Everest

Optoelectronic devices ranging from light-emitting diodes and solar cells to semiconductor lasers and detectors are an integral part of our daily life and are expected to play an increasingly important role in the future, e.g. for fighting climate change or for ultrafast optoelectronic data processing. As the basic operating principles of optoelectronic devices are well known, their improvement while important is often incremental. Finding new avenues to implement novel functionalities is therefore crucial. Cavity quantum electrodynamics (QED) promises radically new ways to innovate optoelectronic devices by exploiting the quantum mechanical principles of strong and ultra-strong light-matter coupling in systems embedded in an optical cavity of high quality. In this regime, the photons couple to the matter so strongly that the formation of new hybridized states known as polaritons, which are new quasiparticles with partly electronic and photonic properties, is caused. This can lead for example to very small effective masses. Their energy spectrum is a mixture of that of electrons and photons with avoided crossings where the energy levels split into lower and upper polariton branches. The energy difference between them is called polariton splitting and is a direct measure for the light-matter coupling. By taking this coupling to the extreme, the polariton splitting becomes comparable to the unperturbed electronic transition energy. Thus, the sole presence of a photon cavity can induce modifications of ground and excited states. Therefore, the behavior of electronic devices can be modified, since their function is determined by the position and alignment of the energy levels. The ultra-strong light matter coupling therefore opens up new operating principles for cavity-control of electronic processes, a quantum engineering strategy for devices that is still in its infancy. This project`s first objective is to develop a new concept of a proximity sensor based on cavity electrodynamics. The conductance of the device, that can be measured in real time and with very high precision, will be affected by the proximity of a conducting/dielectric surface, up to few hundred nm distances. The result will be the start of a new family of sensor devices. The second one is fundamental in nature and aims at understanding how virtual photons i.e. the sole presence of a cavity affect the energy levels of a system and therefore its electronic properties. We will tackle the open scientific and technological challenge to identify and isolate the most relevant processes and to harness them for (opto)-electronic devices.

(Opto)electronic devices such as light-emitting diodes, solar cells, lasers, and detectors are integral to daily life and are expected to become even more important as photonic technologies play a central role in data and AI centers. Cavity quantum electrodynamics (QED) offers fundamentally new ways to innovate optoelectronic devices. The main objective of the EVEREST project was to investigate a novel class of devices whose electronic properties are improved or enabled through interaction with vacuum-field photons. This approach exploits strong light-matter coupling in microcavity systems, where the coupling strength leads to hybrid light-matter states known as polaritons. In the ultra-strong coupling regime, vacuum-induced modifications of quantum states become significant and can measurably alter device behavior. We studied two types of structures: resonant tunneling devices and multi-quantum well (MQW) structures. Resonant tunneling diodes serve as model nanoelectronic devices, while MQWs are widely used in photonic applications such as quantum well infrared photodetectors. For resonant tunneling devices, we developed a fabrication approach enabling simultaneous investigation of electronic transport and optical properties. Using a double metal wafer-bonding scheme, we realized metallic bottom contacts, micron-sized mesas defined by lithography and reactive ion etching, and metallic top contacts. This geometry allows vertical transport measurements while forming a patch cavity. The same technology was applied to MQW structures designed as bound-to-continuum systems with localized ground states and extended excited states. Optical measurements confirmed a sizable polariton splitting without pushing the upper polariton fully into the continuum. Because the intersubband transition energy was about 110 meV, the double-metal cavities required resonant lengths of approximately 2 m. To electrically contact these small cavities, we implemented an insulating silicon nitride (SiN) superstructure that was selectively opened above the patches and connected to extended contacts. Optical measurements revealed a modified polariton splitting behavior, showing a two-fold splitting around a weighted transition between the intersubband resonance and a SiN optical phonon. To explain these results, we extended the theoretical model to an ultra-strong coupling Hamiltonian that includes coupling of both electrons and SiN phonons to the cavity mode. The strong agreement between theory and experiment demonstrates successful coupling of two independent materials via a shared optical mode. Transport measurements showed that surface depletion strongly influences micron-scale devices, complicating transport analysis. To overcome this, we developed new structures with thinner tunneling barriers and two bound states, allowing higher quantum well numbers and stronger light-matter interaction. Arrays of micro-patch resonators exhibited coupling strengths up to 31% and cavity Q-factors of about 10. Vertical transport measurements revealed pronounced differences between resonant and off-resonant devices, including up to eight-fold transport enhancement at low temperatures. These results demonstrate that cavity vacuum fields can strongly modify electronic behavior in semiconductor quantum devices, opening new avenues for vacuum-field engineering.