Mica coverslips were prepared by coupling a mica sheet to a quartz coverslip with optical glue as described elsewhere 58 (link) . Briefly, coverslips were cleaned with 2% detergent (Hellmanex III, Hellma Analytics) and ethanol before gluing previously cut mica leaflets (5 mm x 5 mm, 1872-CA, SPI) on top of them with a low viscosity optical adhesive (NOA60, Norland products). After curing the adhesive with one hour exposure to UV light, another coverslip was glued on top of the mica with a high-viscosity optical adhesive (NOA63, Norland products), followed by another round of UV exposure. Coverslips were separated to expose a freshly cleaved mica surface just before performing SB experiments.