From Berkeley Engineering: “New single-mode semiconductor laser delivers power with scalability” 

From Berkeley Engineering

At

The University of California-Berkeley

June 29, 2022
Sarah Yang

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Schematic of the Berkeley Surface Emitting Laser (BerkSEL) illustrating the pump beam (blue) and the lasing beam (red). The unconventional design of the semiconductor membrane synchronizes all unit-cells (or resonators) in phase so that they are all participating in the lasing mode. (Image courtesy of the Kanté group)

Berkeley engineers have created a new type of semiconductor laser that accomplishes an elusive goal in the field of optics: the ability to emit a single mode of light while maintaining the ability to scale up in size and power. It is an achievement that means size does not have to come at the expense of coherence, enabling lasers to be more powerful and to cover longer distances for many applications.

A research team led by Boubacar Kanté, Chenming Hu Associate Professor in UC Berkeley’s Department of Electrical Engineering and Computer Sciences (EECS) and faculty scientist at the Materials Sciences Division of the DOE’s Lawrence Berkeley National Laboratory, showed that a semiconductor membrane perforated with evenly spaced and same-sized holes functioned as a perfect scalable laser cavity. They demonstrated that the laser emits a consistent single wavelength regardless of the size of the cavity.

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Top view of a scanning electron micrograph of the Berkeley Surface Emitting Laser (BerkSEL). The hexagonal lattice photonic crystal (PhC) forms an electromagnetic cavity. (Image courtesy of the Kanté group)

The researchers described their invention, dubbed Berkeley Surface Emitting Lasers (BerkSELs), in a study published June 29, 2022 in the journal Nature.

“Increasing both size and power of a single-mode laser has been a challenge in optics since the first laser was built in 1960,” said Kanté. “Six decades later, we show that it is possible to achieve both these qualities in a laser. I consider this the most important paper my group has published to date.”

Despite the vast array of applications ushered in by the invention of the laser — from surgical tools to barcode scanners to precision etching — there has been a persistent limit that researchers in optics have had to contend with. The coherent, single-wavelength directional light that is a defining characteristic of a laser starts to break down as the size of the laser cavity increases. The standard workaround is to use external mechanisms, such as a waveguide, to amplify the beam.

“Using another medium to amplify laser light takes up a lot of space,” said Kanté. “By eliminating the need for external amplification, we can shrink the size and increase the efficiency of computer chips and other components that rely upon lasers.”

The study’s results are particularly relevant to vertical-cavity surface-emitting lasers, or VCSELs, in which laser light is emitted vertically out of the chip. Such lasers are used in a wide range of applications, including fiber optic communications, computer mice, laser printers and biometric identification systems.

VCSELs are typically tiny, measuring a few microns wide. The current strategy used to boost their power is to cluster hundreds of individual VCSELs together. Because the lasers are independent, their phase and wavelength differ, so their power does not combine coherently.

“This can be tolerated for applications like facial recognition, but it’s not acceptable when precision is critical, like in communications or for surgery,” said study co-lead author Rushin Contractor, an EECS Ph.D. student.

Kanté compares the extra efficiency and power enabled by BerkSEL’s single-mode lasing to a crowd of people getting a stalled bus to move. Multi-mode lasing is akin to people pushing in different directions, he said. It would not only be less effective, but it could also be counterproductive if people are pushing in opposite directions. Single-mode lasing in BerkSELs is comparable to each person in the crowd pushing the bus in the same direction. This is far more efficient than what is done in existing lasers where, using the same analogy, only part of the crowd contributes to pushing the bus.

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Schematic showing the “Dirac cones.” Light is emitted synchronously from the entire semiconductor cavity as a result of the Dirac point singularity. (Image courtesy of the Kanté group)

The study found that the BerkSEL design enabled the single-mode light emission because of the physics of the light passing through the holes in the membrane, a 200-nanometer-thick layer of indium gallium arsenide phosphide, a semiconductor commonly used in fiber optics and telecommunications technology. The holes, which were etched using lithography, had to be a fixed size, shape and distance apart.

