Download PDF The Bridge: 50th Anniversary Issue January 7, 2021 Volume 50 Issue S This special issue celebrates the 50th year of publication of the NAE’s flagship quarterly with 50 essays looking forward to the next 50 years of innovation in engineering. How will engineering contribute in areas as diverse as space travel, fashion, lasers, solar energy, peace, vaccine development, and equity? The diverse authors and topics give readers much to think about! We are posting selected articles each week to give readers time to savor the array of thoughtful and thought-provoking essays in this very special issue. Check the website every Monday! Projected Applications of the Laser in the 21st Century Monday, March 8, 2021 Author: J. Gary Eden This year marked the 60th anniversary of the discovery of the laser. Few optical or electronic devices have more significantly and directly impacted the quality of life worldwide and, not surprisingly, the NAE designated the laser as one of the 20 foremost engineering achievements of the 20th century (Constable and Somerville 2003). Most early predictions of laser applications proved to be inaccurate (a well-known hazard of all prognostications), but a myriad of unforeseen and yet transformative applications became reality. The laser was the engine for spectacular advances in areas from surgery and medical therapeutics to materials processing (e.g., cutting, welding, film deposition, annealing), information storage and retrieval, metrology, and time and frequency standards. It has also given birth to entirely new industries such as optical communications and laser radar. Looking forward to the next 50 years, it is likely that new classes of lasers will profoundly transform several applications. Living Cells and Tissue One indication of future potential is the recent realization of lasing from living cells expressing green fluorescent protein (GFP; Gather and Yun 2011). Although these experiments required an optical cavity and external pump source several orders of magnitude larger than the cells themselves, they nevertheless demonstrated that laser emission can be produced from a fluorescent protein grown in a single cell. Figure 1 shows the green laser spectrum and map of the emission intensity generated from GFP in a mammalian cell. When combined with the demonstration of a wide variety of nanolasers (such as quantum dots) over the past 2 decades (Geiregat et al. 2019; Klimov et al. 2000; Ma and Oulton 2019), this breakthrough suggests that in situ biomedical lasers will be introduced and developed into a routine optical diagnostic of both neurological and bio-chemical processes. The integration of micro- or nanoscale lasers with animal or human tissue will face a number of hurdles, such as the development of new types of optical resonators capable of being chemically interfaced with an arbitrarily chosen cell and yet providing the spectral selectivity (Q) required for laser oscillation to occur. -Delivery of optical pump/electrical power to the cellular or nanoparticle laser medium and access to the optical signal produced by the laser-tissue interaction are other formidable challenges. When solved, the resulting family of lasers will open a door to exploring the local chemical environment of cells and tissue. Autonomous Vehicles The widespread introduction of autonomous vehicles capable of navigating congested urban traffic will require compact, onboard laser-ranging (Lidar) systems of unprecedented precision. Because the mapping resolution of the Lidar system is directly dependent on the temporal widths of the pulses emitted by the system and the bandwidth of the detector(s) receiving the back-scattered radiation, new and compact laser optical systems emitting picosecond pulses and designed to cover large angular intervals quickly will be developed. Instead of reliance on narrow laser beams that are scanned mechanically, it is likely that overlapped, intentionally broad laser beams will be introduced to Lidar systems so as to decrease the time required for the acquisition of one complete scan around the vehicle. In this context, autonomous vehicles currently rely on GPS for navigation but it is quite possible that autonomous navigation of the future will demand an onboard atomic clock driven by an inexpensive laser or a lamp. Accordingly, the mass production of a new generation of low-cost atomic clocks will be a priority for the navigation of both terrestrial and airborne vehicles. X-Ray and Deep-UV Imaging and Photochemistry Another frontier for laser physics and engineering is the development of compact and efficient lasers and incoherent optical sources at short wavelengths, ranging from 1 nm (x-ray region) to 200 nm. The first lasers in the soft x-ray region were reported in the last century, but breaking this wavelength barrier has typically required large laser systems for producing hot plasmas to radiate the desired high-energy photons (Macchietto et al. 1999; Martz et al. 2010; Zhang et al. 1997). Despite the complexity and cost of laser-generated soft x-ray systems at present, a prominent example of potential applications of 1–200 nm photons is the 13.5 nm system manufactured for photolithography by the Cymer subsidiary of ASML. By exploding microdroplets of tin with a high-energy carbon dioxide laser, more than 200 W of average power is produced at 13.5 nm. The technological advances in mirrors, mask design, materials, and laser design necessary to reach this milestone are enormous but this multiyear effort has