Applied Physics Seminar

A laser is an optical device that transforms incoherent input energy (the pump), into coherent outgoing radiation in a specific set of modes of the electromagnetic field, with distinct frequencies. There is a threshold pump energy for the first lasing mode, and above that energy the laser is a non-linear device, and non-linear interactions strongly affect the emission properties of the laser. Surprisingly, the electromagnetic theory of non-linear steady-state multimode lasing remained rather rudimentary until recently. Motivated by the complex laser cavities being developed in modern micro and nano-photonics, we have developed a new formalism, Steady-state Ab initio Laser Theory (SALT), which calculates directly all the steady-state lasing properties without integration of the semiclassical lasing equations in time, and is hence orders of magnitude more efficient, and provides additional analytic insight. The method is based on non-hermitian states of the electromagnetic field, which provide a quantitative and tractable description of arbitrarily complicated laser systems, including extremely open and non-linear examples, such as random lasers. SALT also provides the basis for a quantitative theory of quantum fluctuation properties of the laser, leading to a novel generalization of the Schawlow-Townes formula.

Our reformulation of laser theory emphasizes that a laser cavity is a certain kind of scattering system, with a non-unitary amplifying scattering matrix due to the presence of gain. This approach suggested the possibility of constructing a time-reversed or anti-laser , which we term a coherent perfect absorber (CPA); a linear device in which the gain medium of the laser is replaced with a loss medium such that the cavity will perfectly absorb the incoming (time-reversed) modes of the corresponding laser at threshold. Recently we have experimentally demonstrated such a device in a simple silicon cavity, which acts as an absorptive interferometer, in which narrow-band absorption can be both increased to ~ 99% and reduced to ~30%. Time-reversed optical parametric oscillation can also be studied in the same framework, leading to new insights and possible applications.