World-Changing Science

Harvard University’s Federico Capasso, whose scientific accomplishments with quantum cascade lasers (QCL) and bandgap engineering have transformed the field of photonics across the globe, will receive the 2013 SPIE Gold Medal in August.

“Federico Capasso is a world-changing scientist and engineer,” says SPIE Fellow Vladimir Shalaev of Purdue University (USA).

“His pioneering contributions to bandgap engineering of optoelectronic heterostructure materials and devices; quantum optoelectronic devices culminating in the co-invention of the QCL, a revolutionary infrared light source; and his recent work on plasmonics will always stay in the history of science,” says Shalaev, a professor of electrical and computer engineering.

“With his seminal contributions, Professor Capasso has totally transformed the field of photonics, creating new opportunities that will continue to be pursued long after we are all gone.”

Known as the father of bandgap engineering, Capasso, the Robert Wallace Professor of Applied Physics at the Harvard School of Engineering and Applied Sciences (SEAS) (USA), has permanently changed the way light can be manipulated and controlled, opening up a host of new applications.

Starting with his distinguished career at Bell Labs, Capasso has been a major force in photonics for more than 30 years, making major contributions to solid-state physics devices, materials, and applications.

Considered one of the most important developments in laser physics in the last 20 years, QCLs are used in a wide range of industrial, defense, and security applications including exhaust sensing, explosives detection, medical diagnostics, infrared imaging and countermeasures, and many other precision sensing and spectroscopic applications.

As researchers and manufacturers gain more experience with QCLs, the number of markets and applications continues to increase.

Born in Italy in 1949, Capasso grew up in post-World War II Rome, a city with a long history in science. The physicist Enrico Fermi who, along with Robert Oppenheimer is considered the father of the U.S. atomic bomb, was a native of Rome and a professor of physics at the University of Rome before emigrating in 1938.

After the war, nuclear energy was considered the next great wave in science. Almost every country in Europe was investing in nuclear technology and the demand for physicists was growing.

It was in this environment that the 7-year-old Capasso first read Our Friend the Atom and was inspired by nuclear science. Written in 1956 by Heinz Haber and popularized by a Walt Disney film, the book was an attempt to show the kinder side of atomic power.

Capasso says as a child he was “hooked by the simple style of writing and how complex problems were explained in a simple, graphical way.” Learning to think of scientific concepts in terms of pictures would later inspire his own research and teaching methods at Harvard.

“It was an instant love affair,” Capasso says of his experience reading the book. “I decided then I would become a physicist.”

In 1969, Capasso began his studies at the University of Rome where three faculty members had worked with Fermi. Within two years, the strenuous course load had him reconsidering the idea of being a physicist. His mother persuaded him to continue the uphill battle, however, and during his third year, things finally began to fall into place.

His faculty advisers encouraged him to go into optics – in particular lasers. At the time, the use of lasers was rapidly expanding, and Capasso’s advisers told him training in optics would prove useful in several areas of physics.

“How good that advice was!” Capasso says. “A PhD in either electronic engineering or physics with a focus on optics and photonics gives access to many areas of science and technology and as such provides many career options.”

He finished his studies at University of Rome with several important papers to his credit and a new focus on nonlinear optics.

After a stint doing fiber optics research at Fondazione Bordoni in Rome, Capasso began looking for opportunities to work abroad. He applied for a Rotary Foundation scholarship that would support research outside of Italy for nine months.

Capasso’s father had insisted that he and his sister learn English because the language would open doors to greater opportunities. Being fluent in English almost backfired when Capasso was slated for Belfast, Northern Ireland – not the safest place then for the Catholic Capasso and his new bride, Paola. An appeal to the Rotary board placed Capasso at Bell Labs in New Jersey in 1976 where he would remain for 27 years.

Capasso held several management positions at Bell, including head of the Quantum Phenomena and Device Research and Semiconductor Physics Research departments. He was made a Distinguished Member of Technical Staff in 1984 and a Bell Labs Fellow in 1997.

While at Bell Labs, Capasso observed advances being made in creating diode lasers, avalanche photodiodes, and engineered quantum-well structures. Studying these types of structures led to thoughts of creating ultralow-noise avalanche photodiodes.

His research eventually led to the breakthrough concept of a heterostructure comprised of multiple-graded bandgap regions forming an energy staircase, which became the basis for a new class of electron-transport structures, detectors, and lasers. Capasso and his collaborators continued their research on these models and over the next 10 years wrote more than 100 papers and received more than 20 patents on graded bandgap physics, bandgap-engineered structures, and applications.

