For many microscopy applications, millimeters-long free working distances (LWD) are required. However, the high resolution and contrast of LWD objectives operated in air are lost when introducing glass and/or liquid with the sample. We propose to use spatial light modulation to correct for such beam aberrations caused by refractive index mismatches. Focusing a monochromatic laser beam with a 10 mm working distance air objective (50×, 0.5 NA) through air, glass, and water, we manage to restore a sharp, intense focus (FWHM<2λ) by adaptive beam phase shaping. Our approach offers a practical and cost-effective route to high resolution and contrast microscopy using LWD air objectives, extending their usage beyond applications in air.

Appl. Optics
Ultrafast Spectroscopy

Gjonaj, B., Johnson, P. M., Bonn, M., & Domke, K. F. (2012). Index mismatch aberration correction over long working distances using spatial light modulation. Appl. Optics, 51(33), 8034–8040. doi:10.1364/AO.51.008034