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.

Laser technology heats up with research

http://www.theredstonerocket.com/military_scene/article_e6752784-7a16-11e8-9463-ebba083bdd6a.html

Gas phase optically pumped far infrared lasers were once the most powerful sources of radiation in the challenging terahertz spectral region. Unfortunately, these lasers were enormous, often filling an entire optical table to produce milliwatts of power and requiring a different gas each time a different wavelength was needed.

Consequently, when more compact, more tunable alternative sources of terahertz radiation were discovered, OPFIR lasers were largely abandoned. 

However, in the mid-1980s professor Frank De Lucia, then of the Duke Physics Department, recognized that a more compact version of OPFIR lasers was possible with then student and now adjunct professor Henry Everitt of the Duke Physics Department. They discovered that OPFIR lasers worked at much higher pressures and with much more tunability than their enormous commercial counterparts. 

By 1990 the basic understanding had been worked out, but the complexity of the problem evaded accurate theoretical modeling. Fast forward to 2010 when Everitt, a senior scientist with the Aviation and Missile Research, Development and Engineering Center, partnered with researchers at the Army-funded MIT Institute for Soldier Nanotechnologies to develop such a model in hopes of designing optimized OPFIR laser cavities. 

The breakthrough came in 2016 when graduate student Fan Wang, working with Everitt and MIT faculty Steven Johnson, Marin Soljacic, and John Joannopoulos, reformulated the problem and developed a model that precisely predicted the observed behavior with no adjustable parameters. To their surprise, they discovered that operating the laser in the unprecedented high pressure regime actually improved the efficiency a factor of 10 higher than commercial lasers and within a factor of three of the maximum possible efficiency. This breakthrough, reported in the Proceedings of the National Academy of Science, heralds the possible rebirth of OPFIR lasers as powerful, efficient, tunable sources of terahertz radiation.

Abstract-A high-efficiency regime for gas-phase terahertz lasers

Fan Wang, Dane J. Phillips, Jeongwon Lee,  Henry O. Everitt

https://www.researchgate.net/publication/325702634_A_high-efficiency_regime_for_gas-phase_terahertz_lasers

We present both an innovative theoretical model and an experimental validation of a molecular gas optically pumped far-infrared (OPFIR) laser at 0.25 THz that exhibits 10× greater efficiency (39% of the Manley–Rowe limit) and 1,000× smaller volume than comparable commercial lasers. Unlike previous OPFIR-laser models involving only a few energy levels that failed even qualitatively to match experiments at high pressures, our ab initio theory matches experiments quantitatively, within experimental uncertainties with no free parameters, by accurately capturing the interplay of millions of degrees of freedom in the laser. We show that previous OPFIR lasers were inefficient simply by being too large and that high powers favor high pressures and small cavities. We believe that these results will revive interest in OPFIR laser as a powerful and compact source of terahertz radiation.

Abstract-Dimensionality Reduction for Identification of Hepatic Tumor Samples Based on Terahertz Time-Domain Spectroscopy

 Haishun Liu,  Zhenwei Zhang, Xin Zhang,  Yuping Yang, Zhuoyong Zhang, Xiangyi Liu, Fan Wang,  Yiding Han, Cunlin Zhang

https://ieeexplore.ieee.org/document/8333778/

Terahertz time-domain spectroscopy (THz-TDS) combining with chemometrics methods was proposed for the identification of hepatic tumors. Two linear compression methods, principle component analysis and locality preserving projections (LPPs), and a nonlinear method, Isomap, were used to reduce the dimensionality of the measured dataset. Comparing two-dimensional (2-D) data reduced by these three dimensionality reduction techniques, only 2-D Isomap plot could separate the distances between two classes for the THz time-domain data and LPP had capacity of distinguishing two types of samples building on frequency-domain data. The best classification accuracies from 2-D time-domain data were 99.81±0.30% and 99.69±0.61% given by Isomap probabilistic neural network (PNN) and Isomap support vector machine (SVM), respectively, while the best classification results of 2-D frequency-domain data were 100.00±0.00%, 99.75±0.32% provided by LPP-PNN, LPP-SVM. The results showed that Isomap and LPP are appropriate techniques to reflect the nonlinear manifold of the THz data. The THz technology either in time-domain or frequency-domain coupled with Isomap-PNN or LPP-PNN could offer a potential procedure to identify hepatic tumors.

Abstract-Single Nanowire Photoconductive Terahertz Detectors

Kun Peng , Patrick Parkinson , Lan Fu , Qiang Gao , Nian Jiang , Yanan Guo , Fan Wang , Hannah J Joyce , Jessica L Boland , Hark Hoe Tan , Chennupati Jagadish , and Michael B Johnston

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/nl5033843

Publication Date (Web): December 9, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/nl5033843

Spectroscopy and imaging in the terahertz (THz) region of the electromagnetic spectrum has proven to provide important insights in fields as diverse as chemical analysis, materials characterization, security screening and non-destructive testing. However, compact optoelectronics suited to the most powerful terahertz technique – time-domain spectroscopy – are lacking. Here, we implement single GaAs nanowires as microscopic coherent THz sensors and for the first time incorporated them into the pulsed time-domain technique. We also demonstrate the functionality of the single nanowire THz detector as a spectrometer by using it to measure the transmission spectrum of a 290 GHz low pass filter. Thus nanowires are shown to be well suited for THz device applications, and hold particular promise as near-field terahertz sensors.