Rapid chemically selective 3D imaging in the mid-infrared

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.

Fig. 1.

(a) Schematic representation of the setup. (b) Beam image on the CCD Si chip with MIR (2850 cm^‒1, 3500 nm) and gate NIR pulses (8333 cm^‒1, 1200 nm). The inset shows a gate pulse DTA image at the same scale. (c) Spatial cross section of the beam image on CCD Si chip with (NTA, orange line) and without MIR pulse (DTA, blue line). Inset: temporal cross-correlation of MIR and gate pulse, indicating a 110 fs pulse width (gray line—Gaussian fit).

Fig. 2.

(a)–(c) Tomographic imaging of the structured metal surface of a one cent US coin (Union Shield). (a) 3D reconstruction; (b) and (c) frames measured at height h = 30 µm and h = 0 µm, respectively. (d)–(f) Tomographic imaging of stacked cellulose acetate sheets, a weakly reflecting polymer structure. (d) 3D reconstruction; (e) and (f ) 2D frames taken at the top of each sheet (∆h = 105 µm). Total 3D scan time is 1 s.

Fig. 3.

Sketch of penetration experiment arrangement through (a) 3 mm thick GaAs wafer and (d) 190 µm water layer. (b) 3D reconstruction of one cent US coin (Union Shield) through 3 mm GaAs wafer. (c) Tomographic imaging of stacked cellulose acetate sheets through 3 mm GaAs wafer. Imaging of one cent US coin (Union Shield) through 190 µm water layer (380 µm in double pass) at (e) 2850 cm^‒1 and (f ) 2600 cm^‒1.

Fig. 4.

3D imaging of stacked cellulose acetate sheets with printed letters. (a) 3D reconstruction of the structure. (b) FTIR transmission spectrum of cellulose acetate (blue line) and real part of the refractive index obtained through a Kramers–Kronig transformation (orange dotted line). Rectangles represent Gaussian pulse width of ∼150 cm^‒1. (c) and (d) 3D imaging at 2875 cm^‒1; (e) and (f ) 3D imaging at 2600 cm^‒1. Total image acquisition time is 1 s.

Fig. 5.

3D imaging of a resin structure manufactured through projection-based photolithography technique. (a) 3D reconstruction of resin structure. (b) FTIR absorption spectrum of the resin (blue line) and real part of the refractive index obtained through a Kramers–Kronig transformation (orange dotted line). Rectangles represent Gaussian pulse width of ~150 cm^‒1. (c) and (d) 3D imaging at 2775 cm^‒1; (e) and (f ) 3D imaging at 2450 cm^‒1. Structure height is ~50 µm. Images have been corrected for non-spectroscopic, spectral NTA efficiency variations (see Supplement 1 Fig. S2). Total image acquisition time is 1 s.

Fig. 6.

Imaging of different lysozyme crystals on mica glass. (a) 3D reconstruction of lysozyme crystal cluster at 2875 cm^‒1. (b) 3D reconstruction of a single crystal. 2D image of the crystal’s top face taken at (c) 2875 cm^‒1, (d) 2600 cm^‒1, and (f ) 2450 cm^‒1. Images have been corrected for non-spectroscopic, spectral NTA efficiency variations (see Supplement 1 Fig. S2). FTIR absorption spectrum of lysozyme is shown on the far right.