lasers must be kept at very low temperatures to function.

Researchers led by Qing Hu and Zbig Wasilewski have now partially overcome this barrier by developing a QCL that produces light in the THz region with only a modicum of cooling. At 250 K, the new laser’s maximum operating temperature is significantly higher than the previous maximum of 210 K that Jérome Faist’s group at ETH Zurich in Switzerland achieved in 2019 – a record that was itself much higher than the 2012 record of 200 K.

Quantum cascade lasers

The temperature-sensitive nature of QCLs stems from the way they are constructed. Unlike standard semiconductor lasers, which generate photons when electrons and holes combine inside a material with a given electronic energy band gap, QCLs consist of tailor-made quantum wells and barriers made of thousands of thin layers of semiconductors. Each electron that travels through the device “cascades” through a series or “staircase” of these quantum wells (QWs) as it passes through the semiconductor layers. In the process, the electron emits multiple photons at frequencies that are set by the structure of the layers.

At higher temperatures, electrons tend to “leak” over the barriers of the QWs, disrupting the laser’s output. Hu and colleagues’ breakthrough came after they managed to reduce this leakage by developing new semiconductor band structures – an improvement which, in turn, enabled them to double the height of the barriers. They also devised a novel configuration in which lower lasing levels of each step of the quantum-well staircase are quickly depopulated of electrons by scattering phonons (vibrations of the quantum lattice) into a ground state. This state then serves as the “injector” of electrons into the upper level of the next step, Hu explains, and the process repeats so that lasing can occur.

Complex structures

The structures the team created are very complex and contain close to 15 000 interfaces between QWs and barriers, half of which are less than seven atomic layers thick, Wasilewski explains. The quality of these interfaces is, he adds, paramount to the THz laser’s performance.

One near-term application for the new THz source would involve the real-time imaging of skin during skin-cancer screenings, Hu says. Cancer cells show up “very dramatically” in THz light, he explains, because they contain more water and blood than normal cells, and water strongly absorbs THz signals. The technology could also be used to detect drugs like methamphetamine and heroin and explosives like TNT since these molecules have a spectral fingerprint within the THz frequency range too.

The researchers, who report their work in Nature Photonics, say that in the future it should be possible to generate THz radiation with a QCL without the need for a cooler at all. They now plan to further increase their device’s maximum operating temperature while also lowering its lasing threshold to reduce heat dissipation. “This will enable a compact and portable laser system operating in a continuous way, which is more useful in applications,” Hu tells Physics World. “For example, a single-frequency continuous laser based on a distributed feedback structure is essential for high-resolution spectroscopy and sensing.”

Hu and his colleagues are also developing compact THz imaging systems with a high dynamic range, which is required to penetrate thicker layers of material and allows for faster imaging. “We are also developing broadband THz radiation amplifiers, akin to the amplifiers used at the front end in cell phones and radio receivers,” he says. “Since most THz signals are quite weak, be they from the universe (90% of photons in the universe are in the THz range) or from earthly sources, amplification will greatly ease the demanding requirements for detection and subsequent signal processing.”

A Terahertz QCL Without the Cryogenics

 

Patricia Daukantas

https://www.osa-opn.org/home/newsroom/2020/november/a_terahertz_qcl_without_the_cryogenics/

The development of the terahertz quantum cascade laser (QCL) nearly two decades ago held great promise for applications ranging from explosives detection to skin cancer screening. However, the need for bulky cryogenic cooling equipment to make the QCL work properly limited the use of such devices in the field.

Left: The tiny terahertz quantum cascade laser compared in size to two coins. Right: The laser chip with a thermoelectric cooler on a block. In the background is a cryocooler. [Image: Khalatpour et al., MIT and University of Waterloo]

Now, researchers at two North American universities have developed a high-power, compact terahertz QCL with a maximum operating temperature of 250 K (Nat. Photon., doi: 10.1038/s41566-020-00707-5). The millimeter-scale source of 4-THz radiation, paired with a portable thermoelectric cooler, can generate sufficient power to produce images with room-temperature cameras and detectors.

The terahertz allure

Scientists have long known that the spectral region with wavelengths between 1 and 10 THz is full of interesting chemical and biological fingerprints. Illegal drugs, protein structures and atomic oxygen in the Martian atmosphere all show up in terahertz spectroscopy. Unfortunately, electronic devices don’t yield much power at frequencies above 1 THz, and conventional semiconductor photonic devices cannot operate below roughly 10 THz. Researchers have various methods for generating either narrowband or broadband terahertz radiation, but QCLs generating high-power coherent radiation in this spectral region would eliminate the need for upconversion or downconversion from other frequencies.