In 1994, Capasso and a team of researchers demonstrated the first QCL at Bell Labs. This fundamentally new light source featured an emission wavelength that could be designed to cover the mid- to far-infrared spectrum by tailoring the active region layer thickness. The QCL was a novel laser concept that represented a radical departure from conventional solid-state lasers and made many IR diode lasers obsolete.

“The mid-infrared spectrum is of paramount scientific and technological importance since many molecules have their telltale absorption fingerprints in this region,” says SPIE Fellow Malvin C. Teich, professor emeritus at Boston University (USA). “Yet, until the advent of the QCL, this ‘molecular fingerprint’ region lacked suitable compact, continuously tunable, and high-power single-mode lasers.”

Through Capasso’s collaborations with dozens of researchers in industry and academia, QCLs have become commercially available in a variety of forms for chemical sensing, trace gas analysis, spectroscopy, and other applications.

Capasso in his Harvard lab. (Photos by Eliza Grinnell).

Capasso discusses the development of the QCL and its applications in a 2010 interview at SPIE Photonics West. See the interview on SPIE TV.

Bell Labs became Lucent Technologies in 1996. A year later, Capasso became head of the Semiconductor Physics department, and he served as vice president of physical research from 2000 to 2002. By then Lucent was already feeling the effects of the telecom meltdown and in 2003, Capasso was persuaded by his former boss at Bell Labs, Venkatesh Narayanamurti, then a dean at Harvard, to join SEAS.

At Bell Labs, Capasso had learned the importance of research focused on actual application and how science and industry can blend. This experience, combined with the emphasis on cross-disciplinary research in an academic environment, made Harvard the perfect venue for Capasso.

At Harvard, Capasso leads his own research group of talented graduate students and post-docs. The Capasso Group’s current research projects include expanding the capabilities of QCLs; nanomechanical devices based on the control of the Casimir force; and working with plasmonics and metamaterials to shape the wavefront of lasers.

With QCLs, the group researches mode-locked QCLs and new QCL-based terahertz light sources utilizing difference-frequency generation.

Recently his group discovered a generalization of the laws of reflection and refraction, which form the basis of new planar photonic devices such as flat lenses without optical aberrations.

Applying his experience in science and industry, he started Eos Photonics in 2009. Eos designs and manufactures commercially available broadband QCLs for chemical sensing applications.

IR spectroscopy tools are used in the industrial, scientific, and defense markets, and devices on submounts are delivered to system integrators and researchers.

The company is also developing QCL array-based spectroscopy products for the security, industrial monitoring, material identification, and earth science sectors.

“I take a wide approach to science,” Capasso says. “I have many different areas of interest and that has always been my style.

“To be creative, you can’t focus too narrowly on a topic,” he says. “I tell my students, if you want to be successful in science these days, if you want to be marketable — and those two things go together to some extent — you need to become an excellent problem solver. You can’t be a Jack-of-all-Trades. Yet, you also shouldn’t be too specialized. You should be learning a set of tools so that you can do research in a number of different areas.”

Capasso believes that scientists will be called on more and more to solve the huge problems facing the world. If students are too focused in a single area, he says, they will miss some potentially big opportunities for research, so they need the flexibility to be able to move around in various areas.

An active SPIE member since the 1980s and an SPIE Fellow since 1990, Capasso has long been a fixture at SPIE conferences where he has been involved in organizing symposia, giving presentations, and teaching courses.

His research group presented 20 papers at SPIE Photonics West in February, including three invited papers and a keynote presentation on “Broadband mid-infrared wavefront engineering with optical antenna metasurfaces.”

At SPIE Optics + Photonics in August, Capasso will be a plenary speaker at the NanoScience + Engineering symposium. His talk Monday, 26 August, "Molding Optical Wavefronts: Flat Optics based on Metasurfaces," will cover a variety of flat optical components, including lenses, polarizers, vortex plates, coatings, holograms, and couplers with polarization invariant coupling efficiency.

He encourages his students to attend scientific conferences because of the exposure to a variety of subjects and fellow scientists.

“Optics is one of the most exciting fields,” Capasso says. “Every year, there seems to be a new surprise in optics, in both science and technology. Optics is widely applicable in so many fields, like medicine, physics, chemistry, and engineering.”

According to Capasso, optics has a privileged position in science. He points out the laser as an example, citing its wide use in a number of fields.

“I always tell students, if you gain skills in optics and photonics, you’ll be able to use them in many areas of science and technology,” Capasso says. “You can’t go wrong becoming a very strong optical scientist because you can work in many different fields.”

Almost 40 years later, Capasso is still following the advice of his university mentors who encouraged him to study optics – and passing that enthusiasm on to his students.

In honor of his world-changing achievements, Capasso will be awarded the SPIE Gold Medal, the highest honor the Society bestows, on Wednesday 28 August at SPIE Optics + Photonics.

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