Harmonic holography for three-dimensional cellular imaging

Luminescent markers play a key role in imaging techniques for life sciences since they provide a contrast mechanism between signal and background. We describe a new type of marker using second harmonic generation (SHG) from noncentrosymmetric BaTiO_3 nanocrystals. These nanoparticles are attractive due to their stable, non-saturating and coherent signal with a femtosecond-scale response time and a broad flexibility in the choice of excitation wavelength. We use the coherent SHG signal from BaTiO_3 nanoparticles for three-dimensional (3D) imaging without scanning. We built a harmonic holographic (H^2) microscope which records digital holograms at the second harmonic frequency. Highresolution 3D distributions of these SHG markers in mammalian cells are successfully captured and interpreted by the H^2 microscope.