High-speed 2D and 3D mid-IR imaging with an InGaAs camera Eric O. Potma, * Dave Knez, and Dmitry A. Fishman † Department of Chemistry, University of California, Irvine, CA 92697 Martin Ettenberg, Matthew Wizeman, Hai Nguyen, and Tom Sudol Princeton Infrared Technologies, Momouth Jct., NJ 08852, US (Dated: July 5, 2021) Recent work on mid-infrared (MIR) detection through the process of non-degenerate two-photon absorption (NTA) in semiconducting materials has shown that wide-field MIR imaging can be achieved with standard Si cameras. While this approach enables MIR imaging at high pixel densities, the low nonlinear absorption coefficient of Si prevents fast NTA-based imaging at lower illumination doses. Here we overcome this limitation by using InGaAs as the photosensor. Taking advantage of the much higher nonlinear absorption coefficient of this direct bandgap semiconductor, we demon- strate high-speed MIR imaging up to 500 fps with under 1 ms exposure per frame, enabling 2D or 3D mapping without pre- or post-processing of the image. I. INTRODUCTION The process of non-degenerate two-photon absorp- tion (NTA) forms an attractive strategy for the de- tection of low energy photons with wide bandgap semiconductors.[1, 2] In NTA the energy needed for the generation of charge carriers in the semiconductor is de- termined by the sum of the energies of a long wavelength signal photon and a shorter wavelength gate photon. In particular, NTA has made it possible to detect signals in the mid-infrared (MIR) wavelength range, which roughly spans 3 μm–12 μm, with Si-based detectors.[3–5] When applied to imaging, detecting MIR light with Si detec- tor technologies offers several advantages compared to the use of low bandgap MIR cameras, including much lower thermal noise and significantly higher pixel densi- ties. For instance, NTA-based imaging with a Si CCD camera has enabled 4 Mpx MIR mapping with 100 ms exposure times.[6] In addition, using a femtosecond gate pulse, axial optical slicing of the 3D MIR image can be achieved, allowing tomographic mapping with contrast based on the sample’s MIR spectroscopic transitions.[7] NTA-based imaging with Si cameras offers a promising route for MIR mapping at high pixel densities. However, Si is an indirect semiconductor, and its nonlinear absorp- tion coefficient is unfavorably low compared to the ones of direct bandgap materials. Another limitation of Si is its shallow (linear) absorption edge, which translates into a spectral response of the camera that displays a tail on the low energy side, extending from 900 nm to well over 1100 nm. The shallow absorption edge profile limits flexible tuning of gate pulse energies due to one-photon absorption, which renders Si detectors incompatible for NTA-based MIR detection at energies below ∼ 900 cm -1 . The Urbach tail of direct bandgap semiconductors, on the other hand, generally displays a much steeper profile, and * epotma@uci.edu † dmitryf@uci.edu would thus enable NTA detection over a more extended MIR tuning range. In this work we push the efficiency of NTA-based MIR imaging by selecting a detector based on a direct bandgap semiconducting material. A careful examination of two- photon absorption efficiencies as well as the practical tun- ing range for MIR detection identifies InGaAs as an ideal candidate for NTA applications. Using an InGaAs cam- era, we achieve MIR imaging with frame rates that are two orders of magnitude faster than previously shown for Si CCD cameras. We show high-speed 2D and 3D imag- ing with 1Mpx frames at 100 fps and 40 kpx frames at 500 fps, using exposure times as low as 60 μs per frame. We demonstrate that these new imaging capabilities en- able direct in situ detection of several mechanical and physio-chemical processes. II. II. SEMICONDUCTORS NONLINEARITY SCALING RULE In the case of linear absorption in semiconducting ma- terials, where one photon excites an electron from the valence to the conduction band, the carrier population in the conduction band scales linearly with incoming optical power. In two-photon absorption, two photons are required to produce a similar transition if the sum of their energies exceeds the bandgap. For degener- ate two-photon absorption (DTA), the carrier population scales quadratically with incident optical power. To esti- mate the efficiency of such process, an elegant quantum- mechanical model based on Keldysh theory has been de- veloped in the 1980s.[8–10] Using a two-band model and a second-order perturbation approach, the DTA coefficient can be expressed as follows:[11] α DTA = K  E p F DTA (x) n 2 E 3 g (1) F DTA (x)= (2x - 1) 3/2 2 5 x 5 , x = ω E g (2) arXiv:2107.00720v1 [physics.optics] 1 Jul 2021