Electrically-injected ultra-low threshold polariton laser – Plug-and-Bose

The main goal of “Plug-and-Bose” is to fabricate electrically-injected polariton lasers operating at room-temperature (RT) and to evaluate both theoretically and experimentally their performance as potential low-power-consumption coherent sources.
All commercial semiconductor laser diodes today are based on the same physical mechanism: the inversion of the electronic population within an active medium into which electrons and holes are injected from electrical contacts, followed by stimulated photon emission. But fundamental studies have demonstrated that coherent light could also be emitted from semiconductor sources based on a new physical paradigm, namely polariton lasers. Polaritons result from the strong coupling of excitons (bound electron-hole pair) and photons. Contrary to the particles in the electron-hole plasma of conventional semiconductor lasers, polaritons are bosons and when their density exceeds a certain threshold they can condense into a single state which emits coherent light, resulting in polariton lasing. The “gain mechanism” is based on stimulated polariton scattering. This process can take place at a much lower particle density, and light sources with thresholds one to two orders of magnitude smaller than the corresponding conventional laser diodes have been predicted and observed. Such a laser could address applications where a drastic reduction of the energy-consumption is required. For example, recent calculations have set a standard of few tens of femto-joules of electrical energy per optical bit to implement optical links in data communications, which represents a stringent requirement for conventional nanolasers.
The maximum operation temperature of a polariton laser depends on the exciton stability, which is material-dependent. Using GaN and ZnO wide bandgap semiconductors, optically-pumped polariton lasers have been demonstrated at RT and up to 450 K. The fabrication of an electrically-injected device operating at RT and showing a low threshold current would represent a major achievement. Indeed, for very short-range communications (e.g. intra-chip), no optical source “standard” has emerged up to now. Emission in the ultra-violet is one possible option considering that low parasitic signals or cross-talks are expected in this spectral range.
Still, some pending questions have to be answered in order to demonstrate the interest of a wide-bandgap polariton laser for applications in data communications. Among them: i) what is the best design and lowest threshold power of an electrically-driven polariton microlaser operating above RT? ; ii) can the polariton emission be modulated (directly or externally), and at which bit rates?
The Plug-and-Bose project will tackle these questions by demonstrating a GaN- or ZnO-based electrically-injected polariton laser operating above room temperature with a significantly reduced lasing threshold. We will realize a Vertical Cavity Surface Emitting Polariton Laser and evaluate the possibilities to achieve an Edge Emitting Polariton Laser. The vertical microcavity approach, the most mature in polaritonics, has been already developed by our consortium with optical pumping, while the waveguide geometry -barely explored- appears very attractive for the implementation of electrical injection, and its integration with photonic circuits. In a more prospective part of the project, the modulation capabilities of the polaritonic emission will be evaluated both experimentally and theoretically. The laser response under direct modulation relies on the polariton dynamics, different from that of a classical laser diode, but the first-generation devices may be limited by the diode parasitics. Alternatively, we propose to evaluate in the waveguide geometry the temporal dynamics under polariton amplification conditions.