Periodic Reporting for period 2 - POLLOC (Polariton logic)
Reporting period: 2021-10-01 to 2023-09-30
Dennard scaling, which describes that the performance of a transistor can be increased by scaling while keeping a constant power envelope, has fuelled the semiconductor industry roadmap for decades. However, since its basic breakdown around 2005, due to several fundamental physical reasons, the clock frequency of processors has stalled at a few GHz. Optical transistors could overcome this: Light consists of electromagnetic wave oscillating at a frequency of hundreds of terahertz (THz), imposing no speed barriers. All-optical logic that would be able to work with single photons would correspond to sub-attojoule switching energies, about two orders of magnitude less than current CMOS transistors.
The main impediment, however, is to efficiently couple photons with electronic excitations that can mediate nonlinear interactions. A solution to this is using an optical microcavity to surround the active material and enhancing light-matter interaction and leading to the creation of half-light half-electron-hole quasiparticles, i.e. polaritons. The nonlinearity provided by stimulated scattering of polaritons can be exploited to realize ultrafast amplifiers, and so-called polariton condensates enabled all-optical transistors and switching functionality, albeit at cryogenic temperature and high optical power requirements. Recently, inorganic lead halide perovskites have emerged as materials with truly exceptional optoelectronic properties that combine all the important ingredients for achieving strong-light matter coupling and strong exciton-exciton interaction.
The PoLLoC project's primary goal was to establish a groundbreaking scalable technology platform for all-optical digital logic circuits based on polariton quasiparticles. The objectives were to enhance the effective photon interaction strength through optimized perovskite compounds, realize single digital logic gates with a switching energy of less than 100 attojoules and high-speed operation in the picosecond range, and implement more complex polariton logic circuitry.
Over the project duration, we have shed light on many unknown fundamental photophysical properties and mechanisms of lead halide perovskites that helped to engineer the light-matter coupling and improve the robustness. We have successfully demonstrated the operation of multi-input universal logic gates, along with ultra-fast sub-picosecond switching speeds and highly efficient all-optical logic down to the ultimate limit of single photon input signals. Furthermore, we have demonstrated transistor action and cascadability of transistor devices by developing novel in-plane grating resonator structures, paving the way for scalable, complex polaritonic circuitry.
The main impediment, however, is to efficiently couple photons with electronic excitations that can mediate nonlinear interactions. A solution to this is using an optical microcavity to surround the active material and enhancing light-matter interaction and leading to the creation of half-light half-electron-hole quasiparticles, i.e. polaritons. The nonlinearity provided by stimulated scattering of polaritons can be exploited to realize ultrafast amplifiers, and so-called polariton condensates enabled all-optical transistors and switching functionality, albeit at cryogenic temperature and high optical power requirements. Recently, inorganic lead halide perovskites have emerged as materials with truly exceptional optoelectronic properties that combine all the important ingredients for achieving strong-light matter coupling and strong exciton-exciton interaction.
The PoLLoC project's primary goal was to establish a groundbreaking scalable technology platform for all-optical digital logic circuits based on polariton quasiparticles. The objectives were to enhance the effective photon interaction strength through optimized perovskite compounds, realize single digital logic gates with a switching energy of less than 100 attojoules and high-speed operation in the picosecond range, and implement more complex polariton logic circuitry.
Over the project duration, we have shed light on many unknown fundamental photophysical properties and mechanisms of lead halide perovskites that helped to engineer the light-matter coupling and improve the robustness. We have successfully demonstrated the operation of multi-input universal logic gates, along with ultra-fast sub-picosecond switching speeds and highly efficient all-optical logic down to the ultimate limit of single photon input signals. Furthermore, we have demonstrated transistor action and cascadability of transistor devices by developing novel in-plane grating resonator structures, paving the way for scalable, complex polaritonic circuitry.
Within the framework of the PoLLoC project, our efforts were concentrated on the synthesis of various perovskite compounds, resulting in the successful development of quantum nanomaterials. These nanomaterials exhibit the desired absorption profile and a high photoluminescence quantum yield, which is crucial for their applications in strong-coupling scenarios.
Our exploration into the optical properties of these compounds involved investigations at both the ensemble and single-particle/photon levels. These experimental studies were complemented by theoretical modelling and provided insights into the intrinsic optical characteristics of excitons and trions within these novel compounds that are key for many optoelectronic applications. Moreover, we were able to demonstrate polariton condensation, associated with nonlinearity through stimulated scattering.