For most of the past decade, the maximum operating temperature of terahertz QCLs has not crept above 200 or 210 K, despite numerous attempts to improve the technology. A few years ago, scientists at the Massachusetts Institute of Technology (MIT), USA, found that high temperatures exacerbate carrier leakage over the tiny aluminum gallium-arsenide barriers inside QCLs.

Breaking barriers by making them higher

University of Waterloo scientist Zbig R. Wasilewski with a student in the lab. [Image: University of Waterloo]

In the current set of experiments, Qing Hu of MIT and Zbig R. Wasilewski of the University of Waterloo, Canada, and their students heightened the semiconductor barriers to reduce carrier leakage inside their QCL system. The team observed that this design required high-precision molecular beam epitaxy to fabricate—an important concern when refining the technology and someday making it commercially available.

The researchers tested one of their QCLs, cooled by a single-stage thermoelectric cooler, and plotted its output power versus current. The team also captured beam pattern images with a room-temperature camera and with the QCL cooled with a three-stage thermoelectric module.

MIT researchers use graphene and boron nitride to convert terahertz waves to usable energy

https://www.graphene-info.com/mit-researchers-use-graphene-and-boron-nitride-convert-terahertz-waves-usable

Researchers at MIT are working to develop a graphene-based device that may be able to convert ambient terahertz waves into a direct current. The MIT team explains that any device that sends out a Wi-Fi signal also emits terahertz waves —electromagnetic waves with a frequency somewhere between microwaves and infrared light. These high-frequency radiation waves, known as “T-rays,” are also produced by almost anything that registers a temperature, including our own bodies and the inanimate objects around us.

Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. However, to date, terahertz waves are wasted energy, as there has been no practical way to capture and convert them into any usable form. This is exactly what the MIT scientists set out to do.

Their design takes advantage of the quantum mechanical, or atomic behavior of graphene. They found that by combining graphene with another material, in this case, boron nitride, the electrons in graphene should skew their motion toward a common direction. Any incoming terahertz waves should “shuttle” graphene’s electrons, like so many tiny air traffic controllers, to flow through the material in a single direction, as a direct current.

“We are surrounded by electromagnetic waves in the terahertz range,” says lead author Hiroki Isobe, a postdoc in MIT’s Materials Research Laboratory. “If we can convert that energy into an energy source we can use for daily life, that would help to address the energy challenges we are facing right now.”

Over the last decade, scientists have looked for ways to harvest and convert ambient energy into usable electrical energy. They have done so mainly through rectifiers, devices that are designed to convert electromagnetic waves from their oscillating (alternating) current to direct current.

Most rectifiers are designed to convert low-frequency waves such as radio waves, using an electrical circuit with diodes to generate an electric field that can steer radio waves through the device as a DC current. These rectifiers only work up to a certain frequency, and have not been able to accommodate the terahertz range.

A few experimental technologies that have been able to convert terahertz waves into DC current do so only at ultracold temperatures — setups that would be difficult to implement in practical applications.

Instead of turning electromagnetic waves into a DC current by applying an external electric field in a device, Isobe wondered whether, at a quantum mechanical level, a material’s own electrons could be induced to flow in one direction, in order to steer incoming terahertz waves into a DC current.

Such a material would have to be very clean, or free of impurities, in order for the electrons in the material to flow through without scattering off irregularities in the material. Graphene, he found, was the ideal starting material.

To direct graphene’s electrons to flow in one direction, he would have to break the material’s inherent symmetry, or what physicists call “inversion.” Normally, graphene’s electrons feel an equal force between them, meaning that any incoming energy would scatter the electrons in all directions, symmetrically. Isobe looked for ways to break graphene’s inversion and induce an asymmetric flow of electrons in response to incoming energy.

Looking through the literature, he found that others had experimented with graphene by placing it atop a layer of boron nitride, a similar honeycomb lattice made of two types of atoms — boron and nitrogen. They found that in this arrangement, the forces between graphene’s electrons were knocked out of balance: Electrons closer to boron felt a certain force while electrons closer to nitrogen experienced a different pull. The overall effect was what physicists call “skew scattering,” in which clouds of electrons skew their motion in one direction.

Isobe developed a systematic theoretical study of all the ways electrons in graphene might scatter in combination with an underlying substrate such as boron nitride, and how this electron scattering would affect any incoming electromagnetic waves, particularly in the terahertz frequency range.

He found that electrons were driven by incoming terahertz waves to skew in one direction, and this skew motion generates a DC current, if graphene were relatively pure. If too many impurities did exist in graphene, they would act as obstacles in the path of electron clouds, causing these clouds to scatter in all directions, rather than moving as one.

