2016 Balzan Prize for Applied Photonics
Fotonica come Design – Rome, 18.11.2016 Forum (Video – Italian + Text – English)
During my career, I have had the opportunity to work in different fields, almost always straddling science and applications, and in large part influenced by the 27 years (starting in 1976) I spent at the legendary Bell Laboratories. One of the most extraordinary institutions in the field of sciences and high-tech, this is where I started my career as a young researcher, and where I ended up as Vice President of the Division of Physical Research. Even if the work of management led to stress and frustration at times, it gave me a bird’s eye view from one of the outposts in science that were beginning to open up. It also complemented my vision of physics as being in the trenches of research, so to speak. What an experience to be a witness in real time to great scientific and technological developments, even if almost all of them without exception were not recognized as such until many years later!
Bell Labs was an unequalled institution in the history of man in terms of the quality and quantity of discoveries and inventions that made a decisive contribution to the birth of the Information Age and that collectively constitute an incredible series of contributions in the most diverse fields: physics, chemistry, astrophysics, biophysics, electronics, photonics, computer science, mathematics and so on, not to mention its eight Nobel Prizes. The secrets to Bell Labs’ success were obvious enough to the careful observer: emphasis on solving important problems rather than on academic disciplines; freedom to do research with variable horizons ranging from five to twenty years but with an eye towards exploiting opportunities with technological impact; recruiting the best young researchers who had just finished their doctoral degrees rather than hiring people in a limited sector in order to fill a position; a strong meritocracy and a healthy mix of competition and collaboration; and – last but not least – the rejection of the mentality (unfortunately still present in certain academic sectors ) that creates a marked distinction between pure and applied research, often giving precedence to the former.
In my second career as professor at Harvard, which started in 2003, I had the good fortune ever since the beginning to work in a unique organization in the academic panorama, the Division (now School) of Engineering and Applied Sciences, comprising a wide range of disciplines: computer science, mechanical and electronic engineering, bioengineering, physics and applied mathematics, atmospheric chemistry and environmental sciences, without divisions into separate departments. Here reigned a culture and horizontal scheme of organization that strongly encouraged collaboration between students and professors. This was a great intellectual stimulus for me, and enabled me to begin research in new fields as well as to dive into teaching, also experimenting with new didactic methods. The distinctions between disciplines (chemistry, physics, biology, etc.) for the most part have historical origins, that is to say, they are linked to the necessity to organize knowledge in a manageable way. They are also connected to the birth of the modern university, with its division into specialized departments into specific branches of knowledge. Nature, however, makes no distinctions between physical, chemical or biological phenomena.
If there is a common thread running through my scientific adventures, it is this: I have become a Designer: of new, man-made materials which have led me to discover new phenomena and to design new devices, down to the “ultimate design” – of quantum fluctuations in the void. I will illustrate all of this through examples drawn from my research.
The different eras that have marked the history of humanity have been identified and defined as such thanks to the emergence of new materials, or to a different use of already existing materials, which have revolutionized society, making inventions and the introduction of innovative technology possible, which have turned out to be advantageous for the quality of life and economic development. Just think of the Stone Age, the Bronze Age, the Iron Age, and more recently, the Age of Steam, the use of which had a decisive impact on the development of industrial society.
The invention of the transistor in Bell Laboratories in 1947, and of the integrated circuit or chip at Texas Instruments and at Fairchild in 1958-59 define the era that we can define as the Silicon Age, after the basic material of the chip, or the Information Age. The demonstration of the modern semiconductor laser at the end of the 1960s in Russia and in the United States also played a decisive role in this era. This device made it possible to transmit tens of billions of bits of information per second, over thousands of kilometres, in the form of ultrashort pulses of light, using low attenuation optical fibres. The semiconductor laser, together with optical fibres, including optical amplifiers, is the element that revolutionized telecommunications. It was the first laser to be employed on a large scale. In order to function properly, it depends critically on the nature of the interface between two different materials, like gallium arsenide and an alloy known as “gallium-aluminium arsenide”. This apparatus is a sandwich into whose central material carriers of negative electrical charge (electrons) and positive ones (wells) are injected by applying cable tension, and from there they emit, neutralized, laser light of the appropriate wavelength.
