High-efficiency SHG and polarization manipulation of femtosecond higher-order Poincaré sphere beam

Nonlinear frequency conversion is a well-established technique for extending the laser wavelength range. However, achieving nonlinear frequency conversion and polarization manipulation for vector beams with non-uniform polarization distributions remains a challenge due to the intrinsic polarization selectivity of optical nonlinear processes. This study demonstrates high-efficiency second harmonic generation (SHG) of femtosecond vector beams using orthogonal dual-BBO crystals. A cascaded group-delay compensation crystal is employed to correct the temporal asynchrony between the two orthogonally polarized second harmonic components generated sequentially. A 100 fs cylindrical vector beam at a near-infrared wavelength of 800 nm was efficiently converted into its second harmonic at 400 nm, achieving a maximum single-pass SHG efficiency of approximately 23%. The polarization distribution of the vector-polarized second harmonic (SH) beam exhibited a strong correlation with its fundamental-harmonic (FH) counterpart. Arbitrary polarization states on the higher-order Poincaré sphere, characterized by a doubled topological charge (lSH = 2lFH), were demonstrated through polarization modulation of the seeded FH beam, with SHG efficiency consistently maintained above ∼20%.