Figure 1. Red-light-induced bleaching of the NV⁻ fluorescence.

(a) Experimental protocol. On a green laser sample scanning, we station the red beam at a fixed spot for a predefined, variable time, to finally image the area via a red beam scan. During the fixed-point illumination and scanning times the green beam is off. (b) NV ionization takes place via a two-step process in which the excess electron is ejected into the conduction band (CB) by the consecutive absorption of two photons of wavelength λ≤632 nm (steps 1 and 2). NV⁰ recharging takes place on absorption of an electron from the valence band (VB) following excitation with λ≤575 nm (steps 3 and 4). (c) The central figure displays the sample fluorescence as a function of the red illumination time t_R for three different excitation powers before the readout scan. The top/down inserts correspond to fluorescence images obtained on scanning the red beam over an area xy on the focal plane around the fixed point. The time lapse t_R before sample scanning is indicated in the lower right corner of each image. In all images, the green laser power is 750 μW; note that the red laser power is 500 μW during t_R and 100 μW during the readout scanning, thus leading to different fluorescence intensities. Each image has 100 × 100 pixels and the integration time per point is 1 ms both throughout the scanned images and the fluorescence time traces in the central plot.