Circular dichroism with multiple photoelastic modulators
Photoelastic modulators (PEMs) are the beating hearts of circular dichroism spectropolarimeters. They serve to modulate the light polarisation with high purity, efficiency, and stability, while operating over a broad spectral range. Recently, polarimeters have been deployed that use two and even four freely running PEMs. These instruments are particularly well suited to the analysis of complex anisotropic media whose chiroptical properties are a small part of the perturbation to the polarisation state of light. The advantages and challenges of using more complex polarisation modulation schemes are discussed herein.
Circular dichroism (CD) spectroscopy, the measurement of the difference in the wavelength dependent absorbance of left- and right-circularly polarised light (LCPL and RCPL, respectively) by suitably dissymmetric solutions is an essential analytical tool of the stereochemist and biophysicist (1). The phenomenon of CD was first detected by the Parisian student Aimé Cotton during his doctoral research (2, 3).
To observe what has now become his eponymous Cotton effect, young Aimé examined tartrate solutions of copper and chromium through orthogonally oriented quarter wave plates (QWPs) fitted side by side. QWPs transform linearly polarised light into LCPL or RCPL. Cotton's QWPs were linearly birefringent sheets of mica that achieved the aforementioned transformation by shifting the relative phases of the orthogonal x and y linear components that propagate through an anisotropic crystal by π/2 with respect to one another.
The resulting electric vector of light emergent from such a crystal will trace out a left- or right-handed helix in space. Cotton's paired, misoriented micas enabled him to see the effects of transmitt ...