“With many impurities, this skewed motion just ends up oscillating, and any incoming terahertz energy is lost through this oscillation,” Isobe explains. “So we want a clean sample to effectively get a skewed motion.”

They also found that the stronger the incoming terahertz energy, the more of that energy a device can convert to DC current. This means that any device that converts T-rays should also include a way to concentrate those waves before they enter the device.

With all this in mind, the researchers drew up a blueprint for a terahertz rectifier that consists of a small square of graphene that sits atop a layer of boron nitride and is sandwiched within an antenna that would collect and concentrate ambient terahertz radiation, boosting its signal enough to convert it into a DC current.

“This would work very much like a solar cell, except for a different frequency range, to passively collect and convert ambient energy,” Fu says.

The team has filed a patent for the new “high-frequency rectification” design, and the researchers are working with experimental physicists at MIT to develop a physical device based on their design, which should be able to work at room temperature, versus the ultracold temperatures required for previous terahertz rectifiers and detectors.

“If a device works at room temperature, we can use it for many portable applications,” Isobe says.

He envisions that, in the near future, terahertz rectifiers may be used, for instance, to wirelessly power implants in a patient’s body, without requiring surgery to change an implant’s batteries. Such devices could also convert ambient Wi-Fi signals to charge up personal electronics such as laptops and cellphones.

“We are taking a quantum material with some asymmetry at the atomic scale, that can now be utilized, which opens up a lot of possibilities,” Fu says.

Cryptographic “tag of everything” could protect the supply chain

MIT researchers’ millimeter-sized ID chip integrates a cryptographic processor, an antenna array that transmits data in the high terahertz range, and photovoltaic diodes for power.

Image: courtesy of the researchers, edited by MIT News

Rob Matheson

http://news.mit.edu/2020/cryptographic-tag-supply-chain-0220

Tiny, battery-free ID chip can authenticate nearly any product to help combat losses to counterfeiting.

To combat supply chain counterfeiting, which can cost companies billions of dollars annually, MIT researchers have invented a cryptographic ID tag that’s small enough to fit on virtually any product and verify its authenticity.

A 2018 report from the Organization for Economic Co-operation and Development estimates about $2 trillion worth of counterfeit goods will be sold worldwide in 2020. That’s bad news for consumers and companies that order parts from different sources worldwide to build products.

Counterfeiters tend to use complex routes that include many checkpoints, making it challenging to verifying their origins and authenticity. Consequently, companies can end up with imitation parts. Wireless ID tags are becoming increasingly popular for authenticating assets as they change hands at each checkpoint. But these tags come with various size, cost, energy, and security tradeoffs that limit their potential.

Popular radio-frequency identification (RFID) tags, for instance, are too large to fit on tiny objects such as medical and industrial components, automotive parts, or silicon chips. RFID tags also contain no tough security measures. Some tags are built with encryption schemes to protect against cloning and ward off hackers, but they’re large and power hungry. Shrinking the tags means giving up both the antenna package — which enables radio-frequency communication — and the ability to run strong encryption.

In a paper presented yesterday at the IEEE International Solid-State Circuits Conference (ISSCC), the researchers describe an ID chip that navigates all those tradeoffs. It’s millimeter-sized and runs on relatively low levels of power supplied by photovoltaic diodes. It also transmits data at far ranges, using a power-free “backscatter” technique that operates at a frequency hundreds of times higher than RFIDs. Algorithm optimization techniques also enable the chip to run a popular cryptography scheme that guarantees secure communications using extremely low energy.   

“We call it the ‘tag of everything.’ And everything should mean everything,” says co-author Ruonan Han, an associate professor in the Department of Electrical Engineering and Computer Science and head of the Terahertz Integrated Electronics Group in the Microsystems Technology Laboratories (MTL). “If I want to track the logistics of, say, a single bolt or tooth implant or silicon chip, current RFID tags don’t enable that. We built a low-cost, tiny chip without packaging, batteries, or other external components, that stores and transmits sensitive data.”

Joining Han on the paper are: graduate students Mohamed I. Ibrahim and Muhammad Ibrahim Wasiq Khan, and former graduate student Chiraag S. Juvekar; former postdoc associate Wanyeong Jung; former postdoc Rabia Tugce Yazicigil, who is currently an assistant professor at Boston University and a visiting scholar at MIT; and Anantha P. Chandrakasan, who is the dean of the MIT School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science.

System integration

The work began as a means of creating better RFID tags. The team wanted to do away with packaging, which makes the tags bulky and increases manufacturing cost. They also wanted communication in the high terahertz frequency between microwave and infrared radiation — around 100 gigahertz and 10 terahertz — that enables chip integration of an antenna array and wireless communications at greater reader distances. Finally, they wanted cryptographic protocols because RFID tags can be scanned by essentially any reader and transmit their data indiscriminately.

