Achieving control over light−matter interaction in custom-tailored nanostructures is at the core of modern quantum electrodynamics. In strongly and ultrastrongly coupled systems, the excitation is repeatedly exchanged between a resonator and an electronic transition at a rate known as the vacuum Rabi frequency Ω R . For Ω R approaching the resonance frequency ω c , novel quantum phenomena including squeezed states, Dicke superradiant phase transitions, the collapse of the Purcell effect, and a population of the ground state with virtual photon pairs are predicted. Yet, the experimental realization of optical systems with Ω R /ω c ≥ 1 has remained elusive. Here, we introduce a paradigm change in the design of light−matter coupling by treating the electronic and the photonic components of the system as an entity instead of optimizing them separately. Using the electronic excitation to not only boost the electronic polarization but furthermore tailor the shape of the vacuum mode, we push Ω R /ω c of cyclotron resonances ultrastrongly coupled to metamaterials far beyond unity. As one prominent illustration of the unfolding possibilities, we calculate a ground state population of 0.37 virtual photons for our best structure with Ω R /ω c = 1.43 and suggest a realistic experimental scenario for measuring vacuum radiation by cutting-edge terahertz quantum detection. KEYWORDS: Quantum electrodynamics, ultrastrong coupling, terahertz, metamaterials I n the strong coupling regime of quantum electrodynamics (QED), where the vacuum Rabi frequency Ω R exceeds the dissipation rates of the electronic excitation and the resonator, new eigenmodes called cavity polaritons emerge. This universal principle is found in a large variety of systems, ranging from atoms 1 to excitons in semiconductors, 2,3 molecules, 4 mid-IR plasmonic structures, 5−9 circuit QED systems at GHz frequencies, 10−13 and structures in the THz spectral range. 14−16 In ultrastrongly coupled structures, Ω R becomes comparable to the resonance frequency ω c itself; the rotating-wave approximation of light−matter interaction falters, and antiresonant coupling terms describing the simultaneous creation of correlated light and matter excitations become relevant. 17−19 Most prominently, the ground state is theorized to be a modified squeezed quantum vacuum with a finite population of correlated virtual photon pairs. 17,19 For sufficiently large values of the relative coupling strength Ω R /ω c ≳1, subcycle switching of Ω R 6,9 may release these photons 17,19,20 in analogy to Unruh− Hawking radiation emerging at the event horizon of black holes. 21 These spectacular perspectives have fuelled the quest of the QED community for ever greater relative coupling strengths, ultimately aiming for Ω R /ω c beyond unity.The key strategy for boosting Ω R /ω c , also referred to as g/ω c , comprises increasing the dipole moment of the electronic transition, decreasing the resonator mode volume and ω c , or enhancing the overlap of the photonic mode and the electroni...

The combination of continuous miniaturization of electronics and the demanding reliability requirements for industrial and automotive electronics is one big challenge for emerging packaging technology. One aspect is to increase the understanding of the damage under environmental loading. Therefore, the solder joints of a wafer-level chip-scale package assembled on a printed circuit board (PCB) have been analyzed after a temperature cycling test. In the case of the investigated package, a limited number of joints did not form a proper mechanical connection with the PCB copper pad. Although not intended in the first place, these circumstances cause a detachment of those joints within the first few thermal cycles. However, this constellation offers a unique opportunity to compare the solder joint microstructure after thermomechanical loading (connected joints) with pure thermal loading (detached joints) located directly next to each other. It is shown that microstructure aging effects can be directly linked to regions in the joint with increased loading. This is particularly the case for detached joints, which could almost retain their initial microstructure up to the effect of the high-temperature part of the thermal profile. By means of finite element simulation, it is further possible to quantify the increased loading on adjacent joints if isolated solder balls detach from the board. In one case presented, the lifetime of the corner joint was calculated to reduce up to 85% only.