UC Irvine

The mid-infrared (MIR) part of the electromagnetic spectrum corresponds to an energy range that includes the resonances of molecular bond vibrations. For this reason, MIR light is extensively used to characterize samples, as the absorption of particular MIR frequencies results in a spectrum corresponding to the mode vibrations of chemical motifs. The ability to exploit this in the context of imaging would allow both the morphological character and chemical composition of samples to be visualized simultaneously. Unfortunately, MIR imaging technologies still struggle to gain widespread adoption as analytical tools, as they fail to provide high-definition images quickly over the whole MIR spectrum. The work contained in this thesis seeks to overcome this hurdle by transferring the information carried by the MIR radiation to the visible-to-near-infrared part of the electromagnetic spectrum. In the latter spectral range, camera technologies are fast, sensitive and ever increasing in definition. Although the linear spectral responsivity of such cameras excludes the direct detection of MIR radiation, a nonlinear absorption process can be used to convert MIR light to charge carriers in the camera's light sensitive element. In this approach, two pulses of different energy, one in the MIR and one in the near infrared (NIR) are timed to simultaneously reach the detector, enabling the excitation of charge carriers through nondegenerate two-photon absorption (NTA). The use of NTA effectively turns a visible/NIR camera into a MIR imager. This thesis explores this principle to generate MIR images of a variety of samples, where the image contrast is provided by the spectroscopic fingerprints of molecular vibrations. The work in this thesis includes the first demonstration of the NTA principle for the purpose of widefield MIR imaging, followed by specific implementations that leverage the physical properties of ultrashort laser pulses to provide 3D imaging as well as hyperspectral imaging capabilities. In addition, this work discusses a rigorous theory of NTA in indirect semiconductors such as silicon. Together, the work presented herein provides a comprehensive picture of the physics and applications of the NTA process for rapid MIR imaging of a broad range of sample targets.