Compact, chip-based lasers have conquered much of the electromagnetic spectrum, from ultraviolet to infrared, enabling technologies from digital communications and barcode readers to laser pointers and printers. Diode lasers depend on the process of electron-hole recombination: an electron from the conduction band recombines with a hole in the valence band, and in the process a single photon is emitted.
Diode lasers are limited to about 2.5 µm wavelength because the wavelength is determined by the recombination energy, or bandgap, of the material system used to fabricate the device. Different material combinations result in different bandgaps, but there is a limit to the materials that can be used to make a diode laser. The quantum cascade laser is a special kind of semiconductor laser, usually emitting mid-infrared light. Such a laser operates on laser transitions not between different electronic bands but on intersubband transitions of a semiconductor structure.
In 1994, researchers at AT&T Bell Labs created a new kind of laser in which the semiconductor’s structure, not just its chemistry, determined the wavelength. Called a quantum cascade laser (QCL), it contained hundreds of layers of semiconductors of precise thicknesses. Electrons injected into the structure cascade down hundreds of energy steps, shedding a photon at each one.
In contrast to diode lasers, Quantum cascade lasers are comprised of dozens of alternating layers of semiconductor material, forming quantum energy wells that confine the electrons to particular energy states. As each electron traverses the lasing medium it transitions from one quantum well to the next, driven by the voltage applied across the device. At precisely engineered locations, called the “active region,” the electron transitions from one valence band energy state to a lower one and in the process emits a photon. The electron continues through the structure and when it encounters the next active region it transitions again and emits another photon. The QCL may have as many as 75 active regions, and each electron generates that many photons as it traverses the structure.
Multiplication of these quantum heterostructures leads to a cascade effect with each electron responsible for the emission of multiple photons, thereby achieving higher quantum efficiencies than conventional laser diodes. The higher optical gain is obtained at the expense of a higher required electrical voltage. The operation voltage can easily be of the order of 10 V, whereas few volts are sufficient for ordinary laser diodes. Those photons were infrared in the first QCL, but in 2002 researchers in Italy and the United Kingdom created QCL lasers that emitted terahertz photons.
Applications include detecting chemical warfare agents and toxic industrial chemicals, monitoring building air quality, measuring greenhouse gases for atmospheric research, monitoring and controlling industrial processes, analyzing chemicals in exhaled breath for medical diagnostics, and many more. Compact, portable trace gas sensors enable operation in a wide range of platforms, including handheld units for use by first responders, fixed installations for monitoring air quality, and lightweight sensors for deployment in unmanned aerial vehicles (UAVs). The most important application for QCLs is in gas sensing and measurement. Systems based on widely-tunable QCLs can be used to measure multiple gas species, and narrowly targeted systems can detect and measure gas concentrations in the parts-per-trillion range.
At present QCLs are still somewhat specialized devices. Manufacturing is difficult to optimize, and small batch sizes result in high unit cost. As the value of these devices is more widely realized and more applications are created, QCLs will become more readily available and affordable.
Quantum cascade laser (QCL) technology
As the transition energies are defined not by fixed material properties but rather by design parameters (particularly by layer thickness values of quantum wells), quantum cascade lasers can be designed for operating wavelengths ranging from a few microns to well above 10 μm, or even in the terahertz region. By careful design of the quantum wells, lasing from 2.75 μm to 161 μm (1.9 THz) has been observed.
Epitaxially grown semiconductor structures provide periodically repeated stacks of quantum well heterostructures (for example, semiconductor superlattices) allowing for discrete intersubband transition of electrons injected within the conduction band of such devices. Distributed feedback QCLs (DFB QCL) are capable of wavelength tuning up to a few tens of cm-1. External cavity QCLs (EC-QCL) expand the tuning range to ~1000 cm-1, and are used for sensing and measuring multiple gas species. These types of QCLs are used for both local and remote sensing of numerous gas species, including CO, CO2, NH3, CH4, NOX, and SO2.
