BROADBAND FOCUSING FOR EXTREME MULTIMODAL MICROSCOPY

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Project Summary Virtually all optical microscopes for biological imaging are based on refractive objective lenses. The performance of these lenses approaches the theoretical limit, however, their use is limited to the visible to near-infrared spectral range. Even within this range, their performance is only guaranteed over a relatively narrow range, and broadband use is invariably affected by chromatic aberrations. Another problem is the group delay dispersion that these lenses introduce to short optical pulses, which reduces the efficiency of nonlinear optical (NLO) signal generation in the microscope. Taken together, these shortcomings seriously compromise the imaging properties of several NLO imaging modalities such as three-photon excited fluorescence and third-harmonic generation. In addition, refractive objectives simply cannot be used for NLO techniques that incorporate excitation light in the mid-infrared (MIR) range, such as photothermal imaging and sum-frequency generation, promising technologies based on MIR molecular contrast. The only viable alternative is the all-reflective Schwarzschild-Cassegrain (SC) objective, which is inherently achromatic but suffers from a non-ideal pointspread function and a center obscuration that limits throughput. Because of these limitations, SC lenses have not found widespread use in biological imaging applications. This lack of performance is also the reason why advances in exciting new MIR-based NLO imaging technologies have been stifled: there simply are no high-performance high numerical focusing options available to support these emerging imaging technologies. In this project, we develop a novel all-reflective high numerical aperture lens that overcomes all limitations of the SC focusing lens. Based on a non-concentric design, this new design features perfect color correction from the ultra-violet to the mid-infrared, exhibits a wide field of view, dramatically reduces group delay dispersion and significantly improves throughput by eliminating the center obscuration all together. This lens not only advances existing NLO modalities that rely on broadband radiation, but also enables new technologies such as photothermal imaging and SFG microscopy that have thus far suffered from low performance focusing optics.