Second harmonic generation observed by dielectric relaxation at high electric fields: SHG without optics

Area of Science:

• Nonlinear optics
• Dielectric spectroscopy
• Materials science

Background:

• Polar liquids exhibit unique dielectric properties.
• Second harmonic generation (SHG) is a nonlinear optical phenomenon sensitive to symmetry breaking.
• Propylene glycol is a representative polar glass-forming liquid.

Purpose of the Study:

• To investigate second harmonic generation (SHG) in a polar liquid (propylene glycol).
• To explore the influence of DC bias fields on SHG.
• To correlate SHG with nonlinear dielectric permittivity and theoretical predictions.

Main Methods:

• Applying a DC bias field to break inversion symmetry in propylene glycol.
• Measuring second harmonic generation (SHG) signals under varying DC and AC electric fields.
• Analyzing results in the static limit and comparing with third-order nonlinear susceptibility theory.
• Investigating SHG detectability across different frequencies related to dipole orientation.

Main Results:

• Highest SHG signals were observed when the DC bias field (EB) amplitude equaled the AC peak field (E0).
• Static limit SHG measurements closely matched theoretical predictions based on third-order nonlinear susceptibility.
• SHG signals were detectable at frequencies where dipole orientation influences permittivity.
• The study demonstrated SHG's presence immediately upon DC bias field application.

Conclusions:

• Second harmonic generation in polar liquids is controllable via DC bias fields.
• The amplitude matching condition (EB = E0) maximizes SHG signals.
• SHG can serve as an indicator of anisotropy, applicable to interfaces.
• Impedance spectroscopy at twice the fundamental frequency offers an alternative to optical methods for assessing SHG-induced anisotropy.