A breakthrough in developing multi-watt terahertz lasers

A phase-locking scheme for plasmonic lasers is developed in which traveling surface-waves longitudinally couple several metallic microcavities in a surface-emitting laser array. Multi-watt emission is demonstrated for single-mode terahertz lasers in which more photons are radiated from the laser array than those absorbed within the array as optical losses. Credit: Yuan Jin, Lehigh University

https://phys.org/news/2020-06-breakthrough-multi-watt-terahertz-lasers.html

Terahertz lasers could soon have their moment. Emitting radiation that sits somewhere between microwaves and infrared light along the electromagnetic spectrum, terahertz lasers have been the focus of intense study due to their ability to penetrate common packaging materials such as plastics, fabrics, and cardboard and be used for identification and detection of various chemicals and biomolecular species, and even for imaging of some types of biological tissue without causing damage. Fulfilling terahertz lasers' potential for us hinges on improving their intensity and brightness, achieved by enhancing power output and beam quality.

Sushil Kumar, associate professor in Lehigh University's Department of Electrical and Computer Engineering, and his research team are working at the forefront of terahertz semiconductor 'quantum-cascade' laser (QCL) technology. In 2018, Kumar, who is also affiliated with Lehigh's Center for Photonics and Nanoelectronics (CPN) reported on a simple yet effective technique to enhance the power output of single-mode lasers based on a new type of "distributed-feedback" mechanism. The results were published in the journal Nature Communications and received a lot of attention as a major advance in terahertz QCL technology. The work was performed by graduate students, including Yuan Jin, supervised by Kumar and in collaboration with Sandia National Laboratories.

Now, Kumar, Jin and John L. Reno of Sandia are reporting another terahertz technology breakthrough: they have developed a new phase-locking technique for plasmonic lasers and, through its use, achieved a record-high power output for terahertz lasers. Their laser produced the highest radiative efficiency for any single-wavelength semiconductor quantum cascade laser. These results are explained in a paper, "Phase-locked terahertz plasmonic laser array with 2 W output power in a single spectral mode" published yesterday in Optica.

"To the best of our knowledge, the radiative efficiency of our terahertz lasers is the highest demonstrated for any single-wavelength QCL to-date and is the first report of a radiative efficiency of greater than 50% achieved in such QCLs," said Kumar. "Such a high radiative efficiency beat our expectations, and it is also one of the reasons why the output power from our laser is significantly greater than what has been achieved previously."

To enhance the optical power output and beam quality of semiconductor lasers, scientists often utilize phase-locking, an electromagnetic control system that forces an array of optical cavities to emit radiation in lock step. Terahertz QCLs, which utilize optical cavities with metal coatings (claddings) for light confinement, are a class of lasers known as plasmonic lasers that are notorious for their poor radiative properties. There are only a limited number of techniques available in prior literature, they say, that could be utilized to improve radiative efficiency and output power of such plasmonic lasers by significant margins.

"Our paper describes a new phase-locking scheme for plasmonic lasers that is distinctly different from prior research on phase-locked lasers in the vast literature on semiconductor lasers," says Jin. "The demonstrated method makes use of traveling surface waves of electromagnetic radiation as a tool for phase-locking of plasmonic optical cavities. The efficacy of the method is demonstrated by achieving record-high output power for terahertz lasers that has been increased by an order of magnitude compared to prior work."

Traveling surface waves that propagate along the metal layer of the cavities, but outside in the surrounding medium of the cavities rather than inside, is a unique method that has been developed in Kumar's group in recent years and one that continues to open new avenues for further innovation. The team expects that the output power level of their lasers could lead to collaborations between laser researchers and application scientists toward development of terahertz spectroscopy and sensing platforms based on these lasers.

This innovation in QCL technology is the result of a long term research effort by Kumar's lab at Lehigh. Kumar and Jin jointly developed the finally-implemented idea through design and experimentation over a period of approximately two years. The collaboration with Dr. Reno from the Sandia National Laboratories allowed Kumar and his team to receive semiconductor material to form the quantum cascade optical medium for these lasers.

The primary innovation in this work, according to the researchers, is in the design of the optical cavities, which is somewhat independent from the properties of the semiconductor material. The newly acquired inductively-coupled plasma (ICP) etching tool at Lehigh's CPN played a critical role in pushing the performance boundaries of these lasers, they say.

