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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - POLARSENSE (Polarimetry with spatially coherent detection for ultrasensitive detection)

Teaser

Summary

The POLARSENSE project proposed a new ground-breaking approach for the detection and quantification of optical activity that permits ultrasensitive chiral discrimination through optical means. Its objective was to set up a coherent detection method that exploited the spatial coherence of light, similarly as in double beam interferometers exploit the temporal coherence of light. Switching from temporal coherence to spatial coherence supposed a paradigm shift for the optical coherent detection technique and it allowed using a single light beam instead of having to deal with two different beams. With this new technique, two different parts of the beam will acquire different polarizations and will then recombine coherently. The formed interference pattern depends on the optical properties of the sample, and they can be recovered after an holographic analysis. This spatially coherent detection method can be implemented in a Mueller matrix polarimeter capable of detecting minute differences of polarization states associated to chiral signals.

The main goal of the POLARSENSE project was to achieve an unprecedented sensitivity in detection of optical activity, a wide applicability for virtually any kind of sample and in a wide spectral range. This has been achieved with a new technique for chiral sensing that is based on snapshot circular dichroism polarimetry, which permits recovering the chiroptical properties of a sample from analysis of an interference pattern formed in polarimetric experiment with a single beam.

Molecular chirality is a key area for large added value products: The pharmaceutical industry alone has worldwide sales of single enantiomer drugs of anywhere up to 410 billion Euros according to recent estimates. It is therefore not surprising that the necessity of producing enantiomerically pure compounds is now a basic tenet of the Chemical Industry. It is also a growing area for fundamental scientific research, given the unknown nature of the origins of homochirality in biological systems and the increasing number of applications of optical activity in photonics.

Work performed

The most relevant work and research results obtained in the Polarsense project are the following ones:
- A theoretical method to reconstruct a complete Mueller matrix for partial Mueller matrix experiments has been designed. This method allows working with both depolarizing and non-depolarizing schemes.
- A formalism to describe the coherent recombination process of the beam formulated in terms of covariance vectors and matrices. This formalism uses an analogy between deterministic matrix states associated with optical media and quantum mechanical wave functions, showing it is possible to construct a general formalism that accounts for the additional terms resulting from the coherency effects that average out for incoherent treatments. This has been applied to study the dipolar response of systems based on coupled oriented dipoles.
- An experimental determination of circular dichroism with snapshot ultrasensitive method in the visible spectral range. The method demonstrates an accurate and sensitive circular dichroism measurement from a single camera snapshot, making it compatible with real-time spectroscopy.
- A method to split the left and right circular polarization of unpolarized or polarized light beam. This method is based on an old design proposed by A. Fresnel in 1822. It makes use of a series of prisms of alternating handedness. When the light traverses this polyprism system it gets refracted at each interface with a different refractive angle according to its handedness, thus producing a macroscopically visible light splitting.

Final results

The results of POLARSENSE will significantly impact the way the chiroptical effects are studied, in particular for circular dichroism spectroscopy. We are developing techniques and methods that allow more reliable and accurate measurements, for example throught snapshot method that greatly reduce the measurement time and are compatible with real-time mesaurements. We have shown that it is possible to combine this more new specific technique for circular dichroism with a general approach based on Mueller matrix polarimetry to study different optical samples that are of interest for the organic chemistry, molecular biology, crystallography and photonics communities.