San Francisco, CA--In session 8240-15 at Photonics West 2012, scientists from the Technical University of Denmark (DTU; Lyngby, Denmark) today presented their new approach to mid-IR imaging, which reduces noise by a factor of a billion when compared to conventional mid-IR imaging methods (such as microbolometers and low-bandgap semiconductors like indium antimonide and mercury cadmium telluride). The DTU device also detects single mid-IR photons, serving as a photon-counting imager.
In short, the device upconverts the incoming IR radiation using a lithium niobate (LiNbO3) nonlinear crystal via three-wave mixing, then images the upconverted light with a conventional silicon camera. A conventional IR camera produces on the order of 108 electrons per second, while a silicon camera produces only on the order of 0.1 electrons per second—thus the factor of 109. Light at wavelengths of 1.9 to 4.5 µm can be upconverted and imaged; the viewing angle of the prototype is 12°.
The incoming mid-IR signal is mixed with laser light, producing the upconversion. The upconversion itself is noise free, because upconverted photons are produced only as a result of incoming mid-IR photons; since LiNbO3 is transparent with an absorbance and emissivity of only 0.005/cm at a 3 µm wavelength, it adds virtually no thermal noise. In fact, the researchers had to heat the LiNbO3 crystal to 160°C to see measurable thermal noise.
The setup easily detects single photons; a blackbody object cooled to -10°C was imaged as its individual mid-IR photons were captured.
Jepp Dam, the presenter and one of the DTU researchers, noted that the conversion hardware is only about $10,000 in cost; combine that with a $1000 silicon camera, and you have a complete low-noise mid-IR imager. (The nonlinear crystal itself needs no optics in front of it.)
(Note: the DTU researchers presented technical details of the nonlinear upconversion technique—which is based on the dependence of the phase-matching condition on the angle of propagation of the two interacting fields with the optical axis of the crystal—in their immediately prior session 8240-14.)
