Terahertz Second-Harmonic Generation from Lightwave Acceleration of Symmetry-Breaking Nonlinear Supercurrents

C. Vaswani, M. Mootz, C. Sundahl, D. H. Mudiyanselage, J. H. Kang, X. Yang, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, L. Luo, I. E. Perakis, C. B. Eom, and J. Wang

Phys. Rev. Lett. 124, 207003 – Published 19 May 2020

Abstract

We report terahertz (THz) light-induced second harmonic generation, in superconductors with inversion symmetry that forbid even-order nonlinearities. The THz second harmonic emission vanishes above the superconductor critical temperature and arises from precession of twisted Anderson pseudospins at a multicycle, THz driving frequency that is not allowed by equilibrium symmetry. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting nonzero integrated pulse area inside the superconductor leads to light-induced nonlinear supercurrents due to subcycle Cooper pair acceleration, in contrast to dc-biased superconductors, which can be controlled by the band structure and THz driving field below the superconducting gap.

Received 3 December 2019
Revised 29 March 2020
Accepted 23 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.207003

Physics Subject Headings (PhySH)

Second order nonlinear optical processes Superconducting RF Superconducting phase transition

Low-temperature superconductors Nonequilibrium systems

Methods in superconductivity Optical absorption spectroscopy Terahertz spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. Vaswani^1,*, M. Mootz^2,*, C. Sundahl^3,*, D. H. Mudiyanselage¹, J. H. Kang³, X. Yang¹, D. Cheng¹, C. Huang¹, R. H. J. Kim ¹, Z. Liu ¹, L. Luo¹, I. E. Perakis², C. B. Eom³, and J. Wang ^1,†

¹Department of Physics and Astronomy and Ames Laboratory-U.S. DOE, Iowa State University, Ames, Iowa 50011, USA
²Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294-1170, USA
³Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA

^*These authors contributed equally to this work.
^†jwang@ameslab.gov jgwang@iastate.edu

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 124, Iss. 20 — 22 May 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) SHG by THz lightwave acceleration of superfluid momentum, $p_{s} (t)$ . (b) THz emission for two $E$ -field strengths, $21.7 kV / cm$ (blue) and $2.5 kV / cm$ (black). Inset: 0.5 THz multicycle phase-locked THz pulse and spectrum. (c) THz emission at 4.2K normalized to the emission at 20K for various THz $E$ -field strengths (traces offset for clarity). (d) The THz SHG contribution after subtracting the pump leakage (main text). Inset: SHG emission from the substrate.
Reuse & Permissions
Figure 2
(a) THz SHG signals for various $E$ -field strengths at 4.2 K (traces offset for clarity). (b) THz SHG at 1 THz as function of the $E$ -field strength normalized by $E_{\max}$ used. The grey line shows a linear fit to the data. Inset: THz pump-THz probe differential transmission $Δ E / E_{0}$ as a function of THz driving field $(E_{THz} / E_{\max})^{2}$ for time delay $Δ t_{p p} = 100 ps$ (maximum signal size) shows weak quench of SC state marked by a dash line. (c) THz third harmonic generation THG at 1.5 THz as function of the cube .of the $E$ -field strength normalized by $E_{\max}$ used. Inset: THz emission for field strength $11 kV / cm$ showing the THG signal.
Reuse & Permissions
Figure 3
(a) THz-SHG signals scaled to the $E$ field transmittance at various temperatures normalized by the 20 K data (see text, traces offset for clarity). (b) Temperature dependence of the integrated spectral weight of THz SHG signals.
Reuse & Permissions
Figure 4
Gauge-invariant quantum kinetic simulation of dynamical symmetry breaking and nonlinear supercurrent photogeneration by THz lightwave propagation and interference effects. (a) Dynamics of THz-light-induced nonlinear supercurrent $J (t)$ , calculated without (black line) and with propagation effects (red line), together with the representative 0.5 THz pump oscillating electric field used in the calculations (shaded area). (b) Calculated THz SHG for various $E$ -field strengths. (c),(e) Calculated nonlinear spectra over a range of frequencies, in linear and semilogarithmic scale; the linear and THG peaks are indicated by vertical solid lines, while SHG is denoted by vertical dashed line. (d) Calculated nonperturbative THz SHG at 1 THz as a function of the square of the $E$ -field strength normalized by $E_{quench}$ at which the SC order parameter, asymptotically reached, becomes completely quenched (inset). Note there are still SC coherences left at $E_{quench}$ since a part of the Fermi surface remains gapped, different from temperature tuning above $T_{c}$ in Fig. 3. (f) Fluence dependence of the zero-frequency component of the transmitted nonlinear $E$ field for three different electron hopping strengths $t_{1}$ that characterize the flatness of the electronic bands. Inset: DOS for the different $t_{1}$ used.
Reuse & Permissions

Physical Review Letters