DSMP microscope

A custom-built diode-pumped ytterbium-ion-doped potassium gadolinium tungstate (Yb:KGW) crystal-based oscillator was used as the laser source. It operates at 1028 nm and it generates ~400 fs pulses at 14.3 MHz repetition rate⁴⁷. The laser was coupled to a custom-built laser-scanning SHG microscope, as described previously³⁶. A 20 × 0.75 numerical aperture (NA) air objective (Carl Zeiss, Thornwood, New York, USA) was used to focus the beam onto the sample, and a custom 0.85 NA objective (Omex Technologies, Wheeling, Illinois, USA) was used to collect the SHG signal in the transmission geometry. The SHG signal was detected using a single-photon-counting photomultiplier tube (Hamamatsu H10682-210, Hamamatsu, Bridgewater, New Jersey, USA). A BG 39 filter and a 510–520 nm band-pass interference filter (Edmund Optics, Regina, Saskatchewan, Canada) were used to separate the SHG from the incident light. The microscope polarimeter consisted of a polarisation-state generator (PSG) inserted before the excitation objective and a polarisation-state analyser (PSA) placed after the collection objective. The PSG consisted of a fixed linear polariser (Thorlabs, Newton, New Jersey, USA), a half-waveplate (HWP) and a quarter-waveplate (QWP). The PSA contains a HWP, a QWP (all wave plates are from Eksma Optics, Lithuania) and a linear polarizer (Thorlabs, Newton, New Jersey, USA). The PSG was set to produce nine different incoming polarisation states (HLP, VLP, +45, −45, RCP, LCP, −22.5, REP, LEP), for each of which the PSA was set to measure the SHG signal at six different polarisation states (0, 90, +45, −45, RCP, LCP) in order to determine the SHG Stokes vector.