# Measurement of infrared optical constants with visible photons

## TL;DR

This paper introduces a novel infrared spectroscopy technique using visible light and nonlinear interference of correlated photons, enabling measurement of infrared optical constants without specialized infrared detectors.

## Contribution

The authors present a new method for infrared spectroscopy that employs visible photons and quantum interference, eliminating the need for infrared sources and detectors.

## Key findings

- Infrared optical constants can be inferred from visible photon interference patterns.
- The method does not require prior knowledge of sample properties in the visible range.
- It broadens applicability to various samples with simpler equipment.

## Abstract

We demonstrate a new scheme of infrared spectroscopy with visible light sources and detectors. The technique relies on the nonlinear interference of correlated photons, produced via spontaneous parametric down conversion in a nonlinear crystal. Visible and infrared photons are split into two paths and the infrared photons interact with the sample under study. The photons are reflected back to the crystal, resembling a conventional Michelson interferometer. Interference of the visible photons is observed and it is dependent on the phases of all three interacting photons: pump, visible and infrared. The transmission coefficient and the refractive index of the sample in the infrared range can be inferred from the interference pattern of visible photons. The method does not require the use of potentially expensive and inefficient infrared detectors and sources, it can be applied to a broad variety of samples, and it does not require a priori knowledge of sample properties in the visible range.

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Source: https://tomesphere.com/paper/1706.04739