But including all those functions would normally require building a fairly large chip. Instead, the researchers came up with “a pretty big system integration,” Ibrahim says, that enabled putting everything on a monolithic — meaning, not layered — silicon chip that was only about 1.6 square millimeters.

One innovation is an array of small antennas that transmit data back and forth via backscattering between the tag and reader. Backscatter, used commonly in RFID technologies, happens when a tag reflects an input signal back to a reader with slight modulations that correspond to data transmitted. In the researchers’ system, the antennas use some signal splitting and mixing techniques to backscatter signals in the terahertz range. Those signals first connect with the reader and then send data for encryption.

Implemented into the antenna array is a “beam steering” function, where the antennas focus signals toward a reader, making them more efficient, increasing signal strength and range, and reducing interference. This is the first demonstration of beam steering by a backscattering tag, according to the researchers.

Tiny holes in the antennas allow light from the reader to pass through to photodiodes underneath that convert the light into about 1 volt of electricity. That powers up the chip’s processor, which runs the chip’s “elliptic-curve-cryptography” (ECC) scheme. ECC uses a combination of private keys (known only to a user) and public keys (disseminated widely) to keep communications private. In the researchers’ system, the tag uses a private key and a reader’s public key to identify itself only to valid readers. That means any eavesdropper who doesn’t possess the reader’s private key should not be able to identify which tag is part of the protocol by monitoring just the wireless link.  

Optimizing the cryptographic code and hardware lets the scheme run on an energy-efficient and small processor, Yazicigil says. “It’s always a tradeoff,” she says. “If you tolerate a higher-power budget and larger size, you can include cryptography. But the challenge is having security in such a small tag with a low-power budget.”

Pushing the limits

Currently, the signal range sits around 5 centimeters, which is considered a far-field range — and allows for convenient use of a portable tag scanner. Next, the researchers hope to “push the limits” of the range even further, Ibrahim says. Eventually, they’d like many of the tags to ping one reader positioned somewhere far away in, say, a receiving room at a supply chain checkpoint. Many assets could then be verified rapidly.

“We think we can have a reader as a central hub that doesn’t have to come close to the tag, and all these chips can beam steer their signals to talk to that one reader,” Ibrahim says.

The researchers also hope to fully power the chip through the terahertz signals themselves, eliminating any need for photodiodes.

The chips are so small, easy to make, and inexpensive that they can also be embedded into larger silicon computer chips, which are especially popular targets for counterfeiting.

“The U.S. semiconductor industry suffered $7 billion to $10 billion in losses annually because of counterfeit chips,” Wasiq Khan says. “Our chip can be seamlessly integrated into other electronic chips for security purposes, so it could have huge impact on industry. Our chips cost a few cents each, but the technology is priceless,” he quipped.

Laser combo opens up futuristic terahertz technology

Experimental setup showing the system components and highlighting the path followed by the quantum cascade laser light (red) and terahertz radiation (blue). (Courtesy: Arman Amirzhan, Harvard SEAS)

https://physicsworld.com/a/laser-combo-opens-up-futuristic-terahertz-technology/

Researchers have created a new terahertz radiation emitter with highly-sought-after frequency adjustment capability. The compact source could enable the development of futuristic communications, security, biomedical and astronomical imaging systems.

The high bandwidth, high resolution, long-range sensing and ability to visualize objects through materials, makes terahertz electromagnetic frequencies much-coveted. However, the costliness, bulk, inefficiencies and lack of tunability of traditional terahertz emitters has stymied these promising avenues. This new combined laser terahertz source, product of a collaboration between researchers at Harvard, the US Army, MIT and Duke University, paves the way for future technologies, from T-ray imaging in airports and space observatories, to ultrahigh-capacity wireless connections.

“Existing sources have limited tunability, not more than 15-20% of the main frequency, so it’s fair to say that terahertz is underutilized,” explains co-senior author Federico Capasso from Harvard University. “Our laser opens up this spectral region, and in my opinion, will have revolutionary impact.”

The team has now described the theoretical proof and demonstration of this widely tunable and compact terahertz laser system (Science 10.1126/science.aay8683).

Perfect partnership

Capasso is no stranger to laser technology. He invented a compact tunable semiconductor laser, the quantum cascade laser (QCL), which is used commercially for chemical sensing and trace gas analysis. The QCL emits mid-infrared light, the spectral region where most gases have their characteristic absorption fingerprints, to detect low concentrations of molecules.

