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

Abstract

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.

Deterministic Construction of Plasmonic Heterostructures in Well‐Organized Arrays for Nanophotonic Materials

“…A schematic of this microscope is provided in Figure S8 (Supporting Information). A linear polarizer was placed in the beam path to create high extinction ratio of the linearly polarized input excitation, and then a pulse replica generator was used to generate LCP and RCP excitation . The fundamental was focused to the sample with a single aspherical lens (numerical aperture, NA = 0.23), and the SH signal was collected with a 1.25 NA, 100× oil immersion compound objective.…”

Advances in multi-dimensional super-resolution nonlinear optical microscopy

“…The experimental setup for nonlinear optical microscopy has been described in detail previously [7][8][9][10][11]. Figure 1 provides the optical layout for nonlinear optical (NLO) measurements.…”

“…As described in Section V, we routinely achieve sub-15 attosecond inter-pulse phase stability, which is persistent for longer than two hours and more than sufficient to acquire circular dichroism (CD) images. 9 Depending on the type of measurement, image frame exposure times vary from milliseconds to seconds.…”

“…A superposition of the two orthogonal pulses, shifted by sub-cycle temporal delays, can manipulate the polarization state of the laser. For example, inducing a positive (or negative) 667-attosecond inter-pulse delay, one quarter of an optical cycle at 800 nm, can generate left (or right) handed circularly polarized light [9]. This PRG method has been shown to generate pulses with a minimum time step of ~5.7 attoseconds, and is phase stable to ~33 mrad over the course of two hours [9].…”

“…Our group has approached this problem -the incompatibility of optical imaging and spectroscopy length scales -by developing interferometric nonlinear optical imaging methods that utilize phase-stable laser pulses and array detection [8][9][10][11][12]. Statistical localization of the signal point spread function obtained from an imaging array yields information about position, while Fourier analysis of the interferometrically collected data provides spectroscopic outputs.…”