  Infrared (IR) spectroscopy is an indispensable tool for many practical
applications including material analysis and sensing. Existing IR spectroscopy
techniques face challenges related to the inferior performance and the high
cost of IR-grade components. Here, we develop a new method, which allows
studying properties of materials in the IR range using only visible light
optics and detectors. It is based on the nonlinear interference of entangled
photons, generated via Spontaneous Parametric Down Conversion (SPDC). In our
interferometer, the phase of the signal photon in the visible range depends on
the phase of an entangled IR photon. When the IR photon is traveling through
the media, its properties can be found from observations of the visible photon.
We directly acquire the SPDC signal with a visible range CCD camera and use a
numerical algorithm to infer the absorption coefficient and the refraction
index of the sample in the IR range. Our method does not require the use of a
spectrometer and a slit, thus it allows achieving higher signal-to-noise ratio
than the earlier developed method.
