Development of low-cost, compact chiroptical imaging systems

Circular dichroism spectroscopy is a key probe of the structural and optical properties of chiral materials, however, commercial circular dichroism spectrometers are large, prohibitively expensive and rarely offer environmental control of the sample under test. Using Fresnel rhombs as inexpensive broadband quarter-wave plates, we demonstrate two novel, low-cost (<£2000) and portable imaging systems controlled by our own bespoke open-source control software which are capable of spatially mapping the circular dichroism of chiral solid state films. By coupling these imaging systems with a temperature controlled stage, we show that we can rapidly identify the thermal processing conditions required to maximise circular dichroism in chiral solid state films by measuring circular dichroism in situ during thermal annealing of a sample under test. The accuracy and spatial resolution of these circular dichroism imagers are cross-compared against our previous studies using an existing circular dichroism imaging system at the Diamond Light Source and are shown to be in good agreement, with a sensitivity down to 250 mdeg and a spatial resolution of 100 μm.


Derivation of Ellipticity
The electric field of elliptically polarised light (purple ellipse, Figure S1) may be considered an unequal sum of the electric field of a left-(red circle, Figure S1) and right-handed circularly polarised electric field (blue circle, Figure S1).

Figure S1
The electric field of elliptically polarised light (purple) expressed as the sum of a left- (red) and right-handed (blue) circularly polarised electric field.The ellipticity of the resultant electric field is shown here as θ.
The ellipticity of a given polarisation, θ, is defined as the angle between the points on the ellipse which intersect the major and minor axes.This is given by where and are the magnitudes of the electric fields corresponding to the left-and right-handed circularly polarised components of the elliptical polarisation.This may be expressed in terms of the intensity of the left-and right-handed circularly polarised light as follows: where we have used the relation For a sample illuminated by unpolarised or linearly polarised light, the incident intensity of left-and right-handed circularly polarised light are equal ( ).For an optically thick absorbing sample (i.e., a  0 sample without optical interference effects), the intensity of left-or right-handed circularly polarised light transmitted through the sample is given by where is the absorbance of sample.Applying this result to equation S2 we find: = )} []   (S6) where and are the absorbances of the sample under right-and left-handed circularly polarised light and is the difference between and (i.e., ).Finally, this may be converted from radians to the common ellipticity units of degrees, yielding: For small values of , this simplifies to:

Measurement of Ellipticity and Absorption Dissymmetry
As discussed in the manuscript, is calculated as follows: Similarly, for : [ log 10 ( 0 ) -log 10 (  ) + log 10 ( 0 ) -log 10 (  )] = 2 log 10 (  ) -log 10 (  ) [ 2log 10 ( 0 ) -log 10 (  ) -log 10 (  )] Measurements of require measurements of , and -the measurement of incident intensity cannot be avoided, unlike the case of .While the argument could be made that a calibration ∆ measurement could be made beforehand using a blank substrate to enable the measurement of , this  0 does not account for:  Any differences between the position of the sample and the substrate, especially given the 100 µm spatial resolution of the system. Any changes in light intensity following the initial calibration.Often the intensity of the illumination source must be modified during annealing to account for changes in absorption due to phase changes within the sample.
For this reason, measurements of obtained using a calibration sample will suffer from considerably   more uncertainty than that of a measurement of ellipticity.
FigureS2demonstrates the agreement between the precise and approximate equations for ellipticity as ( 0 ) -log 10 (  ) -log 10 ( 0 ) + log 10 (  ) = log 10 (  ) -log 10 (  ) and are the intensities of left-and right-handed circularly polarised light transmitted through the     sample under test and is the intensity of either left-or right-handed circularly polarised light incident  0 on the sample.The above equation demonstrates that the initial intensity of light does not need be measured for to be calculated -only the transmitted intensities (as measured by the cameras) are Δ required.

FigureFigure S5
Figure S4 A direct cross-comaprison between CD images of spatially patterned solid state F8BT:[P]aza[6]H captured using beamline 23 at the diamond light source (a) and the one camera CD imaging system (b).

Figure
Figure S6 CD, g abs , and absorption spectra of the cellulose nanocrystal film under rotation around the optical axis and sample flipping.At the wavelength of interest (405 nm), almost no change in any of the

Figure
Figure S7 A screenshot of the custom open-source python graphical user interface used to control the

Table S1
List of parts used to assemble the two-and one-camera CD imaging systems.Prices are accurate at the time of writing, September 2022.