Special OQE Seminar
Friday, October 18, 2019
10:00 AM
Grier A, 34–401A
Professor Jacob B. Khurgin
Johns Hopkins University
Hosted by: Professor Kevin O’Brien
Some myths and realities in nanophotonics:
(1) Excited carriers in metals: from icy cold to comfortably warm to scalding hot
The field of plasmonics in recent years has experienced a certain shift in priorities. Faced with undisputable fact that loss in metal structures cannot be avoided, or even mitigated (at least not in the optical and near IR range) the community has its attention to the applications where the loss may not be an obstacle, and, in fact, can be put into productive use. Such applications include photo-detection, photo-catalysis, and others where the energy of plasmons is expended on generation of hot carriers in the metal. Hot carriers are characterized by short lifetimes, hence it is important to understand thoroughly their generation, transport, and relaxation in order to ascertain viability of the many proposed schemes involving them.
In this talk we shall investigate the genesis of hot carriers in metals by investigating rigorously and within the same quantum framework all four principle mechanisms responsible for their generation: interband transitions, phonon-and-defect assisted intraband processes, carrier-carrier scattering assisted transitions and Landau damping. For all of these mechanisms we evaluate generation rates as well as the energy (effective temperature) and momenta (directions of propagation) of the generated hot electrons and holes. We show that as the energy of the incoming photons increases towards the visible range the electron-electron scattering assisted absorption becomes important with dire consequences for the prospective “hot electron” devices as four carriers generated in the process of the absorption of a single photon can at best be characterized as “lukewarm” or “tepid” as their kinetic energies may be too small to overcome the potential barrier at the metal boundary. Similarly, as the photon energy shifts further towards blue the interband absorption becomes the dominant mechanism and the holes generated in the d‑shell of the metal can at best be characterized as “frigid” due to their low velocity. It is the Landau damping process occurring in the metal particles that are smaller than 10nm that is the most favorable on for production of truly “hot” carriers that are actually directed towards the metal interface.
We also investigate the relaxation processes causing rapid cooling of carriers. Based on our analysis we make predictions about performance characteristics of various proposed plasmonic devices.
(2) Non-magnetic optical isolators: what works and what does not?
Optical Isolator is a key component of photonic circuits and systems. An optical isolator requires non-reciprocal propagation i.e. breaking time inversion symmetry. Time symmetry cannot be broken in a linear optical system without magnetic field and/or gain and loss, hence all the practical isolators at this point are based on Faraday (magneto optic) effect which makes it difficult to develop isolators for planar integrated photonic circuits. Therefore, in recent years a strong effort has been mounted to develop non-magnetic isolators. A number of schemes had been proposed and demonstrated, such as devices with temporal modulation, acousto-optic and opto-mechanical isolators, various nonlinear schemes and parity time schemes with gain and loss.
In this talk we review performance characteristics of all these schemes and find them lacking any advantages in comparison to magnetic isolators. Most of the proposed schemes are severely limited in bandwidth and require high power consumption. Moreover, often they are not true optical isolators but are “optical diodes” in the sense that they do not offer full isolation.
We then make a case for the optical isolator based on second and third order nonlinearities that have good isolation and high dynamic range and offer detailed analysis of this exciting family of devices.