  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.