This research represents a paradigm shift in how such single-wavelength terahertz lasers with narrow beams are developed and will be developed going forward in future, says Kumar, adding: "I think the future of terahertz lasers is looking very bright."

PF-SNOM Characterization Technique Reveals 3D Shape of Polariton Interaction Around Nanostructures

The holy grail of novel materials is believed to be nanostructures. For instance, the wonder material - graphene - is a one layer of carbon atoms that are organized in a hexagonal pattern, and thanks to its strength, transparency, conductivity, and flexibility, it can possibly lead to more efficient solar cells, faster and smaller microchips and electric circuits, high-density batteries and capacitors, and transparent displays.

This is an image of Xiaoji Xu, Assistant Professor, Department of Chemistry, Lehigh University. (Image credit: Douglas Benedict/Academic Image)

https://www.azonano.com/news.aspx?newsID=36192

Another quality that makes graphene and other nanomaterials so unique is their ability to create a physics phenomenon known as a polariton, says Xiaoji Xu, assistant professor in the Department of Chemistry at Lehigh University.

An intense coupling of electromagnetic waves with a magnetic or electric dipole-carrying excitation results in quasiparticles called polaritons. Some refer to this as light-matter coupling. These polaritons allow nanostructures to confine and compress light around the material. For future computing and optical communications, the ability to compress light is very important to scale down devices. In fact, it could lead to sensing at a scale less than one nanometer, which is significant for realizing biomedical advancements in detection, prevention, and treatment of various diseases.

For those who are exploring these materials, the challenge would be to how to expose and define the polaritons at the nanoscale because this cannot be done by traditional microscope, says Xu.

Now, Xu and his co-workers have discovered a technique to expose the 3D shape of the polariton interaction that takes place around a nanostructure. Their method improves upon the standard spectroscopic imaging method called scattering-type scanning near-field optical microscopy (s-SNOM). The new technique, known as peak force scattering-type scanning near-field optical microscopy (PF-SNOM), functions through a combination of time-gated light detection and peak force tapping mode. The scientists have described their work in an article titled:

The authors state in the paper: "PF-SNOM enables direct sectioning of vertical near-field signals from a sample surface for both three-dimensional near-field imaging and spectroscopic analysis. Tip-induced relaxation of surface phonon polaritons are revealed and modeled by considering tip damping.""Tomographic and multimodal scattering-type scanning near-field optical microscopy with peak force tapping mode" (DOI: 10.1038/s41467-018-04403-5) published online in Nature Communications on May 21, 2018. Besides Xu, Le Wang, Haomin Wang, and Devon S. Jakob, Ph.D. students in Xu's lab, are the paper's co-authors.

According to the team, PF-SNOM also provides an enhanced spatial resolution of 5 nanometers, instead of the usual 10 nanometers provided by the conventional s-SNOM technique.

"Our technique could be beneficial to scientists studying nanostructures enabling them to better understand how the electrical field is distributed around a given nanostructure," says Xu.

The researchers’ PF-SNOM characterization technique is more direct than current methods and at the same time obtains the polaritonic, electrical, and mechanical information. With a single measurement, multiple modes of information can be achieved, which is indeed a special advantage, explains Xu.

The advancement of the PF-SNOM characterization method emerged from the researchers’ analysis of gap mode—when a pair of plasmonic structures approaches within a few nanometers, a large improvement of the plasmon intensity is observed in the gap between both the structures as energy is moved from one structure to the other structure. Thanks to their ability to close this gap mode response in simulations, the team decided to extend it to the non-gap mode as well - when increasing the distance between the sample and the atomic force microscopy (AFM) probe tip.

An interesting fact is that when the researchers started their experiments they anticipated a different result, but during the simulations, they noticed a unique shape of light scattering and observed a clear enhancement of the gap mode.

"It turned out that we could section the light in different tip-samples distances and use those signals to view the near-field response at different layers and in vertical directions," says Wang.

He adds: "Though this work was done with infrared, in principle it could also be extended to other frequencies, such as visible and terahertz."

