Q-plate enabled spectrally diverse orbital-angular-momentum conversion for stimulated emission depletion microscopy

Spin to orbital angular momentum (OAM) conversion using a device known as a q-plate has gained recent attention as a convenient means of creating OAM beams. We show that the dispersive properties of a q 1∕2 plate, specifically its group index difference Δng for ordinary and extraordinary polarization light, can be tuned for achieving single-aperture, alignment-tolerant stimulated emission depletion (STED) nanoscopy with versatile control over the color combinations as well as laser bandwidths. Point spread function measurements reveal the ability to achieve single-aperture STED illumination systems with high throughput (transmission >89%) and purity (donut beam extinction ratios as high as 18.75 dB, i.e., 1% residual light in the dark center of the donut beam) for a variety of color combinations covering the entire visible spectrum, hence addressing several of the fluorescent dyes of interest in STED microscopy. In addition, we demonstrate dual-color STED illumination that would enable multiplexed imaging modalities as well as schemes that could use wide bandwidths up to 19 nm (and hence ultrashort pulses down to ∼50 fs). Switching between any of these color settings only involves changing the bias of the q-plate that does not alter the alignment of the system, hence potentially facilitating alignment-free, spectrally diverse multiplexed nanoscale imaging.