Abstract
We use a hemispheric sapphire lens in combination with an off-axis parabolic mirror to demonstrate high-resolution vibrationally resonant sum-frequency generation (VR-SFG) microscopy in the mid-infrared range. With the sapphire lens as an immersed solid medium, the numerical aperture (NA) of the parabolic mirror objective is enhanced by a factor of 1.72, from 0.42 to 0.72, close to the theoretical value of 1.76 ( = nsapphire). The measured lateral resolution is as high as 0.64 μm. We show the practical utility of the sapphire immersion lens by imaging collagen-rich tissues with and without the solid immersion lens.
Highlights
Optical microscopy with mid-infrared (MIR; 2.5-25 μm) light enables spectroscopic imaging with contrast based on molecular vibrational modes
With the sapphire lens as an immersed solid medium, the numerical aperture (NA) of the parabolic mirror objective is enhanced by a factor of 1.72, from 0.42 to 0.72, close to the theoretical value of 1.76 ( = nsapphire)
We show the practical utility of the sapphire immersion lens by imaging collagen-rich tissues with and without the solid immersion lens
Summary
Optical microscopy with mid-infrared (MIR; 2.5-25 μm) light enables spectroscopic imaging with contrast based on molecular vibrational modes. Several linear and nonlinear optical imaging techniques with vibrational contrast have been developed for biological applications in the MIR region with high sensitivity [1,2,3,4,5,6,7]. Vibrationally resonant sumfrequency generation (VR-SFG) microscopy is a second-order nonlinear optical imaging technique, which is sensitive to samples with noncentrosymmetry [6,7,8,9,10,11,12]. The molecular modes in VR-SFG are excited with an optical frequency ω1 in the MIR range, followed by an upconversion with a second optical χfr(2e)qiusefnrecqyuωen ciyn the visible/near-infrared range to generate a dependent in the MIR range, the signal grows visible signal at ω1 + stronger when the ω1. Since frequency approaches resonances of SFG-active molecular vibrational modes
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