Terahertz QCLs are now being commercialized – some emitting in the range of 100 µm to 150 µm. More complex gas molecules absorb at these longer wavelengths, and greater measurement accuracy is possible with QCL-based systems than with current technologies.
The longer wavelength devices still require cryogenic cooling, but room temperature operation is possible to at least 16 μm. Commercial availability has concentrated in the mid-infrared (3.5 – 13 μm). In a quantum cascade laser, the mentioned quantum well structure is embedded in a waveguide, and the laser resonator is mostly of DBR or DFB type. There are also external-cavity lasers, where a wavelength tuning element such as a diffraction grating is part of the resonator.
Whereas continuously operating room-temperature devices are normally limited to moderate output power levels in the milliwatt region (although more than a watt is possible), multiple watts are easily possible with liquid-nitrogen cooling. Even at room temperature, watt-level peak powers are possible when using short pump pulses. The power conversion efficiency of quantum cascade lasers is typically of the order of a few tens of percent. Recently, however, devices with efficiencies around 50% have been demonstrated, although only for cryogenic operation conditions.
New designs of high power, continuous wave/room temperature (CW/RT) QCLs in the MWIR and LWIR regions have been demonstrated. At present, the highest power that has been reported in the MWIR region is about 5 W with device electrical power input to optical power output conversion efficiency (wall plug efficiency, WPE) of about 20% for TEC cooled devices with operation at ~ 20°C. Under similar operating conditions, CW/RT power outputs in excess of 1.4 W and WPE of 10% have been achieved at a wavelength of ~7.2 μm. Maxion Technologies is developing high-performance quantum-cascade (QC) lasers in the mid-IR spectral region for Army and other DoD and DHS infrared countermeasure (IRCM) applications.
The objectives are to: (1) develop and demonstrate a Band-IV (around 4.8-micron wavelength) QC laser operating quasi-cw (at 25% duty cycle) near room temperature with a peak output power in excess of 2 W (average power of 0.5 W), and (2) Illustrate the feasibility of power-scaling techniques for IRCM applications using QC laser technology by demonstrating a suitable laser-beam combining scheme with these lasers.
New lasers that fire terahertz beams could propel medical imaging and contraband detection
One key region of the spectrum had remained untamed with diode lasers in the terahertz band, which lies between infrared light and microwaves. Efficient sources of terahertz radiation can penetrate opaque objects and probe chemical fingerprints inside. But compact terahertz lasers have only worked at ultralow temperatures, limiting them mostly to laboratory settings.
Those devices needed to be chilled to 50 K, but in 2019, researchers led by physicist Jérôme Faist at ETH Zurich unveiled a terahertz QCL made up of hundreds of alternating layers of gallium arsenide and aluminum gallium arsenide (AlGaAs) that works at 210 K. It still required bulky and expensive cryogenic coolers, however. In Nov 2020 issue of Nature Photonics, researchers report creating a grain-of-rice–size terahertz laser on a chip that operates at 250 K, or –23°C, within reach of a plug-in cooler the size of a cracker.
“This is a great achievement,” says Miriam Vitiello, a condensed matter physicist at the Nanoscience Institute of Italy’s National Research Council. “It has been a long-term goal in the community to push up the temperature of terahertz lasers,” she adds. “There is now a plethora of applications that can be done,” from medical imaging to explosives detection at airports.
At higher temperature the electrons leap the barriers between layers rather than cascading through the structure one step at a time. “Over-the-barrier electron leakage was the killer,” says Qing Hu, an electrical engineer at the Massachusetts Institute of Technology. So Hu and his colleagues added more aluminum to the AlGaAs barriers in hopes of better confining the electrons. Hu’s team also had to prevent electrons from interacting in a way that caused them to leak through the AlGaAs barriers.
