Abstract
The emerging technique of mid-infrared optical coherence tomography (MIR-OCT) takes advantage of the reduced scattering of MIR light in various materials and devices, enabling tomographic imaging at deeper penetration depths. Because of challenges in MIR detection technology, the image acquisition time is, however, significantly longer than for tomographic imaging methods in the visible/near-infrared. Here we demonstrate an alternative approach to MIR tomography with high-speed imaging capabilities. Through femtosecond nondegenerate two-photon absorption of MIR light in a conventional Si-based CCD camera, we achieve wide-field, high-definition tomographic imaging with chemical selectivity of structured materials and biological samples in mere seconds.
Highlights
Supplementing optical imaging with spectroscopic information enables the identification of objects based on their morphology and on their chemical composition
We show that nondegenerate two-photon absorption (NTA)-enabled tomography allows background-free 3D MIR imaging of weakly reflective interfaces of organic materials, objects underneath a 3 mm thick GaAs wafer, and even targets hidden under a 190 μm layer of water, using an average power density for the MIR illumination as low as 0.4 mW/cm2
MIR absorption of materials is measured with a commercial Fourier transform infrared (FTIR) spectrometer (Jasco 4700) either in transmission mode or by using an attenuated total reflection (ATR) accessory equipped with a diamond crystal
Summary
Supplementing optical imaging with spectroscopic information enables the identification of objects based on their morphology and on their chemical composition. These obstacles have spurred many developments that aim to convert the information encoded in MIR light into vis/NIR radiation [13,14,15,16,17,18,19,20] Such spectral conversion enables the use of mature detector technology based on Si or other wide bandgap semiconductor materials [21,22,23,24,25,26,27]. We apply this principle in a massively parallel fashion through the use of a 1.4 Mpx Si CCD camera, omitting the need for lateral scanning altogether and enabling the acquisition of 3D images in mere seconds or faster—acquisition rates that are up to 2 orders of magnitude higher than the present standard This novel detection strategy permits 3D MIR imaging at high sensitivity. To emphasize the chemical selectivity of MIR tomography, we demonstrate 3D images of polymer structures and protein crystals with spectroscopic contrast based on fundamental vibrational transitions in the 2000−3000 cm−1 range
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