Nonreciprocal Terahertz Second-Harmonic Generation in Superconducting NbN under Supercurrent Injection

Area of Science:

• Condensed Matter Physics
• Nonlinear Optics
• Superconductivity

Background:

• Second-harmonic generation (SHG) is a key nonlinear optical process.
• Superconductors typically exhibit even parity, prohibiting SHG.
• Breaking symmetries in materials can enable novel optical responses.

Purpose of the Study:

• To investigate giant second-harmonic generation (SHG) in a superconductor.
• To explore the role of broken time-reversal (T) and space-inversion (P) symmetries in THz SHG.
• To understand the underlying mechanisms of THz SHG in NbN thin films.

Main Methods:

• Fabrication of thin niobium nitride (NbN) films.
• Injection of supercurrent to break P and T symmetries.
• Measurement of THz frequency SHG signal and its phase dependence on supercurrent direction.
• Temperature-dependent analysis of the SH signal.

Main Results:

• Observed giant SHG in NbN thin films at THz frequencies.
• Demonstrated phase flipping of the SH signal with supercurrent reversal, confirming nonreciprocal response.
• Identified a sharp resonance in temperature dependence attributed to vortex motion in an anharmonic potential.
• Achieved a maximum SHG conversion ratio of ≈10⁻² with a 25 nm NbN film.

Conclusions:

• NbN thin films exhibit giant THz SHG due to broken P and T symmetries.
• Supercurrent injection is crucial for enabling this nonreciprocal nonlinear optical response.
• Vortex dynamics play a significant role in the observed resonant SHG behavior.