Andrea Cavalleri

Abstract:

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.

Abstract:

Optical excitation of large-amplitude apical oxygen phonon oscillations has been shown to renormalize the electronic properties of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+x}$ , 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 $2\mathrm{\Delta }\simeq 30\mathrm{c}{\mathrm{m}}^{-1}$ 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.

Abstract:

Photoexcited ${\mathrm{K}}_3{\mathrm{C}}_{60}$ 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 $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6.48}$ . Here, we search for transient magnetic field expulsion in ${\mathrm{K}}_3{\mathrm{C}}_{60}$ by measuring Faraday rotation in a magneto-optic material placed in its vicinity. Unlike in the case of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6.48}$ , inhomogeneous, metallic ${\mathrm{K}}_3{\mathrm{C}}_{60}$ 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 ( ${\chi }_v>-0.1$ ). On this basis, we conclude that the photoinduced phase has weaker magnetic susceptibility than superconducting ${\mathrm{K}}_3{\mathrm{C}}_{60}$ 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 $0.8T_c$ , for which ${\chi }_v>-0.1$ . A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for ${\mathrm{K}}_3{\mathrm{C}}_{60}$ with the current resolution.

Abstract:

Polaritons are quantum mechanical superpositions of photon states with elementary excitations in molecules and solids. The light-matter admixture causes a characteristic frequency-momentum dispersion shared by all polaritons irrespective of the microscopic nature of material excitations that could entail charge, spin, lattice or orbital effects. Polaritons retain the strong nonlinearities of their matter component and simultaneously inherit ray-like propagation of light. Polaritons prompt new properties, enable new opportunities for spectroscopy/imaging, empower quantum simulations and give rise to new forms of synthetic quantum matter. Here, we review the emergent effects rooted in polaritonic quasiparticles in a wide variety of their physical implementations. We present a broad portfolio of the physical platforms and phenomena of what we term polaritonic quantum matter. We discuss the unifying aspects of polaritons across different platforms and physical implementations and focus on recent developments in: polaritonic imaging, cavity electrodynamics and cavity materials engineering, topology and nonlinearities, as well as quantum polaritonics.

Abstract:

Ultrafast switching of ferroic phases is an active research area with technological potential. Yet, some key challenges remain, ranging from limited speeds in ferromagnets to intrinsic volatility of switched domains owing to depolarizing fields in ferroelectrics. Unlike these ferroic systems, ferroaxial materials host bistable states that preserve spatial-inversion and time-reversal symmetry and are therefore immune to depolarizing fields but also difficult to manipulate with conventional methods. We demonstrate photo-induced switching of ferroaxial order by engineering an effective axial field composed of circularly driven terahertz phonon modes. A switched ferroaxial domain remains stable for many hours and can be reversed back with a second terahertz pulse of opposite helicity. The effects demonstrated in this work may lead to the development of a robust platform for ultrafast information storage.