But it wasn’t until a conference in 2017 when Capasso met Henry Everitt, senior technologist with the US Army and adjunct professor at Duke University, that the idea to apply the widely tunable QCL to a laser with terahertz ability, formed.

Everitt, alongside Steven Johnson’s group at MIT, theoretically calculated that terahertz waves could be emitted with high efficiency from gas molecules held within cavities much smaller than those currently used on the optically pumped far-infrared (OPFIR) laser – one of the earliest sources of terahertz radiation. Like all traditional terahertz sources, the OPFIR was inefficient with limited tunability. But, guided by the theoretical calculations, Capasso’s team were able to use the QCL to dramatically increase the terahertz tuning range of a nitrous oxide (laughing gas) OPFIR laser.

“The same laser is now widely tunable – it’s a fantastic marriage between two existing lasers,” says Capasso.

Universal use

In initial experiments with the shoe-boxed sized QCL pumped molecular laser – QPML – the researchers demonstrated that the terahertz output could be tuned to produce 29 direct lasing transitions between 0.251 and 0.955 THz.

Artistic view of the QCL pumped terahertz laser showing the QCL beam (red) and the terahertz beam (blue) along with rotating molecules inside the cavity. The figure shows emission spectra of each gas. The QCL is continuously tunable and the terahertz laser emits at discrete frequencies and different ranges depending upon the gas used in the laser. (Courtesy: Arman Amirzhan, Harvard SEAS)

It was Johnson and Everitt’s theoretical models that highlighted nitrous oxide as a strongly polar gas with predicted terahertz release in the QPML. Similarly, a whole menu of other gas molecules have been predicted for terahertz generation at different frequencies and tuning ranges. Using this menu, it should be possible to select a gas laser appropriate for almost any application.

“This is a universal concept, because it can be applied to other gases,” says Capasso. “We haven’t quite reached one terahertz, so next thing is to try a carbon monoxide laser and go up to a few terahertz, which is very exciting for applications!”

Both Capasso and Everitt are particularly keen to use their laser to look skywards and sensitively identify unknown spectral features in the terahertz region. The team is developing higher power terahertz QPMLs for astronomical observations, while also eagerly working towards other commercial applications.

New laser opens up large region of the electromagnetic spectrum

http://www.spacedaily.com/reports/New_laser_opens_up_large_region_of_the_electromagnetic_spectrum_999.html

The terahertz frequency range - which sits in the middle of the electromagnetic spectrum between microwaves and infrared light - offers the potential for high-bandwidth communications, ultrahigh-resolution imaging, precise long-range sensing for radio astronomy, and much more.

But this section of the electromagnetic spectrum has remained out of reach for most applications. That is because current sources of terahertz frequencies are bulky, inefficient, have limited tuning or have to operate at low temperature.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with MIT and the U.S. Army, have developed a compact, room temperature, widely tunable terahertz laser.

"This laser outperforms any existing laser source in this spectral region and opens it up, for the first time, to a broad range of applications in science and technology," said Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and co-senior author of the paper.

"There are many needs for a source like this laser, things like short range, high bandwidth wireless communications, very high-resolution radar, and spectroscopy," said Henry Everitt, Senior Technologist with the U.S. Army CCDC Aviation and Missile Center and co-senior author of the paper.

While most electronic or optical terahertz sources use large, inefficient and complex systems to produce the elusive frequencies with limited tuning range, Capasso, Everitt and their team took a different approach.

To understand what they did, let's go over some basic physics of how a laser works:

In quantum physics, excited atoms or molecules sit at different energy levels - think of these as floors of a building. In a typical gas laser, a large number of molecules are trapped between two mirrors and brought to an excited energy level, aka a higher floor in the building. When they reach that floor, they decay, fall down one energy level and emit a photon. These photons stimulate the decay of more molecules as they bounce back and forth leading to amplification of light. To change the frequency of the emitted photons, you need to change the energy level of the excited molecules.

So, how do you change the energy level? One way is to use light. In a process called optical pumping, light raises molecules from a lower energy level to a higher one - like a quantum elevator. Previous terahertz molecular lasers used optical pumps but they were limited in their tunability to just a few frequencies, meaning the elevator only went to a small number of floors.

The breakthrough of this research is that Capasso, Everitt and their team used a highly tunable, quantum cascade laser as their optical pump. These powerful, portable lasers, co-invented by Capasso and his group at Bell Labs in the 1990s, are capable of efficiently producing widely tunable light. In other words, this quantum elevator can stop at every floor in the building.

The theory to optimize the operation of the new laser was developed by Steve Johnson, Professor of Applied Mathematics and Physics at MIT and his graduate student Fan Wang.

The researchers combined the quantum cascade laser pump with a nitrous oxide - aka laughing gas - laser.