Now, Hu’s team has shown that by tailoring its layered structure even more precisely—some layers were just seven atoms thick—it could make electrons behave at temperatures warm enough to be reached with standard compact thermoelectric coolers. What’s more, Hu says, the same strategy should enable the team to eventually make room temperature terahertz lasers. Room temperature terahertz sources could be paired with terahertz detectors that also work at room temperature, which Vitiello and other researchers are now developing. That marriage could lead to technologies such as terahertz imagers able to distinguish skin cancer from normal tissue without a biopsy or watch airline passengers and cargo for hidden explosives, illegal drugs, and even pharmaceutical fakes. Faist says: “We have hoped for this for a very long time.”
High-Efficiency Quantum Cascade Laser Leads to Research Contract for Intraband
Intraband announced in Feb 2019 that it has received a Department of Defense (DOD) Navy STTR Phase II contract to develop quantum cascade lasers (QCLs) with 40% wall-plug efficiency for the mid-wave infrared (MWIR) wavelength region. Previously, Intraband and UW-Madison researchers had reported record-high internal QCL efficiency values and that the upper limit in QCL wall-plug efficiency can be significantly raised based on their patented Step-Taper Active approach to QCL design. In turn, Intraband team’s proposal led to winning the $1,000,000 three-year program.
High wall-plug efficiency has been key to the dramatic success of conventional near-infrared lasers for marking, welding, and medical procedures. The low input-power requirement together with the small size of the semiconductor-laser chip enable systems that combine many of those lasers together to provide the extremely-high optical power levels needed for those applications. The QCL is now poised to bring these same advantages to the MWIR spectral region which includes important “windows” in atmospheric absorption where infrared light may be transmitted over many kilometers, enabling free-space communication and environmental gas sensing. MWIR light sources are key to many applications in the medical, industrial, and defense industries.
Dan Botez, the project’s Principal Investigator and Intraband co-founder, states “Our approach to efficiency is to use band-gap and interface-roughness engineering to get the most out of every injected electron while minimizing device operating voltage. Industry-proven, metal-organic chemical-vapor deposition (MOCVD) allows us to use material compositions not possible with other crystal-growth methods while providing the best device reliability.” Intraband President, Rob Marsland, comments, “The system developers are very excited about the prospect of QCLs with dramatically higher wall-plug efficiency. They can begin to implement QCL solutions now knowing that there is room for continuing improvement.”
Scientists create most efficient quantum cascade laser ever
A team of UCF researchers has produced the most efficient quantum cascade laser ever designed – and done it in a way that makes the lasers easier to manufacture. Quantum cascade lasers, or QCLs, are tiny – smaller than a grain of rice – but they pack a punch. Compared to traditional lasers, QCLs offer higher power output and can be tuned to a wide range of infrared wavelengths. They can also be used at room temperature without the need for bulky cooling systems. But because they’re difficult and costly to produce, QCLs aren’t used much outside the Department of Defense.
A University of Central Florida team led by Assistant Professor Arkadiy Lyakh has developed a simpler process for creating such lasers, with comparable performance and better efficiency. The results were published recently in the scientific journal Applied Physics Letters.
“The previous record was achieved using a design that’s a little exotic, that’s somewhat difficult to reproduce in real life,” Lyakh said. “We improved on that record, but what’s really important is that we did it in such a way that it’s easier to transition this technology to production. From a practical standpoint, it’s an important result.”
That could lead to greater usage in spectroscopy, such as using the infrared lasers as remote sensors to detect gases and toxins in the atmosphere. Lyakh, who has joint appointments with UCF’s NanoScience Technology Center and the College of Optics and Photonics, envisions portable health devices. For instance, a small QCL-embedded device could be plugged into a smartphone and used to diagnose health problems by simply analyzing one’s exhaled breath. “But for a handheld device, it has to be as efficient as possible so it doesn’t drain your battery and it won’t generate a lot of heat,” Lyakh said.
The method that previously produced the highest efficiency called for the QCL atop a substrate made up of more than 1,000 layers, each one barely thicker than a single atom. Each layer was composed of one of five different materials, making production challenging.The new method developed at UCF uses only two different materials – a simpler design from a production standpoint.