The researchers explained that the periodic holes in the membrane became Dirac points, a topological feature of two-dimensional materials based on the linear dispersion of energy. They are named after English physicist and Nobel laureate Paul Dirac, known for his early contributions to quantum mechanics and quantum electrodynamics.

The researchers point out that the phase of light that propagates from one point to the other is equal to the refractive index multiplied by the distance traveled. Because the refractive index is zero at the Dirac point, light emitted from different parts of the semiconductor are exactly in phase and thus optically the same.

“The membrane in our study had about 3000 holes but theoretically it could have been 1 million or 1 billon holes, and the result would have been the same,” said study co-lead author, Walid Redjem, an EECS postdoctoral researcher.

The researchers used a high-energy pulsed laser to optically pump and provide energy to the BerkSEL devices. They measured the emission from each aperture using a confocal microscope optimized for near-infrared spectroscopy.

The semiconductor material and the dimensions of the structure used in this study were selected to enable lasing at telecommunications wavelength. Authors noted that BerkSELs can emit different target wavelengths by adapting the design specifications, such as hole size and semiconductor material.

Other study authors are Wanwoo Noh, co-lead author who earned his Ph.D. degree in EECS in May 2022; Wayesh Qarony, Scott Dhuey and Adam Schwartzberg from Berkeley Lab; and Emma Martin, a Ph.D. student in EECS.

The Office of Naval Research provided the primary support for this study. Additional funding came from the National Science Foundation, the Berkeley Lab, the Moore Inventor Fellows program and UC Berkeley’s Bakar Fellowship.

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The College of Engineering, also known informally as Berkeley Engineering or CoE, is one of the fourteen schools and colleges at the University of California, Berkeley. Established in 1931, the college is considered among the most prestigious engineering schools in the world, ranked third by U.S. News & World Report and with an acceptance rate of 8%. Berkeley Engineering is particularly well known for producing many successful entrepreneurs; among its alumni are co-founders and CEOs of some of the largest companies in the world, including Apple, Boeing, Google, Intel, and Tesla.

The college is currently situated in 14 buildings on the northeast side of the central campus, and also operates at the 150 acre (61 ha) Richmond Field Station. With the Haas School of Business, the college confers joint degrees and advises the university’s resident startup incubator, Berkeley SkyDeck.

Departments

Aerospace Engineering
Bioengineering (BioE)
Civil and Environmental Engineering (CEE)
Development Engineering (DevEng)
Electrical Engineering and Computer Sciences (EECS)
Engineering Science
Energy Engineering
Engineering Mathematics and Statistics (EMS)
Engineering Physics
Environmental Engineering Science (EES)
Industrial Engineering and Operations Research (IEOR)
Materials Science and Engineering (MSE)
Mechanical Engineering (ME)
Nuclear Engineering (NE)

The College of Letters and Science also offers a Bachelor of Arts in computer science, which requires many of the same courses as the College of Engineering’s Bachelor of Science in EECS, but has different admissions and graduation criteria. Berkeley’s chemical engineering department is under the College of Chemistry.

Research units

All research facilities are managed by one of five Organized Research Units (ORUs):

Earthquake Engineering Research Center – research and public safety programs against the destructive effects of earthquakes
Electronics Research Laboratory – the largest ORU; advanced research in novel areas within seven different university departments, organized into five main divisions:
Berkeley Sensor & Actuator Center
Berkeley Wireless Research Center
Berkeley Northside Research Group
Micro Systems Group
Engineering Systems Research Center – focuses on manufacturing, mechatronics, and microelectro mechanical systems (MEMS)
Institute for Environmental Science and Engineering – focuses on applying basic research to current and future environmental problems
Institute of Transportation Studies – sponsors research in transportation planning, policy analysis, environmental concerns and transportation system performance

