Applications

Nonlinear microscopy is a powerful technique for imaging inside living organisms with a submicrometer resolution at millimeter depths. In conjunction with genetically-encoded calcium indicators and opsins, multiphoton fluorescence (MPEF) microscopy has revolutionized neuroimaging and is becoming a standard tool in neuroscience, while label-free methods such as second- and third-harmonic generation (SHG and THG), and coherent anti-Stokes and stimulated Raman scattering (CARS and SRS) have been developed into ultrasensitive structural and chemical imaging techniques.

The nonlinear optical processes of nonlinear microscopy require high light intensities, which can be reached at low average power when ultrashort light pulses are tightly focused. This feature is exploited to provide optical sectioning and to improve imaging contrast deep inside scattering tissues. Multiphoton excitation occurs when two or more photons simultaneously pass in the vicinity of a molecule, and their combined energy is used for excitation leading to fluorescence. The simultaneous arrival of several photons can also result in a harmonic generation – radiation at double or triple of the excitation laser frequency. Harmonic generation is an intrinsic label-free contrast determined by and used to characterize the molecular order and homogeneity of the sample.

Higher-order, three- and four-photon-excited (3PEF and 4PEF), fluorescence microscopy deserves special attention as it enables imaging at depths that cannot be achieved with conventional microscopy techniques, especially in strongly scattering samples such as the brain. Most importantly, modern laser sources can operate at the required repetition rates and deliver the necessary pulse energy for real-time functional brain imaging at biologically-relevant depths.

CRONUS-3P is an OPA-based laser source that was developed specifically for nonlinear microscopy. It provides µJ-level sub-85 fs pulses at repetition rates of up to 2 MHz and tunable from 1.25 µm to 1.8 µm, thus covering the biological transparency windows at 1.3 µm and 1.7 µm for 3PEF microscopy. CRONUS-3P has integrated group delay dispersion (GDD) control, ensuring optimal pulse duration at the sample, and optional automated beam steering to guarantee laser pointing stability.