Measurements of complex refractive indices of photoactive yellow protein

TL;DR

This paper introduces a new optical method using quantitative phase microscopy and Fourier light scattering to accurately measure the complex refractive indices of photoactive yellow proteins across visible wavelengths, revealing state-dependent optical properties.

Contribution

The study presents a novel technique combining quantitative phase microscopy with a wavelength swept source to measure the complex refractive index of photoactive proteins over a broad spectrum.

Findings

Measured real and imaginary refractive indices of PYP over 461-582 nm.

Quantified the change in CRI with blue light excitation.

Explained CRI deviations using Kramers-Kronig relations.

Abstract

A novel optical technique for measuring the complex refractive index (CRI) of photoactive proteins over the wide range of visible wavelengths is presented. Employing quantitative phase microscopy equipped with a wavelength swept source, optical fields transmitted from a solution of photoactive proteins were precisely measured, from which the CRIs of the photoactive proteins were retrieved with the Fourier light scattering technique. Using the present method, both the real and imaginary RIs of a photoactive yellow protein (PYP) solution were precisely measured over a broad wavelength range (461 - 582 nm). The internal population of the ground and excited states were switched by blue light excitation (445 nm center wavelength), and the broadband refractive index increments of each state were measured. The significant CRI deviation between in the presence and absence of the blue excitation was quantified and explained based on the Kramers-Kronig relations.