Developments in heterostructure devices and materials have been extraordinary in the past thirty years, opening new horizons in science and technology. At the end of the 1960s at Bell Laboratories, my collaborator and friend Alfred Y. Cho invented a new technique for growing ultrathin films (down to a single layer of atoms). Called molecular beam epitaxy (MBE), the technique made it possible to create heterostructures of great scientific and technological interest that otherwise would have remained on paper only. For the first time, it was possible to create ultrathin sandwiches of two different materials of a few nanometres in thickness, called quantum wells, where the electrons acquire levels of energy in a way that is analogous to atoms and molecules, whose value depends on the choice of thicknesses, a phenomenon observed for the first time in Bell Laboratories and in the laboratories of IBM in 1974. Quantum wells have found widespread applications in photonics and electronics.
I realized that MBE would have made it possible to create new materials and devices with enormous technological potential. I saw that by properly combining different semiconductor materials (the so-called heterostructures), by controlling their thicknesses and the chemical composition in varying the distance in the material, I would have been able to design new artificial materials and devices whose electrical and optical properties could be varied, almost at will, for specific applications.
My first important result in the budding field of artificial materials with properties that could be engineered were obtained at Bell Laboratories. This was the invention at the beginning of the 1980s of the so-called stair-step heterostructures, the first step towards quantum cascade lasers, which we realized in 1994. Imagine a multi-layered material designed in such a way as to create a series of stair-steps of energy for electrons. In the presence of an electrical field, when an electron reaches a step, it gains kinetic energy equal to its height. I thought of the step as a sort of trampoline to launch the electrons, like a ski-jumper who gains energy as he leaves the ramp. My intention was to launch an electron and make it reach the highest speed possible. In repeating this process at each step, I expected to be able to reduce the transit time of the electrons through the device. This type of step, that is to say, an energetic leap between two different materials, is used in in superfast transistors today. Then I got the idea to use the stair-step of energy in a new type of light detector. Here, at every step, an electron generated by an incidental photon on the device would gain enough kinetic energy to be able to use it to create a second electron because of the crash with one of the atoms of the crystal, and so on in the so-called process of avalanche multiplication. I showed that theoretically this detector was much more sensitive than existing avalanche detectors.
Then I had the idea to create a stair-step device in which each step of energy was designed in such a way as to create a photon every time it was crossed by an electron. Let’s imagine that the staircase consists of a properly engineered quantum well. An electron tunnel injected at a higher level of energy takes a quantum leap at a lower one, emitting a laser photon. The electron can be recycled after the first leap, re-injecting it into a successive, identical stage and so on. In this way, an electron generates a cascade of laser photons equal to the number of stages: the quantum cascade laser! This number typically varies by a few dozen up to one hundred. Quantum cascade lasers can emit, according to the thicknesses of the quantum wells, wavelengths that go from a few microns up to 300 microns, thus covering most of the infrared spectrum. In this zone most of the molecules absorb light, so the quantum cascade laser has found many uses in detecting traces of gas or low concentration vapours (parts per billion in volume). The applications are countless: from high precision measurements of greenhouse gases in the atmosphere, important for modelling climate change, to the diagnostics of processes of combustion in turbines, where the concentration of carbon monoxide (CO) and nitrogen (NO); from the control of environmental contamination and industrial or medical processes, for example, diagnosis through breath analysis or safety-related ones, like detecting explosives. Quantum cascade lasers and related instruments are currently available on the market from about thirty firms. A few years ago, with some young colleagues of my group, I created a start-up (EOS Photonics), which has now merged with another company (Pendar Technologies) to commercialize a new portable ultrasensitive sensor for traces of gas.
