Simultaneous amplitude and phase modulation for wide-field nonlinear microscopy applications

Abstract: In wide-field nonlinear microscopy, wavefront modulation by means of phase-only spatial light modulators (SLMs) allows achieving simultaneous two-photon excitation and fluorescence emission from specific region-of-interests (ROIs) of biological specimens. This is basically accomplished at the illumination path of the microscope by the reconstruction of computer generated holograms (CGHs) onto the sample plane. However, as two-photon absorption (TPA) is inherently an intensity-square dependent process and iterative Fourier transform algorithms (IFTAs) can only approximate the illumination of selected ROIs with the reconstructed CGHs, both signal acquisition and/or image formation can be largely affected by the spatial irregularities of the illumination patterns. In addition, the speckle associated with the superposition of coherent light at the selected ROIs prevents illumination strategies based on CGHs to be successfully used for large-area (more than 50x50 $\mu$m2) excitation tasks. To overcome these limitations, we propose an alternative complex illumination method (CIM) able to generate simultaneous nonlinear excitation of large-area ROIs with full control over the amplitude and phase of the optical wavefront. We experimentally demonstrate spatially uniform illumination, as well as structured illumination with user-defined intensity levels onto micrometric but large-area ROIs. Furthermore, a proof-of-concept experiment on wide-field second harmonic generation (SHG) is provided. We believe that the proposed CIM could find applications in wide-field nonlinear microscopy, particularly for speed up signal acquisition time or improve two-photon image formation.