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. One consequence of this is that the surface of a topological insulator is populated by exotic edge states with unusual properties. In the photonic context, these edge states can be used to realize a fundamentally new class of electromagnetic wave-guides, which are able to transport light around sharp corners without backscattering.

Our group has made numerous contributions to this exciting field. For instance, we showed theoretically how a periodic lattice of coupled ring resonators can function as a photonic topological insulator with tunable “topological phase transitions” [Physical Review Letters (2013), Physical Review B (2014)]. We then used this theory to design and implement a “topological pump”, an experiment that directly probes and provides evidence for the fact that a bandstructure is topologically nontrivial [Physical Review X (2015)]. More recently, we have been exploring the intersection of nonlinear optics and topological photonics [Physical Review Letters (2016), New Journal of Physics (2017)].

We have also collaborated extensively with experimental groups, at NTU and around the world, to implement systems such as topological spoof surface plasmons, optical “Weyl modes”, and more.

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)]. Currently, we are interested in applying the principle to complex optical media. For instance, we have shown theoretically that in weakly-absorbing “random media”, such as foam, paint, or tissue, it should be possible to use a specially-designed input wave-front to 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)].

PT symmetry and Non-Hermitian Photonics

Recently, there has been significant scientific interest in the properties of photonic devices containing optical amplification (gain) and/or absorption (loss). Such systems are said to be “non-Hermitian”, as they do not conserve the electromagnetic energy flowing through them. Although non-Hermitian systems have long been treated as imperfect variants of “Hermitian” (energy-conserving) systems, researchers now realize that they can also possess features completely different from the Hermitian case.

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 been working on bringing the ideas of band topology into non-Hermitian photonics. For instance, we have theoretically analyzed a novel class of topological edge states that occur only in non-Hermitian lattices [Physical Review Letters (2017)], and we have experimentally implemented a non-Hermitian version of a topological pump [Physical Review B (2017)].