A molecular vibration is a periodic motion of the atoms of a molecule relative to each other. The molecular vibration frequency range from less than 1013 hertz to approximately 1014 hertz; and with corresponding wavelengths of around 3 to 10 micrometers. This range corresponds to the Mid Infrared Range (MIR) of the electromagnetic spectrum, roughly covering the light wavelength. The chemical composition of a material can be determined by detecting and imaging this MIR, which produces images with contrast based on the chemical specificity.
Detecting MIR light is difficult when compared to visible light due to multiple factors.
- Present MIR camera technologies offer excellent sensitivity to MIR and, at the same time, affected by thermal noise
- Fastest MIR camera has low pixel density limiting high definition images
Several strategies have been developed to shift the MIR into visible range to overcome the problems with MIR camera so that a modern Silicon-based (Si) camera can detect them. However, technology to shift the MIR to the visible range is rather complicated. At present, the most direct way to achieve color conversion is through a non-linear optical crystal. When MIR light and another near-infrared light are coincident in the crystal, visible light is generated due to a sum-frequency generation process (SFG). SFG conversion has its drawbacks; numerous crystal orientations are required to produce an image on the Silicon-based camera.
A team of scientists from the University of California, Irvine led by Dr. Dmitry Fishman and Dr. Eric Potma, established a method for detecting MIR image with a silicon-based camera using non-linear optical properties of the silicon chip itself. Specifically, they used the process of non-degenerate two-photon absorption (NTA). When the MIR light illuminates the sensor, the NTA process with the help of an additional NIR ‘pump’ beam, triggers the generation of photo-induced charge carriers in Si. Since there are no non-linear optical crystals involved, the MIR imaging becomes very simple. This study is published in Light Science & Applications.
This principle enabled the team to perform vibrational spectroscopy measurements of organic liquids by employing a simple Si photodiode as the detector. The team replaced the photodiode with a charge-coupled device (CCD) camera. Through the NTA process, they were able to capture images of several polymer and biological materials and living nematodes on a 1392 x 1040 pixel sensor at 100ms exposure. The team was able to detect small changes in optical density in the image even though the technology is not optimized for NTA.
According to David Knez, one of the team members in this research, this approach’s simplicity, and versatility allow for broader adoption. The NTA process can reduce the analysis time in various fields, such as pharmaceutical quality assurance, geologic mineral sampling, or microscopic inspection of biological samples.
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