"By optimizing the laser cavity and lenses, we were able to produce frequencies spanning nearly 1 THz," said Arman Amirzhan, a graduate student in Capasso's group and co-author of the paper.

"Molecular THz lasers pumped by a quantum cascade laser offer high power and wide tuning range in a surprisingly compact and robust design," said Nobel laureate Theodor Hansch of the Max Planck Institute for Quantum Optics in Munich, who was not involved in this research. "Such sources will unlock new applications from sensing to fundamental spectroscopy."

"What's exciting is that concept is universal," said Paul Chevalier, a postdoctoral fellow at SEAS and first author of the paper. "Using this framework, you could make a terahertz source with a gas laser of almost any molecule and the applications are huge."

"This result is one of a kind," said Capasso. "People knew how to make a terahertz laser before but couldn't make it broadband. It wasn't until we began this collaboration, after a serendipitous encounter with Henry at a conference, that we were able to make the connection that you could use a widely tunable pump like the quantum cascade laser."

This laser could be used in everything from improved skin and breast cancer imaging to drug detection, airport security and ultrahigh-capacity optical wireless links.

"I'm particularly excited about the possibility of using this laser to help map the interstellar medium," said Everitt. "Molecules have unique spectral fingerprints in the terahertz region, and astronomers have already begun using these fingerprints to measure the composition and temperature of these primordial clouds of gas and dust. A better ground-based source of terahertz radiation like our laser will make these measurements even more sensitive and precise."

Everitt is also an Adjunct Professor of Physics at Duke University. The research is published in Science.

Researchers generate terahertz laser with laughing gas

A new shoebox-sized laser produces terahertz waves (green squiggles) by using a special infrared laser (red) to rotate molecules of nitrous oxide, or laughing gas, packed in a pen-sized cavity (grey).

Courtesy of Chad Scales, US Army Futures Command

http://news.mit.edu/2019/tunable-terahertz-laser-laughing-gas-1114
Jennifer Chu

Device may enable “T-ray vision” and better wireless communication.

Within the electromagnetic middle ground between microwaves and visible light lies terahertz radiation, and the promise of “T-ray vision.”

Terahertz waves have frequencies higher than microwaves and lower than infrared and visible light. Where optical light is blocked by most materials, terahertz waves can pass straight through, similar to microwaves. If they were fashioned into lasers, terahertz waves might enable “T-ray vision,” with the ability to see through clothing, book covers, and other thin materials. Such technology could produce crisp, higher-resolution images than microwaves, and be far safer than X-rays.

The reason we don’t see T-ray machines in, for instance, airport security lines and medical imaging facilities is that producing terahertz radiation requires very large, bulky setups or devices, many operating at ultracold temperatures, that produce terahertz radiation at a single frequency — not very useful, given that a wide range of frequencies is required to penetrate various materials.

Now researchers from MIT, Harvard University, and the U.S. Army have built a compact device, the size of a shoebox, that works at room temperature to produce a terahertz laser whose frequency they can tune over a wide range. The device is built from commercial, off-the-shelf parts and is designed to generate terahertz waves by spinning up the energy of molecules in nitrous oxide, or, as it’s more commonly known, laughing gas.

Steven Johnson, professor of mathematics at MIT, says that in addition to T-ray vision, terahertz waves can be used as a form of wireless communication, carrying information at a higher bandwidth than radar, for instance, and doing so across distances that scientists can now tune using the group’s device.

“By tuning the terahertz frequency, you can choose how far the waves can travel through air before they are absorbed, from meters to kilometers, which gives precise control over who can ‘hear’ your terahertz communications or ‘see’ your terahertz radar,” Johnson says. “Much like changing the dial on your radio, the ability to easily tune a terahertz source is crucial to opening up new applications in wireless communications, radar, and spectroscopy.”

Johnson and his colleagues have published their results today in the journal Science. Co-authors include MIT postdoc Fan Wang, along with Paul Chevalier, Arman Amirzhan, Marco Piccardo, and Federico Capasso of Harvard University, and Henry Everitt of the U.S. Army Combat Capabilities Development Command Aviation and Missile Center.

Molecular breathing room

Since the 1970s, scientists have experimented with generating terahertz waves using molecular gas lasers — setups in which a high-powered infrared laser is shot into a large tube filled with gas (typically methyl fluoride) whose molecules react by vibrating and eventually rotating. The rotating molecules can jump from one energy level to the next, the difference of which is emitted as a sort of leftover energy, in the form of a photon in the terahertz range. As more photons build up in the cavity, they produce a terahertz laser.

