Nonlinear optical control of Josephson coupling in cuprates

In High-T_C cuprates superconducting Cu-O planes alternate with insulating layers along the crystallographic c-axis, making the materials equivalent to Josephson junctions connected in series. The most intriguing consequence is that the out-of-plane superconducting transport occurs via Cooper pairs tunneling across the insulating layers and can be predicted by the Josephson tunneling equations. Nonlinear interaction between light fields and the superconducting carriers serves as a powerful dynamical probe of cuprates, while offering opportunities for controlling them in an analogous fashion to other stimuli such as pressure and magnetic fields. The main goal of this thesis work is to use intense transient light fields to control the interlayer superconducting transport on ultrafast time scales. This was achieved by tuning the wavelength of such light pulses to completely different ranges, in order to either directly excite Josephson Plasma Waves in the nonlinear regime, or efficiently melt the competing charge and spin order phase, which in certain cuprates quenches the Josephson tunneling at equilibrium. In a first study, I have utilized strong field terahertz transients with frequencies tuned to the Josephson plasma resonance (JPR) to coherently control the c-axis superconducting transport. The Josephson relations have a cubic nonlinearity which is exploited to achieve two related, albeit slightly different, phenomena. Depending on the driving pulse, solitonic breathers were excited with narrow-band multi-cycle pulses in La_1.84Sr_0.16CuO₄ while broad-band half-cycle pulses were employed to achieve a parametric amplification of Josephson Plasma Waves in La_1.905Ba_0.095CuO₄. These experiments are supported by extensive modeling, showing exceptional agreement. A comprehensive study illustrates the strong enhancement of the nonlinear effects near the JPR frequency. Then, I turned to investigate the competition between superconductivity and charge- and spin-order (the so called stripe phase) in La_1.885Ba_0.115CuO₄. I have demonstrated selective melting of the stripe phase through the irradiation with high photon energy pulses, which results in a transient enhancement of the c-axis superfluid density. The dependence of the effect on the wavelength of the pump pulse suggests a dominant energy scale which is at play with superconductivity, supporting the competing nature between the stripe and the superconducting order.