Abstract
Graded pitch profiles are found in structurally chiral materials like cholesteric liquid crystals (CLC) and in the cuticle of some scarab beetles. In most cases, the pitch profile is determined from electron microscopy techniques. Recently, it was shown that approximate pitch profiles in the cuticle of scarab beetles can be retrieved through an analysis of the spectral dependence of maxima and minima in normalized Mueller-matrix data. The analysis relies on basic concepts of interference in thin films, properties of optical modes in chiral systems, and the condition for circular Bragg reflection. In this work, the consistency of the procedure is demonstrated by analysis of normalized Mueller matrices of circular Bragg reflectors calculated for three predefined pitch profiles with (1) a stepwise decrease, (2) a stepwise increase and, (3) an exponential increase. The procedure does not require knowledge of the full Mueller matrix and can be used for non-destructive analysis of pitch in CLC, beetle cuticle and similar structures.
Highlights
Cholesteric liquid crystals (CLC) and the cuticle of some scarab beetles are structural and optical analogs [1]
The consistency of the procedure is demonstrated by analysis of normalized Mueller matrices of circular Bragg reflectors calculated for three predefined pitch profiles with (1) a stepwise decrease, (2) a stepwise increase and, (3) an exponential increase
Since the bandwidth of circular Bragg reflection of cholesteric liquid crystals (CLC) is limited by their birefringence, the broadening of selective reflection has been achieved by different methods producing a varying pitch across the thickness of the sample [12,13,14,15,16,17,18,19,20,21,22]
Summary
Cholesteric liquid crystals (CLC) and the cuticle of some scarab beetles are structural and optical analogs [1]. Since the bandwidth of circular Bragg reflection of CLC is limited by their birefringence, the broadening of selective reflection has been achieved by different methods producing a varying pitch across the thickness of the sample [12,13,14,15,16,17,18,19,20,21,22] In these cases, the graded pitch profile has been quantified from cross-section electron microscopy images. J. Opt. 21 (2019) 125401 that spectral analysis of maxima in minima in Mueller-matrix data can be used to determine approximate pitch variation across the cuticle of narrow- and broad-band circular Bragg reflectors [23,24,25,26]. We investigate the consistency of the procedure to determine approximate pitch profiles of circular Bragg reflectors from Mueller-matrix data.
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