Parametric resonance of Josephson plasma waves: A theory for optically amplified interlayer superconductivity in YBa2Cu3O6+x

Abstract

Nonlinear interactions between collective modes play a definitive role in far out of equilibrium dynamics of strongly correlated electron systems. Understanding and utilizing these interactions is crucial to photocontrol of quantum many-body states. One of the most surprising examples of strong mode coupling is the interaction between apical oxygen phonons and Josephson plasmons in bilayer YBa 2 Cu 3 O 6+x superconductors. Experiments by Hu et al. [Nat. Mater. 13, 705 (2014)] and Kaiser et al. [Phys. Rev. B 89, 184516 (2014)] showed that below T c , photoexcitation of phonons leads to enhancement and frequency shifts of Josephson plasmon edges, while above T c , photoexcited phonons induce plasmon edges even when there are no discernible features in the equilibrium reflectivity spectrum. Recent experiments by von Hoegen et al. (arXiv:1911.08284) also observed parametric generation of Josephson plasmons from photoexcited phonons both below T c and in the pseudogap phase. In this paper, we present a theoretical model of three-wave phonon-plasmon interaction arising from changes of the in-plane superfluid stiffness caused by the apical oxygen motion. Analysis of the parametric instability of plasmons based on this model gives frequencies of the most unstable plasmons that are in agreement with experimental observations. We also discuss how strong parametric excitation of Josephson plasmons can explain pump-induced changes in the terahertz reflectivity of YBa 2 Cu 3 O 6+x in the superconducting state, including frequency shifts and sharpening of Josephson plasmon edges, as well as appearance of a new peak around 2 THz. An interesting feature of this model is that overdamped Josephson plasmons do not give any discernible features in reflectivity in equilibrium, but can develop plasmon edges when parametrically excited. We suggest that this mechanism explains photoinduced plasmon edges in the pseudogap phase of YBa 2 Cu 3 O 6+x .

Generalized Fresnel-Floquet equations for driven quantum materials

“…We use our model to fit experimental data presented in reference [39] (reprinted here with the author's permission). Parameters used in this section to produce the figures are tabulated in Appendix C. We consider first a low temperature state in the superconducting regime, T = 10K, and model pumping as parametrically driving Josephson plasmons [18,19]. Using our simple model, we find excellent agreement with experiments shown in Fig.…”

“…Particularly striking are recent observations of light amplification in the photoexcited insulator SiC and the superconductor K 3 C 60 above its equilibrium T c [15][16][17]. Furthermore, in the case of pumped YBa 2 Cu 3 O 6+δ discussed above, strong experimental evidence has accumulated indicating that an effective photo-induced edge arises from parametric amplification of Josephson plasmons rather than a modification of the effective coupling [18,19] (see discussion in Section-IV of this paper). Prior work also demonstrated higher harmonic generation from Higgs and other collective modes [20][21][22] and nonlinear effects including parametric amplification of superconducting plasma waves [23][24][25][26].…”

“…Our goal in this section is to introduce a simple model for Floquet materials and discuss special features in reflectivity that appear in this model close to parametric resonances. In the next section we will demonstrate that features discussed in this section are ubiquitous, and can be found in more accurate models [19,31]. We model the Floquet medium by assuming that the presence of an oscillating field inside the material results in a time periodic refractive index, n 2 driven (t) = n 2 (ω)+δn 2 drive cos (Ω d ).…”

“…is the coefficient of the light reflected at the idler frequency. The Fresnel-Floquet problem in equation (19) together with equations ( 17) and ( 18) form a closed set of equations that can be solved to determine the reflectivity R = |r s | 2 .…”

Graphene as a source of entangled plasmons

“…For example, monolayer hexagonal boron nitride [97,98] should exhibit strong second order optical nonlinearity due to broken inversion symmetry of the crystal lattice, and would be a natural platform for generating entanglement between the long lived hyperbolic phonon polaritons [99][100][101][102][103][104][105][106][107][108]. Similar nonlinear processes exist for optical phonons in SiC [109], Josephson plasmons in layered superconductors [110][111][112][113] and the collective modes in excitonic insulators [114][115][116][117][118].…”