Harvard, MIT, Duke, US Army develop for tunable QCL Terahertz Technology
Researchers from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with the Massachusetts Institute of Technology and the U.S. Army, have developed a compact, room-temperature, widely tunable terahertz laser. The research is published in Science.
Researchers have created a new terahertz radiation emitter with coveted frequency adjustment capability. The compact source could enable the development of futuristic communications, security, biomedical, and astronomical imaging systems. Terahertz electromagnetic frequencies have been sought after for their range of applications, such as high bandwidth, high resolution, long-range sensing, and ability to visualize objects through materials. However, the costliness, bulk, inefficiencies, and lack of tunability of traditional terahertz emitters has stymied growing markets. This new combined laser terahertz source, made possible through the collaboration of researchers at Harvard, MIT, Duke, and the U.S. Army, 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 to 20% of the main frequency, so it’s fair to say that terahertz is underutilized,” said co-senior author Federico Capasso from Harvard University. “Our laser opens up this spectral region, and in my opinion, will have a revolutionary impact.” 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,” commented 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, such as short-range, high-bandwidth wireless communications, very high-resolution radar, and spectroscopy,” said Henry Everitt, senior technologist with the U.S. Army CCDC Aviation & Missile Center and co-senior author of the paper. Everitt is also an adjunct professor of physics at Duke University. 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.
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 have taken a different approach. The team says that their “breakthrough” was achieved by the use of a highly tunable, quantum-cascade laser (QCL) as the 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. The theory to optimize the operation of the new laser was developed by Steven 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 laser based on nitrous oxide. “By optimizing the laser cavity and lenses, we were able to produce frequencies spanning nearly 1THz,” 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 Hänsch of the Max-Planck Institute for Quantum Optics in Munich, Germany, who was not involved in this research. “Such sources will unlock new applications from sensing to fundamental spectroscopy.”
Capasso is the inventor of 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.
At a conference in 2017, Capasso met Henry Everitt, senior technologist with the U.S. Army and adjunct professor at Duke University. Through their discussion the idea was birthed to apply the widely tunable QCL to a laser with terahertz ability. 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 was able to use the QCL to dramatically increase the terahertz tuning range of a nitrous oxide OPFIR laser.
“The same laser is now widely tunable,” Capasso said. “It’s a fantastic marriage between two existing lasers.” In initial experiments with the shoebox-size 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.
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. “This is a universal concept, because it can be applied to other gases,” Capasso said. “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.” Capasso and Everitt are hoping to use their laser to look skyward and identify unknown spectral features in the terahertz region. The team is now developing high-power terahertz QPMLs for astronomical observations.
Broad area quantum cascade laser with high brightness
Scientists at the Air Force Research Laboratory have recently invented a broad-area quantum cascade laser with fundamental transverse mode operation, which allows most of the power to be contained in a near-diffraction-limited beam with high brightness. The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.
Quantum cascade lasers are unipolar semiconductor lasers that use optical transitions between electronic sub-bands to produce light. They can be used for chemical sensing applications including pollution monitoring, gas sensing, medical diagnostics through breath analysis, and the remote detection of toxic chemicals and explosives. However, increasing the power of broad-area quantum cascade lasers results in the operation of high-order transverse modes that result in mode competition, beam steering, and loss of brightness.
Air Force researchers have developed a broad-area quantum cascade laser that sustains fundamental transverse mode operation to attain higher brightness in a single beam that is relatively easy to operate. They employ a method that suppresses the high-order transverse modes by generating a lateral constriction in the waveguide in the form of short trenches defined by the focused ion beam milling technique. This modification allows fundamental transverse mode operation to be sustained even when the cavity width is enlarged to produce higher power.
By extracting the fundamental mode and providing emission along the optical axis, the laser attains higher brightness in a single beam that is relatively easy to operate. They Can be designed to emit in the mid-infrared and long-infrared wavelength ranges of the electromagnetic spectrum which has applications in the areas of remote sensing, long-wave imaging, communications, aircraft countermeasures, etc.
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