Major research centers and programs

Jacobs Institute for Design Innovation
Berkeley Institute of Design
Berkeley Multimedia Research Center
Center for Information Technology Research in the Interest of Society (CITRIS)
Center for Intelligent Systems – developing a unified theoretical foundation for intelligent systems.
Consortium on Green Design and Manufacturing
Digital Library Project
UCSF/Berkeley Ergonomics Program
International Computer Science Institute – basic research institute focusing on Internet architecture, speech and language processing, artificial intelligence, and cognitive and theoretical computer science
Intel Research Laboratory @ Berkeley
Integrated Materials Laboratory – facilities for research in nano-structure growth, processing, and characterization
Microfabrication Laboratory
The Millennium Project – developing a hierarchical campus-wide “cluster of clusters” to support advanced computational applications
Nokia Research Center @ Berkeley
Pacific Earthquake Engineering Research Center
Partners for Advanced Transit & Highways – researching ways to improve the operation of California’s state highway system
Power Systems Engineering Research Center

The The University of California-Berkeley is a public land-grant research university in Berkeley, California. Established in 1868 as the state’s first land-grant university, it was the first campus of the University of California system and a founding member of the Association of American Universities . Its 14 colleges and schools offer over 350 degree programs and enroll some 31,000 undergraduate and 12,000 graduate students. Berkeley is ranked among the world’s top universities by major educational publications.

Berkeley hosts many leading research institutes, including the Mathematical Sciences Research Institute and the Space Sciences Laboratory. It founded and maintains close relationships with three national laboratories at DOE’s Lawrence Berkeley National Laboratory, DOE’s Lawrence Livermore National Laboratory and DOE’s Los Alamos National Lab, and has played a prominent role in many scientific advances, from the Manhattan Project and the discovery of 16 chemical elements to breakthroughs in computer science and genomics. Berkeley is also known for student activism and the Free Speech Movement of the 1960s.

Berkeley alumni and faculty count among their ranks 110 Nobel laureates (34 alumni), 25 Turing Award winners (11 alumni), 14 Fields Medalists, 28 Wolf Prize winners, 103 MacArthur “Genius Grant” recipients, 30 Pulitzer Prize winners, and 19 Academy Award winners. The university has produced seven heads of state or government; five chief justices, including Chief Justice of the United States Earl Warren; 21 cabinet-level officials; 11 governors; and 25 living billionaires. It is also a leading producer of Fulbright Scholars, MacArthur Fellows, and Marshall Scholars. Berkeley alumni, widely recognized for their entrepreneurship, have founded many notable companies.

Berkeley’s athletic teams compete in Division I of the NCAA, primarily in the Pac-12 Conference, and are collectively known as the California Golden Bears. The university’s teams have won 107 national championships, and its students and alumni have won 207 Olympic medals.

Made possible by President Lincoln’s signing of the Morrill Act in 1862, the University of California was founded in 1868 as the state’s first land-grant university by inheriting certain assets and objectives of the private College of California and the public Agricultural, Mining, and Mechanical Arts College. Although this process is often incorrectly mistaken for a merger, the Organic Act created a “completely new institution” and did not actually merge the two precursor entities into the new university. The Organic Act states that the “University shall have for its design, to provide instruction and thorough and complete education in all departments of science, literature and art, industrial and professional pursuits, and general education, and also special courses of instruction in preparation for the professions”.

Ten faculty members and 40 students made up the fledgling university when it opened in Oakland in 1869. Frederick H. Billings, a trustee of the College of California, suggested that a new campus site north of Oakland be named in honor of Anglo-Irish philosopher George Berkeley. The university began admitting women the following year. In 1870, Henry Durant, founder of the College of California, became its first president. With the completion of North and South Halls in 1873, the university relocated to its Berkeley location with 167 male and 22 female students.

Beginning in 1891, Phoebe Apperson Hearst made several large gifts to Berkeley, funding a number of programs and new buildings and sponsoring, in 1898, an international competition in Antwerp, Belgium, where French architect Émile Bénard submitted the winning design for a campus master plan.

20th century

In 1905, the University Farm was established near Sacramento, ultimately becoming the University of California-Davis. In 1919, Los Angeles State Normal School became the southern branch of the University, which ultimately became the University of California-Los Angeles. By 1920s, the number of campus buildings had grown substantially and included twenty structures designed by architect John Galen Howard.