Improving the design of these gas lasers has been hampered by unreliable theoretical models, the researchers say. In small cavities at high gas pressures, the models predicted that, beyond a certain pressure, the molecules would be too “cramped” to spin and emit terahertz waves. Partly for this reason, terahertz gas lasers typically used meters-long cavities and large infrared lasers.  

However, in the 1980s, Everitt found that he was able to produce terahertz waves in his laboratory using a gas laser that was much smaller than traditional devices, at pressures far higher than the models said was possible. This discrepancy was never fully explained, and work on terahertz gas lasers fell by the wayside in favor of other approaches.

A few years ago, Everitt mentioned this theoretical mystery to Johnson when the two were collaborating on other work as part of MIT’s Institute for Soldier Nanotechnologies. Together with Everitt, Johnson and Wang took up the challenge, and ultimately formulated a new mathematical theory to describe the behavior of a gas in a molecular gas laser cavity. The theory also successfully explained how terahertz waves could be emitted, even from very small, high-pressure cavities.

Johnson says that while gas molecules can vibrate at multiple frequencies and rotational rates in response to an infrared pump, previous theories discounted many of these vibrational states and assumed instead that a handful of vibrations were what ultimately mattered in producing a terahertz wave. If a cavity were too small, previous theories suggested that molecules vibrating in response to an incoming infrared laser would collide more often with each other, releasing their energy rather than building it up further to spin and produce terahertz.

Instead, the new model tracked thousands of relevant vibrational and rotational states among millions of groups of molecules within a single cavity, using new computational tricks to make such a large problem tractable on a laptop computer. It then analyzed how those molecules would react to incoming infrared light, depending on their position and direction within the cavity.

“We found that when you include all these other vibrational states that people had been throwing out, they give you a buffer,” Johnson says. “In simpler models, the molecules are rotating, but when they bang into other molecules they lose everything. Once you include all these other states, that doesn’t happen anymore. These collisions can transfer energy to other vibrational states, and sort of give you more breathing room to keep rotating and keep making terahertz waves.”

Laughing, dialed up

Once the team found that their new model accurately predicted what Everitt observed decades ago, they collaborated with Capasso’s group at Harvard to design a new type of compact terahertz generator by combining the model with new gases and a new type of infrared laser.

For the infrared source, the researchers used a quantum cascade laser, or QCL — a more recent type of laser that is compact and also tunable.

“You can turn a dial, and it changes the frequency of the input laser, and the hope was that we could use that to change the frequency of the terahertz coming out,” Johnson says.

The researchers teamed up with Capasso, a pioneer in the development of QCLs, who provided a laser that produced a range of power that their theory predicted would work with a cavity the size of a pen (about 1/1,000 the size of a conventional cavity). The researchers then looked for a gas to spin up.

The team searched through libraries of gases to identify those that were known to rotate in a certain way in response to infrared light, eventually landing on nitrous oxide, or laughing gas, as an ideal and accessible candidate for their experiment.

They ordered laboratory-grade nitrous oxide, which they pumped into a pen-sized cavity. When they sent infrared light from the QCL into the cavity, they found they could produce a terahertz laser. As they tuned the QCL, the frequency of terahertz waves also shifted, across a wide range.

“These demonstrations confirm the universal concept of a terahertz molecular laser source which can be broadly tunable across its entire rotational states when pumped by a continuously tunable QCL,” Wang says.

Since these initial experiments, the researchers have extended their mathematical model to include a variety of other gas molecules, such as carbon monoxide and ammonia, providing scientists with a menu of different terahertz generation options with different frequencies and tuning ranges, paired with a QCL matched to each gas. The group’s theoretical tools also enable scientists to tailor the cavity design to different applications. They are now pushing toward more focused beams and higher powers, with commercial development on the horizon.

Johnson says scientists can refer to the group’s mathematical model to design new, compact and tunable terahertz lasers, using other gases and experimental parameters.

“These gas lasers were for a long time seen as old technology, and people assumed these were huge, low-power, nontunable things, so they looked to other terahertz sources,” Johnson says. “Now we’re saying they can be small, tunable, and much more efficient. You could fit this in your backpack, or in your vehicle for wireless communication or high-resolution imaging. Because you don’t want a cyclotron in your car.”

This research was supported in part by the U.S. Army Research Office and the National Science Foundation.