Abstract-Research team demonstrates terahertz semiconductor laser with record-high output power

Left to right: Research contributors and Lehigh electrical and computer engineering graduate students Ji Chen, Liang Gao and Yuan Jin stand in the Terahertz Photonics laboratory of Sushil Kumar in the Sinclair Building at Lehigh University. Credit: Sushil Kumar, Lehigh University

https://phys.org/news/2018-04-team-terahertz-semiconductor-laser-record-high.html#jCp

The ability to harness light into an intense beam of monochromatic radiation in a laser has revolutionized the way we live and work for more than fifty years. Among its many applications are ultrafast and high-capacity data communications, manufacturing, surgery, barcode scanners, printers, self-driving technology and spectacular laser light displays. Lasers also find a home in atomic and molecular spectroscopy used in various branches of science as well as for the detection and analysis of a wide range of chemicals and biomolecules.
Lasers can be categorized based on their emission wavelength within the electromagnetic spectrum, of which visible light lasers—such as those in laser pointers—are only one small part. Infrared lasers are used for optical communications through fibers. Ultraviolet lasers are used for eye surgery. And then there are terahertz lasers, which are the subject of investigation at the research group of Sushil Kumar, an associate professor of Electrical and Computer Engineering at Lehigh University.

Terahertz lasers emit radiation that sits between microwaves and infrared light along the electromagnetic spectrum. Their radiation can penetrate common packaging materials such as plastics, fabrics and cardboard, and are also remarkably effective in optical sensing and analysis of a wide variety of chemicals. These lasers have the potential for use in non-destructive screening and detection of packaged explosives and illicit drugs, evaluation of pharmaceutical compounds, screening for skin cancer and even the study of star and galaxy formation.

Applications such as optical spectroscopy require the laser to emit radiation at a precise wavelength, which is most commonly achieved by implementing a technique known as "distributed-feedback." Such devices are called single-mode lasers. Requiring single-mode operation is especially important for terahertz lasers, since their most important applications will be in terahertz spectroscopy. Terahertz lasers are still in a developmental phase and researchers around the world are trying to improve their performance characteristics to meet the conditions that would make them commercially viable.

Top: A scanning electron microscope image of a high-power surface-emitting terahertz semiconductor laser with hybrid gratings. Multiple lasers are fabricated on a Gallium Arsenide semiconductor chip. Each laser is approximately 1.5mm long, 10 microns thick and varies in width between 0.1mm to 0.2mm. Bottom: Artistic illustration of the terahertz laser in operation. The laser's semiconductor material is sandwiched between metallic layers on both top and bottom. A periodic grating is introduced in the top metallic layer in the form of apertures from where light could leak out. An interplay of second- and fourth-order Bragg gratings (manifested as alternating single and double slits) leads to intense radiation from alternating periods of the periodic structure, combining coherently into a high quality single-lobed laser beam in the surface-normal direction. Credit: Sushil Kumar, Lehigh University

As it propagates, terahertz radiation is absorbed by atmospheric humidity. Therefore, a key requirement for such lasers is an intense beam such that it could be used for optical sensing and analysis of substances kept at a standoff distance of several meters or more, and not be absorbed. To this end, Kumar's research team is focused on improving their intensity and brightness, achievable in part by increasing optical power output.

In a recent paper published in the journal Nature Communications, the Lehigh team—supervised by Kumar in collaboration with Sandia National Laboratories—reported on a simple yet effective technique to enhance the power output of single-mode lasers that are "surface-emitting" (as opposed to those using an "edge-emitting" configuration). Of the two types, the surface-emitting configuration for semiconductor lasers offers distinctive advantages in how the lasers could be miniaturized, packaged and tested for commercial production.

The published research describes a new technique by which a specific type of periodicity is introduced in the laser's optical cavity, allowing it to fundamentally radiate a good quality beam with increased radiation efficiency, thus making the laser more powerful. The authors call their scheme as having a "hybrid second- and fourth-order Bragg grating" (as opposed to a second-order Bragg grating for the typical surface-emitting laser, variations of which have been used in a wide variety of lasers for close to three decades). The authors claim that their hybrid grating scheme is not limited to terahertz lasers and could potentially improve performance of a broad class of surface-emitting semiconductor lasers that emit at different wavelengths.

