Broadband mid-infrared frequency upconversion and spectroscopy with an aperiodically poled LiNbO3 waveguide
Spotlight summary: What makes tomatoes look red or green? When sunlight, composed of all of the visible colors, hits carotenoids in a ripe tomato, the red light is picked out and scattered toward our eyes. By contrast, chlorophyll in unripe tomatoes dominantly scatters away green light. In that way, our eyes work like coarse, clumsy chemical sensors. In fact, humans evolved color vision partly for this purpose: to distinguish between ripe and unripe fruit. Using commercial detectors instead of eyes, many chemicals can be distinguished in this way, based on which wavelengths they scatter or absorb, a process which is called spectroscopy.
Past the red end of the visible spectrum is infrared (IR) light. Some materials which are transparent in the visible spectrum (like glass), scatter or absorb strongly in the infrared. So for example, with a good detector we can easily distinguish proteins, lipids or other cell components using spectroscopy.
The trouble is that good detectors are hard to find. Visible light can be sensed not only by our eyes, but by a range of cheap, widely available materials. But infrared detectors are expensive, and generally need to be cooled with liquid nitrogen. They’re not as sensitive to low light levels, which means that samples tend to look dark, and ultimately, chemical details can’t be resolved.
A possible solution is to convert the IR light into visible light, where conventional detectors can be used. This technique relies on nonlinear optics, in which a material (usually a crystal) is used to combine multiple low-energy IR photons into one high-energy visible photon. Unfortunately, nonlinear optics isn’t easy either. Typically, the crystal has to be cut into slices of equal thickness, and every second slice flipped backward to meet a condition known as phase matching. Even then, the crystal will only work for a tiny range of wavelengths, so trying to convert multiple wavelengths at once is impossible.
Neely et al. discovered that they could instead cut each slice of the crystal slightly larger than the last. When they flipped every second slice and reassembled the crystal, they found that it worked for a wide range of IR wavelengths. In essence, each small section of the crystal was effective for a small wavelength range. Put all together, they could convert the long span of IR light necessary for spectroscopy to the visible spectrum (or just a bit toward the near-infrared), and detect it with a high-sensitivity conventional detector.
They show convincing results from a proof-of-principle spectroscopy experiment on methane, the main component of natural gas. Interestingly, methane is commonly used to artificially ripen tomatoes during shipping. Coincidence?! You decide.
Technical Division: Light–Matter Interactions
ToC Category: Nonlinear Optics
|OCIS Codes:||(140.7090) Lasers and laser optics : Ultrafast lasers|
|(190.7220) Nonlinear optics : Upconversion|
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