Terahertz pulse drives strontium titanate into hidden ferroelectric phase

Left: The initial configuration of STO, in which a central titanium ion (Ti4+) is surrounded by oxygen ions (red) and strontium ions (gray, Sr2+); there is no dipole moment and the crystal is paraelectric. Center: A THz pulse drives the soft lattice vibrational mode, causing positive and negative ions to move in different directions as shown by the arrows. Right: The resulting ferroelectric crystalline phase with lower-symmetry geometry and a dipole moment (μ). Credit: Science

By Kendra Redmond
https://www.cambridge.org/core/journals/mrs-bulletin/news/terahertz-pulse-drives-strontium-titanate-into-hidden-ferroelectric-phase

With an intense pulse of terahertz radiation, researchers from the Massachusetts Institute of Technology (MIT) and the University of Pennsylvania (Penn) have induced an ultrafast phase transition in a metal oxide. As reported in a recent issue of Science, the transition reveals a hidden phase in which the ferroelectric crystal structure displays different properties than in other phases of the material, suggesting a path toward the collective, coherent control of materials structures. 

The physical properties of a solid are largely controlled by the collective vibrations (phonons) of its crystal lattice. Ferroelectric structural phase transitions are usually associated with the so-called “soft” vibrational mode, which induces a new lattice structure that has reduced crystal symmetry and leads to electric polarization.  

A decade ago, a collaboration led by Keith Nelson at MIT and Andrew Rappe at Penn published a theoretical study suggesting that in some materials, terahertz-frequency (THz) radiation could excite a resonance in the soft mode of the crystal that would move the ions in the lattice from their position in one ferroelectric domain structure to their position in another, changing the direction of the ferroelectric polarization. 

In this new research, Nelson, Rappe, and their colleagues studied whether they could directly drive the soft mode in strontium titanate (SrTiO₃, STO) to activate a hidden ferroelectric phase. Hidden phases are metastable collective states of matter not usually accessible on a material’s equilibrium phase diagram. They are of special interest because hidden phases in conventional materials sometimes give rise to exotic physical properties. 

STO is a dielectric material with a cubic perovskite structure at room temperature. Unlike many perovskites, STO does not transition to a ferroelectric material at any point on its equilibrium phase diagram. Even at its critical temperature of 36 K, the material is paraelectric because quantum fluctuations prevent long-range ordering. As a result, the researchers call STO “a textbook example” of a material in the quantum paraelectric (QPE) phase. 

In an experiment at MIT led by then-graduate student Xian Li, the research team sent a single-cycle THz pump pulse through STO, followed by an optical pulse that probed the material’s response. They repeated this process for several different temperatures and THz field strengths. Spectroscopic analyses revealed characteristic signals of lower crystal symmetry, as well as nonlinear increases in dipole ordering and phonon amplitude as a function of field strength. These results demonstrate a QPE-to-ferroelectric phase transition. The new phase was maintained for about 10 ps. 

A complementary theoretical investigation by the Penn colleagues explored whether a single pulse could induce ferroelectricity in STO. The researchers ran a molecular dynamics simulation in which a rapid electric field pulse was applied to a supercell consisting of many units of the lattice. After running the simulation over a range of field strengths, they found that a THz pulse on the order of 200 kV/cm or greater can stimulate a soft mode response that drives ions to new positions in a ferroelectric lattice structure. This work also revealed how other vibrational modes adapt to the soft mode change, stabilizing the hidden polar phase. 

This research “highlights the unique capability of light to selectively deform a material lattice through vibrational resonances,” says Andrea Cavalleri, director of the Max Planck Institute (MPI) for the Structure and Dynamics of Matter and a professor at the University of Oxford. Cavalleri is familiar with this approach, as he recently led a separate research effort at MPI to influence the electric polarization of STO. The MPI team irradiated STO with a mid-infrared pulse over a range of temperatures and frequencies. They also saw signs of lattice deformation, signs that were most pronounced when the pulse was resonant with the highest-frequency vibrational mode of STO. Follow-up experiments suggested that the IR pulse induced a transition to a metastable ferroelectric phase that persisted for several hours. These results were published alongside the MIT-Penn results in Science. 

“[B]y driving a specific lattice deformation with a single-cycle terahertz pulse, Li et al. have shown that a ferroelectric order forms on ultrafast timescales. Gaining control of technologically relevant properties such as ferroelectricity on short timescales could open up new strategies for next-generation high-speed devices,” says Cavalleri. Rappe agrees. “These studies launch the age of ultrafast reconfiguration of nonlinear optics, paving the way for rapidly reconfigurable optical devices,” he says. 

The MIT-Penn team plans to explore applied facets of this research going forward. “We’ve dreamed for many years about coherent control over collective material structure,” says Nelson. “It is becoming possible to use THz fields to control crystal lattice structure, ferroelectric order, magnetic order, and electronic state (such as insulating or metallic). Next must be control over combinations of these properties in complex materials like high-temperature superconductors, in which changes in the properties are strongly coupled to each other,” he says. 

Read the abstract in Science.