The report discusses experimental results for a monolithic single-mode terahertz laser with a power output of 170 milliwatts, which is the most powerful to date for such class of lasers. The research shows conclusively that the so-called hybrid grating is able to make the laser emit at a specific desired wavelength through a simple alteration in the periodicity of imprinted grating in the laser's cavity while maintaining its beam quality. Kumar maintains that power levels of one watt and above should be achievable with future modifications of their technique—which might just be the threshold needed to be overcome for industry to take notice and step into potential commercialization of terahertz laser-based instruments.

A spectrum of potential

A sharper focus for quantum cascade lasers

http://www.lehigh.edu/engineering/research/resolve/2017v1/briefs-kumar-quantum-cascade-lasers.html

Once the preferred weapon of B-movie madmen and space-fantasy heroes alike, the laser—a device that generates an intense beam of coherent electromagnetic radiation by stimulating the emission of photons from excited atoms or molecules—has grown a bit domesticated of late.

Known for document printing and home theaters, here and there it pops up in medical journals and military news, but it's basically been reduced to reading barcodes at the grocery checkout: A technology that's lost its mojo.

Read the full version of this story in the Lehigh Engineering News Center.

But lasers are still cool, insists Sushil Kumar of Lehigh University, with vast potential for innovation we've just begun to tap. And with support from the National Science Foundation, he's on a mission to prove it.

Kumar, an associate professor of electrical and computer engineering, focuses specifically on lasers that arise from a relatively unexploited region in the electromagnetic spectrum, the terahertz (THz), or far infrared, frequency. A researcher at the forefront of THz semiconductor 'quantum-cascade' laser technology, he and his colleagues have posted world-record results for high-temperature operation and other important performance characteristics of such lasers.

His goal is to develop devices that open up a wide array of possible applications: chemical and biological sensing, spectroscopy, detection of explosives and other contraband materials, disease diagnosis, quality control in pharmaceuticals, and even remote-sensing in astronomy to understand star and galaxy formation, just to name a few.

Yet despite the known benefits, Kumar says that terahertz lasers have been underutilized and underexplored; high cost and functional limitations have stymied the innovation that would lead to such usage.

Focusing on a solution

Kumar believes he's on track to truly unleash the power of THz laser technology; he recently received a grant from the NSF, "Phase-locked arrays of high-power terahertz lasers with ultra-narrow beams," with a goal of creating THz lasers that produce vastly greater optical intensities than currently possible—and potentially removing barriers to widescale research and commercial adoption.

“The terahertz region of the electromagnetic spectrum is significantly underdeveloped due to lack of high-power sources of radiation,” he explains. “Existing sources feature low output power and other undesired spectral characteristics which makes them unsuitable for serious application.”

His current project aims to develop terahertz semiconductor lasers with precise emission frequency of up to 100 milliwatts of average optical power—an improvement of two orders of magnitude over current technology—in a narrow beam with significantly less than five degrees of angular divergence.

Kumar works with quantum cascade lasers (QCLs). These devices were originally invented for emission of mid-infrared radiation. They have only recently begun to make a mark at THz frequencies, and in that range they suffer from several additional challenges. In this cutting-edge environment, Kumar's group is among a select few in the world making progress toward viable and low-cost production of these lasers.

Kumar's intended approach will significantly improve power output and beam quality from QCLs. A portable, electrically-operated cryocooler will provide the required temperature cooling for the semiconductor laser chips; these will contain phase-locked QCL arrays emitting at a range of discrete terahertz frequencies determined by the desired application.

In previous work, Kumar and his group showed that THz lasers (emitting at a wavelength of approximately 100 microns) could emit a focused beam of light by utilizing a technique called distributed feedback. The light energy in their laser is confined inside a cavity sandwiched between two metallic plates separated by a distance of 10 microns. Using a box-shaped cavity measuring 10 microns by 100 microns by 1,400 microns (1.4 millimeters), the group produced a terahertz laser with a beam divergence angle of just 4 degrees by 4 degrees, the narrowest divergence yet achieved for such terahertz lasers from a single laser cavity.

Kumar believes most companies that currently employ mid-infrared lasers would be interested in powerful, affordable terahertz QCLs, and that the technology itself will spawn new solutions.

"The iPhone needed to exist before developers could write the 'killer apps' that made it a household product," he says. "In the same way, we are working toward a technology that could allow future researchers to change the world in ways that have yet to even be considered."

