Researchers are putting electron oscillations, or plasmons, to work and paving the way for instruments that could improve food safety and devices for future mobile communications. Additionally, plasmonic-enabled nanolasers could provide a means to link chips or subsections of chips, which would enable computers to become faster, smaller, and more efficient. However, scientists and engineers must first overcome challenges related to nanomanufacturing, high optical intensities, and optical losses. Possibilities and obstacles both arise from the interaction between photons and plasmonic structures, which consist of conductors sized and shaped to concentrate electromagnetic fields. An example of plasmonic potential can be seen in work underway at UCLA, where researchers are seeking to exploit widespread spectral attributes. A rendering of a plasmonic terahertz source. Plasmonic nanostructures confine carriers to a small space, which boosts the terahertz source radiation power. Courtesy of Mona Jarrahi/UCLA. “Many molecules have unique terahertz spectral signatures, and many materials are transparent in the terahertz regime,” said Mona Jarrahi, UCLA professor of electrical and computer engineering. The terahertz spectrum, which lies between the IR and microwave, could be useful in spectroscopy, giving scientists and industry another tool to investigate materials. However, sources and detectors based on current technology perform poorly in the terahertz, a consequence of limited transistor switching speed and a lack of materials with the right characteristics. Jarrahi and her group therefore turned years ago to an approach based on a photoconductor, which — to be useful — must quickly and efficiently convert light to terahertz radiation, and terahertz radiation into an electrical signal. One approach to enhancing efficiency is to use plasmonic structures that concentrate charge carriers into a nanoscale space, Jarrahi said. The tight confinement helps because the carrier transit time from creation to collection must be less than a terahertz oscillation, or roughly a picosecond. In this short period, charge carriers move only about 100 nm. Since plasmonic structures can increase carriers within that tiny volume, they increase efficiency. According to Jarrahi, photoconductive sources and detectors based on 2D plasmonic gratings could improve the efficiency of terahertz spectroscopy systems by three orders of magnitude over a nonplasmonic approach. And a 3D plasmonic grating would create a more significant boost, she said. Plasmonic nanostructures increase terahertz detection sensitivity of a photoconductive detector, which, with plasmonic sources, can improve the efficiency of terahertz spectroscopy systems by orders of magnitude. Courtesy of Mona Jarrahi/UCLA. When asked about challenges, Jarrahi pointed to the difficulty of making devices based on these structures, which can be expensive to fabricate in volume. But in some applications, the cost may not be important because no alternative approaches exist. “Many sensing markets don’t have a solution,” she said. So one of Jarrahi’s former students, Nezih Tolga Yardimci, formed a startup called Lookin Inc. The company, which commercializes plasmonic terahertz spectroscopy technology, detects aflatoxin as its initial application. Aflatoxin is a poison produced by fungi and a health hazard found in such foods as tree nuts. A large enough concentration could kill, and low-level exposure for a long time can cause liver damage, which is why the FDA set the allowable aflatoxin limit to less than 20 parts per billion (ppb). (Regulators in Japan and Europe put the limit at 5 ppb.) The concept of next-generation (6G) terahertz communication links and their integration into fiber optic networks (a), with a plasmonic modulator (b). Researchers demonstrated that such a modulator could operate at the necessary speeds, a capability that today’s conventional transistor-based approaches struggle with. Courtesy of Sandeep Ummethala/ Karlsruhe Institute of Technology and the Institute for Photonics and Quantum Electronics. Plasmonic terahertz technology could play a role in next-generation mobile communications networks. Current testing methods are destructive and slow, Yardimci said. So producers sample only a small portion of their product and are unable to know immediately whether the product has problems. Terahertz spectroscopy could enable food to be screened more thoroughly because the aflatoxin detection process does not harm the product and would be much faster. “The system will be high throughput. The test will be less than a second,” Yardimci said of the technology under development. He said a prototype unit should be available within a few years. The goal is to have both a large fixed tester and a smaller field-deployable handheld. The technology could also be extended to screen for additional fungi-produced poisons. Known as mycotoxins, these hazards have the potential to appear in a wide variety of foods consumed by people and animals. Consequently, technology that improves detection could have a significant impact. Besides spectroscopy, plasmonic terahertz technology could also play a role in next-generation mobile communications networks. Today, carriers are deploying 5G, with research and development underway for 6G technology, which will have data rates as high as a terabit per second, hundreds of times that of 5G. The 6G networks will also have lower latency and therefore be more responsive. These