been rewarded. These 13.5 nm incoherent sources are responsible for regaining the momentum of the semiconductor industry, following Moore’s law to the 10 nm level and beyond. Continued industrial and university research and development to build on the existing soft x-ray and deep-UV source base will culminate, over the next few decades, in efficient and compact sources at discrete wavelengths throughout the 1–200 nm region. History has shown that the availability of new sources of electromagnetic radiation invariably leads not only to previously inaccessible areas of fundamental research but also to new processes and products as well. Of particular interest are promising opportunities in microscopy, materials analysis, thin film processing, and photochemistry. Furthermore, because many applications of short-wavelength radiation do not require the laser property of coherence, for example, incoherent sources such as lamps and the ASML photolithographic exposure and stepper systems mentioned above will play a major role in transitioning short-wavelength radiation sources to industry. As one early example, a series of flat lamps emitting at several wavelengths in the deep ultraviolet has been introduced, and those emitting at 222 nm are being manufactured for the disinfection of surfaces and room air during the covid-19 pandemic (Anderson 2020). Similar lamps operating at 172 nm (hu = 7.2 eV) have enabled photolithography at this wavelength and are capable of directly fabricating optical components in polymers and multilayer nanostructures in various organic materials (Mironov et al. 2020). Figure 2 shows two laser confocal microscope images (in false color) of 3- and 4-level nanostructures formed in polymethyl methacrylate by a 3-minute exposure of the surface, through a mask, with a 172 nm lamp. The periodic pattern of square posts is an optical grating and each color-coded layer is 240 nm thick. Nanolithography with deep-UV lamps reduces the cost of optical exposure systems by several orders of magnitude, relative to existing systems, while eliminating the requirement for processing in vacuum and rinsing with toxic solvents. These results provide a window into the capabilities of future nanolithographic systems. Recent developments provide only a hint of the impressive advances that undoubtedly lie ahead. Because it is in the ultraviolet that photon energies begin to match the energies necessary to break most chemical bonds, the potential benefit to humanity and industry of developing 1–200 nm wavelength sources of high average power (1–100 W) and efficiency is staggering. Reaching this goal will necessarily entail dropping the cost of generating a photon (or a mole of photons) by 2 or more orders of magnitude, allowing photons to be regarded as chemical reactants similar to the conventional liquids or gases that are the mainstay of existing chemical syntheses. In short, inexpensive deep-UV and soft x-ray -photons will usher in photochemistry on an industrial scale. Since the energies of photons are well defined, photochemical processes not accessible to thermally activated (Arrhenius) chemistry are expected to become available, increasing product yield and specificity. Conclusion Several new classes of lasers and lamps will surely be developed over the next 50 years, and the mid- to far-infrared regions are particularly attractive because of their value in imaging and environmental sensing. Regardless of the laser and incoherent sources that will become available, it is certain that these novel sources of photons will broaden dramatically the commercial, environmental, and healthcare applications for which the renown of the laser is already considerable. References Anderson M. 2020. UV light might keep the world safe from the coronavirus—and whatever comes next. IEEE -Spectrum, Oct. Constable G, Somerville B. 2003. A Century of Innovation: Twenty Engineering Achievements That Transformed Our Lives. Washington: Joseph Henry Press. Gather MC, Yun SH. 2011. Single-cell biological lasers. Nature Photonics 5:406–10. Geiregat P, Van Thourhout D, Zeger H. 2019. A bright future for colloidal quantum dot lasers. NPG Asia Materials 11:41. Klimov VI, Mikhailovsky AA, Xu S, Malko A, Hollingsworth JA, Leatherdale CA, Eisler HJ, Bawendi MG. 2000. Optical gain and stimulated emission in nanocrystal quantum dots. Science 290(5490):314–17. Ma R-M, Oulton RF. 2019. Applications of nanolasers. Nature Nanotechnology 14:12–22. Macchietto CD, Benware BB, Rocca JJ. 1999. Generation of millijoule-level soft x-ray laser pulses at a 4 Hz repetition rate in a highly-saturated tabletop capillary discharge amplifier. Optics Letters 24(16):1115–17. Martz D, Alessi D, Luther BM, Wang Y, Kemp D, Berrill M, Rocca JJ. 2010. High energy 13.9 nm table-top soft x-ray laser at 2.5 Hz repetition rate excited by a slab-pumped Ti:sapphire laser. Optics Letters 35(10):1632–34. Mironov AE, Kim J, Huang Y, Steinforth AW, Sievers DJ, Eden JG. 2020. Photolithography in the vacuum ultra-violet (172 nm) with sub-400 nm resolution: Photoablative patterning of nanostructures and optical components in bulk polymers and thin films in semiconductors. Nanoscale 12:16796–804. Zhang J, MacPhee AG, Lin J, Wolfrum E, Smith R, Danson C, Key MH, Lewis CLS, Neely D, Nilsen J, and 3 -others. 1997. A saturated x-ray laser beam at 7 nanometers. -Science 276(5315):1097–100. About the Author:Gary Eden (NAE) is the Intel Alumni Endowed Chair Emeritus in the Department of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign.