Communication: SHG-detected circular dichroism imaging using orthogonal phase-locked laser pulses

We demonstrate a novel method for second harmonic generation-detected circular dichroism (CD) imaging based on the use of phase-locked, temporally delayed femtosecond laser pulses. The polarization state of the fundamental wave was controllably changed over 2π rad by using a birefringent delay line, which provided attosecond inter-pulse delays for orthogonal phase-locked replicas; the achievable phase stability was 14 as. By introducing either a positive or negative delay of ∼667 as, we induced a ±π/2 phase shift between the orthogonally polarized pulses, resulting in left circularly polarized or right circularly polarized light. CD imaging performance using the pulse sequence was compared to results obtained for plasmonic nanoantennas using a rotating quarter-wave plate. The pulse sequence is expected to simplify polarization-resolved optical imaging by reducing experimental artifacts and decreasing image acquisition times. This method can be easily extended to other CD spectroscopy measurements.