Making lasers cool again

Lehigh University laser researcher boldly goes into uncharted THz territory

In this illustration of a terahertz plasmonic laser, the laser cavity is enclosed between two metal films (with periodic slits on the top film). The colors represent coherent SPP light waves. One wave is confined inside the 10-micron-thick cavity. The other, with a large spatial extent, is located on top of the cavity.

https://www.eurekalert.org/pub_releases/2016-10/lu-mlc103116.php

Once the preferred weapon of B-movie madmen and space-fiction heroes alike, the laser -- a device that generates an intense beam of coherent electromagnetic radiation by stimulating the emission of photons from excited atoms or molecules -- has grown a bit domesticated of late.

These days, it has a steady job in industry, and spends its spare time printing documents in home offices and playing back movies in home theaters. Here and there it pops up in medical journals and military news, but it's basically been reduced to reading barcodes at the grocery checkout -- a technology that's lost its mojo.

But lasers are still cool, insists Sushil Kumar of Lehigh University, with vast potential for innovation we've just begun to tap. And with support from the National Science Foundation (NSF), he's on a mission to prove it.

Kumar, an associate professor of electrical and computer engineering, focuses specifically on lasers that arise from a relatively unexploited region in the electromagnetic spectrum, the terahertz (THz), or far infrared, frequency. A researcher at the forefront of THz semiconductor 'quantum-cascade' laser technology, he and his colleagues have posted world-record results for high-temperature operation and other important performance characteristics of such lasers.

His goal is to develop devices that open up a wide array of possible applications: chemical and biological sensing, spectroscopy, detection of explosives and other contraband materials, disease diagnosis, quality control in pharmaceuticals, and even remote-sensing in astronomy to understand star and galaxy formation, just to name a few. (Pretty cool stuff...the folks back at the checkout line would be impressed.)

Yet despite the known benefits, Kumar says that terahertz lasers have been underutilized and underexplored; high cost and functional limitations have stymied the innovation that would lead to such usage. Kumar, however, believes he's on track to truly unleash the power of THz laser technology; he recently received a grant from the NSF, Phase-locked arrays of high-power terahertz lasers with ultra-narrow beams, with a goal of creating THz lasers that produce vastly greater optical intensities than currently possible -- and potentially removing barriers to widescale research and commercial adoption.

Focusing on a solution

According to Kumar, the terahertz region of the electromagnetic spectrum is significantly underdeveloped due to lack of high-power sources of radiation. Existing sources feature low output power and other undesired spectral characteristics which makes them unsuitable for serious application. His current project aims to develop terahertz semiconductor lasers with precise emission frequency of up to 100 milliwatts of average optical power -- an improvement of two orders of magnitude over current technology -- in a narrow beam with signifcantly less than five degrees of angular divergence.

Kumar works with quantum cascade lasers (QCLs). These devices were originally invented for emission of mid-infrared radiation. They have only recently begun to make a mark at THz frequencies, and in that range they suffer from several additional challenges. In this cutting-edge environment, Kumar's group is among a select few in the world making progress toward viable and low-cost production of these lasers.

Kumar's intended approach will significantly improve power output and beam quality from QCLs. A portable, electrically-operated cryocooler will provide the required temperature-cooling for the semiconductor laser chips; these will contain phase-locked QCL arrays emitting at a range of discrete terahertz frequencies determined by the desired application.

In previous work, Kumar and his group showed that THz lasers (emtting at a wavelength of approximately 100 microns) could emit a focused beam of light by utilizing a technique called distributed feedback. The light energy in their laser is confined inside a cavity sandwiched between two metallic plates separated by a distance of 10 microns. Using a box-shaped cavity measuring 10 microns by 100 microns by 1,400 microns (1.4 millimeters), the group produced a terahertz laser with a beam divergence angle of just 4 degrees by 4 degrees, the narrowest divergence yet achieved for such terahertz lasers.

Kumar believes most companies that currently employ mid-infrared lasers would be interested in powerful, affordable terahertz QCLs, and that the technology itself will spawn new solutions.

"The iPhone needed to exist before developers could write the 'killer apps' that made it a household product," he says. "In the same way, we are working toward a technology that could allow future researchers to change the world in ways that have yet to even be considered."