In 1917, one of the nation’s first ROTC programs was established at Berkeley and its School of Military Aeronautics began training pilots, including Gen. Jimmy Doolittle. Berkeley ROTC alumni include former Secretary of Defense Robert McNamara and Army Chief of Staff Frederick C. Weyand as well as 16 other generals. In 1926, future fleet admiral Chester W. Nimitz established the first Naval ROTC unit at Berkeley.

In the 1930s, Ernest Lawrence helped establish the Radiation Laboratory (now DOE’s Lawrence Berkeley National Laboratory (US)) and invented the cyclotron, which won him the Nobel physics prize in 1939. Using the cyclotron, Berkeley professors and Berkeley Lab researchers went on to discover 16 chemical elements—more than any other university in the world. In particular, during World War II and following Glenn Seaborg’s then-secret discovery of plutonium, Ernest Orlando Lawrence’s Radiation Laboratory began to contract with the U.S. Army to develop the atomic bomb. Physics professor J. Robert Oppenheimer was named scientific head of the Manhattan Project in 1942. Along with the Lawrence Berkeley National Laboratory, Berkeley founded and was then a partner in managing two other labs, Los Alamos National Laboratory (1943) and Lawrence Livermore National Laboratory (1952).

By 1942, the American Council on Education ranked Berkeley second only to Harvard University in the number of distinguished departments.

In 1952, the University of California reorganized itself into a system of semi-autonomous campuses, with each campus given its own chancellor, and Clark Kerr became Berkeley’s first Chancellor, while Sproul remained in place as the President of the University of California.

Berkeley gained a worldwide reputation for political activism in the 1960s. In 1964, the Free Speech Movement organized student resistance to the university’s restrictions on political activities on campus—most conspicuously, student activities related to the Civil Rights Movement. The arrest in Sproul Plaza of Jack Weinberg, a recent Berkeley alumnus and chair of Campus CORE, in October 1964, prompted a series of student-led acts of formal remonstrance and civil disobedience that ultimately gave rise to the Free Speech Movement, which movement would prevail and serve as precedent for student opposition to America’s involvement in the Vietnam War.

In 1982, the Mathematical Sciences Research Institute (MSRI) was established on campus with support from the National Science Foundation and at the request of three Berkeley mathematicians — Shiing-Shen Chern, Calvin Moore and Isadore M. Singer. The institute is now widely regarded as a leading center for collaborative mathematical research, drawing thousands of visiting researchers from around the world each year.

21st century

In the current century, Berkeley has become less politically active and more focused on entrepreneurship and fundraising, especially for STEM disciplines.

Modern Berkeley students are less politically radical, with a greater percentage of moderates and conservatives than in the 1960s and 70s. Democrats outnumber Republicans on the faculty by a ratio of 9:1. On the whole, Democrats outnumber Republicans on American university campuses by a ratio of 10:1.

In 2007, the Energy Biosciences Institute was established with funding from BP and Stanley Hall, a research facility and headquarters for the California Institute for Quantitative Biosciences, opened. The next few years saw the dedication of the Center for Biomedical and Health Sciences, funded by a lead gift from billionaire Li Ka-shing; the opening of Sutardja Dai Hall, home of the Center for Information Technology Research in the Interest of Society; and the unveiling of Blum Hall, housing the Blum Center for Developing Economies. Supported by a grant from alumnus James Simons, the Simons Institute for the Theory of Computing was established in 2012. In 2014, Berkeley and its sister campus, University of California-San Fransisco, established the Innovative Genomics Institute, and, in 2020, an anonymous donor pledged $252 million to help fund a new center for computing and data science.

Since 2000, Berkeley alumni and faculty have received 40 Nobel Prizes, behind only Harvard and Massachusetts Institute of Technology among US universities; five Turing Awards, behind only MIT and Stanford University; and five Fields Medals, second only to Princeton University (US). According to PitchBook, Berkeley ranks second, just behind Stanford University, in producing VC-backed entrepreneurs.

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