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Many fundamental molecular vibrations have energies in the mid-infrared (MIR) window—a wavelength region that stretches from approximately 2 to 10 μm. For this reason, the MIR range is of particular interest for spectroscopic imaging. The ability to generate images with chemical selectivity is of direct relevance to a myriad of fields, including the implementation of MIR-based imaging for biomedical mapping of tissues1-3, inspection of industrial ceramics4, stand-off detection of materials5, mineral sensing6, 7, and environmental monitoring8, among others.
Given its unique analytical capabilities, it is perhaps surprising that MIR-based imaging is not a more widely adopted technology for chemical mapping. The relatively scarce implementation of MIR imaging has been due in part to the lack of bright and affordable light sources in this range, although recent developments in MIR light source technology have largely overcome this problem9-11. Nonetheless, a remaining limitation is the performance and high cost of the MIR cameras. Current cameras are based on low bandgap materials, such as HgCdTe (MCT) or InSb, which inherently suffer from thermally excited electronic noise12. Cryogenic cooling helps to suppress this noise, but it renders the MIR camera a much less practical and affordable detector than mature Si-based detectors for the visible and near-IR. Electronically cooled MCT detectors are a promising alternative, although the matrixes of such detector arrays are still of low density and are not yet on par with high definition Si-based CCD cameras.
Recognizing the attractive features of Si-based cameras, several strategies have been developed that aim to convert information from the MIR range into the visible/NIR range, thus making it possible to indirectly capture MIR signatures with a Si detector. A very recent development is the use of an entangled MIR/visible photon pair, which allows MIR imaging and microscopy utilizing nonlinear interferometry for detecting visible photons entangled to their MIR counterparts on a Si-based camera13-15. Another strategy accomplishes the MIR-to-visible conversion by using a nonlinear optical (NLO) response of the sample, such as in third-order sum-frequency generation (TSFG) microscopy16. Photothermal imaging, which probes the MIR-induced changes in the sample with a secondary visible beam, is another example of this approach17-21. An alternative but related method is the acoustic detection of the MIR photothermal effect, which has recently been demonstrated22. Another technique uses a nonlinear optical crystal placed after the sample to up-convert the MIR radiation with an additional pump beam through the process of sum-frequency generation (SFG)23-29. The visible/NIR radiation produced can be efficiently registered with a high bandgap semiconductor detector. Elegant video-rate MIR up-conversion imaging has recently been accomplished with a Si-based camera at room temperature, offering an attractive alternative to imaging with MCT focal plane arrays30. A possible downside of SFG up-conversion techniques is the requirement of phase matching of the MIR radiation with the pump beam in the NLO medium. This requirement implies crystal rotation to enable the multiple projections needed for capturing a single image and postprocessing for each measured frame for image reconstruction.
An alternative to utilizing an optical nonlinearity of the sample or a dedicated conversion crystal for indirect MIR detection (SFG up-conversion) is the use of the NLO properties of the detector itself. In particular, the process of non-degenerate two-photon absorption (NTA) in wide bandgap semiconductor materials has been shown to permit the detection of MIR radiation at room temperature with the help of an additional visible or NIR probe beam31-34. In NTA, the signal scales linearly with the MIR intensity with detection sensitivities that rival those of cooled MCT detectors31. Compared with SFG-based up-conversion, NTA does not depend on phase matching and avoids the need for an NLO crystal altogether, offering a much simpler detection strategy. Moreover, the nonlinear absorption coefficient drastically increases with the energy ratio of the interacting photons35-39, allowing detection over multiple spectral octaves. Although NTA has been shown to enable efficient MIR detection with single pixel detectors, its advantages have not yet been translated to imaging with efficient Si-based cameras. Here, we report rapid, chemically selective MIR imaging using NTA in a standard CCD camera at room temperature.
The nature of nonlinear absorption enhancement for direct-band semiconductors has been modelled with allowed-forbidden transitions between two parabolic bands37-40. The nonlinear absorption coefficient α2 for photon energies ħωpump and ħωMIR can be written as40:
$$ \begin{array}{l}\alpha _2\left( {\omega _{\mathrm{p}},\omega _{{\mathrm{MIR}}}} \right) = K\frac{{\sqrt {E_{\mathrm{p}}} }}{{n_{\mathrm{p}}n_{{\mathrm{MIR}}}E_{\mathrm{g}}^3}}F\left( {x_{\mathrm{p}},x_{{\mathrm{MIR}}}} \right)\\ F = \frac{{\left( {x_{\mathrm{p}} + x_{{\mathrm{MIR}}} - 1} \right)^{3/2}}}{{2^7x_{\mathrm{p}}\left( {x_{{\mathrm{MIR}}}} \right)^2}}\left( {\frac{1}{{x_{\mathrm{p}}}} + \frac{1}{{x_{{\mathrm{MIR}}}}}} \right)^2,x_{\mathrm{p}} = \frac{{\hbar \omega _{\mathrm{p}}}}{{E_{\mathrm{g}}}},x_{{\mathrm{MIR}}} = \frac{{\hbar \omega _{{\mathrm{MIR}}}}}{{E_{\mathrm{g}}}}\end{array} $$ (1) where Ep is the Kane energy parameter, np and nMIR are refractive indices and K is a material independent constant. The function F accounts for the change in the nonlinear absorption as the ratio between the pump and MIR photon energies is adjusted, with dramatic enhancements when the pump energy is tuned closer to the bandgap energy Eg. For an indirect bandgap semiconductor, such as Si, optical transitions can be understood as a nonlinear process that involves three interacting particles—two photons and a phonon. Several models have been considered to describe multiphoton absorption in Si, including earlier "forbidden-forbidden" models41 and more recently suggested "allowed-forbidden" and "allowed-allowed" pathways42. The latter two models agree well with degenerate absorption experiments43. For the case of NTA, experiments demonstrate enhancement behaviour similar to those seen in direct-bandgap semiconductors44, 45, with the "allowed-allowed" pathways providing the best description46. Modest numbers of acquired and derived nonlinear absorption coefficients of only a few cm/GW have classified Si as a rather inefficient material for NTA. For this reason, attempts to develop MIR detection strategies based on Si detectors have been scarce46. In this work, we show that despite previous concerns, detecting MIR radiation through NTA in silicon is not only feasible but readily provides a very practical approach for MIR imaging with standard cameras.