Ultrafast giant enhancement of second harmonic generation in a strongly correlated cobaltite YbBaCo4O7

Abstract

We report the observation of ultrafast photoinduced giant enhancement of optical second harmonic generation (SHG) efficiency in cobaltite YbBaCo4O7. Upon femtosecond pumping at energies above the band gap, the system exhibits an ultrafast enhancement in SHG intensity, reaching up to 60% higher than the initial value, then decays into a metastable state maintaining the enhancement. The enhancement scales linearly with pump fluence but shows no dependence on pump polarization. A pure electronic process sets in within the first ~200 fs and is accompanied by a pronounced anisotropic amplification of nonlinear susceptibility. We propose this anomalous SHG enhancement originates from ultrafast electronic band renormalization arising from dynamical modification of multi-electron correlations. In stark contrast to conventional asymmetric systems where SHG is typically suppressed upon photoexcitation, our experimental findings shed a new light on ultrafast optical control nonlinear properties in quantum materials.

Figures (4)

• Figure 1: The ultrafast SHG enhancement of Yb114. (a) The crystal structure of Yb114. (b) The optical experimental setup for the SHG measurement. (c) The static SHG intensity as the function of probe fluence. The insert panel: the angular dependent SHG pattern(PP configuration) under different probe fluence without pump incident. Here, the SHG intensity for the 2.4 $mJ/cm^2$ probe is magnified 9-fold to facilitate a clear comparison with the 7.2 $mJ/cm^2$ curve. (d) The 800 nm interband excitation dynamics of Yb114, upper panel: $\Delta I_{SHG}/I_{SHG}$ (along 60$^{\circ}$ in pp configuration), lower panel: $\Delta R/R$. The black dashed line denotes the peak position of the excitation. The solids are fitted lines. The pump fluence is 10 $mJ/cm^2$.
• Figure 2: The pump wavelength dependence of SHG dynamics in Yb114. tr-SHG along 60$^\circ$ for pp configuration under various pumping incidents at (a) THz (1 $THz$, 1.1 $MV/cm$), (b) MIR(8 $\mathrm{\mu m}$, 1.3 $mJ/cm^2$ upper panel and 12 $\mathrm{\mu m}$, 0.9 $mJ/cm^2$ lower panel), (c) NIR(800 nm, 6 $mJ/cm^2$ upper panel and 1300 nm, 6 $mJ/cm^2$ lower panel). The grey solid line is the normalized dynamical SHG spectrum and the brown dashed line depicts the pump light waveform. Insert of (a): The static optical conductivity spectrum of Yb114. The filled circles denote the pump wavelengths. (d) The relative change of SHG upon 1.55 eV pumping at time delay 200 fs and 20 ps for different pump fluences.
• Figure 3: The rotational anisotropy and the dynamics of SHG spectra under 800 nm pumping. The initial state and excitation state SHG pattern for (a) PP and (b) PS configurations, respectively; (c-d) The time resolved SHG in typical directions for (c) PP mode and (d) PS mode respectively. Here for all figures, the pumping fluence is 10 $mJ/cm^{2}$ and the incident probe wavelength is fixed at 800 nm. The solids are fitted lines.
• Figure 4: The fluence dependence properties under 800 nm excitation. THe normalized second order susceptibility components at 200 fs (solid) and 20 ps (hollow) post-excitation.