improvements may be possible, in part, because 6G will operate at a higher carrier frequency — in an unallocated part of the spectrum in the terahertz. Base stations will send and receive wirelessly using this spectrum, while converting data to and from an optical signal suitable for transmission over existing fiber infrastructure. To handle the conversion, researchers at Germany’s Karlsruhe Institute of Technology demonstrated a plasmonics-based modulator and reported on it in Nature Photonics1. A plasmonic nanolaser small enough to be used for chip-to-chip applications and within chips. Lasing occurs where a nanowire crosses a grating, as shown in a schematic of a ZnO (zinc oxide) nanowire lying on an Ag (silver) grating. The stimulated emission is scattered from the boundaries of the 1D-surface plasmon polariton cavity. Courtesy of Tien-chang Lu/National Chiao Tung University. Using nanofabrication, the researchers created structures that converted terahertz signals to optical in a plasmonic device with a silicon waveguide trench that was 75 nm wide. This dimension, and the conversion region, are several orders of magnitude smaller than would be the case with a traditional photonic approach, said Christian Koos, a professor who leads the research effort. The demonstration converter achieved speeds of more than 300 GHz, at least 10× faster than is possible with current technology. This performance was due mainly to the size of the device. “Roughly speaking, modulators get faster the smaller the active volume becomes,” Koos said. The researchers tested their proof-of-concept device over a limited distance. According to Sandeep Ummethala, graduate student and the paper’s lead author, it should be possible to achieve a reach close to that required by a commercial setting by increasing the power of the terahertz source. However, although making a 75-nm trench poses no problem for current nanofabrication technology, the plasmonic device needs to minimize optical losses, which creates special requirements on the trench. “You need the right material, and it’s important for the slot sidewalls to be as smooth as possible,” Ummethala said. Confining light to a small volume means it is more challenging to hold the photons in that region. It also means that when light is forced into such a small space, photon intensity increases, which is another issue. The challenge is to cut optical losses while managing a high photon flux. This will require research and development in fabrication techniques and material systems. Koos said the researchers have been contacted by large companies about their project, in part because the technique is straightforward. In the base station, the plasmonic device and some hardware are all that are needed, unlike conventional methods, which require extensive electronic and optical signal processing. Because of the simplicity of the plasmonic modulator approach, 5G base stations may eventually use it. The same technology could have other uses, such as in the making of a fast analog-to-digital convertor. These convertors are present in the interface between sensing devices that measure the real world, and in the processors that manipulate data in the digital realm. An ultrafast analog-to-digital convertor has broad applicability. One mentioned by Koos is the ability to measure the shape of electron pulses as they race near the speed of light inside a particle accelerator. Terahertz applications are not the only possible use of plasmonic techniques. For instance, plasmonic structures are being used to help bridge size gaps between electronics and photonics. Tien-chang Lu, a photonics professor at National Chiao Tung University in Taiwan, said semiconductor device dimensions are measured in nanometers. However, a laser needs a cavity size that is a multiple of its wavelength, or many hundreds of nanometers. Lu and his group worked around this limitation by creating a grating and placing a nanowire on top. Using plasmonics, the researchers made this combination lase, with the emission happening only where the 35-nm × 1.5-µm nanowire ran over the grating trench. The total volume was five orders of magnitude smaller than that of a conventional photonic laser, according to Lu. “The plasmonic nanowire laser can shrink the lasers to tens of nanometers,” he said. With a better match to semiconductor dimensions, such lasers could potentially be used to carry signals between and inside chips. This would eliminate speed and power constraints imposed by the current practice of moving electrons around. Lu said the biggest obstacle to possible applications is the problem of creating an electrically pumped nanowire laser. Achieving this would eliminate the need for another source to pump in light and initiate lasing. Electrical pumping could also allow electrical modulation. Lu said it may be possible to do this, pointing to a recent Nano Letters paper about a graphene-insulator-metal structure2. The aim now is to show that the approach leads to lasing that can be modulated by applying an external current. Then, Lu said, the group will “try to build a plasmonic circuit that contains light source, modulator, waveguide, and detector, all based on plasmonic waves with an extremely small footprint.” References 1. S. Ummethala et al. (August 2019). THz- to-optical conversion in wireless communications using an ultra-broadband plasmonic modulator. Nat Photonics, Vol. 13, pp. 519- 524. 2. H. Li et al. (2019). Nano Lett, Vol. 19, No. 8, pp. 5017-5024.