The objective of this research is to develop optical computing components that are fast, energy efficient, and integrateable into a larger photonic system. The approach is to build on the recent advancements in the understanding and use of the polaritonic states of strongly light-mater-coupled systems, and to demonstrate a room temperature low-threshold polariton laser and a polariton-mediated all-optical switch, both primary components of an envisioned all-optical computing architecture.
Although a laser and an optical switch are conceptually quite different, the project will show that the underlying physics of strong light-matter coupling can be leveraged in both cases to create devices with ultra-low thresholds for switching and lasing. Polaritonic effects will be strongly enhanced in the record-high optically-absorptive molecular aggregates, that investigators recently demonstrated, enabling first demonstrations of compact, room-temperature polaritonic structures. The proposed polaritonic devices are also scalable and integrateable, and could ultimately form the building blocks of an integrated all-optical computing architecture.
Optical computing has the potential to create a paradigm shift in the way information is transmitted and manipulated, by creating all-optical data networks and computational circuits with nearly unlimited bandwidth and unprecedented energy efficiency. This long-standing goal has thus far been difficult to achieve due to a lack of appropriate material sets, light-matter interactions, and device designs. Recent advancements, however, that this proposal builds on, can enable construction of optical computing components that for the first-time utilize the physical phenomenon of the strong light-matter coupling, delivering a technological breakthrough, and intellectually stimulating a new field of research.
The goal of this work was to develop a room temperature low-threshold polariton laser and polariton mediated all-optical switch based on the physics of strong coupling of organic J-aggregates to an optical microcavity. Success of this work would allows us to pursue future all-optical data network and computational circuits which would allow for unlimited bandwidth and high energy efficiency. Strong coupling limit of cavity QED is reached when matter inserted inside an optical microcavity exchanges energy with a resonant mode of the cavity more rapidly than the combined rate at which energy escapes the system. In this limit, the microcavity and matter component form a composite quantum system with two new eigenstates that are superpositions of the initially uncoupled states. The matter component of the coupled system can be a gas of atoms trapped inside the cavity, a superconducting qubit, or a thin film containing excitons in the form of an inorganic quantum well, quantum dot or organic material. When excitons, a bound state of an electron and a hole are used, the superposition states are referred to as exciton-polaritons. Many of the intriguing properties of strongly coupled systems arise because of the hybrid nature of polaritons, which are quasiparticle composites of a photon and a matter excitation. In particular, the relatively small stimulated emission cross-section of the exciton can be greatly enhanced by coupling to a photon (which has a size scale of wavelength/2), suggesting that polaritonic structures could be candidates for low-threshold lasers. Likewise, the non-interacting nature of photons can be modified by strong coupling to excitons, opening the possibility of all-optical switching mediated by polaritons. While most demonstrations of strong coupling have been done in gaseous, superconducting, or inorganic semiconductor systems, organic materials offer particular advantage for realizing technological applications of strong light-matter coupling, as they enable observation of polariton states at room temperature. In particular, a class of organic materials known as J-aggregates are especially suited for polaritonic systems due to their high oscillator strength, narrow linewidth absorption, small absorption-to-photoluminescence Stokes shift, and large exciton binding strength. Prior to this program, we demonstrated strong coupling of ultra-high absorption J-aggregate thin films to optical microcavities and have also demonstrated the first electrically pumped polaritonic device. Within this program we accomplished the first demonstration of lasing from a polaritonic cavity using J-aggregates as the strong coupling material. The advance which enabled lasing to be achieved is a new excitation method we developed, which we call intracavity pumping (see figure 1). This excitation scheme allows for the injection of polaritons directly into the polariton mode thus circumventing much of the annihilation loss and slow polariton relaxation that has previously prevented lasing in J-aggregate microcavities. Equally as important, the new cavity architecture, which separates strong coupling and gain into two materials, presents a general and flexible design for polariton devices. This cavity design and pumping scheme allows for the use of a wide range of materials, both organic and inorganic, to be integrated into the cavity. The work demonstrates that the strong QED coupling phenomenon is observable in room temperature systems that can be fabricated and tested much more easily than any previously fabricated lasing QED coupled structure. In the process we discovered a new fabrication technology that enabled us to pattern, with nanometer precision, the microcavity structures. This thin-film contact-patterning using PDMS (polydimethylsiloxane) stamps (see Fig. 2) is a general new technique for fabricating large-area nanostructures that will have a broad impact on development of new molecular (and nanostructured) materials technologies. Contact patterning provides a photolithography-free, potentially scalable approach to subtractive patterning of wide range of molecular organic films of nanoscale thickness. We have made much progress during this research program. From improving J-aggregate films to coming up with new schemes for efficient laser pumping to nano patterning organic microcavities. We have also explored photonic crystal nanobeam microcavities (see Fig. 3) in TiO2 material system with an aim to improve the effect of nonlinearity in the system. Such structures not only produce high Q/V cavities but also require a much smaller footprint. All the effort brings us a step closer towards achieving the ultimate goal of an all-optical data network. Though this is just the beginning, the findings on exciton polariton lasing cross mutiple disciplines and can be further developed by engagement of chemists who will develop new material sets for strong QED coupling, thin films materials growers who would perfect the crystaline thin film structures used for enhanced optical response, nanofabricators who would minimize the size of the lasing domains to the dimensions of a single grain of the strongly coupled materials hence further reducing the operating threshold, physicists who can use this model system to study coherence and strong coupling, and engineers who want to implement optical logic systems based on this phenomenon.