Plasmonic Lasers Get a Sharper Focus

In the Lehigh team’s “antenna feedback” approach, the plasmonic-laser cavity is enclosed between two metal films, with periodic slits on the top film. One SPP wave is confined inside the 10-micron-thick cavity; the other, with a larger spatial extent, is located on top of the cavity and coupled both to it and the far field, allowing a strong, narrow-beam emission. [Image: Sushil Kumar]

Stewart Wills

Lasers based on coherent surface plasmon polaritons (SPPs)—subwavelength oscillations of electrons that are excited when incident light hits a metal-dielectric interface—hold promise for ultraminiaturized, chip-scale optics, and also as a possible platform for terahertz quantum cascade lasers (QCLs). But there’s a catch: SPP lasers, precisely because of their subwavelength apertures, tend to have divergent radiation patterns, making it tough to produce a sharp, directional beam.

Now, a research team led by Sushil Kumar of Lehigh University, Penn., USA, has devised an “antenna feedback” scheme that reportedly can provide single-mode operation and strong, highly directional far-field coupling in such SPP lasers, bringing them “closer to practical applications” (Optica, doi:10.1364/OPTICA.3.000734). The team’s work includes a proof-of-concept terahertz QCL based on the scheme that, according to the study, achieved the narrowest beam yet reported for such a QCL.

The pros and cons of “spasers”

SPP lasers—also called plasmonic lasers or “spasers”—operate by confining light energy as coherent SPP oscillations in subwavelength cavities (commonly parallel-plate Fabry-Perot-type cavities in with a length greater than the subwavelength cavity width). Their subwavelength dimensions make these lasers intriguing for certain applications in integrated photonics and nanophotonics. Parallel-plate cavities with SPP modes are also used for terahertz QCLs (which have some interesting potential applications in biosensing and standoff detection of dangerous materials), as they can show low-threshold, high-temperature performance at those frequencies.

It turns out, however, that it’s difficult to extract light from the plasmonic energy trapped in the spaser cavity. And when light can be made to leak out, it tends to be low in power and highly divergent, which limits its usefulness in actual applications.

A plasmonic phased array?

Kumar’s team found a potential solution through a distributed-feedback approach that the team has dubbed “antenna feedback,” and that Kumar compares to the action of phased-array antennas in microwave communication systems. The team demonstrated by numerical modeling that a grating of slits on one side of the subwavelength Fabry-Perot resonator, spaced at a specific value, would allow a single SPP mode within the cavity to diffract outside of the cavity in the surrounding medium, through Bragg diffraction. The energy outside of the cavity builds up with positive feedback (again the result of the selection of the grating period).

As a result, a second intense SPP wave develops in the medium outside of the cavity that remains coupled to the cavity’s metal cladding but also can form a highly directional beam outside of it. “The narrow-beam emission,” the team writes, “is due in part to the cavity acting like an end-fire phased-array antenna at microwave frequencies.”

In a proof of concept, the team implemented the antenna-feedback scheme in a terahertz QCL, using a box-shaped cavity consisting of two 100-m by 1400-m metallic plates, separated by a distance of 10 m. The researchers report that the resulting laser showed a beam divergence as small as 4 degrees by 4 degrees—“the narrowest beam reported for any terahertz QCL to date,” according to the study.

Applications in security and elsewhere

The researchers note that terahertz QCLs in particular have some interesting applications in security and standoff detection. At a recent innovation conference, they pointed out that “approximately 80 to 95 percent of explosives, and all commonly used ones, have unique and identifiable terahertz signatures.”

But, while their experiments focused particularly on terahertz QCLs, they stress that the antenna-feedback scheme should be applicable to plasmonic lasers of any operating wavelength that operate with Fabry-Perot cavities. That, in turn, could aid help make other applications of plasmonic lasers, in areas such as nanophotonics, more feasible, according to the scientists.

Abstract-Random Nanowire Configurations Increase Conductivity Over Heavily Ordered Configurations

http://www.photonicsonline.com/doc/random-nanowire-configurations-increase-conductivity-0001

Researchers at Lehigh University have identified for the first time that a performance gain in the electrical conductivity of random metal nanowire networks can be achieved by slightly restricting nanowire orientation. The most surprising result of the study is that heavily ordered configurations do not outperform configurations with some degree of randomness; randomness in the case of metal nanowire orientations acts to increase conductivity.

