Reconfigurable Kirigami Optics and Chiral Phonons

Abstract

All atoms and chemical bonds vibrate at natural frequencies associated with their physical properties. Scientists therefore use resonance-based spectroscopy to investigate the structural characteristics and dynamics of molecules in several research areas. However, it has been arduous to observe and identify complex vibrational modes of biomolecules and tissues with excitations in the terahertz (THz) range. In this work, we report on the development of THz circular dichroism spectroscopy enabled by kirigami polarization modulator and their applications for probing mesoscale chiral architectures and vibrations from the (bio) materials. Also, we show that hyperspectral THz chiroptical spectroscopy enables registration and attribution of chiral phonons in microcrystals of numerous amino acids and dipeptides. Terahertz circular dichroism (TCD) offers multifaceted spectroscopic capabilities for understanding of biomaterials, biomolecules, and pharmaceuticals because the energy of THz photons enables probing the ‘soft’ oscillatory vibrations of biomolecules with distinct chirality. However, the lack of dynamic polarization modulators is impeding the proliferation of TCD spectroscopy. In the Chapter 3 of this dissertation, we show that tunable optical elements fabricated from patterned plasmonic sheets with periodic kirigami cuts make possible polarization modulation of THz radiation under application of mechanical strain. A herringbone pattern of microscale metal stripes enables dynamic range of polarization modulation exceeding 80 degree repeatable over thousands of cycles. Upon out-of-plane buckling, the plasmonic stripes function as reconfigurable semi-helices of variable pitch aligned along the THz propagation direction. Several biomaterials, exemplified by elytra of Chrysina gloriosa beetles, revealed distinct TCD fingerprints associated with the helical substructure in the bio-composites. Chiral phonons, complex lattice vibrations modes with mirror asymmetry, have been known only for a small number of low-dimensional inorganic nanostructures. Abundant chiral phonon modes can also be expected for crystals of many biomolecules but experimental and theoretical toolbox on their observation and identification is unknown. Besides much larger variety of vibrational modes, chiral phonons in biological crystals can also be medically relevant. In the Chapter 4 of the dissertation, we show that terahertz absorption (TA), circular dichroism (TCD), and optical rotation dispersion (TORD) provide effective tools for the registration and identification of chiral phonons in micro-crystals of 20 proteinogenic L- and D-amino acids (AAs). Theoretical predictions and molecular dynamics simulations of chiral phonon in AA crystals provided direct evidence for the molecular origins of TCD and TORD spectra, which are dominated by collective motions of AA molecules. Generality of these findings can be highlighted by the observation of chiral phonons in crystals of dipeptides cystine and carnosine, which also demonstrates direct relevance of chiral phonons for medical and pharmaceutical applications.