Optics and Photonics

The past two decades have seen a revolution in the science of light. Photonic (light-manipulating) structures can now be engineered with size comparable to the optical wavelength, allowing us to exploit the wave nature of light to achieve a variety of novel effects. Devices such as photonic crystals, on-chip optical circuits, and microcavity lasers have important technological roles in communications, sensing, and computing. They are also fruitful platforms for basic scientific research.

Topological Photonics

photonic topological insulator A photonic topological insulator consisting of a lattice of coupled ring resonators. Light is injected on the left (red and blue areas showing the positive/negative values of the electric field), and flows around the edge of the lattice similar to “topologically-protected” electron edge states in topological insulator materials.

Researchers in photonics have long been inspired by a conceptual analogy between electromagnetic waves in patterned dielectric media and quantum mechanical electron waves in solids. In the late 1980s, Eli Yablonovitch and Sajeev John conceived of the photonic crystal as a photonic analog of a conventional electronic insulator. Photonic crystals are now a staple of nanophotonics, with applications ranging from low-loss optical resonators to slow-light wave-guides.

In recent years, researchers have begun to study photonic analogs of insulating materials known as topological insulators, which are mathematically distinct from conventional insulators. A topological insulator possesses a bandgap, but its bands are topologically distinct from the bands of a conventional insulator; it is impossible to smoothly deform one into the other, in the same way that a torus cannot be deformed into a sphere without tearing its surface. As a result, the surface of a topological insulator is populated by “topological edge states” that possess highly unusual properties, such as the ability to travel around sharp corners without backscattering.

Some contributions we have made to this exciting field include:

Topological Acoustics and Electronics

Apart from photonics, topological edge states can occur in other types of systems that host classical (non quantum mechanical) waves, including sound waves and electrical waves. We have made numerous contributions in these areas, in collaboration with Zhang Baile, including:

Non-Hermitian Photonics

Photonic devices containing optical amplification (gain) and/or absorption (loss) do not conserve the electromagnetic energy flowing through them. These so-called “non-Hermitian” systems have long been treated as imperfect variants of “Hermitian” (energy-conserving) systems. Recently, however, researchers have realized that non-Hermitian systems can possess distinct and noteworthy features.

For instance, when gain and loss are simultaneously present in equal and opposite amounts in two halves of a photonic structure, it is said to be PT (parity-time) symmetric. The exotic properties of PT symmetric photonic structures have been studied in great detail by many research groups. Several years ago, we showed theoretically that by tuning the frequency and/or the gain and loss in a PT symmetric structure, one can induce a spontaneous symmetry breaking transition between PT symmetric scattering eigenmodes (which conserve energy) and PT broken pairs of eigenmodes (one damping and one amplifying) (Physical Review Letters 2011). In the most extreme case, a PT symmetric structure can simultaneously function as a coherent perfect absorber and a laser!

We have also worked on bringing the ideas of band topology into non-Hermitian photonics. Our contributions in this area include:

Coherent Perfect Absorbers

CPA Animation In this simulation, a small disc (much smaller than the optical wavelength) made of optically absorbing material is surrounded by larger discs made of non-absorbing dielectric. When a specially-designed waveform is directed at the structure, it is perfectly absorbed.

Coherent perfect absorption is a phenomenon in which a photonic structure absorbs all of a specially-designed incoming optical wave. The absorption is “perfect”: all of the input light energy is delivered into the material of the photonic structure. (The energy subsequently flows out to an external reservoir, e.g. in the form of heat or electric current.)

Theoretically, coherent perfect absorption is tied to the time-reversal symmetry of electromagnetism, which states that if one process is allowed, then so is a time-reversed process which reverses all currents, spins, and magnetic fields. Several years ago, we published a theoretical paper pointing out that since the field emitted by a laser is a purely-outgoing electromagnetic wave, the time-reversal symmetry operation maps a laser into an “anti-laser” structure, which exhibits perfect absorption of a certain incoming wave — the time reverse of the laser field (Physical Review Letters 2010).

Coherent perfect absorption is a generalization of the concept of “critical coupling”. We demonstrated the principle experimentally (Science 2011). We have extended the concept in various ways. For instance, we have shown theoretically that in weakly-absorbing “random media”, such as foam, paint, or tissue, a specially-designed input wave-front can achieve extremely strong absorption even if the medium is ordinarily “white” (Physical Review Letters 2010).

We have also written a review article on the topic (Nature Reviews Materials 2017).