The study, Conductivity of Nanowire Arrays under Random and Ordered Orientation Configurations, is published in the current issue of Nature's journal Scientific Reports. The research was carried out by Nelson Tansu, Daniel E. '39 and Patricia M. Smith Endowed Chair Professor in Lehigh's Center for Photonics and Nanoelectronics and Department of Electrical and Computer Engineering, and lead author Milind Jagota, a Bethlehem-area high school student.

Transparent conductors are needed widely for flat screen displays, touch screens, solar cells, and light-emitting diodes, among many other technologies. Currently, Indium Tin Oxide (ITO) is the most widely used material for transparent conductors due to its high conductivity and high transparency. However, ITO-based technology has several issues. The material is scarce, expensive to manufacture and brittle, a particularly undesirable characteristic for anything being used in this modern age of flexible electronics.

Researchers searching for a replacement for ITO are increasingly employing random networks of metal nanowires to match ITO in both transparency and conductivity. Metal nanowire-based technologies display better flexibility and are more compatible with manufacturing processes than ITO films. The technology, however, is still in an early phase of development and performance must be improved. Current research is focused on the effect of rod orientation on conductivity of networks to improve performance.

In this work, Lehigh researchers developed a computational model for simulation of metal nanowire networks, which should speed the process towards idealizing the configuration of nanowires. The model predicts existing experimental results and previously published computational results.

The researchers then used this model to extract results for the first time on how conductivity of random metal nanowire networks is affected by different orientation restrictions of varying randomness. Two different orientation configurations are reported.

In the first, a uniform distribution of orientations over the range (?θ, θ) with respect to a horizontal line is used. In the second, a distribution of orientations over the range [?θ] _ [θ] is used, also with respect to a horizontal line. In each case θ is gradually decreased from 90° to 0°. Conductivity is measured both in directions parallel and perpendicular to alignment.

Researchers found that a significant improvement in conductivity parallel to direction of alignment can be obtained by slightly restricting orientation of the uniform distribution. This improvement, however, comes at the expense of a larger drop in perpendicular conductivity. The general form of these results matches that demonstrated by researchers experimenting with carbon nanotube films. Surprisingly, it was found that the highly ordered second case is unable to outperform isotropic networks for any value of θ; thus demonstrating that continuous orientation configurations with some degree of randomness are preferable to highly ordered configurations.

Prior research in this field has studied the effects of orientation on conductivity of 3D carbon nanotube composites, finding that a slight degree of alignment improves conductivity. Computational models have been used to study how percolation probability of 2D random rod dispersions is affected by rod orientation. Others have developed a more sophisticated computational model capable of calculating conductivity of 3D rod dispersions, again finding that a slight degree of axial alignment improves conductivity.

"Metal nanowire networks show great potential for application in various forms of technology," said Jagota. "This computational model, which has proven itself accurate through its good fit with previously published data, has demonstrated quantitatively how different orientation configurations can impact conductivity of metal nanowire networks."

"Restriction of orientation can improve conductivity in a single direction by significant amounts, which can be relevant in a variety of technologies where current flow is only required in one direction," said Tansu. "Surprisingly, heavily controlled orientation configurations do not exhibit superior conductivity; some degree of randomness in orientation in fact acts to improve conductivity of the networks. This approach may have tremendous impacts on improving current spreading in optoelectronics devices, specifically on deep ultraviolet emitter with poor p-type contact layer."

This work is supported in part by the National Science Foundation, the Daniel E. '39 and Patricia M. Smith Endowed Chair Professorship Fund, and Center for Photonics and Nanoelectronics at Lehigh University.

Tansu's research team at Lehigh works in the Laboratory for Emerging Photonics and Nanostructures and focuses on the physics and device technologies of semiconductor nanostructures for photonics and energy-efficiency applications. Tansu and his team employ fundamental knowledge derived from physics and chemistry in solving problems in engineering with technological impact. Several of the key applications they pursue include energy efficiency and renewable energy technologies including solid state lighting, solar cells, solar hydrogen, thermoelectricity, as well as deep UV emitters, terahertz photonics and semiconductor lasers for communications.

SOURCE: Lehigh University