Abstract
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.
Highlights
Stimulated emission depletion (STED) microscopy is one of the most successful far-field imaging techniques to date, allowing access to resolution beyond the diffraction limit [1,2]
To achieve a high-quality donut beam, it is well known that vortex modes with orbital angular momentum (OAM) or, equivalently, helical phase are required, and it is critical that the handedness for both the helical phase and circular polarization be the same [3]
STED microscopy, while a very attractive technique for realizing super-resolution, suffers from practical drawbacks: the STED beam needs to be a pure spin-orbit aligned OAM beam and the Gaussian excitation and STED beam need to be spatially coaligned with very high precision
Summary
Stimulated emission depletion (STED) microscopy is one of the most successful far-field imaging techniques to date, allowing access to resolution beyond the diffraction limit [1,2]. We define an extinction ratio (ER; zero for an OAM and unity for a Gaussian mode) by measuring the ratio of the intensity at the beam center (see Supplement 1, Section 3, Fig. S2) with and without the qplate in the beam path.
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