Michele Buzzi

Is it feasible to alter the ground-state properties of a material by engineering its electromagnetic environment? Inspired by theoretical predictions^1-12, experimental realizations of such cavity-controlled properties without optical excitation are beginning to emerge^13-19. Here we devised and implemented a new platform to realize cavity-altered materials. Single crystals of hyperbolic van der Waals (vdW) compounds provide a resonant electromagnetic environment with enhanced density of photonic states and prominent mode confinement^20-24. We interfaced hexagonal boron nitride (hBN) with the molecular superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br (κ-ET). The frequencies of infrared hyperbolic modes (HMs) of hBN (refs. ^25,26) match the infrared-active carbon-carbon (C=C) stretching molecular resonance of κ-ET implicated in superconductivity²⁷. Nano-optical data supported by first-principles molecular Langevin dynamics simulations confirm the presence of resonant coupling between the hBN hyperbolic cavity modes and the C=C stretching mode in κ-ET. Meissner-effect measurements using magnetic force microscopy (MFM) demonstrate a strong suppression of superfluid density near the hBN/κ-ET interface. Non-resonant control heterostructures, including RuCl₃/κ-ET and hBN/Bi₂Sr₂CaCu₂O_8+x (BSCCO), do not show the pronounced superfluid suppression. These observations suggest that hBN/κ-ET realizes a cavity-altered superconducting ground state. Our work highlights the potential of dark cavities devoid of external photons for engineering electronic ground-state properties of complex quantum materials.

The generation of tunable narrowband pulses is increasingly being pursued in terahertz science, for example, to study the nonlinear response of individual modes of solids and molecules. Here, we extend the chirp-and-delay method to achieve collinear phase-matched difference-frequency generation in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA-S), which results in tunable narrowband terahertz pulses. In this configuration, the fundamental frequency of a Ti:sapphire amplifier is used—eliminating the need for optical parametric amplifiers typically required for THz generation in other organic crystals. Chirped-pulse excitation suppresses multiphoton absorption in BNA, improving stability and extending crystal lifetime. The source delivers THz transients tunable from ~0.25 THz to ~2 THz with adjustable spectral width.

Single-shot electro-optic sampling (EOS) is a powerful method enabling the measurement of weak terahertz signals that would otherwise require prohibitively long acquisition times. This is generally achieved by encoding the EOS time delay into a spatial, angular, or frequency coordinate. In general, angular-encoding techniques operate well up to 3 THz but become more challenging for larger bandwidths, due to dispersion and imaging imperfections. Here, we demonstrate a reliable angular-encoding single-shot EOS implementation that reaches frequencies beyond 6 THz. Diffraction simulations are used to design the experimental setup and adapt this technique to commercial reflection gratings, removing the need for custom-built echelon mirrors. Furthermore, we show that, contrary to earlier reports, group delay dispersion from angular dispersion does not reduce the bandwidth of single-shot EOS.

Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of YB a 2 C u 3 O 6 + x , inducing a superconducting-like optical response above equilibrium T C . All of the evidence collected so far has been based on the changes of the terahertz frequency c -axis response. In these measurements, the capacitive interlayer coupling was seen to transform into a superconducting-like inductive response. This assignment was strengthened by recent measurements of ultrafast magnetic field expulsion. Here, we report an experimental determination of the transient in-plane optical properties, which has so far been elusive due to the high equilibrium reflectivity and the need to evaluate minute changes in the optical response. We report the appearance of a photoinduced in-plane optical gap 2 Δ ≃ 30 c m − 1 and a divergent imaginary conductivity, both consistent with photoinduced superconductivity. A global fit to these data suggests that in- and out-of-plane electronic properties never completely equilibrate during the dynamics.

Photoexcited K 3 C 60 displays several properties reminiscent of equilibrium superconductivity, including transient optical spectra, pressure dependence, and I – V characteristics. However, these observations do not decisively establish nonequilibrium superconductivity, which may be further evidenced by transient magnetic field expulsion measurements, as shown recently in driven YB a 2 C u 3 O 6.48 . Here, we search for transient magnetic field expulsion in K 3 C 60 by measuring Faraday rotation in a magneto-optic material placed in its vicinity. Unlike in the case of YB a 2 C u 3 O 6.48 , inhomogeneous, metallic K 3 C 60 powders reduce the size of the effect. With the ∼50 nT magnetic field resolution achieved in our experiments, we provide an upper limit for the photoinduced diamagnetic volume susceptibility ( χ v > − 0 . 1 ). On this basis, we conclude that the photoinduced phase has weaker magnetic susceptibility than superconducting K 3 C 60 at zero temperature. Yet, from recent nonlinear transport measurements in this granular material, we expect a light-induced state similar to the equilibrium superconductor near 0.8 T c , for which χ v > − 0 . 1 . A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for K 3 C 60 with the current resolution.

The generation of tunable narrowband pulses is increasingly being pursued in terahertz science, for example to study the nonlinear response of individual modes of solids and molecules. Here, we extend the chirp-and-delay method to achieve collinear phase-matched difference-frequency generation in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA-S), which results in tunable narrowband terahertz pulses. In this configuration, the fundamental frequency of a Ti:sapphire amplifier is used - eliminating the need for optical parametric amplifiers typically required for THz generation in other organic crystals. Chirped-pulse excitation suppresses multiphoton absorption in BNA, improving stability and extending crystal lifetime. The source delivers THz transients tunable from 0.25 THz to 2.1 THz with adjustable spectral width.

A long-standing challenge in ultrafast magnetism and functional materials research, in general, has been the generation of a universal, ultrafast stimulus able to switch between stable magnetic states. Solving this problem would open up many new opportunities for fundamental studies, potentially impacting future data storage technologies. Ideally, step-like magnetic field transients with infinitely fast rise time would serve this purpose. Here we develop a new approach to generate ultrafast magnetic field steps by quenching supercurrents in a superconductor. We achieve magnetic field steps with millitesla amplitude, picosecond rise times and slew rates approaching 1 GT s^–1. We test the potential of this technique by coherently rotating the magnetization in a ferrimagnet. Although in the current geometry, the magnetic field step is not sufficient to achieve complete switching, suitable improvements in the device geometry could make these magnetic steps both larger and faster. We foresee new applications ranging from quenches across phase transitions to complete switching of magnetic order parameters.

Optically enhanced superconductivity in K3C60 is supported by transient optical spectra, by pressure responses, and by ultrafast nonlinear transport measurements. However, the underlying physics and in fact the similarity or dissimilarity to most properties of equilibrium superconductivity are not clear. In this paper, we study the ultrafast voltage response of optically driven K3C60 thin films. Photo-conductive switches are used to measure changes in voltage as a function of time after irradiation, both below and above Tc. These measurements can be understood if one considers the role of granularity in the photo-induced transport response. They reveal fast voltage changes associated with the kinetic inductance of the in-grain carriers and a slower response that may be attributed to Josephson dynamics at the weak links. Fits to the data yield estimates of the in-grain photo-induced superfluid density after the drive and the dynamics of phase slips at the weak links. This work underscores the increasing ability to make electrical measurements at ultrafast speeds in optically driven quantum materials and demonstrates a striking new platform for optoelectronic device applications.