To seamlessly integrate these novel perovskite compounds into the fabrication process, we developed suitable encapsulation techniques. This was done to safeguard the perovskite compounds from degradation caused by parasitic processes, such as chemical modification, moisture exposure, or the generation of defects, among others. A pivotal aspect of the overall project was the development of a silicon photonic fabrication process, ensuring the optimal creation of high-index-contrast grating resonator structures for in-plane logic circuits.
The fabricated structures underwent testing, using either the newly synthesized perovskites or a well-known model material (ladder-type polymer MeLPPP), to assess their performance. Through the optimization of the optical properties of our polariton transistor, we obtained experimental evidence of single-photon nonlinearity. Moreover, we successfully demonstrated an all-optical universal gate (NOR gate) with multiple input ports. Finally, we achieved coupled in-plane high-contrast grating resonators, where the output of one resonator serves as the input for the next, exhibiting transistor action and cascadability.
The results have been disseminated in 40 publications (including the journals Science, Nature, Nature Communications, and Advanced Materials) and 72 presentations at international conferences and other venues, plus the organisation of two topical symposia and summer schools and numerous public outreach and press events. The grounds for technological exploitation have been established in the form of a foundational patent and process modules for foundry fabrication.
Our exploration into the optical properties of these compounds involved investigations at both the ensemble and single-particle/photon levels. These experimental studies were complemented by theoretical modelling and provided insights into the intrinsic optical characteristics of excitons and trions within these novel compounds that are key for many optoelectronic applications. Moreover, we were able to demonstrate polariton condensation, associated with nonlinearity through stimulated scattering.
To seamlessly integrate these novel perovskite compounds into the fabrication process, we developed suitable encapsulation techniques. This was done to safeguard the perovskite compounds from degradation caused by parasitic processes, such as chemical modification, moisture exposure, or the generation of defects, among others. A pivotal aspect of the overall project was the development of a silicon photonic fabrication process, ensuring the optimal creation of high-index-contrast grating resonator structures for in-plane logic circuits.
The fabricated structures underwent testing, using either the newly synthesized perovskites or a well-known model material (ladder-type polymer MeLPPP), to assess their performance. Through the optimization of the optical properties of our polariton transistor, we obtained experimental evidence of single-photon nonlinearity. Moreover, we successfully demonstrated an all-optical universal gate (NOR gate) with multiple input ports. Finally, we achieved coupled in-plane high-contrast grating resonators, where the output of one resonator serves as the input for the next, exhibiting transistor action and cascadability.
The results have been disseminated in 40 publications (including the journals Science, Nature, Nature Communications, and Advanced Materials) and 72 presentations at international conferences and other venues, plus the organisation of two topical symposia and summer schools and numerous public outreach and press events. The grounds for technological exploitation have been established in the form of a foundational patent and process modules for foundry fabrication.
Fully inorganic lead halide perovskites materials have attracted a huge community due to their truly exceptional optoelectronic properties. Our project has advanced the synthesis of several forms of perovskite nanocrystals and layered materials. Together with theoretical models and carefully controlled synthesis, we were able to unravel the role of electronic correlations that lie at the heart of excitonic interactions, leading even to peculiar phenomena like single-photon superradiance and superabsorption. These fundamental insights will not only be extremely valuable to further engineer and optimize this material class for optical computing, but have deep implications and provide new opportunities for many other applications like displays, lighting, detectors and photovoltaics.
In view of photonic device architectures, we achieved to establish silicon high contrast gratings as compact, universal resonators for photonic integrated circuits, even in the visible wavelength regime despite the strong absorption of silicon. We implemented the first polariton condensate that works on-chip with integrated elements, enabling fully planar light in-/output, providing the essential building block of a photonic transistor in a scalable architecture. Harnessing its nonlinearity, we are able to realize all-optical logic gates that can operate at room temperature with picosecond speed and input signals down to attojoule energy, orders of magnitude faster, more efficient and more compact than previous architectures based on semiconducting optical amplifiers. We expect that these proof-of-principle demonstrations spark future developments towards ultrafast all-optical accelerators to boost the computational performance.
In view of photonic device architectures, we achieved to establish silicon high contrast gratings as compact, universal resonators for photonic integrated circuits, even in the visible wavelength regime despite the strong absorption of silicon. We implemented the first polariton condensate that works on-chip with integrated elements, enabling fully planar light in-/output, providing the essential building block of a photonic transistor in a scalable architecture. Harnessing its nonlinearity, we are able to realize all-optical logic gates that can operate at room temperature with picosecond speed and input signals down to attojoule energy, orders of magnitude faster, more efficient and more compact than previous architectures based on semiconducting optical amplifiers. We expect that these proof-of-principle demonstrations spark future developments towards ultrafast all-optical accelerators to boost the computational performance.