At Harvard my group had the opportunity to open a new road in the design of materials and devices. Thanks to the pioneering work of the great physicist Faraday, had been known for almost 150 years that the nanoparticles of noble metals take on different colours with variations in their form, size and material (silver, gold, etc). At the basis of this phenomenon of resonance are the oscillations of the electrons of the metal, the so-called plasmons, thus the nanoparticle behaves like a minuscule antenna that responds differently to incident light with variations in the wavelength. Our first simple application, which had a wide appeal, was the use of a double nanoantenna realized on the facet of a minuscule commercial semiconductor laser to concentrate the light in one spot of nanometric dimensions for uses like high-density optical storage.
This was followed by a fundamental study on the optical properties of colloids, spherical dielectric particles covered with a metallic shell, arranged in clusters of various forms. We designed an heptameter with anti-resonant optical properties: at a precise wavelength, the cluster spread light in opposition to the phase of the incident light, thus partially blocking the reflection, an example of the famous Fano resonance, or Fano minimum, after the famous physicist who had worked with Fermi.
Later we were motivated by the request of one of my colleagues, Professor Jim Anderson, famous for his pioneering studies in atmospheric physics, if we could make a collimated quantum cascade laser, that is to say, with minimum divergence, without having to use an external lens so that it could be used on a drone, where the space available is minimal. And so we made, so to speak, a metallic contact lens directly on the laser facet so that the laser beam came out almost perfectly collimated. It worked like a microwave or highly directional radio frequency antenna, an early example of optical frequencies and how the wavefront of light emitted by a source can be designed.
These experiments in plasmonics led us to realize that by properly structuring a surface with a series of thin metallic antennae separated by a distance that was less than the wavelength of the light (what was then called a metasurface by analogy with three-dimensional metamaterials), we could create a new class of flat optical components capable of transforming the beam of incident light into any cast or reflected beam of light, that is to say, with an arbitrary wavefront. The first results led to a powerful generalization of the law of light refraction, or Snell’s Law, known for more than 400 years and valid for flat, homogeneous surfaces. Starting from Fermat’s Principle (light takes the path of least time), we found that the condition necessary for incident light at a certain degree can be refracted by the metasurface according to a pre-assigned angle. It is properly engineered to produce a constant phase gradient of the light transmitted, along the surface. It should be noted that in this way metasurfaces with negative refraction can be designed, that is to say that, in crossing them the light is bent by the part opposite the perpendicular of the surface, an effect that we had observed experimentally in metasurfaces constituted by an ensemble of gold antennae on silicon. In a later experiment, we demonstrated that by decorating a surface with the same type of antenna, V-shaped but radially arranged, we could create in transmission beams of light in spiral form, known as optical vortices, with intensity concentrated in a circular crown.
The most important result of these studies from the point of view of applications came from our first demonstration of a flat lens that could focalize light in the near-infrared without the spherical aberration typical of curved lenses that impedes the rays from being all focalized in the same point, thus producing out-of-focus images. However, we realized that a metasurface constituted of metallic antennae was not useful for producing optical components given the considerable loss of light due to absorption. We resolved the problem by using – instead of metal antennae – dielectric nanostructures with the same functionality to modify the phase of incident light in the visible spectrum in such a way as to make all of the rays converge on the same focal point, but without losing light. The material we used was Titanium dioxide, widely used in various kinds of technology, patented with a new process invented in my group. In this way we have recently created flat lenses in the visible spectrum, in particular in red, green and blue, which focalize light in one point with the precision of half a wavelength, the so-called diffraction limit. The quality of focalization is the same as that of a high quality commercial microscope lens.
This work led to enormous interest on the part of many industries. Flat lenses could be produced with the same planar processes used to create the silicon chips used in cell phone cameras, where there are typically five or six traditional curved lenses. They should be able to lead to a great reduction of production costs since they are thinner, hence with a consequent reduction of the thickness of the cell phone and greater facility in assembly and optical alignment. There are many possible applications – practically anywhere lenses are used: laptops, displays including portable ones for virtual reality, or the so-called “augmented reality”, correction of eyesight, microscopes and so on.
