Voltage-controlled phase matching in quadrupole second-harmonic generation.

Area of Science:

• Nonlinear optics
• Liquid crystal physics
• Materials science

Background:

• Second-harmonic generation (SHG) is a key nonlinear optical process.
• Nematic liquid crystals with inversion symmetry are typically centrosymmetric, forbidding quadrupole SHG.
• Electric fields can induce dipole-allowed SHG in centrosymmetric materials.

Purpose of the Study:

• To investigate phase-matched (PM) quadrupole optical second-harmonic generation (SHG) in nematic liquid crystals.
• To demonstrate voltage control over molecular orientation for achieving PM conditions.
• To differentiate quadrupole SHG from electric-field-induced dipole-type SHG.

Main Methods:

• Utilized nematic liquid crystals with inversion symmetry.
• Applied voltage control to manipulate molecular orientation and refractive indices.
• Employed time-gated measurements under a pulsed electric field.
• Separated quadrupole SHG from dipole-type SHG.

Main Results:

• Observed strong phase-matched quadrupole SHG.
• Achieved PM conditions in two distinct configurations via voltage control.
• Successfully distinguished quadrupole SHG from induced dipole-type SHG.

Conclusions:

• Demonstrated the feasibility of phase-matched quadrupole SHG in liquid crystals with inversion symmetry.
• Showcased voltage-induced molecular reorientation as a method to control nonlinear optical responses.
• Validated time-gated measurements for isolating specific SHG mechanisms.