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Register a new userNonreciprocal Terahertz Second Harmonic Generation in Superconducting NbN under Supercurrent Injection
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Added by
Sachiko Nakamura Dr.
Publication date
2020
fields
Physics
and research's language is
English
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Giant second-harmonic generation (SHG) in the terahertz (THz) frequency range is observed in a thin film of an s-wave superconductor NbN, where the time-reversal ($mathcal{T}$-) and space-inversion ($mathcal{P}$-) symmetries are simultaneously broken by supercurrent injection. We demonstrate that the phase of the second-harmonic (SH) signal flips when the direction of supercurrent is inverted, i.e., the signal is ascribed to the nonreciprocal response that occurs under broken $mathcal{P}$- and $mathcal{T}$-symmetries. The temperature dependence of the SH signal exhibits a sharp resonance, which is accounted for by the vortex motion driven by the THz electric field in an anharmonic pinning potential. The maximum conversion ratio $eta_{mathrm{SHG}}$ reaches $approx10^{-2}$ in a thin film NbN with the thickness of 25 nm after the field cooling with a very small magnetic field of $approx1$ Oe, for a relatively weak incident THz electric field of 2.8 kV/cm at 0.48 THz.
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Higgs mode in superconductors, i.e. the collective amplitude mode of the order parameter does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. On the contrary to this common understanding, here, we demonstrate that, if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at $omega=2Delta$ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.
Nonreciprocal devices that allow the light propagation in only one direction are indispensable in photonic circuits and emerging quantum technologies. Contemporary optical isolators and circulators, however, require large size or strong magnetic fields because of the general weakness of magnetic light-matter interactions, which hinders their integration into photonic circuits. Aiming at stronger magneto-optical couplings, a promising approach is to utilize nonlinear optical processes. Here, we demonstrate nonreciprocal magnetoelectric second harmonic generation (SHG) in CuB2O4. SHG transmission changes by almost 100% in a magnetic-field reversal of just 10 mT. The observed nonreciprocity results from an interference between the magnetic-dipole- and electric-dipole-type SHG. Even though the former is usually notoriously smaller than the latter, it is found that a resonantly enhanced magnetic-dipole-transition has a comparable amplitude as non-resonant electric-dipole-transition, leading to the near-perfect nonreciprocity. This mechanism could form one of the fundamental bases of nonreciprocity in multiferroics, which is transferable to a plethora of magnetoelectric systems to realize future nonreciprocal and nonlinear-optical devices.
Recent advances in time-domain terahertz (THz) spectroscopy have unveiled that resonantly-enhanced strong THz third-harmonic generation (THG) mediated by the collective Higgs amplitude mode occurs in s-wave superconductors, where charge-density fluctuations (CDF) have also been shown to contribute to the nonlinear third-order susceptibility. It has been theoretically proposed that the nonlinear responses of Higgs and CDF exhibit essentially different polarization dependences. Here we experimentally discriminate the two contributions by polarization-resolved intense THz transmission spectroscopy for a single-crystal NbN film. The result shows that the resonant THG in the transmitted light always appears in the polarization parallel to that of the incident light with no appreciable crystal axis dependence. When we compare this with the theoretical calculation here with the BCS approximation and the dynamical mean-field theory for a model of NbN constructed from first principles, the experimental result strongly indicates that the Higgs mode rather than the CDF dominates the THG resonance in NbN. A possible mechanism for this is discussed such as the retardation effect in the phonon-mediated pairing interaction beyond BCS.
We report terahertz (THz) second harmonic generation (SHG) in superconductors (SC) with inversion symmetric equilibrium states that forbid even-order nonlinearities. Such SHG signal is observed in single-pulse emission by periodic driving with a multi-cycle THz electric field tuned below the SC energy gap and vanishes above the SC critical temperature. 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 non-zero integrated pulse area inside the SC drives lightwave nonlinear supercurrents due to sub--cycle Cooper pair acceleration, in contrast to d.c.-biased superconductors, which can be controlled by the bandstructure and the THz pump field.
Optical conductivity of a moderately disordered superconducting NbN film was investigated by terahertz time-domain spectroscopy in external magnetic field applied along the film plane. The film thickness of about 5 nm was comparable with the coherence length, so vortices should not form. This was confirmed by the fact that no marked difference between the spectra with terahertz electric field set perpendicular and parallel to the external magnetic field was observed. Simultaneous use of Maxwell-Garnett effective medium theory and the model of optical conductivity by Herman and Hlubina proved to correctly reproduce the terahertz spectra obtained experimentally in a magnetic field of up to 7 T. This let us conclude that the magnetic field tends to suppress the superconductivity, resulting in an inhomogeneous state where superconducting domains are enclosed within a normal-state matrix. The scattering rate due to pair-breaking effects was found to linearly increase with magnetic field.
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