In the last years at Bell Laboratories, I began to study the Casimir Effect, a manifestation of quantum vacuum fluctuations. In quantum physics there is no vacuum, as understood in the traditional sense of the word. In its place, there is a continuous activity of creation and disappearance of particles (photons, etc.), which for this reason are called virtual particles in continuous effervescence of the “vacuum” that produces very interesting effects. Among these is the Casimir Effect, predicted by Hendrik Casimir as early as 1948: two metallic plates with no electrical charge place in a vacuum at a certain distance attract each other with a quantum force that is inversely proportion to the fourth power of the distance between the plates, with a constant of proportionality that contains the speed of light and Planck’s constant. The paradoxical aspect is that two electrically neutral particles can attract each other.
This apparently very mysterious phenomenon can be understood by analogy with the force that, unexplainably in the era of the great sailing ships, seemed every now and make them collide if they were close to one another in particularly rough seas, a phenomenon only recently understood and analysed quantitatively. The physical reason for this effect is that only waves with wavelengths (the distance between two crests) less than the distance separating two ships can exist between them. In areas of water outside the ships, waves of any length are possible, and hence exert a greater force than that of the waves between them. The Casimir Effect is entirely analogous, even if its origin is purely quantum: here the virtual photons have associated electromagnetic waves that in the space between the metallic places can take on wave lengths that are not greater than the space between them, while outside the plates, in free space, waves of every length are possible. This gives way to a net force of attraction between the plates that is in fact the Casimir force.
For many years this effect was considered a mere theoretical curiosity. Intrigued, I did a simple calculation with Casimir’s formula, and showed that if I could place two plates in a vacuum at a distance of ten nanometres, a pressure of attraction equal to one atmosphere would be developed between the two plates, a totally respectable value. I knew that at the time at Bell, there was a particular kind of technology called MEMS (MicroElettroMechanical Systems), that is, microelectromechanical systems consisting of silicon chips with mobile mechanical parts. As a form of technology, MEMS have been commercially available for some time: it will suffice to say that air-bags in automobiles are controlled by MEMS devices sensitive to deceleration. I realized that MEMS could be very useful to for a high-precision measurement of the Casimir force as well as for applications of this “exotic” force.
We built a contraption consisting of a gold-coated silicon plate that could revolve around a central linchpin. As the metallized sphere drew near to the plate, the Casimir force of attraction between the two metallic surfaces made the plate rotate. A high-precision measurement of the miniscule angle of rotation gave a direct measurement of the Casimir force, which we achieved with great precision. This was also the first application to micromechanics of this quantum force of electromagnetic origin. The idea has always been to design and engineer nature! Casimir forces depend on the form of the interacting surfaces, not just on the nature of the materials. Given that the origin of these forces are quantum vacuum fluctuations, we must design the latter in such a way as to alter Casimir forces. In short, we have passed from materials engineering to vacuum engineering!
An even more exotic effect, repulsive Casimir force, attracted my attention when I arrived at Harvard. It is a fact that, if instead of two metallic plates we consider two made of different materials, one metal like gold and the other dielectric like glass, and they are separated by a liquid with appropriate optical properties, a repulsive force will be developed between them, and this time it will be due to the quantum fluctuations of the materials. Many had tried to measure this force, but without success. Our extremely difficult experiment lasted three years, and at the end, we measured the repulsive Casimir force between a sphere covered with gold film, immersed in Bromobenzene, and a glass plate, in agreement with the theory. This effect will be able to find applications in reducing arthritis and in general in modifying the flow of a liquid in a space between two materials or in the development of new sensors balancing gravity. Exploiting this quantum levitation effect years ago, we invented ultra-sensitive devices that could rotate and transfer with minimal friction.
If there is a lesson that I have learned after many years of research and management, it is the futility of scientific and technological predictions or of the predicted economic and hence social impact of discoveries and inventions. Even researchers themselves – most of the time – do not know what turns their studies might take. Therefore, imagine what would happen if planners of research and managers could never do better! For that matter, technology is even less predictable than science, since its success depends greatly on economic and political factors and on life and less on psychological barriers to its acceptance.
I would like to conclude by expressing my gratitude to the Balzan Foundation for this prestigious prize.